Effects of Hydraulic Loading Rate and Filter Length on the Performance of Lateral Flow Sand Filters for On-Site Wastewater Treatment
Publication: Journal of Hydrologic Engineering
Volume 16, Issue 8
Abstract
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 (), medium (), and coarse ()-grained sands with hydraulic conductivities ranging from to 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 five-day biochemical oxygen demand (), 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 to 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.
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Acknowledgments
This research was funded by the Canadian Foundation for Innovation, National Sciences and Engineering Research Council of Canada, the Nova Scotia Innovation and Research Trust, the Canadian Mortgage and Housing Corporation, and Nova Scotia On-site Wastewater Research Program. Also a special thanks to Bruce Curry, Rick Scott, and Don Waller of the Centre for Water Resources Studies (CWRS) at Dalhousie University, John McCabe at Nova Scotia Agricultural College, and Wastewater Consulting Engineer Don Reid for their technical assistance in construction and instrumentation of the site.
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Published In
Journal of Hydrologic Engineering
Volume 16 • Issue 8 • August 2011
Pages: 639 - 649
Copyright
© 2011 American Society of Civil Engineers.
History
Received: Apr 21, 2010
Accepted: Dec 13, 2010
Published online: Jul 15, 2011
Published in print: Aug 1, 2011
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