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Sedimentation (centralised)

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

Sedimentation is a simple, physical pre-treatment of water prior to application of other purification treatments such as filtration and disinfection. 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 held undisturbed, the more the suspended solids and pathogens will settle to the bottom of the container. Adding coagulants can accelerate the sedimentation process. Three common chemicals used for this purpose are aluminium sulphate, polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate. Some native plants like prickly pear cactus, Moringa seeds, broad beans and Fava beans have all been traditionally used as natural coagulants 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
Certain contaminants such as lead and barium can be also effectively removed by coagulation
Disadvantages
Plain sedimentation is not sufficient in purifying water. Additional measures need to be applied
Maximum effectiveness requires a careful control of coagulant dose and pH, and consideration of the quality of the water being treated
Some coagulants (e.g. polyelectrolytes) are expensive
Sedimentation without using coagulants is very time-consuming
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
Coagulants may be toxic if used improperly
Sedimentation is not effective in removing dissolved chemicals unless specific coagulants are added
In Out

Freshwater

Freshwater, Drinking Water

Introduction

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

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

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).


Working Principle

  Illustrations of different stages of sedimentation with the aid of coagulant. Source: MECC (n.y.)

Illustrations of different stage s of sedimentation with the aid of coagulant. Source: MECC (n.y.)   

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.


Basic Design Principle & Technical Installations

 Centralised Water Treatment Cycle. Source: EPA (n.y.)

Centralised Water Treatment Cycle. Source: EPA (n.y.) 

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.


Simple, centralised sedimentation tank. Source: BAREFOOT ECONOMICS (n.y.)

Simple, centralised sedimentation tank. Source: BAREFOOT ECONOMICS (n.y.) 

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.


Large, centralised, high-tech sedimentation basins. Source: MIDVAALWATER (n.y.)

Large, centralised, high-tech sedimentation basins. Source: MIDVAALWATER (n.y.) 

Costs

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.

Health Aspects

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).

 

Bacteria

Viruses

Protozoa

Helminths

Turbidity

Laboratory

>90 - >99%

>90 - >99%

>90 - >99%

>90 - >99%

80-99.5%

Field

<90%

 

 

 

95%

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.

At a Glance

Working Principle

With or without adding coagulants, fresh water is let standing still in a chamber or basin until solid particles have settled to the bottom.

Capacity/Adequacy

Can reduce turbidity to a level that further purification measures can efficiently deal with.

Performance

Depending on their size, sedimentation chambers can suit both rural and urban drinking water systems.

Costs

Low to high

Self-help Compatibility

High without coagulation. Rather low with coagulation

Reliability

High, particularly with coagulants.

Main strength

Simple, physical process

Main weakness

Plain sedimentation is not sufficient as purification measure

Applicability

 

Sedimentation is used to remove solids from water with high sediment content/turbidity. The process of plain sedimentation is easy to perform and requires a minimum of material and skills. However, it is very time-consuming and thus requires considerable large chambers or basins to operate effectively.
If coagulants are used, operational staff needs to be adequately trained to carry out jar tests to determine coagulant dosage. Coagulants can be expensive to buy (particularly polyelectrolyte) and need accurate dosing equipment to function efficiently. 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 findings of 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 seeds in water treatment but there is no evidence that indicates any acute or chronic effects on humans, particularly at the low doses required for water treatment (FOLKARD et al. 1998).
The frequency for performing maintenance tasks depends on the initial turbidity level. However, the sedimentation chamber needs to be emptied and cleaned on a regular basis to avoid overfilling and microbial contamination regardless of the turbidity level.
Library references

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]

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]

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]

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]
Further Readings

Compendium of Sanitation Systems and Technologies (Arabic)

This is the Arabic version of the Compendium of Sanitation Systems and Technologies. The Compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

TILLEY, E. ; ULRICH, L. ; LUETHI, C. ; REYMOND, P. ; SCHERTENLEIB, R. ; ZURBRUEGG, C. ; (2014): 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag) PDF

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]

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]

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]
Case Studies
Training Material

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]

Alternative Versions to

No Structure Described.