2. Suspended solids. A known volume of the effluent, after proper shaking, is filtered through a gooch crucible which is already dried and weighed. The residue on the gooch crucible is dried at 110° C for 2 hours and weighed.
3. Ammoniacal nitrogen. Ammonium salts, when they react with formalin, produce the corresponding acid. This acid is estimated by treating against standard sodium hydroxide using Thymol Blue as indicator. From the volume of acid produced, the ammoniacal nitrogen is calculated in mg/1.
4. Chemical Oxygen Demand (COD). A known volume of effluent (filtered) is refluxed with potassium dichromate and sulphuric acid, and the excess of dichromate is titrated with ferrous ammonium sulphate. The amount of oxidizable organic matter is proportional to the potassium dichromate consumed.
5. Fluoride. Fluoride is separated from the other constituents of water by distillation of Fluosilicic acid from a solution of the sample in an acid with a higher boiling point. The liberated HF is absorbed in either distilled water or alkali and titrated against standard Thorium Nitrate solution with Methyl Thymol Blue as an indicator.
6. Dissolved Oxygen (DO). Oxygen dissolved in water is made to react with MnSO4. KI- KOH solution is added followed by H2SO4 (conc.) Liberated Iodine is titrated against standard sodium thiosulphate solution.
7. Biological Oxygen Demand (BOD). A known volume of sample is added to the dilution water to have 2 percent strength of sample. Initial DO of this diluted sample is determined. Then, the diluted sample is incubated for 5 days at 20°C. The DO is again determined after this period. The difference between these two DO figures represents the Biochemical Oxygen Demand.
Determination of P2O2 has not been outlined as this is a well-known method.
With the help of modern technology which is in operation in the development various scrubbers is balanced in the heat and water balance such that most of the liquid effluents are recycled back into the system.
In the urea plant, the dust which emanates from the pilling tower is scrubbed and re-used in the absorbers. Similarly, in the complex fertilizer plant, the reaction fumes and the dust containing NP are scrubbed and recycled back into the system so as to minimize liquid effluents.
In the new plants being installed at Trombay and other placed in this country strict control has been exercised to ensure that effluents are kept at the minimum level consistent with available modern technological advances.