TRANSFORMING BASE SUBSTANCES INTO GOLD
The Chemistry of Water Treatment Chemicals
by Rami E. Kremesti M.Sc., CSci, CEnv, CWEM
Scale and Corrosion Inhibitors: Usually some kind of blend of phosphonates, phosphates, polymers and azoles, or Silicate, Sodium molybdate, Zinc based corrosion and scale inhibitors, or Aliphatic amine corrosion inhibitors which are sometimes film-forming chemicals. Nitrites are also used as oxidizers for Iron based metallurgy which create a uniform Iron oxide protective film. Please note that while Silicates are useful as scale inhibitors for steel water pipes they would have the opposite effect in RO systems where silica fouls the membranes. For RO systems, calculations are made based on water chemistry and the species that is at risk of forming scale is known and based on this the proper anti-scalant is used.
Flocculants: Sometimes flocculants such as polyacrylate polymer formulations are dosed before sand filters to aid in the agglomeration process of suspended solids and to improve the efficiency of the filtration process. Flocculants are also dosed in clarifiers after coagulation to aid in the settling process of small suspended solids.
Biocides can be divided into 2 main groups; oxidising biocides and non-oxidising biocides:
Oxidizing Biocides: Chlorine or Bromine, Peroxide (ex. Peracetic acid) or Ozone Based.
Non-Oxidizing Biocides:
THPS: Tetrakis hydroxymethyl-phosphonium sulfate
ITA: Iso-ThiAzoline
BNP: 2-Bromo-2-Nitro Propan-1,3-diol
Glutaraldehyde:
DBNPA: 2,2-dibromo-3-nitrilopropionamide is a quick-kill biocide that easily hydrolyzes under both acidic and alkaline conditions. It is preferred for its instability in water as it quickly kills and then quickly degrades to form a number of products, depending on the conditions, including ammonia, bromide ions, dibromoacetonitrile, and dibromoacetic acid.
Quaternary ammonium chloride:
Quaternary ammonium salts are used as disinfectants, surfactants, fabric softeners, and as antistatic agents (e.g. in shampoos). In liquid fabric softeners, the chloride salts are often used. In dryer anticling strips, the sulfate salts are often used. Spermicidal jellies also contain quaternary ammonium salts.
Polyquats are a variety of engineered polymer forms which provide multiple quat molecules within a larger molecule.
Benzalkonium chloride (above) is a common type of quat salt used as a biocide, a cationic surfactant, and as a phase transfer agent.
Biodispersants do not kill organisms but they do break up any deposits of microorganisms allowing the biocides to attack the organisms more effectively. They also inhibit the attachment of microorganisms to metal surfaces.
Antifoams: Mineral Hydrocarbons or Silicone based or Mineral oil based antifoams. Note that Mineral based oils are the light distillate fraction of Petroleum oil which contains mostly alkanes.
Descalers: Sulfamic acid, stabilized HCl, Citric Acid, Phosphoric acid, Formic acid, Acetic acid.
For Closed Cooling Circuits:
Molybdate / nitrite blend, Phosphate, Nitrite and borate, nitrite based blends are not suitable for Aluminum based systems due to high pH corrosion of Al. Ofcourse some smart chemists know that for all ferric metallurgy, all you need is demin water and a little NaOH to bring the pH up to 10 and you will have no problems with corrosion, scale, bacteria or fouling. The trick is that the system needs to be air tight as Demin water with dissolved oxygen in it is aggressive to steel.
Anti-Freeze: Mono ethylene glycol, Mono propylene glycol
Chemicals Used In Sewage Treatment
One of the main chemicals used is for control of Phosphorus. Keep in mind that Total Phosphorus can be either organic or inorganic, dissolved or in the form of suspended solids. Chemicals such as Ferric Chloride or Poly Aluminum Chloride are dosed to react with Inorganic Phosphorus and form precipitates. They are also coagulants so they will help lower suspended solids. Note that organic phosphorus can be more difficult to remove.
In sewage treatment plants where the incoming water is low in alkalinity, chemicals such as NaHCO3 or Na2CO3 may be dosed to increase the alkalinity to neutralize the acids generated by septicity or nitrification. Sodium BiCarbonate is a buffer: it can react with acids as well as with bases.
Ofcourse, it is know universally that sometimes the effluent from sewage treatment plants is disinfected using Sodium Hypochlorite.
Experiments performed by Kremesti Environmental have proven that adding Magnesium Chloride to raw sewage reduces ammonia in the effluent. This could be owing to the formation of Struvite or the stimulation of nitrifying bacteria which use Magensium as a nutrient.
Chemicals Used in Boiler Water Treatment:
There are several categories of chemicals used. Take a look at our presentation on Boiler Water Treatment for a better understanding of the subject.
The main issues are control of pH for increased corrosion resistance using chemicals such as Ammonia and Sodium Phosphate. Then there are the oxygen scavengers.
Lower pressure boilers use dispersants in their steam drums to keep the solids in suspension. Some boilers inject Oxygen gas to form a strong corrosion resistant Hematite layer on the steel.
In Drinking Water Treatment, sometimes lead corrosion inhibitors are added to the water to protect the owners of old homes that have lead pipes from lead poisoning. This is normally some kind of phosphate or phosphoric acid. Ocourse we all know that adding a little Sodium Fluoride to drinking water helps with the protection of teeth from cavities.
In reactors that are used to remove heavy metals, sometimes NaOH or CaOH2 are added to precipitate heavy metals. Note that the Ksp curve of the heavy metal or metals in questions need to be studied carefully as some heavy metals precipitate at lower pH ranges and redissolve at higher pH ranges.
Swimming Pool water treatment uses biocides, algaecides, pH adjusting chemicals as well as coagulants. Read our page on Swimming Pool Water Treatment.
Anerobic Digesters sometimes need attention to the water chemistry. Buffers are important as some metabolic processes lower the pH which becomes toxic to the micro-organisms. Also macro and micro nutrients need to be available. ZVI (Zero Valence Iron) or GAC are sometimes added which help as electron donors and ammonia adsorbers respectively.
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