Sedimentation (centralised)

Plain sedimentation (also referred to as settlement or simple gravity sedimentation) is a simple, physical, low-cost pre-treatment of water prior to application of other purification methods such as filtration (e.g. slow sand filtration) and disinfection (e.g. chlorination, ozonation). It removes undesirable small particulate suspended matter (sand, silt and clay) and some biological contaminants from water under the influence of gravity. The longer the water is stored or hold undisturbed, the more the suspended solids and pathogens will settle to the bottom of the container. Adding coagulants can accelerate the sedimentation process.

This factsheets focuses on sedimentation as a step in centralised water purification plants. For sedimentation as part of point of use water purification on household level see: Sedimentation (POU).

The suspended particles in water vary considerably in source, composition, charge, particle size, shape and density. The smaller particles present in water are kept in suspension by the action of physical forces on the particles themselves. One of the forces playing a dominant role in stabilisation results from the surface charge present on the particles. Most solids suspended in water possess a negative charge and since they have the same charge sign, repel each other when they come close together. Therefore they will remain in suspension rather than clump together and settle. However, under the influence of gravity, larger particles will settle to the bottom and can be separated from the water. The optimal holding time depends on the initial turbidity and water quality requirements for subsequent treatment steps. This process is referred to as plain sedimentation or settlement.

Adding chemical or natural coagulants to the water can quicken the sedimentation process. Aluminium sulphate, polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate are three common types of chemicals used for coagulation. Some examples of natural coagulants are prickly pear cactus, Moringa seeds, broad beans and Fava beans. Some products like “PUR” contain both coagulants and disinfectant. After sedimentation, the water should be filtered (e.g. slow sand filtration, rapid sand filtration, etc.) to further remove suspended materials and pathogens.

Most often, centralised sedimentation chambers form an integral part of a treatment cycle combining pure sedimentation with coagulation flocculation, filtration, disinfection and storage facilities. For more information on this treatment cycle on the household level, see: Household Water Treatment and Safe Storage HTWS.

Sedimentation can be done by simply filling jars or tanks and holding the water undisturbed for a while and then decanting the water from the sediment. This process may be suitable for water treatment on a household level. For community or urban water supply, the sedimentation process needs to be operated continuously.

The design of sedimentation facilities for community or urban water supply range from simple, small sedimentation chambers to large, highly technological sedimentation basins. The former are often made of concrete rectangular tanks. Inflowing water horizontally passes one or more separated sub-chambers while more and more particles sediment.

Large sedimentation basins are often round-shaped. The freshwater inlet is located in the centre of the basin from where the water radially flows to the circular outlet at the rim.

Water purification measures are associated with capital and operational costs. Many centralised water treatment plants in urban areas in developed countries are costly as they make use of high-end technology. However, effective treatment does not necessarily require such a level of investment. Simple sedimentation chambers are associated with very low construction costs. Equally, spending for operation and maintenance is minimal given the turbidity level of the initial water is not too high. Hereby, maintenance tasks mainly include interrupting the inflow, emptying the chamber and removing the settles material. However, the use of coagulants may be costly depending on water quality and the availability of required chemicals.

Plain sedimentation is very effective in reducing water turbidity, however, it is not consistently effective in reducing microbial contamination. Holding the water undisturbed for a few hours allows the larger, denser particles (e.g. sand, silt) to sediment. The longer the water remains motionless inside the sedimentation chamber, the more fine particles start to settle. Since many pathogens including bacteria, viruses, protozoa and helminths are attached to suspended particles, the reduction of turbidity also improves the microbiological quality (AMAGLOH & BENANG 2009). Removing turbidity comes along with an improved visual quality of the water and thus increases its acceptance by consumers.

Yet, very fine solids (i.e. clay) and most dispersed viruses and bacteria are too small to be settled by simple gravity sedimentation. For this reason, coagulants are used helping to effectively remove fine particles and to shorten the sedimentation process. The use of Moringa Oleifera seeds for water treatment is efficient in reducing 80% to 99.5% of turbidity accompanied by 90% to 99.99% bacterial reduction (LEA 2010).

Summary of Treatment efficiency of sedimentation with the aid of natural & chemical coagulants. Adapted from CAWST (2009)

The effectiveness of coagulants has a complex dependence on the type of coagulant, the characteristics of the fresh water (e.g. temperature, pH, and the specific proportions of particles that constitute the suspended solids). The best approach for determining the treatability of a water source and determining the optimum parameters (i.e. the most effective coagulant, the required dose rates, pH) is by use of a jar tester. Plain sedimentation or settling is not effective for removing dissolved chemicals from the water.

Usually, purified water is either stored in tanks or fed into a distribution system before being consumed or utilised. In order to make sure water quality still meets drinking water standards after storage or distribution, sedimentation needs to be combined with further purification procedures. For this reason, filtration and chlorination is often applied even though water quality after purification is sufficient. The residual concentration of chlorine will also prevent the water from being re-contaminated.

In case of water purification in emergencies, filtration may be left out temporarily, however, chlorination has to be applied at all events.