The basic technology generally used for the treatment of Sewage is activated sludge process. This applies to both small and large processing plants and the difference lies in the arrangement and enhancement of the various sections of the process. The activated sludge process is a natural process and nature offers us a unique solution to treat sewage. Nature has provided a special balance in this process in that the micro organisms present when the food levels are high, will also consume the largest amount. This allows the quick breakdown of the BOD levels to more reasonable levels. Once these levels are reached, other microorganisms, which are heavier and less mobile, will reduce the BOD levels further, until the final acceptable standards are obtained The fact that the last organisms are large and heavy, allows us in practical terms to settle these organisms out very efficiently, producing a clear _liquor.
To balance the process, we can identify four major sections in a activated sewage plant system:
- Collection and anaerobic storage
- Aeration of the Sludge
- Setting of the sludge removing all solids
- Chlorination and phosphor removal to bring the final effluent up to the required standard.
Biological treatment is the process of utilizing naturally occurring living organisms to degrade, stabilize and destroy organic contaminants. These microorganisms use the waste as their source of energy and carbon. Biological treatment technologies are restricted to organic wastes, and therefore have limited application. It is appropriate at this point to review some principles of biological process. All living organisms require a source of energy and carbon to be able to develop and reproduce. Many organisms (autographic) get their carbon from inorganic compounds (such as CO2), while other organisms (heterotrophic) use organic compounds to get their carbon. Aerobic and anaerobic metabolic pathways are used by microorganisms to degrade organic waste. During aerobic respiration, the organism utilizes oxygen to break down complex organic compounds into simple in organic salts, carbon dioxide and water. These microorganisms require an electron acceptor (oxygen in the case of aerobic), nutrients such as nitrogen (N) and phosphorous (P), and other trace elements. One of the most important characteristics of the waste is its biodegradability. Microorganisms can either directly use the contaminated waste and gain energy and carbon from it; or, with the help of another substance they can co-metabolically break down the contaminated waste. The biodegradability of a waste can be measured in the laboratory through BOD (5) / COD tests. BOD (biological oxygen demand) is a test through which contaminants can be categorized according to their biodegradability. COD (chemical oxygen demand) is a measure of the oxygen required to oxidize all compounds in water, both organic and inorganic.
The technologies described in this section are chemical treatment technologies that are used for the cleanup of uncontrolled hazardous waste sites. In general, chemical treatments alter the structure of the waste constitutions to render them less hazardous than their original form. The objectives in using chemicals and chemical reaction are to either immobilize, mobilize for extraction, or detoxify the contaminants. A chemical technology may achieve one or all of the above tasks. Before describing any of the treatments in detail, a few points must be emphasized.