TERTIARY-TREATMENT OF WASTE-WATER IN BIOLOGICAL PONDS

Stabilization ponds for tertiary treatment of wastewater are characterised by low concentration of biomass and low rates of biodegradation processes. As in such condition the power requirements for effective mixing of the pond's content are 10-15 times superior to those necessitated by oxygen supply, it is sometimes considered that the introduction of artificial aeration into the pond's system is uneconomical. Nevertheless, analysis of the factors determining performance and efficiency of ponds shows that power requirement for aeration could be significantly increased by raising the oxidation capacity of the pond, while those for mixing purposes could be noticeably reduced by choosing the suitable equipment and appropriate manner of aeration. The first becomes possible through maintaining the highest concentration of biomass admissible from the standpoint of better separation of suspended solids from the effluent and by intensifying biochemical processes of treatment. The second is achievable by using intermittent aeration, dependent on the settleability of biomass, and suitable aerating equipment. This paper considers some analytical and practical aspects of such approach to the problem of determining detention time and capacity of an artificially aerated pond that could ensure required quality of the effluent expressed in terms of BOD and SS concentration parameters. All the main engineering features of this approach, such as the arrangement of a pond in three units(operated in series), two of which are aerated and the third one is designated for SS separation; the augmentation of pond's depth up to 5-6 m in order to reduce its surface area and, subsequently, the heat losses through it; the determination of biomass concentration on the basis of its settleability and of admissible SS concentration in the effluent; the employment of floating mechanical aerators designed to perform "to and fro motion" in its zone of influence, are reflected in a proposed series of equations, forming the method of calculation of aerated stabilization ponds.