PakWater Care Services: Safe Drinking Water For All

Safe Drinking Water for Small Communities

A Global Issue

According to the WHO/UNICEF Joint Monitoring Program on Water Supply and Sanitation, globally an estimated 884 million people lack access to an improved water source. Out of these, 743 million live in rural areas1. Many of the remaining 2.5 billion rural dwellers are served by small water supplies that are managed by operators who lack adequate training, with significant gaps in adequate risk management practices.

The greatest risk is the potential for an outbreak of infectious disease such as acute diarrheal illness. Every year, 2.2 million deaths are attributed to diarrhea alone,

With the vast majority of deaths among children under the age of five. Some 88% of diarrhea cases are attributed to unsafe water, inadequate sanitation and hygiene.

Small community water supplies are a concern faced by both developed and developing countries. One in ten Europeans, between 40 and 50 million people, receive drinking-water from small or very small systems,including private wells. Globally, and particularly in sub-Saharan Africa and Oceania, small water supplies found in rural areas significantly lag behind their urban counterparts in the quality of their drinking-water.

Safe Drinking Water in Pakistan

Background Information about Pakistan:

Population: 160m

Area: 796,000km²

Infant mortality: 98/1000

Life expectancy: 63.4 years

Water supply coverage: 91%

Sanitation coverage: 59%

Below poverty line: 32.6%

Development index: 134

Adult literacy: 54%

Introduction

As everybody knows water is essential for life of man, plants and animals. From the beginning of civilization humans have settled close to water sources. Unfortunately in many countries water is scarce or contaminated. Providing a better water supply can significantly improve the quality of life and is a source of, and the condition for, a socio-economic development.

Some diseases in poor or developing countries are related to insufficient or unsafe water, together with local factors as climate, density of population, local practices etc.

To control these diseases a sufficient amount of safe drinking water is important. This implies not only improve the design and planning of water supplies, but also sanitation and hygiene behaviour. This can be obtained raising the demand and introducing sanitation programs. Improvements in water services can be made by outsiders (politicians, planners, engineers) but they have to operate in partnership with the community.

Better water distribution allows avoiding the presence of stagnant water or wastewater, where insects carrying the above mentioned diseased can be present. Better water distribution can also bring no need for women or children for carrying water. This allows more free time to dedicate to better activities, as childcare, animal rising or vegetable gardening.

In developing countries communities that want to establish and run an improved water supply vary greatly. It is important not to overlook the different nature and history of small communities. There is no standard solution, but different solutions for different communities. Planning and making decisions on the pros and cons, the implications of each option and choosing the best option considering the kind of community is crucial for the success of the project.

Planning and management

During the last two decades it has been recognized that water supply improvements alone do not bring optimum health and development impact in developing countries. Other complementary activities needed are better sanitation provisions, changes in hygiene provisions and linkages with other livelihood inputs.

Community participation in water projects is certainly very important. There is need of inclusive approach avoiding marginalization of the poor. This can be gained through programs, that are series of integrated activities directed to the establishment and continue functioning and use of water supply services. The challenge of a program is social, organizational and administrative. It is important that agencies and partners work together with communities group and users and plan their activities on a mutual agreement.

To meet long-term health benefits of environmental engineering it is important to enhance the demand for better water use, sanitation and hygiene. The new systems have to be and remain better than the alternatives in terms of economic and social costs and benefits. Program teams have to seek the values of local experiences and viewpoints to understand what local people really want and can use and sustain.

The community water supply designs should be holistic, so to meet all the basics needs of people, expandable, in view of community growth with access to the community improved water supply, and upgradeable, in view of a socio-economic growth and a need of later upgrading. Standardization, even if often more cost-effective, is not always a good choice because it can imply competition between different brands, poor incentive for the involvement in the private sector and the technology may not respond to the needs and preference of the users.

Funding

Small communities often find it difficult to obtain the capital to construct improved water supplies. Usually the central or provincial government organize and finance multi-communities programs and the fund may be partly revolving, using repayments or earlier loans. The communities candidates for a loan or a grant, or a combination of both, are asked to submit a pre-proposal to the program. Communities are not homogeneous entities, they often consist in the middle classes and the poor, marginalized groups. To help and support all the groups it is important to identify all of them at the very start of the project and to ensure their equal participation. All the groups should participate to the formulation of the preliminary plans to the program level. Projects must be based on the existing water supply already available for the community.

Once a proposal has been selected and resources have been assigned, the next stage is detailed planning and design.

When each community has developed its own detailed plan, through the decision making process at a program level it is decided which plans are financed though a loan, a grant or a combination of the two. The project funds are then transmitted in instalments to the special project bank account that each community has established

Maintenance

The staff in charge of management and maintenance of the water supply varies depending on the size of the project. For small water supply systems, selected technicians and the management committee are trained during and after the construction. For larger and multi-village systems with a community base management staff are generally professionally trained and hired by the community water board.

Support

All team members must be conscious of gender and poverty conditions, and they should be able to overcome or reduce inequalities between women and men and rich and poor. The team needs to be able to combine specific knowledge, expertise and skills of local people with those of the team itself. Technical options will have socio, cultural and organizational implications that the technical staff must take into account. On the other end social staff needs to have a basic understanding of the technical implications of community choices.

At the support level, technical-social teams from the private sector can be chosen by a program to support work with communities. For technical work the community may decide to use their own procurements, use their own artisans and/or hire contractors, who will be guided by the support program. At the higher level, managers and other superiors should support and reward the ability of the personnel to integrate local people, the quality of process work and the nature of long-term results.

Human Requirement of Water:

The exact amount of water a human need is highly individual, as it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity. The reference daily intake for water is 3.7 liters per day for human males older than 18, and 2.7 liters for human females older than 18 including water contained in food, beverages and drinking water. Food contributes 0.5 to 1 liter and the metabolism of protein, fat, and carbohydrates produces another 0.25 to 0.4 liters, which means that 2 to 3 liters of water for men and 1 to 2 liters of water for women should be taken in as fluid.

A person requires about 30 gallons of water per day for all uses combined e.g. cooking washing, bathing etc. [3.78 Litters = 1 U.S Gallon]

Water quality

The basic requirements for drinking water are that it should be clear (low turbidity), not salty, free from offensive taste or smell, free from chemicals that may cause corrosion or encrustation, free of heavy metals, with not excessive sodium, sulphate and nitrate but above all free from pathogenic organisms as bacteria and viruses which may cause disease. The WHO has published guidelines to help counties to set qualities standards with which domestic water supplies should comply. These standards are often considered as long-term goals rather than rigid standards.

High turbidity implies the presence of particles or colloidal material, which provide adsorption sites for chemicals that might be harmful.

Colour is often due to natural organics or dissolved inorganic compound such as iron and manganese. Organic colour when disinfected with chlorine will produce harmful chlorinated organics, iron or manganese in high quantities in drinking water, making it not healthy to drink. Low pH can increase corrosion of pipe works, while a too high pH can lead to calcium carbonate deposition and encrustations.

Some chemicals as ammonia, calcium, chloride, fluoride, magnesium, nitrates, sodium, potassium, sulphate and zinc can be present in large quantities. Excessive levels have a harmful effect on health, but in many cases limited quantities are necessary for the maintenance of living organisms and low concentrations are therefore desirable in water supplies.

In assessing an existing or potential water supply, efforts should be made to take suitable samples of the water and to have them analysed as fully as possible.

Pumping

Human and animal power is often the most readily available power for pumping water for small communities in developing countries, particularly in rural areas. Under suitable conditions wind power is of relevance. Diesel engines and electric motors should be used only if the necessary fuel and electricity supplies are readily available. Prevailing local conditions and management capacities determine the type of pump that is most suitable and sustainable. Participation by representatives of the different users groups in selecting and trying the pumps, help to ensure that the type chosen is suitable to them.

The main applications of pumps in small community water supply systems are pumping water from wells, surface water intakes, or into storage reservoirs and distribution system.

There are different types of pumps and to chose the most suitable one for a specific purpose the following technical criteria needs to be considered:

Rate of delivery required
Vertical distance from pumping to delivery level
Variation of water level expected at the source
Durability of basic components
Availability and cost of spares
Ease of maintenance

Reciprocating pump: it is the type of pump most commonly used for small water supplies. It can be divided in suction pumps (the plunger and its cylinder are located above the water level, they can be operated by arms or legs), lift pumps (the cylinder and plunger are located below the water level in the well), force pumps (same as suction pump but enclosed at the top so that it can be used to force the water to elevations higher than the pump). Suction pumps gives a discharge up to 7 m, lift pumps can lift water from wells as deep as 180 m or even more.

Rotary pump: this pump uses a continuous chain of small buckets, discs, knots or a single-thread helical rotor to carry water from the bottom of the well to the top. The investment cost of this pump is low and is therefore attractive as a family pump. Drive arrangements for this kind of pump are manual operation, electric motors, diesel and petrol engines.

Axial flow pump: Radial fins or blades are mounted on an impeller or wheel, which rotates in a stationary enclosure. The rotating impeller lift the water mechanically. The fixed guide blades ensure that the water flow has no whirl velocity when it enters or leaves the impeller.

Centrifugal pump: the essential component of a centrifugal pump is the impeller and the casting. When rotated at a sufficient speed the impeller imparts kinetic energy to the water, the casting is so shaped this kinetic energy is partly converted in useful pressure, which forces the water into the delivery pipe. The water leaving the eye of the impeller creates suction. An impeller and the matching section of the casting create a stage. More stages can be used if the required pressure is higher than a single stage can produce.

Air-lift pump: an air-lift pump raises water by injecting small, evenly distributed bubbles of compressed air at the foot of discharge pipe fixed on the well. This requires an air compressor. Because the mixture of air and water is lighter than the water outside the discharge pipe, the mixture is forced upward by the hydrostatic head.

Hydraulic ram: the ram utilises the energy contained in a flow of water running through it, to lift a small water volume to a higher level. The principle used is that of a pressure surge, developed when a moving mass of water is suddenly stopped. The hydraulic ram needs no external source of power, it requires very little and infrequent maintenance. It needs water running at a high speed: it will work at its best if the supply head is about one-third of the delivery head.

Groundwater withdrawal

For community water supply systems groundwater is almost always the preferred source, and its use is probably still very much below the potential in many countries.

Knowledge of the manner in which water exists in the water-bearing ground formations can give successful prospecting for groundwater. Available hydrological information about the study area should be collected and collated. To provide data to form a basis for drawing up a hydro geological map, a survey of the study area should be made, preferably towards the end of the dry season. This hydro geological map should show distribution of aquifers, springs, depth of water tables and piezometric levels. Geophysical investigations (i.e. resistivity measurements) are very useful in understanding the distribution and quality of groundwater. Sometimes it is necessary to drill small boreholes for post-prospecting purposes to supplement the data obtained from surface geophysical methods. To obtain the maximum amount of information from a borehole, geophysical logging may be necessary.

The oldest and simplest method of groundwater withdrawal is to dig a hole in the ground to a depth below the water table. The aquifer must be tapped over a greater area of contact if more withdrawal capacity is needed. This may be done enlarging the width of excavation through galleries or increasing the depth building dug wells or boreholes. Infiltration galleries are divided in ditches and drains. Ditches are just a cut in the ground to make the aquifer accessible from the surface. Drains have pores, perforations or open joints allowing the groundwater to enter. Galleries are very expensive and difficult to build, so they should only be uses where the groundwater table is at a shallow depth (no more than 5-8 meters below the ground surface).
Dug wells are made simply by digging a hole in the ground and Usually no special equipment or skills are required for their construction. Dug wells usually have a limited capacity, so their use is restricted to individual households and other small scale water supplies. They provide water storage as well. The depth to which a well can and should be dug largely depends on the type of ground and on the fluctuation of the water table. Private wells are generally less than 10 m deep; dug well for communal use have often a depth of 20-30 m.
A borehole has a casting consisting of pipes in the non water bearing formations, and a perforated or slotted screen section in the aquifer. Boreholes should be used when the groundwater table is at a considerable depth below the ground surface. They can be constructed to 200 m or deeper depending on the method used. Borehole construction is greatly influenced by local factors and relatively unknown underground conditions. Several drilling and construction techniques have been developed for use in these different environments. Also, the selection of the correct material required is of extreme importance. Boreholes are very suitable for drinking water supply because simple precautions will be adequate to safe-guard the water against contamination.
In some cases either vertical or horizontal water collectors, or a combination of the two, can be appropriate. When groundwater is withdrawn there is always a lowering of the groundwater table. The possible effect of an appreciable lowering of the groundwater table should be carefully investigated.

Surface water intake and small dams

Most of the more convenient source of water for small communities is frequently a natural stream or river close by. A river intake should be sited where there is an adequate flow and the level allows gravity supply to minimize pumping costs. The quality of the water is also important so the water intake should be upflow of density populated or farming areas or of cattle watering places. Intake design should avoid clogging and when the river transport rolling stones or boulders a protection in concrete, stone or brick of the intake may be necessary. At the water intake a screen is usually placed, to remove to remove floating or suspended matter of large and small size. The bottom of the intake structure should be at least 1 m above the riverbed. A submerged weir may have to be constructed downstream of the intake to ensure that the necessary depth of water is available even in dry periods.

The quality of lake water is influenced by self-purification through aeration, bio-chemical processes and settling of suspended solids. In deep lakes, wave and turbulence will not affect the deeper strata. As there is no mixing, a thermal stratification will develop, which can be fairly stable and should be taken into account when choosing the location and depth of a lake water intake for water supply purposes. Deep lakes will have towards the bottom water with a low nutrient content and good chemical quality that will be same throughout the full depth. Provision should be made to withdraw the water at some depth below the surface.

River and lake intakes should be periodically checked and floating material and debris should be periodically removed from the screens and weir. Checks for any damage of intake, bank protection and weir from heavy materials or from heavy flow from debris need to be made.

Water treatment

In many situations treatment of raw water is necessary to make it suitable for drinking and domestic use. In most developing countries small towns and rural communities are not able to run complicated water systems that surmount local capacity and feasible regional support structures. The construction and running costs, and the operational and maintenance needs are key factors that must be considered carefully when planning and designing a small water treatment plant.

Water treatment should be combined with other strategies as watershed and land use management to protect surface and ground water, selection and protection of the best available water sources, adequate and well-maintained distribution system. A good drinking water quality depends on more than water quality enhancement or water treatment processes. The types of risk existing in the supply source and the institutional and socio-economic conditions prevailing in the target community determines the level of water treatment technology. The best approach is the multi-stage water treatment: successive stages progressively remove contaminants from the raw water and consistently produce safe and wholesome final water. Strengths and weaknesses of each treatment stage should be quantified and balanced, so that all contaminants are effectively removed at a feasible cost. The final stage of the water treatment will be disinfection. It is effective only if the previous stages have removed most of the waterborne pathogens and reduced solids or other contaminants. This should allow the use of only a small dose of disinfectant.

The main health risk related to water supply systems that use surface water is contamination with waste water. This introduce a big variety of bacteria, viruses and protozoa and can cause waterborne diseases. All pathogenic organisms as well as high risk chemical substances such as heavy metals, fluoride, arsenic, nitrate and organic constituent must be removed. Other substances that needs to be removed or considerably reduced are suspended solids causing turbidity, iron and manganese compounds imparting a bitter taste or staining laundry, and excessive carbon dioxide corroding concrete and metal parts. For small community water supplies other quality characteristics such hardness, TDS and organic content would generally be less important. Click here for quality guidelines for drinking water.

Some water treatment processes serve a single purpose and others have a multiple applicability. Often a treatment result can be obtained in different ways. The following table summarize the removal of some water contaminants by various treatment processes. This comparison is obviously general because there are many factors to take into account. A detailed description of water treatment processes for small community water supplies will follow.

Treatment processes	Sedimentation	Coagulation	Flocculation, sedimentation and filtration	Slow sand filtration	Multistage filtration	Chemical oxydation: disinfection
Water contaminants
Bacteria	0	0	+++	++++	++++	++++
Viruses	0	0	+++	++++	++++	++++
Giarda cysts	0	0	+++	++++	++++	++
Cryptosporidium oocystis	0	0	+++	+++	+++	+
Turbidity	0	+	++++	++++	++++	0
Suspended solids	0	+++	++++	++++	++++	0
Taste and odour	++	0	+++	+++	+++	+
Iron and Manganese	++	+	+++	+++	+++	++
Fluoride	0	0	+	0	0	0
Arsenic	0	0	++	+	+	0
Heavy metals	++	0	++	++	++	+
Dissolved Oxygen	+	0	0	-	-	0
Carbon dioxide	-	0	0	+	+	0
Colour and organics	0	0	++	++	++	+

0 : no effect
+ : positive effect
- : negative effect

Groundwater treatment:
Groundwater, if properly withdrawn, will be free from turbidity and pathogenic organisms and if it originates from a clean sand aquifer, other hazardous substances will also be absent. In this case only disinfection is recommended, groundwater can be used without any further treatment. When groundwater comes from an aquifer containing organic matter, the oxygen content may be depleted and the water may be able to dissolve iron, manganese and heavy metals. Aeration and filtration can remove these substances. Sometimes groundwater contains also fluoride, arsenic and salts or may be polluted by hazardous waste and if no alternative source of water is available, these polluted sources may have to be used. In this case the source should be treated with chemical coagulation and flocculation, iron exchange and different filtration technologies, including GAC. Unfortunately many of these treatment processes involve expensive technologies, but they are necessary to make the water suitable for drinking and domestic purposes.

Surface water treatment:
Water in surface sources originates partly from groundwater outflows and partly from rainwater that has lowed over the ground to the receiving bodies. The groundwater outflow will bring dissolved solids into the surface water, while the surface run-off is the main contributor of turbidity and organic matter, as well as pathogenic organisms. In surface water bodies the dissolved mineral particles will remain unchanged, but the organic impurities are degraded. Unpolluted surface water of permanently low turbidity may be purified by slow sand filtration, or by direct rapid filtration followed by chlorination. SSF has the advantage of low operational requirements. When the turbidity of the water is high, or when algae are present, SSF units would rapidly clog. In this case pre-treatment is needed, such as sedimentation, coarse (gravel) media-filtration, rapid filtration or a combination of more of these processes. Chemical coagulation and flocculation can improve the removal by settling and filtering of colloidal suspended particles.

During water treatment it is important not only to have an assessment of the raw water quality, but also performance efficiencies and treatment objectives for the treatment plant.

Aeration

Aeration is the treatment process whereby water is brought into intimate contact with air. Aeration is widely used for the treatment of groundwater having too high iron and manganese content. Ferrous and manganese compounds will react with the atmospheric oxygen brought into the water through aeration. They will be transformed into insoluble ferric and manganic oxide hydrates that can be subsequently removed by sedimentation or filtration. It is important to know that when the water contains organic matter, the formation of iron and manganese precipitates through aeration is likely to be not very effective. In this case it might be necessary to use chemical oxidation, alkalinity variation or special filters. These methods however are expensive and complex, so often they are not suitable for rural communities in developing countries.

The intimate contact between water and air for drinking water treatment is mostly achieved by dispersing the water through the air in thin sheets or fine droplets (water fall aerators) or by mixing the water by disperse air (bubble aerators).

Coagulation and flocculation

Coagulation and flocculation provide the water treatment process by which finely divided suspended and colloidal matter in the water is made to agglomerate and form flocs. Colloidal particles are midway in size between dissolved solids and suspended matter and are kept in suspension by a balance between electrostatic repulsion and hydration. Colloids usually have a surface charge due to the presence of a double layer of ions around each particle, and this charge is responsible for electrostatic repulsion. This electrostatic repulsion between these negative charges cancels out the electronic attraction forces that would attach the particles together. Some chemicals (called coagulants) are able to reduce the range of electrostatic repulsion, by compressing the double layer of ions around the colloidal particles. They enable the particles to flocculate, forming flocs that can grow to a sufficient size and specific weight to allow their removal by settling, filtration or flotation.

The substances that are frequently removed by coagulation and flocculation are those that cause turbidity and colour. Generally water treatment processes involving the use of chemicals are not suitable for small community water supplies. they should only be used when the needed treatment result cannot be achieved with another treatment process using no chemicals.

Sedimentation

Sedimentation is the settling and removal of suspended particles that take place when the water stands still in, or flow slowly through a basin. Turbulence is generally absent or negligible, and particles having a specific weight (density) higher than that of the water are allowed to settle. These particle will deposit on the bottom of the tank forming a sludge layer and the water reaching the outlet of the tank (generally place on the top on the side opposite to the feed) will be in a clarified condition. Settling tanks need to be regularly cleaned to remove the sludge layer formed on the bottom.

In small community water supplies dissolved air flotation (DAF) can be used particularly for the flotation of algae, which can give rise to the filtration problem if not reduced. DAF consist in the injection in the tank of fine bubbles that make it possible to collect and remove fine light particles, such as flocs containing colour or algae. The basic technology in rather complex and can involve more steps, such as chemical addition and mixing, flocculation, injection of water saturated with air under pressure, nozzles for the pressure release, a filtration tank and rapid filtration. For this reason it would not normally be advisable to use this process in small community water supply.

Multi stage filtration technology

The multi stage filtration technology (MSF) is a combination of slow sand filtration (SSF) and coarse gravel filtration (CGF). This combination allows the treatment of water with considerable levels of contamination and it is a robust and reliable treatment method that can be maintained by operators with low levels of formal education. It is much better suited than chemical water treatment to the conditions in rural communities and small and medium municipalities.

Slow sand filtration:
The water treatment in SSF is the result of a combination of physio-chemical and biological mechanisms that interact in a complex way. Soluble matter in the sand bed is consumed by bacteria and other micro-organisms. The main physical mechanisms contributing to particle removal are surface straining, interception, transport, and attachment and detachment mechanism. A SSF unit basically consists of a structure containing flow control and drainage systems, a supernatant water layer and a filter bed. For continuous supply there should be at least two units operating in parallel. To maintain the proper filtration rate through the filter bed outlet and inlet- controlled flow can be used. In the outlet controlled flow the outlet valve is gradually opened to compensate the increase in the head loss over the filter media. The supernatant water level is always kept close to maximum. In the inlet-controlled filters an increase in head loss is compensated by an increase in the height of the supernatant water. The layer of supernatant water provides the static head necessary for the passage of water through the sand bed. The adequate selection of sand include size grading, characterized by the effective size diameter (d₁₀) and the uniformity coefficient (u_c = d₆₀/d₁₀). The sand to be put into the SSF should be clean and free of clay, earth and organic material. The presence of dust seems produce high initial head losses and to limit the essential development of an active and effective microbial population in the filter bed.
SSF units must operate continuously, since this contributes to a better quality effluents and a smaller filtration area required. After several weeks or months of running, the SSF unit will gradually become clogged. By scraping of the top layer of the filter bed, the hydraulic conductivity will be restored after a secondary ripening period (usually 0-10 days). Scrape sand should be washed and stored. After several filter runs this activity leads to a gradual reduction of the sand bed depth unit a minimum value (0.3 - 0.5 m) is reached. Then re-sanding become necessary.
Great differences exists in the application of SSF technology, depending on water quality standards, raw water quality, type and level of pre-treatment and local conditions. Even if high removal efficiencies can be obtained, SSF alone cannot always produce water of high standard. Limitations can be due to the presence of the following contaminants and parameters:

Suspended solids or turbidity: suspended solids can create major increases in head loss and adverse conditions for the biomass active in the filtering bed.
Iron and manganese: high concentrations of iron (> 1 mg/l) may contribute significantly to the clogging of the SSF unit.
Algae: a massive algal growth can cause a quick reduction of the permeability of the filtering bed.
Organic colour and organic carbon: TOC and COD usually are not easily removed by SSF.
True colour: true colour is removed by SSF only in the range of 25-30%.
Heavy microbiological contamination.
Low temperature: low temperature increases the viscosity of the water and reduces the biological activity in the sand bed.
Nutrients: the micro-organisms active in the sand bed require nutrients (carbon, nitrogen, phosphorous and sulphur) for their metabolism and growth. The total absence of nutrients in the incoming water can decrease the SSF efficiency.
Dissolved Oxygen: with low levels of dissolved oxygen the filter skin can develop anaerobic conditions. This can create serious water quality problems, such as bad smell and taste.

Surface waters presenting relatively moderate to high levels of contamination could not be treated directly by SSF units.

During the last few decades pre-treatment alternatives have been developed to extend the application of SSF to poorer water sources without requiring skilled staff, complex mechanical equipment or chemical supplies, i.e. sedimentation, prolonged storage in basins or coarse media filtration (CMF).

Coarse media filtration:
In coarse media filters porous media such gravel and sand are used as pre-treatment. In dynamic gravel filters the water enters the unit and passes through the fine gravel to the drainage system. With moderate levels of SS in the source water, this fine gravel would eventually clog. If quick changes occur, the clogging may be much faster. Eventually the gravel bed will be blocked and the total water volume will just flow over the clogged surface are to waste, protecting the subsequent treatment steps that are more difficult to maintain. Depending on the flow direction in the layer of gravel, the second treatment step are upflow, downflow or horizontal flow systems. Head loss in CMF is small, usually a few centimetres, with a maximum value around 0.30 m. Since the CMF units in small water supplies systems deal with low flow and low pressure values, some simplified valves, gates, and weirs can be used together with more commercial hydraulic devices. The main criteria for CMF design are removal efficiency and head loss related to particle retention in the filtering bed. The filtering media should have a large surface area to enhance particle removal and a high porosity to allow the accumulation of the separated solids. Tests showed that neither the roughness nor the shape of the filter materia had a great influence on filter efficiency. Gravel is the commonly used material, but broken bricks, palm fibre and plastic material can also be used.
Operation on CMF units required a frequent (at least daily) control of the influent and effluent flow and the quality of filtered and raw water. Maintenance is associated mainly with the cleaning process. To facilitate it, a minimum of two units should be constructed in parallel. Frequent cleaning of CMF is recommended to limit head loss development and to avoid operational problems.

In general, performance findings of MSF are very satisfactory. Nevertheless, the performance may be different in different regions of the world. Much depends on the characteristics of the raw water (turbidity, SS, particle size distribution, temperature, true colour) and partly on climatic seasonal fluctuations. MSF can adapt itself to the type of raw water and the concentration of contamination. MSF technology has a great potential to reduce the physical-chemical and the bacteriological risk associated with surface water sources.

The cost efficiency of MSF systems increases with the size of the system. The operational and maintenance cost is mainly determined by labour cost. Capital and running costs increase with increasing contamination levels in their raw water types.

Rapid filtration

Sand is also the media commonly used in rapid filtration, but much coarser sand than in SSF is used, with an effective grain size in the range 0.4 - 1.2 mm. The filtration rate is much higher, generally between 5 and 15 m³/m²*h. This coarse sand allow the impurities to penetrate deep into the filter bed, giving to it an high capacity to store deposited impurities. Cleaning of rapid filters is effected by backwashing: an high rate flow of clean water is sent back through the filter bed. This backwash water carries away the deposited material that was clogging the filter. Cleaning of rapid filters is quick and can be done as necessary as required, even everyday. The limit is imposed only by the amount of clean water used.

Rapid filtration as a variety of applications: it can be used for removal of iron and manganese, generally combined with aeration, as a pre-treatment to reduce the load on the following SSF or for treating water that has been clarified by coagulation, flocculation and sedimentation. Rapid filters can work by gravity or under pressure, the water can flow upwards or downwards and they can be placed in series or in parallel.

Operation: during filtration the water enters the filter through an inlet valve, moves down towards the filter bed, flows through it, passes the under drainage system placed on the bottom and flows out through another valve. Due to gradual clogging of the pores the filter's bed resistance against the downward water flow will progressively increase. This will reduce the filtration rate unless it is compensated by a rising raw water level above the filter bed. A filter rate control device will allow the filters to operate with a constant raw water level, providing an adjustable resistance to the water flow. They open gradually and automatically to compensate the filter's bed increasing in resistance, to keep constant operating conditions. When the filter rate controller is fully opened filter clogging cannot be further compensated and the filtration rate will fall. The filter is taken out of service for backwashing. Another option is operating with constant water level. In this case the filtration rate can be controlled by the raw water feeding rate, that can be easily adjusted to meet the demand for filtered water.
Further options widely used in Europe and North America are variable water level with constant or declining rate control. The last is the most simple system and it is recommended for small water treatment plants in developing countries. In this kind of RF, all filters are in open connection with both raw and filtered water conduits. Consequently, they have approximately the same raw water level and filtered water level, so that they operate under the same head. The filtration rate, instead, will be different: higher in the filters just cleaned by backwashing and lowest for the longest underway in its current filler run. The supply of water should be high enough to meet the demand for filtered water. During filtration the filters bed gradually become clogged and raw water level in all filters will rise. The filter unit that has been in operation for the longest period of time will reach the maximum allowable water level first, so will be stopped and backwashed. After cleaning this filter will have the lowest resistance, so a considerable portion of water will pass through it, reducing the load on the other filters. When the maximum raw water level is reached in a second filter this one will be backwashed and so forth.

For the design of a RF, the parameters that need to be selected are: grain size of the filter material, thickness of filter bed, depth of supernatant water and rate of filtration. These design factors should be based on experience obtained in existing plants, or on the result obtained with a pilot plant. The filter tank is generally made of reinforced concrete, rectangular and with vertical walls. An ample concrete cover should be provided to protect the reinforcement bars against corrosion. Numerous drain systems have been developed in the past, but their are either to expensive or unable to ensure and even distribution of the water over the full undersize of the filter bed. Considerable attention should be give to the composition of sand.

Because of their complex design and construction and the need of an expert operation, RF are not very suited for application in a village-scale water treatment plant, especially if used as final filters in the treatment of turbid river water, where bacteriological safety should then be secured by chlorination, with all its associated difficulties. The greatest difficulty encountered in village-scale rapid filtration is the backwashing process: it is uneconomical to use a wash water pump. The only effective and convenient application of RF is as roughing filtration to be used when the turbidity of the inflow to slow sand filters exceed 25 mg/l. Such pre-treatment protect the sand filters from being rapidly clogged.

Desalination technology

Driven by scarcity, competition for water between municipal users and irrigation may increase dramatically. Some experts predict that desalinated water will become an important water source in this century, as 70% of the population live within 50 miles from the sea. Desalination could provide sustainable water supply to many municipalities and industries. It could be the technology of choice to develop industry in region of water scarcity in Asia, Africa and South America.

Water is generally divided in three categories, depending on is total dissolved solids (TDS) content. Freshwater generally cover water with a TDS up to 1.000 mg/l, brackish water from 1.000 to 10.000 mg/l and seawater above 35.000 mg/l. "Difficult" brackish water has a TDS concentration above 10.000 mg/l.

Desalination technology can basically be divided in two types: thermal desalting technology and membrane desalting technology. The thermal technology includes:

Multi-stage flash distillation (MSF): water is heated up to 110 Celsius and flows subsequently through chambers (stages) of decreasing pressure. As a result, the water flashes off to produce vapour that is condensed through a heat exchange with feed water. In this way the evaporation (condensation) heath is recovered. In practice about 9 tonne of distillate is produced with one tonne of steam. Scaling can be a problem, but not so relevant as for multi-effect distillation.
Multi-effect distillation (MED): distillation takes place in a series of chambers (or effects) operating at progressively lower pressures, thus ensuring that the temperature at which seawater boils is lower at each subsequent effect. Heath exchanger tubes in the first effect are heated by steam from a boiler or a turbine. Cold seawater is either sprayed or distributed over the surface of the evaporator tube in a thin film to promote rapid boiling and evaporation. Steam flowing into the tubes condensed (inside the tubes) into pure water. The seawater film formed outside the tubes, boils as it absorbs heat from the steam inside the tubes. The steam from the seawater is introduced into the heat exchanger tubes into the next effect. In this process usually there are 8-16 effects. Scale formation is the main problem that needs to be avoided. In the MED process up to 15 tonnes of distillate can be produced per tonne of steam.
Vapour compression (VC): seawater, preheated in a heath exchanger by the outgoing streams of concentrate and freshwater, is sprayed onto the heat exchanger tubes. It boils and partially vaporises. The vapour produces in drawn up into the compressor where it is compressed, raising its saturation temperature. The vapour condenses inside the heath exchanger tubes and releases its condensation heat to evaporate the preheated and recycled seawater outside. The capacity of vapour compression plants depends is limited by the size of the compressors. The units are typically used for small-scale applications in remote locations.

Membrane technology includes:

Reverse Osmosis (RO): RO represents today the fasted growing segment of the desalination market. The hearth of any RO system is a semi-permeable membrane that allows the fluid that is being purified to pass though it, rejecting a high percentage of unwanted constituents. In RO systems the dynamic pressure difference must be greater that osmotic pressure in order to reverse the flow and force the water from the seawater side through the membrane to the pure water side. The permeate water flow is proportional to this difference. Pre-treatment of feed water is necessary to avoid fouling and/or scaling, which result in lower permeability and higher pressure required. Typically pre-treatment consists of filtration to remove SS and dosing with acid or acid/antiscalant to prevent precipitation, but it depends on the location. RO desalination membranes are made from a variety polymeric materials and can be in two possible configurations: hollow fibre or spiral wound. A pump is used to pressurise the feed water: the feed pressure is usually 2-3 times greater than the osmotic pressure. An important parameter in the design of a reverse osmosis system is the recovery rate, defined as the ratio of permeate flow to the feed flow.

In all types of desalting technology, saline water is separated into two streams: a freshwater stream with low salt concentration and brine or concentrate stream with a high salt concentration. Both distillation and membrane processes are widely used for seawater desalination, and RO is also applied for brackish and low salinity water. Electrodialisys (EC) is only suitable for fresh or brackish water. Energy consumption in RO and ED for brackish and low salinity water is much lower than in distillation processes. About 50% of the newly installed seawater desalination capacity is based on reverse osmosis technology. The remarkable growth in this technology is due to the lower energy consumption, lower specific investment cost, shorter plant construction time and easy capacity extension of membrane systems compared with distillation systems.

Plants for thermal desalination require large amounts of thermal energy in the form of steam and electrical energy (2-4 kWh/m³). They are generally coupled to power plants where steam is available to provide thermal energy. The total energy consumption of desalination systems is high compared to RO systems. RO systems require only electrical energy (3-6.5 kWh/m³). Significant improvements in energy recovery and RO membranes have recently reduced this energy to about 2.5-3 kWh/m³.

Dual desalination plants (thermal distillation + power plant) are not convenient in rural and small towns situations, because the high rate capital cost against production volumes. Membrane desalting technologies are not common in small communities water supplies because of the required mechanical parts and sensitive membranes. These technologies can be considered in situations where sufficient fresh water cannot be found and and water has to be supplied by trucks from far away. In such situations, desalinate water may be cheaper. Before deciding to opt for desalination technology, all other water source options should be reviewed for long-term feasibility and sustainability.

Disinfection

Disinfection is the destruction or complete inactivation of harmful micro-organisms present in the water. It is considered the last stage in water treatment. In small communities it can be applied at household level, if the population is scattered, or at a central level in communities with an high population density. Disinfection can be physical or chemical.

Physical disinfection:
At family level the two physical disinfection methods most used are:

Boiling: it is very effective because it destroys pathogenic micro-organisms. Problems are the taste of the water, often not good and that it takes a long time for the water to cool.
Solar disinfection: solar disinfection uses pasteurisation that works on a time/temperature relationship to destroy water germs or pathogenic organisms. Solar disinfection depends on many parameters, such as latitude, altitude, time of the day etc, so it never gained popularity.

The most practical physical method that can be used at central level is ultraviolet irradiation. Low-pressure mercury lamps are used to produce UV radiation with a wavelength of 254 nm, which has a disrupting effect on the DNA of micro-organisms and viruses, killing them in a short time. The effective intensity of the radiation depends on the condition of the water: parameters as turbidity, organic matter, iron and manganese content strongly affect the disinfection, since these elements absorb UV radiation. This UV systems are very simple to use and maintain, nevertheless they are not jet used in developing countries. This may change in the coming years.

Chemical disinfectants
Chemical are used to destroy micro-organisms. The chemical used in developing countries should be easy to handle, transport and store, destroy germs and pathogenic micro-organisms, dissolve rapidly in water, provide a residual effect and be easy to detect and measure. They obviously should not be toxic be and they should not impart any bad taste, colour or odour to the water. The most common disinfectant used in water treatment are:

Ozone: it is becoming very widespread in industrialized countries, having the advantage not to give any objectionable taste or colour to water. Ozone is not really suitable in developing countries because of its high installation and operational costs, the need of proper maintenance and operation and the need for a continuous power supply.
Iodine: it is widely used for individual water supplies and for small water batches, but it has not gained widespread used in water treatment at a more comprehensive stage (communities or municipalities). The main reason are that it is suspected to be harmful for iodine-sensitive people if used on a long term, it is much more expensive than chlorine (up to three times) and it is very volatile in aqueous solutions.
Bromine: It is a more effective than chlorine and iodine throughout the pH range, but it has not been widely used as disinfectant for drinking water. This is due to the fact that it is not easy to manipulate, it is not found Since there is little experience with respect to its application, its use for small water supplies is not recommended.
Potassium permanganate: it is very effective, but it leaves stains in the container, so it is not recommended for community water supplies.
Silver: it has germicidal properties and it is not harmful for humans if in low quantities (20- 75 microg/LT). It is directly dosed from solutions or by direct electrolysis. The drawbacks are that is not so quick, it is not a very good virucide, and it is very expensive, even ten times more expensive than low-cost disinfectants. Click here for more information about copper-silver disinfection.
Chlorine: water disinfection by chlorination was perhaps the most important technological development in the history of water treatment. Its introduction together with filtration in developing countries have considerably increased the life expectancy. The characteristics that make it highly valuable are its broad-spectrum germicidal potency and its good persistence in water distributions systems, the simplicity of the equipment needed for its dosage, and its cost-effectiveness and availability even in remote locations. The substances in the family of chlorine most commonly used for water disinfection are chlorinated lime, high concentration hypochlorites and sodium hypochlorite. Click here for more information about chlorine as disinfectant. The amount of chlorine added to the water is called dose, normally measured in mg/l. Residual chlorine is the amount of chlorine left after a contact time of generally 30 minutes. WHO recommends for a proper disinfection a concentration of residual chlorine between 0.5 and 5 ppm. It is important that this upper limit is not exceeded.

Disinfection systems: small community water systems should be designed for simple operation and management by local people. Households may collect the disinfectant in liquid form from a central distribution point and use it to disinfect their own containers. The chemical can be brought from outside or produced locally through a generator. It is important to clean and disinfect always new tanks and pipes before they are brought to use. New wells and small water bodies used for water supply should also be disinfected to avoid bacteria contamination. It is often not easy to feed the disinfectant at a constant rate. If the water bodies are open it is better to apply disinfection at household level.

Water transmission

Water transmission refers to the transportation of the water from the source to the treatment plant and to the area of distribution. It can be realized through free-flow conduits, pressurized pipelines or a combination of the two. For small community water supplies pressurized pipelined are most common, since they are not very limited by the topography of the area to be traversed. Free-flow conduits (canals, aqueducts and tunnels) are preferred in hilly areas or in areas where the required slope of the conduit more or less coincides with the slope of the terrain. Routes need always to be checked with community members.

To design a water transmission system it is important to know the water demand on the maximum consumption day and the number of hours the transmission may operate each day. Pressure is relevant only for pressurized pipelines. Consumer connections to transmission pipelines should be limited, so the pressure can be kept close to the minimum. A few meters water column is required to prevent intrusion of pollution through faulty joints or parts. Long distances or specific topography can cause high pressures. To limit the maximum pressure it is possible to separate the routes in two sections using a break-pressure tank. Construction of surge tanks, air vessels or water towers as well as the selection of proper material can prevent the development of critical pressures like water hammers. The velocity in water transmission lines should be included between a minimum to prevent water stagnation and a maximum to control head losses and to reduce the effects of water hammer. Pressurised transmission mains generally have a flow included between 1 and 2 m/s.

If the water has to be transported over long distances and/or higher elevations, transmission by pump is required. The pumping head is the sum of the static head plus the friction head loss for the design flow rate. The head loss for a specific flow rate can be calculated for different pipe diameters. The diameter chosen should represent the least-cost choice taking into account the capital investment, the maintenance costs and the energy costs for pumping. In small community water supplies often a considerable head is required. The pump most frequently selected are the centrifugal (radial-flow) type wet-pit pr dry-pit type. The wet-pit type has the pumps immersed in the water, the dry-pit has the pump in a dry room separated by a wall.

The selection of the material has a strong influence on the investment cost. Ductile iron and steel are the strongest pipe materials and are the best choice when high pressures are expected. It is anyway recommended to keep the maximum pressure low in pipes because the costs of valve and fittings increase considerably for higher pipe pressure classes. Compared with metal pipes cement asbestos pipes are lighter and easy to handle. They are widely used in sizes up to 300 mm. Asbestos has no carcinogenic effect when used to supply drinking water, but it is dangerous if inhaled. For this reason alternative materials have been introduces, like PVC, PE or DI. PVC pipes have a good resistance to corrosion and are easy to joint, but they loose strength when exposed to direct sunlight for long periods. High density PE is very suitable for small diameter pipes because it can be supplied in coil, reducing the number of joints needed. Polyethylene does not deteriorate when exposed to direct sunlight. For pipelines of diameters up to 200 mm PVC and PE are preferable if high working pressures are not expected. If low pressure can be maintained these pipes are can be used also for diameters up to 500-600 mm. Cast iron, ductile iron and steel are used only for large diameter mains or where very high pressures are required.

Water distribution

Water distribution systems transport water from the source or the water treatment point to the users. The flow varies greatly from the peak hours, when a lot of water is used for washing or drinking, to the minimum consumption hours, at night. In small community distribution systems it is better to built a balancing storage tank, because the supply of diesel or electricity to pumps is usually unreliable. In small communities water is generally supplied for domestic and household water requirements, including irrigation is some cases. A minimum pressure of 15 m of water column is generally sufficient to avoid intrusions of polluted seepage water.