Analysis of surface waters: By Hubert Hellmann. Pp. 275. Ellis Horwood, Chichester. 1987. £45.00

Abstract

Assertions have been made that the macrofauna in percolating filters influence the efficiency of treatment by controlling the growth of film, thus reducing the tendency toward blocking. The present study was designed to establish the effect of the two types of macroinvertebrates—psychodid flies and enchytraeid worms—separately and in mixed populations, on the efficiency of percolating filters. Four groups of triplicate perspex filters were supplied with artificial sewage at a constant temperature of 15°C, one group being inoculated with psychodid larvae, one with enchytraeid worms, one with mixtures of flies and worms, and one being kept free as a control. The film in the control group grew rapidly and blocked the filter, causing ponding and producing anaerobic conditions. The efficiency of treatment, measured in terms of BOD and organic carbon removal, decreased from 90 per cent to 40 per cent, some of the carbon being removed by anaerobic decomposition.The two groups containing fly populations showed increased efficiencies (> 90 per cent) within 2 months of inoculation, with the larvae successfully controlling the film accumulations. The filter group containing worms achieved similar high treatment efficiences (> 90 per cent), but not until 5 months after inoculation, the worm populations increasing more slowly because of longer developmental periods. The difference in efficiency of the control group and groups with macrofauna in terms of BOD, organic carbon and ammonia removal, and nitrate production were statistically highly significant (P < 0.001). The settlement of solids in the effluents from the control group was shown to be significantly slower (P < 0.001) than the settlement of solids from the filter groups containing macro-invertebrates.