Salient data and features of slow sand filters

(for more detailed information see Ref. [15, 16, 17])

Design criteria

filtration velocity	vF	0.1 - 0.2 - (0.3 - 0.4) m/h
area per filter bed	A	10 - 50 - (100) m²
number of filter beds		minimum of 2
height of supernatant water	hW	1 - (1 5) m
depth of filter sand bed	hf	(0.6) - 0.8 - 1 m
depth of underdrains system and filter support	hs	(0.2) - 0.3 - 0.5 m
specification of filter sand
effective size	d10	0.15 - 0.35 - (0.6) mm
uniformity coefficient	UC 2 - 5
specification of filter support
size/depth	1-1.5 mm/10 cm
(size of the support medium should be roughly 4 x the size of	4 - 6 mm /10 cm
the medium to be supported)	15 - 15 mm / 15 cm

Fig. 3/1 Main Features of a Slow Sand Filter

Common design faults and their consequences (see also Fig. 13)

1. Inappropriate or missing flow rate control installations ® filter often overloaded or operated at frequent flow rate changes.

2. Water pressure in effluent line lower than the top level of the sand bed ® generation of negative pressure (vacuum) in the sand bed resulting in air release and additional filter resistance.

3. Inappropriate sand size and depth of filter bed ® poor effluent quality (coarse sand, small depth) or short filter runs requiring frequent cleaning (sand too fine).

4. Missing supernatant drainage system ® long drainage periods for dewatering the filter box will affect the biology in the sand bed

5. Slow sand filter beds with areas larger than 50 m² ® long cleaning periods will reduce or kill the biological filter activity

6. Missing installations for watering the sand bed from bottom to top ® air binding in the sand bed resulting in an initially high filter resistance.

7. Installations not properly protected against unauthorized handling.

Common operational problems

1. Turbidity and suspended solids concentration in the raw water too high for SSF application. Turbidity should preferably be less than 10 turbidity units and the suspended solids concentration lower than 2 5 mg/l to achieve reasonable filter operation.

2. Missing auxiliary equipment such as tools and sand washing installations. Failing to clean and replace the sand will lead to exhaustion of the sand bed.

3. Untrained caretakers who do not understand the SSF process are generally not motivated to operate the treatment plant properly.