Using wicking beds to combine WSUD and Urban Ag.
Biofilta is serious about changing the way cities feed themselves while putting storm water and food waste to good use. To help this happen, we have designed a household scale super productive 40m2 urban farm that captures and stores rooftop rainwater, and diverts that water into an advanced wicking garden system for food production. This approach demonstrates how cities can close loops, and re-purpose abundant storm water and food “waste” streams into valuable resources to help cities feed themselves in the process
The system is ergonomic, reduces weeding, and the water efficient design requires watering once a week in summer and less in winter. It also takes composted food waste from households and uses that input to grow food, which will reduce the amount of waste going to landfill. The household farm is designed to reduce the time taken to grow food in a closed loop system and to allow the busy modern city dweller to reconnect with food and become an urban farmer.
Underlying this initiative are some deeper insights into the role an urban garden can have on our broader ecosystem.
WATER SENSITIVE
A 200 m2 roof area in Melbourne was modelled using a typical year’s worth of real rainfall in Melbourne and is expected to produce 109,000 litres of runoff per year taking into account some evaporation and losses. If this same area was a green revegetated pasture, nearly all of this flow would have been soaked into the ground and some of it would slowly find its way into the streams as a base flow. Hence, due to development of typical house, there is now 100,000 litres of storm water rushing into our waterways per annum. Multiply this across thousands of houses and you quickly see that the rivers and creeks receive a lot more volume from urban areas than would naturally occur.
While it may sound good to provide more water, ecosystems have developed over long periods of time with intermittent peaks of flow, not constantly every time it rains as happens when small rain events hit urban catchments. These flows also mobilise sediments that carry suspended solids and nutrients to the creeks and Bays that are detrimental to sensitive environments and cause algal blooms.
Where do these solids and nutrients come from? Well, rainfall carries airborne pollutants from dust to our roof and that runs off to the waterway. Annually, this load comprises 3.02kg of suspended solids (the gunk you find in your gutters), 16.7g of phosphorus and 242g of total nitrogen from our 200m2 roof.
So, for urban areas, reducing the volume of outflow is a really good thing from an environmental perspective and removing these nutrient loads is also good so they don’t accumulate quickly in our marine environments where nature tries to restore balance by creating algal blooms.
Our 40m2 garden will, based on our own urban water consumption figures, consume an average of 4.8litres per square metre of garden (higher rate in hot periods). This equates to a base consumption of 192 litres per day for a mix of lettuce, onions, silver beet, mint and other seasonal vegetables and herbs.
If we connect a 10,000 litre water tank to capture the rainfall and add the water demand pattern from the garden, we see something pretty cool:
The cool part is that in growing food during the year, we can meet the annual demand of the garden with a 97% reliability from rainfall and only use potable water for 3% of the water demand.
Further, from a volumetric perspective, our tank and garden reduces the flow to the Council drains by 70% which could mean that if everyone did it, issues like nuisance flooding in streets would be significantly reduced and current pipe assets would provide a greater level of service.
Best Practice Environmental targets in Victoria are met if you can provide treatment for stormwater that results in an 80% reduction in Total Suspended Solids, 45% reduction in Total Phosphorus and 45% reduction in Total Nitrogen. From capturing and reusing the rainfall from the roof, Best Practice targets are exceeded for nutrient removal and instead of feeding algae in the waterway, these nutrients can become food for veggies and feed us instead.
FOOD PRODUCTION
Based on our trial gardens over the past year, only 40m2 of growing area is required to produce 640kg of food per annum, or enough for the yearly dietary consumption of 5 adults. Value of produce per annum could be over $5,000 per annum through the production of a diverse range of herbs and vegetables.
Given that only 5% of Australians eat the recommended 130kg of vegetables per annum, our 40m2 garden would likely feed more than the average household and provide opportunity for food sharing within the neighbourhood or extended family. Invite the relatives for a BBQ and everyone leaves with a bunch of silver beet and some onions. The power of this food sharing is something that really excites us.
Growing local also helps to develop more resilient communities that are connected with their food and less reliant on imported goods.
CLOSED LOOP
Soil based gardening can be used to close the nutrient loop by turning green waste into compost and this in turn input into the garden to recycle the nutrients into more food. How many kg’s of lettuce or vegetable scraps do you typically throw into the bin per week? By composting this green waste, you can create a closed loop system and become an essential part of our society achieving greater environmental sustainability.
COST
Excluding seedlings, soil and water tanks we expect this particular household farm system to cost under $10,000. Before you run away screaming, consider that depending on the varieties and volumes of produce grown, the system can achieve payback times of between 1.5 – 3 years or a return on investment in the range of 30 – 60%. Lets say the return is in the lower range of 30% pa. Where can you find a better return on investment?
Not every garden needs to cost $10,000 to get this type of return. The same percentage return applies to 1m2 or 100m2 if farmed correctly.
POWER
If saving water means being water sensitive, then the model garden is power sensitive too as our bottom watered, advanced wicking garden beds can be irrigated under gravity from the tank.
This low energy system reduces the ongoing cost of maintaining an urban garden and harnesses the free energy stored in the tank.
More power that can be better stored in batteries from the solar panels and used for other purposes than pumping water.
SUMMARY
Growing food in wicking beds using captured rainwater from the roof can have substantial beneficial environmental impacts on the downstream environment by reducing the volume of storm water runoff and nutrient loads.
Our model 40m2 garden could produce the recommended amount of vegetables for 5 adults each year ( and in reality, more like 10 ).
The benefits of urban farming can be significant if the accumulated effect of lot scale water sensitive urban design is taken into account.
Growing food at home in wicking garden beds is very cost effective and can represent an excellent return on investment from reduced supermarket spend.
Biofilta is currently building household, community and school urban farms and is currently taking orders for the large format wicking systems shown in the model garden. We can also offer a full design and delivery service for full landscape setup.
If you are interested or have queries, please contact Marc Noyce at Biofilta- mnoyce@biofilta.com.au