This paper concentrates on estimating the possibility of, and mathematical analysis of, the use of biological fluidised beds (BFB) in the treatment of night soil with low dilution rates. An experiment was conducted using a continuous type reactor at 20°C, varying F/M ratio from 0.12 to 0.37 and dilution ratio from 2 to 10, with matted reticulated polypropylene sheets for the solid supports. The results obtained showed that the use of BFBs for the treatment of night soil would be more effective than any other biological treatment process. It was observed that the optimum dilution ratio was about 5-fold, and the optimum hydraulic retention time (HRT) was about 17 hrs. It was estimated that, in comparison with other biological treatment processes, the reactor volume and the quantity of dilution water could be reduced by 70% and 80%, respectively. The experimental results for the BFBs can be analysed by the same mathematical models applied to completely mixed activated sludge processes. The substrate removal rates obtained using the equations of McKinney (Km) and Eckenfelder (Ke) were 1.784/hr and 2.0 x 10−3 l/mg.day, and substrate was removed very rapidly compared to conventional type biological treatment processes. The biomass yield coefficient (a5), the endogenous respiration rate (b), the synthesis oxygen demand rate (a5'), and the endogenous respiration oxygen demand rate (b') were 0.349, 0.0237/day, 0.495 and 0.0336, respectively.

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