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Introduction

Slow Sand Filtration

Settling

Physical Filtration

Biological Filtration

Climate Considerations

The slow sand filter and settling tank design are open to the atmosphere and as such are subject to atmospheric temperature and weather. Therefore the designs cannot be employed in regions where temperatures drop below zero. If the water in the filter freezes it will no longer function, in order to run through low temperatures an expensive, heated building would need to be constructed.

Design Modifications

Flow rate for the settling basin and filter must be kept at a level which maintains a constant head. The design contains an additional weir for overflow which allows the design to accept a higher flow rate than required. The extra water entering the system will simply overflow and can be re-routed to the source.

For source waters with high turbidity, the filter can clog much quicker than expected. In this situation, the settling basin can be employed to settle out particles before the effluent is directed to the filter bed. This will improve filter performance and extend the filter life.

Alternative Sieve Apparatus

Filter material must have a nominal diameter that corresponds to an adequate hydraulic conductivity. If a sand with a small enough diameter is not available for the filter, a sieve may be used to separate the smaller particles and create an adequate filter material. In areas where standard sieves are not available, a sieve apparatus similar to the settling basin design could be used to separate the smaller particles. The settling velocity of a particle is proportional to the particle diameter and the flow rate across the settling basin. By controlling this rate with basin dimensions and a constant head reservoir, all particles greater than a given diameter will be settled in the basin and smaller particles will remain in the effluent. By removing larger particles in previous basins, particles settled in subsequent sieves will be an adequate size for the filter material. By maintaining a constant head with water that has suspended sand particles, the basin will collect the design diameter which can be collected and used in for the filter material. The sieve design parameters are outlined in the excel file and design drawings are provided.

Filter Maintenance

Maintenance of the filter is required. Aside from the necessary concrete cracking maintenance (which is not described here), the filter bed will require periodical maintenance as well. As water is purified by passing through the sand bed, suspended matter and coliforms which attach to the sand particles will clog the filter and reduce flow rate. When flow rate decreases to a level which prevents the necessary amount of water from being filtered, maintenance is required. When this happens, the top 2-3cm of sand should be scraped off and removed from the filter. After subsequent scrapings, the sand level may drop below the required amount (1-1.5m). At this time, more sand should be added to the filter to ensure the length of filtration is adequate. The removed material can be put back into the water upstream to dilute and filter out particles once more.

Materials

The slow sand filter is based on hydraulic permeability and head differential. If concrete is not available for the structure of the treatment facility, any material which is less permeable than sand could be used to wall the filter. However it would also have to support the pressure of up to 3 meters of water and 1.5 meters of sand. It may be possible to dig a pit for the filter and line it with a clay (much less permeable than sand) and rely on the soil surrounding the filter to resist the water and sand pressure. However this is much less stable than a concrete structure and may require heavy excavation equipment.

In areas with extreme turbidity, some settling aids may be employed to increase floc size and rate of sedimentation. Chemical additives are likely too expensive for poor communities, however other, natural additives may be employed to accomplish the same purpose. One such material is the Moringa oleifera seed extract. This water-soluble material in suspension can be added to source water before flowing into the settling basin to increase sedimentation of turbid waters.

Environmental Considerations

The environmental impacts of this technology come from material choices. Concrete production results in large quantities of CO2 emissions, mostly due to the production of cement. Fine and coarse aggregates may need to be quarried depending on the location. However, blasting may not be required in some developing communities where sand and gravel may be taken from the surrounding area. In this case, the environmental impact and cost are lowered for both the filter and concrete production.

The concrete design used for this treatment system does not use any pozzolans. A pozzolan is used to replace some cement with waste material from an industrial process. Fly ash and soda ash are common materials which can replace some cement and reduce both cost and environmental impact. These materials may not be readily available in developing countries and were not included in the design however they should be used when available.

Design Drawbacks

The design offered below has been engineered based on water filtration and does not have any structural analysis done. The concrete structure will need to be designed to the standards of the community it is developed for. Foundations may need to be designed based on the soil conditions on site. Steel reinforcement may also need to be installed depending on the structure size.

All costs, both financial and environmental are based on Canadian numbers and as such may not be accurate to the area where the filter will be built. If the treatment plant were to be properly designed for a specific area, research would need to be done to input into the excel sheet provided.