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Tuesday, April 15, 2008

Sewage disposal and its treatment – the ultimate recycling of used water:




Sewage disposal and its treatment – the ultimate recycling of used water:

In urban areas sewage is created by residences, institutions, hospitals and commercial and industrial establishments. Sewage treatment and disposal system is an important function for any city planner, in order to recycle the used water. It is the ultimate return of used water to the environment. Discharge to the environment must be accomplished without transmitting diseases, endangering aquatic organisms, impairing the soil.

Wastewater is treated to remove contaminants or pollutants that affect water quality. The treated wastewater is recycled / reused for gardening, irrigation, flushing etc. Disposal system has arrangements to distribute the used water either to aquatic bodies such as oceans, rivers, lakes, ponds, or lagoons or to land by absorption systems, groundwater recharge, and irrigation. Waste water must be mixed, diluted and absorbed before it is discharged to the general body of water, so that the receiving environments do not lose its beneficial usable characteristics; such as drinking, bathing, recreation, aquaculture, irrigation, groundwater recharge, industry etc.

Water quality standards relate to the esthetics and use of the receiving environment for public water supply, recreation, maintenance of aquatic life and wildlife, or agriculture. The parameters of water quality, which define the physical, chemical, and biological limits, include floating and settleable solids, turbidity, color, temperature, pH, dissolved oxygen, biochemical oxygen demand (BOD), toxic materials, heavy metals, and nutrients.

Sewage treatment is an artificial process to which sewage is subjected in order to remove or alter its objectionable constituents and to render it less dangerous from the standpoint of public health. It can be treated close to where it is created (in septic tanks, bio-filters or aerobic treatment systems), or may be collected and transported via a network of pipes and pump stations to a municipal treatment plant. Typically, sewage treatment involves three stages, called (a) primary, (b) secondary and (c) tertiary treatment.

(a) Primary sewage treatment removes larger floating objects through screening and sedimentation. The incoming wastewater flows through one or more screens and then enters a grit chamber where it slows down enough to allow sand, gravel, and other inorganic matter to settle out. In treatment plants where only primary treatment occurs, the effluent is chlorinated and discharged into circulation in a water source. The sludge, or sedimentation of larger solids, is removed, dried, and disposed of. Primary treatment removes 50 to 65 percent of suspended solids and decreases biological oxygen demand (BOD) by 25 to 40 percent. Primary treatment alone is not considered adequate for protection of the environment or people's health.

(b) Secondary treatment relates to processes similar to natural biological decomposition. Aerobic bacteria and other microorganisms are used to break down organic materials into inorganic carbon dioxide, water, and minerals. Trickling filters, which are made from a bed of rocks with a microbial covering, are used to absorb the organic material present in the water. Activated sludge processes can be used in place of trickling filters. The level of suspended solids and BOD in wastewater after primary and secondary treatment has been decreased by 90 to 95 percent. This level of treatment is not effective in removing viruses, heavy metals, dissolved minerals, or certain chemicals.

(c) Tertiary treatment is an advanced level of treatment. This form of treatment can decrease the level of suspended solids and BOD to approximately 1 percent of what was present in the raw sewage prior to primary treatment. Advanced treatment processes consist of several biological, chemical, or physical mechanisms. Sewage treatment aims to destroy pathogenic organisms. Since primary and secondary treatments do not destroy a significant number of organisms, chlorination, which is effective in killing bacteria, is used to disinfect treated effluent.

Most advanced wastewater treatment systems include denitrification and ammonia stripping, carbon adsorption of trace organics, and chemical precipitation. Evaporation, distillation, electro-dialysis, ultra-filtration, reverse osmosis, freeze drying, freeze-thaw, floatation, and land application, with particular emphasis on the increased use of natural and constructed wetlands, are being studied and utilized as methods for advanced wastewater treatment to improve the quality of the treated discharge to reduce unwanted effects on the receiving environment.

(d) Private sewage treatment, usually a septic system, is constructed on-site and is maintained by the private homeowner. In this case, the septic tank holds the solid materials while the water goes to a leach field or absorption field. The solids undergo decomposition, and on a regular basis, generally every three years, are pumped from the holding tank. This will vary according to use and capacity.

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