10. Urban water supply

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Contents

1. Scope

2. Environmental impacts and protective measures

2.1 Overview
2.2 Water abstraction

2.2.1 Groundwater
2.2.2 Surface water

2.3 Conveyance and treatment of raw water
2.4 Piped distribution
2.5 Consequential impacts of urban water supply projects
2.6 Environmental protection measures and recommended options

3. Notes on the analysis and evaluation of environmental impacts

3.1 Limits and guidelines in Germany and other industrialised countries.
3.2 Other national guidelines
3.3 Rating of environmental impacts

4. Interaction with other sectors

5. Summary assessment of environmental relevance

5.1 Appraised water resources and multi-sectoral use
5.2 Evidence for efficient water use in existing or planned urban water supply schemes coupled with efficient disposal
5.3 Curative measures for inefficient water use in existing urban water supply schemes and inefficient disposal.
5.4 Important planning considerations for environment-orientated urban water supply projects

6. References

1. Scope

What is meant by urban water supply is facilities for meeting the water requirements of an urban population, of the public sector, and of trade and industry. The distribution of the water may take place via either distribution systems (piped supply) or non-piped supply points (e.g. wells).

In many countries the term "urban" is not necessarily related to the size of the community in question and for that reason the type of supply is defined as follows

In the context of development efforts, consumers in groups 2) and 3) above should be accorded priority treatment, as also should consumer group 1) where there are plans for them to be connected to the piped supply. To the figures in the overview table must be added allowances, considerable in some cases, for wastage and for the water losses that affect the majority of existing piped supplies. The figures considered for sizing the component parts of an urban water supply system should be peak demand figures (e.g. at the day and hour of maximum demand). In many countries, it is only seldom that allowances for water supplied for fire-fighting (the peak measured demand) will enter into the calculations.

Water abstraction breaks down into the following sub-divisions:

- abstraction from groundwater sources,
- abstraction from surface water sources.

Hybrid forms of abstraction should also be allowed for:

- abstraction via bank river intakes using infiltration drains
- artificial infiltration with recovery.

The following are the component parts of the urban water supply layout:

- abstraction (wells, infiltration galleries, spring tappings, abstraction structures, storage basins/reservoirs)
- treatment (e.g. iron removal, chlorination, desalination)
- storage of the treated water
- distribution system (pipe network, long-distance supply facilities).

In the case of artificial infiltration with recovery, this layout has inserted into it at the upstream end the

- infiltration system (basins, recharge wells, drain ducts).

2. Environmental impacts and protective measures

2.1 Overview

What should be considered in connection with urban water supply are the environmental impacts both on the volume of water available and on the quality of the water.

In many countries, and particularly in zones of varying climate, the problem of water availability is beginning to take precedence over the problem of water quality.

As with the parts of the urban water supply system, impacts can be broken down into the following groups:

- impacts from water abstraction
- impacts from conveyance and treatment of raw water
- impacts from piped distribution.

In addition to the above, there are also secondary impacts in the form of

- consequential effects of an urban water supply system.

2.2 Impacts from water abstraction

2.2.1 Groundwater

The abstraction of groundwater will cause a change in the aquifer water balance and there are a large number of consequential effects that this may have. The balance is between

- inflow-side components (groundwater recharging from precipitation and surface water, subsurface inflow from adjoining aquifers, artificial infiltration) and
- outflow-side components (outflow to surface water, drains, abstraction intakes, etc.).

It is essential to remember that, due to hydraulic interaction, the changes caused by water abstraction in components on both sides of the equation may even be lasting ones (e.g. an increase in the inflow from adjoining aquifers).

Thought must also be given to the interaction between availability and use and between groundwater and surface water. Greater use of surface water may cause a reduction in infiltration into the subsoil, and the remaining volume of surface water may be more heavily polluted in particular ways. The consequence may be an increasing requirement for groundwater use (2.2.2).

The environmental impacts that a change in the components contributing to the balance may have are:

a) Quantitative depletion of groundwater resources

Increasing quantitative depletion of groundwater resources results from:

- increasing consumption of drinking water due to a growing population and an improvement in the standard of the supply
- more rearing of livestock
- increasing demand for industrial water (for trade and industry)
- wasting of water
- water losses from defective distribution systems.

Other factors that need to be taken into account are ones that lead to a temporary or permanent reduction in groundwater resources, such as declines in precipitation in aquifer watersheds (due to deforestation, steppification). It should also be borne in mind that, under the traditional urban water supply strategy, it is peak demand that has to be met, but that this demand often occurs in the dry season. The high consumption during dry periods and the vast water losses from some piped systems, only a proportion of which is returned to the groundwater, then give rise (seasonally) to a particularly severe depletion of groundwater resources.

b) Long-term changes in groundwater quality

These may be caused by:

- mobilisation (leaching out) and subsequent spread of previously immobile pollutants
- increases in flow velocity (e.g. in natural gypsum beds or man-made pollutant deposits)
- changes in groundwater flow (resulting in interception of charges that previously flowed harmlessly away, inducement of infiltration from contaminated surface waters)
- inducement of widespread infiltration from overlying or underlying groundwater storeys in which groundwater quality is poorer.
- entry of pollutants due to the use of fertilizers and pesticides
- intrusion of salt water into aquifers close to coasts
- deterioration in groundwater quality caused by seepage of untreated waste water from open, unsealed roadside ditches, leaking sewers or poorly built cesspits, or by seepage of pollutants and toxins from liquid industrial and commercial waste.
- charging with minerals from irrigated areas, caused by the high evaporation in arid and semi-arid areas and subsequent entry into the groundwater as a result of periodic mobilisation.
- leakage of pollutants from storage depots and transport systems for liquid and mineral products.

c) Localised and extensive lowering of the water table

In the case of groundwater abstraction, lowering of the water table is inevitable for hydraulic reasons. However, the size and physical distribution of the lowering will depend on local conditions, e.g. the positions of the wells, the structure and nature of the aquifer, recharge conditions. Typical consequential impacts of water table lowering are:

- drying up of ecologically important wetlands,
- reduction in soil moisture content (field capacity), with plant- specific impacts on plant cover (change in the natural and cultivated flora, e.g. steppification) and with consequential effects on the fauna,
- total depletion of groundwater resources during sustained dry spells (drying up of wells),
- drying up of springs and watercourses,
- soil settlement.

The environmental impacts of water table lowering are usually less severe where there was a low water table (> 10 m) even before abstraction.

Environmental protection measures to minimise the harmful effects of groundwater collection will be concerned mainly with selection of suitable locations for wells and the design and modes of operation of the wells. The adverse impacts of excessive abstraction of groundwater may also be mitigated or prevented by efficient use of water, by seasonal control of water consumption (rainy season vs. dry season) and by introducing and operating systems of consumption-dependent tariffs and charges.

To boost the efficiency of environmental protection measures in dealing with the impacts of groundwater abstraction, it will be necessary not only to carry out the appropriate hydrogeological reconnaissances and to assess the total water balance (groundwater and surface water), but also to provide continuously operating measuring and monitoring facilities, the purpose of which will be:

- to ensure an ongoing improvement in evaluations of and statements on questions of hygiene and hydrogeology,
- to watch for changes in the groundwater supplies (volume and quality) by constantly checking groundwater levels, groundwater quality and volumes of groundwater abstracted,
- to keep a constant watch for waste of water of all kinds and for water losses from piped urban water supplies, by means of continuously operating measuring facilities (district water consumption, consumption from stand pipes and domestic connections), and to take action to counter both of these (by repairing damage in good time, by tariff setting and by penalising the wasting of water),
- to apply restrictions on the water allocation to other, competing user groups in order to ensure that a supply is available for human beings (emergency supply),
- to put in hand rehabilitation work on existing parts of the urban water supply system (to replace defective water mains and domestic service pipes, faulty taps and cocks, and overflowing cisterns and domestic storage tanks),
- to monitor the efficient execution of rehabilitation work by checking the results.

2.2.2 Surface water

The use of surface water will cause a change in the water balance and this, as in the case of groundwater abstraction, may have a wide range of consequential impacts. What will need to be considered in this case are two-way effects between surface water and groundwater availability and use. Also of importance are the following factors:

- In some regions, more surface water may become available in the future, due for example to changes in the (micro)climate (such as an increase in precipitation due to the effect of artificially created reservoirs), to an increase in surface runoff caused by changes in the vegetation in the surface water catchment area (deforestation), to building over (roads, buildings) producing greater surface runoff, or even to the discharge of (cleaned) wastewater from towns and smaller communities into the surface water.
- In other regions, a climate-related decline in precipitation may occur, and in this way surface water runoff may be reduced and the quality of the water degraded, in which case the situation will be even worse in countries where surface water is not available throughout the year anyway.
- Increasing abstractions from flowing waters (by means of river intakes) will cause a reduction in the availability of water in many regions, particularly at periods of low water, and in the self-cleaning action of the body of water and in infiltration into the subsoil.
- If demand for water increases and the quantity available in flowing or dormant surface waters is reduced and at the same time the quality of the water is degraded, a requirement often arises for water to be brought into the region in need from remote areas or for the demand for water to be met from more plentiful or less plentiful groundwater resources. In particular borderline cases, emergency situations may arise, i.e. where supply of even the minimum amount of water required for the human population can be guaranteed only at high cost.

a) Quantitative depletion of surface water resources

The demand-side components listed in 2.2.1 are likely to cause an increase in the use of surface water. Account should also be taken of climatic changes and changes in the vegetative cover in the watershed, as these may result in some regions in reductions in the amount of surface water available or in an adverse time-related runoff distribution (greater runoff at periods of high water with a higher charge of suspended matter and sediment, but lower runoff at periods of low water).

What is often lacking for checking the volume of runoff, the extent of the resources and the volumes abstracted is an adequate network of measuring stations within the watersheds (for precipitation) and at particular points on the bodies of water (for level), and expert staff to analyse the measurements and monitor the multi-sectoral use of surface water resources and to draw up water balance sheets (for ground and surface water) and water management plans.

b) Changes to ecosystems caused by water abstraction

Relatively large reductions in flow, particularly at times of low water, may have impacts on all the ecological processes in a body of water or watercourse and on its shores or banks. Biotopes of value to the landscape or ecology may be adversely affected or even totally destroyed; under certain circumstances the ecological equilibrium, with its balanced variety of floral and faunal species, may be altered. However, such impacts only occur when the abstraction of water, measured against the total flow, is substantial, i.e. such that an ecosystem no longer receives its minimum water requirement. Also, the impacts of water abstraction are, as a rule, not spread over a wide area but (depending on the topographical situation) confined to small areas (strips along banks and shores, floodplain meadows).

c) Intrusion into the water supply of unknown or undetected hazardous constituents

The use of surface water to provide a water supply is fundamentally a problem of water quality. In properly designed treatment plants, suitable monitoring facilities ensure that safe feed into the distribution system is possible. However, a risk of damage to health and impacts on hygiene may occur if pollutants remain undetected in the water, the pollutants being for example the result of uncontrolled discharge of substances into the water. The pollution may possibly take the form of a concentrated dose of a discharge which at other times is continuous and relatively harmless (e.g. when toxic pollutants are being drained off). Another risk is that, due to their low detectability, constituents may evade the existing monitoring and testing facilities. Substances that are difficult to detect in this way include a range of industrial solvents that are considered to be carcinogenic even at extremely low concentrations if continuously ingested by humans. Where there is a risk of exposure to such pollutants, the requirements to be met in water-protection zones must be particularly stringent, as also must the checks made in the zones, and provision must also be made for sensitive early-warning measuring devices to be introduced in stages and for embargoes on abstraction to be applied.

In the event of surface water abstraction, the following protective measures need to be borne in mind:

- the introduction of suitable measuring and monitoring systems to keep a watch on water levels, runoff volumes, charges of sediment, sand and suspended matter, chemical, physical and biological water quality, and pollutant charges, and also to monitor an extremely wide diversity of parameters applicable to ecosystems in the watersheds,
- collection and analysis of the data acquired by the measuring and monitoring systems, and preparation of hydrogeological appraisals,
- collection and analysis of hydrogeological data, including continuous measurements made at observation and producing wells in regions where use is made of both groundwater and surface water resources, with the object of producing water budgets to show the volumes of water available for use and of checking that distribution conditions are being complied with.
- monitoring of water quality and of the self-cleansing action of surface waters,
- analysis of data to allow the introduction in good time of protective regulations, statutory provisions to safeguard resources, and conditions governing supply in emergencies,
- appraisal of existing uses of surface water, for the purpose of preventing harm to persons further downstream as a result of fresh abstractions of surface water and/or the discharge of used water,
- the prevention of waste of water, the introduction of restrictions on water allocated, and the execution of rehabilitation work on the drinking water distribution system (see section 2.2.1 on Groundwater).

2.3 Conveyance and treatment of raw water

When raw water is conveyed in open channels, and particularly when it is withdrawn from contaminated or hygienically unsatisfactory surface waters, it can be expected that health problems will arise as a result of illicit use of the raw, contaminated water and of human beings coming into contact with it in other ways.

In the course of treatment of raw water, adverse environmental impacts may arise due to incorrect plant operation (inattentiveness by operating staff, absence of alarm devices) or as a result of, for example, the disposal of sludge from settling basins, of filter cakes, and of chemicals from stocks held (e.g. disposal of old stock), excessively high doses of chemicals (e.g. chlorine), and the disposal of alkali concentrates used in desalination processes.

Factors of significance in connection with the treatment of raw water are therefore the efficiency of the treatment process, the operation of the monitoring and alarm facilities, and the possibility of gearing treatment to the seasonal variation in the quality of the raw water. Another factor that has an important bearing on the possibility of achieving proper treatment of the water (meaning the pumping and pretreatment of the raw water, metering in of chemicals, flocculation, filtering and disinfection, and analysis) and on the possibility of guaranteeing hygiene in treatment plants is the standard of training of the staff employed in such plants.

Environmental protection measures that may be envisaged are as follows:

- measures to prevent access to systems conveying raw water for the purpose of extracting water for use (as drinking water) by humans, and/or warning the population of the dangers of using contaminated water,
- codes governing the quality of the discharge from treatment plants, with due consideration for the seasonal capacity of receiving waters and the rights of use and expected requirements of persons further downstream,
- installation or retrofitting of environmental protection facilities in water treatment plants, such as detention basins, sprinkler systems for chlorine stations, secure storage for fuels and chemicals.
- installation of measuring and monitoring facilities for monitoring water flowrates and quality and for reporting incidents in the course of water treatment (e.g. damage to a tank of chlorine gas).

2.4 Piped distribution

Where the environmental relevance of distribution lies is in the following impacts:

a) Due to the poor technical standard of the urban water supply system in many countries and particularly the poor technical standard of the distribution pipes (inferior materials and bad laying as a result of mistaken low-cost policies), the incidence of defects is very high in buried pipes. In industrialised countries, the average incidence is 0.2 to 0.3 defects per km per year, whereas in other countries figures of up to 9.1 defects per km per year have been found.

Water losses from dilapidated distribution pipes are often many times greater than consumption.

b) Simply due to high water losses, it is often the case that the capacity of urban water supply plants is exceeded well before they achieve their designed output to consumers. It then becomes impossible to maintain a 24 hour supply and an intermittent supply is introduced.

c) When the supply is interrupted at times (intermittent urban water supply), the consequent lack of outward pressure allows contaminated water to makes its way into the distribution network through fractures in buried pipes, the contaminated water coming for example from ditches carrying wastewater, leaking roadside channels carrying wastewater, leaking sewage pipes, defective/overflowing settlement basins, badly designed dumps for waste and toxic materials, etc. This constitutes a risk to the state of health of the population.

d) Water may become foul due to stagnation in runs of pipe where the hydrodynamics of the system are poor or in clean-water tanks in the distribution system through which there is insufficient flow.

e) Contamination of the water in dilapidated distribution systems is often so bad that the water, despite being heavily disinfected (e.g. by high chlorine dosage rates) at the input to the distribution network, becomes so contaminated with organic matter on its way from the input to the consumer that there is a permanent health risk.

The following are suitable measures which can be applied to minimise the impacts of piped distribution:

- critical assessment of the techniques for reducing water losses developed in industrialised countries and adaptation of these techniques to meet the particular circumstances in the country and the special requirements that exist (e.g. use of leak detectors on pipes where the pressure is low, quantitative determination of water losses from intermittent water supplies, execution of measurements by district metering to determine water losses in distribution districts only sparsely equipped with gate valves and hydrants).
- introduction of appropriate measuring and monitoring systems and pipe network improvements (e.g. installation of essential gate valves) to allow a constant watch to be kept on water consumption, water waste, illegal extraction of water, and water losses by monitoring the supply being fed to distribution districts and the pressure within the districts and to check the effectiveness of improvements to the pipe network (reductions in water losses, etc.).
- monitoring of the incidence of defects in the distribution districts in the urban water supply system.
- establishment of priorities for the permanent upgrading of the distribution system in the urban water supply system (early detection and repair of defects and rehabilitation or replacement of sections of the pipe network where there is evidence of a high incidence of defects etc.).
- improvement of the standard of materials used and the standard of the laying work in the distribution system.
- introduction of a continuous water supply (meaning adequate 24-hour pressure in the pipe network) once the distribution system has been upgraded.
- monitoring of the bacteriological quality of the water (e.g. for excess chlorine) at the consumer connections/stand pipes.

2.5 Consequential impacts of urban water supply projects

The purpose of an urban water supply system is to distribute reasonable quantities of water of a satisfactory hygienic standard to consumers. Using good drinking water eliminates the health risks that occur because the water being drunk is unhygienic. However, any rise in water consumption also means an increase in wastewater arisings and thus, in the absence of appropriate provisions for disposal, a greater potential danger to health posed by an increase in water-borne diseases.

In the current state of the art, 100% of the water from an urban water supply system is produced to good drinking water quality standards, whereas in fact only 5 to 15% of the water needs to be of drinking water quality. It is therefore on cost grounds as well that it is important to make sparing use of drinking water. By introducing suitable (consumption-related, cost-covering) tariffs and possibly even by having separate distribution networks for drinking and other water it will be possible to achieve sparing, efficient use of hygienically acceptable water.

A special problem is posed by the unhygienic treatment of the water as it is being transported from stand pipe to user and its unhygienic storage in homes, and/or by defective domestic installations (e.g. defective roof tanks) that pose a permanent threat of disease.

Adverse consequential impacts of urban water supply projects arise mainly as a result of errors and shortcomings, such as:

- shortcomings in the quality of the materials used and in the standard of work done,
- shortcomings in operation, maintenance and rehabilitation,
- overrunning of the designed capacity of the urban water supply system as a result of waste and losses of water,
- shortcomings in the instruction given to the population and particularly to women, who often bear responsibility for hygiene-related matters such as transporting water, storing water in the home, cleaning, and the preparation of food.

A frequent source of dissatisfaction among consumers is a decline in the standard of the supply caused by defects. Grievances of this kind then lead to increasing unwillingness to pay bills and hence to a falloff in income from the sale of water and, what is more, to such things as lack of interest in motivating and instructional campaigns (to involve the population, to promote efficient use of water and to provide education in hygiene and health).

There are special demands that the planning and execution of maintenance and rehabilitation measures, based on the collection and analysis of data and information, must be expected to meet. This applies particularly to non-visible parts of the water supply system such as buried pipes. Serious mistakes are often made here, such as replacing old pipes (i.e. ones more than 50 years old) when the incidence of defects in old pipes of this kind is often lower than in pipes laid in the past 20 years.

In many cases new water abstraction and treatment works are built before dilapidated drinking water distribution networks have been upgraded.

One basic consideration that should be borne in mind is that the consequential impact of a proper urban water supply system will be beneficial to the state of health of the population when it is not simply waste water disposal but solid waste disposal, housing conditions, and food hygiene etc. too that are improved with the aim of producing a permanent effect on the state of health and living conditions of the population. The following aspects also deserve special attention in this connection:

- changing the population's traditional attitudes to the scarcity and importance of water as a resource (water is not a "free" commodity),
- enlightening and involving target groups, and particularly women, with respect to the costs and value of a proper urban water supply system and improved sanitary conditions and what they can expect from them.

To minimise the consequential effects of projects in the field of urban water supply, all the facilities should be planned, constructed, operated and maintained to a standard appropriate to local conditions and in line with the current state of the art. There must be a guarantee that the operation of water supply systems (for water abstraction and distribution) can be maintained for the full 24 hours in order to prevent any contamination of the water being distributed. It must be ensured that the water distributed is used sparingly, either by introducing and making active use of metering and monitoring facilities and/or by bringing into force appropriate tariffs and charges commensurate with the sparing use of water.

At the same time, provisions for waste water disposal and other sanitary provisions will also need to be made.

By the proper maintenance and rehabilitation of existing water supply facilities, and particularly of buried water pipes with their known susceptibility to defects, it will be possible both to reduce water losses and to prevent consumer dissatisfaction (caused by disruptions to the supply resulting from frequent repair work and by an intermittent supply) plus any related drop in income from charges for water.

Other essential prerequisites for preventing adverse consequential impacts are as follows:

- the introduction of measuring and monitoring systems for logging flowrate and pressure parameters and for the early detection of defects in water supply systems (distribution networks),
- the introduction of measuring and monitoring systems for monitoring the quality of the drinking water being distributed,
- the involvement of the population, and particularly women, in a very wide variety of watchkeeping tasks such as reporting defects (leaks) and water waste, and the giving of instruction in good hygienic use of water (containers for carrying water, the carrying itself, and storage of water in the home),
- the systematic introduction of improvements in systems which are to be integrated into new systems in the future,
- the introduction of efficient operating and maintenance systems,
- the planning of expansions from a practical point of view,
- the avoidance of past errors and of the uncritical acceptance of techniques from industrialised countries.

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