Surface water supplies usually have conventional water treatment facilities including filtration, which removes potentially pathogenic microorganism’s, particulates and followed by disinfection.
Coagulation and flocculation’s are often the first step in water treatment and commonly known as chemical treatment. It is used to remove fine suspended matter by chemical treatment which chemicals are added in to the water, then the positive charge of these chemicals neutralizes the negative charge of dirt and other dissolved particles in the water that causes formation of absorbent bulky precipitate.
The rate of sedimentation of the particles depends on the settling time allowed and nature of the suspended material. The main purpose of coagulation and flocculation is to permit settleable floc to be deposited and thus reduce the concentration of suspended solids that must be removed by filters (WHO; Geneva, 1996).
During sedimentation, floc settles to the bottom of the water supply, due to its weight. This settling process is called sedimentation.
The main factors that affect sedimentations are shape, size and weight of the floc, viscosity , temperature of the water, depth and areas of the basins, surface over now rate, velocity of flow and inlet and outlet design. The flotation is an alternative to sedimentation and has advantages when the amount of floc is small (WHO; Geneva, 1996).
Filtration is the process of separation of solid particle from liquid by means of a partition with pores of such size that they do not allow passage the particles while permitting the free flow of the liquid (Abdel et al.
In drinking water treatment processes slow sand and rapid sand filters are conventionally used by developed or developing countries. Sand filter is basically used to physically filter out the suspended particles from the water, but the application of a sand filter has expanded because numerous researchers have revealed its advantages in treating water. According to numerous studies the main advantage of a sand filter is that it not only separates the suspended solids and particles from the water but also other chemical constituents, including micro pollutants (T.L. Zearley, R.S, 2012), nitrogen compounds (Y.Cai, D. Li,Y et al., 2014), heavy metals (D. Pokhrel, T, 2009), biological contaminants (pathogenic microorganisms, such as human pathogenic viruses and bacteria) and pesticides (M.J. Hedegaard et al., 2014).
The biological sand filters (BSF) can be classified as slow sand filter (SSF) and the rapid sand filter (RSF). The sand bio filtration systems can be commonly designed in various shapes either in a vessel, column, a concrete tank or rectangular or square polyethylene water tank, it depends on the scale of treatment and it can vary from a household to a full scale drinking water production plant. The main difference between the two biological sand filters systems are the rate of filtration. The SSF have a slower filtration rate (typically 0.1 to 0.2 mh−1) compared to the RSF (5-7.5 mh−1) (EPA, 1995).\nSlow sand filtration
Slow sand filtration is a process to remove suspended particles from the water due to the water slowly passes through a bed of sand, but its degree of removal can be depended on the thickness of the bed, character and size of the filter sand, and the size and quantity of the suspended partic1es and the typical depth of sand filter bed is ranges 0.5 to 1.2 m (WHO; Amman, 1993).
Generally, slow sand filter improves the microbiological and physico-chemical water quality and it is particularly important to removes pathogens by means of adsorption and subsequent predation and it is also very efficient in removing parasites. The efficiency of slow sand filtration will be significantly reduced at water temperatures below 6°C (WHO; Geneva, 1997).
Therefore, slow sand filtration is preferred when the turbidity of the raw water is less than 100 NTU; it is suitable for developing countries and small rural systems but it requires sufficient available land or larger area than a rapid gravity filter of the same capacity.
Rapid sand filtration is frequently used in large treatment works after coagulation and flocculation and sedimentation and before disinfection and it is not suggested for small community water supplies, because it requires regular operation and maintenance , necessary controls and careful attention to obtain satisfactory results. It is preferred when the turbidity of the raw water is greater than 100 NTU or highly variable and remains high for a considerable period in a year (WHO; Amman, 1993).
Generally, rapid sand filters have low rates of microbial removal but quite efficiently in removal of suspended solids than slow sand filter, filters are quickly blocked by excessive sedimentation or surface straining in their upper layers and it is considerably higher operational costs than slow sand filtration (WHO; Geneva, 1997).