Anaerobic ponds are deep treatment ponds that exclude oxygen and encourage the growth of bacteria, which break down the effluent. It is in the anaerobic pond that the effluent begins breaking down in the absence of oxygen "anaerobically". The anaerobic pond acts like an uncovered septic tank. Anaerobic bacteria break down the organic matter in the effluent, releasing methane and carbon dioxide. Sludge is deposited on the bottom and a crust forms on the surface as show in Fig. 2.
Fig. 2 Operation of the Anaerobic Pond.
Anaerobic ponds are commonly 2-5 m deep and receive such a high organic loading (usually >100g BOD/m3 d equivalent to >3000 kg/ha/d for a depth of 3 m). They contain an organic loading that is very high relative to the amount of oxygen entering the pond, which maintains anaerobic conditions to the pond surface. Anaerobic ponds don’t contain algae, although occasionally a thin film of mainly Chlamydomonas can be seen at the surface. They work extremely well in warm climate (can attain 60-85% BOD removal) and have relatively short retention time (for BOD of up to 300 mg/l, one day is sufficient at temperature >20oC). Anaerobic ponds reduce N, P, K and pathogenic micro-organisms by sludge formation and the release of ammonia into the air. As a complete process, the anaerobic pond serves to: * Separate out solid from dissolved material as solids settle as bottom sludge. * Dissolve further organic material.
* Break down biodegradable organic material.
* Store undigested material and non-degradable solids as bottom sludge. * Allow partially treated effluent to pass out.
* BOD Removal Rates and Factors
First the concept of Biological Oxidation Demand (BOD) should be introduced. Organic compounds in wastewater may be used as food for bacteria, which can biochemically digest or oxidize the organic compounds to produce energy for growth. This oxidation of organic material, if done under aerobic conditions (i.e. in the presence of oxygen), "consumes" oxygen and produces carbon dioxide. An organic waste can therefore be said to have a biochemical oxygen demand, i.e. the amount of oxygen required by aerobic bacteria to oxidize it. The term BOD is used to refer to the organic material in a waste and can be used in quantitative expressions relating to organic material, i.e. the expression g BOD or kg BOD describes an amount of organic material. The amount of BOD in a specific volume of wastewater is the concentration or strength of the wastewater and is expressed in terms such as g/m3 or mg/L or parts per million of BOD (all numerically equivalent). The loading rate of organic waste to a treatment system or a receiving environment (i.e. land) is expressed as a mass of BOD/ volume (or area) of treatment system per unit of time: i.e.- g BOD/m3/day for loading rate of an anaerobic pond; g BOD/m2/day to a facultative pond or to land. BOD is measured in a five-day test of oxygen consumption. The BOD value derived from this test is usually expressed as the BOD5 of the wastewater. Small ponds that receive a reasonably high input of plant nutrients generally develop ecosystems that feature algal populations that produce oxygen in excess of the respiration requirements of the algae. This "excess" oxygen can be used by bacteria to oxidize biodegradable organic matter (quantified as BOD5) entering the pond. This principle forms the basis of natural-aeration waste stabilization ponds, wherein bacterial degradation of organic waste provides carbon dioxide and nutrients to sustain algal photosynthesis and production of oxygen that the bacteria then use. In anaerobic ponds BOD removal is achieved (as in septic tanks) by sedimentation of settleable solids and subsequent anaerobic digestion in the resulting sludge layer: this is particularly intense at temperatures above 150C when the pond surface literally bubbles with the release of biogas (around 70 percent methane...
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