Effects of loading rate and filter length on the performance of sloping sand filters used for on-site wastewater treatment

Approximately 400,000 people in Nova Scotia depend upon on-site sewage disposal systems. Many of these systems are malfunctioning for various reasons, leading to contamination of surface waters, ground waters, shellfish areas and recreational areas (Havard et al., 2008). The need for alternative wastewater treatment systems such as sloping sand filters is growing. Virtually no field studies have been conducted to assess the validity of currently used design guidelines for sloping sand filters. Preliminary investigations have indicated that the hydraulic behavior of these types of systems is poorly understood, and that current designs may be too conservative. The overall objective of this research is to develop and test design guidelines for sloping sand filter systems which maximize treatment performance while minimizing construction costs. The hydraulic characteristics and treatment performances of sloping sand filters operated at different loading rates and varying lengths were assessed within this study.Results indicate that the regular length filters (1--6) achieved at least 89% and 95% concentration reduction of total suspended solids and 5-day biochemical oxygen demand during the high loading rate period. The filters also achieved at least 5-log removal for Escherichia coli during this loading regime, yet there were some unacceptable spikes in outlet concentrations (>200 CFU/100 mL, based on provincial regulations and federal guidelines for recreational water) which were cause for concern for surface discharge. The shorter filters provided comparable levels of treatment performance to regular length filters under similar operating conditions, outperforming them with respect to total Kjedalh nitrogen and ammonia-nitrogen (>2.4% and 0.8%, respectively). Nutrient removal of phosphorus is lessening for all filters over time. All filters show great capacity for nitrification. Based on these results, recommendations to improve sloping sand filter design include further research into the following: long term monitoring (>5 years) with respect to hydrology, hydraulic characteristics and treatment performance adding a component in the sloping sand filter treatment train to aid in bacteria reduction (ultraviolet, ozone or chlorine disinfection), phosphorus removal and denitrification (blast furnace slag, alum sludge, bone char or oyster shell filter materials) considering subsurface discharge to address bacteria removal instead of surface discharge regarding varying lengths of sloping sand filter toes and varying loading rates.Experiments were conducted at an on-site wastewater treatment research facility located in Bible Hill, Nova Scotia, Canada. Eight full scale sloping sand filter systems were installed at this facility. Six filters were designed in 2004 according to Nova Scotia Department of Environment and Labour guidelines at 8 m long by 1.5 m wide by 1 m deep, installed at varying slopes ranging from 5% to 30% with small, medium and coarse grained sands. Initial findings indicated that saturated flow conditions did not exist within the filters, an assumed design condition, and as such, the loading rate was increased from 100 L/d to 200 L/d in 2006, double the rate of the current provincial guidelines. The remaining two filters were installed in 2007 and are 5.5 m long by 1.5 m wide by 1 m deep and are continuously loaded at 100 L/d.