Factors Influencing the Seasonal Accumulation of Solids in Bacteria Beds Treating Domestic Sewage

Abstract

The literature relating to the bacteria bed method of sewage purification has been reviewed. This includes an account of its historical development and a discussion of the biological and physical factors influencing its performance. The ecology and performance of two bacteria beds at the Langley Mill works of the Upper Tame Main Drainage Authority were studied over a period of two years. The beds treated domestic sewage at a similar overall rate of loading but at different frequencies of dosing. It was found that accumulation of film in winter was greater in the high frequency dosed bed. There were also considerable differences in the relative proportions of the various species of macroinvertebrate grazers between the two beds. In spite of differences in the ecology, the rates of purification were mostly very similar, apart from a slightly better level of nitrification in the low-frequency dosed bed.  Laboratory experimental filters were constructed to simulate the conditions of the operational beds and their performance at different temperatures was studied both in the presence end in the absence of macrograzers, represented by the dipteran Psychoda. It was found that at 5°c the grazing activity of _Psychoda was suppressed, allowing film to accumulate, while at 20°C the flies bred actively and mainteined the film at a low level. This supported the belief that film accumulation in winter is caused by the suppression of the macrograzers. However, in the absence of macroinvertebrates film levels were also greater at 5°C than at 20°C, indicating that there is a direct effect of temperature on film accumulation as well. Studies on the carbon dioxide output of the laboratory filters were made, and by comparing these with the corresponding rates of removal of organic matter conclusions were drawn concerning the relative significance of physical and metabolic processes in sewage purification.