Document(s) 4 of 19

Abstract

Policies on wastewater use have tended to focus on treatment before use and the implementation of strict regulations. But in many low-income developing countries untreated urban wastewater is used for irrigation. Clear policy guidelines on how to optimise the benefits and minimise the risks of this practice are lacking. A better estimate of the extent of wastewater irrigation is needed before the reality of its use can become an agenda item for policy and decision-makers. Secondary data and results of nationwide assessments should be aggregated to obtain a global estimate of use. For this, a common typology of wastewater use is needed that would need to address such aspects as: the direct use of urban wastewater versus the dilution of urban wastewater by natural surface water before use, the relative contributions of domestic wastewater, industrial effluent, and stormwater to urban wastewater, the extent to which the wastewater is treated, and the use of wastewater in formal irrigation schemes, or as informal irrigation by smallholders without external support or control. A typology of wastewater irrigation and a database structure for consolidation of results are proposed. It is intended that this should be developed into a framework for a global assessment of the extent and importance of wastewater irrigation.

Introduction

With the increasing scarcity of freshwater resources that are available to agriculture, the use of urban wastewater in agriculture will increase, especially in arid and semi-arid countries. The major challenge is to optimise the benefits of wastewater as a resource of both the water and the nutrients it contains, and to minimise the negative impacts of its use on human health. From the environmental aspect there are potentially positive and negative impacts that should be considered. International guidelines for use and quality standards of wastewater in agriculture exist (Mara and Cairncross, 1989). These standards can only be achieved if the wastewater is appropriately treated. Because of high treatment costs, most cities in low-income developing countries will not have wastewater treatment facilities in the foreseeable future. However, while the use of untreated wastewater has become a routine practice in most developing-country cities, policies on its use have not taken this reality into consideration. Such policies range from active enforcement of legislation that totally prohibits the use of untreated wastewater, to turning a blind eye. Clearly, there is a need for better-informed decision-making.

To put wastewater use onto the international policy agenda there is a need to describe the importance of wastewater for integrated water resources management (IWRM), agricultural production, and to the livelihoods of poor urban, peri-urban, and rural populations. At present there are no clear estimates of the extent of irrigation with urban wastewater. Some people say it is an insignificant source of water for agriculture because the amounts of water diverted to cities and later disposed as wastewater are small in relation to the amount of water needed for agriculture in most developing countries. Others claim that worldwide, more than 20 million ha are irrigated with urban wastewater, and that wastewater has an important impact on agricultural productivity and livelihoods.

The International Water Management Institute (IWMI) proposes to lead a collaborative global assessment of the extent of wastewater use and has already initiated nationwide assessments in Vietnam and Pakistan. By linking up with other interested international and national institutions, a global database will be built that will be accessible in the public domain. This Global Assessment of the Extent of Wastewater Irrigation is linked to the Global Irrigated Area Mapping proposed by IWMI (Droogers, 2002), and the CGIAR’s Comprehensive Assessment of Water Management in Agriculture (CGIAR, 2001a) which is a key component of the knowledge base for the Dialogue on Water, Food and Environment (CGIAR, 2001b).

This chapter aims to promote a common understanding of the characteristics of wastewater and its use in order to provide a framework for a global database of wastewater irrigation.

Definition of Wastewater

Definitions and concepts of wastewater are given in various reports and textbooks (Metcalf and Eddy, 1995; Westcot, 1997; Asano and Levine, 1998; Martijn and Huibers, 2001). In this report it is assumed that urban wastewater (Fig. 2.1) may be a combination of some or all of the following:

• Domestic effluent consisting of blackwater (excreta, urine and associated sludge) and greywater (kitchen and bathroom wastewater)

  

  Fig. 2.1. Urban wastewater components.

• Water from commercial establishments and institutions, including hospitals
• Industrial effluent
• Stormwater and other urban runoff.

The actual proportion of each constituent within any given urban sewage load will vary due to spatial and temporal differences. For instance, monsoon climatic patterns will have a marked effect by diluting wastewater during heavy rains with the converse effect during hot and dry summers when there is more evaporation.

In irrigation sometimes the term marginal quality water is used. This refers to water whose quality might pose a threat to sustainable agriculture and/or human health, but which can be used safely for irrigation provided certain precautions are taken. It describes water that has been polluted as a consequence of mixing with wastewater or agricultural drainage (Cornish et al., 1999). It can also include water with a high salt content. Such water can also be considered wastewater in the context of this chapter, but is not included in the Pakistan and Vietnam national assessments mentioned above.

The Need for a Typology of Wastewater Use

All kinds of variations in wastewater use are possible and it is to be expected that different uses will have different impacts on agricultural productivity, the environment, and human health. Appropriate policy decisions and technical interventions are likely to depend on the nature and characteristics of the wastewater and the way in which it is being used. A typology that can effectively capture these characteristics is required to ensure that those involved in this field are aware of the important differences that exist, and are able to identify where a given research finding, policy instrument, or technical intervention will and will not find relevant application. Cornish and Kielen (Chapter 6, this volume) propose a framework describing wastewater sources and use. The search for a single, all-embracing definition that says what is included and what is excluded from the notion of wastewater irrigation appears futile. Rather, a typology or a classification of the most common forms of wastewater use in irrigation must be developed. It is important that such a typology can be readily understood by all those involved in building the global database. Obviously, a typology that is so complex and sub-divided that every single situation requires a separate definition should be avoided. Instead, a certain minimum number of basic ‘types’ need to be agreed. Once a typology is agreed upon, then it is possible to debate, which ‘types’ of wastewater irrigation will be included and which excluded from the global assessment.

Typology of Wastewater Use

The following three types of wastewater use are the most relevant (Fig. 2.2):

Direct use of untreated wastewater is the application to land of wastewater directly from a sewerage system or other purpose-built wastewater conveyance system. Control exists over the conveyance of the wastewater from the point of collection to a controlled area where it is used for irrigation (Westcot, 1997). The irrigation source is wastewater that is directly taken from the sewerage system, or from stormwater drains that carry large sewage flows. An example of this situation is that found in Haroonabad, Pakistan, where untreated wastewater from a sewerage outlet

Fig. 2.2. Basic types of wastewater use.

is directly disposed on land where it is used for vegetable production (van der Hoek et al., 2002). Another type of such use is when numerous informal irrigators draw water directly from the sewers or open drains, upstream of the site where disposal or treatment occurs. For example, this happens in Nairobi, Kenya, where farmers block sewers deliberately causing them to overflow (Cornish and Kielen, Chapter 6, this volume).

Direct use of treated wastewater is the use of treated wastewater where control exists over the conveyance of the wastewater from the point of discharge from a treatment works to a controlled area where it is used for irrigation. Many countries in the Middle East make use of wastewater stabilisation ponds to remove pathogens from wastewater. The effluent from the ponds is used for irrigation. To describe such a situation the term reclaimed water is often used, meaning water that has received at least secondary treatment and is used after it flows out of a domestic wastewater treatment facility. It must be noted that in many cases wastewater can only be considered partially treated to the design standard because the levels of wastewater production far exceed treatment capacity.

Indirect use of wastewater is the planned application to land of wastewater from a receiving water body. Municipal and industrial wastewater is discharged without treatment or monitoring into the watercourses draining an urban area. Irrigation water is drawn from rivers or other natural water bodies that receive wastewater flows. There is no control over the use of water for irrigation or domestic consumption downstream of the urban centre. As a consequence, many farmers indirectly use marginal quality water of unknown composition that they draw from many points downstream of the urban centre. In other cases the water is abstracted at one or two well defined sites for use in a formal irrigation system. An example of indirect use of untreated urban wastewater is found in Kumasi, Ghana, where large parts of the urban development have no operational sewerage or drainage network. A river passes through the urban centre and is progressively polluted by diffuse urban runoff. The water from this polluted river is abstracted by many users at many points downstream of the urban centre (see also Cornish and Kielen, Chapter 6, this volume).

Asano and Levine (1998) make the distinction between wastewater reuse which is the beneficial use of reclaimed (treated) wastewater and wastewater recycling, which normally involves only one use or user, who captures the effluent from the user and directs it back into the use scheme. Please note the assumption in this description that it is always treated wastewater. Wastewater reuse implies that the wastewater is used a second time. In fact, it is the water, not the wastewater that is being reused. Wastewater use therefore seems to be a better term than reuse, because the wastewater is generally used only once. Wastewater use can take place at the household level or off-site when there is a sewerage system.

As wastewater use can be defined as the deliberate application of urban wastewater for a beneficial purpose, it is in most cases planned, either by state agencies or farmers. However, there are also situations where natural rivers passing through cities become so heavily polluted with wastewater that they become de facto sewers. Asano (1998) describes the diversion of water from a river downstream of a discharge of wastewater as an incidental or unplanned reuse. Asano states that indirect reuse normally constitutes unplanned reuse whereas direct reuse normally constitutes planned reuse. There are important exceptions to this definition. For example, the effluent from the As-Samra treatment plant in Jordan ends up in an irrigation scheme after dilution in an intermittent stream locally known as a wadi and in a reservoir (McCornick et al., Chapter 14, this volume). Although the scheme was never planned to use wastewater, it is clearly an irrigation scheme with planned development and managed by an irrigation agency that levies water tariffs. Along the Musi River in India irrigation schemes controlled by the Irrigation Department depend primarily on urban wastewater from the city of Hyderabad (Buechler, Chapter 3, this volume).

The distinction between planned and unplanned use does not seem to be of much practical relevance for the typology. Instead, it is suggested that the typology should indicate the main reason for use of the wastewater by farmers. In many cases the wastewater supply is more reliable than other sources of irrigation water, or it may even be the only source of water that is available to farmers. In other cases it is the nutrients in the wastewater that make it attractive to farmers.

Another distinction that is often made is between formal and informal use. The concepts of formal and informal irrigation are, to some extent, synonymous with planned and unplanned irrigation. Formal irrigation could refer to the presence of an irrigation infrastructure or to a certain level of permission and control by state agencies. In most cases this will apply to a single point type of abstraction. If the abstraction of wastewater is at numerous scattered points, then it is unlikely that there is an irrigation infrastructure, and probably no control by state agencies, hence the wastewater use is informal.

Nationwide Assessments

The question is, ‘Can the proposed typology be meaningfully applied at the national level? While there are only limited data available on the extent of wastewater irrigation, the salient features of wastewater use in some countries and the applicability of a typology can be described.

Pakistan

Pakistan has a rapidly growing population, that is expected to increase from 139 million in 1998 to 208 million in 2025. By that time, about 50% of the population will live in urban centres. In almost all towns in Pakistan that have a sewerage system, the wastewater is directly used for irrigation. IWMI has made a nationwide survey of wastewater use in Pakistan and the results indicate that 32,500 ha are directly irrigated with wastewater (Ensink et al., 2004). A negligible proportion of this wastewater is treated and no clear regulations exist on crops that can be irrigated with wastewater. Vegetables are the most commonly irrigated crops, because they fetch high prices in the nearby urban markets. The wastewater used for irrigation is valued by farmers mainly because of its reliability of supply. In some cases the wastewater is auctioned by the municipal council to the highest bidder, often a group of richer farmers who then rent out their fields to poor landless farmers. Under these conditions, the use of untreated wastewater is considered a win-win situation by both the authorities that are responsible for wastewater disposal and the farmers who get a reliable supply of water with high nutrient content. There are therefore very few incentives to invest scarce resources in wastewater treatment.

India

The situation in the semi-arid parts of India is not much different from that in Pakistan, except that industrial effluent probably plays a bigger role. India has a population of one billion people (as of the 2001 census), with a population increase of 181 million during the 1990s alone. More than 28% of this population lives in cities with a percentage decadal growth in the urban population at 31%. Strauss and Blumenthal (1990) estimated that 73,000 ha were irrigated with wastewater in India. Surely, the typology used to obtain this estimate must have been different from the one used for China, where Mara and Cairncross (1989) estimated 1.3 million ha were irrigated with wastewater. Most wastewater irrigation in India occurs along rivers, which flow through such rapidly growing cities as Delhi, Kolkata, Coimbatore, Hyderabad, Indore, Kanpur, Patna, Vadodara, and Varanasi. Many of the Indian peninsular rivers would not have much or any flow during most of the year if they were not used to funnel wastewater away from cities to peri-urban and rural areas. In such cases this can hardly be considered disposal in surface waters as it is, in fact, disposal in a natural conveyance channel. Along the rivers the water is diverted via anicuts (weirs) to canals and often to tanks and then channelled to the fields for irrigation. If such uses were included, a much higher figure than 73,000 ha would be obtained, since for the area along one river, the Musi in Andhra Pradesh alone there are approximately 40,500 ha irrigated with wastewater.

Vietnam

Vietnam has a centuries-old tradition of using human waste in agriculture and aquaculture. Hanoi and other cities in the Red River delta have natural ponds that collect wastewater and drainage water from cities. These ponds are used for aquaculture and as sources of irrigation water, and also play an important role in flood control. While there are hardly any conventional treatment facilities, the natural ponds are likely to provide at least some purification of the wastewater. The ponds generally discharge wastewater directly into irrigation canal systems and rivers. Wastewater from city drains is also pumped into irrigation canal systems at certain times of the year, and at locations where there is insufficient irrigation water. Ongoing IWMI research in Vietnam shows that the area irrigated directly with urban wastewater is limited, but that indirect use after passage through natural ponds is widespread.

Mexico

Mexico accounts for about half of the 500,000 ha irrigated with wastewater in Latin America. Much of the recent scientific work on health impacts and other aspects of wastewater use has been done in Mexico. In most cases the wastewater is used at some distance from the urban centre in a formal irrigation setting. The bulk of the untreated wastewater from Mexico City goes to Mezquital, immediately north of the Mexico Valley where it is used for irrigation via an extensive network of irrigation canals. This is probably the largest and longest-standing wastewater use system in the world.

Jordan

In Jordan most of wastewater from urban areas is treated and used in agriculture. The As-Samra plant is one of the largest wastewater treatment plants in the world. It is a wastewater stabilisation pond system, consisting of 32 ponds occupying 200 ha and serving about half the population of the country. The benefits of this system have been well described. For example, aubergine yield under trickle irrigation with the effluent from the system was twice the average Jordanian aubergine production under freshwater irrigation using conventional fertilisers (Al-Nakshabandi et al., 1997). This could be considered direct use of treated wastewater. However, much of the effluent is transported over long distances and is blended with rainwater stored in a reservoir. So indirect use of treated wastewater also takes place. A second point is the effectiveness of the treatment plants. Some treatment plants are clearly overloaded and the effluent from such plants could at best be called ‘partially treated’ if it is directly used. The effluent that is transported over some distance from overloaded plants receives a form of additional unintended natural treatment. There is no information on water quality from nationwide assessments so it is suggested that a very simple categorisation that includes ‘treatment, but largely dysfunctional’ is a possibility.

Global Database

Initially the database generated from the proposed global assessment of wastewater irrigation should provide estimates of the national and global areas irrigated with wastewater. As the database expands and more results of nationwide surveys become available, the possibilities for further analyses should be explored. For example, the area irrigated with wastewater in a country and the crops grown could be related to the total area irrigated and total agricultural production. A further step would be to estimate the impact of wastewater use on agricultural production, the economy in general, and livelihoods. Different scenarios could then be developed and their impact on agricultural production and the economy modelled.

Table 2.1 suggests a basic set of data requirements for the global database. Primary data collection will only be possible in a limited number of countries and the level of detail is therefore determined to a large extent by the availability of secondary data. To avoid the diversity of real-life situations being squeezed into a rigid format, any city-level description would need an additional description to the standard item scored.

Table 2.1. Proposed database outline for a global assessment of the extent of wastewater use in agriculture.

A reality check of data on the extent of area irrigated with urban wastewater can be obtained from a few typical scenarios that could apply to most countries. For example, assuming an annual rate of irrigation of 500 mm and per capita sewerage production of 100 l/day, a city of one million people would produce enough wastewater to irrigate an area of 7000 ha using efficient irrigation methods (Strauss, 2001).

Table 2.2 provides an overview of the information on the extent of wastewater irrigation that is currently available from a limited number of sources.

Limitations of the Typology

The proposed typology, like every typology, has limitations. It clearly focuses on those situations where (part of) cities have a conveyance system for wastewater, either sanitary sewers or stormwater drains that carry large sewage flows. There are, of course, many cities that do not have purpose-built sewers or drains. These obviously have a serious sanitation problem, but one could argue that for them the issue of wastewater use does not arise. Certain well-known types of wastewater use such as informal backyard (on-site) use of wastewater will have to be excluded from a global assessment because data are unlikely to be available. To document such practices, detailed case studies are likely to be more relevant than a global assessment. Certain types of on-site use are receiving increasing attention. These include the use of greywater, community-controlled decentralised wastewater disposal and use systems, and ecological sanitation. Obviously, in the countries where nationwide surveys can be organised, more details of wastewater use, on-farm conditions, and characteristics of the irrigators (men, women, children, socioeconomic status, ownership of land, land and water rights, etc.) can be collected.

Indirect wastewater use implies that there is a certain retention time and that certain processes take place before the water is used for irrigation. These include a certain die-off and removal of pathogens from the wastewater before its final use by the farmer. After a period of retention and at some distance downstream from the urban centre it is expected that the water quality improves, to the extent that it should no longer be called wastewater. However, there are at present no criteria to distinguish between: river water of good quality, polluted river water, and wastewater. In fact, the alternative to direct use of untreated wastewater is often the disposal of this wastewater in natural rivers and the two would be expected to have opposite effects on surface water quality. The disposal of untreated wastewater in rivers is an environmental problem, while one of the advantages of direct use of wastewater is that environmental (water) pollution is reduced.

Conclusions

In the foreseeable future, many towns in developing countries will continue or expand the direct or indirect irrigation of crops with untreated wastewater. Current government policies focus on regulation of wastewater use and wastewater treatment and are unable to offer practical solutions to the users. An important input into more realistic policies on wastewater use is information on the area irrigated with urban wastewater at national and global levels. Such macro-level estimates can only be obtained when there is a common understanding of the different types of wastewater use.

Table 2.2. Information currently available on the extent of wastewater irrigation from a limited number of sources.

Table 2.2. Continued.

Table 2.2. Continued.

Table 2.2. Continued.

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Document(s) 4 of 19