A design approach for vertical flow constructed wetlands based on selection of not only bed size, but also substrate characteristics, configuration, plant species/variety and management is piloted in a trial system to treat sugar beet processing wastes. In the beet processing season these wastes are hot, low in suspended solids and relatively high in ammoniacal nitrogen.
The pilot system consists of a small, coarse substrate first stage followed by a larger, fine substrate second stage. Results in the beet processing season show 87.3% COD removal, 87.7% TSS removal and a 79.5% reduction in ammoniacal nitrogen. Treating larger volumes of cooler stored wastes out of season, the mean removal efficiencies were 73.9% for COD, 88.0% for TSS and 93.4% for ammoniacal nitrogen.
TSS removal m−2 was found to be independent of influent concentration and temperature, whilst COD removal m−2 was found to be independent of temperature, but not influent concentration. Removal of ammoniacal nitrogen appeared to be dependent upon influent concentration and, in particular, upon temperature. Infiltration rates were shown to be negatively correlated to length of dosing cycle. Simple models of substrate permeability based on substrate particle size distribution are seen to be inadequate for describing infiltration rates in vertical downflow systems, even those treating low strength wastes.