|Technologies for the Provision of Basic Infrastructure in Low-income Settlements (HABITAT)|
|V. Appropriate infrastructure technology for low-income settlements|
Providing a water-supply system for a community involves tapping the most suitable source of water, ensuring that the water will be tit for domestic consumption and supplying it in adequate quantities.
1. Water quality and quantity
As indicated above, water-quality standards for domestic consumption in developing countries generally follow closely or are the same as the WHO International Standards for Drinking Water. However, it has often been suggested that these standards might be difficult to achieve under certain conditions, where there is only one source of water and the cost to treat the water to the required standard is unaffordable. While it is possible that the human body can develop tolerance to physical or chemical elements in the water, it is not advisable to assume that it can also acquire immunity to water-borne diseases. However, some authors and countries are proposing and adopting bacteriological water quality standards less stringent than those of WHO, with the object of eliminating the need for final disinfection. In rural areas particularly, the complexity of chemical dosing and the cost of chlorinating chemicals will make the chance of maintaining continuous disinfection rather small, and the health risk of accepting upgraded but unchlorinated water supply must be balanced against the problems and costs of maintaining continuous chlorination. Supplying slightly polluted water without disinfection will also incur additional increased costs for storage and boiling water in the home, and these should be offset against the cost of ensuring a completely safe supply. The new WHO Guidelines take these factors into account and provide realistic objectives in terms of the level of pollution which affects human health in an unacceptable manner. Flexible approaches to bacteriological standards have been followed, for example, in the Cameroon Community Development Project and in China.
Standards for the quantity of water to be supplied cannot be uniform over different regions, since they are subject to particular physical and socio-cultural conditions. Water demand or water consumed also depend on the type of service provided. Thus, in areas with household connections, the demand will tend to be higher than in settlements provided with communal water taps. It has also been confirmed that the distribution of communal taps and the walking distance from any household will have an effect on the level of water consumption. However, designers should be aware that, beyond a certain point, any increase in the number of communal water taps will not have a significant effect on consumption levels. Many surveys of water consumption and use have been carried out in several parts of the world, and based on these it is possible to estimate that average levels of demand in lowincome settlements range between 20 and 40 lcpd for communal taps with maximum walking distances of 200 metres and from 40 to 60 lpcd for household connections. The demand for multiple household taps is likely to be slightly higher.
2. Water sources
Water is available in the atmosphere, underground, in rivers and lakes and in the seas In theory and in practice, any of these sources can be tapped for domestic water supplies, but the cost implications must be considered.
Rainwater harvesting and condensation of water from the atmosphere
Water in the atmosphere can be tapped through the mist or air moisture or through rain-catchment. Chile and Peru are carrying out investigation condensation of to recover water held in coastal fogs quite common in that region. Pilot structures for the catchment of air moisture have proved the feasibility of this technology where other water sources are not available. Other countries, such as China, Cyprus and Kenya. have traditional techniques for moisture catchment, but much work needs to be done on this technology before cost effective systems can be developed. The interception of rainwater before it reaches the ground has the advantage that the water may be collected with minimum contamination. The amount of water which can be collected is determined by the amount of rainfall and the size of the collection area The use of roofs for water collection is widespread in developing countries with regular rainfall, and, under satisfactory climatic conditions, collection of rainwater from roofs can supplement other water sources at minimal cost.
Some people object to the use of rainwater because of its contamination with bird droppings, dust and other deposits on the roof. This objection can be overcome by the installation of simple devices to separate the first flush of water from the remainder to be stored. In countries such as China, courtyards have been used to collect rainwater, but, in these cases, there is the risk of contamination, and the water has to receive some type of treatment before its use for domestic purposes.
Ground catchments are ideal for collecting surface runoff, and, if this technique is adopted, the ground is usually compacted and often coated with a non-toxic material to reduce water loss through seepage. This type of collection requires a degree of protection of the catchment area to prevent gross pollution of the water.
Rainwater is normally stored in depressions or specially built containers (traps), and the collected water can be treated using silt traps and sand filters prior to storage. There is a whole array of above-ground and below-ground reservoir designs, and their suitability for a particular situation must be decided on the basis of quality and cost. Water harvested through adequate catchment arrangements does not normally require disinfection. The WHO International Reference Centre (IRC) in The Hague has produced a training module on rainwater harvesting for drinking-water supply.
Spring tapping and protection
Springs with a regular flow of water constitute one of the most economical sources of water for domestic use. In the Latin American highlands, springwater supplies account for a large percentage of rural water supplies and they are also common sources in Asian and African countries. Since most springs produce uncontaminated Groundwater there is no need for disinfection or any other treatment. Civil works consist basically of concrete or masonry structures to intercept the water at the point at which it surfaces, and the same structure is designed to protect the water from external contamination, avoid clogging of the spring "eye" and intercept silt or other material which might otherwise pass into the water-distribution system. Spring catchment units require little maintenance that cannot be done by unskilled personnel. However, the yield from springs is normally small, and their use for the supply of water to large settlements is not feasible in most cases.
Groundwater is normally free of bacteriological contamination and, although its chemical characteristics vary from one source to another, its use does not present a health hazard, unless there are extremely high concentrations of dissolved solids or specific elements. Groundwater is normally abstracted from wells or infiltration galleries, and the type of well and the technique adopted for its construction are dictated by ground conditions, characteristics of the aquifer and costs.
Large-diameter excavated wells normally have a circular section and are lined with masonry or concrete, or have wooden walls. Such wells are adopted where it is not possible to obtain or maintain the drilling and pumping equipment necessary for small-diameter wells, where it is desired to use a type of water-raising system requiring more space than is available in a small-diameter well, where there are cheap labour and local skills, where the aquifer is not very deep, or where it is necessary to store water in low-permeability aquifers. When compared with small diameter wells, large-diameter wells have the disadvantages of requiring long construction periods, posing safety hazards during construction and operation, being easy to contaminate and giving generally low production rates.
There are many construction techniques and types of equipment for small-diameter wells, sometimes called tubewells. The elaborate methods requiring sophisticated drilling equipment and skilled labour will not be dealt with in this review. A summary of methods for drilling small-diameter wells is given in table 11.
Wells, whatever the type, have to be located and protected to minimize the risk of pollution through direct access of contaminated surface water, seepage of contaminated water from the surface, infiltration of polluted groundwater, access of refuse to the well shaft, or unsanitary water-drawing systems. Only rarely will groundwater require treatment, and, if it does, this will mainly take the form of disinfection. Low-yield wells serving low-density human settlements normally have the water abstraction and distribution system at the surface, immediately above or beside the well. Under these conditions, the criteria for their operation follow similar principles to those for communal water taps, in addition to the physical factors mentioned above. High-yield wells normally require supply reservoirs and piped distribution systems, and require the installation of sophisticated pumping equipment.
Table 11.Summary of methods for drilling small-diameter wells
The systems for drawing water from wells can be classified as: hand devices using buckets or similar containers; hand-operated pumps; or non-hand-operated pumps. Hand devices using buckets are used in large-diameter wells. Despite the fact that several systems have been developed to avoid direct contact between the bucket and the drawer of water, the risk of polluting the well water is still high.
Hand pumps can easily be installed on either large-diameter or small-diameter wells to deliver water above the wells. Most of these pumps are of the constant-displacement reciprocating-piston type and, therefore, are easy to manufacture, install and maintain. Hand pumps are widely used in developing countries, and many, including Bangladesh, India, Malawi and the United Republic of Tanzania, are implementing large rural water-supply programmes based on this technology. In general, hand-purmp programmes in Latin America are not as intensive or as well developed as those currently being implemented in Africa and Asia. There are many designs for hand pumps, some better than others, but the basic characteristics of a successful hand pump are that it can be manufactured and maintained locally, requires a minimum of maintenance and is durable. The important principle when implementing a hand-pump programme is to ensure that there will be the institutional capacity (if possible, with community participation) and financial resources to take care of the inspection and maintenance of the pumps. A pump design should be chosen on the principle that parts which wear out often should be inexpensive and easy to replace by unskilled personnel. The World Bank has supported a programme of testing hand pump designs to arrive at a ranking of available pumps on the basis of village-level operation and maintenance (VLOM) criteria.
There are several combinations of non hand-operated pumps and complementary driving systems or power sources. First, pumps can be of the constant-displacement type (reciprocating piston pumps, rotary pumps and helical-rotor pumps) or of the variabledisplacement type (centrifugal pumps, jet pumps etc.). All these types have their advantages and disadvantages and a set of situations or uses for which they are most suitable. Most of them, at present, are not manufactured in developing countries, tend to be costly and require skilled personnel for their maintenance - all factors limiting their use in developing countries. The type of energy available is one of the factors affecting the design of pumping units, and, apart from human labour, the main sources of energy are wind, solar energy, conventional electrical supplies and intemal-combustion engines.
Wind is a cheap source of energy worthy of consideration in small water-pumping systems. Windmills have a relatively high capital cost, but recurrent costs are low, and the level of technology is suitable for low- income communities with unskilled labour Solar energy is a promising alternative to traditional energy sources especially in regions with good solar irradiation Energy from the sun is captured and transformed into electrical energy through the use of arrays of photovoltaic cells. The only drawback with this renewable energy source is the cost of the equipment and the experimental nature of many of the systems designed to harness it Both wind and solar energy sources are subject to atmospheric factors in their operation, and, since the reliability of wind and solar energy is based on average statistical factors. water supplies depending on these forms of energy should be designed with safety factors which are mainly reflected in the storage of water. Electrical motors and internal-combustion engines have been used for many years, both in developed and developing countries, but, in spite of the fact that they are not considered to be sophisticated equipment, they still require skilled personnel and institutional back-up for their maintenance. internal-combustion engines require more maintenance than electric motors and must always be attended by an operator. Another consideration is their high initial cost, as well as the costs of spares and fuel or electricity. When studying energy sources for a water-pumping system, it is important that the economic comparisons consider the whole water-supply system and not only the driving units of power source.
Fresh surface water
At present, surface water constitutes the main source of water for domestic and industrial consumption. Water from rivers and lakes has a greater chance of being polluted than the other sources already considered, and, therefore, surface water for domestic consumption almost always requires some form of treatment.
Techniques for surface-water catchment are not necessarily complicated but they normally involve civil works, and thus technical supervision is often required. One way of reducing the pollution load when abstracting surface water is to construct at infiltration gallery under the bed of or alongside the body of the surface water.
Sea water has a dissolved-solids concentration of approximately 3 to 4 per cent (30,000 to 40,000 mg/l). Because of its high salinity, sea water cannot be used directly for drinking purposes or for preparation of food. However, it can be used for other domestic tasks such as flushing toilets and washing clothing and utensils.
This is now practiced in places such as Fiji and Hong Kong and could be introduced in other coastal settlements to supplement scarce freshwater sources.
Water with high salinity can be treated to drinking water standards, although, of the several processes available, most involve sophisticated equipment and are beyond the economic and institutional capacity of the majority of developing countries.
Desalination can be accomplished in at least five different ways: distillation; freezing, reverse osmosis; ion transport; and chemical methods.
Of these methods, only distillation so far offers the possibility of using a low-cost technology. Several countries have experimented with and used solar stills to treat sea or brackish water, including Australia, Israel, Mauritania, the United States of America, and several North African countries. A solar still consists basically of a basin in which a black porous plastic wick floats, with a sloping cover of lightweight glass sealed at the edges. The sun's rays pass through the transparent cover with little reflection or absorption and are converted into heat when they strike the black surface. Water evaporates from the wick, and the vapour diffuses to the cover where it is condensed to liquid which drains into a trough. The main disadvantages of solar stills are the low yield of distilled water per unit area and an absolute dependence on weather conditions. Solar stills have, however, proved successful when supplying individual households or small communities, owing to their low cost and relatively simple maintenance requirements. For large settlements, the required area of evaporation, extensive pipework and complex maintenance make solar stills an inconvenient choice.
3. Water treatment
Need for treatment
One of the criteria for selecting a water source is the quality of the water, and designers tend to favour sources that require a minimum of treatment. As mentioned before, groundwater or water from springs usually requires little or no treatment at all. In most cases, treatment is reduced to disinfection and, occasionally, to the removal of iron and manganese by aeration. With surface water, the situation is quite different: suspended matter, dissolved inorganic matter.
Organic matter causing colour and taste and, finally, pathogenic and disease-carrying organisms are typical contaminants. These undesirable materials should be removed, taking as criteria health impacts, costs and the effect of treatment on consumer demand. The first decision to be taken by the designer of a water supply is whether to treat the water or not, taking into account the facts that water treatment tends to complicate the operation of a system and to increase costs. For these reasons, treatment processes should be included only when strictly necessary to bring the water to realistic, affordable and potable levels of quality.
Simple covered storage can help in the removal of the schistosome larvae (bilharzia) in developing countries where schistosomiasis is a problem To achieve this. water should be stored for two days and protected from further external contamination In medium sized water supply systems however, protected tanks of this capacity might not be feasible Large storage reservoirs are normally designed as part of impounding systems but they are mainly unprotected and subject to pollution However, open reservoirs have beneficial effects in reducing the concentration of suspended solids and pathogenic micro-organisms. The "treatment" capacity of storage units should be evaluated for each particular case, even though the storage volume is primarily determined by water-flow regulation considerations. and "treatment" is an added benefit.
Sedimentation is normally adopted as pre-treatment before filtration.The main objective is to eliminate solids by plain gravity sedimentation or, when this is not possible. encourage the formation of large settlable solids by the addition of coagulants to the water Sedimentation in itself is a simple process but requires specially designed basins The addition of coagulants represents a substantial increase in capital and maintenance costs. because it is necessary to buy the chemicals. store them and provide facilities for their dilution dosage and mixing In addition the correct dosage of chemicals as a function of the flow or volume of water to be treated and water quality must be determined. These characteristics of raw water are constantly changing during the normal operation of a treatment plant requiring the regular assessment of water quality and corresponding changes in chemical dosage. It has been observed in many rural treatment plants in Africa and Latin America that chemical dosages are not applied correctly, leading to wastage of chemicals or to low quality of the water supplied The sludge collected in sedimentation basins has also to be disposed of by a sanitary method thus creating additional problems in larger water undertakings or where there are not suitable places for studge disposal
There are several ways to overcome some of these problems but, unfortunately they are of very restricted application. In certain rural areas of India and Africa. plant coagulants are used instead of chemicals, thus reducing the cost of coagulant and its storage and, in addition reducing the amount of sludge produced. Sedimentation basins can be increased in size and made less expensive by the use of local building materials and appropriate building technologies, and the same can be said of the mixing and dosing unit
Another option its the complete elimination of sedimentation basins by adding coagulants directly before filtration This technique requires non-conventional filter systems which will be discussed later Finally, under certain conditions sedimentation basins card be replaced by coarse media pre- filters or roughing filtration Even so, for large water supply situations, there is no well-tested alternative to treatment by coagulation, and the problems of operation also remain
Technical literature normally classifies filters into two categories - slow sand filters and rapid filtration units. Slow sand filters (SSF) are widely used in rural settlements all over the world. They can eliminate turbidity and at least 99 per cent of the bacteria and viruses in water, through a process of mechanical straining and biological activity in the surface of the filter. Among the main advantages of SSFs are: low capital cost and easy construction; simple maintenance and low recurrent costs; no energy requirement; acceptance of reasonable variations in raw-water quality; elimination of water wastage; and, in most cases, elimination of the need for disinfection of the effluent. The cost of an SSF can be reduced by the use of low-cost building materials and techniques or by the use of innovative technologies such as the filter media in the two-stage filter, developed at the Asian Institute of
Technology in Bangkok, which has been installed in the Philippines, Thailand and Viet Nam. 3 In this filter, the first stage (coconut fibre) serves as a roughing filter' and the second stage (burnt rice-husks) is similar to an SSF. Coarse-media pre-filters can be used to replace sedimentation using coagulants, and research on this system has proved that 60-70 per cent of the suspended solids and 80 per cent of the conform organisms in raw surface water can be removed.
In rapid filtration units (RFU), water passes through a bed of sand or other filter medium by gravity or under pressure. These filters rely on mechanical straining for the removal of suspended solids and some organisms. When the filter medium becomes clogged, it is cleaned by backwashing. This system requires skilled personnel for its operation, is relatively expensive, involves fairly elaborate civil works and pipework, and requires the use of mechanical equipment such as backwashing pumps. Backwashing water represents a substantial amount of the production - approximately 4 per cent - and, in places with scarce water resources, this might not be acceptable. There are many types of RFUs which can be used, depending on the direction of flow, type of filter medium, system of backwashing, system of filter control (flow or heady, type of pre-filter treatment etc. The current level of technical development of RFUs makes them unsuitable for water treatment to supply low-income rural and urban settlements in developing countries, where the community must provide operation and maintenance.
The disinfection of water aims at the inactivation of pathogenic micro- organisms and the provision of residual protection to the water against possible contamination during its transport from the point of treatment to the consumer. There are several techniques for disinfection, with the application of chemical disinfectants (especially chlorine compounds) being the simplest and, so far, the cheapest method for consideration in supplying water to low-income settlements in developing countries. Simple chlorinators which dose chlorine compounds at constant rates can be made using very simple technology but they need to be calibrated properly for efficient use. Disinfection using chlorine in pure gas form requires complex equipment and presents problems in the acquisition, transporting and storage of chlorine. Further, its use is not appropriate without skilled personnel and adequate institutional back-up.
Disinfection is mainly applied to surface waters, and it is essential for turbidity to be reduced to acceptable levels if disinfecting chemicals are to be effective.
Small-diameter wells do not normally require disinfection, but, it it is decided to disinfect, this has to be done at certain points in the storage or distribution system.
So far, there is no reliable and efficient method discovered for disinfecting small- diameter wells equipped with a hand pump. For large-diameter wells, experiments have been conducted with the "pot chlorinator". This consists of a porous or perforated container which contains bleaching powder (any other dry chlorine compound can also be used) mixed with sand. The container is immersed under water in the well, and the chemical is released slowly. The use of these chlorinators has proved successful in countries such as China and India, but gross contamination of shallow wells will not be controlled using this technique.
If it is decided not to treat or only partially to treat polluted water before distribution to the consumer, it is obvious that water must receive household treatment, if health risks are to be avoided. The objectives and principles of household treatment are similar to those in water treatment facilities. The planner should acknowledge that, by transferring the treatment function to the household, he is also transferring costs. The clarification of or removal of turbidity from water at the household level can be achieved by filtration, storage, coagulation or sedimentation.
Traditional filtration is practiced in several countries of the developing regions - e.g.. Bangladesh, Ecuador, India, the Islamic Republic of Iran, and Sudan. The filtration process normally involves passing water through sieves, cloths, vessels of dry or porous stones, or plant material. In some areas of Nigeria and Sudan, water is stored for five months before use, allowing for sedimentation and natural purification. Short-term storage, for 12 to 24 hours, to allow sedimentation is also practiced in countries such as Kenya as a pre-treatment for further purification.
There is a great variety of plant material that can be used to produce coagulation, but this treatment is only feasible for small volumes of water where there is a constant supply of appropriate raw material.
4. Distribution system
When water is not supplied to the consumer directly from the source, it is necessary to construct a water-distribution system that will bring water close to every household. Since the distribution network is one of the most expensive items in a water-supply system, the standards for design and service level should be carefully studied and chosen for every situation. The choice of service level, which can range from a simple water main and a few communal water taps to a complex network with multiple house connections. must take into account: the quantitative and qualitative water demand; the investment and running costs; the ability of the consumers to pay the real price for the water; the financial resources available for investment or subsidies; and the capacity available for managing the installations.
In most instances, national governments or local authorities have standards and codes of practice which regulate the design and construction of the different components of a water-supply system. Most of these standards are extremely conservative, specifying levels of quality in materials and equipment that make low-cost projects unacceptable. It is essential that current codes of practice, which fix required storage volumes, minimum pipe pressures, consumption peak factors etc., be revised to allow flexibility and leave room for modification or variation, based on the study of individual physical and socio-economic conditions for a particular water-supply system. Several methods have been devised to optimize the service level, the standards and the technical parameters of a water supply system as a function of cost and user-convenience. Low-cost design or technology is not synonymous with a low-quality product, and planners should strive to achieve the correct balance between these considerations.
An appropriate water-distribution system is the result of careful decisions regarding service levels, use of cost-effective materials and techniques, and design of the different components of the distribution system. The basic knowledge to achieve this goal is available, but what is needed is film political and institutional commitment to apply it in the project-implementation process. Currently, common knowledge and practice indicate that water supply through communal water taps (or standpipes) is cheaper than through individual house connections. Thus, a very simple standpipe for use in rural areas should cost $20-$50, and a standpipe for urban use, with concrete support construction, two to four taps and a platform with a drainage facility, is likely to cost $200-$500. 2 However, data from 27 countries show that standpipes are very often over-designed and cost much more than commonly estimated. Consequently, attention should be paid to the design, cost, location and level of service to be provided from standpipes. The relatively high cost of some standpipe systems implies that it should be possible to upgrade the water-supply service to house connections, thus making it possible to achieve, with only a marginal increase in cost, better cost recovery (tariff collection), less water wastage, less risk of contamination, and saving in productive time dedicated to water collection.
Finally, special attention should be given to existing water-distribution facilities when upgrading low income settlements. It is well known that water wastage in the distribution system sometimes amounts to a substantial proportion of the produced water. Investment in the repair of leaks and control equipment can often increase the amount of water available to consumers at a minimal cost.