Wastewater Treatment Facilities and Processes

Recover Clean Energy and Clean Energy from Wastewaters.

Biomass to Biogas Using Cascade Technology

Solid Wastes to Clean Energy Using Cascade technology

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WASTEWATER TREATMENT

Wastewater (sewage) consists of liquid and solid waste products collected by drains and pipes called sewers. Wastewater contains microorganisms, some of which are pathogenic to humans. The proper treatment of wastewater prevents the dispersal of these pathogenic microbes, such as

coliform bacteria, staphylococci, fungi, protozoa, and helminthic larvae. Most wastewater treatment plants incorporate three broad levels of processing: physical or primary treatment, biological or secondary treatment, and chemical or tertiary treatment.

Subterranean conduits, called sewers, bring wastewater to the grit chamber in the treatment facility for preliminary physical processing. The wastewater passes through a bar screen which traps large debris. Insoluble material passing the screen, such as sand, eggshells, and gravel settles in the grit chamber and is moved out of the chamber by a rotating auger. This insoluble waste is often transferred to a drying field.

Ninety percent of the wastewater from the grit chamber flows into the primary clarifier, a type of settling tank. Here light, insoluble waste collects on the surface of the wastewater. This scum is drawn into v-notched collectors, called weirs, located around the outer, upper edge of the

clarifier. The heavier solids (sludge) settle and collect at the bottom of the clarifier. Both scum and sludge are transferred to the anaerobic digester. The wastewater is transferred to aerobic biological processing units.

Microorganisms are the principal actors in biologic processing. The anaerobic digester is a closed tank in which anaerobic bacteria resident in the sludge digest or break down the organic matter present (dead organisms, insoluble vegetative matter, and so on). This fermentation process goes on for weeks. The end products of fermentation are converted by anaerobic bacteria to methane gas (CH₄) which collects at the top of the digester. This gas can be recovered and used by cogeneration units at the treatment plant to produce electricity. The unconverted indigestible sludge is pumped to drying fields, developed as a fertilizer, or put in landfills or on

specified areas of ocean floors.

Aerobic digestion of organic matter occurs in the trickling filter. Here the wastewater is sprayed over fist-sized stones (called media) on which a microbial (zoogleal) film of aerobic bacteria and fungi develops. Aerated wastewater passes down through the rocks, creating a virtual food chain on the media. Bacteria and fungi feed on particulate organic matter; these small microorganisms are consumed by larger microorganisms (protozoa). As the amount of available nutrients decrease, the number of microorganisms decreases.

In technical terms, there is a reduction of biological loading and biochemical oxygen demand (BOD). In time, the remaining organic matter in the wastewater is oxidized and mineralized, forming ammonia, nitrate, phosphate, sulfate, and carbon dioxide. The oxidation of wastewater may also occur in large ponds.

Another form of aerobic digestion of organic matter in the wastewater is the activated sludge system. Here wastewater, containing numerous microorganisms, is discharged into a tank in which air diffuses up from the bottom. The aerated, flowing water encourages the growth and

reproduction of bacteria which necessarily consume the available organic matter. In effect, the activated sludge system speeds up the process that ultimately reduces the biological loading. Some of the wastewater cycles back to the tank (return activated sludge or RAS); eventually the wastewater is piped off to the secondary clarifier by way of a mixer.

Chemical processing of wastewater begins with the addition of chemicals, such as polymers, coagulants, and flocculents to the waste water in a rapid mixer tank. These chemicals bind the microorganisms in the wastewater, forming insoluble clumps (floc) in the secondary clarifier. The floc settles to the floor of the tank, accumulating as sludge. The sludge in the secondary clarifier may be transferred to an anaerobic digester or it may be transferred to drying fields

Wastewater is transferred from the secondary clarifier to the chlorine contact basin. Here sodium hypochlorite or chlorine gas is injected into the flowing wastewater for disinfection. Filtration of the wastewater is sometimes used as a tertiary treatment method.

If the treated effluent is to be discharged into a lake, river or ocean, it must be dechlorinated with sulfur dioxide or sodium bisulfite. If the effluent is to be used for reclamation (irrigation), the chlorine is usually dissipated during conduction of the water in the miles of transmission pipes from the treatment plant to the reclamation site.