| dc.description.abstract | The Slow Sand Filter (SSF) is an effective device for water treatment with its proven excellent
purification characteristics ill terms of turbidity and microorganisms removal, yielding potable
water with no other treatment necessary. However, its large size and prohibitive cost has inhibited
its wide-spread use, particularly within the poorer communities of developing countries, where the
availability of clean water is limited or non-existent.
This thesis presents the results of the research into the reduction in the size of a conventional slow
sand filter,without affecting its excellent purification characteristics.
It is realized that the retention time of the water within the sand bed is the vital parameter, enabling
the production of potable water. With this in mind, a baffle arrangement was utilized within the
sand bed (similar to a shell and tube heat exchanger), thereby significantly increasing the retention
time for a given sand depth.
The pilot SSF was constructed from perspex sheet with a cross-section of 20 em x 30 em with a
sand depth of 70 cm, Four filter runs were conducted: three with and one without baffles. The
baffles were also manufactured from perspex, held together by a stainless steel framework. The
baffle arrangements used had 24%,32% and 42% spacings based on the hydraulic diameter (sand
free) with eleven (11), eight (8) and six (6) baffles respectively. Leakage paths, between the baffles
and the main section, were eliminated using non-toxic modeling clay as a sealant.
. The sand was graded and cleaned and had an effective diameter of 0.188 mm and the flow velocity
through the sand bed was maintained at 0.2 m/hr (288 l/day), controlled by an outlet valve.
All experiments were run over a period of between nine (9) and seventeen (17) days, during
which both the influent water and effluent water (at various sand depths) were analyzed for
turbidity, coliform, colour and pH, at approximately 48 hours intervals.
It was discovered that only 50 em of the bed depth was required, when the baffle arrangements of
24%to 32% spacing were incorporated, to produce potable water, well within that defined by the
.World Health Organization (WHO). The 42% baffle spacing required a 65 em sand depth to
produce the same water quality. In addition, the length of the filter runs were, with the turbidity of
the influent used, within the expected period for a conventional SSP. For the un-baffled
arrangement,the full sand bed depth of70 em (accepted minimum depth for a SSF) yielded poor,
un-potablewater quality.
These results clearly indicate that potable drinking water can be produced, without post
chlorination, from a sand depth significantly less than the hitherto accepted minimum, thus
permitting a significant size and cost reduction over the conventional design.
A full cost analysis is given for the new baffle design and comparisons are made with a
conventional filter. The new design cost is approximately half that of a conventional filter.
Data are also given on the head loss characteristics of both the baffled and un-baffled
(conventional) arrangements. These characteristics were used to develop models for the
predication of the head loss in a baffled clean sand bed and for a filter run period. | en |
