DCOM Volume I Appendix G
1 Appendix G: Additional Details of Various Unit Operations
1.1 Horizontal-Flow Roughing Filters (HRF)
1.1.1 Features of HRF
The filter box comprises of 2 to 4 compartments filled with gravel media of different sizes, coarsest in the first compartment where the bulk of solids will be retained, to finest in the last compartment which acts as polisher, removing the last traces of solids; The raw water enters via an inlet channel across the filter width and falls over a weir into an inlet chamber which allows for settlement of coarse solids and separation of floating material. Water passes into the first and subsequent filter compartments through openings in the separation walls. Treated water is collected in an outlet chamber and discharged over a weir- (for flow measurement) to an outlet channel and then to the SSF The length of the filter box depends on raw water quality, hydraulic loading and media size. Height should be limited to about 1.5m to allow for easy removal of media for manual cleaning. The width depends on the required filter capacity. A minimum of 2 HRF units are provided, to allow for one being out of service for cleaning, and an adjacent area for gravel cleaning should be provided,
Design Considerations for HRF
- Characteristics of the raw water quality in both dry and rainy seasons i.e.; Turbidity;
- Suspended solids or filterability; and E-coli
- Raw water intake site location, elevation, high/low flows and water levels;
- Treatment plant site, elevation, foundation conditions;
- Hours of operation of intake, HRF, SSF and distribution;
- Availability of raw materials, gravel, sand, rock; and
- Requirements for pre-sedimentation with or without combined raw water balancing tank.
