Waste‐water treatment as a fermentation process

Abstract

In this age of cash shortage, the Water Authorities need research which will, either in the long or short term, enable them to cut costs without sacrificing, or even improving upon, the effectiveness and reliability of the process. Cutting aeration periods is an obvious way of achieving this and now is an appropriate time to reexamine the possibilities and to do this both with and without primary settlement. Also as nitrification is increasingly necessary to restrict the ammonia concentrations in the rivers, investigation of means of maintaining reliability of nitrification in short aeration periods is of particular importance.It is now known that nitrification is not the all or nothing situation predicted by the theory of Downing, Painter and Knowles but that in practice, due to variability in the conditions partial nitrification does occur and that as the sludge age is shortened nitrification becomes increasingly less complete. It is suggested that automatic control of plant, particularly of mixed liquor suspended solids level, could offset this tendency for reduced reliability of nitrification as aeration periods are cut. A plant designed to operate with automatic control of mixed liquor suspended solids is shortly to be commissioned in the Southern Division of the Yorkshire Water Authority and hopefully this principle will be proved.The major factors affecting the reliability of the process are, of course, the biological and physical characteristics of the sludge. The phenomenon of bulking is particularly harmful and it is encouraging that it is now receiving particular attention with some prospect of elucidating the reasons why the growth of bulking species suddenly accelerates to make them the dominant organisms. Equally, it could be valuable to know how to accelerate the death of filamentous organisms and one wonders whether mechanical agitation has any advantage in this respect. Visual inspection of flocs formed from sludges subjected to varying degrees of agitation in different plants at Manchester Sewage Treatment Works 15 years ago indicated that this might be a possibility.Investigation of the physical properties of activated sludge has been somewhat neglected though it is a field which could be very rewarding. Very little has been done on the mechanisms which bind flocs together and on the mechanical shear and coagulating effects in activated sludge plants. Extended aeration plants operate with low mechanical shearing conditions and yet they notoriously produce effluents containing high solids. This is no doubt due to them operating with a sludge having a low binding energy and which is hence easily dispersed.