Effects of Hydraulic Loading Rate and Filter Length on the Performance of Lateral Flow Sand Filters for On-Site Wastewater Treatment

摘要

On-site treatment of residential wastewater is important for rural and remote regions where centralized wastewater treatment may not be feasible. Sand filtration is a proven method for secondary treatment of septic tank effluent (STE) and can be an economical option where soil conditions do not favor the installation of a typical disposal field. This study investigates the hydraulics and treatment performance of eight lateral flow sand filters (LFSFs) receiving domestic STE at the Bio-Environmental Engineering Centre (BEEC) in Truro, Nova Scotia,Canada. The main objectives of this study were to evaluate the long-term treatment performance associated with the LFSFs and to investigate the effects of filter length and wastewater loading on performance. Previous research conducted at the BEEC indicated that current technical guidelines used in Nova Scotia for the design of LFSFs may be conservative and that the design of these systems could be optimized. The hydraulics and treatment performance associated with: (1) two shortened length filters (5.5 m versus 8 m standard length), and (2) six standard length filters receiving double the recommended hydraulic loading rate were evaluated. Results were compared to the previously documented performance of standard length filters operating at the recommended loading rate. Filters included fine (d_(10)= 0.15 mm), medium (d_(10) = 0.17 mm), and coarse (d_(10) = 0.30 mm)-grained sands with hydraulic conductivities ranging from 1.5 × 10~(-4) to 1 × 10~(-3) ms~(-1)at slopes of 5 and 30%. Hydraulic residence times (HRT) were determined for each filter several times during the study period using a conservative tracer. Average removal efficiencies ranged from 82.4 to 96.9% for total suspended solids (TSS), 96.7 to 98.4% for fiveday biochemical oxygen demand (BOD_5), 4.3 to 5.2 log reduction for Escherichia coli (E. coli), 43.7 to 93.4% for total phosphorous (TP), and 40.9 to 57.2% for total nitrogen (TN). Water quality analysis indicated that both the shortened filters and the filters receiving higher hydraulic loading rates provided a similar level of treatment for most parameters. This suggests that sand filtration is not only a physical treatment process but can be attributed mainly to Fbiological degradation of contaminants that occurs within the biomat. Filter length and loading rate adjustments did not appear to affect the HRT of the filters, suggesting that the biomat also plays an important role in the hydraulic functioning of the filters. Based on performance and tracer study results, LFSFs appear to maintain their hydraulic function and provide adequate contaminant removal at linear loading rates ranging from 66 Lm~(-1) d~(-1) to 132 Lm~(-1) d~(-1) of domestic STE. Long-term performance monitoring must continue in order to adequately assess the effects of filter length and wastewater loading on treatment performance and to further evaluate current design guidelines.