13.1 Filter box
Filter structures can generally be located below or above
ground, as illustrated in Fig. 45. The respective choice depends on soil
characteristics, available construction material and hydraulic profile. On a
flat surface, gravity flow often requires the structures to be placed below
ground. This, however, might cause some problems or additional costs for
adequate drainage of the washwater. A partially buried filter will require less
excavation work and provide support to the sidewalls by the back-filled soil.
Fig. 45 Location and Materials of
Roughing Filter Boxes
Roughing filters are usually shallow structures of about
0.6 m (intake and dynamic filters) to 1 - 2 m (roughing filters). The size of
the filter box is dependent on hydraulic capacity (see also Section 9.4) and
filter length. The filter boxes should not be too big (maximum filter area for
vertical-flow roughing filters should amount to 25 30 m², maximum cross
section area for horizontal-flow roughing filters about 4 - 6 m²), to avoid
problems with high washwater discharge rates. Furthermore, the filter box should
also not be too high (preferably around 1 m) to allow easy removal of the filter
material during manual cleaning.
A trench excavated in impervious soil, such as clay, silt or
lateritic ground, presents a low-cost solution for a filter box. The trench
has sloping sidewalls which do not exceed the slope stability of the
water-saturated soil (slope less than 1:1). Lining of the base and sidewalls
prevents clean filter material from mixing with the surrounding ground. A layer
of sand, prefabricated slabs, in-situ applied coatings (concrete lining,
ferro-cement, lime mortar) or in exceptional circumstances (e.g. refugee camps),
prefabricated plastic linings or the use of geotextiles, are the most
appropriate materials to use.
A watertight box has to be constructed in permeable ground or
if the filter is installed above ground. In such cases, vertical sidewalls
are recommended. Burnt clay bricks with cement mortar lining, concrete bricks or
reinforced concrete should be used for such filter boxes. The foundation and
floors of the box need special attention to avoid cracks caused by uneven soil
settlement. Finally, watertight expansion joints will probably have to be made
for long filter boxes constructed for horizontal-flow roughing filters.
Alternatively, long filter boxes resting on difficult ground can be split into
two or more separate units with flexible pipes interconnecting the compartments.
U-shaped units can also reduce the total length of filter boxes. In such a
layout, inlet and outlet are to be placed on the same filter side, and the box
divided into two equal parts by a longitudinal separation wall.
The filter box should be tested for watertight-ness,
preferably before it is filled with filter material. Leaks can be detected
and repaired more easily in an empty structure. Special attention must be paid
to the joints at the floor-wall interface or the inlet and outlet boxes fixed to
the filter box. Watertight joints require water stoppers made of PVC or rubber.
Other weak points in the structure include the pipe sealings which possibly need
additional reinforcement to prevent cracking of the walls, and seep rings to