Wastewater treatmentt and use in agriculture

5. Irrigation with Wastewater

Wastewater treatmentt and use in agriculture by M.B. Pescod
Type and Wear Professor of Environmental Control Engineering
and Head, Department of Civil Engineering
University of Newcastle-upon-Tyne
Newcastle-upon- Tyne, UK

5.1 Conditions for Successful Irrigation

5.2 Strategies for Managing Treated Wastewater on the Farm

5.3 Crop Selection

5.4 Selection of Irrigation Methods

5.5 Field Management Practices in Wastewater Irrigation

5.6 Planning for Wastewater Irrigation

List of tables

List of figures

Success in using treated wastewater for crop production will largely depend on adopting appropriate strategies aimed at optimizing crop yields and quality, maintaining soil productivity and safeguarding the environment. Several alternatives are available and a combination of these alternatives will other an optimum solution for a given set of conditions. The user should have prior information on effluent supply and its quality, as indicate in Table 21, so ensure the formulation and adoption of an appropriate on-farm management strategy.

Basically, the components of an on-farm strategy in using treated wastewater will consist of a combination of:

• crop selection
• selection of irrigation method, and
• adoption of appropriate management practices.

Furthermore, when the farmer has additional sources of water supply, such as a limited amount of normal irrigation water, he will then have on option to use both the effluent and the conventional source of waster in two ways, namely;

• by blending conventional water with treated effluent, and
• using the two sources in rotation.

These are discussed briefly in the following sections:

The different types of irrigation methods have been introduced in Section 5.1.4. Under normal conditions, the type of irrigation method selected will depend on water supply conditions, climate, soil, crops to be grown, cost of irrigation method and the ability of the farmer to manage the system. However, when using wastewater as the source of irrigation other factors, such as contamination of plants and harvested product, farm workers, and the environment, and salinity and toxicity hazards, will need to be considered. There is considerable scope for reducing the undesirable effects of wastewater use in irrigation through selection of appropriate irrigation methods.

The choice of irrigation method in using wastewater is governed by the following technical factors:

• the choice of crops.
• the wetting of foliage, fruits and aerial parts.
• the distribution of water, salts and contaminants in the soil.
• the case with which high soil water potential could be maintained.
• the efficiency of application, and
• the potential to contaminate farm workers and the environment.

Table 26 presents an analysis of these factors in relation to four widely practised irrigation methods, namely border, forrow, sprinkler and drip irrigation. A border (and basin or any flood irrigation) system involves complete coverage of the soil surface with treated effluent and its normally not an efficient method of irrigation. This system will also contaminate vegetable crops growing near the ground and root crops and will expose farm workers to the efluent more than any other method. Thus, from both the health and water conservation points of view, border irrigation with wastewater is not satisfactory.

Furrow irrigation, on the other hand, does not wet the entire soil surface. This meethod can reduce crop contamination, since plants are grown on the ridges, but complete health protection cannot be guaranted. Contamination of farm workers is potencially medium to high, depending on automation. If the effluent is transported through pipes and delivered into individual furrows by means of gated pipes, risk to irrigation workers will be minimum.

The efficiency of surface irrigation methods in general, borders, basins, and furrows, is not greatly affected by water quality, although the health risk inherent in these systems is most certainly of concern. Some problems might arise if the effluent contains large guaranties of suspended solids and these setle out and restrict flow in transporting channels, gates, pipes and appurtenances. The use of primary treated sewage will overcome many of such problems. To avoid surface ponding of stagnant effluent, land levelling should be carried out carefully and appropriate land gradients should be provided.

Sprinker, or spray, irrigation methods are generally more efficient in terms of water use since greater uniformity of application can be achieved. However, these overhead irrigation methods may contaminate ground crops, fruit trees and farm workers. In addition, pathogens contained in aerosolized effluent may be transported downwind and create a health risk to nearby residents. Generally, mechanized or automated systems have relatively high capital costs and low labour costs compared with manually-moved sprinkler systems. Rough land levelling is necessary for sprinkler systems, to prevent excessive head losses and achieve unifornity of wetting. Sprinkler systems are more affected by water quality than surface irrigation systems, primarily as a result of the clogging of orifices in sprinkler heads, potential leaf burns and phytotoxicity when water is saline and contains excessive toxic elements, and sediment accumulation in pipes, valves and distribution systems. Secundary wastewater treatment has generally been found to produce an effluent suitable for distribution through sprinklers, provided that the effluent is not too saline. Further precautionary measures, such as treatment with granular filters or microstrainers and enlargement of nozzle orifice diameters to not less than 5 mm. are often adopted.

Localized irrigation, particularly when the soil surface is covered with plastic sheeting or other mulch, uses effluent more efficiently, can often produce higher crop yields and certainly provides the greatest degree of health protection for farm workers and consumers. Trickle and drip irrigation systemsare expensive, however, and require a high quality of effluent to prevent clogging of the emitters through which water is slowly released into the soil. Table 27 presents water quality requirements to prevent clogging in localized irrigation systems. Solids in the effluent or biological growth at the emitters will create problems but gravel filtration of secondary treated effluent and regular flushing of lines have been found to be effective in preventing such problems in Cyprus (Papadopoulos and Stylianou 1988). Bubber irrigation, a technique developed for the localized irrigation of tree crops avoids the need for small emitter orifices but careful setting is required for its successful application (Hillel 1987). When compared with other systems, the main advantages of trickle irrigation seem to be:

1. increased crop growth and yield achieved by optimizing the water, nutrients and air regimes in the root zone.
2. high irrigation efficiency -no canopy interception, wind drift or conveyance losses and minimal drainage losses.
3. minimal contac between farm workers and effluent.
4. low energy requeriments - the trickle systems requires a water pressure of only 100-300 k Pa (1-3 bar).
5. low labour requirements -the trickle system can easily be automated, even to allow combined irrigation and fertilization (sometimes terms fertigation).

Apart from the high capital costs of trickle irrigation systems, another limiting factor in their use is that they are only suited to the irrigation of row crops. Relocation of subsurface systems can be prohibitively expensive.

Clearly, the decision on irrigation system selection will be mainly a financial one but it is to be hoped that the health risks associated with the different methods will be taken into account. As pointed out in Section 2.1, the method of effluent application is one of the health control measures possible, along with crop selection, wastewater treatment and human exposure control. Each measure will interact with the others and thus a decision on irrigation system selection will have an influence on wastewater treatment requirements, human exposure control and crop selection (for example, row crops are dictated by trickle irrigation). At the same time the irrigation techniques feasible will depend on crop selection and the choice of irrigation system might be limited if wastewater treatment has already been decided before effluent use is considered.