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Sedimentation

Author/Compiled by
Raju Shrestha (Environment and Public Health Organization (ENPHO))
Dorothee Spuhler (seecon international gmbh)
Executive Summary

Sedimentation is recommended as simple pre-treatment of water prior to application of other purification treatments such as filtration and disinfection methods. It removes undesirable small particulate suspended matters (sand, silt and clay) and some biological contaminants from water under the influence of gravity. The longer the water is sedimented, the more the suspended solids and pathogens will settle to the bottom of the container. Adding special chemicals or some natural coagulants can accelerate sedimentation. Three common chemicals used are aluminium sulphate, polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate. 'PUR' is a powder product containing both coagulants and disinfectant. Some native plants like prickly pear cactus, Moringa seeds, broad beans and Fava beans have all been traditionally used as natural coagulant to help sediment water in a number of countries in Africa and Latin America.

Advantages
Simple and low cost water pre-treatment technology
Coagulants reduce the time required to settle out suspended solids
Natural coagulants can sometimes be obtained for free or at a low cost
Coagulation can also be effective in removing protozoa, bacteria and viruses, particularly when polyelectrolyte is used. Some bacteria and viruses can also attach themselves to the suspended particles in water that cause turbidity. Therefore, reducing turbidity levels through coagulation may also improve the microbiological quality of water
Certain contaminants such as lead and barium can be also effectively removed by coagulation
Disadvantages
If only settling or plain sedimentation is practised, it removes only partially turbidity and some microorganisms
Maximum effectiveness requires careful control of coagulant dose and pH, and consideration of the quality of the water being treated, as well as mixing
Costs are variable depending on coagulant; some coagulants like polyelectrolyte are expensive to buy. Effectiveness of coagulants varies from one to another
Without using coagulants, a long sedimentation time is needed
Training is required for coagulant dosage, jar testing and frequent monitoring
Natural coagulants are not available in a usable form and need to be prepared
May be toxic if used improperly
Except with the use of specific coagulants, it is not effective for removing dissolved chemicals from the water
In Out

Freshwater

Drinking Water

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.

Working Mechanism

Sedimentation Process. Source: CASIDAY et al. (1999)

Sedimentation Process. Source: CASIDAY et al. (1999)

 Illustrations of different stages of sedimentation with the aid of coagulant. Source: MECC (N.Y.)

Illustrations of different stages of sedimentation with the aid of coagulant. Source: MECC (N.Y.)

Illustrations of different stages of sedimentation with the aid of coagulant. Source: MECC (N.Y.) 

PUR Sachet using procedure. Source: UNKNOWN

PUR Sachet using procedure. Source: UNKNOWN

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.


Effectiveness

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).

 

 

Bacteria

 

Viruses

 

Protozoa

 

Helminths

 

Turbidity

 

Laboratory

 

>90 to >99%1

 

>90 to >99%1

 

>90 to >99%1

 

>90 to >99%1

 

80-99.5%2

 

Field

 

< 90%3

 

 

 

 

95%2

 

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.
 

Applicability

Sedimentation is used to remove solids from water. It is suitable for water with high sediment content. It is easy to perform and requires a minimum of materials and skill. It can be done with as little as two or more simple storage vessels such as pots and buckets by manual transfer. Typically, at least two containers are needed to settle water: one to act as the settling vessel and another to be the recipient of the supernatant water after the settling period. Care must be taken to avoid disturbing the sedimented particles when recovering the supernatant water by decanting or other methods. Staffs need to be adequately trained to carry out jar tests to determine coagulant dosage. For better results, the coagulants should be rapidly and thoroughly mixed in water. Coagulants can be expensive to buy (particularly polyelectrolyte) and need accurate dosing equipment to function efficiently.

When water is sedimented in vessel, the sediment should be removed and the vessel should be cleaned after each use. More rigorous physical or chemical cleaning is needed to avoid the microbial colonization of the vessel surface.

Some communities have opted not to use aluminium based coagulants because of unsubstantiated reports that claim that the aluminium in drinking water poses a risk to public health despite of scientific evidences (SRINIVASAN et al. 1999). Based on the WHO (2008), there is no evidence of health risk.Studies have been carried out to determine the potential risks associated with the use of M. Oleifera seed in water treatment but there is no evidence to suggest any acute or chronic effects on humans, particularly at the low doses required for water treatment (FOLKARD et al. 1998).

Library references

Water Clarification using Moringa Oleifera Seed Coagulant

This technical brief gives an overview of the application of an indigenous, naturally derived coagulant, namely seed material from the multi-purpose tree Moringa Oleifera, which offers an alternative solution to the use of expensive chemical coagulants.

FOLKARD, G. ; SUTHERLAND, J. ; SHAW, R. ; (1998): (= WELL Technical Briefs , 60 ). Loughborough: Water and Environmental Health at London and Loughborough (WELL) URL [Accessed: 02.08.2010]

Lesson 9: Colloids and Coagulation

MECC (= Mountain Empire Community College. Water and Wastewater Distance Learning ). Big Stone Gap: Mountain Empire Community College (MECC) URL [Accessed: 03.08.2010]

Guidelines for Drinking-water Quality, Third Edition

This volume of the Guidelines for Drinking-water Quality explains requirements to ensure drinking-water safety, including minimum procedures and specific guideline values, and how those requirements are intended to be used. The volume also describes the approaches used in deriving the guidelines, including guideline values. It includes fact sheets on significant microbial and chemical hazards.

WHO (2008): Third Edition incorporating the First and Second Addenda. Geneva: World Health Organization (WHO) URL [Accessed: 23.04.2012]

Chemical Coagulants

A three-page factsheet containing information on treatment efficiency, operating criteria and other relevant information on chemical coagulants.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]
Further Readings

Chemical Coagulants

A three-page factsheet containing information on treatment efficiency, operating criteria and other relevant information on chemical coagulants.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

PUR

A four-page factsheet containing product information on the procedure of using PUR flocculant and disinfectant powder, its treatment efficiency, operating criteria and other relevant information’s related to PUR.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

Natural Coagulants

A four-page factsheet containing information on treatment efficiency, operating criteria and other relevant information related to natural coagulants.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

Settling

A three-page factsheet containing introduction, operation procedure, treatment efficiency, operating criteria and other information related to settling.

CAWST (2009): (= Household Water Treatment and Safe Storage Fact Sheets ). Calgary: Centre for Affordable Water and Sanitation Technology (CAWST) URL [Accessed: 03.08.2010]

Flocculant/Disinfectant Powder

A two-page factsheet on PUR flocculant and disinfectant powder with field and lab, containing information on effectiveness, advantages and limitations and implementation examples.

CDC ; USAID (2008): (= CDC Household Water Treatment Options in Developing Countries Factsheets ). New York: Center for Disease Control and Prevention (CDC) and United States Agency for International Development (USAID) URL [Accessed: 01.04.2010]

Water Clarification using Moringa Oleifera Seed Coagulant

This technical brief gives an overview of the application of an indigenous, naturally derived coagulant, namely seed material from the multi-purpose tree Moringa Oleifera, which offers an alternative solution to the use of expensive chemical coagulants.

FOLKARD, G. ; SUTHERLAND, J. ; SHAW, R. ; (1998): (= WELL Technical Briefs , 60 ). Loughborough: Water and Environmental Health at London and Loughborough (WELL) URL [Accessed: 02.08.2010]

Smart Disinfection Solutions

This booklet, part of the Smart Water Solutions series provides a wide range of methods and products for home water treatment in rural areas.

NWP (2010): Examples of small-scale disinfection products for safe drinking water. (= Smart water solutions ). Amsterdam: KIT Publishers URL [Accessed: 07.07.2010]

Household Treatment and Storage

This technical brief presents the current options for safe storage and overview of seven types of point of use water treatment options. It also explains the working mechanism, and discusses the microbiological effectiveness of ceramic candle filter as well as community acceptance and sustainability.

OXFAM (2008): (= Technical Brief No. 4 ). Oxford: OXFAM URL [Accessed: 21.07.2010]

Water Clarification Using Moringa oleifera

Technical information on water clarification using Moringa Oleifera, with preparation techniques of the coagulant solution, method of treatment, dosage, advantage and disadvantages of the natural coagulant with useful links.

SCHWARZ, D. ; (2000): Eschborn: GATE-GTZ URL [Accessed: 03.08.2010]

Application of Indigenous Materials in Drinking Water Treatment

In this study, volcanic ash and Moringa oleifera (M. oleifera) were investigated as indigenous materials for drinking water treatment based on problems identified at Kampala and Masaka water treatment plants in Uganda.

KALIBBALA, H.M. ; (2007): Stockholm: Royal Institute of Technology (KTH) URL [Accessed: 26.10.2011]

Conservation et Traitement de l Eau a Domicile

This practical guide provides a review of different processing techniques and adequate water conservation at home and is structured around 10 key questions that should be posed before choosing a suitable solution.

DESILLE, D. ; (2013): Paris: Programme Solidarite Eau (PSeau) URL [Accessed: 06.06.2013]
Case Studies

Application of the Purified Moringa Oleifera Coagulant for Surface Water Treatment

This study investigated the effectiveness of the Moringa oleifera coagulant for the removal of turbidity, bacteria, and natural organic matter (NOM) from natural surface water. The results obtained were compared with inorganic coagulants of alum and ferric chloride.

ABALIWANO, J.K. ; GHEBREMICHAEL, K.A. ; AMY, G.L. ; (2008): Watermill Working Paper Series No. 5). Delft: United Nations Educational, Scientific and Cultural Organization, Institute for Water Education (UNESCO-IHE) URL [Accessed: 03.08.2010]

Moringa Seed and Pumice as an Alternative Natural Material for Drinking Water Treatment

This research thesis presents an investigation on the suitability of pumice and seeds of Moringa Oleifera as natural materials for drinking water treatment based on the problem identified on the Stretta Vaudetto water treatment plant in Eritrea.

GHEBREMICHAEL, K.A. ; (2004): (= Doctoral Thesis ). Stockholm: Royal Institute of Technology (KTH) URL [Accessed: 03.08.2010]
Training Material

Household Water Treatment Manual

This manual on household water treatment system includes various useful topics on safe water and multi barrier approach (source protection, sedimentation, filtration, disinfection and safe storage).

CAWST (2008): Calgary: Centre for Affordable Water and Sanitation Technology (CAWST)

Evaluating Coagulant for Water Treatment. Presentation

This document is an overview presentation on different coagulants, discussing different case studies with field results and some lessons learned. Presented in PNWS-AWWA Section Conference.

DUNCAN, K. WISE, D. (2008): American Water Works Association, Pacific Northwest Section (PNWS-AWWA) URL [Accessed: 03.08.2010]

Household water treatment and safe storage in emergencies

This document is intended as a general manual on household water treatment and storage in emergencies. Methods of treatment but also promotion are presented, including factsheets, a decision tree and very comprehensive illustrations.

IFRC (2008): pdf presentation. Geneva: International Federation of Red Cross and Red Crescent Societies (IFRC) URL [Accessed: 23.04.2012]

The Three-Pot Water Treatment System

This poster is part of the series of Water, Sanitation and Hygiene posters designed by the Water, Engineering and Development Center of Loughborough University.

SKINNER, B.H. ; SHAW, R. ; (2013): Poster. (= WEDC Posters, 002 ). London: Water, Engineering and Development Center (WEDC) URL [Accessed: 28.08.2013]

Alternative Versions to

No Structure Described.