MODELING BIOLOGICAL AERATED FILTERS FOR WASTE-WATER TREATMENT

Biological aerated filters (BAFs) are submerged three-phase fixed-medi a reactors for wastewater treatment. A major characteristic of BAF rea ctors is the use of granular media which allows solids separation as w ell as secondary or tertiary biological treatment in one unit. The aim of this work was to design a simple empirical model relating influent soluble chemical oxygen demand (sCOD) to effluent sCOD and reactor he ight, verify experimentally the suitability of this model and ascertai n the relationship between model constants and reactor performance. Th e theoretical model was based on that designed for trickling filters a s both fixed-film processes show a similar plug flow pattern. Two reac tors were set up to run parallel treating settled domestic sewage usin g media identical in size and shape, except one was less dense than wa ter (relative density 0.92) and the other was denser than water (relat ive density 1.05). The reactors were run upflow with liquid flowrates of 0.29-0.58 m(3) d(-1) (0.2-0.5 litres min(-1)) and an air:liquid rat io of 10:1. After 4 weeks from start-up, steady-state was reached. Fro m this point, samples were taken at different heights along the reacto rs at timed intervals and profiles of sCOD removal against reactor hei ght were produced. This analysis was repeated for a number of differen t flowrates and organic loadings (0.57-1.40 kg sCOD m(-3) d(-1)). The resulting data was then used with the empirical model, which was based on a first-order reaction, to calculate the values of k (overall pro cess constant) and n (media factor). A higher value of k was found fo r the floating media (55) compared with the value found for the sunken media (33). This indicated the greater efficiency of sCOD removal in the floating media. The values of the media constants were similar, sh owing the similarity in the media shape and size. (C) 1997 Elsevier Sc ience Ltd.