|Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.)|
|Part 1: General aspects of roughing filter application|
|4. Solid matter separation|
Let us now examine the first treatment step; i.e., the separation of solid matter. We might be confronted with a great variety of solids as observed in our bucket filled with turbid river water. The variety, illustrated in Fig. 5, is greatly dependent on the type of surface water and whether natural purification processes can separate part of the solids or possibly generate undesirable particulate matter by organic growth. Natural purification should largely be integrated into the treatment design when determining surface water source and intake location.
Fig. 5 Solid Matter Content for Separation
Sedimentation and filtration processes are mainly applied for solid matter separation. These shall be discussed in detail in the next two sections.
Yet, let us focus first on the peculiarities of the various types of surface water and their impact on the different solids in the raw water:
· The properties of the drained catchment area and the characteristics of the surface water influence the type and concentration of solid matter in the raw water. Flow velocity and rate of erosion determine the amount of settleable solids carried by the water. Flowing and still surface waters greatly differ with respect to the encountered type of solid matter. The turbulent flow of a water course may carry coarse settleable solids, which settle in gently flowing or impounded surface water. Algae found in ponds and lakes contribute to the suspended solids concentration of the water.
· Flowing surface water is often subjected to drastic quantitative and qualitative changes. The annual rainfall distribution influences the seasonal surface water fluctuation mainly with regard to turbidity and solids concentration. Flowing surface water will usually carry settleable solids at varying concentrations during different periods of time. During the dry season, small upland rivers are generally low in turbidity, however, they can exhibit high short-term turbidity peaks during heavy rainfalls. Larger lowland rivers may be of moderate turbidity throughout the year but with relatively long periods of increased turbidity levels.
· In still surface water, amount and type of solid matter change only gradually in the course of a year. In fact, the large volume of stored water in lakes, reservoirs and ponds preconditions the water quality. Coarse inorganic particles settle at the bottom of the receiving water body, light organic solid debris tend to float on the water surface, and dissolved organic matter may be transformed by photosynthetic processes to algae and plankton. Hence, each still water source acts as a first pretreatment step since the incoming and stored water is exposed to natural purification. As a result, impounded water is generally characterised by smaller water quality fluctuations. This higher stability of the raw water quality usually facilitates treatment plant operation.
· Flowing surface water carries solids of different sizes, such as coarse sand and silt to fine clay. Due to the irregular flow conditions, the solids are unevenly distributed over the cross section of a river bend. Coarse solids drift towards the outer side of the bend whereas the fine solids are washed to the inner side of a river bend and form a silting zone. Selecting an appropriate location for the intake structure contributes to reducing the levels of fine particles which are difficult to remove in treatment processes. The intake should, therefore, be placed at the outer or erosion side of a river bend in order to reduce the abstraction of fine matter and to avoid the silting of intake works.
· Surface water can also carry coarse floating matter which may block or even damage part of the water supply installations. The undesirable material is thus retained right from the beginning either by screens or by a scumboard. Fixed screens (e.g. a coarse screen followed by a finer one) are most commonly used to avoid blockage and excessive headlosses.
In short, if surface water is used as raw water source in a water supply scheme, preference should be given to still water provided excess amounts of algae or colour do not create special treatment problems. Natural purification processes reduce in particular the solid matter concentration by sedimentation, and smaller water quality variations often decrease and simplify the required degree of treatment. Flowing surface water frequently exhibits rapid water quality changes which render water treatment more difficult.