Chapter 5 - Stream pollution and effluent standards

Abstract

This chapter focuses on the process of organic stream pollution and the standard principles related to effluent discharge. In case of discharge of an organic effluent into a river, two processes take place, which include oxidation of the organic waste and reaeration; and the introduction of oxygen into the water. Before the effluent is discharged, the river contains dissolved oxygen (DO). The effluent reduces the initial DO concentration progressively to satisfy the biochemical oxygen demand (BOD). In the zone of degradation, the decomposition of the effluent dominates, and so the DO concentration drops rapidly. The sediment accumulation in the immediate vicinity may be large due to the settling of suspended material in the effluent. During active decomposition, reaeration is approximately equal to the rate of decomposition and in the zone of recovery, the rate of reaeration is larger than the rate of decomposition. As the effluent oxygen demand drops and the DO deficit becomes large, atmospheric oxygen diffuses into the water body at a greater rate, thereby increasing the quantity of DO. During the clear water phase, the DO returns to its original value, with the BOD being virtually eliminated. However, as there are increased levels of nutrients in the water in this phase, it may eventually lead to eutrophication. The limits or standards placed on effluent discharges have been traditionally specified in terms of effluent volume, BOD and suspended solids (SS), in order to safeguard public health, and protect water, so that it is suitable for subsequent use in domestic, agricultural, or industrial circumstances. The water quality standards should also ensure that needs of commercial, game, and course fisheries have been taken into account.