tend to be more effective when used in clarifying applications, hence a combination of the two processes can provide a suitable filtration system, which can be adapted to many applications. Assessment of main process characteristics Process Characteristics such as filtration rate and filter media are important design considerations and greatly impact filter performance, as a result continuous monitoring and assessment is necessary to ensure greater control over the process quality. Treated flow rate The flow rate is defined as the ratio of the driving force over the filter resistance. The two conventional types of depth filter designs: the rapid and slow filters operate with velocities of 5–15 m/h and 0.1-0.2 m/h respectively; whereas pressurised sand filters have design flow rates of 238 L/min[14]. During operation the filter rate decreases due to increasing filter resistance as particulates get lodged within the media. The rate of filtration affects the rate of clogging with high filter rates causing faster build up. Pilot tests demonstrate that the higher the filter rate the lower the filter area whilst increasing filter rate reduces the time to breakthrough, reduces the time to head loss (increases head loss) and results in shorter runs and lower optimum depths. They also demonstrate that higher filter rates can be achieved by using larger diameter media and increased media depth. High filtration rates depend on media design with the highest filtration rate design in service at 13.5gpm/ft2. Backwashing in depth filters Backwashing is an important operation employed to remove filtered solids as this build up causes resistance to filtration to increase with time. Backwashing involves inverting the direction of liquid flow while using clean liquid. This process is employed for times in the range of 5–15 minutes with typical flow rates per unit area in the range of 6.8- 13.6 L/m2.s. Most designs typically employ backwashing once per day of operation. The operation of depth filters is inherently cyclic due to the necessity of solids removal build up during the process, as such two or more units are typically used so that backwashing does not interfere with the filtration. Effective backwashing occurs when the filter medium is fluidized. Fluidization flow rates generally fall in the range of 20-50 gpm/ft2. Separation efficiencies Rates of removal for pressurized sand filters with media typically in the range of 0.3- 0.5 mm have been reported to be at 95 of particles as small as 6 µm