Comparison of backwashing with conventional cleaning methods in slow sand filters for small-scale communities

Abstract

The application of slow sand filtration technology for domestic water supply was investigated, with particular attention to the issue of periodic maintenance (cleaning the filter layer). The study used raw lake water with a high algal density to allow results to be obtained in a short time. Two filters were constructed with the same filter layer characteristics and were operated in parallel under the same conditions (filtration rate 0.16 m/h and hydraulic load variable maximum 0.80 m). Periodic maintenance was performed when the hydraulic load on the filtration layer reached 0.80 m. One filter was cleaned conventionally and the other by backwash. Nine filtration runs were performed, with an average duration of 14 d. Both filters provided significant improvements in water quality: filtered water turbidity less than 1 NTU, apparent color average of 15 Pt/Co, and removal of total coliforms and Escherichia coli around 1.5 log. In general, the quality of filtered water from the two filters was similar. However, the results obtained indicated that the backwash facilitates the operation of slow sand filters by simplicity (opening one valve) and time consumption (7 min), and ensures the water quality in terms of the parameters evaluated, justifying increased use of this technology, especially in small communities and rural areas. Nevertheless, this technology is applicable in small filters (up to 93 m²), because uniform backwash does not occur in filters with large dimensions.

Keywords:

• Decentralized water treatment
• Slow sand filter
• Backwash
• Cleaning filter

Acknowledgments

We thank the National Council for Scientific and Technological Development (CNPq) and Support Program for Centers of Excellence (PRONEX) for resources and the Catarinense Water and Sanitation Company (CASN) for providing the physical space for the pilot systems.