Modelling biological aerated filters for wastewater treatment

Biological aerated filters (BAFs) are submerged three-phase fixed-media reactors for wastewater treatment. A major characteristic of BAF reactors is the use of granular media which allows solids separation as well 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 height, verify experimentally the suitability of this model and ascertain the relationship between model constants and reactor performance. The theoretical model was based on that designed for trickling filters as both fixed-film processes show a similar plug flow pattern. Two reactors were set up to run parallel treating settled domestic sewage using media identical in size and shape, except one was less dense than water (relative density 0.92) and the other was denser than water (relative 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 ratio of 10:1. After 4 weeks from start-up, steady-state was reached. From this point, samples were taken at different heights along the reactors at timed intervals and profiles of sCOD removal against reactor height were produced. This analysis was repeated for a number of different 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 process constant) and n (media factor). A higher value of k∗ was found for 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, showing the similarity in the media shape and size.