WATER TREATMENT CHEMICAL
For the chemical treatment of water, a great variety of chemicals can be applied. Below, the different types of water treatment chemicals are summed up:
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 useable 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.
Foam is a mass of bubbles created when certain types of gas are dispersed into 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:
In laboratory tests, a maximum tolerable microbial population limit in systems is determined. When these data are known in many cases the number of bacteria and other microrganisms needs serious reduction. This can be accomplished by addition of biocides; chemical compounds that are toxic to the present microrganisms. Biocides are usually slug fed to a system to bring about rapid effective population reductions from which the microrganisms cannot easily recover. There are various different biocides, some of which have a wide range of effect on many different kinds of bacteria. They can be divided up into oxidising agents and non-oxidising agents.
Below are list of Oxidising Agents:
- Chlorine Dioxide
- Chlorinated phenolics
- Copper Salt
- Organo-sulphur Compounds
- Quaternary Ammonium Salts
BOILER WATER CHEMICALS
Boiler water chemicals include all chemicals that are used for the following applications:
- Oxygen scavenging
- Scale inhibition
- Corrosion inhibition
- Alkalinity control
When referring to coagulants, positive ions with high valence are preferred. Generally aluminium and iron are applied, aluminium as Al2(SO4)3– (aluin) and iron as either FeCl3 or Fe2(SO4)3 -. One can also apply the relatively cheap form FeSO4, on condition that it will be oxidised to Fe3+ during aeration.
Coagulation is very dependent on the doses of coagulants, the pH and colloid concentrations. To adjust pH levels Ca(OH)2 is applied as co-flocculent. Doses usually vary between 10 and 90 mg Fe3+/ L, but when salts are present a higher dose needs to be applied.
Corrosion is a general term that indicates the conversion of a metal into a soluble compound.
Corrosion can lead to failure of critical parts of boiler systems, deposition of corrosion products in critical heat exchange areas, and overall efficiency loss.
That is why corrosion inhibitors are often applied. Inhibitors are chemicals that react with a metallic surface, giving the surface a certain level of protection. Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film.
There are five different kinds of corrosion inhibitors. These are:
- Passivity Inhibitors (Passivators)
- Cathodic Inhibitors
- Organic Inhibitors
- Precipitation Inducing Inhibitors
- Volatile Corrosion Inhibitors (VCI)
Disinfectants kill present unwanted microrganisms in water. There are various different types of disinfectants:
- Chlorine (dose 2-10 mg/L)
- Chlorine Dioxide
Every disinfection technique has its specific advantages and its own application area. In the table below some of the advantages and disadvantages are shown :
|No.||Technology||Environmentally Friendly||Byproduct||Effectivity||Investment||Operational Cost||Fluid||Surface|
|2||UV||++||++||+||+ / –||++||+||++|
|3||Chlorine Dioxide||+ / –||+ / –||++||++||+||++||—|
|4||Chlorine Gas||—||—||–||+||++||+ / –||—|
|5||Hypochlorite||—||—||–||+||++||+ / –||—|
To promote the formation of flocs in water that contains suspended solids polymer flocculants (polyelectrolytes) are applied to promote bonds formation between particles. These polymers have a very specific effect, dependent upon their charges, their molar weight and their molecular degree of ramification. The polymers are water-soluble and their molar weight varies between 105 and 106 g/ mol.
There can be several charges on one flocculent. There are cationic polymers, based on nitrogen, anionic polymers, based on carboxylate ions and polyampholytes, which carry both positive and negative charges.
NEUTRALIZING AGENTS (ALKALINITY CONTROL)
In order to neutralize acids and basics we use either sodium hydroxide solution (NaOH), calcium carbonate, or lime suspension (Ca(OH)2) to increase pH levels. We use diluted sulphuric acid (H2SO4) or diluted hydrochloric acid (HCl) to decline pH levels. The dose of neutralizing agents depends upon the pH of the water in a reaction basin. Neutralization reactions cause a rise in temperature.
Chemical oxidation processes use (chemical) oxidants to reduce COD/BOD levels, and to remove both organic and oxidisable inorganic components. The processes can completely oxidise organic materials to carbon dioxide and water, although it is often not necessary to operate the processes to this level of treatment.
A wide variety of oxidation chemicals are available. Examples are:
- Hydrogen Peroxide
Oxygen scavenging means preventing oxygen from introducing oxidation reactions. Most of the naturally occurring organics have a slightly negative charge. Due to that they can absorb oxygen molecules, because these carry a slightly positive charge, to prevent oxidation reactions from taking place in water and other liquids.
Oxygen scavengers include both volatile products, such as hydrazine (N2H4) or other organic products like carbohydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, but also non-volatile salts, such as sodium sulphite (Na2SO3) and other inorganic compounds, or derivatives thereof. The salts often contain catalysing compounds to increase the rate of reaction with dissolved oxygen, for instance cobalt chloride.
Municipal water is often pH-adjusted, in order to prevent corrosion from pipes and to prevent dissolution of lead into water supplies. During water treatment pH adjustments may also be required. The pH is brought up or down through addition of basics or acids. An example of lowering the pH is the addition of hydrogen chloride, in case of a basic liquid. An example of bringing up the pH is the addition of natrium hydroxide, in case of an acidic liquid.
The pH will be converted to approximately seven to seven and a half, after addition of certain concentrations of acids or basics. The concentration of the substance and the kind of substance that is added, depend upon the necessary decrease or increase of the pH.
Ion exchange resins need to be regenerated after application, after that, they can be reused. But every time the ion exchangers are used serious fouling takes place. The contaminants that enter the resins will not be removed through regeneration; therefore resins need cleaning with certain chemicals.
Chemicals that are used are for instance sodium chloride, potassium chloride, citric acid and chlorine dioxide.
Chlorine dioxide cleansing serves the removal of organic contaminants on ion exchange resins. Prior to every cleaning treatment resins should be regenerated. After that, in case chlorine dioxide is used, 500 ppm of chlorine dioxide in solution is passed through the resin bed and oxidises the contaminants.
Scale is the precipitate that forms on surfaces in contact with water as a result of the precipitation of normally soluble solids that become insoluble as temperature increases. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate.
Scale inhibitors are surface-active negatively charged polymers. As minerals exceed their solubility’s and begin to merge, the polymers become attached. The structure for crystallisation is disrupted and the formation of scale is prevented. The particles of scale combined with the inhibitor will than be dispersed and remain in suspension.
Examples of scale inhibitors are phosphate esters, phosphoric acid and solutions of low molecular weight polyacrylic acid.