NEED FOR WASTEWATER TREATMENT
Sewage is composed of 99.7 to 99.9% of liquid depending upon the per-capita water supply rate; more per-capita water supply means more percentage of liquid content in sewage. Sewage contains only 0.1 to 0.3% of solids, but these solids are responsible for all kinds of problems associated with sewage. In particular organic solids, which account for 70% of total solids, are liable for decomposition by microorganisms. Unless wastewater is properly treated to remove these organic solids before disposal, it will cause environmental degradation. It is therefore, necessary to treat the wastewater before any kind of disposal.
Discharge the wastewater by dilution in to water bodies was considered as the method of disposal for long. But ‘dilution is the solution to pollution’ is no longer an acceptable option in most situations. To prevent water pollution the wastewater should be treated before disposal into the environment. Purpose of wastewater treatment is to remove or alter solids in the wastewater in order to minimize the impact of discharging wastewater into the environment.
The biochemical oxygen demand (BOD), total suspended solids (TSS) faecal coliforms (FC) and nutrients are important parameters for consideration while designing the wastewater treatment because excessive BOD and TSS levels could cause severe and readily apparent problems in receiving waters, such as dissolved oxygen (DO) deficits that led to odors and fish kills, and sludge deposits that suffocated benthic organisms. By removing BOD and TSS, other contaminants are also removed and other benefits are realized; so even today, effluent standards of some countries contain only limits for BOD and TSS only. However, effluent standards of many countries now contain limits on other contaminants as well.
The most common nutrients of concern are phosphorus and nitrogen. Earlier removing phosphorits and nitrogen could only be done through expensive, advanced methods. But now there are ways to accomplish enhanced removals of nutrients in conventional biological treatment plants with relatively minor operational and structural adjustments. The treatment systems used for municipal wastewater can be different from the systems used for industrial wastewaters as they can pose special problems which require innovative applications of the technologies available. Additionally, industrial wastes are sometimes pretreated before being discharged to a sewer, as opposed to being totally treated for direct discharge to the environment.
The unit operations and processes used in the wastewater treatment consist of physical, chemical and biological systems.