Full-scale trials of recycled glass as tertiary filter medium for wastewater treatment

Pilot-scale trials at a domestic wastewater treatment works compared the performance of three grades of recycled glass (coarse, medium and fine) when used as tertiary filter media for total suspended solids removal (TSS). Fine glass produced the best effluent quality but blinded rapidly and coarse glass could process three times the flow but with a reduction in final effluent quality. The medium glass offered a compromise with similar flow characteristics to the coarse glass, yet still achieve good solids removal, albeit less than the fine glass. Full-scale studies compared the performance of medium glass with the sand medium that is typically used in this application. There was little difference between them in terms of TSS removal, and they both removed around 75% of TSS from the influent, provided that the solids concentration did not exceed 70 mg/l. However, the glass media had superior flow characteristics and was able to treat an additional 8–10% of the influent following the backwash cycle. Over the study period, the influent to the filters had an average TSS concentration of 38 mg/l and produced an effluent with an average of 15 mg TSS/l. In order to design for an average TSS concentration of 20 mg/l, the maximum solids loading on the medium should not exceed 0.25 kg solids/m3/h. Selecting recycled glass as a tertiary filter medium will give a 10% reduction in the amount of media required, compared to sand. It also carries with it the benefits of using a recycled material, and although these are more difficult to quantify they include: reduced CO2 emissions and use of a more sustainable product that promotes favourable publicity and positive environmental reporting.