Measurements of the level of pollution
There are many  factor for finding  the level of pollution in the waste waters that generally find out the quantity of organic matter suspended and/or mix in the water, and the quantity of important inorganic nutrients like phosphorus and nitrogen  present. The important measurements are: (i )ammoniac nitrogen content
 (ii) biochemical oxygen demand (BOD, (iii) biological indicators (iv) chemical oxygen demand (COD),
 (v) phosphate content and (vi)suspended solids.

The procedure of waste water treatment

After the level of pollutants in waste water is find out,  following 5 stages are applying for water treatment: (i) preliminary treatment, (ii) primary treatment, (iii) secondary treatment, (iv) tertiary treatment, and finally (v) sludge treatment. The first four stages are commonly apply for  both aerobic and anaerobic procedure where  sludge treatment is mainly for anaerobic.

Aerobic reactors or digesters

Biological treatment of settled sewage (sewage that has undergone primary treatment) can be depend on a (i) fixed film digester or (ii) a dispersed growth digester under aerobic conditions.

(i) Fixed film digesters: In these digesters, the biological factors or micro-organisms are present in the form of a film on large discs or filter particles. There are basically two type of: (i) trickling filter digesters and (ii) rotating biological contactors.

(ii) Dispersed growth digesters: In this type of processes, the microbial population is spread throughout the sewage being treated. The activated sludge process is a example of this; it uses a huge aerated vessel for a big scale oxidation of liquid wastes. The micro-organisms are spread as flocs throughout the waste water being treated. Waste water constantly flows into the pot and treated water flows out at a predefined speed to obtain optimal digestion of the dissolved organics. With the treated water, the microbial flocs also flow out. The water that is treated is carried into ponds with huge surface area from where it flows out at very little speed. As a outcome, the suspended solids including the microbial flocs settle down; the sediment is called activated sludge as it contains microbes for aerobic digestion. A part of the activated sludge is continually returned to the digester pot to restore the microbial population that is lost due to washout; the remaining portion of sludge is disposed off.