Aerobic Based Water Treatment
Waste water contribute organic materials and inorganic nutrients to river waters; this is called eutrophication. This promotes microbial and plant growth which depletes oxygen from the water resulting in death of fish and other fauna. Sometimes algal and plant growth may be so profuse as to physically choke the river. Waste waters may also add inert particulate matter which settles in the river, sometimes it may be hot and raise the river water temperatures. All these factors influence the composition and the density of flora and fauna of the rivers.
The process of waste water treatment
Once the level of pollutants in waste water is determined, treatment of the water proceeds through the following 5 stages: (i) preliminary treatment, (ii) primary treatment, (iii) secondary treatment, (iv) tertiary treatment, and finally (v) sludge treatment. The first four stages are common to both aerobic and anaerobic process, while sludge treatment is mainly anaerobic.
Measurements of the stage of pollution
There are several parameters for determining the level of pollution in the waste waters which usually determine the amount of organic matter suspended and/or dissolved in the water, and the amounts of important inorganic nutrients like nitrogen and phosphorus present. The important measurements are: (i) biochemical oxygen demand (BOD), (ii) chemical oxygen demand (COD), (iii) suspended solids, (iv) ammonical nitrogen content, (v) phosphate content and (vi) biological indicators.
Aerobic reactors or digesters
Biological treatment of settled sewage (sewage that has undergone primary treatment) may be based on a (i) fixed film digester or (ii) a dispersed growth digester under aerobic conditions.
(i) Fixed film digesters: In these digesters, the biological components or micro-organisms are present in the form of a film on filter particles or large discs. These are basically of two types: (i) trickling filter digesters and (ii) rotating biological contactors.
(ii) Dispersed growth digesters: In such processes, the microbial population is dispersed throughout the sewage being treated. The activated sludge process is a typical example; it uses a large aerated vessel for a large scale oxidation of liquid wastes. The micro-organisms are distributed as flocs throughout the waste water being treated. Waste water continuously flows into the vessel and treated water flows out at a predetermined rate to obtain optimal digestion of the dissolved organics. The microbial flocs also flow out with the treated water. The treated water is carried into ponds with large surface area from where it flows out at very low flow rates. As a result, the suspended solids including the microbial flocs settle down; the sediment is called activated sludge as it contains microbes for aerobic digestion. A portion of the activated sludge is regularly returned to the digester vessel to restore the microbial population lost due to washout; the remaining portion of sludge is disposed off.
The micro-organisms involved in aerobic digestion and their activities are the same as those found in nature. The organic material (biodegradable component) is oxidised to CO2 and H2O, and biomass and nitrogenous compounds are also produced. In waste waters, however, the organic materials are present in much higher concentrations than in nature. Therefore, the microbial population and activities are increased accordingly by (i) providing large surface area for biofilm formation and O2 exchange in fixed film processes, especially trickling filters, and (ii) returning a part of the activated sludge to the digester vessel and making effective aeration arrangements.