Specialty chemical

Main Categories:

• Algaecides
• Antifoams
• Biocides
• Boiler water chemicals
• Coagulants
• Corrosion inhibitors
• Disinfectants

• Flocculants
• Neutralizing agents
• Oxidants
• Oxygen scavengers
• pH conditioners
• Resin cleaners
• Scale inhibitors

Algaecides:

Algaecides are chemicals that kill algae and blue or green algae, when they are added to water. Examples are copper sulphate, iron salts, rosin amine salts and Benzalkonium chloride. Algaecides are effective against algae, but are not very usable for algal blooms for environmental reasons.
The problem with most algaecides is that they kill all present algae, but they do not remove the toxins that are released by the algae prior to death.

Antifoams:  Silicon & glycol basis

Foam is a mass of bubbles created when certain types of gas are dispersed into a liquid. Strong films of liquid than surround the bubbles, forming large volumes of non-productive foam.
The cause of foam is a complicated study in physical chemistry, but we already know that its existence presents serious problems in both the operation of industrial processes and the quality of finished products. When it is not held under control, foam can reduce the capacity of equipment and increase the duration and costs of processes.
Antifoam blends contain oils combined with small amounts of silica. They break down foam thanks to two of silicone's properties: incompatibility with aqueous systems and ease of spreading. Antifoam compounds are available either as powder or as an emulsion of the pure product.

Powder
Antifoam powder covers a group of products based on modified polydimethylsiloxane. The products vary in their basic properties, but as a group they introduce excellent antifoaming in a wide range of applications and conditions.
The antifoams are chemically inert and do not react with the medium that is defoamed. They are odourless, tasteless, non-volatile, non-toxic and they do not corrode materials. The only disadvantage of the powdery product is that it cannot be used in watery solutions.

Emulsions
Antifoam Emulsions are aqueous emulsions of polydimethylsiloxane fluids. They have the same properties as the powder form, the only difference is that they can also be applied in watery solutions.

Biocides:

Biocides

Boiler feed water

A boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications.

The water circuit of a water boiler can be summarized by the following pictures:

The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure.

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment. For more information check the boiler water treatment web page.

In the following table you can find a list of the common boiler feed water contaminants, their effect and their possible treatment.

Boiler water treatment

The treatment and conditioning of boiler feed water must satisfy three main objectives:

• Continuous heat exchange
• Corrosion protection
• Production of high quality steam

External treatment is the reduction or removal of impurities from water outside the boiler. In general, external treatment is used when the amount of one or more of the feed water impurities is too high to be tolerated by the boiler system in question. There are many types of external treatment (softening, evaporation, , membrane contractors etc.) which can be used to tailor make feed-water for a particular system. Internal treatment is the conditioning of impurities within the boiler system. The reactions occur either in the feed lines or in the boiler proper. Internal treatment may be used alone or in conjunction with external treatment. Its purpose is to properly react with feed water hardness, condition sludge, scavenge oxygen and prevent boiler water foaming.

External treatment

The water treatment facilities purify and deaerate make-up water or feed water. Water is sometimes pretreated by evaporation to produce relatively pure vapor, which is then condensed and used for boiler feed purposes. Evaporators are of several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point. Sometimes to increase the efficiency the vapor from the first tank is passed through coils in a second tank of water to produce additional heating and evaporation. Evaporators are suitable where steam as a source of heat is readily available. They have particular advantages over demineralization, for example, when the dissolved solids in the raw water are very high.

Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions. Since simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation type softening. One of the most common and efficient combination treatments is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity and in some cases silica, and subsequent treatment with a cation exchange softener. This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.
Chemical treatment of water inside the boiler is usually essential and complements external treatment by taking care of any impurities entering the boiler with the feed water (hardness, oxygen, silica, etc.). In many cases external treatment of the water supply is not necessary and the water can be treated only by internal methods.

Internal treatment

Internal treatment can constitute the unique treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when good quality raw water is available. The purpose of an internal treatment is to

1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale;

2) condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal;

3) provide anti-foam protection to allow a reasonable concentration of dissolved and suspended solids in the boiler water without foam carry-over;

4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion.

In addition, as supplementary measures an internal treatment should prevent corrosion and scaling of the feed-water system and protect against corrosion in the steam condensate systems.

During the conditioning process, which is an essential complement to the water treatment program, specific doses of conditioning products are added to the water. The commonly used products include:

• Phosphates-dispersants, polyphosphates-dispersants (softening chemicals): reacting with the alkalinity of boiler water, these products neutralize the hardness of water by forming tricalcium phosphate, and insoluble compound that can be disposed and blow down on a continuous basis or periodically through the bottom of the boiler.
• Natural and synthetic dispersants (Anti-scaling agents): increase the dispersive properties of the conditioning products. They can be:
• Natural polymers: lignosulphonates, tannins
• Synthetic polymers: polyacrilates, maleic acrylate copolymer, maleic styrene copolymer, polystyrene sulphonates etc.
• Sequestering agents: such as inorganic phosphates, which act as inhibitors and implement a threshold effect.
• Oxygen scavengers: sodium sulphite, tannis, hydrazine, hydroquinone/progallol-based derivatives, hydroxylamine derivatives, hydroxylamine derivatives, ascorbic acid derivatives, etc. These scavengers, catalyzed or not, reduce the oxides and dissolved oxygen. Most also passivate metal surfaces. The choice of product and the dose required will depend on whether a deaerating heater is used.
• Anti-foaming or anti-priming agents: mixture of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry over of fine water particles in the steam.

The softening chemicals used include soda ash, caustic and various types of sodium phosphates. These chemicals react with calcium and magnesium compounds in the feed water. Sodium silicate is used to react selectively with magnesium hardness. Calcium bicarbonate entering with the feed water is broken down at boiler temperatures or reacts with caustic soda to form calcium carbonate. Since calcium carbonate is relatively insoluble it tends to come out of solution. Sodium carbonate partially breaks down at high temperature to sodium hydroxide (caustic) and carbon dioxide. High temperatures in the boiler water reduce the solubility of calcium sulphate and tend to make it precipitate out directly on the boiler metal as scale. Consequently, calcium sulphate must be reacted upon chemically to cause a precipitate to form in the water where it can be conditioned and removed by blow-down. Calcium sulphate is reacted on either by sodium carbonate, sodium phosphate or sodium silicate to form insoluble calcium carbonate, phosphate or silicate. Magnesium sulphate is reacted upon by caustic soda to form a precipitate of magnesium hydroxide. Some magnesium may react with silica to form magnesium silicate. Sodium sulphate is highly soluble and remains in solution unless the water is evaporated almost to dryness.

There are two general approaches to conditioning sludge inside a boiler: by coagulation or dispersion. When the total amount of sludge is high (as the result of high feed-water hardness) it is better to coagulate the sludge to form large flocculent particles. This can be removed by blow-down. The coagulation can be obtained by careful adjustment of the amounts of alkalis, phosphates and organics used for treatment, based on the fee-water analysis. When the amount of sludge is not high (low feed water hardness) it is preferable to use a higher percentage of phosphates in the treatment. Phosphates form separated sludge particles. A higher percentage of organic sludge dispersants is used in the treatment to keep the sludge particles dispersed throughout the boiler water.
The materials used for conditioning sludge include various organic materials of the tannin, lignin or alginate classes. It is important that these organics are selected and processed, so that they are both effective and stand stable at the boiler operating pressure. Certain synthetic organic materials are used as anti-foam agents. The chemicals used to scavenge oxygen include sodium sulphite and hydrazine. Various combinations of polyphosphates and organics are used for preventing scale and corrosion in feed-water systems. Volatile neutralizing amines and filming inhibitors are used for preventing condensate corrosion.

Common internal chemical feeding methods include the use of chemical solution tanks and proportioning pumps or special ball briquette chemical feeders. In general, softening chemicals (phosphates, soda ash, caustic, etc.) are added directly to the fee-water at a point near the entrance to the boiler drum. They may also be fed through a separate line discharging in the feed-water drum of the boiler. The chemicals should discharge in the fee-water section of the boiler so that reactions occur in the water before it enters the steam generating area. Softening chemicals may be added continuously or intermittently depending on feed-water hardiness and other factors. Chemicals added to react with dissolved oxygen (sulphate, hydrazine, etc.) and chemicals used to prevent scale and corrosion in the feed-water system (polyphosphates, organics, etc.) should be fed in the feed-water system as continuously as possible. Chemicals used to prevent condensate system corrosion may be fed directly to the steam or into the feed-water system, depending on the specific chemical used. Continuous feeding is preferred but intermittent application will suffice in some cases.

Scaling in boilers

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots. Leading to local overheating. Scaling mechanism is the exceeding of the solubility limits of mineral substances due to elevated temperature and solids concentration at the tube/water interface. The deposition of crystalline precipitates on the walls of the boiler interferes with heat transfer and may cause hot spots, leading to local overheating. The less heat they conduct, the more dangerous they are.

Common feed water contaminants that can form boiler deposits include calcium, magnesium, iron, aluminum, and silica. Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate. 
The values corresponding to their thermal conductivity are:

Steel 15 kcal/m2.h per degree C
CaSO4 1-2 kcal/m2.h per degree C
CaCO3 0.5-1 kcal/m2.h per degree C
SiO2 0.2-0.5 kcal/m2.h per degree C

Scaling is mainly due to the presence of calcium and magnesium salts (carbonates or sulphates), which are less soluble hot than cold, or to the presence of too high concentration of silica in relation to the alkalinity of the water in the boiler. 
A carbonate deposit is usually granular and sometimes of a very porous nature. The crystals of calcium carbonate are large but usually are matted together with finely divided particles of other materials so that the scale looks dense and uniform. Dropping it in a solution of acid can easily identify a carbonate deposit. Bubbles of carbon dioxide will effervesce from the scale. 
A sulphate deposit is much harder and more dense than a carbonate deposit because the crystals are smaller and cement together tighter. A Sulphate deposit is brittle, does not pulverize easily, and does not effervesce when dropped into acid. 
A high silica deposit is very hard, resembling porcelain. The crystal of silica are extremely small, forming a very dense and impervious scale. This scale is extremely brittle and very difficult to pulverize. It is not soluble in hydrochloric acid and is usually very light coloured.
Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in hot acid giving a dark brown coloured solution.

The first anti-scaling preventative measure is to supply good quality demineralized water as make–up feed water. The purer the feed water is, the weaker the driving mechanism to form scale. Scale-forming minerals that do enter the boiler can be rendered harmless by internal chemical treatment. A long-established technique is to detach the hardness cations, magnesium and calcium, from the scale forming minerals and to replace them with sodium ions.

Presence of Silica

Silica can vaporize into the steam at operating pressures as low as 28 bars. Its solubility in steam increases with increased temperature; therefore, silica becomes more soluble as steam is superheated. The conditions under which vaporous silica carryover occurs have been thoroughly investigated and documented. Researchers have found that for any given set of boiler conditions using demineralized or evaporated quality make-up water, silica is distributing between the boiler water and the steam in a definite ratio. This ratio depends on two factors: boiler pressure and boiler water pH. The value of the ratio increases almost logarithmically with increasing pressure and decreases with increasing pH.
If the silica enters the boiler water, the usual corrective action is to increase boiler blowdown, to decrease it to acceptable levels and then to correct the condition that caused the silica contamination.

Corrosion in boilers

Corrosion is the reversion of a metal to its ore form. Iron, for example, reverts to iron oxide as the result of corrosion. The process of corrosion, however is a complex electro chemical reaction and it takes many forms. Corrosion may produce general attach over a large metal surface or it may result in pinpoint penetration of metal. Corrosion is a relevant problem caused by water in boilers. Corrosion can be of widely varying origin and nature due to the action of dissolved oxygen, to corrosion currents set up as a result of heterogeneities on metal surfaces, or to the iron being directly attacked by the water. 
While basic corrosion in boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses, acid conditions, and specific chemical corrodents may have an important influence and produce different forms of attack. It is necessary to consider the quantity of the various harmful substances that can be allowed in the boiler water without risk of damage to the boiler. Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide.
Starting form these figures, and allowing the amount that can be blown down, the permitted concentration in the make-up water is thus defined.

Corrosion is caused principally by complex oxide-slag with low melting points. High temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct contact with the metal. Deposits also promote the transport of oxygen to the metal surface. 
Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.
Corrosion is caused by the combination of oxide layer fluxing and continuous oxidation by transported oxygen

Cracking in boiler metal may occur by two different mechanisms. In the first mechanism, cyclic stresses are created by rapid heating and cooling and are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. The second type of corrosion fatigue cracking occurs in boilers with properly treated water. In these cases corrosion fatigue is probably a misnomer. These cracks often originate where a dense protective oxide film covers the metal surfaces and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are most often circumferential on the tube.

Corrosion control techniques vary according to the type of corrosion encountered. Major methods include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses trough design and operational practices.
Deaeration and recently the use of membrane contractors are the best and most diffused ways to avoid corrosion removing the dissolved gasses (mainly O2 and CO2).

For further information about the different types of corrosion check the following web pages:

• Galvanic corrosion
• Caustic corrosion
• Acidic corrosion
• Hydrogen embrittlement
• Oxygen attack
• Carbon dioxide attack

Protection of steel in a boiler system depends on temperature, pH, and oxygen content. Generally, higher temperatures, high or low pH levels and higher oxygen concentrations increase steel corrosion rates. Mechanical and operation factors such as velocities, metal stresses, and severity of service can strongly influence corrosion rates. Systems vary in corrosion tendencies and should be evaluated individually.

Foaming and priming in boilers

Boiler water carry-over is the contamination of the steam with boiler-water solids. Bubbles or froth actually build up on the surface of the boiler water and pass out with the steam. This is called foaming and it is caused by high concentration of any solids in the boiler water. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are particularly conducive to foaming. In theory suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking and builds up foam. It is believed that the finer the suspended particles the greater their collection in the bubble.

Priming is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler-water. 
Some mechanical entrainment of minute drops of boiler water in the steam always occurs. When this boiler water carryover is excessive, steam-carried solids produce turbine blade deposits. The accumulations have a composition similar to that of the dissolved solids in the boiler water. Priming is common cause of high levels of boiler water carryover. These conditions often lead to super heater tube failures as well. Priming is related to the viscosity of the water and its tendency to foam. These properties are governed by alkalinity, the presence of certain organic substances and by total salinity or TDS. The degree of priming also depends on the design of the boiler and its steaming rate.

The most common measure to prevent foaming and priming is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foaming and anti-priming agents, mixtures of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry-over of fine water particles in the stream, can be very effective in preventing carry-over due to high concentrations of impurities in the boiler-water.

Oxygen attack in boilers

Without proper mechanical and chemical deaeration, oxygen in the feed water enters the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. Oxygen in water produces pitting that is very severe because of its localized nature. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability.

Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems: iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area. Oxygen attack is an electrochemical process that can be described by the following reactions:

Anode: Fe è Fe2+ + 2e-

Cathode: ½ O2 + H2O + 2e- è 2 OH-

Overall: Fe + ½ O2 + H2O è Fe(OH)2

In this reaction a temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in a rapid and severe corrosion.

The acceptable dissolved oxygen level for an

The acceptable dissolved oxygen level for any system depends on may factors, such as feed water temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. In general, the limit value of oxygen in make-up water can be stared 0.10 mg/kg

For a complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration. Membrane contractors are also a possibility.

Carbon dioxide attack in boilers

Carbon dioxide exists in aqueous solutions as free carbon dioxide and the combine forms of carbonate and bicarbonate ions. Corrosion is the principal effect of dissolved carbon dioxide. The gas will dissolve in water, producing corrosive carbonic acid:

H2O + CO2 çè H2CO3 çè H+ + HCO3-

The low pH resulting from this reaction also enhances the corrosive effect of oxygen.

In boiler systems, corrosion resulting from carbon dioxide is most often encountered in the condensate system. Because feed water deaeration normally removes carbon dioxide from the boiler feed water, the presence of the gas in condensate is typically due to carbonate and bicarbonate decomposition under boiler conditions. For an approximation is estimated that feed water with a total alkalinity of 100 mg/l as calcium carbonate could be expected to generate a carbon dioxide level of 79 mg/l in the steam (alkalinity multiplied by a factor 0.79). Such a high carbon dioxide level would create a very corrosive condensate.

Carbon dioxide corrosion is frequently encountered in condensate systems and less commonly in water distribution systems.

Find information about the main problems occurring in boilers: scaling, foaming and priming, and corrosion. 
For a description of the characteristics of the perfect boiler water click here. 
Check also our web page about boiler feed water and boiler water treatment, in particular through deaeration (deaerating heaters or membrane contractors).

Membrane contractors for boilers

Recently membrane contractors have been utilized to remove the dissolved gasses (O2 and CO2) in boiler feed water. Widely used in the semiconductor, power, pharmaceutical and other industries to control dissolved gasses in water systems, their use in boiler feed water degasification systems has grown steadily since the development of new industrial grade devices. They have displaced the vacuum tower, forced draft deaerator, and oxygen scavengers around the world for over 10 years.

Membrane contractors are constructed using micro porous hydrophobic membranes. The membrane is used to bring a gas and liquid in direct contact without mixing. Contractors operate by lowering the pressure of gas in contact with the liquid to create a driving force to remove the dissolved gases from the water.
Contactors are used extensively for deaeration of liquids in the microelectronics, pharmaceutical, power, food & beverage, industrial, photographic, ink and analytical markets.
The running cost of a membrane system includes electricity and seal water for a vacuum pump. A typical membrane system designed to degas the water outlined in this example can reach payback in less than two years.

Chemical treatment is also widely used to control dissolved oxygen in a boiler. Any chemical that is added to the water can increase the frequency of blow down, which affects operating costs of the boiler. In smaller low-pressure boilers (< 4500 lb/h and < 3.45 bars), chemical treatment alone may be used. A combination of steam deaeration and chemicals is most often used for larger high-pressure boilers.

Dissolved oxygen and carbon dioxide control in boiler feed water protects the boiler from corrosion. Membrane contractors can be used to replace or supplement the chemical treatment program, which is often used to control dissolved oxygen. The contractors can minimize the volume of chemicals added to the feed water and offer savings to the end user by reducing chemicals and energy costs.

Find extra information about boiler water treatment and deaeration.
Check also our web page about he main problems occurring in boilers: scaling, foaming and priming, and corrosion. 
For a description of the characteristics of the ideal boiler water click here.

Deaeration in boilers

In order to meet industrial standards for both oxygen content and the allowable metal oxide levels in feed water, nearly complete oxygen removal is required. This can be accomplished only by efficient mechanical deaeration supplemented by a properly controlled oxygen scavenger.

Deaeration is driven by the following principles: the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas at the liquid surface, decreases with increasing liquid temperature; efficiency of removal is increased when the liquid and gas are thoroughly mixed.

Deaeration can be performed using a physical medium such as deaerating heaters or vacuum deaerators or a chemical medium such as oxygen scavengers (polishing treatment) or catalytic resins. Membrane contractors are increasingly being used. Carbon dioxide is often removed using a physical medium.

The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level and improve plant thermal efficiency by raising the water temperature. In addition, they provide feed water storage and proper suction conditions for boiler feed water pumps.

Pressure deaerators can be classified under two major categories: tray type and spray type.

The Acrolein consist of a shell, spray nozzles to distribute and spray the water, a direct contact vent condenser, tray stacks and protective interchamber walls. The chamber is constructed in low carbon steel, but more corrosion-resistant stainless steels are used for the spray nozzles and the other parts.

Incoming water is sprayed into steam atmosphere, where it is heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases (principally oxygen and free carbon dioxide) are released to the steam as the water is sprayed into the unit. Seals prevent the recontamination of tray stack water by gases from the spray section. Water falls from tray to tray, breaking into fine droplets of film, which intimately contact the incoming steam.

The steam heats the water to the steam saturation temperature and removes the very last traces of oxygen. Deaerated water falls to the storage space below, where a steam blanket protects it from recontamination. It is usually stored in a separate tank.

The steam enters the deaerators through ports in the tray compartment, flows down through the tray stack parallel to the water flow. A very small amount of steam condenses in this section as the water temperature rises to the saturation temperature of the steam. The rest of the steam scrubs the cascading water. Before leaving the tray compartment, the steam flows upward between the shell and the interchamber walls to the spray section. Most of the steam is condensed and becomes part of the deaerated water. A small portion of the steam, which contains the non-condensable gas released from the water, is vented to the atmosphere. It is essential that sufficient venting is provided at all times or deaeration will be incomplete. Steam flow through the tray stack may be cross-flow, counter-current, or co-current to the water.

The Spray type deaerating heaters
 consist of a shell, spring-loaded inlet spray valves, a direct contact vent condenser section and a steam scrubber for final dearetion; the shell and steam may be low carbon steel, the spray valves and the direct contact vent condenser section are in stainless steel. The incoming water is sprayed into a steam atmosphere and heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases are released to the steam, and the heated water falls to water seals and drains to the lowest section of the steam scrubber. The water is scrubbed by a large volume of steam and heated to the saturation temperature prevailing at that point. As the water-steam mixture rises in the scrubber, the deaerated water is a few degrees above the saturation temperature, due to a slight pressure loss. In this way a small amount of flashing is produced, which aids in the release of dissolved gases. The deaerated water overflows from the steam scrubber to the storage section below.

Steam enters the deaerator through a chest on the side and flows to the steam scrubber. After flowing into the scrubber it passes up into the spray heater section to heat the incoming water. Most of the steam condenses in the spray section to become a part of the deaerated water. A small portion of the gases is vented to the atmosphere to remove the non-condensable gases.

Vacuum deaeration is used at temperatures below the atmospheric boiling point to reduce the corrosion rate in water distribution systems. A vacuum is applied to the system to bring the water to its saturation temperature. Spray nozzles break the water into small particles to facilitate gas removal and vent the exhaust gases. Incoming water enters through spray nozzles and falls through a columns packed with Raschig rings to other synthetic packing. In this way, water is reduced to thin films and droplets, which promote the release of dissolved gases. The released gases and water vapor are removed through the vacuum, which is maintained by steam jet eductors or vacuum pumps, depending on the size of the system. Vacuum deaerators remove oxygen less efficiently that pressure units.

Corrosion fatigue at or near welds is a major problem in deaerators. It is the result of mechanical factors, such as manufacturing procedures, poor welds and lack of stress-relieved welds. Operational problems such as water/steam hammer can also be a factor.

Find extra information about boiler feed water and boiler water treatment.
Check also our web page about the main problems occurring in boilers: scaling, foaming and priming, and corrosion. 
For a description of the characteristics of the perfect boiler water click here.

Galvanic-corrosion

Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal alloy. The most common type of galvanic corrosion in a boiler system is caused by the contact of dissimilar metals, such as iron and copper. This differential cells can also be formed when deposits are present. Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction, including: scratches in a metal surface, differential stresses in a metal, differences in temperature, conductive deposits.

Pitting of boiler tube banks has been encountered due to metallic copper deposits. Dissolved copper may be plated out on freshly cleaned surfaces, establishing anodic corrosion areas and forming pits.

This process is illustrated by the following reactions.

Using hydrochloric acid as the cleaning solvent, magnetite is dissolved and yields an acid solution containing both ferrous (Fe2+) and ferric (Fe3+) chloride. The latter are very corrosive to steel and copper.

Fe3O4 + 8 HCl è FeCl2 + 2 FeCl3 + 4H2O

Metallic or elemental copper in boiler deposits is dissolved in the hydrochloric acid solution by the following reaction:

FeCl3 + Cu è CuCl + FeCl2

Once cuprous chloride is in solution, it is immediately redeposited as metallic copper on the steel surface according to the following reaction:

2CuCl + Fe è FeCl2 + 2 Cu

Thus, hydrochloric acid cleaning can cause galvanic corrosion. A complexing agent is added to prevent the copper from redepositing on the steel surface. The following chemical reaction results:

FeCl3 + Cu + Complexing agent è FeCl2 + CuCl

This can take place as a separate step or during acid cleaning. Both iron and copper are removed from the boiler, and the boiler surface can then be passivated.

Find information about the main problems occurring in boilers: scaling, foaming and priming, and corrosion. 
For a description of the characteristics of the perfect boiler water click here. 
Check also our web page about boiler feed water and boiler water treatment, in particular through deaeration (deaerating heaters or membrane contractors).

Ultrapure water

Ultrapure water (UPW) is water that has been purified to very strict specifications, containing by definition only H20, and H+ and OH- ions in equilibrium. Therefore, ultrapure water conductivity is about 0,055 uS/cm at 25oC, also expressed as resistivity of 18,2 MOhm.

Ultra-pure water is mainly used in the semiconductor and pharmaceutical industry.

Ultrapure water production often is produced through processes like

 membrane filtration or ion exchange to reach the ultimate conductivity of 10 uS/cm.

The demineralized water is then processed through a high performance Mixed Bed or by Electrodeionization

Cooling water quality

Cooling water quality depends on the type of heat exchanger or engine to be cooled. Very generally, it should have the following quality:

Deionized/Demineralized Water

Distilled,deionized and demineralized water and measuring of the purity

Demineralization: Any process used to remove minerals from water, however, commonly the term is restricted to ion exchange processes. [1]

Ultra pure water: Highly-treated water of high resistivity and no organics; usually used in the semiconductor and pharmaceutical industries [4]

Deionization entails removal of electrically charged (ionized) dissolved substances by binding them to positively or negatively charged sites on a resin as the water passes through a column packed with this resin. This process is called ion exchange and can be used in different ways to produce deionized water of various qualities.

• Strong acid cation + Strong base anion resin systems
  These systems consist of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form (see picture below). Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions. The decationised water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O). [2]
  These systems remove all ions, including silica. In the majority of cases it is advisable to reduce the flux of ions passed to the anion exchanger by installing a CO2 removal unit between the ion exchange vessels. This reduces the CO2 content to a few mg/l and brings about a reduction of the following strong base anion resin volume and in the regeneration reagent requirements.
  In general the strong acid cation and strong base anion resin system is the simplest arrangement and a deionized water that may be used in a wide variety of applications can be obtained with it.
• Strong acid cation + weak base anion + Strong base anion resin systems
  This combination is a variation of the previous one. It provides the same quality of deionized water, while offering economic advantages when treating water which contains high loads of strong anions (chlorides and sulphates). The subtitle shows that the system is equipped with an extra weak base anion exchanger before the final strong base anion exchanger. The optional CO2 removal unit may be installed either after the cation exchanger, or between the two anion exchangers (see picture below). The regeneration of the anion exchangers takes place with caustic soda (NaOH) solution first passing through the strong base resin and then through the weak base resin. This method requires less caustic soda than the method described before because the remaining regeneration solution after the strong base anion exchanger is usually sufficient to regenerate the weak base resin completely. Moreover, when raw water contains a high proportion of organic matter, the weak base resin protects the strong base resin. [3

• 
  
  Mixed-bed Deionization
  In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The two resins are mixed by agitation with compressed air, so that the hole bed can be regard as an infinite number of anion and cation exchangers in series (mixed bed resin). [2,3]
  
  To carry out regeneration, the two resins are separated hydraulically during the loosening phase. As the anion resin is lighter than the cation resin it rises to the top, while the cation resin falls to the bottom. After the separation step the regeneration is carried out with caustic soda and a strong acid. Any excess regenerant is removed by rinsing each bed separately. 
  The advantages of mixed bed systems are as follows:
  
  - the water obtained is of very high purity and its quality remains constant throughout the cycle,
  - pH is almost neutral,
  - rinse water requirements are very low.
  
  The disadvantages of mixed bed systems are a lower exchange capacity and a more complicated operating procedure because of separation and remixing steps which have to be carried out. [3]

Next to the ion exchange systems deionized water can be produced with reverse osmosis plants. Reverse osmosis is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the color, taste or properties of the fluid. Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 Daltons.
RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. This process achieves rejections of 99.9+% of viruses, bacteria and pyrogens. Pressure in the range of 50 to 1000 psig (3.4 to 69 bar) is the driving force of the RO purification process. It is much more energy-efficient compared to phase change processes (distillation) and more efficient than the strong chemicals required for ion exchange regeneration. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected. [4]

Measuring of the purity

Water purity may be measured in various ways. One can attempt to determine the weight of all of the dissolved material ("solute"); this is most easily done for dissolved solids, as opposed to dissolved liquids or gases. In addition to actually weighing the impurities, one can estimate their level by the degree to which they increase the boiling point or lower the freezing point of water. The refractive index (a measure of how transparent materials bend light waves) is also affected by solutes in water. Alternately, water purity can be quickly estimated on the basis of electrical conductivity or resistance — very pure water conducts electricity poorly, so its resistance is high. [2]

pH-value

Pure water by definition is slightly acidic and distilled water will test out around pH 5.8. The reason is that distilled water dissolves carbon dioxide from the air. It dissolves carbon dioxide until it is in dynamic equilibrium with the atmosphere. That means that the amount being dissolved balances the amount coming out of solution. The total amount in the water is determined by the concentration in the atmosphere. The dissolved carbon dioxide reacts with the water and finally forms carbonic acid.

2 H2O + CO2 --> H2O + H2CO3 (carbonic acid) --> (H30+) (charged acidified water) + (HCO3-) (charged bicarbonate ion)

Only recently been produced distilled water has a pH-value of approximately 7, but affected by the presence of carbon dioxide it will reach a slightly acidic pH-value within a couple of hours.
Additional, it is important to mention that the pH of ultra-pure water is difficult to measure. Not only does high-purity water rapidly pick up contaminants - such as carbon dioxide (CO2) - that affect its pH, but it also has a low conductivity that can affect the accuracy of pH meters. For instance, absorption of just a few ppm of CO2 can cause the pH of ultra-pure water to drop to 4.5, although the water is still of essentially high quality.

The most accurate estimation of the pH of ultra-pure water is obtained by measuring its resistance; for a given resistance, the pH must lie between certain limits. For example, if the resistance is 10.0 MWcm, the pH must lie between 6.6 and 7.6. The relationship between the resistance and pH of high-purity water is shown in the figure below. [2]

Electrical resistivity versus pH of deionized water [2]

Compared with other beverages deionized water has apparently a slightly acidic pH-value.

According to the Merck Manual the human body uses buffers to balance the pH. If a person consumes something acid, the blood will produce more bicarbonate and less carbon dioxide to neutralize the acidity. Likewise, the blood will produce more carbon dioxide and less bicarbonate if an alkaline substance is consumed. So drinking distilled water, will not put a human body in an acidic state.

Drinking water production

Drinking water can be produced from any natural sources like groundwater, lakes and rivers (surface waters) or seawater.

Drinking water standards are set by the World Health organisation or by the European Union.

Drinking water must be free of suspended solids, microorganisms and toxic chemicals. Mineral concentration recommendation vary from country to country but most of the minerals have a maximum concentration recommended to ensure safe, equilibrated and pleasant water to drink.

For municipal drinking water, a special focus is carried on the corrosivity and scaling potential of the water to maintain distribution piping in good shape. Typical pH 8, TAC 8 and TH 8 are applied, when possible.

For bottled water, taste can vary upon calcium, magnesium, sulfate and iron content.

For the answers to your questions on drinking water please check out our drinking water FAQ.

Here an example of a drinking water purification process.

A drinking water purification process

Drinking water production from surface water

On this page you will find an explanation of a drinking water purification process. All process steps are numbered and the numbers correspond with the numbers in the schematic representation of the drinking water process found below. This is a summing up of the process steps:

a: Prefiltration

1) The uptake of water from surface waters or groundwater and storage in reservoirs. Aeration of groundwater and natural treatment of surface water usually take place in the reservoirs. Often softening and pH-adjustments already happen during these natural processes.

2) Rapid sand filtration or in some cases microfiltration in drum filters.

b: Addition of chemicals

3) pH adjustment through addition of calcium oxide and sodium hydroxide.

4) FeCl3 addition to induce flocculation for the removal of humic acids and suspended particulate matter, if necessary with the addition of an extra flocculation aid. Flocs are than settled and removed through lamellae separators. After that the flocs are concentrated in sludge and pumped to the exterior for safe removal of the particulates and sludge dewatering.

5) Softening in a reservoir, through natural aeration or with sodium hydroxide, on to 8,5 oD. This is not always necessary. For instance, in case natural filtration will be applied, softening takes place naturally.

c: Natural filtration

6) Drinking water preparation step that is specific for the Netherlands: Infiltration of the water in sand dunes for natural purification. This is not applied on all locations The water will enter the saturated zone where the groundwater is located and it will undergo further biological purification. As soon as it is needed for drinking water preparation, it will be extracted through drains.

d: Disinfection

7) Disinfection with sodium hypochlorite or ozone. Usually ozonation would be preferred, because ozone not only kills bacteria and viruses; it also improves taste and odour properties and breaks down micro pollutants. Ozone diffuses through the water as small bubbles and enters microrganisms cells by diffusion through cell walls. It destroys microrganisms either by disturbance of growth or by disturbance of respiratory functions and energy transfers of their cells. During these processes ozone is lost according to the reaction O3 -> O2 +(O).

e: Fine filtration

8) Slow sand (media) filtration for the removal of the residual turbidity and harmful bacteria. Sand filters are backwashed with water and air every day.

9) Active carbon filtration for further removal of matter affecting taste and odour and remaining micro pollutants. This takes place when water streams through a granular activated carbon layer in a filter. Backwash is required regularly due to silting up and reactivation of an active carbon filter should be done once a year.

f: Preservation and storage

10) Addition of 0.3 mg/L sodium hypochlorite to guarantee the preservation of the obtained quality. Not all companies chlorinate drinking water. The water will eventually be distributed to users through pipelines and distribution pumps.

11) Aeration for recovery oxygen supply of the water prior to storage. This is not always applied.

12) Remaining water can be stored in drinking water reservoirs.

In the following schematic representation of the drinking water preparation process dotted arrows represent the incoming chemicals and red arrows represent the outgoing flows.

Schematic representation of the drinking water preparation process

Water is not always infiltrated in sand dunes during treatment. Holland clearly illustrates this:
- In Rotterdam water is stored in reservoirs in the Biesbosch, where it undergoes natural treatment
- In Amsterdam the water was stored and naturally treated in sand dunes on to the year 2000, now it is stored in reservoirs
- In The Hague the water is still stored and naturally treated in sand dunes

Drinking water standards

Regulations concerning the quality of the water intended for consumption:

Process water

Process water covers the wide range of boiler feed water, cooling water for heat exchangers or engine, chemicals dilution, etc...

It should typically have a conductivity ranging from 0,1 to 50 uS/cm, with little to no hardness to avoid scaling in heating system.

Oxygen and carbon dioxide should be removed to prevent corrosion

Depending on your application, the water quality requirements can vary:

Boiler feed water characteristic

Cooling water quality

Tap water or fresh groundwater are the most widely used source of water to produce process water.

Our process water treatment plant can combine various technology, depending on the purity required:

Process water covers the wide range of boiler feed water, cooling water for heat exchangers or engine, chemicals dilution, etc...

It should typically have a conductivity ranging from 0,1 to 50 uS/cm, with little to no hardness to avoid scaling in heating system.

Oxygen and carbon dioxide should be removed to prevent corrosion

Depending on your application, the water quality requirements can vary:

Boiler feed water characteristic

Cooling water quality

Tap water or fresh groundwater are the most widely used source of water to produce process water.

Our process water treatment plant can combine various technology, depending on the purity required:

·        

The World Health Organization


The World Health Organization (WHO), set up some guidelines for drinking-water quality which are the international reference point for standards setting and drinking-water safety. The latest guidelines drew up by the WHO are those agreed to in Geneva, 1993.

Click here for the WHO's drinking-water standards. 

You will notice that there is no guideline for some of the elements and substances which are taken into account. This is because there have not been sufficient studies about the effects of the substance on the organism, and therefore it is not possible to define a guideline limit. In other cases, the reason for a non-existing guideline is the impossibility of that substance to reach a dangerous concentration in water, due to its insolubility or its scarcity.

• The European Union
  
  The European Union drew up the Council Directive 98/83/EC on the quality of water intended for human consumption, adopted by the Council on 3 November 1998. This was drawn up by reviewing the parametric values of the old Drinking Water Directive of 1980, and strengthening them where necessary in accordance with the latest available scientific knowledge (WHO guidelines and Scientific Committee on Toxicology and Ecotoxicology). This new Directive provides a sound basis for both the consumers throughout the EU and the suppliers of drinking water.
  
  Click here for the EU's drinking water standards.

These were the main changes in the parametric values:

- Lead: The guideline was reduced from 50 µg/l to 10 µg/l, and a transition period
of 15 years was defined to allow replacing of lead distribution pipes.

- Pesticides: The values for individual substances and for total pesticides were 
retained (0.1µg/l / 0.5µg/l), plus additional, more stringent values were
introduced for certain pesticides (0.03µg/l).

- Copper: The value was reduced from 3 to 2 mg/l.

- Some new standards were introduced for new parameters like trihalomethanes,
trichloroethene and tetracholoroethene, bromate, acrylamide etc.

WHO's drinking water standards 1993

WHO's Guidelines for Drinking-water Quality, set up in Geneva, 1993, are the international reference point for standard setting and drinking-water safety.

Organic compounds

Pesticides

Disinfectants and disinfectant by-products

EU's drinking water standards

Council Directive 98/83/EC on the quality of water intented for human consumption. Adopted by the Council, on 3 November 1998:

Chemical parameters

Indicator parameters

Microbiological parameters

Drinking water FAQ Frequently Asked Questions

The question library on water related issues

What is in our drinking water?

Drinking water, like every other substance, contains small amounts of bacteria. Most of these bacteria are common ones and they are generally not harmful. Chlorine is usually added to drinking water to prevent bacterial growth while the water streams through pipelines. This is why drinking water also contains minimal amounts of chlorine.
Water mostly consists of minerals and other inorganic compounds, such as calcium.
If you want to find out what substances your tap water consists of and whether it is totally safe to drink you can have a specialized agency check it out for you.

Where does drinking water come from?

Drinking water can come from different resources. For one, it can be pumped from the ground through wells. This groundwater is than purified, so that it will contain no more contaminants and is suited to drink. Drinking water can also be prepared directly from surface water resources, such as rivers, lakes and streams. Usually surface water has to undergo many more purification steps than groundwater to become suited to drink. Preparing drinking water out of surface water is much more expensive due to this. Still 66% of all people are served by a water system that uses surface water.
Part of our drinking water is pumped from the ground, usually under sand dunes. In sand dunes water can also be infiltrated. As it sinks into the ground through the dunes it is naturally purified. This costs much less money than the purification of surface water. Part of our drinking water originates from dune water.

How is drinking water purified?

Treating water to make it suitable to drink is much like wastewater treatment. In areas that depend on surface water it is usually stored in a reservoir for several days, in order to improve clarity and taste by allowing more oxygen from the air to dissolve in it and allowing suspended matter to settle out. The water is then pumped to a purification plant through pipelines, where it is treated, so that is will meet government treatment standards. Usually the water runs through sand filters first and sometimes through activated charcoal, before it is disinfected. Disinfection can be done by bacteria or by means of adding substances to remove contaminants from the water. The number of purification steps that are taken depend on the quality of the water that enters the purification plant. In areas with very pure sources of groundwater little treatment is needed.

What dangers can there be in drinking water?

There are several problems that can endanger the quality of drinking water. A number of these problems are summed up here.

Someone can detect coliform bacteria in drinking water. Coliform bacteria are a group of microrganisms that are normally found in the intestinal tract of humans and other warm-blooded animals, and in surface water. When these organisms are detected in drinking water this suggests contamination from a subsurface source such as barnyard run-off. The presence of these bacteria indicates that disease-causing microrganisms, known as pathogens, may enter the drinking water supply in the same way if one does not take preventive action. Drinking water should be free from coliform.

Yeasts and viruses can also endanger the quality of drinking water. They are microbial contaminants that are usually found in surface water. Examples are Giardia and Cryptosporidium. Giardia is a single cell organism that causes gastrointestinal symptoms. Cryptosporidium is a parasite that is considered to be one of the most significant causes of diarrhoeal disease in humans. In individuals with a normal immune system the disease lasts for several days causing diarrhoea, vomiting, stomach cramps and fever. People with weakened immune systems can suffer from far worse symptoms, caused by cryptosporidium, such as cholera-like illnesses.

Nitrate in drinking water can cause cyanosis, a reduction of the oxygen carrying capacity of the blood. This is particularly dangerous to infants under six months of age.

Lead can enter the water supply as it leaches from copper pipelines. As the water streams through the pipes, small amounts of lead will dissolve in the water, so that it becomes contaminated. Lead is a toxic substance that can be quickly absorbed in the human systems, particularly those of small children. It causes lead poisoning.

Legionella is a bacterium that grows rapidly when water is maintained at a temperature between 30 and 40 degrees for a longer period of time. This bacterium can be inhaled when water evaporates as it enters the human body with aerosols. The bacteria can cause a sort of flue, known as Pontiac fever, but it can also cause the more serious deathly illness known as legionellosis.

How is drinking water quality protected?

All countries have their own legal drinking water standards. These prescribe which substances can be in drinking water and what the maximum amounts of these substances are. The standards are called maximum contaminant levels. They are formulated for any contaminant that may have adverse effects on human health and each company that prepares drinking water has to follow them up. If water will be purified to make it suitable to drink it will be tested for a number of dangerous pollutants, in order to establish the present concentrations. After that, one can determine how much of the contaminants have to be removed and if necessary purification steps can be progressed.

Is bottled water safer than tap water?

Many people worry about getting sick from tap water, because of articles on the news and in the papers, for instance about Legionella outbreaks. They may either drink bottled water or install expensive water purification systems as a result of this. However, studies have indicated that many of these consumers are being ripped off due to the expenses of bottled water and in some cases they may end up drinking water that is dirtier then they can get from their taps. To be safe, consumers that buy bottled water should determine wheather the company that supplies them with water belongs to the International Bottled Water Association (IBWA) and lives up to the testing requirements of drinking water. The IBWA sends inspectors to its companies annually, to ensure that a plant produces safe drinking water.
People can also spare themselves the costs of bottled water and have their tap water tested by local health authorities or private labs. If any contaminants are discovered they can buy a unit that removes the contaminant in concern, but for most households this is not necessary because their tap water is safe enough.


Drinking water production from surface water

 
On this page you will find an explanation of a drinking water purification process. All process steps are numbered and the numbers correspond with the numbers in the schematic representation of the drinking water process found below. This is a summing up of the process steps:

a: Prefiltration

1) The uptake of water from surface waters or groundwater and storage in reservoirs. Aeration of groundwater and natural treatment of surface water usually take place in the reservoirs. Often softening and pH-adjustments already happen during these natural processes.

2) Rapid sand filtration or in some cases microfiltration in drum filters.

b: Addition of chemicals

3) pH adjustment through addition of calcium oxide and sodium hydroxide.

4) FeCl3 addition to induce flocculation for the removal of humic acids and suspended particulate matter, if necessary with the addition of an extra flocculation aid. Flocs are than settled and removed through lamellae separators. After that the flocs are concentrated in sludge and pumped to the exterior for safe removal of the particulates and sludge dewatering.

5) Softening in a reservoir, through natural aeration or with sodium hydroxide, on to 8,5 oD. This is not always necessary. For instance, in case natural filtration will be applied, softening takes place naturally.

c: Natural filtration

6) Drinking water preparation step that is specific for the Netherlands: Infiltration of the water in sand dunes for natural purification. This is not applied on all locations The water will enter the saturated zone where the groundwater is located and it will undergo further biological purification. As soon as it is needed for drinking water preparation, it will be extracted through drains.

d: Disinfection

7) Disinfection with sodium hypochlorite or ozone. Usually ozonation would be preferred, because ozone not only kills bacteria and viruses; it also improves taste and odour properties and breaks down micro pollutants. Ozone diffuses through the water as small bubbles and enters microrganisms cells by diffusion through cell walls. It destroys microrganisms either by disturbance of growth or by disturbance of respiratory functions and energy transfers of their cells. During these processes ozone is lost according to the reaction O3 -> O2 +(O).

e: Fine filtration

8) Slow sand (media) filtration for the removal of the residual turbidity and harmful bacteria. Sand filters are backwashed with water and air every day.

9) Active carbon filtration for further removal of matter affecting taste and odour and remaining micro pollutants. This takes place when water streams through a granular activated carbon layer in a filter. Backwash is required regularly due to silting up and reactivation of an active carbon filter should be done once a year.

f: Preservation and storage

10) Addition of 0.3 mg/L sodium hypochlorite to guarantee the preservation of the obtained quality. Not all companies chlorinate drinking water. The water will eventually be distributed to users through pipelines and distribution pumps.

11) Aeration for recovery oxygen supply of the water prior to storage. This is not always applied.

12) Remaining water can be stored in drinking water reservoirs.

In the following schematic representation of the drinking water preparation process dotted arrows represent the incoming chemicals and red arrows represent the outgoing flows.

Schematic representation of the drinking water preparation process

Water is not always infiltrated in sand dunes during treatment. Holland clearly illustrates this:
- In Rotterdam water is stored in reservoirs in the Biesbosch, where it undergoes natural treatment
- In Amsterdam the water was stored and naturally treated in sand dunes on to the year 2000, now it is stored in reservoirs
- In The Hague the water is still stored and naturally treated in sand dunes

Ammonium removal

Ammonium removal for drinking water applications

Challenge when using well-groundwater with regards to ammonium removal

Ammonium concentrations in groundwater aquifers have increased over the years due to several reasons with the most important the intensive use of fertilizers in agriculture. In many areas, these concentrations are impressively high compared to the World Health Organization’s standard which sets that ammonium concentration in drinking water should be less than 0.2mg/l.  Concentrations higher than this limitation could be harmful for humans and animals due to the conversion of ammonium to nitrate which is toxic for organisms.

Except from regular-used processes, like ion exchange, it is found that reverse osmosis desalination membranes can be applied to achieve surprisingly high ammonium removalfrom drinking water. This rejection can reach even the 97%, depending on the membrane that is used, the operational characteristics of the system as well as the quality of feed water.  Based on the water characteristics, RO membranes can be applied giving the advantage of removing not only ammonium but also a variety of dissolved solids, simultaneously. However, a challenge with this application is the inaccurate prediction by most of the calculation software that are used in the market and are available from the membrane manufacturers. This could lead to confusion and mistrust for this advanced technology.

Crow-chem is here to help you.  We have an extended experience and a wide range of membrane technologies that can guarantee you the efficient ammonium removal from drinking water in a cost-efficient way. Our high qualified team, which includes civil, environmental, chemical, mechanical and electrical engineers can provide you the best solution for your problem with ammonium, considering all the parameters that affect ammonium removal. Dealing daily with water purification issues of our clients from more than 140 countries worldwide makes us the ideal team to support you with the best service in terms of quality and availability.

Irrigation water

Water use for irrigation

Agriculture is by far the largest water use at global level. Irrigation of agricultural lands accounted for 70% of the water used worldwide. In several developing countries, irrigation represents up to 95% of all water uses, and plays a major role in food production and food security. Future agricultural development strategies of most of these countries depend on the possibility to maintain, improve and expand irrigated agriculture

On the other hand, the increasing pressure on water resources by agriculture faces competition from other water use sectors and represents a threat to the environment.

Water is a resource that may create tensions among countries down and upstream. Irrigated agriculture is driving much of the competition since it accounts for 70-90% of water use in may of these regions.

Source: Turkmenistan and Uzbekistan figures from David R. Smith "Climate Change, Water Supply, and Conflict in the Aral Sea Basin", paper presented at the "Pri-Aral Workshop 1994", San Diego State University, March 1994: Others from Peter H. Gleick, Water in Crisis (NY, Oxford University Press, 1993)

Within the European Union (EU) agriculture represents around 30% of total water abstraction. The intensity of irrigation in different countries obviously varies depending on the climate, the crops cultivated and the farming methods. For example, the role of irrigation is completely different in Southern European countries, where irrigation is essential for agricultural production, compared to Central and Western Europe. 

In fact the major part of irrigated land in Europe is located in the South with Spain, Italy, France, Greece and Portugal accounting for 85% of the total irrigated area in the EU. For example, in Spain irrigated agriculture accounts for 56% of total agricultural production, occupying only 18% of the total agricultural surface.

Water resources for irrigation

Water used for agriculture comes from natural or other alternative sources.

Natural sources includes rainwater and surface water (lakes and rivers). These resources must be used in a sustainable way. 

Rain water resources rely on the atmospheric conditions of the area. Surface water is a limited resource and normally requires the construction of dams and reservoirs with a significant environmental impact.

Alternative sources of irrigation water are the reuse of municipal wastewater and drainage water.

However the use of recycled water for irrigation may have some adverse impacts on the public health and the environment. This will depend on the recycled water application, soil characteristics, climate conditions and agronomic practises. Therefore it is important that all these factors are taken into account in the management of recycled water.

Lets study this a little bit further.

Reuse of water for irrigation

Water reuse for irrigation is a normal practice worldwide. In Europe, for example there is a large project in Clermont-Ferrand, France since 1997 where more than 10.000m3/day of tertiary treated urban wastewater are reused for irrigation of 700Ha of maize. In Italy more than 4000 Ha of various crops are irrigated with recycled water. Spain also counts with several similar projects.

The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. For example, the physical and mechanical properties of the soil, ex. soil structure (stability of aggregates) and permeability, are very sensitive to the type of exchangeable ions present in irrigation waters. Check irrigation water quality

Related pages:

Bicarbonate hazard of irrigation water

Irrigation water lab analysis

Nutrients in irrigation water

Salinity hazard irrigation

SAR hazard of irrigation water

Toxic ions hazard of irrigation water

For more information check the following pages: groundwater contamination, source of groundwater pollution, contaminants (seawater intrusions, nitrates, arsenic, iron), reducing groundwater contamination.

Irrigation water quality

The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. For example, the physical and mechanical properties of the soil, ex. soil structure (stability of aggregates) and permeability, are very sensitive to the type of exchangeable ions present in irrigation waters.

Irrigation water quality can best be determined by chemical laboratory analysis. The most important factors to determine the suitability of water use in agriculture are the following:

- PH

- Salinity Hazard

- Sodium Hazard (Sodium Adsorption Ration or SAR)

- Carbonate and bicarbonates in relation with the Ca & Mg content

- Other trace elements

- Toxic anions

- Nutrients

- Free chlorine

Parameters of reuse water with agronomic significance

Source of information: Valentina Lazarova Akiçca Bahri; Water Reuse for irrigation: agriculture, landscapes, and turf grass; CRC Press.

Menu of Options for Improving Irrigation Water productivity

Sources: Amy L. Vickers, Handbook of Water Use and Conservation (Boca Raton, FL: Lewis Publishers, in press); J.S. Wallace and C.H. Batchelor, "Managing Water Resources for Crop Production", "Philosophical Transactions of the Royal Society of London: Biological Science, vol. 352, pp.937-47 (1997)

Related pages:

Bicarbonate hazard of irrigation water

Irrigation water lab analysis

Nutrients in irrigation water

Salinity hazard irrigation

SAR hazard of irrigation water

Toxic ions hazard of irrigation water

For more information check the following pages: groundwater contamination, source of groundwater pollution, contaminants (seawater intrusions, nitrates, arsenic, iron), reducing groundwater contamination.

Carbonates & bicarbonates hazard of irrigation water

High carbonate (CO3=) and bicarbonate (HCO3-) increases SAR index (around >3-4mEq/L or >180-240mg/L). Let's explain why:

Bicarbonate and carbonate ions combined with calcium or magnesium will precipitate as calcium carbonate(CaCO3) or magnesium carbonate (MgCO3) when the soil solution concentrates in drying conditions.

The concentration of Ca and Mg decreases relative to sodium and the SAR index will be bigger. This will cause an alkalizing effect and increase the PH. Therefore when a water analysis indicates high PH level, it may be a sign of a high content of carbonate and bicarbonates ions.

Residual Sodium Carbonate (RSC)

The RSC has the following equation:

RSC=(CO3-+HCO3-)-(Ca2++Mg+2)

It is another alternative measure of the sodium content in relation with Mg and Ca. This value may appear in some water quality reports although it is not frequently used.

If the RSC < 1.25 the water is considered safe

If the RSC > 2.5 the water is not appropriate for irrigation.

Some practices to solve problem of carbonates and bicarbonates in irrigation water

Related pages:

Bicarbonate hazard of irrigation water

Irrigation water lab analysis

Irrigation water quality

Nutrients in irrigation water

Salinity hazard irrigation

SAR hazard of irrigation water

Toxic ions hazard of irrigation water 

Laboratory analysis for irrigation water

Waste Water Treatment :

Industrial waste water treatment using various technologies:

Clarification

Clarification consists in removing all kind of particles, sediments, oil, natural organic matter anc colour from the water to make it clear.A clarification step is the first part of conventional treatment for waste and surface water treatment.
It usually consist in:

- Screening
- Physical chemical treatment is a generic term for Coagulation-Flocculation
- Sedimentation or Flotation, upon particles properties and water type
- Fine filtration

For industrial effluents, Centrifugation is applied for heavy particles removal

Check our clarification equipment

Sludge Treatment

Biological/chemical waste water treatment reduces the solved and unresolved pollutants existing in the waste water. These are to be regained in the sewage sludge at the end of the water treatment.

The sludge treatment is necessary to reduce and to amliorate the sludges, which are produced within the biological wastewater treatment.

·        

General

Sludge sorts
Sludge components
Sludge parameters

• Sludge treatment
  
  Stabilisation
  Thickening
  Dewatering
  Sludge drying

• Additional informative pages:

Filter presses for sludge treatment

Centrifugation and centrifuges

Sludge sorts

·         Biological wastewater treatment produces different sorts of sludge within the individual process steps. In the wastewater linguistic usage the following terms are used for sludge.

Raw sludge
Raw sludge is untreated non-stabilized sludge, which can be taken from wastewater treatment plants. It tends to acidify digestion and produces odour.

• Primary sludge
  Primary sludge is produced through the mechanical wastewater treatment process. It occurs after the screen and the grit chamber and consists of unsolved wastewater contaminations. The sludge amassing at the bottom of the primary sedimentation basin is also called primary sludge. The composition of this sludge depends on the characteristics of the catchment area. Primary sludge consists to a high portion of organic matters, as faeces, vegetables, fruits, textiles, paper ect. The consistence is a thick fluid with a water percentage between 93 % and 97 %.

Activated Sludge
The removal of dissolved organic matter and nutrients from the wastewater takes place in the biological treatment step. It is done by the interaction of different types of bacteria and microorganisms, which require oxygen to live, grow and multiply in order to consume the organic matter. The resulting sludge from this process is called activated sludge. The activated sludge exists normally in the form of flakes, which besides living and dead biomass contain adsorbed, stored, as well as organic and mineral parts.
The sedimentation behaviour of the activated sludge flakes is from great importance for the function of the biological treatment. The flakes must be well removable, so that the biomass can be separated from the cleaned wastewater without problems and a required volume of activated sludge can be pumped back into the aerated part

• Return activated sludge
  The activated sludge flows from the biological aeration basin into the final clarifier. The activated sludge flakes settle down to the bottom and can be separated from the cleaned wastewater. The main part of the separated sludge, which is transported back to the aeration basin, is called return activated sludge.

• Excess sludge, secondary sludge
  To reach a constant sludge age the unused biomass has to be removed from the biological treatment system as excess sludge. The excess sludge contains not-hydrolysable particulate materials and biomass due to metabolisms.

• Tertiary sludge
  Tertiary sludge is produced through further wastewater treatment steps e.g. by adding a flocculation agent.

• Bulking sludge, Floating sludge

• Digested sludge
  Digested sludge accrues during the anaerobic digestion process. It has a black colour and smells earthy. As a function of the stabilization degree anaerobic sludge exhibits an organic portion of the solid from 45 to 60 %.

Sludge components

The main characteristic of the activated sludge is the occurrence of microorganisms, which take up solved food over their body surface or a cell mouth thereby contributing to wastewater cleaning. The biocoenosis of the activated sludge gives information over the condition of the activated sludge and the cleaning achievement.

Bacteria
Bacteria are simple, colourless, onecelled plants that use soluble food and are capable of self reproduction without sunlight. As decomposers they fill an indispensable ecological role of decaying organic matter in stabilizing organic wastes in treatment plants. They are responsible for activated sludge growing in domestic wastewater treatment. A wide variety of bacteria can be found in a sludge flake.

Beside bacteria, a number of species of protozoa such as flagellate-, ciliated- and amoebae protozoa have been identified in activated sludge. Protozoa are single-celled organisms that can consume food such as bacteria and particulate matter. 
These organisms are partly on the activated sludge flakes. Other protozoa move lively on or between the activated sludge flakes. The nematodes or rotifers are ranked among the multi-cellular organisms.

The sludge abstracted after the wastewater treatment process contains in the unloaded stabilized condition:



Carbon (50-70 %), 
Hydrogen (6,5-7,3 %), 
Oxygen (21-24 %), 
Nitrogen (15-18 %), 
Phosphorus (1-1,5 %) and 
Sulphur (0-2,4 %). 

Water is the main component of sludge. Its amount depend on the sludge sort (primary-, secondary or tertiary sludge) and the way of stabilisation (aerobic, anaerobic). Raw sludge has a water content of 93 % to 99 %. Therefore a dewatering (up to approx. 35 % dry substance content) or drying (to over 85 % dry substance content) can be necessary for a further utilization.
The second main component is the dry substance, which is made up of organic and inorganic substances.

Beside the main parts, sludge contains a large variety of trace components that have been separated from the wastewater. Organic and inorganic trace elements, which have its origin in wastewater, are found enriched in the sludge.

Sludge parameters

Sludge parameters are the basis for the supervision and the construction of sludge treatment plants. They provide the engineer with important notes over the existing (in-) organic portions, the settling behavior, the dewatering and the heat value of the sludge.

·  Total suspended solids TSS 
Mixed liquid from the reactor, the treated water, and the feed is filtered and heated in a 105°C oven. In this process, because of the low temperature, only the water is evaporated and nothing is burned. This is then weighed to conclude how much solids were in the feed, the treated water and the reactor. 

·  Volatile suspended solid VSS 
The filtrate is taken from the 105 degree C oven and then placed in a 600 degree C oven where the organic materials burn and the left over inorganic materials are weighed. 

·  Sludge volume index SVI
Mixed liquid is taken from the reactor and taken to a slow mix to settle. After half an hour the sludge volume is measured. The SVI units are (ml/g).

Sludge Treatment Processes

Sludge treatment in general

The goals of sludge treatment are:

·         Stabilisation for a controlled degradation of organic ingredients and odour removal

·         Volume and weight reduction

·         Hygiene - the deadening of pathogen organisms

·         Ameliorating of sewage sludge characteristics for the further utilization or disposal

The accompanying procedures are used to reach these goals. 

The sludge consistency and appearance is changed through treatment. It must be pointed out that also the heating value of the sludge changes with the way of the treatment. Anaerobic treated sludge has a lower heating value than raw sludge because of sewage gas production. This point has to receive attention by a thermal disposal.

Stabilisation

Aerobic stabilisation can be performed simultaneously in an activated sludge plant whereby primary and secondary sludges are continuously aerated for long periods of time. In aerobic digestion the microorganisms extend into a respiration phase where materials previously stored by the cell are oxidized, resulting in a reduction of the biologically degradable organic matter. Thus, aerobic stabilisation of the entire excess sludge (including primary sludge) is energy consuming. Additionally, it calls for extra reactor volume. 
Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth. Anaerobic stabilisation processes work at normal temperatures (< 40°C) or within the range of thermophile bacteria, where 50-65°C are reached alone by the heat development of the biochemical processes. The chemical stabilisation of the sludge by means of wet oxidation or addition of quicklime and thermal stabilisation under high temperature and pressure, are applied less often.

The anaerobic sludge digestion takes place in the highly visible digesters