Wastewater Treatment Glossary: Water Treatment Chemicals
What Are Water Treatment Chemicals?
Water treatment chemicals is the collective term for a broad family of chemical agents used in water treatment. They are widely applied in the petroleum, chemical, metallurgical, transportation, light industry and textile sectors. The family includes corrosion inhibitors, scale inhibitors, biocides, flocculants, purifying agents, cleaning agents and pre-filming agents. In practice, compounded formulations are commonly used, or several classes of chemicals are applied together. Two points therefore deserve attention: an unsuitable combination can produce antagonistic effects that reduce or destroy performance, while a well-designed combination exploits synergy - the enhanced effect produced when several agents are present at the same time. In addition, most water systems are open systems with a certain discharge volume, so the environmental impact of each chemical must be taken into account.
Common water treatment chemicals include: flocculants, ferrous sulfate heptahydrate, polymeric iron salts, calcium hydroxide, ferric chloride hexahydrate, biocides and algae-control agents, chlorine dioxide, scale and corrosion inhibitors, polyacrylamide (cationic, anionic and non-ionic), polyaluminium chloride, polyaluminium ferric chloride and ferrous sulfate.
1. Corrosion Inhibitors
Corrosion inhibitors are chemicals that, when dosed into water in an appropriate concentration and form, prevent or slow the corrosion of metal materials and equipment. They offer good performance, low dosage and convenient use. There are many classes and grades: by compound type they divide into inorganic and organic inhibitors; by the reaction they suppress they divide into anodic, cathodic and mixed inhibitors; and by the mechanism of the protective film they form on the metal surface they divide into passivating-film, precipitation-film and adsorption-film types. Passivating inhibitors commonly used in water treatment include chromates, nitrites and molybdates; precipitation-film inhibitors include polymeric phosphates and zinc salts; adsorption-film inhibitors include organic amines.
2. Scale Inhibitors and Dispersants
The earliest scale dispersant was polyacrylic acid (and its sodium salt). It performs well against calcium carbonate scale but has very limited inhibiting effect on calcium phosphate deposition. Polycarboxylic acid scale inhibitors of this type provide both coagulation and dispersion and also distort the crystal lattice as crystals grow, which prevents scale from depositing firmly on metal heat-transfer surfaces. Acrylic acid copolymers were later found to inhibit both types of deposit very effectively and gradually replaced polyacrylic acid and its sodium salt. Organic polyphosphonates are anionic corrosion inhibitors and non-stoichiometric chelating scale inhibitors: they show a pronounced threshold effect on calcium, magnesium, zinc and iron ions and also act synergistically with other agents, which is why they are widely used in cooling water treatment. Organic phosphonates are cationic corrosion inhibitors whose scale-control mechanism is mainly lattice distortion. They inhibit corrosion of iron while also controlling calcium scale; the dosage is lower than that of phosphonates and they hydrolyse, so there is no accumulation of chemicals or blowdown problem, and the hydrolysis products are biodegradable.
Since the 1980s, copolymers carrying sulfonic acid and phosphonic acid groups have been developed. They combine strong scale inhibition and dispersion with a degree of corrosion inhibition. Liang Haiyan and co-workers synthesised a copolymer containing phosphonic, carboxylic and sulfonic groups; it combines the advantages of organic phosphonic, carboxylic and sulfonic groups, delivering good resistance to calcium carbonate and calcium phosphate scale together with good particle dispersion in water. It can be regarded as a multifunctional agent suitable for industrial circulating cooling water and boiler feedwater in the steel, metallurgical and chemical industries.
Also called anti-scaling agents, scale inhibitors are chemicals that suppress the formation of scale from scale-forming salts of calcium and magnesium. They include natural inhibitors such as tannins and lignin derivatives; inorganic inhibitors such as sodium hexametaphosphate and sodium tripolyphosphate; and organic polymeric inhibitors, among which the polymeric types perform best and have the greatest development potential. Two classes of organic polymeric inhibitors are widely used: (1) organic phosphonic acids such as EDTMPS (ethylenediamine tetramethylene phosphonic acid) and HEDP (hydroxyethylidene diphosphonic acid); and (2) polycarboxylic acids such as polyacrylate salts and hydrolysed polymaleic anhydride. Both work through lattice distortion together with dispersion-coagulation, and are widely applied in oilfield water, boiler water and industrial cooling water systems.
3. Biocides and Slime-Control Agents
Also called biocides, algae-control agents, slime stripping agents or anti-slime agents, these chemicals suppress the growth of bacteria, algae and other microorganisms in water so that biological slime does not form. They are usually divided into oxidising and non-oxidising types. Oxidising biocides include chlorine, sodium hypochlorite, bleaching powder (mainly calcium hypochlorite) and chlorine dioxide. Among non-oxidising biocides, the most effective and widely used are chemicals that destroy bacterial cell walls and cytoplasm, such as quaternary ammonium salts. Dodecyl dimethyl benzyl ammonium chloride or bromide, for example, often combines biocidal, stripping and corrosion-inhibiting functions and has good development prospects; it is already used in oilfield water and industrial cooling water.
4. Flocculants
Flocculants remove or reduce turbidity and suspended solids in water and accelerate the settling of impurities and sludge. The earliest inorganic flocculants used were alum and ferric chloride. Organic and polymeric flocculants are the type that will be widely used in drinking water and wastewater treatment. They divide into anionic flocculants such as carboxymethyl cellulose and sodium polyacrylate; cationic flocculants such as polyvinylamine; and non-ionic flocculants such as polyacrylamide. Their action is mainly achieved through charge neutralisation and adsorption bridging.
5. Purifying Agents
Purifying agents are a specialised class used in oilfield water treatment. They remove mechanical impurities and oil from oily wastewater: in addition to the separation of suspended solids and mechanical impurities achieved by flocculants, they also provide oil-water separation. Such agents therefore contain the usual flocculant components - aluminium salts, polyacrylamide and so on - together with surfactants. Purification performance is usually measured with a membrane filter and expressed as the membrane filtration factor: the better the purification effect, the higher the factor.
6. Cleaning Agents and Pre-Filming Agents
Cleaning agents are chemicals with a cleaning function. In the pretreatment stage of water treatment, chemicals are often needed to remove deposits from metal surfaces, including corrosion products, scale and microbial slime. According to the cleaning requirement, they include acid cleaners such as hydrochloric acid, sulfuric acid, hydrofluoric acid and citric acid, and passivating agents such as sodium benzoate. Sodium sulfosuccinate di(alpha-ethylhexyl) ester is a surfactant used as a dedicated cleaning agent to remove oil, grease and light rust from metal surfaces.
Pre-filming agents form a protective film on metal surfaces in advance during pretreatment. Pre-filming serves two purposes: first, to raise the dosage during the initial period of chemical corrosion control; second, to establish a film so that a small maintenance dose of inhibitor during normal operation is enough to maintain and repair it, saving chemicals and cost. Common pre-filming agents include sodium hexametaphosphate with zinc salts, and sodium tripolyphosphate.
To make the most of water treatment chemicals, a series of supporting products is also required: cleaning agents, defoamers, oxygen scavengers, oil-flotation agents and pre-filming agents. Most of these auxiliary agents are compounded products with a marked synergistic action. Cleaning agents, for instance, are generally compounded surfactants and are sometimes compounded with corrosion inhibitors or chelating agents. Pre-filming formulations include sodium hexametaphosphate with zinc sulfate, or simply the routine treatment chemicals at 10-30 times the normal concentration.
7. Ferrous Sulfate Heptahydrate
Molecular formula: FeSO4·7H2O. Molecular weight: 278.05. Basic properties: sky-blue or green monoclinic crystals or crystalline powder; relative density 1.8987; melting point 64 degrees C; odourless, astringent; effloresces in air with the surface turning to white powder; oxidises readily in moist air to form a brownish-yellow basic ferric sulfate on the surface; soluble in water and glycerol, insoluble in alcohol; reducing in nature. It changes from the heptahydrate to the tetrahydrate at 56.6 degrees C, to the monohydrate at 64.6 degrees C, loses six molecules of water of crystallisation at 90 degrees C, loses all of it at 300 degrees C to become the anhydrous salt, and decomposes at red heat releasing SO2 and SO3. It is corrosive and fluoresces in dry air. The anhydrous form is a white powder that turns blue-green on contact with water.
Uses: (1) as a flocculant with good flocculating performance and strong decolourising ability; besides flocculation it also decolourises, removes heavy metal ions, removes oil, removes phosphorus and disinfects, with particularly clear effects on decolourisation and COD removal in printing and dyeing wastewater and on ferrite co-precipitation in electroplating wastewater; (2) as a food additive, pigment, raw material for the electronics industry, hydrogen sulfide odour control agent, soil conditioner, foliar fertiliser and industrial catalyst.
Packaging and storage: 50 kg polyethylene-lined woven bags; protect from moisture and rain; never store or stack together with toxic or hazardous substances.
8. Ferric Sulfate
Product overview: ferric sulfate (FS), formula Fe2(SO4)3, is a pale white amorphous fine powder solid. It is readily soluble in water; a 10% (by weight) aqueous solution is an opalescent colloid that gradually becomes a yellowish-brown transparent solution on standing. It is hygroscopic.
Basic uses: as a water purifying agent, mainly for drinking water purification and for industrial water, industrial wastewater and municipal sewage treatment. It provides effective turbidity removal, decolourisation, oil removal, deodorisation, disinfection, algae removal, phosphorus removal, grease removal, and removal of COD, BOD and heavy metal ions. It can also be used as a pigment, pharmaceutical, catalyst and mordant.
Product features, compared with other inorganic flocculants: (1) a low-molecular-weight trivalent iron flocculant with stable performance; (2) high effective iron content, 24% for this product and up to more than 27% at best; (3) stronger charge neutralisation capacity and higher positive charge density than polymeric iron and its composite products; (4) higher coagulation efficiency and demulsification performance, widely used in drinking water purification - at the same concentration its aqueous solution is lighter in colour than polymeric iron and does not cause the water to turn yellow even when overdosed.
Performance advantages: good coagulation with abundant, dense flocs and fast settling, with little sludge; good treated water quality, free of aluminium, chlorine and other harmful heavy metal ions, no transfer of iron ions into the water phase, no yellowing of the water, non-toxic and reliable; marked decolourisation, turbidity removal, deodorisation, algae removal and removal of COD, BOD and harmful heavy metal ions; clearly better than traditional aluminium salts when treating low-temperature, low-turbidity, slightly polluted and high-turbidity raw water, at a chemical cost about 20% lower than aluminium salts; a wide applicable pH range (4-11, optimum 6-9) with little change in pH or total alkalinity after treatment; and in sludge dewatering it can fully replace ferric chloride and other inorganic salts - combined with polyacrylamide it gives rapid solid-liquid separation, sludge that releases easily and low cake moisture, clearly reducing dewatering costs.
9. Calcium Hydroxide
Calcium hydroxide is the conventional alkaline agent for wastewater treatment. Compared with 30% sodium hydroxide solution, the price per tonne of sodium hydroxide solution is comparable to that of a tonne of calcium hydroxide powder, yet in treatment performance calcium hydroxide powder costs less than 50% of sodium hydroxide solution and is also superior in coagulation and decolourisation - which is why it is recognised worldwide as the preferred alkaline neutralising agent. Specification: Ca(OH)2, 325 mesh pass rate of 95% or more.
Functions: (1) neutralises acidic wastewater or heavy-metal wastewater as a strongly alkaline agent, bringing acidic wastewater to neutrality; (2) absorbs SO2 from boiler flue gas so that sulfur emissions meet environmental standards; (3) acts as a coagulation aid on colloidal particles in wastewater and, as a weighting agent for particle nuclei, accelerates separation of insoluble matter; (4) effectively removes anions such as phosphate, sulfate and fluoride; (5) breaks the binding of some metal ions by complexing or chelating agents such as sulfamates; (6) destabilises and demulsifies emulsion wastewater by adjusting pH.
10. Polyacrylamide (PAM)
Polyacrylamide is a water-soluble high polymer, insoluble in most organic solvents, with good flocculating properties that reduce friction between liquids. By ionic character it divides into non-ionic, anionic, cationic and amphoteric types.
Flocculation: PAM flocculates suspended matter through charge neutralisation and bridging adsorption.
Adhesion: it bonds materials through mechanical, physical and chemical action.
Drag reduction: PAM effectively reduces the frictional resistance of fluids - a trace addition reduces drag by 50-80%.
Thickening: PAM thickens in neutral and acidic conditions; above pH 10 it hydrolyses readily, and thickening becomes more pronounced when it forms a semi-network structure.
11. Ferric Chloride Hexahydrate
English name: ferric chloride hexahydrate. Formula: FeCl3·6H2O. Properties: odourless with an astringent taste, strongly hygroscopic and deliquescent, deliquescing in air to a reddish-brown liquid; very soluble in water, giving a strongly acidic solution that coagulates proteins; soluble in ethanol and acetone, also in liquid sulfur dioxide, ethylamine and aniline; insoluble in glycerol and phosphorus trichloride. Uses: mainly as a water treatment chemical; also as an etchant in printing plate making and electronic circuit boards, a chlorinating agent in metallurgy, an oxidising agent and mordant in the dye industry, a catalyst and oxidising agent in organic synthesis, a raw material for other iron salts and pigments, and in mining beneficiation.
