Waste Water Treatment in Municipal Sewage Works

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Municipal waste water is a term applied to the effluents which are drained by sewerage systems and are treated in sewage treatment works. According to the definition given by the International Public Health Organization, municipal waters are spent waters flowing from settlements and represent water-borne domestic refuse and pollutants from houses, and commercial and industrial premises as well as surface and ground waters that have entered the sewerage system. Depending on the origin, type and quality characteristics of the contaminants, waste waters can be subdivided into three categories—domestic, industrial and run-off (atmospheric).

Traditionally the treatment of municipal waste waters is a combination of physical and biological processes. The overall scheme of treatment usually includes three main stages:

(1) physical separation;

(2) biological treatment; and,

(3) tertiary polishing, which may be accomplished by physical, physico-chemical or biological methods or some combination of these.

The minimum treatment usually consists of physical and biological processes, although physico-chemical techniques may also be used in municipal works. The latter have been widely adopted for some time and the results are not necessarily inferior to biological treatment in terms of efficiency. Tertiary polishing of municipal wastes may be carried out after either biological or physico-chemical treatment. Any one of a variety of methods may be selected, depending on the subsequent use of the fully treated municipal effluent.

Urban waste waters – treatment for use in steam and power generation

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Search for alternative water resources is urgent for any regions of the world. Desalinated sea water and polished municipal waste waters are successfully used world-wide for technical water supplies. Whereas city waste waters are more universal water resources since they can be used both at coastal and inland industries. Thermal power stations and large boiler houses are the standouts among the energy industries. Transferring them to feeds derived from treated municipal waste waters can significantly relieve the stressed water supply balance in many cities. However, municipal waste waters, even after complete biological and tertiary treatment at municipal sewage treatment works, remain contaminated with characteristic impurities such as dissolved organic substances, compounds of ammoniacal and organic nitrogen, surfactants and others. For thermal power stations and boiler houses residual concentration of dissolved organic matter (DOM) poses threat. At thermal power stations operating at high and supercritical pressure, DOM is capable of deteriorating the operating parameters of anion-exchange filters. Residual concentration of DOM, after the deionization process, may cause the problems of the water-chemical regimes of steam generators and turbines. The entrainment of DOM in boiler water can cause foaming and carry-over of water particles into the steam outlet. We have dealt with the problems outlined above along with other issues with respect to the use of urban waste waters at thermal and nuclear power stations. We have developed technologies suitable for converting conventional and atomic power stations to feeds derived from treated municipal waste waters. Quality of the polished effluent, drawn from town works, typically does not meet the requirements for water use at power stations. We have developed several different types of final tertiary treatment depending on the destination of the water (e.g. cooling system, primary cycle or heating system).

Industrial Waste Management

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We have provided consulting services to such water consuming industries as power generation, ferrous and nonferrous metal refining, and chemical, petrochemical, oil refining, and pulp and paper. These projects were focused on preventing pollution by wastewater by constructing closed systems in industrial plants based on the following principles: the need for an all-embracing water economy, including water supply, drainage and the treatment of wastes; the main use of treated industrial and municipal waste waters and surface drainage; regenerating spent process liquors and other waters in local treatment works; extracting and recovering valuable components during the processes of effluent treatment. Treating industrial wastewaters, in preference to discharging them into municipal sewers was challenging for these industries.

We have designed twin drainage systems for oil refineries to handle the various effluent and waste water streams. The primary drainage systems were used for draining and treating storm water and general industrial waste waters. The secondary systems were for draining and treating emulsions and chemically polluted wastes. These contained oil products, sulfur-bearing compounds, alkalis, salts and various other organic and inorganic reagents. We also provided consulting services to synthetic rubber, isoprene (produced by both condensation of isobutylene with formaldehyde and two-stage dehydration methods) and synthetic fatty acids manufacturers. We developed new multistage treatment schemes. This included a novel first stage where volatile organic components were neutralized with alkali, extracted and distilled. In the second stage the high molecular weight compounds were rectified. Finally, the effluent passed through a biological treatment process. Volatile hydrocarbons and organic compounds, which are largely unaffected by biochemical oxidation, were treated by sedimentation, treatment with ferrous sulfide and lime, and sorption on activated carbon. After this, the partially treated liquors were sent on for biological processing together with the other, less polluted, wastes from the mills. Treatment alternatives of the acidic waste waters from fatty acid production included extraction, ion exchange, adsorption and/or azeotropic and extractive rectification. The sodium sulfate was recovered from the sulfate wastes by recrystallization in spray dryers, amongst other processes. Granulated sodium sulfate was produced, containing 90% of the primary material. The fatty acids and aldehydes present underwent biological treatment as the final stage of processing.