Why Bother To Re-Use Greywater?
Greywater re-use for garden irrigation provides several beneficial results. It utilises a valuable on-site resource, which is otherwise wasted, it conserves fresh water, which can remain in natural ecosystems, and it reduces the load on wastewater disposal systems (both on-site and centralised). This last point is important as many on-site septic tank systems in Australia are failing, resulting in effluent surfacing in the leach field area. This presents a significant health and environmental pollution risk for occupants and the local catchment. Direct greywater re-use for garden irrigation diverts much of the low-pollutant water load from a septic tank system, allowing the system to function under far less stress.
The Design
We found the key to satisfying the local Council was to engage a reputatable designer, Nigel Jones, who has designed environmentally responsible homes for Tasmania since 1980, as well as large scale wetlands for organisations such as wineries. Nigel calculated the total fixture load for the building, allowed for gains and losses through rainfall and evaporation, and designed the wetlands to provide a minimum retention time of 4.2 days.
Rainwater is collected from the roof to supply the kitchen, shower and laundry (all with standard water saving fixtures), with water from the kitchen sink passing through a grease trap. A waterless composting toilet is installed, with the excess liquid drain connected directly to the waste water line.
The waste water line enters the first stage of the hybrid-wetland, being a series of six second-hand fish bins that were reclaimed from a local fish processing plant, and form the Sub Surface Flow [SSF] wetlands. These bins are full of gravel and have no free water at the surface. As Nigel points out, “Introducing native reeds in the gravel draws oxygen into their root zones providing opportunities for aerobic bacteria and other organisms to feed on suspended organic material and prey on pathogens.” The effluent moves through the system via sections of slotted PVC pipe, which is positioned over HDPE pipe welded in place to the opposite top and bottom corners of each bin.
The outflow from the first stage is directed to the second stage wetland, being two fish bins that form the Free Water Surface [FWS] wetlands, or ponds. Nigel states that “Here the effluent is exposed to high levels of ultra-violet light in a varied environment possessing a greater range of plant and animal species and a high surface to volume ratio, facilitating higher levels of dissolved oxygen. The second pond enables further settlement, UV exposure, storage and ultimately re-use through a sub-soil irrigation system.
Constructing the Wetlands
We purchased the bins second hand from a local fish farm, Tassal, making sure there were no large cracks or holes that were not able to be repaired by a plastic welder. The site was surveyed and a trench dug with a small excavator to create four level areas, each area housing two bins, with the next pair of bins at least 20mm lower than the previous pair. Here is the constructed system awaiting plastic welding.
A 5cm bed of 7mm gravel was laid before positioning the bins, and then the HDPE fittings were welded to the bins.
Slotted PVC pipes were then placed over the lower HDPE fittings in the first stage wetland and surrounded with 20mm gravel before the remainder of the first stage bins were filled with 7mm gravel, allowing enough space to position the slotted PVC pipes over the top HDPE fittings. We used a level to make sure the slotted PVC pipes were level before surrounding them with 20mm gravel.
The first stage wetlands were then planted with reeds and plants such as Phragmites australis, Juncus sp, Dianella tasmanica and Diplarenna latifolia. Tassel Cord Rush in pots were supported inside the final two bins (second stage wetland), to ensure that they remained 150mm below the water surface. The slotted PVC pipes were not required in these two bins.
Once the waste water line was connected, we commissioned the wetlands by adding some worm tea and flooded the system to test for leaks, as well as to give the plants time to establish a deep root system prior to the introduction of the waste water. The completed wetlands are nicely tucked away in the landscape. All we want to do now is paint some wildlife inspired patterns on the bins :o)
Maintenance
The wetlands require some regular maintenance to; ensure that the water and gravel levels are maintained, check for signs of a healthy system, remove weeds and dead plant material, check for leaks or cracks, remove and compost dry canes from the previous year, and ensure that water plants do not take up the entire surface of the second stage wetland.
Because the effluent is light, it may not contain enough nutrients to support the reeds. A weak solution of fertiliser in spring, such as worm tea, should continue to support healthy plant life. Eventually the reeds may become too established and require partial removal or replacement; however it is anticipated that this will take many years.
Summary of Benefits
· Rain water is captured for domestic use, then treated for re-use in irrigation
· Used fish bins are diverted from landfill
· Habitat is created for many species of aquatic invertebrates
· Fish may be introduced to the second stage wetland once the water is shown to be of adequate quality
· Mature reeds can be harvested annually for compost and basket weaving
· Because the tubs are above ground, no storm water enters the system
· All maintenance is managed by the owner, no external contractors are involved
· The modular nature of the system permits the addition of separate units to accept additional loads in the future, without compromising the established wetland
· Low embodied energy through using second hand materials
· Council approved (in fact, they look forward to seeing more of these in the future!)
Costings
Design and implementation of the wetlands was much more affordable than many commercial greywater treatment systems (Biolytix systems are $10-12,000 fully installed):
· Design $600
· Permit application $200
· 8 fish bins & delivery $330
· Plants $146
· Blue metal $342
· Earthworks $595
· HDPE fittings $50
· Plastic welding $300
· PVC slotted pipes $89
· Total $2,652
NB: A soil test was required for the Special Plumbing Permit; however this was already done as part of the site assessment for the house building permit application
Thanks again for all your visualising so we could get the permit to install this system, it’s a real milestone and hopefully many others can benefit from the design, as well as Councils be educated by it.