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.
| Maximum suspended solids concentration in pre-settled water(mg/L) | > 300 high | 300-100 medium | 100 low |
|---|---|---|---|
| Filtration rate m/h | 0.5 | 0.75-1 | 1-1.5 |
| Filter length (m) for medium size (mm) | |||
| 20mm | 3-5 | 3* | 3* |
| 15mm | 2-5 | 2-4 | 2-3 |
| 10mm | 2-4 | 2-3 | 2 |
| 5mm | 1-2 | 1-2 | 1 |
| Maximum suspended solids concentration in HRF effluent(mg/L) | 5 | 2-3 | 2 |
- This gravel size can possibly be omitted. (Source: http://www.nzdl.org/gsdlmod?)
Design Criteria of HRF The criteria for design of HRF are as follows (also illustrated in Table 3.25 and Appendix B:
Filtration rate (m/s) ranges between 0.3 and 1.5 m/h. It is defined here as hydraulic load (m³/h) per unit of vertical cross section area (m²) of the filter; Filter Box : -length 9-12 m -Height 1-1.5 m -width 2-5 m per unit
Media size: -coarsest 35- 40 mm -finest 4 mm Length of compartments (see table 2 below) Required effluent quality foe SSF: -10 turbidity units 2-5 mg/L 2-5 mg/L SS Openings in compartment walls; 10-20% of cross- sectional area, evenly distributed Other criteria, such as expected efficiency of Suspended Solids (SS) removal, length of filter runs, required maximum head loss, etc, may be estimated by weans of a nomogran if necessary. The outlet weir level should be 30-40 cm below the top level of filter material in order to allow for head loss across the filter, which builds up as the length of filter run increases and the filter loads up with silt Filter material ranges between 20 and 4 mm in size, and is usually distributed as coarse, medium and fine fraction in three subsequent filter compartments. Filter length is dependent on raw water turbidity and usually lies within 5 to 7 m. Due to the comparatively long filter length. Horizontal-flow roughing filters can handle short turbidity peaks of 500 to 1,000 NTU.
