Performance and Hydraulics of Lateral Flow Sand Filters for On-Site Wastewater Treatment

This paper describes the performance of six lateral flow sand filters (LFSFs) for their treatment of septic tank effluent in Truro, Nova Scotia, Canada. This report presents LFSF performance data collected during the first year of monitoring (Sept. 2004–Sept. 2005). The objectives of this initial study were to: (i) Evaluate the performance of LFSFs in field conditions and determine the influence of temperature and external hydrologic processes on treatment processes; (ii) evaluate the effects of slope and sand characteristics on LFSF performance; and (iii) characterize the hydraulic operation of LFSF systems in field conditions. Six LFSFs were constructed according to the Nova Scotia Department of Environment and Labour’s (NSDEL) design guidelines. Fine $(d_{10}=0.15\mathrm{mm})$ , medium $(d_{10}=0.17\mathrm{mm})$ , and coarse $(d_{10}=0.30\mathrm{mm})$ sands were tested at 5 and 30% slopes. The hydraulic conductivity of these sands ranged from $1.5\times {10}^{-4}$ to $1\times {10}^{-3}\mathrm{m}{\mathrm{s}}^{-1}$ . Each LFSF was loaded with approximately $100\mathrm{L}{\mathrm{d}}^{-1}$ of septic tank effluent for 1 year and samples were collected monthly. Average removal efficiencies for all LFSFs met NSDEL requirements: biological oxygen demand $(>98.5\%)$ , total suspended solids $(>95.5\%)$ , and E. coli ( $>5.4$ log reduction). Phosphorus removal ranged from 98% in the fine sand to 71.2% in the coarse sand filter. Nitrification was favored because the filters were operating under aerobic and unsaturated conditions. Therefore, denitrification was limited causing elevated nitrate effluent concentrations. Total nitrogen removal ranged from 60 to 66%. The LFSFs provided consistent year-round treatment and did not appear to be impacted greatly by slope, temperature, or external hydrologic influences.