Sedimentation

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 out of the water.

Sedimentation is a simple, low cost pre-treatment technology to reduce settable solids and some microbes from water under the influence of gravity prior to application of other purification methods. It also improves the visual qualities of the water and increases its acceptance by consumers. The longer the water is stored, the more the suspended solids and pathogens will settle to the bottom of the container. 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 the 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. Coagulants are dosed in solution at a rate determined by raw water quality. After sedimentation, the water should be filtered to further remove suspended materials and pathogens.

Much of the suspended material can be removed by simply allowing the water to stand and settle for some time. This retention time (from one hour up to two days, the longer the better) is required to settle particles in the bottom. Storing water for at least one day will also promote the natural die-off of some bacteria.

Coagulants enhance sedimentation because they neutralise the surface charge of suspended particles. Particles that cause turbidity (e.g. silt, clay) are generally negatively charged, making it difficult for them to clump together because of electrostatic repulsion. But chemical coagulant particles are positively charged, and they chemically attracted to negative turbidity particles, neutralising the latter’s negative charge and accumulate to form larger particles (flocs), which settle faster.

Natural coagulants contain significant quantities of water-soluble proteins, which carry an overall positive charge when in solution. The proteins bind to the predominantly negatively charged particles that cause turbidity and form flocs. The flocs can be settled out or removed by filtration. Bacteria and viruses can attach themselves to the suspended particles in water. Therefore, reducing turbidity levels through coagulation may also improve the microbiological quality of water.

Plain sedimentation often is effective in reducing water turbidity, but it is not consistently effective in reducing microbial contamination. Storing water for as little as a few hours will sediment the large, dense particles like inorganic sands and silts, large microbes and any other. Overnight or 1-2 days longer settling times will remove larger microbes, including Helminth ova and some parasites, some microbes, such as certain algae, and the larger clay particles. Most viruses and bacteria and fine clay particles are too small to be settled out by simple gravity sedimentation.

Sedimentation by using coagulant reduces the time required to settle out suspended solids and is very effective in removing fine particles. Some bacteria and viruses can also attach themselves to suspended particles. Therefore, reducing turbidity level through coagulation may also improve some microbiological quality (bacteria, viruses, protozoa and helminths) of water (AMAGLOH& BENANG 2009).

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 the coagulants has a complex dependence on the type of coagulant used, the nature of the raw water, being affected by such things as temperature, pH and especially the specific proportions of organic, inorganic and biological particles that constitute the suspended solids as well as mixing. The best approach for determining the treatability of a water source and determining the optimum parameters (most effective coagulant, 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.