Water quality & greywater – soaps & detergents
As greywater is water from the shower and bath, basins and washing machine it may contain soap, hair, lint, grease and household cleaning products. Despite this, it is still a suitable and beneficial source of water for irrigation. Greywater cannot be stored for later use.
A greywater system must include the ability to easily turn off the discharge into the irrigation system, so that water with low microbiological quality (e.g. from washing nappies or if someone is sick), or high levels of chemicals (e.g. from solvents or harsh cleaning chemicals) flows straight to sewer. This is normally done by switching off the pump or by manually opening the diverter valve (depending on the system).
Residents with a greywater system should always use “eco”, “garden-friendly” or “greywater-friendly” products that are biodegradable and low in salts and phosphorus. There has been an increase in the availability of these products, partly due to the removal of phosphorus from detergents. Using greywater- friendly shampoos, soaps and detergents minimises the potential negative impact that greywater can have on some plant species and soil types.
Chemicals in the water
Alkalinity and dissolved salts
Soaps and detergents are highly alkaline (i.e. pH is >10), to help dissolve dirt and grease. Most plants prefer a more neutral pH of between 6 and 7. As salts, typically including sodium but also calcium and magnesium, build up in the soil profile there is a loss of permeability and the ability to absorb water. This is more problematic with heavy soils (loam and clay) rather than free draining (sandy) soils. Regardless of your soil type, the following considerations can help minimise issues related to pH and salt accumulation.
• Powdered detergents generally contain the most salt as it is used as a ‘builder’ or ‘filler’ to carry the active ingredients. Concentrated powders contain less, and liquid detergents the least.
• The potential impact depends on the amount used. To avoid overload don’t use more detergent than necessary for the size of the wash and hardness of the water. You may be able to reduce the amount used by 50% for lightly soiled loads and get the same results.
• Turning the system off in the winter (i.e. diverting the water to the overflow and sewer) gives the soil a rest and allows rainwater to naturally flush out any salt build up.
• Increasing the amount of organic matter in the soil, such as using compost, can buffer pH change and improve soil health.
Phosphorus and Nitrogen are nutrients essential to plant growth. However, their presence in wastewater and also in garden runoff from excessive domestic application of fertiliser, contributes to high nutrient levels in rivers. The use of phosphates in both powdered and liquid laundry detergent has been banned in Australia since 2014, although they are still used in dishwasher detergent. A range of other more benign ingredients such as sodium citrate is now used to replace phosphorus. Look for ‘NP’ on the label to show that the pack contains “No Phosphorus”.
Bleaches and disinfectants
Avoid running bleaches and disinfectants through a greywater system and onto your garden as these kill beneficial soil micro- organisms. Always divert greywater to sewer when using these types of products.
Fats and oils, as well as paints, can cause clogging and reduce soil permeability so avoid running these through a greywater system.
Water quality and plant health
The greywater irrigation system needs to accommodate existing plants and vegetation for retrofit situations, and new plantings for new homes. An experienced greywater professional can provide advice at the design stage to ensure a successful outcome.
Periodic inspection of the health of plants and soil receiving greywater is always good practice and should be part of a maintenance regime. This is particularly important for some Western Australian natives species which might be phosphorus-sensitive, such as members of the Proteaceae family (e.g. Banksias, Grevilleas and Hakeas).