The Loam Ranger – Irrigating with effluent
Dear Loam Ranger,
I have the opportunity to irrigate with treated effluent. Is it safe?
Generally yes, but there are many variables that determine how much you can apply before problems start to appear.
What sort of effluent?
The first variable is what sort of effluent it is. The most commonly available forms of effluent in rural areas are sewage effluent (near towns) and dairy effluent (in dairying areas), but other industries produce effluent, including tanneries, food processors and abattoirs. The source of the effluent will broadly determine the contaminants and nutrients present in it. In particular, sewage effluent will contain bacteria, so the degree of processing is important (tertiary processing is better than secondary). Keep in mind that both local councils and the NSW Department of the Environment, Climate Change and Water control the disposal of effluent, so you will need to ask your council for guidance.
What nutrients does it hold?
If the effluent contains plant nutrients in useful amounts, such as nitrogen or phosphorus, these can offset your costs of fertiliser. But there is a limit to how much you can apply. One nutrient will always determine the maximum amount of effluent you can apply: the nutrient that is present in the highest amount or that supplies most of the requirements will always “fill up” the soil first. At this point you have to stop applying the effluent, even if you need more water, because a nutrient excess will always cause problems. For example, nitrogen can leach into groundwater and streams and cause algal blooms, and burn plant roots.
What salts does it hold?
Effluents always contain salts, which can build up to toxic levels in soils with repeated application. It may be necessary to dilute the effluent with clean water so as not to exceed the crop’s tolerance to salt. Alternatively, if other components are not critical, you can irrigate to excess so as to wash accumulated salt down the profile.
What contaminants does it hold?
Heavy metals and complex organic chemicals are common contaminants of effluents. All soils have a certain capacity to “absorb” heavy metals (such as lead, copper, zinc and cadmium), making them unavailable to plant roots, but at some point this capacity will be filled, and all subsequent applications of effluent will result in free metals that the crop will take up. (This capacity is the soil’s cation exchange capacity, which is determined largely by the clay content and the organic matter content.) These metals can then kill the crop or make the produce unsafe to eat.
Organic chemicals (such as polycyclic aromatic hydrocarbons and dioxins) are less of a problem for the crop, but if they leach into groundwater and streams, they can kill fish and other water inhabitants.
What microorganisms does it hold?
Sewage effluent and dairy effluent both contain potentially huge numbers of bacteria and other microorganisms. These are not important to the crop, but they can cause sickness in humans, livestock and wildlife if ingested, either during application or on uncooked produce. Tertiary treatment (by ozone or ultraviolet light) can reduce bacterial loads to near drinkable levels. If the effluent doesn’t receive tertiary treatment, you can still render it safe by holding it in a shallow pond for a few days to allow the sun’s UV light to destroy the microorganisms.
Devise an irrigation management plan
Given all these variables, it is essential that you get the effluent tested for nutrients, salts, contaminants and bacteria. You must also get your soil tested to determine its capacity to receive the limiting nutrient or contaminant. This one component will determine the maximum quantity of effluent you can apply, how often you can apply it, and the maximum life of the site to receive the effluent. Without this information, you risk losing the ability to use the site to produce anything, and contaminating local waterways.
Once you have all this information, you can draw up an irrigation management plan that determines how much to apply and when. Given the limiting nutrient or contaminant, you can then work out how much of other nutrients you need to add. If the supply of effluent is seasonal, this plan will tell you how much extra irrigation you’ll need to source at other times of the year.
Further reading
Dairy Australia. 2008. Effluent and Manure Management Database for the Australian Dairy Industry.
SESL. 2007. Make best use of recycled water.
SESL. 2007. Glossary of water quality terms.
SESL. 2008. Salinity (incl. classes).
SESL. 2008. How salinity is measured.
SESL. 2008. Managing irrigation water quality.
SESL. 2009. Irrigation in drought.
SESL. 2009. Recycling irrigation runoff.
SESL. 2009. Turfgrass irrigation – principles and practices.