S. Braatz and A. Kandiah
Susan Braatz is Forestry Officer in the Forestry Resources Division of FAO.
Arumugam Kandiah is Senior Officer in the water Resources Development and Management Service of FAO.
The steady increase in the amount of water used and waste water 1 produced by urban communities and industries throughout the world poses potential health and environmental problems. Countries are seeking safe, environmentally sound and cost-efficient ways to treat and dispose of waste waters. At the same time, increased attention is being focused on the role that forestry, traditionally a rural-based sector, can play in improving the urban and pert-urban environment. One opportunity to combine these two concerns is the use of municipal waste water (both sewage and industrial effluent) to irrigate forests, forest plantations, greenbelts and amenity trees. This article discusses some of the experiences to date and various issues related to the use of waste water for forest and tree irrigation. Although the use of sewage sludge on forests and trees both for the purposes of waste disposal and fertilization has also been adopted successfully in many places and merits increased attention, it is beyond the scope of this article.
1 In this article, waste water,, refers to the liquid waste discharged from homes, commercial premises and industrial plants to individual disposal systems or to municipal sewer pipes. Sometimes the term is used to mean surplus irrigation water after it has flowed through the irrigated fields, hut this definition is not included here.
The use of waste water for irrigation is probably as ancient as humankind's cultivation of land. Large-scale, controlled waste water use for irrigation, however, only dates back to last century when so-called sewage farms ² were established in parts of Europe, Australia, India and the United States for the purpose of disposing of waste water and preventing river pollution. Although agricultural crops were produced on these farms, crop production was a secondary consideration. There are also examples of farms where tree plantations were irrigated with waste water, such as the El-Gabal El-Asfar farm located about 30 km northeast of Cairo where a tree plantation, originally of 200 ha, was established in 1911 to dispose of the city's sewage water. In the mid-1980s, the forest was converted to citrus, cereal and vegetable production. The farm by that time had increased in size to 1 260 ha.
² In some cases "sewage farms" refer to sewage treatment compounds, but the common use of the term connotes a farm established for the disposal of waste water through the irrigation of crops and sometimes trees.
During this century, and particularly within the past two decades, the practice of irrigating agricultural crops with municipal waste water has become more widespread, especially in arid and semiarid areas of both developed and developing countries. The controlled use of untreated and treated waste water in irrigation is now quite common in Europe, the United States, Mexico, Australia, China, India and the Near East as well as, to a lesser extent, in Chile, Peru, Argentina, the Sudan and South Africa (Bartone and Arlosoroff, 1987). In China, for example, over 1.33 million ha, mainly croplands, are irrigated with waste water. Mexico City's waste water use scheme is the largest in the world (90 000 ha of irrigated land) and waste water use is practiced throughout the country in most cities with a sewage system. Land treatment with treated sewage effluent is quite common in dry areas of the United States; for example, 7 to 8 percent of the total volume of municipal waste water produced in the State of California is being used for agriculture, landscape irrigation (golf courses, lawns, roadside plantings, etc.) and ground water recharge India had a reported 73 000 ha in sewage farms in 1988, many of which were not managed rigorously, however, and which consequently posed potential health and environmental problems. Over the past two to three decades, a number of schemes for the use of waste water in crop and landscape irrigation (parks, roadsides, school grounds, golf courses, etc.) have been launched in the Near East and parts of North Africa, both as a means to dispose of waste water effectively and to conserve and recycle water (see Box).
Purification of waste water in Tunisia.
Experiments are being carried out on use of this water to irrigate tree and food crops.
Whereas the use of waste water for agricultural and landscape irrigation is now quite widespread in many regions, waste water irrigation of trees has been much less explored and exploited. Shade and amenity (street) trees and urban green areas are irrigated with treated sewage effluent transported by tanker in some cities (e.g. Cairo, Tehran and others in the Near East, India and the United States) and the literature gives examples of effluent use in production forestry. For instance, some communities in Egypt use sewage or drainage water after primary treatment to irrigate woodlots. The most commonly used species are Casuarina glauca, Eucalyptus camaldulensis and Tamarix aphylla. These provide for local fuelwood consumption and the production of poles for sale on the local markets (El-Lakany, 1995). Overall, however, large-scale use of waste water for the irrigation of tree plantations or forests is still relatively limited and, where it is practiced, it is generally more for reasons of waste disposal and treatment than for enhanced forestry production. The next sections of this article consider the potential benefits of using waste water for irrigation in general, the particular benefits for trees and the various conditions under which it is feasible.
Waste water use in the Near East: Kuwait
Untreated sewage was used in Kuwait for many years to irrigate forestry plantations. Contents of septic tanks were transported by tanker and used on government-controlled forest plantations to which public access was prohibited. For example, two shelterbelts (of Tamarix aphylla, Eucalyptus camaldulensis and Acacia salicina) were successfully established, one along the Kuwait-Jahara road, the other around Jahara for environmental protection of the township (Armitage, 1985). A programme of sewage treatment and effluent use for irrigated agriculture was launched by the government in the 1960s. Subsequently, secondary and tertiary sewage treatment plants were built and research was conducted on crop production using the treated effluent. In 1977 the Ministry of Public Works began the preparation of a master plan fin the effective use of all treated effluent in the country until the year 2010. Implementation of the plan began in 1985. The first priority is the production of fodder and hay and vegetable crops. The second is the development of environmental protection forestry, and commercial timber production is to be developed if trials prove successful. The plan calls for the irrigation of 2 700) ha of intensive agriculture, 9 000 ha of environmental forest. (e.g. shelterbelts and sand dune stabilization trees) and 213 ha of commercial forest by the year 2010. The projected use of waste water by 2010 is 125 million m³, up from the 27 million m³ used in 1985 (Cobham and Johnson. 1985).
There are many potential benefits offered by the use of municipal waste water for irrigation purposes, including the safe and low-cost treatment and disposal of waste water; the conservation of water and recharge of groundwater reserves; and the use of nutrients in the waste water for productive purposes. The irrigation of trees may provide additional benefits. In areas where water is relatively plentiful, crop or forestry irrigation may be adopted primarily for treatment and disposal purposes. In arid and semi-arid zones, the issues of groundwater recharge and tree and crop production may be equally or more important.
Safe, low-cost treatment and disposal
If insufficiently treated or improperly disposed of, waste water constitutes a source of pollution and a health hazard. However, the cost of treatment using conventional methods (see Box) is high, prohibitively so for most developing countries. As a result, countries are experimenting with other forms of treatment, among which are land application methods, including irrigation. When practiced properly, these are simple, low-cost and effective means both to dispose of waste water and to improve its quality. Where waste water is already treated, this practice can further improve its quality at a low cost. When limited pretreatment is available, it may provide a means of disposal which poses the least risk of disease and environmental damage. The irrigation of trees is likely to pose fewer health risks and be more socially acceptable than the irrigation of crops.
Waste water treatment
Conventional of treating water are described here in order of intensity (and cost):
Primary treatment. This is a simple sedimentation process in which organic and inorganic solids are allowed to settle and can thus he removed from the water. This reduces the biological oxygen demand by 25 to 50 percent, the total suspended solids from 50 to 70 percent and the oil and grease content by 55 to 65 percent. Some organic nitrogen, phosphorus and heavy metals are also removed. Primary effluent may be of en acceptable quality for the irrigation of trees, orchards, vineyards, fodder crops and some processed food crops.
Secondary treatment. The most common level of treatment in industrialized countries, this method consists of further processing (after the primary treatment) to remove the rest of the organic matter and suspended solids using biological processes (i.e. metabolism by aerobic micro-organisms, mainly bacteria). Secondary treatment is required when the risk of public exposure to waste water is high (e.g. in the case of food crops). It is also required in many industrialized countries to prevent environmental pollution. Much of the nitrogen and phosphorus remains after such treatment, however and if the effluent is discharged into water bodies with an insufficient diluting capacity it may lead to water pollution.
Tertiary treatment. This is a more sophisticated and costly process which removes specific waste eater constituents such as nitrogen, phosphorus, additional suspended solids, heavy metals and dissolved solids. When public exposure to the effluent is high (e.g. when spray irrigation is used in public parks and golf courses), tertiary treatment is carried out to minimize the risk of disease. When irrigation using treated sewage effluent is intended, the removal of plant nutrients during the treatment process is a disadvantage in terms of plant productivity.
Disinfection. This is often carried out after secondary or tertiary treatment to kill viruses and other pathogens which may remain in the water. It consists in the addition of a chemical (usually chlorine) to the water.
Various non-conventional methods of waste purification are being adopted in many countries as alternatives or supplements to conventional treatment. These methods include natural biological treatment systems such as stabilization ponds and land treatment, including irrigation. They tend to be less costly and simpler to operate and maintain but effective in the purification of nutrients' other chemical constituents and pathogens. In some places, overland treatment is used for the purification of ran sewage while in others it is used for the upgrading of primary secondary or even tertiary effluent. Waste water irrigation of crops or trees is particularly attractive in the face of growing water demands and rising costs of artificial fertilizers. It represents a means of conserving water and nutrients and applying them for productive purposes. Irrigation and other land application methods, however do require considerably more land than in-plant (conventional processing, so a lack of available or affordable land may be a constraint in some urban and pert-urban areas.
Improved use and conservation
Water conservation is a key issue in arid and semi-arid areas. Often groundwater is the only source of water available and, in many places, reserves are being overlapped. Where waste water would otherwise be discharged into rivers and thus be lost from the local system, its use in irrigation contributes to groundwater recharge, thus maintaining it in the system and enabling its subsequent reuse.
When water supplies are limited, the general tendency is to allocate water for priority uses, i.e. for drinking and domestic use and later for agriculture. This is also true of waste water. For this reason, irrigated agriculture has received far more attention than irrigated forestry. Health hazards and cultural and aesthetic values, however, limit agricultural use of waste water (particularly of food crops) in some places and may make forestry a more appropriate option. In addition, under some circumstances irrigated forestry is economically competitive with irrigated agriculture and possibly even more profitable (Armitage, 1985). Integrating trees with irrigated agriculture in the form of windbreaks or boundary plantings, for example, may well be the most economically attractive option in many places.
Nutrients for crop production
Raw sewage and even secondary treated effluent are rich in mineral nutrients needed for plant growth (nitrogen, phosphorus, potassium and micro-nutrients). Experiments have repeatedly demonstrated an increased productivity of crops or trees when irrigated with waste water as compared with clean water. These nutrients represent a resource of considerable value when compared with the equivalent cost of fertilizer. The application of waste water at rates which ensure a balance between nutrient input and plant uptake will promote optimal plant growth while limiting the risks of pollution. Calculations have been made in Australia on nutrient inputs and use by tree plantations irrigated with waste water (CSIRO, 1995).
With the nitrogen content of an effluent at 10 to 30 mg/litre (average 20 mg/litre) and the phosphorus content at 4 to 10 mg/litre (average 7 mg/litre), and assuming an average annual waste water application rate of 8 000 m³/ha, the total annual input of nitrogen (N) is 160 kg/ha and of phosphorus 56 kg/ha. A young plantation growing rapidly can take up 120 to 150 kg N/ha and about 12 kg P/ha per year; therefore, sufficient levels of these nutrients will be available for potential maximum growth.
A balance is struck between input and uptake for nitrogen which, in nitrate form, is mobile and poses the greatest risk of leaching and contaminating the groundwater. Any excess phosphorus will be held in the soil and does not constitute a pollution risk.
The benefits of "greening" urban and peri-urban areas for environmental protection, amenity, recreation and production purposes are being increasingly recognized. Although all cities benefit from having trees in the urban landscape, the benefits are perhaps most obvious in the arid and semi-arid tropics where natural vegetation is sparse and protection against sandstorms and desiccating winds is needed, and where high temperatures make shade as much a matter of health as of comfort. In these zones, tree planting requires irrigation at least in the establishment phase, if not throughout the life of the trees. Cities which wish to increase forest plantations, greenbelts or amenity trees in and around the urban area but which are unable to justify the use of scarce freshwater resources for irrigation may be able to use waste water for this purpose. The irrigation of large stands forest plantations, greenbelts and urban green space - can contribute substantially to the objective of safe waste disposal and treatment. Because small stands or dispersed trees require relatively limited amounts of water, their primary benefits will come from the other environmental services they can offer (pollution and noise mitigation, shade, beautification) rather than from waste water disposal.
The use of municipal waste water for irrigating forests and tree plantations is still relatively limited, while sound, well designed and systematic research on the topic is even more limited. However, it is possible to give some examples of research which help illustrate the benefits of waste water-irrigated forestry and various conditions under which it is feasible.
A popular plantation irrigated with waste water in India
Pioneering studies on the application of treated municipal waste water on forest lands as a means of purification and groundwater recharge were carried out in central Pennsylvania in the United States from 1963 to 1977 (Sopper in FAO, 1978). Sewage effluent which had undergone secondary treatment (see Box) was sprayed on three different forest areas: a mixed hardwood stand consisting mainly of oaks (Quercus spp.), a plantation of red pine (Pinus resinosa) and a sparse stand of white spruce (Picea glauca).Different application rates were used, ranging from 2.5 to 15 cm per week over varying periods from 16 weeks in the growing season to all 52 weeks of the year. The chemical composition of the effluent was compared with concentrations in the soil and soil water. The results indicated that:
i) the controlled irrigation of forests with up to 2.5 cm/ha of effluent per week over the year can effectively filter out nitrogen, phosphorus and other constituents, rendering the water of an acceptable quality for drinking;
ii) at an application rate of 15 000 m³/ha per year (about 2.5 cm per week), 95 percent of the effluent applied is recharged to the groundwater reservoir; and
iii) nutrients in the effluent were responsible for an increase in tree growth (measured by diameter) of 80 to 186 percent. The research results demonstrate that treated municipal waste water can be recycled through forest ecosystems with the benefits of restoring the water to drinking quality, recharging groundwater reserves and increasing tree growth.
A model of waste water purification similar to that tested in Pennsylvania has been adopted in parts of Spain; for example, in communities along the Rio Cinca in the municipality of Monzón (Instituto Nacional pare la Conservación de la Naturaleza, n.d.; Navarro, 1977). Starting in 1955, reforestation with poplars (Populus euroamericana) irrigated with river water was carried out on 14 ha to stabilize the banks of the Rio Cinca and contain floodwaters. From the early 1960s, this plantation was irrigated exclusively with untreated waste water. In 1977 it was reported (Hernandez, 1977) that 7 km of river bank had been stabilized, the previous unproductive soils had been much improved, tree production was higher than had been predicted and the return on investment through cutting and sales of the timber was good. In 1983, faced with the need to expand treatment facilities, the municipality decided not to build a sewage treatment plant but instead to expand the system of land treatment with irrigated forestry plantations, which it called filtros verdes (green filters). By 1987, filtros verdes had been established along the river in four different municipalities on a total of 245 ha. The land needed for the irrigation scheme was calculated on the basis of at least 1 ha per 200 persons. By June 1995, 396 ha were being irrigated, with many more planned or in various stages of development. Each month, 1 800 m³/ha of irrigation water is applied. The irrigation water is a mixture of waste water and river water, with adjustments made during the dormant season in both the amount of irrigation water applied and the ratio of waste water to freshwater. This system has been found to be an acceptable way to dispose of waste water and, at the same time, to provide for very rapid wood production. Poplars, which were found to be the most suitable species in terms of nutrient uptake, ability to withstand flooding in the dormant period and economic wood production, are managed on a 12-year rotation for timber. This system is being adopted elsewhere in Spain.
Australia is also increasingly using land application as an effective way to dispose safely and productively of secondary treated waste water, whose discharge into rivers has been the cause of algae blooms and eutrophication in some places. Tree plantations are effective in removing nitrogen and phosphorus from treated effluent before it enters the water system and they offer additional benefits in the greening of Australia and the support of local forest industries. The area of tree plantations irrigated with effluent has increased from 500 ha in 1991 to about 1 500 ha in 1995. There are now more than 60 effluent-irrigated plantations varying in size from one to several hundred hectares (CSIRO, 1995). Despite the rapid increase in the area under effluent-irrigated plantations, however, much is still to be learned about optimal management of the systems to ensure environmental safety and maximize economic returns.
Systematic studies on the irrigation of forest plantations with secondary treated waste water are being carried out by CSIRO in a six-year research project (1991-1997) in Wagga Wagga in the Murray Darling Basin, New South Wales. Data are being collected on the rate of water use and nutrient accumulation by various tree species, seasonal variations in the trees' water use and growth, risks of groundwater contamination and the sustainability and economic viability of the plantations. The effectiveness of various water distribution systems is also being studied. Guidelines for plantation design, establishment and management will be developed based on the results of the research. Various species and end uses (sawlogs, veneer, chipwood, pulp) are being considered to determine which are compatible with the primary objective of minimizing pollution as well as the secondary one of maximizing the value of forestry production.
In India, where 10 000 million litres of domestic waste water are generated per day, only 37 percent of waste water undergoes primary treatment and only 8 percent undergoes secondary treatment. Most inland towns and cities dispose of about two-thirds of their sewage waters on the land.
Various government agencies in India have done work on waste water use in the irrigation of tree plantations (Shende et al., 1988; Das and Kaul, 1992). One example is the studies initiated in 1981 by the Central Soil Salinity Research Institute in Karnal (Haryana State) to evaluate the suitability of utilizing raw sewage to irrigate forest trees (CSSRI, 1989). Three species of forest trees were tested: Eucalyptus tereticornis, Populus deltoides and Leucaena leucocephala. Trees were grown on ridges of 1 m in width and 50 cm in height and untreated sewage was distributed through shallow, 2 m wide trenches. The amount of sewage used was adjusted to the age and species of plants and to ensure that standing water disappeared within 12 to 18 hours of application to avoid foul odours, mosquito breeding and waterlogging. Three irrigation treatments (15 cm of sewage water daily, 15 cm of sewage water fortnightly, 15 cm of sewage water monthly) were tested and compared against a control (15 cm of tubewell water monthly). The experiments were continued for five years and gave the following results:
· For all three species, trees irrigated monthly with sewage water showed higher growth than trees irrigated with well water at the same frequency: the eucalyptus was 6 percent taller after 48 months; the leucaena was 12 percent taller after 36 months; and the poplar was 4 percent taller after 24 months.
· Faster growth was exhibited with frequent watering; the eucalyptus and leucaena grew fastest when watered every fortnight and the poplar grew fastest when watered daily.
While the trees showed no negative signs over the course of five years resulting from a high nutrient input, the long-term effects of irrigating with untreated waste water on the soils, however, have still to be determined. Related research by India's National Environmental Engineering Research Institute (NEERI) found that agricultural crops undergoing prolonged irrigation with untreated waste water gave lower yields than crops irrigated with primary or secondary effluent, leading to a recommendation that land application be preceded by a combination of oxidation/stabilization ponds and/or aerated lagoons. Clearly, additional research is needed on the effects of nutrient loading and management of the waste water-soil-crop system and the species to be irrigated need to be selected carefully.
Although some countries have had a long experience in the use of municipal waste water for irrigation, historically the objective has been largely disposal-oriented. Only relatively recently has greater emphasis been put on the productive and purification aspects of this practice.
The prevailing tendency has been to allocate waste water to agricultural uses. Existing quality standards and guidelines for waste water use in irrigation apply agricultural crops; no detailed standards and guidelines have been developed for trees. There are compelling reasons to examine more closely the feasibility of irrigating forests and tree plantations with waste water. These include health factors (the reduced risk posed by food crop contamination with pathogens or heavy metals), environmental considerations (benefits provided by greenbelts and other urban and pert-urban forestry plantings), economic benefits (the value of forest products and environmental services although habitually undervalued) and social and aesthetic concerns. These factors, combined with a high demand in many developing country cities for fuelwood and construction materials to supply their growing numbers of low-income urban dwellers as well as rising market prices for timber, have led to a growing interest in the possibility of waste water-irrigated tree plantations.
A test plantation of eucalyptus (background) end poplars (foreground) irrigated with waste water in India
This interest has recently led some countries to initiate research. To date, research done on tree plantations irrigated with waste water indicates that this is potentially both a cost-effective means of treatment and a productive use of waste water, provided sufficient land is available. As with the irrigation of any crop, however, there are a number of design and management factors to be addressed. Trees differ from agricultural crops in their perennial nature and, for some species and products, long rotation periods; their water and nutrient use; end uses; susceptibility to mineral toxicities and salinity; and other factors. Irrigation techniques suitable for trees and water quality standards for the irrigation of trees must be developed.
In designing a system, careful attention must be paid to site characteristics, rates of water use, the suitability of water distribution and application methods, species selection and market potentials. Nutrient dynamics, the buildup of salts in the soil and the effect of salinity on tree health and growth rates are also critical issues. It has been suggested that agroforestry, or the integration of trees with crops, can help balance nutrient inputs from waste water application with uptake by plants; because the nutrient demand of tree plantations decreases after the initial years of rapid growth, introducing crops in later years may enable the nutrient input to remain constant without risk of nutrient overloading. The use of agroforestry practices in waste water irrigation systems, however, has barely been investigated. Clearly, there is some tantalizing potential for using waste water for irrigated forestry and agroforestry, but much research is still to be done before this potential can be realized.
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