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Make best use of recycled water

In Australia's new, even drier climate, recycled water has become a valuable resource. Opportunities exist for wastewater producers and water users to benefit from each other. If testing shows that effluent is suitable for irrigation, it could provide a year-round, reliable source of both water and nutrients for plants. Growers get vital water, and wastewater producers can dispose of it beneficially.

Before water can be recycled on land, the following factors must be considered. Sydney Environmental & Soil Laboratory can advise on every one of them.

Reuse or disposal?

Reuse is the use of wastewater for a beneficial purpose, such as irrigation or cleaning. Disposal is the getting rid of unwanted wastewater. Because of water's scarcity in Australia, beneficial reuse has more value than disposal.

Legislation is very strict on the disposal of wastes. In NSW, the Protection of the Environment (Operations) Act 1997 provides harsh penalties for the illegal disposal of wastes. SESL has the expertise to ensure that you comply with all legislative requirements.

Water quality

The suitability of recycled water for irrigation depends on its salt content, its nutrient content and the degree of treatment used to reduce harmful organisms.

Water quantity versus area of use

The amount of water supplied must not exceed the capacity of the land to receive it. Where plenty of land is available and the irrigation volume is less than the difference between rainfall and evaporation, reuse becomes sustainable. But authorities consider the supply of large amounts of water to small areas as disposal, not reuse, and look unfavourably on this practice. A highly permeable soil (in which water flows easily) is preferable, but soils with lower permeability can be used if large enough areas allow the water to be spread out without causing waterlogging.

Because the difference between rainfall and evaporation changes seasonally, storage may be required in cool or wet seasons.

Water availability

The ideal situation is where the water supply can be matched with the water demand. A town sewage treatment plant can provide a year-round supply of irrigation water for cropping or pasture production. A small or intermittent effluent supply can reduce reliance on town water. But water that becomes available in large quantities only occasionally must be stored for use.

Plant species

Species composition is critical. Fast-growing, deep-rooted trees make best use of large volumes of water. Pastures and grass, especially where the production is removed off-site, respond well to the nutrients in recycled water, allowing a reduction in the use of fertiliser and town water. Selecting species with a high water use will improve the ability of a site to handle water disposal. Phosphorus-sensitive Australian natives are unsuitable if the effluent contains high phosphorus levels. Species with low water requirements may also be unsuitable if large volumes of water must be accommodated. Ideally, a reuse site should be specifically designed with these factors in mind.

Soil characteristics

The suitability of a site for effluent reuse depends on the physical and chemical properties of the soil. By law, these properties must be assessed before any reuse can go ahead.

Good drainage and good soil structure are essential in any irrigation system where water with a moderate to high degree of salinity, such as effluent, is being applied. Well structured soils generally have a good rate of conductivity of water through the profile and are well drained. This is important for effluent disposal, as large volumes of water must be applied without creating runoff or waterlogging. Soils with low permeability should usually be avoided for disposal, but may be well suited to irrigation.

Salt build-up in the soil can be prevented or reversed through irrigation management and leaching. Soils with poor drainage are prone to salt accumulation.

Nutrients in the water

Irrigating with recycled water has the potential to contaminate waterways with nutrients. This applies particularly to nitrate, which is highly soluble and will move downwards (leach) in a permeable soil towards the water table. Nitrate removal must rely on plant uptake, so the amounts of nitrate and water applied need to be considered. Disposal of nutrient-rich water on permeable soil carries the risk of contaminating groundwater.

Phosphate sorption capacity of the soil

Phosphate is a highly reactive chemical that clings readily to soil particles. This process, termed adsorption (as distinct from absorption), can work against growers by soaking up all the superphosphate they apply. But it can work in favour of effluent reuse by tying up excess phosphate supplied in the water.

Most soils have some ability to adsorb phosphate and prevent it from leaching. This property is called the phosphate sorption or retention capacity of the soil, and it can be measured so that effluent loads can be calculated so as not to exceed it. Clay and soils rich in iron and aluminium oxides (e.g. subsoils) have the highest phosphate sorption capacity. Sandy soils and soils with a low clay content usually have a low capacity. On these sites it may be necessary to remove phosphorus by regular harvesting of crops or fodder to maximise the longevity of the site.

Soil depth

Deeper soils (>1.5 m) and deep water tables (>3 m) are preferred for irrigation of recycled water. These conditions stimulate deep root systems, which are better able to use recycled water. Where groundwater is shallow, waterlogging can be induced, so subsoil drainage should be installed to ensure good soil aeration.

Site location

The location of the site where recycled water is applied is critical. It must lie a specified minimum distance from water courses. If flooding can occur, large off-site storage tanks must be installed to hold the water until the soil has drained. Erosion can be an issue, but can be controlled if adequate vegetation cover is maintained. Steep sites may produce runoff or erosion, but trickle irrigation can avoid these.

The site must also lie a minimum specified distance from dwellings and public roads. If the effluent has not been treated to a high standard, the site should be fenced off from the public.