Slow Sand Filtration Treatment of Agriculturally-Impacted Water

Abstract

Agricultural runoff and influence to surface water is a widely experienced problem across the world, particularly in treating that water for potable consumption. Although many effective methods have been researched, there are few that can be considered suitable for rural regions, in-home and on-site water treatment, and extremely smallscale design without requiring significant chemical use, operator skills and sophistication, and large-scale to be economically viable. This research focuses on the use of slow sand filtration (SSF) as one of the most effective methods for potable water treatment that can meet all of the above criteria. The experiments were designed using rapid small scale column testing theory, which is a directly scalable approach to developing a robust and effective system for treating water. The SSF laboratory experiments were divided into two parts: preliminary experiments and full-scale experiments. The preliminary experiments were designed to test the removal efficiency of agriculturally-associated concentrations of orthophosphate and iron. The full-scale experiments were design simulate the realistic SSF plant which can test the removal efficiency with different concentration of additive in raw water. Raw water samples were collected from Wascana Lake as representative of complex water quality associated with both rural and urban contamination and agricultural influence. In the experiments, these raw water samples are subjected to several operational, water quality and environmental factors, including temperature, pH and filtration rate, to determine the impacts on removal efficiency and overall performance of the SSFs. The results of the preliminary experiments indicate that 25°C and alkaline conditions had the most significant impact on orthophosphate removal. Factors such as lower temperature (5°C) and acidic condition created an inhibited impact on the results, leaving higher concentration of orthophosphate in the effluent. For iron removal, the pH, temperature and filtration rate were all noted to impact removal efficiency. Higher temperature (25°C), increased alkalinity, and higher filtration rate resulted in higher effluent concentrations of iron. The results of the full-scale experiments indicate the interaction of phosphate, nitrate, iron and humic acids in SSF. The data show that increasing phosphate and nitrate in the raw water can slightly improve the performance of nitrate and phosphate removal but no obvious effect in humic acids and iron removal. The higher iron concentration can help improve humic acids removal. The experimental results clearly demonstrate that temperature and pH control in water treatment design are essential for high performance and robust and reliable production of high quality potable water.