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Original:Slow sand filtration water treatment plants
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
Before embarking on a treated water supply project, it is essential to evaluate and define the level of organisation in the community. If it is insufficient or non- existent then the operation and maintenance of the system will most likely be neglected and eventually abandoned.
The community must participate in every stage of the installation of the water supply service, including the selection of the technical option, the construction and supervision of the works and the management of the service.
In the design stage, the community must study the proposed solutions and decide upon the best technical alternative and the most appropriate service. Consequently, they must be aware of the costs of the system – including their own contribution to the construction and the rates to be paid, the advantages and disadvantages of each alternative and their operation and maintenance responsibilities (UNDP 1998).
The following factors must be borne in mind for a preliminary study to define possible water treatment solutions:
- Weather conditions : The temperature has a substantial influence on the performance of water treatment systems and the intensity and duration of the rain influences the quantity and quality of the source.
- Watershed characteristics : Human and natural factors, such as discharges of residual water or run-off from chemically treated farmland, can seriously affect the quality of the water.
- Quality of the water : A physical, chemical and bacteriological analysis of the water is necessary in order to determine its quality and the level of treatment required.
- Location of the plant : The land must be easily accessible, having a natural slope of between 5 and 10% and having no exposure to natural hazards or to subterranean water nearby. A written agreement with the owner must be signed.
- Characteristics of the community : It is necessary to know the customs and beliefs that could affect the acceptance of the system, the
characteristics of existing organisations, the availability of natural materials and human resources, and the level of schooling.
- Existence of water-related diseases.
The selection of water treatment processes as a possible solution for supplying drinking water to rural communities depends on the quality of the raw water.
P = Pre-sedimentation S = Sedimentation PC = Gravel Pre-filter in layers with an ascending flow PS = Gravel Pre-filter in series with vertical or horizontal flows FL = Slow sand filter C = Sieve
- If the water contains faecal coliforms and turbidity levels that exceed the values shown in the table, it is advisable to look for other water sources.
- At the entrance to the plant, a triangular weir structure should be considered to measure the flow, in which the desired water should be marked with paint. The operator can open the inlet valve to reproduce the flow required to operate the system.
Aeration consists of blending the water with the air in order to:
- Increase the oxygen content in the water
- Increase the pH in the water by reducing its carbon dioxide content
- Remove the iron, magnesium, hydrogen sulphate, methane and various volatile organic compounds responsible for the flavour and smell of the
water (Hofkes et al, 1987).
The simplest aeration methods are described below:
Multiple tray aerator
This is an economical solution that occupies little space. It consists of four to eight trays with mesh bottoms, placed one on top of another with a 30-50cm space between them. The water is evenly distributed in the first tray through porous pipes, or by stopping the gush with a bar (as shown in the figure), so that the water trickles into the tray at a rate of 0.02 m3/s per square metre of the tray’s surface. Small droplets are then sprayed into the next tray. The trays can be made of various materials: asbestos-cement sheets, plastic pipes with small diameters or parallel wooden slats.To obtain a finer spray, the aerator trays can be filled with thick gravel approximately 10cm deep.
This consists of either four or six steps, each 30cm high and having a capacity of approximately 0,01 m3/sec per metre of width. An obstacle is often placed at the edge of each step to produce turbulence and thus improve the aeration efficiency,
Many designs of water treatment plants fail to take into account the differences in the quality of water, particularly during the rainy season when turbidity increases, obstructing filters and reducing their effectiveness. Hence the need to previously clear the water through different processes, including pre-filtration, which not only removes turbidity but also algae and bacteria, lengthening the life of the filters and reducing the cleaning frequency (Collins et al, 1991).
Although the efficiency of ascending and descending flow pre-filters is similar, the former are easier to maintain as the hydraulic retro-washing operation is simpler.
The alternative of an ascending flow in layers is recommended for water with less than 50 UT turbidity with short peaks. On the other hand, the series pre- filter is better for poor quality water as it has a better capacity for storing solids (AARAUV 1985).
There are two alternatives for achieving a uniform flow in the pre-filtration units:
- Structure with rectangular tanks equipped with manual sluice valves to isolate the units. Although a small investment is required, this is the most comfortable solution for the operator and the most reliable in the long term.
- Water measuring devices at the entrance of each unit and the subsequent regulation of the intake valve. No additional investment required.
Gravel pre-filter in layers, with an ascending flow
The intake pipe goes through the centre of the pre-filter to the bottom of the unit. From this pipe, the water is distributed evenly throughout the unit by means of secondary porous pipes. The filtration zone consists of four layers of gravel of different sizes, with the thickest gravel forming the bottom layer and the finest gravel the top layer.
The water goes through the filtration zone in an ascending manner and is collected by the outlet pipe which has 12.7mm diameter holes at 50mm from the centre and 0.40m from the gravel filtration bed. This pipe can be larger than the intake pipe to reduce the head loss and ease the outgoing flow.
The drainage zone is at the bottom of the pre-filter. The floor of the unit has a 12.5% incline to ease the discharge of sediment to the cleaning water discharge channel. Slabs or bricks are placed on the discharge channel to support the gravel, at intervals of two or three centimetres (CRHEA 1991; Marrón 1998).
The filtration speed is selected depending on the quality of the water: It increases in accordance with the quality at a rate of 1 to 1.5 m/h.
- The speed ratio in the secondary and main pipes that distribute the water in the bottom of the unit should be Vp/Vs < 0.462 for an even distribution. The diameter of the secondary pipes is thus obtained.
- The loss of volume in the bed during the normal pre-filtration operation is approximately 20cm.
- The pre-filtration-filtration system has a limited capacity for assimilating sudden differences in the quality of raw water.
Other gravel pre-filtration models with ascending flow layers
Series vertical flow gravel pre-filtration
Horizontal flow gravel pre-filtration
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