In filtration, the partially treated water is passed through a medium such as sand or anthracite, which acts as a 'strainer', retaining the fine organic and inorganic material and allowing clean water through. The action of filters is complex and in some types of filter biological action also takes place. Sand filters are used in water treatment to remove the fine particles which cannot be economically removed by sedimentation. They have been effective in removing Cryptosporidium, a protozoan parasite.
Mechanical straining of the water is only a minor part of the filtration process, as the main process by which particles are retained is adsorption. In adsorption, the particles adhere to the filter material or previously adsorbed particles. If a particle passes close to a solid surface, there may be either electrical attraction or repulsion, depending on the surface charges of both the particle and the solid surface.
Filtration in water treatment can be carried out using simple slow sand filters or, as is more usual for flocculated water, rapid gravity sand filters.
A slow sand filter consists of a shallow basin in which about a metre of sand rests on a gravel base, underneath which there is a system of collection pipes and channels for the filtered water (Figure 28). The water to be treated flows down through the filter bed and, as it does so, a layer a few millimetres thick of algae, plankton and other microscopic plant life forms on the top. This layer is known as the Schmutzdecke, which is German for film or deck of dirt. In this layer, fine filtration takes place. In order to preserve this layer, the temperature and velocity of the inflow must be carefully controlled. Some biodegradation also takes place on the Schmutzdecke, resulting in a reduction of the organic matter, nitrate and phosphate which may be present in the water. The flow rate is normally in the range 0.1–0.2 m3 m−2 h−1. This means that a filter of, say, 21 m2 would produce a maximum of 0.2 × 21 = 4.2 m3 of water per hour.
When the rate of filtration begins to tail off after a month or two, the filter is drained and the top 2 cm of sand is removed to be replaced by fresh sand. Slow sand filters are expensive to build and operate, and require a large amount of space. They cannot be used for coagulated waters because of rapid clogging.
Slow sand filters have been largely replaced by rapid gravity sand filters, which are particularly effective for water treated with coagulants and are less expensive than slow sand filters (Figure 29). The flow is much greater than in slow sand filters, being 4–8 m3 m−2 h−1; hence a smaller filter (requiring a smaller space) will be adequate. Because of the high rate of flow, no Schmutzdecke is formed and hence little or no biodegradation takes place in these filters. The filter is cleaned at intervals of 24–48 hours by pumping water and air (to assist in scouring) under pressure backwards through the filter to wash out the trapped impurities. This process is called backwashing. Unlike slow sand filters which tend to produce water with a particularly low bacterial count, rapid filters produce water with high bacterial counts, increasing the necessity to follow them with disinfection before supplying the water to the public.
In many treatment plants where slow sand filtration is the key processing stage, rapid gravity filtration is employed prior to the slow sand filter in a process called double sand filtration. In this arrangement, the rapid gravity filters reduce the load of solid matter in the water before it goes to the slow sand filters. This allows a greater overall rate of treatment and the slow sand filters do not then need to be cleaned so often.
A variation of the filtering process is the use of a layer of large anthracite grains (1.2–2.5 mm) on top of a layer of smaller (0.5–1.0 mm) sand grains, which are denser and have a smaller 'intergrain' pore size. Anthracite-sand filters tend to clog less rapidly because some of the floc adheres to the larger anthracite grains before the water filters through the sand. This means that increased filtration rates are possible without deterioration in filtrate quality.
List the advantages and disadvantages of slow sand filters and rapid gravity filters.
Slow sand filter
Advantages: a very efficient physical, chemical and bacterial filter; produces water of consistent quality.
Disadvantages: expensive to build and maintain; takes up a large area; since efficiency depends on microbial growth, the temperature and speed of flow must be carefully controlled.
Rapid gravity filter
Advantages: fast; relatively cheap to install and operate; occupies less space than an equivalent flow slow sand filter.
Disadvantages: water has to be treated with coagulant before being passed through the filter; may not produce water of a satisfactorily high bacteriological quality, especially from heavily polluted water.