In recent years, the issues that have been always under debate in these regions are the total costs of water and wastewater treatment as well as the operation and maintenance costs. For most of the water and wastewater treatment, additional chemicals for the treatment as well as skilled man power are the major controversial problems. As Dastanaie et al. (2007) stated this fact is even more highlighted when dealing with small scale societies with low population where implementation of a multipart treatment system is not economically justified. Therefore, in order to reduce the problem faced, the oldest and conventional treatment process such as roughing filtration always been unremitting introduced.
2.1 Roughing Filtration
Filtration is one of the oldest methods in removing contaminant from water. Filtration can be declared as the simplest treatment method because it can be constructed from the local materials. This treatment depends on the media for purification without additional chemicals during the treatment process (Nkwonta, 2010).
There are two types of filtration method which are slow sand filtration and roughing filtration. Slow sand filtration and roughing filtration are essential for removal of impurities, pollutants and any particulate material generated during the treatment process. The media size for slow sand filters can be very fine which are 0.15 to 0.35mm diameter while roughing filter uses coarse media that are greater than 2.0mm diameter (Losleben, 2008). Slow sand filtration is more efficient for the treatment of raw water which has low turbidity. This is because, when surface water is very turbid, the effectiveness of sand filter will be reduced. Hence, as Jayalath (1994) stated, roughing filter is used as pretreatment systems prior to the sand filtration.
According to Younger (2001), roughing filter can be used to polish wastewater before it is discharged to the environment. Thus, the role of roughing filtration is for wastewater to flow through a large compartment filled with filter media under certain filtration rates. The performance of roughing filters depend on various factors such as filter design, media sizes, media types and parameters of water quality (Boller, 1993; Collins et al., 1994). Forster et al. (1987) reported that roughing filters are low cost treatment methods for water and wastewater since no additional chemical is used. The reduction in turbidity from an average level of 23.05 NTUs to 22.82 NTUs using chemicals such as lime can cost $20 while roughing filters can reduce the same quantity with only $10.
Nevertheless, adsorption as well as chemical and biological processes can be happened in roughing filtration due to the large compartment of the filter with the slow filtration rate. Therefore, besides solid matter separation, roughing filters also improve bacteriological water quality and change some other water quality parameters such as colour or amount of dissolved organic matter (Wegelin, 1996).
2.2 Type of Roughing Filter
There are few types of roughing filter such as horizontal roughing filter (HRF), vertical up-flow roughing filter (URF) and vertical down-flow roughing filter (DRF) as shown in Figure 2.1. The selection of the types of roughing filter in treatment method is based on the raw water quality for instance, turbidity, suspended solids, color, iron and fecal coliform levels (Wegelin, 1996).
Figure2.1. Typical types of roughing filter (Wegelin, 1996)
2.2.1 Vertical Roughing Filter
Vertical-flow roughing filters can be either for water to flow from top to bottom or from bottom to top. As shown in Figure 2.1, there is down-flow vertical roughing filter or up-flow roughing filter in series and up-flow roughing filter in layer. Although up-flow and down-flow roughing filters perform similarly, up-flow roughing filters are recommended for ease of cleaning (Wegelin, 1996). The advantages of vertical roughing filter is incorporate a simple self cleaning mechanism and has less space occupancies when compared to horizontal flow roughing filters (Dastanaie et al, 2007).
2.2.2 Horizontal Roughing Filter
Horizontal roughing filter consists of filter media arranged in series. The filter medium can be coarse, fine or other different configurations in arrangement. In order to improve the performance of horizontal roughing filter, media configuration can be used instead of individual compartments. In the media configuration, typical roughing filters will have gravel of different sizes in one, two or three compartments and the arrangement of the media size would be from the coarsest to the finest. The gravel in the middle bed would be intermediate between the first and the third filter beds. Figure 2.2 shows the design guideline of horizontal roughing filter.
Galvis et al (1993) pointed out that horizontal roughing filter can be used for raw water that has relatively high turbidity while at the same time offers a longer filter run time without backwash. Horizontal roughing filter have a large storage capacity compared to vertical roughing filter. The large size of horizontal roughing filter gives advantages where it can provide a huge surface area for pollutant to settle on the top of the media surface when passing through the filter. Horizontal roughing filters also react less sensitively to filtration rate changes, as clusters of suspended solids will drift towards the filter bottom or being retained by the subsequent filter layers (Nkwonta, 2010). Moreover, a modified configuration horizontal roughing filters (DHRF) combined with other water treatment processes can also further improve the performances (Mahvi, 2004).
Figure 2.2 Layout and design of horizontal roughing filters (Wegelin, 1996)
2.3 Roughing Filter Theory and Mechanism
The most influential mechanisms of removal in roughing filters are gravitational sedimentation, interception, and diffusion. However, adsorption, biological as well as biochemical process also occurs in roughing filtration. Figure 2.3 and Figure 2.4 shows the mechanisms of a roughing filter. During the filtration process, water will pass through a coarse media at the first pretreatment stage where the screening of particles is carried out. Particles will undergo coarse solid separation where the larger particles will be stranded at the surface of the media. The larger particles will then settle due to gravity while the finer particles are then separated in a second pretreatment stage. When enough particles are accumulated, interception occurs. Interception enhances the particle removal. Water treatment will end with the removal or destruction of smaller solids and microorganisms. Biological films will be formed at the surface of the media and these biological films will obstruct the flow of solid and microorganisms. Hence, solid matter and microorganisms will face a multitude of treatment barriers (Nkwonta and Ochieng, 2009).
Furthermore, hydrodynamic forces will be responsible for the water to flow through the pore system and by retardation and acceleration of flow, stagnant water will occur near the media surface hence allowing particles to settle. Biological activities will then develop filter to deposit on the filter media. Then a sticky organic film will be developed by bacteria and other microorganisms on the surface of the media in the pores which helps to retain the suspended solids by mass-particle attractions through the Vander Waals forces and electrostatic forces between charged particles (Wegelin, 1996).
Figure 2.3. The mechanisms of roughing filters (Wegellin, 1996)
Figure 2.4. Solid removal in a horizontal roughing filter (Wegellin, 1996)
2.4 Performance of Roughing Filters
There are a few researches on the usage of roughing filters in developing countries such as in Iran, Malaysia, India, Afrika, and Sri Lanka. For instance, Jayalath (2004) indicated that there is a considerable reduction in colour and turbidity for 50â€" 60% where the highest percentage of removal is obtained from the filtration velocities below 1.5 m/h for color and turbidity and below 2 m/h for algae removal. Table 2.1 shows of the performance of roughing filters. As observed in Table 2.1, higher percentage of removal has been achieved for iron and manganese, algae and turbidity.
Table 2.1. Performance of roughing filters (Jayalath, 2004)
