Pre-settlement If water is left in a tank for long enough, particles will settle to the bottom (or, in some cases, float to the top). In the pre-settlement process, water is passed through a tank at a slow rate and suspended solids fall out of suspension to the bottom of the tank (Figure 4). The outlet of the settling tank should be near the top of the structure so that the clear water is removed. Figure 4: Pre-settling basin The solids are removed by either draining the tank or by suction from the bottom. In a small system, a swimming pool vacuum system could be used. A common problem with settling in tanks and ponds is where water ‘short-circuiting’, effectively takes a short cut. To avoid this problem, consider: placing the inlet and outlet as far apart as possible introducing the water to the tank as gently as possible to avoid turbulence installing baffles to direct the flow. Simple settling will not remove very fine-grained particles (including micro-organisms) because they settle extremely slowly. Therefore the water can still be cloudy after Nevertheless, settling reduces the load on downstream treatment processes. Pre-filtration Pre-filters can be worthwhile because they do not require chemicals, have few working parts and are robust. Unfortunately they need frequent cleaning and do not remove fine particles such as silt and clay. It is common to install coarse steel screen and plastic disc filters to remove large items such as small animals, leaves and sand (Figures 5 and 6). Gravel or coarse sand filters are sometimes used to reduce turbidity enough for the downstream processes to work effectively. Infiltration galleries (or bankside filtration) can work in this way (see Figure 7). When the media becomes blocked, the filter is backflushed or drained, depending on the design. Figure 5: Infiltration gallery Figure 6: Disc filter Figure 7: Infiltration gallery
Pre-settlement If water is left in a tank for long enough, particles will settle to the bottom (or, in some cases, float to the top). In the pre-settlement process, water is passed through a tank at a slow rate and suspended solids fall out of suspension to the bottom of the tank (Figure 4). The outlet of the settling tank should be near the top of the structure so that the clear water is removed. Figure 4: Pre-settling basin The solids are removed by either draining the tank or by suction from the bottom. In a small system, a swimming pool vacuum system could be used. A common problem with settling in tanks and ponds is ‘short-circuiting’, where water effectively takes a short cut. To avoid this problem, consider: placing the inlet and outlet as far apart as possible introducing the water to the tank as gently as possible to avoid turbulence installing baffles to direct the flow. Simple settling will not remove very fine-grained particles (including micro-organisms) because they settle extremely slowly. Therefore the water can still be cloudy after Nevertheless, settling reduces the load on downstream treatment processes. Pre-filtration Pre-filters can be worthwhile because they do not require chemicals, have few working parts and are robust. Unfortunately they need frequent cleaning and do not remove fine particles such as silt and clay. It is common to install coarse steel screen and plastic disc filters to remove large items such as small animals, leaves and sand (Figures 5 and 6). Gravel or coarse sand filters are sometimes used to reduce turbidity enough for the downstream processes to work effectively. Infiltration galleries (or bankside filtration) can work in this way (see Figure 7). When the media becomes blocked, the filter is backflushed or drained, depending on the design. Figure 5: Infiltration gallery Figure 6: Disc filter Figure 7: Infiltration gallery
treatment-options-small-drinking-water-supplies-jan14.docx