Recycled glass compared to sand as a media in polishing filters for on-site wastewater treatment

The treatment efficiency of two stratified filters in parallel (one with sand as a media, the other with recycled glass) receiving secondary treated effluent from a single house which also operated as a Bed and Breakfast has been compared over a two year period. A layer of 100 mm of limestone sand was also included in both filters to target phosphorus removal. On average the glass filter performed similarly to the sand filter for all of the monitored parameters. However, within the filters there were some differences in process kinetics between the two systems particularly with respect to nitrogen removal with the glass filter removing almost 1.5 times the total nitrogen load compared to the sand filter. In contrast, phosphorus removal was higher through the sand filter than the recycled glass filter, 51% and 40% respectively. The limestone layer in both filters only accounted for approximately half of the phosphorus removed which was disappointing as adsorption isotherm experiments in the laboratory had shown it to have excellent phosphorus adsorption properties. There was also evidence that the limestone layer was starting to approach saturation particularly in the glass filter with a reduction in removal efficiency apparent over time. Finally, the removal of enteric microorganisms through both filters using the indicator bacteria E. coli was in the range of between 2-3 log removal which, although reasonable compared to other tertiary treatment systems, still left significant numbers of bacteria in the effluent.