Chemical Wastewater: Picking Between Physical, Chemical, Physicochemical and Biological Routes

The chemical industry splits broadly into organic and inorganic production, and the water leaving either type of plant is not a single problem. Chemical products are numerous and their compositions complex, so the wastewater is highly variable: often toxic, resistant to purification, prone to bioaccumulation, strongly oxygen-depleting once it reaches a receiving water, and capable of degrading water quality quickly.

What makes this wastewater difficult

Several properties combine, and they are worth listing because each one points to a different treatment family.

• Feedstock is often solvent or ring-structure compounds, and the by-product spectrum is wide, so the composition is complicated before treatment starts.
• Pollutant concentrations are high, because conversion is incomplete and the solvent media used in production end up in the water.
• Fine-chemical streams contain organics that are toxic to microorganisms, including halogenated compounds, nitro compounds, and dispersants or surfactants with biocidal action.
• A large fraction is poorly biodegradable, with a low B/C ratio.

Inorganic chemical wastewater comes from producing acids, alkalis and salts from mineral raw materials. Cooling water dominates the volume, and the discharge carries acids, alkalis, large salt loads and suspended solids, sometimes with sulfides and other toxic substances. Organic chemical wastewater is more varied again, arising from synthetic rubber, plastics, man-made fibres, dyes, paints and coatings, and pharmaceuticals. It is strongly oxygen-consuming, highly toxic, and, because most of it consists of synthesised organic compounds, both heavily polluting and slow to degrade.

The consequence of discharging it untreated is well documented in China. In Jiawang District, Xuzhou, Jiangsu, a roadside ditch along Nonggu Avenue ran black and malodorous for years after local workshops illegally discharged a mix of chemical wastewater containing caustic soda and hydrogen peroxide together with blood water, fat and other organic pollutants. The affected stretch ran about 12 km, irrigation became impossible and nearby villagers bore the consequences. Across the preceding two years the workshops involved discharged roughly 100 tonnes of caustic soda and 140 tonnes of hydrogen peroxide. Once harmful chemical substances enter a water body, recovery takes a very long time even after discharge stops, and for heavy metals that bioaccumulate the pollution state is effectively permanent.

Characteristic properties that drive selection

Chemical wastewater contains toxic or highly toxic substances such as cyanide, phenol, arsenic, mercury, cadmium and lead. Some compounds resist degradation and accumulate into chronic poisoning, such as organochlorines; others, including polycyclic aromatic hydrocarbons, are regarded as carcinogenic. Inorganic acids and alkalis add corrosivity, and pH swings widely, which damages aquatic life, structures and crops.

Both Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are high, particularly in petrochemical streams carrying organic acids, alcohols, aldehydes, ketones, ethers and epoxides. Once in a water body, that material oxidises further and consumes dissolved oxygen, threatening aquatic life directly. Many streams also carry excessive phosphorus and nitrogen, driving eutrophication, algal blooms and in severe cases red tides with large fish kills. Because reactions often run hot, thermal pollution reduces dissolved oxygen as well. Oil is frequently present too, complicating treatment further.

The four families of method

No single unit removes everything. A given water almost always passes through a system built from several methods, and the methods group by action principle into four categories.

Physical treatment

Physical action separates and recovers pollutants that are suspended, including oil films and oil droplets. Depending on the mechanism it subdivides into gravity separation, centrifugal separation and screening or interception, and in practice covers filtration, gravity separation and centrifugal separation. Physical methods offer simple equipment, low cost, convenient management and stable performance, and are mainly used to remove floating matter, suspended solids, sand and oil.

Chemical treatment

Chemical reaction and mass transfer remove dissolved or colloidal pollutants or convert them into harmless substances. It applies to metal ions, fine colloidal organics, inorganics, nutrients such as nitrogen and phosphorus, emulsified oil, colour, odour, and acids and alkalis. The methods include neutralisation, coagulation, redox and electrochemical treatment. Neutralisation is standard for low-concentration acidic or alkaline water with no recovery value, and is also used routinely to adjust pH ahead of other steps. Coagulation works because the coagulant is an electrolyte: it forms micelles in the water and electrically neutralises colloidal material, producing settleable floc that removes particles in the 10-3 to 10-6 size range along with colour, oil, microorganisms, nutrients, heavy metals and organic matter. Redox converts toxic species into non-toxic or low-toxicity ones, using air oxidation, chlorine oxidation, ozone oxidation or wet oxidation. Electrolysis applies direct current to drive dissolution and redox reactions, subdivided by purification mechanism into electrolytic oxidation, reduction, coagulation and flotation.

Physicochemical treatment

Where physical treatment leaves fine suspended matter and dissolved organics behind, adsorption, ion exchange, membrane separation, extraction, steam stripping and air stripping close the gap.

Biological treatment

Microbial metabolism converts dissolved, colloidal and finely suspended organic pollutants into stable harmless substances. The process is fundamentally one of organic-matter decomposition, carried out mainly by bacteria, with algae and protozoa participating to a lesser degree.

Where micro-electrolysis and hydrolytic acidification fit

Micro-electrolysis has been applied increasingly in recent years, and filler produced by current processes has overcome the earlier hardening and passivation problem, so the media can run continuously and efficiently. For streams with high organic concentration, high toxicity, high colour and poor biodegradability, it cuts colour and COD while raising the B/C ratio, that is, improving biodegradability. It is widely used on printing and dyeing, chemical, electroplating, pulp and paper, pharmaceutical, wool-washing, pesticide, soy-sauce and alcohol wastewater.

The pattern of response is fairly consistent. On dye, printing-dyeing, coking and petrochemical water, the B/C value rises significantly while decolourisation occurs. On petroleum, leather, pulp and paper and wood-processing water, the BOD/COD ratio increases substantially. On electroplating, printing and mining water and other heavy-metal-bearing streams, heavy metals are removed. On organophosphorus and organochlorine agricultural wastewater, biodegradability improves greatly and phosphorus and sulfides are removed.

Beyond the end of pipe

Treatment is only part of the answer. Total pollutant discharge has to be controlled and industrial pollution reduced continuously, with closed-loop recycling and zero discharge pushed in steel, power, chemicals and coal. Water-saving industry needs to be built out, with water-use quotas and conservation standards enforced for key dischargers. Persistent organic pollutants, which make up a relatively high share of chemical wastewater, require specific control. And municipal sewage collection and reuse deserves the same attention, since without improved drainage and treatment the urban water environment keeps deteriorating regardless of what individual plants do.