Received date: 14 July 2013 Accepted date: 10 October 2013
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Oxidation pond is one of the biological systems which are used for the treatment of wastewater. It is considered as the secondary treatment method by which natural purification and stabilization of wastewaters like domestic sewage, trade waste and industrial effluents is accelerated. The biological treatment process in oxidation pond mainly involves an interaction between bacteria, algae and other organisms. It efficiently removes bacteria, biodegradable organics, phosphorous and nitrogen present in the wastewater which is going to be discharged to the receiving streams. In this method, 98% to 99% of BOD reduction in wastewater is often possible.
Oxidation pond, BOD, wastewater, bacteria, algae, protozoa.
Oxidation (stabilization) pond is a simple scientifically designed pond with 2-6 feet depth, where BOD reduction of a wastewater takes place by supporting algal-bacterial growth . These ponds are effective, low-cost and simple technology for the treatment of wastewater before it is discharged to an aquatic ecosystem  and are commonly used in warm climates to purify wastewater. The performance of pond depends on climatological conditions like light, temperature, rain, wind and also the wastewater quality. Primarily these are used as tertiary treatment facilities specially to polish the effluents from conventional treatment plants . These ponds are also used to treat the raw sewage, settled sewage and industrial effluents .
Oxidation pond typically operate in an extended aeration mode with long detention and solids retention time  and is a widely adopted technique for the treatment of domestic and trade wastes. It is one of the methods used extensively in the tropical areas of the world for treating the wastewater . The first oxidation pond "The San Antanio pond" or Mitchell Lake arose due to the practice of land disposal of sewage and industrial effluents for irrigation in large areas of deficient rainfall in the USA . Hospital wastewaters have been reported to be best treated in the oxidation pond . This method would also have disadvantages that it requires extensive land area, potential odour problem, mosquito menace, little control over the effectiveness of the treatment process and the main disadvantage is seepage of effluents into soil which may also lead to ground water pollution. Still, oxidation ponds have proved to be one of the most significant devices of economical waste treatment for small communities and isolated industrial units in Tunisia .
The occurrence of several species of bacteria , fungi , algae [12,13], protozoa [14,15] and viruses  in the oxidation pond has been reported. In the earlier years (1946-1964) it was believed that the symbiotic activity of bacteria and algae alone was responsible for the treatment of wastes in oxidation ponds. However, Gloyna has reported many species of fungi which are involved in the function of waste purification.
Oxidation pond comprises different groups of organisms such as bacteria, algae, protozoa, fungi, viruses, rotifers, nematodes, insects and crustacean larvae etc. These organisms coexist and compete with each other. The bacteria present in the pond decompose the biodegradable organic matter and release carbon dioxide, ammonia and nitrates . These compounds are utilized by the algae, which together with sunlight and photosynthetic process releases oxygen, enabling the bacteria to breakdown more waste and accomplish reduction in BOD levels . Weidemann and Bold  included fungi in the symbiotic cycle and explained that the nutritional aspects of bacteria, algae and fungi are interrelated. These ponds often harbor aquatic weeds and are termed as macrophyte ponds. Initial research on oxidation ponds (1946 to 1960) describes pond activity in terms of mutualistic behaviour of algae and protozoa through photosynthesis .
Studies on the physico-chemical characteristics and the seasonal periodicity of plankton in Indian sewage and treatment of wastewater in oxidation ponds started six decades ago. Studies on microbial succession  and biochemical stratification [22,23,24] have already been carried out. The earlier reports on oxidation ponds have emphasized upon the removal of BOD, bacteria and nutrients [25,26,38].
Intensive work has been carried out on few species of bacteria, fungi, algae and protozoa individually in response to changes in environmental factors like light intensity, temperature and pH [27,39]. Koppen  showed that retardation in the rate of catalytic decomposition of hydrogen peroxide in the presence of catalase may be used for the detection of compounds toxic to aerobes in the biological systems. The analysis of enzyme activity was used to investigate the process of self purification in polluted rivers [29,30].
The bacterial genera commonly present in the oxidation ponds are Achromobacter, Proteus alkaligens, Chromomonas, Zoogloea, Pseudomonas, Chromatium, Thiospirillum, Thiopedia and Rhodothecae . According to the conditions of the oxidation pond aerobic, facultative and anaerobic bacteria grow and stabilize the organic substances present in the wastes through biological processes .
About 15,000 species of algae have been recorded in nature; only few of them are functionally useful in the oxidation process . The algal species belong to the genera Chlorella, Scenedesmus, Euglena [12,21,26] and Microcystis  were shown to be the dominant algae in the oxidation pond because of their high tolerance capacity to extreme environmental conditions. Euglenoids depicted a great deal of adaptability to changes in environmental conditions in ponds .
Protozoans such as microflagellates and ciliates appear during prestabilization phases. Among ciliates, Holotrichales are the most predominant forms , which mainly feed on bacteria and suspended solids . Ganapati and Amin  reported the presence of Vorticella, Macrostoma, Paramecium and Podophrya in the scum formed at the surface of oxidation pond, during prestabilization and overloading conditions. Patil et al.,  recorded 12 species of protozoa from oxidation pond samples belonging to various genera such as Paramecium, Vorticella, Colpodium, Stylonychia, Perispira and Caenomorpha. Nair ) reported 46 species of protozoa from sewage.
In addition to bacteria, algae and protozoa, there are also other organisms such as crustacean larvae, insects, viruses, rotifers, nematodes which interact and compete with each other for food and convert the organic materials of the sewage into simple products in the oxidation ponds. The Tunisian government has laid guidelines for safe reuse of effluents for agriculture with respect to the presence of helminthes and fecal coliforms .
In India and in most of the tropical parts of the world where sufficient sunlight and temperature are available, oxidation pond system is found to be most suitable for the treatment of domestic sewage and trade waste containing nutrients. During the recent past, improved life style and activities like urbanization, industrialization and technology have lead to the enrichment of various pollutants, which may affect the treatment efficiency of oxidation pond. When these pollutants are accumulated in the sediments of oxidation pond, they become toxic to the entire oxidation pond community. In addition, these pollutants circulate in a pathway similar to nutrient cycles in the oxidation pond medium. If the pollutants are over loaded in the sewage, they cause shock loads and degrade the effluent quality.
The authors are thankful to the authorities of Mangalore University for providing laboratory facilities.