Utilising greywater using a mulch basin disposal system.

Greywater is generated by every household but is often overlooked as a valuable resource. Although it can be contaminated with soap, grease, and other impurities, greywater represents a significant source of water that can be put to good use. As the world faces increasing water scarcity, it’s essential to think about how we can conserve this precious resource and make the most of our greywater.

In this article, we will explore the use of mulch basin type greywater disposal systems in permaculture and highlight why every homeowner should consider implementing this approach in their own yard. With its many benefits, including water conservation, soil health, and reduced need for chemical fertilizers, mulch basin greywater disposal systems are an effective way to make the most of this often-overlooked resource.

Introduction to greywater and mulch basins

Greywater refers to household wastewater excluding toilet water. That is the water from your shower, laundry and other household sinks. For those households with composting toilets (like us), greywater is actually the only waste water that you need to worry about.

Our favourite disposal method so far is the mulch basin method. A mulch basin is a type of greywater disposal system that uses the principles of permaculture to reuse greywater on-site.

The mulch basin greywater disposal system involves directing greywater from showers, sinks, and washing machines into a basin filled with wood chips and other organic materials, which acts as a filter for the water.

How does a greywater mulch basin work?

The greywater is directed into the mulch basin where it is filtered through the organic materials, reducing the amount of pollutants in the water.

In addition to the coarse filtration provided by the organic materials, bacteria also play a crucial role in breaking down the grease and other impurities in greywater. The bacteria which live on the organic material within the mulch basin, consume the grease, soap, and other impurities in the greywater, breaking them down into simpler compounds that can be safely absorbed into the soil. This process helps to purify the greywater and reduce the amount of pollutants that can be harmful to plants and the environment.

The clean water is then able to seep into the surrounding soil, providing much-needed moisture and nutrients to plants. This is especially beneficial for moisture loving and nutrient hungry fruit trees like bananas, which are often used in combination with mulch basins to take up the excess nutrients from the water.

Design and Construction

Soil type

The effectiveness of a mulch basin greywater disposal system can vary depending on the type of soil in which it is used. On free-draining soils, the greywater can easily penetrate the soil and be absorbed into the groundwater. This type of soil allows the greywater to move quickly through the soil, reducing the risk of pollution and promoting healthy plant growth.

On the other end of the soil spectrum, clay-based soils have a dense structure that can make it more difficult for greywater to penetrate.Over time the mulch basin can help to break up the soil and promote better absorption of the greywater. Additionally, the organic material in the mulch basin can help to improve the structure of the clay soil, making it more permeable. However the lack of initial permeability will need to be considered when undertaking the volume design.

In either case, the use of a mulch basin greywater disposal system can have a positive impact on soil health and the overall health of the plants growing in the area.

Regulations

In New Zealand, regulations regarding greywater tend to come under the umbrella of Regional Councils. The rules can usually be found in regional OSET (on site effluent treatment) plans alongside rules for septic tanks and composting toilets. Many councils allow a certain amount of greywater discharge to land each day as long as it is done in accordance with their rules which ensure that things like pollution, contamination and smell do not become an issue.

Plant selection

When implementing a mulch basin greywater disposal system, it’s essential to select plants that are appropriate for the local climate and soil type and can thrive in the conditions created by the greywater. Plants with heavy nutrient and moisture needs are best suited to the purpose. In our climate and context, a selection of fruiting subtropical trees (eg. banana, inga bean, cherimoya, tamarillo) and native grasses and flaxes are our first choice.

Our system

Design

After considering the location of our bathroom, kitchen and laundry as well as the slope of our site and the length of potential pipe runs, it was decided that we would install two greywater basins. The first would take water from our bathroom and the second would take water from the kitchen and laundry.

The next step in the process was to establish how much greywater would enter each of our basins in any given 24 hour period. This was a fairly straightforward exercise for us – our water supply is rainwater so we are quite mindful of water use and have a fairly good idea of what we use. For others it may be easier to use some of the standard design figures used by council that work on a litres per person, per day basis.

With some minor calculations we established that our design daily greywater disposal needs would be 80 litres for system one and 100 litres for system two. This was in excess of our predicted usage but it is always good to have a margin or error to account for visitors, wet weather or unexpected circumstances.

With our free draining pumice soils, we were happy that our daily greywater volume could easily be disposed of across a fairly small footprint. However as we intended to use the greywater as irrigation and a nutrient source for subtropical trees like bananas, we upsized the basins to allow for additional planting. System one is currently approximately 1m x 2m and system two is closer to 2m x 2m. Both systems have the ability to be enlarged if necessary.

These numbres equalte to approximately 40mm of water depth each day in basin one (actual depth approx 300mm) and 25mm of water each day in basin two (actual depth approx 400mm). The extra depth of the basins allows for days when use is higher than average or it is raining. It also allows space for extra mulch to be added.

Construction

Construction was as simple as digging a hole in the ground with a level base, using the excess material to form a bund around the top of the hole to prevent stormwater runoff, then filling the hole with coarse mulch. We then ran a pipe from our house to the appropriate basin.

Initally the pipe outlet for system one was run through a spreader system but this turned out to be a source of blockages and did not really function as desired. Upon installing the second system, we left out the spreader and discovered that it worked much better. With the base of basins being level, any significant volumes of water are quite capable of spreading out without assitance.

Planting

Each mulch basin was initially planted with a fast growing cover or biomass crop to provide cover to the exposed soil. For the first basin, this was sorrel, clover and tic bean. For the second basin we used tithonia (mexican sunflower) and one side of the basin already had established flax and manuka. As the initial plantings became established, we added fruit and nut trees at regular intervals around the basins.

Maintenence

After removing the spreader system from the first mulch basin, maintenence has been minimal, only consisting of refreshing mulch which has only been required once for each basin so far (after about 6 months). As the trees around the exterior of the basin mature, it is intended that prunings and leaf drop will provide the fresh mulch that the basins need.

What are the five key benefits of this system?

• Water Conservation The mulch basin greywater disposal system helps conserve water by reducing the amount of fresh water needed for irrigation.
• Soil Health and Fertility Greywater from the mulch basin provides plants with much-needed moisture and nutrients, promoting healthy soil and plant growth. The plants in close proximity to our greywater systems are by far the healthiest, lushest looking plants that we have.
• Reduced Water Usage The use of greywater from the mulch basin reduces the amount of fresh water needed for irrigation, helping to conserve water.
• Reduced Need for Chemical Fertilizers The greywater from the mulch basin provides plants with the necessary nutrients, reducing the need for chemical fertilisers.
• Sustainable Landscape Design The mulch basin greywater disposal system is an excellent example of sustainable landscape design, promoting the conservation of resources and minimizing waste.

If it is possible to utilise a greywater system where you live, the benefits make it very worthwhile. The systems are easy to install and maintain, low cost (unlike conventional septic tanks) and you get to utilise resources in a much more sustainable way.