Sewage Treatment Plant Slow Sand Filter
A slow sand filter consists basically of the following components:
Housing
Water layer
Filter bed
Drainage system
Flow control
Housing
Filters can be constructed in tanks with non-reactive surfaces such as plastic or fiberglass lined galvanized tanks, poly or concrete tanks of various sizes from 44gal drums (205 litres) up to 100,000 litre tanks. It may be advantageous to construct 2 smaller SSF units rather than one large unit so one can be shut down periodically for cleaning or repairs. Table 1 presents the volume of water filtered in a 24 hour period by filters of varying size. The capacity of the filter is determined by the surface area of the filter top and not the overall volume of the filter. Consideration must be made of the flow rate to be used when determining tank size (see Managing the SSF below).
Water layer
The water layer above the filter bed provides the head to push water through the filter bed. It is convenient as a water storage zone and provides an effective temperature buffer to stabilize the filter and protect the biological activity occurring in the top layers of filter bed. A minimum depth of .5 m up to 1.5 m is most commonly used. Research in Australia suggests that this water layer should be maintained at a constant depth by the use of a small pump from an overflow tank or from the filtered water reservoir. This is particularly important if nutrient solution or recycled water is introduced into the filter in pulses and not continuously. The filter should never stand with an exposed dry top layer, or stagnant water which would accumulate only if the filter outlet was closed at the bottom. Continuous filtering assists in the development and maintenance of a healthy filter.
Filter bed
The filter bed consists of a uniform fine particle sand mixture as specified (see Sand Specifications). The most critical design feature of the SSF is using a correct sand or alternative media. The filter bed is built to a depth of 1-1.5 m (or more) with a minimum of .8m on smaller filters. This depth of sand will allow for losses which will occur if the top portion of the sand is removed when particulate matter and algae is cleaned from the top of the sand.
Sand Specifications
Sand is characterized by the diameter of the individual sand grains (eg 0.15-.35 mm) and the effective size of the composite sand, the ES or d10. d10 is defined as the sieve size in mm that permits passage of 10% by weight of the sand. The uniformity coefficient (UC) of a sand is defined as d60/d10.
Sand needs to be of a fine grade ( 0.15-.35 mm is recommended), uniform (the UC should always be less than 3 and preferably less than 2), and be washed free of loam, clay, and organic matter. Fine particles will quickly clog the filters and frequent cleaning will be required. A sand that is not uniform will also settle in volume, reducing the porosity and slowing the passage of water. Sand manufacturers should be able to supply or blend sand to these specifications.
Drainage system
A gravel drainage system is provided at the bottom of the filter to prevent movement of the fine sand into the filter outlet. In European specifications this has consisted of 3 graded layers, 2-8mm,8-16mm,16-32mm. The use of a geo-textile fabric may be considered to support the sand as an alternative to some gravel layers. The bottom layer of gravel supports perforated drainage pipes which may simply bisect the filter or in a large filter form a network of connecting pipes across the base. The use of granulated rockwool as an alternative media to sand can reduce the requirement for the gravel drainage system and thus reduce the depth of the filter. A fine screen over the outlet would still be recommended to prevent rockwool granules from passing into the outflow.
Flow control
A regulating tap should be connected to the filter outlet to control the flow rate. On large filters a flow meter is sometimes installed for use in monitoring the flow rate. The flow rate is specified in terms of litres/hour per unit area of the surface of the filter ( m2). The flow rate through the filter is less than gravitational fall. Keeping the water level above the filter bed constant assists in maintaining an even flow. The flow rate will drop off with the build-up of material on the surface of the filter bed. An open clear pipe (poly tube) fixed to the exterior of the filter can be used to monitor head loss.
Technical parameters:
| TPYE |
TANK SIZE
(mm) |
TOTAL HIGH
(mm) |
FLOW RATE
(m3/hr) |
INLET AND OUTLET
(mm) |
QUARTZ SAND
(t) |
| YLD-MF-500 |
500*1500 |
2350 |
2 |
DN32 |
0.39 |
| YLD-MF- 600 |
600*1500 |
2380 |
2-3 |
DN32 |
0.56 |
| YLD-MF- 700 |
700*1500 |
2400 |
4 |
DN40 |
0.76 |
| YLD-MF- 800 |
800*1500 |
2400 |
5 |
DN40 |
1 |
| YLD-MF- 900 |
900*1500 |
2500 |
6 |
DN50 |
1.3 |
| YLD-MF- 1000 |
1000*1500 |
2600 |
8 |
DN50 |
1.6 |
| YLD-MF- 1200 |
1200*1500 |
2700 |
11 |
DN65 |
2.3 |
| YLD-MF- 1400 |
1400*1500 |
2800 |
15 |
DN65 |
3 |
| YLD-MF- 1500 |
1500*1500 |
2850 |
18 |
DN65 |
3.5 |
| YLD-MF- 1600 |
1600*1500 |
2900 |
20 |
DN80 |
4 |
| YLD-MF- 1800 |
1800*1500
1800*1800 |
3000 |
25 |
DN80 |
5 |
| YLD-MF- 2000 |
2000*1500 |
3100 |
30 |
DN100 |
6 |
| YLD-MF- 2200 |
2200*1500 |
3180 |
38 |
DN100 |
7.5 |
| YLD-MF- 2400 |
2400*1500 |
3330 |
45 |
DN100 |
9 |
| YLD-MF- 2500 |
2500*1500 |
3380 |
50 |
DN100 |
9.7 |
| YLD-MF- 2600 |
2600*1500 |
3430 |
55 |
DN125 |
10 |
| YLD-MF- 2800 |
2800*1500 |
3530 |
60 |
DN125 |
12.5 |
| YLD-MF- 3000 |
3000*1500 |
3630 |
70-80 |
DN125 |
14 |
| YLD-MF- 3200 |
3200*1500 |
3730 |
80~100 |
DN150 |
16 |
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