The need to manage nutrients
Australian farmers are some of the most mechanised and least subsidised in the world – experts at producing good quality food for our population and for export. According to the National Farmers’ Federation, each Australian farmer produces enough food to feed 600 people, 150 domestically and 450 overseas.
Now that Australia is one of the most urbanised societies in the world, there is a lot of food coming into our cities, but what goes back? Food is composed mostly of water, organic carbon and nutrients. In terms of life cycle, both water and carbon find their way back to the farm through cycling in the atmosphere, but nutrients present a very different scenario, as some of them need to be physically shipped back to farms as fertilisers in order to close the resource loop.
The major nutrients required to grow food are nitrogen (N), phosphorus (P) and potassium (K). Due to the large agricultural productivity of Australia, however, as a nation we are large consumers and importers of NPK. According to Fertilizer Australia, each year our farms apply around 850,000 tonnes of N, 282,000 tonnes of P and 130,000 tonnes of K. The efficiencies required in Australian agriculture mean that most of the fertilisers applied tend to be ‘inorganic’ in that the N originates from N-urea, the P as superphosphate from rock phosphate, and K from potash. Modern fertilising techniques use metered application to deliver exact amounts of fertiliser to particular sections of a paddock in order to increase yield and financial returns on investment. This occurs in order to maximise farm profits and to increase the efficiency in which fertilisers are taken up by crops.
The nutrients not taken up by agricultural crops are lost from the farm by a combination of processes including volatilisation, leaching into soils and run-off to creeks and rivers. Current research is being undertaken to develop new fertiliser forms that minimise environmental pollution, but do not limit yield. For example, in response to nutrient discharges impacting the resilience of the Great Barrier Reef, the sugar industry has invested in developing controlled release fertilisers to ensure their contribution to nutrient run-off is minimal.
From farm to table
Once crops are grown, nutrients start a journey towards human consumption. National waste reporting suggests that Australians discard around 15 to 20 percent of food (is not consumed) and, as a result food, waste typically makes up approximately one-third of municipal solid waste (MSW) and one-fifth of commercial and industrial (C&I) waste streams. The National Waste Report estimates that each Australian throws out approximately 415 kilograms of food, but in addition to this, most of the nutrients from the food that is consumed end up in our wastewater.
Waste in cities and the impacts on water
This country is the second highest producer of waste per person in the world; each Australian accounts for around 650 kilograms a year. This is second only to the US, which comes in at around 715 kilograms per person. Meanwhile urban water utilities in Australia quietly manage over 95 percent of the mass of all waste generated in our cities – the other five percent comprises all MSW, C&I and Construction and Demolition wastes, according to the National Waste Report and National Water Commission Performance Report.
It is perhaps no wonder that sewerage costs generally account for over 50 percent of the average water bill in Australia (which varies from state to state). Modern sewage treatment needs to meet its primary objective of sanitation, but must also deal with nutrients in an effective manner prior to disposal.
“The second highest producer of waste per person in the world, each Australian accounts for around 650 kilograms a year.
The oversupply of nutrients to the environment leads to eutrophication, associated algal blooms and, at times, beach closures. For many years the wastewater industry did not intend recovering the nutrient value during the process, focusing on treated effluent disposal to meet environmental regulations. More recently, however, there has been a shift in the potential role of the wastewater industry, which is now aware it could produce valuable resources. For example, there has been a steady increase in the recycling of water from wastewater, particularly through the millennium drought, and all larger cities around the country now capture energy (as methane) through wastewater anaerobic digestion facilities. Such facilities have the ability to become energy and carbon neutral.
Nutrient production from wastewater has long been associated with biosolids, which contain the beneficial residues from wastewater treatment. Biosolids have the advantage of containing a wide range of nutrients and trace elements, but the disadvantage of being low in major nutrient value, variable in composition and often difficult to handle – making them less suited to high-tech agricultural production. Newer technologies are now being developed to recover nutrients in a concentrated form directly from wastewater.
New N & P fertiliser products can already be directly manufactured out of wastewater with a purity far superior to existing commercial fertilisers, some of which contain high levels of cadmium (for instance). Such technologies will gradually be incorporated into upgraded sewage treatment facilities, enabling improved nutrient recycling.
More innovations to close the loop
There are a number of other businesses and initiatives that interact with organic wastes. For example, there are numerous organic sources of nutrients derived from manures and production by-products. Commercially, one can buy products such as Blood and Bone, Dynamic Lifter (composted poultry manure), Neutrog (composted and pelletised organics), compost and potting mix – each of which contribute to closing the loop on nutrients.
In many cities, organic wastes are now starting to be taken to dedicated biodigestion facilities in order to generate electricity. Some examples are EarthPower, Yarra Valley Water and Federation Square, each of which have anaerobic digester facilities that take in food and other organic wastes with the waste-to-energy intent, but also have the ability to recover nutrients.
More recently there has been a resurgence of greening our cities. For instance, the 202020 Vision initiated by the Nursery and Garden Industry Australia and Horticulture Australia is a collaborative plan to increase the amount of green space in our cities by 20 percent by 2020.
Green spaces enhance natural values, can be designed to manage stormwater flows and assist in dissipating extreme heat events caused by urbanisation. Such green spaces may be encouraged in residential living or may be community-based; they may be gardens, lawns or trees.
There are also moves to implement urban agriculture with the aim of producing food for the village from within the village. Such food cooperatives typically seek to bring together people with similar interests in gardening or food to develop community identity and culture. More extreme examples of urban food production include building-integrated agriculture, such as in Singapore, in which entire high-rise buildings produce food via hydroponics and utilise natural advantages of the building to optimise water, nutrients and heating/cooling.
“There has been a shift in the potential role of the wastewater industry, which is now aware it could produce valuable resources.
While it is debatable whether building-integrated agriculture is viable in urban areas with high land value, such forms of agriculture are appearing in Australia. A good example is Sundrop Farms based in Port Augusta, South Australia, which uses solar energy and seawater purification to produce tomatoes on a large scale.
There are many uncertainties as to how food and nutrient cycle will play out in the future. Ideally food should be produced locally and efficiencies need to be gained in water, nutrient and energy usage to grow food. It seems unlikely, however, that large amounts of food will be produced within cities due to the high cost of land. Similarly, it is unlikely that boutique community gardens can achieve the efficiencies required to compete with the produce of professional farmers on a large scale. It may be that urban agriculture and associated farmers’ markets remain a niche domain, but cater more to community well-being and culture.
But the greening of our cities appears to give considerable opportunities when it comes to supplying both water and nutrients by recycling wastewater. This may be achieved through existing approaches of creating recycled water networks or increased sewer mining and perhaps direct underground irrigation. There are moves to install a greater proportion of urine separating toilets, which create a direct means to capture nutrients and also decrease wastewater treatment costs. Peri-urban areas with a decreased land value have many advantages in food production, being able to benefit from both nutrient catchment in cities and in utilising recycled storm and wastewater. The key here is to ensure that urban sprawl over good farming land is managed effectively.
Wastewater treatment facilities seem to be in the box seat in being able to have an impact on returning excess nutrients from cities back to agricultural productivity. As technologies are developed to directly manufacture fertilisers from wastewater, there will be an increased need to work in closely with end-users (farmers) with regards to the fertiliser requirements needed for efficient nutrient uptake by crops. The ability to close the loop on nutrients will work towards the sustainability of food production and create resilience against the need to import large amounts of fertiliser.
The author, Dr Tim Muster, is a principal research scientist with the CSIRO based at the Waite Campus, Adelaide. This article also appears in Issue 2 of Corporate Waste Solutions.