Appropriate uses for greywater


By Water Research Commission

On-site residential greywater reuse is a potentially promising alternative water resource if managed correctly — particularly in low-density, high-income areas where health concerns are less pronounced.

GW System main                                              A complete greywater harvesting system installed at the University of Colorado, US. Credit: Wahaso

Reuse schemes need to be simple and economical, whilst at the same time protecting the environment and public health. Greywater reuse has the potential to result in potable water savings through offsetting the use of such water for non-potable purposes.

From a ‘fit-for-purpose’ perspective, greywater is most appropriate for activities such as toilet flushing and garden watering / irrigation, where human contact is limited. However, even in these cases, there is potential for health risks; for example, in the context of toilet flushing, water droplets are aerosolised and land on nearby surfaces or can be dispersed into the air. The pathogens are then transferred through hand-to-mouth contact after toilet use, if thorough hand-washing is not observed.

Similarly, it is strongly suggested by many international authors that untreated greywater should only be used for subsurface irrigation purposes, to limit the risks involved. In addition to this, discolouration, odours, and negative perceptions about wastewater can also present barriers to its use as an alternative water resource.

Depending on the availability of potable water resources in a specific area, greywater may also be used for small-scale irrigation — with appropriate risk prevention barriers in place. The complexity of installing and managing these greywater systems presents a challenge for broad-scale adoption within urban catchments.

Greywater harvesting is usually heavily decentralised, taking place within the boundaries of individual properties, and the responsibility of managing these systems falls on the property owners. However, in densely populated urban areas and/or informal settlements, management of greywater jointly with other domestic wastewater in a combined collection / treatment system such as constructed wetlands (that is, a centralised system) may be technically and economically more feasible.

ReedbedIn this regard, proactive participation and involvement from the involved ‘community’ is essential for the effective implementation of greywater systems. There are likely to be significant differences between the way greywater is used and managed in serviced and un-serviced settlements.

Reedbeds simulate the function of natural wetlands and assist in the cleaning process of wastewater. Here a gravity-fed system flows through two reedbeds before reaching the storage tank. Credit:

Whether greywater reuse is intended or not, particularly in areas that are un-serviced (and rural or peri-urban areas), control and/or risk aversion measures are required to be put in place to ensure safe disposal of greywater. This is to prevent environmental degradation and reduce exposure of the population to health risks.

The most promising domain for institutionalising greywater reuse appears to be in large buildings such as office blocks, public buildings, and hostels. Greywater can be collected and treated under proper supervision and the large amount of (usually) lightly contaminated water originating from such buildings can be used for irrigation of gardens and sports fields, toilet flushing, and so on.

The water thus saved can make a significant difference, especially during times of water scarcity. If this is to happen, however, the adaptation of building codes for such buildings should be investigated as a matter of priority. The potential to use greywater for a specific end-use is dependent on how and from where the greywater is collected, stored, treated (if at all), and used.

Case study: greywater use in formal housing, Cape Town

A short investigative study was undertaken in Cape Town during 2016, to try and determine how greywater is currently being used and managed in typical upper-income homes, the attitudes of the people towards using these greywater systems, and an indication of some of the health risks that may result.

All the respondents made use of the same type of proprietary greywater diversion device (GDD), which is designed to divert laundry and bath/shower water to a small storage chamber for immediate disposal by irrigation. A randomised sample was selected from a population of 893 homeowners in the study area using a non-probability approach to identify participants — all of whom have had a greywater system installed in their home. Semi-structured interviews were used to elicit information about the way that the home-owners used and managed their greywater systems.

The themes most commonly raised were: the financial benefit of the system, the year-round flourishing gardens, few concerns about exposure to greywater, perceptions about the quality of the greywater, and the ease of use when operating their greywater system.

Eight different houses were then sampled for greywater quality analysis, in which water was sampled over three different days, and a total of 24 nutrient samples were collected. These samples were taken from the greywater tank / chamber at the same time each day and stored in dark glass bottles, at room temperature.

Samples for the testing of Escherichia coli were taken from the same eight houses as the nutrient samples on the day of the third test. It was evident among the interviewed participants that there were similarities in the way home-owners use, perceive, treat, and manage greywater.

Specifically, 95% of the participants were happy with how their greywater has kept their gardens green and reported no concerns to human health. Only one participant removed their system due to foul odours. It was also found that each household had the same primary sources of greywater attached to their system: baths, shower, hand basins, and washing machines.

In each case, greywater is stored and flushed from the greywater chamber on a daily basis and is used to irrigate lawns, flower beds, fruit trees, and vegetable gardens. Only a handful of respondents indicated that they would not use greywater to irrigate edible plants. Those who do use greywater to irrigate vegetable patches, explained that they were conscious of the household products they use to wash and clean. These users would avoid acid-based detergents and rather opt for organic, eco-friendly, and biodegradable products.

The sampling of greywater was a useful indicator of hazardous nutrients and contaminants, even though the sample size was small. These contaminants either enter the system directly through the sources (that is, faecal matter through the laundry greywater), or develop within the system as it experiences temperature, pressure, and oxygen changes.

  1. coli served as an indicator organism of microbial activity, with the results of this study showing a wide range of E. coli concentrations that may be related to household size and the inputs at each source (for example one household included their kitchen basin as a source of greywater). The qualitative analysis indicated that in some instances, greywater could pose a risk to human health, soils, and the environment.

An example of this is that many users do not switch off their system during periods of high rainfall and saturated conditions, resulting in some pooling of greywater in their garden. The results showed that, whilst most respondents were successfully using their greywater and did not report problems with the systems, many residents do not follow some of the basic rules regarding handling of greywater (for example switching off the system during periods of rainfall) — and could thus be exposing themselves to various levels of risk.

This reaffirms the difficulties in quantifying and qualifying greywater risk to humans, soils, and the ecology of the environment. However, assessing the use and management of greywater in a private space that is managed by purpose-built technologies, identifies how these risks are being managed within domestic households. The study therefore provides another snapshot into considerations for guidelines towards safer and sustainable use of domestic greywater.

Uses for which greywater is inappropriate

Specific guidance in respect of greywater use will be provided in another future article, but the general rule is that greywater should never be used in any manner where it may easily come into contact with susceptible individuals and/or ingested. Therefore, it is totally inappropriate for use in the following situations: