3. MSF processes

MSF is predominantly an ecological process. The main physical mechanisms contributing to particle removal are:

• Surface straining of particles that are too large to pass through the pores of the filter media, mainly on the surface of the filter bed.
• Interception, a biological process intercepting bacteria as well as other contaminants. Units innoculated with a ciliary suspension feeder protozoa are more effective and improve as they “ripen”.
• Transport mechanisms (diffusion, sedimentation, interception, inertia, and hydrodynamic action) take smaller particles to the surface of the sand.
• Attachment causes these smaller particles to adhere to the sand grains, while increasing pressure in the filter leads to detachment, transporting the particles deeper into filter.

Levels of contamination can exceed the capacity of an MSF unit, but its limits are higher than for SSF systems, as high as 50 Nefelometric Turbidity Units (NTU) and 120 NTU for short durations. Moderate quantities of algae are usually beneficial, but rapid growth (algal blooms) cause filters to clog very quickly. Decaying algae can produce high concentrations of soluble and biodegradable organic material, leading to smell and taste problems. A low colour level is desirable in water as organic material responsible for colour and chemicals used in the final process can react leading to harmful by-products. MSF will not produce good quality water if the source is very heavily contaminated. Improving the source through catchment protection and waste water treatment is the only long-term solution.

Low water temperature and low levels of nutrients or oxygen in the water all reduce biochemical activity and therefore make the system less efficient. Anaerobic conditions in the filter skin can create serious water quality problems and bad smell and taste.