A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION

Three bench-scale rock filters containing 0.6 m of gravel were used in this investigation. Two of the filters were planted with Sagittaria I ancifolia and Scirpus validus, while the third filter was an unvegetat ed control filter. The wastewater directed through these systems was a synthetic mixture containing nutrient broth as the carbon source. An 80-day experiment was run on the filters using eight combinations of t wo flow rates and four influent biochemical oxygen demand (BOD5) conce ntrations, each combination remaining constant for ten days. These com binations resulted in BOD5 surface loadings from 4.63-30.96 g/day/m2. From other studies of batch and continuous flow rock-plant filter syst ems, the first-order BOD5 reaction rate constant was found to decrease exponentially with detention time. In this study, however, little cor relation was found between BOD5 loading rate and removal percentages, which averaged 69%, 57%, and 47% for the Scirpus, Sagittaria, and cont rol systems, respectively. This is probably due to the relatively smal l change in detention times studied and the short time period of each loading rate. Oxidation/reduction potential (ORP) and dissolved oxygen (DO) measurements within these systems indicated that no free oxygen was available at any depth. Total Kjeldahl nitrogen (TKN) removal was higher in the plant systems relative to the control, with the Scirpus system achieving a higher overall removal than the Sagittaria system.