The Concrete Filter

History

Dr. Manz of the University of Calgary made several modifications to the traditional slow-sand filter to allow for intermittent flow and point-of-use water treatment. He discovered that biological layer living in the sand could survive for extended periods of time if the water level is no more than 5 cm above the bio-film. The organisms living in the filter require a constant supply of oxygen. If the resting water level should be higher than 5 cm, oxygen would no longer readily diffuse to the bio-film, which would suffocate. If the water level drops below the recommended level, then there is a risk that inflowing water will disturb the sand and bio-film. The water height is automatically maintained by a raised under drain pipe that connects to the outlet high in the side of the filters, and filling the filter with sand to a level that is 5 cm below the level of the outlet.

Design

The practical outcome of Manz’s research was therefore a concrete box or cylinder with an outlet pipe embedded in its walls. This vessel is carefully filled with suitable sand. A layer of coarse grains on the bottom prevents the finer sand on the top from blocking the outlet, located at the bottom of the unit. The fine sand has a large surface area, which traps dirt more easily. A diffuser plate is located some distance above the sand. Made from a perforated plate, the diffuser protects the sand and the bio-film it contains from scouring effect that would occur when water is poured into the filter. Between the diffuser plate and the top of the sand is the rest level of the water. Much more detailed information on the physical and biological processes involved in bio-sand filtration can be found elsewhere on this website.

Cost

Usually, concrete bio-sand filters can be made cheaply. The main cost is cement. Depending on the type of filter made (round or square), between a quarter and half a bag of cement are needed in addition to some PVC pipe. The filters are robust and long lasting and are capable of delivering up to 25 liters per hour if the correct sand size is used.

Advantages

The concrete filter is widely used in the developing world, having been introduced by NGOs, often through micro-projects. It is possible to establish profitable small-scale enterprises that build and sell filters to their communities. A small business in Kenya, established by Medair, has sold over 2 thousand filters in 4 years, generating a substantial profit. Several links to articles on this successful project are available at the bottom of this page.

Disadvantages

Because a metal mould is needed for the construction of concrete filters, it is not possible to mass-produce them easily. Each mould can only make one filter every two days. Furthermore, the filters can crack during transport or when moved within the house. They are also very heavy and bulky, making them expensive and awkward to transport.

Re-thinking the concrete filter design

Research conducted by the Universities of North Carolina and California, Davis indicate that the household filter could be re-designed to give better results. The research indicates that improved water quality is associated with increased residence time of water in a filter. To this end, the current concrete filter design itself could be re-thought in order to increase the residence time. This could be done by:

1. Re-designing the filter body to make pore volume equal or greater than the quantity of water used per day on average by an average household
2. Reducing the flow rates – this could be done by:
   • Using sand on the finer end of what is considered acceptable (i.e. the 0.15mm rather than 0.35mm end)
   • Designing a reduced freeboard in the reservoir part of the filter to limit the maximum hydraulic loading and therefore flow rate.

A re-design of the filter would probably end up including all of these aspects anyway because of the nature of household water management. An average rural household will probably use a 20-litre container for transporting water and will want this to be empty soon after reaching the house so that it can be re-used again, so the first criteria in any new filter design would be to maintain a reservoir volume of 20 litres above the pause time water level.

Reducing freeboard on the current concrete filter in order to restrict maximum hydraulic loading would automatically mean less volume of raw water could be put in the reservoir. So then the reservoir would also need to be made wider. Research on hydraulic loading indicates that a maximum of 10 cm above the sand is ideal when combined with using finer sand (0.17 mm). Out of this 10 cm, assuming that a maximum of 5 cm will be taken up with standing water during pause time, this leaves a freeboard of only 5 cm which will need to hold a volume of 20 litres – to do that, the average internal diameter of the reservoir in the round filter would need to be 72 cm.

With such a diameter at the top of the filter, the internal diameter of the base of the filter might be something like 67 cm due to the effect of tapering, with an average internal diameter of around 69 cm. For a sand column of 50 cm, this would mean the sand would have a volume of 187 litres, and if that sand had a porosity of 40% this would mean that water held during pause time would be about 75 litres, which should be more than enough for a household’s drinking and cooking water needs.

The next step might now be to try to construct and pilot some filters taking into account these design parameters, and assess them according to the all important aspect of cost, weight and manoeuvrability. It could be that in reality the costs and weight will be unrealistic for commercial-based field application.

Further reading:

• Mould: Construction guidelines: How to make your own sand filter mould. This document includes blue prints and an illustrated step-by-step construction guidelines and photo guide. Document size approximately 1 MB. Check the Commercial Section for the sale of ready-made moulds;
• Three-roll bender: Construction guidelines: How to build your own three-roll bender. A 3-roll bender is needed to fabricate a filter mould. This guide is useful for metal workshops that don’t already have a bender and want to make their own. COMING SOON…
• Bio-sand filter: Construction guidelines. This document allows you to build your own bio-sand filter, using the mould described in the document above. Size approximately 1.3 MB.

An article published in WaterLines describes the success of a Kenya concrete filter project: The success of household sand filtration, by Adriaan Mol. An evaluation of the same project 4 years later can be found by clicking here.