Slow sand filtration water treatment plants
Practical Action
INTRODUCTION
The limited amounts invested in the treatment of safe water in rural areas are partly
explained by the high cost of water treatment systems. Consequently, the majority of rural
communities are still drinking superficial water that does not meet the required standard of
quality, causing serious health problems.
In many cases, the high cost of water treatment systems and the poor quality of water deter
investment in even the simplest systems of untreated water conveyance by gravity, further
aggravating the health situation and forcing the population – particularly women and children
– to walk long distances to fetch water of a worse quality than that which they could obtain
by conveyance from the headwaters of nearby rivers.
Slow sand filtration systems are a technically viable water treatment solution. Nevertheless,
there are still a number of difficulties involved in the implementation of the technology and
the operation of the system. In addition, the direct cost of the construction is relatively high.
A large percentage of these systems have been abandoned for the following reasons:
• Inappropriate designs, as the variations in the quality of water at different times of
the year were not taken into account.
• The people in charge of operating them are usually members of the community who
have not been adequately trained to operate the system.
• The institutions responsible do not monitor the installations adequately.
• Spare parts are not locally available.
• The sand on the filter bed is not replaced when the minimum thickness has been
reached, after several layers have been scraped off.
The main characteristic of slow sand filtration is that, due to the effect of biological activity,
it efficiently removes pathogenic organisms from raw water, particularly the bacteria and
viruses responsible for transmitting water-related diseases. Furthermore, no chemical
products are required, nor highly qualified, continual supervision.
Slow filtration is undoubtedly the most adequate technology for rural areas. In order to avoid
some of the problems described, however, it is necessary to apply solutions that take into
account local technical and economic capacities, so that the system can actually achieve its
purpose to supply drinking water to rural populations.
The proposal contained in this handbook is a low-cost alternative that is technically adequate
and easily managed by the community. It consists of modified gravel pre-filtration, slow sand
filtration and disinfection units adapted to the rural Amazon environment. In addition to the
design criteria, it contains details of the operation and maintenance tasks required for the
effective operation of the units. Also included is a methodological guide for teams in charge
of training plant managers.
In the proposed design of slow sand filtration units, the filter inlet and outlet structures have
been simplified, the thickness of the sand bed has been reduced and the filter structure has
been changed so that the filter-harrowing method currently used to clean large-scale filters
can be applied.
Reducing the thickness of the sand bed without affecting the efficient removal of bacteria
was made possible by the climate in the Amazon region, as high temperatures favour the
biological activity responsible for eliminating bacteria and pathogenic viruses there.
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