Biological treatment of industrial wastewater is necessary, when they contain large quantities of biodegradable substances. Such biological treatment may be used either with or without acclimatisation. Laboratory tests on wastewater for determining its BOD/COD ratio will help in determining the type of treatment required. Say for example, if this ratio is more than 0.6, the wastewaters are biologically treatable without acclimatisation, if the ratio is less than 0.6 and up to 0.3, then acclimatisation are needed for biological treatment, and if the ratio is less than 0.3 the biological treatment may not be necessary. Acclimatisation consists of the gradual exposure of the wastewaters in increasing concentration to the seed or initial microbiological population under a controlled condition. The criteria for selecting a particular conventional biological treatment process may differ for different types of industrial wastewaters. The system parameters for a given type of industrial wastewater may be determined by laboratory experiments. In the absence of any actual test data, the performance data of’ a similar type of industrial wastewater may be used for design.
It has also been observed that most of the industrial wastewaters do not contain sufficient nutrients for microbiological growth; and hence nutrients like urea (containing nitrogen), super phosphates (containing phosphorous) etc. may have to be added to the reactors. For balanced growth of microorganisms in a biological treatment reactor, the ratios of BOD : Nitrogen : Phosphorous should be 100 : 5 : 1 for aerobic systems, and 100 : 2.5 : 0.5 for anaerobic systems (Garg, 1994).
It can be stated that the selection of a sequence of particular treatment processes depends on the characteristics of the wastewaters, and also upon the permissible requirements of the effluents. The treatment processes needed for different industries will, therefore, generally vary from one industry to another.
The attempts of treating industrial wastewaters, simply by biological methods, though cheaper, have generally failed in the absence of making efforts for pre-recovery of certain pollutants or chemicals from the wastewater by using chemical methods. However, using of simple biological methods has fairly succeeded in industries like fruit processing, dairies, slaughter houses, and textiles. Physical and chemical treatments prior to biological treatment, for separation of troubling pollutants, like chromium, arsenic, cyanide, mercury, several nitrogenous substances, lignin, etc., is very necessary and important for industries like fertilizers, dyes and pigments, pesticides, electroplating, paper and pulp, etc. Suitable Pretreatments to the wastewaters of such industries, before subjecting it to the biological treatment is, thus, the prime requirement for designing and planning the treatment plant for a particular industry.
Possible large scale reuse of the treated water in the industry is another important factor, which must be considered, while deciding the sequence of treatment processes for a particular industry. Such a possible reuse, if can be made possible will help in large scale economy in the industry. The pollution characteristics of certain typical industries along with suggested treatments are broadly reflected in the Table below.
Table Suggested treatment for some industries (Garg, 1994)
Ref. Grag S.K, ” Sewage disposal and air pollution control” 1994