ORGANIC-MATTER ACCUMULATION DURING MATURATION OF GRAVEL-BED CONSTRUCTED WETLANDS TREATING FARM DAIRY WASTEWATERS

The accumulation of organic matter (OM) was investigated after two and five years in a series of four gravel-bed constructed wetlands suppli ed with different hydraulic loading rates (21, 26, 46 and 72 mm d(-1)) of farm dairy wastewaters. At these hydraulic loadings, mean wastewat er loadings of particulate OM (determined as volatile suspended solids ) to the wetlands ranged between similar to 1.7 and 5.8 g m(.)(-2)d(.) (-1) Vertical and horizontal gradients of OM accumulation, measured by ''loss on ignition'', were sampled by stratified coring at 18 sites i n each wetland, and their impact on wastewater residence times investi gated in three of the wetlands using bromide as a conservative tracer. Mean accumulations of OM in the wetlands after five years operation r anged between 6.8 and 14.9 kg m(-2), increasing with wastewater loadin g rate. The annual rates of accumulation during the first two years we re 1.2 to 2-fold higher than those in the subsequent three years. Arou nd 50-60% of the OM occurred within the gravel substratum, the remaind er forming surface sludges, commonly exceeding 50 mm depth over much o f the wetland substratum. OM accumulation in the wetlands considerably exceeded that contributed from applied wastewaters, with wetland plan t derived detritus supplying substantial additional quantities of OM. The effective void space of the wetland substrata was markedly reduced in the highest loaded wetland, with mean wastewater retention time re duced to similar to 50% of its theoretical value (corrected for evapot ranspiration losses). In contrast, the lowest-loaded wetland exhibited retention times close to theoretical values. There was, however, no d irect relationship between OM accumulation and the effective retention times of the wetlands, suggesting other factors, such as differences in OM bulk density, spatial patterns of accumulation and plant root gr owth, and inorganic accumulations, were also influencing their hydrolo gy. (C) 1998 Elsevier Science Ltd. All rights reserved.