As available land area shrinks and human population soars, the demands on available land and water are generating a significant new interest in urban farming. But in many areas of the world, urban farming relies on using a resource which is both a blessing and a curse - wastewater.
For thousands of years, farmers living in and around cities have irrigated their crops with contaminated water. Not only because they had no choice, but because the excreta and urine present in the runoff from settlements actually added nutrient value to the soil and improved plant growth.
In fact such "animal-only" wastewater is better than freshwater for farming, and its use is an underlying reason why arable soils around ancient cities have remained viable despite centuries of intensive cultivation. Today it is estimated some 27 million hectares worldwide is irrigated with urban wastewater.
Recycling nutrients has also meant farmers do not have to buy expensive fertilisers, and irrigating farmland with wastewater also "treats" water that, if discharged directly into a river, would cause eutrophication problems further downstream.
However modern civilisation has upset the balance of this practice in a number of ways.
There are a plethora of contaminants present in urban runoff: heavy metals, toxic solvents, pesticides and herbicides, plastic and rubber particles, and industrial waste. Even after some form of treatment, many of these contaminants are likely to deplete soils or inhibit plant growth. In the worst case, they can turn productive areas barren.
Complicating matters yet further, urban runoff has also introduced exotic diseases, hormones, pests, and mutated microbial life into the waste stream. There are also issues surrounding unprecedented volumes of human waste as urban populations expand beyond historical, locally-supportable levels.
The application of modern standards of health and hygiene has seen a drive to sideline wastewater as a resource in order to delimit the effects of diseases spread through eating waste-contaminated food. Naturally, this has always been the downside of using wastewater for food production. While many communicable diseases do not survive long outside a host body, parasites such as the eggs of hookworms and tapeworms do, as do E.coli bacteria.
So to some extent, the debate around using wastewater today is over the change in standards: afflictions that were accepted as a normal part of everyday life centuries ago are now seen as unacceptable, especially by Western standards, and also curable, and therefore undesirable.
By 2025, nearly 3 billion people will live in regions suffering severe water shortages. A recent survey by the International Water Management Institute of 53 cities in developing nations found that around 80 percent of farms used untreated wastewater for irrigation. Issues surrounding use of wastewater are not about to go away.
With the renewed rise of organic farming coupled with a desire to produce and consume food locally to reduce fossil-fuel and agrichemical use, urban agriculture is in the increase.
Initiatives such as the Transition Towns movement are encouraging an increase in organic farming. While to a large extent in the developed world it is still driven by choice, in some places, such as Cuba [video clip], it is already driven by necessity.
In many Western cities clean water - or, at worst, well-treated wastewater - is still readily available to grow crops. However questions will need to be asked and addressed around quality and supply of water for such uses if current urban farming growth continues.
In less developed cities that lack either treatment infrastructure or suitable supply networks - and 85% of such places have no wastewater treatment - farmers have no alternative, and must generally rely on subjective smell and taste tests to determine whether the water is safe to use.
The good news is there are some simple methods that can significantly reduce risk. These include letting the water settle in a pond, so eggs from worms drop out of the water, and irrigating around the crops rather than on top of them, so residues do not stick to leaves. Ideally when the crop is harvested, it should be washed with fresh, clean water in the market. If these practices are followed, world authorities rate the risk of harm as "minimal".
There are also simple ways of ensuring more available water is utilised. A foot-powered treadle-pump, used throughout eastern India, Nepal, and Bangladesh, is being promoted as one tool that can increase the take of available water significantly without having to rely on machinery or electricity.
Spreading such "grass-roots" technology is vital if the world is to address the impending water crisis, It is predicted that farm water use will increase by 15-20 percent before 2030 to feed the growing population, but environmental scientists say water use needs to fall by 10 percent in that time to protect rivers, lakes, and wetlands, especially with impending consequences of climate change.
Something has to give to balance that equation. Finding ways to increase the use of wastewater for agriculture [pdf] while at the same time making it safer to use may be one way forward.