Greywater is household wastewater from laundry tubs, washing machines, showers, baths and basins. Kitchen sinks and dishwashers are included under the term “greywater” but tend to contain higher levels of contaminants than other greywater sources. Changing domestic lifestyles are driving an increase in water use, creating a need for sustainable options for reuse or disposal of the resulting wastewater.
Many regional and district councils are now considering alternative water management strategies, including rainwater collection and greywater diversion. However, no national legislation or guidelines currently exist for rainwater or greywater practices, and there is a lack of health risk assessment data specific to New Zealand conditions. Given the prevalence of unreported greywater re-use, the CIBR's greywater experts believe research needs to be conducted to address this. A CIBR report exploring the drivers for greywater reuse in New Zealand can be found here.
CIBR's “Greywater-wise” programme was led by ESR. Please feel free to contact ESR if you would like to discuss this research (firstname.lastname@example.org).
This study looked at the effects of greywater re-use on septic tank system efficacy. Greywater diversion from a septic tank should alleviate the burden that is placed on a septic tank particularly those that are old and require maintenance. The study used two different residential properties – one that had a well-functioning septic tank and one that was in need of maintenance and underperforming.
Diversion of the greywater levels that passed through the septic tanks had positive effects on septic tank efficacy, particularly with the underperforming septic tank. Although reducing greywater load was beneficial for septic tank functioning, precautions still need to be taken when dealing with untreated greywater.
The full report [PDF, 1.2 MB] was published in the Water Journal (Water NZ) No. 178 - March 2013.
“Investigating Environmental and Health risks of Greywater water use in New Zealand” was a master’s thesis completed by Morkel Zaayman in 2014, which looked at the environmental impacts of triclosan use. Triclosan is an antimicrobial contaminant commonly found in toothpaste, hand-wash that can ultimately end up in greywater.
The study looked at different soil types that were irrigated with greywater containing triclosan and E. coli – this was used as a microbial indicator species. The aims of this study were to investigate the effects of triclosan on both soil health and microbial community health but also the public health risk related to triclosan and E. coli leaching into the soil.
For all of the soil types that were tested, triclosan and E. coli were present in the leachate which can then lead to groundwater contamination and as such is a public health risk. Triclosan also negatively effects the number of microbial communities present in the soil and microbial biomass.
Thesis link: Morkel Arejuan Zaayman
There are unclear guidelines and regulations in place in New Zealand for greywater disposal which has resulted in unregulated greywater disposal, with greywater disposal known to effect the soil environment and soil microbial communities.
The study, (external link) published in 2016, looked at the environmental health risks and groundwater contamination risks associated with long term (up to 20 years) greywater disposal on soil. This was performed as a case study using a community that were regularly disposing of their greywater onto land.
The study demonstrated that the long term disposal of greywater had both positive and negative effects on the environment. The positive effects were increasing plant growth through an increase in microbial biomass and enzyme (dehydrogenase) activity, whereas the negative effects of greywater disposal relate to the increased levels of pathogens such as E. coli which are a risk to human health. So although greywater disposal onto land is beneficial, it should be applied to land using methods such as sub-surface irrigation to minimise risk.
Microbial survival in mulch
This project investigated the survival ability of the indicator organism E. coli in mulch that had been spiked with synthetic greywater. Greywater can be distributed in gardens via sub-surface irrigation and garden mulches are used as a common cover over the soils.
Three types of mulch were used for this study; pea straw, coconut husk and a bark based mulch. Each mulch was subject to three conditions: a freshwater control, E. coli solution and synthetic greywater + E. coli.
The use of coconut husk and bark based mulches saw a decline in the number of E. coli present. Whereas the use of pea straw mulch demonstrated an increased number of E. coli present.
This study was published in 2017 and demonstrated that there is potential the spread of microbial contamination from greywater to the soil layer covers, that should be considered to safely re-use domestic greywater.
Page last update 28/7/2017