2. MSF Technology

The following principles apply to Multi-stage filtration (MSF):

• There is more than one treatment stage to remove water pollutants and produce water that is consistently fit for drinking;
• Integrated treatment takes into account the potential and limitations of each treatment barrier in removing different types of pollutants
• Terminal disinfection is the final barrier that safeguards the bacteriological quality of the water.

MSF is a combination of coarse gravel filtration (CGF) and slow sand filtration (SSF), and it may also have pre-treatment stages and terminal disinfection built in. A typical system may include dynamic gravel filters (DyGF), coarse gravel filters (CGF) and SSF, followed by a terminal disinfection safety barrier following MSF. Despite ease of operation and maintenance, the result is good water quality.

A slow sand filtration unit contains a filter bed, a supernatant water layer, drainage systems and flow control. SSF water treatment uses a combination of physiochemical and biological mechanisms that interact. Raw or pre-treated water passes through the filter bed under gravity and pressure from the supernatant water above. Some material settles out in the supernatant water layer. Soluble matter is removed by bacteria and other micro-organisms as the water passes through the sand bed, made up of fine sand. The filter bed acts as an organic and aerobic cleansing unit, as zooplankton grazing occurs and the entire biomass respires continuously, using oxygen from the water.

Because of the importance of the continuity of the biological process, an SSF system should include at least two units, allowing a continuous water supply when one filter is off line, by increasing the filtration rate in the other unit(s). The flow in SSF units must be controlled to maintain the proper filtration rate to ensure that the biological process receives oxygen and nutrients. Because of the biological nature of the process, SSF units must operate continuously. The rate of filtration can be slowed, but intermittent operation should be avoided, since oxygen depletion and lack of nutrients compromise biological activity.

After several weeks or months of running, the SSF unit will gradually become clogged with inorganic and organic material, including the biomass formed on top of the filter bed. By scraping off 1cm to 3 cm of this layer, hydraulic conductivity is restored to the previous level of efficiency. Eventually, however, the system needs to be resanded. The TOP describes how this is done.

Design varies around the world and various guidelines are given in this TOP, covering issues such as the depth of the filter layer, the diameter of the sand and the period of time that that system is designed to cover.

Various pre treatment options – the essential characteristic of MSF – are also described, including coarse gravel filters which can be dynamic, upflow or downflow, according to the route that the water takes through the system.

The use of local filtering materials and building materials is recommended wherever possible, with the structure being made from concrete, reinforced concrete or masonry. The system should be built close to the local population and the location should ensure optimal hydraulic performance preferably driven by gravity from source to delivery.

It is not much more expensive to build an extension to an existing system, than to build a bigger system now, so systems should reflect the population size that can be confidently predicted over a time frame of no more than 8-12 years.