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Chapter: 11th Microbiology : Environmental Microbiology

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Recycling of Treated Sewage

Water recycling offers resource and financial savings.

Recycling of Treated Sewage

Water recycling is reusing treated wastewater for beneficial purposes such as agricultural and landscape irrigation, industrial processes, toilet flushing, and replenishing a ground water basin. Water recycling offers resource and financial savings. Recycled water for landscape irrigation requires less treatment than recycled water for drinking purpose. Recycled water can satisfy most water demands, as long as it is adequately treated to ensure water quality appropriate for the use.

The residue that accumulates in sewage treatment plants is called sludge. Treatment and disposal of sewage sludge are major factors in the design and operation of all wastewater treatment plants. Two basic goals of treating sludge before final disposal are to reduce its volume and to stabilize the organic materials. Stabilized sludge does not have an offensive odour and can be handled without causing a nuisance or health hazard.


Sludge Digestion

Treatment of sewage sludge may include a combination of thickening, digestion, and dewatering processes. Among these digestion is mediated by microbes. Sludge digestion is a biological process in which organic solids are decomposed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, having the appearance and characteristics of a rich potting soil.

Most large sewage treatment plants use a two-stage digestion system in which organics are metabolized by bacteria anaerobically (in the absence of oxygen). In the first stage, the sludge, thickened to a dry solids (DS) content of about 5%, is heated and mixed in a closed tank for several days. Acid forming bacteria hydrolyze large molecules such as proteins and lipids, breaking them into smaller water-soluble molecules, and then ferment those smaller molecules into various fatty acids. The sludge then flows into a second tank, where the dissolved matter is converted by other bacteria into biogas, a mixture of carbon dioxide and methane. Methane is combustible and is used as a fuel to heat the first digestion tank as well as to generate electricity for the plant.

Sludge digestion may also take place aerobically The sludge is vigorously aerated in an open tank for about 20 days. Methane gas is not formed in this process.

Digested sewage sludge is usually dewatered before disposal. Sludge-drying beds provide the simplest method of dewatering. A digested sludge slurry is spread on an open bed of sand and allowed to remain until dry. Drying takes place by a combination of evaporation and gravity drainage through the sand.


Sludge Disposal

The final destination of treated sewage sludge usually is the land. Dewatered sludge can be buried underground in a sanitary landfill. It also may be spread on agricultural land in order to make use of its value as a soil conditioner and fertilizer. Since sludge may contain toxic industrial chemicals, it is not spread on land where crops are grown for human consumption.

When a suitable site for land disposal is not available, as in urban areas, sludge may be incinerated. Incineration completely evaporates the moisture and converts the organic solids into inert ash. The ash must be disposed of, but the reduced volume makes disposal more economical. Air pollution control is a very important consideration when sewage sludge is incinerated. Appropriate air-cleaning devices such as scrubbers and filters must be used.


Benefits of Sewage Treatment

·        Water recycling has proven to be effective and successful in creating a new and reliable water supply, while not compromising on public health.

·        Water recycling can help us find ways to decrease the diversion of water from sensitive ecosystems

·        Water users can supplement their demands by using recycled water.

·        Decreases wastewater discharges

·        Reduces and prevents water pollution

·        Recycled water can be further used in Thermal power plant (for cooling), Municipal use, Irrigation and Agricultural use.


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