Urban agriculture is an important aspect of the wider issue of urban sustainability, both by being able to supply food from close-by and by offering livelihoods for city people. Another important issue, as already discussed, is the efficient use of nutrients from the urban metabolism that would otherwise end up as pollutants in rivers and coastal waters.

In many cities attempts are being made to use wastewater in urban food production. This applies particularly to cities in hot and dry places. For instance, in Adelaide, Australia, tens of thousands of hectares of land on the edge of the city are culti­vated using wastewater from the city for irrigation, growing vegetables as well as grapes and fruit. There is some concern about trace quantities of heavy metals that could accumulate in the soil, but it would take decades to cause any problems. Adelaide’s wastewater crop irrigation system is regarded as one of the great success stories of urban agriculture.

In Bristol, Wessex Water has developed its own system for turning sewage into a soil conditioner and fertiliser. It dries the city’s entire sewage output and turns it into small pellets called Biogran, which are then sold to farmers and land reclama­tion companies. Again, trace amounts of heavy metals have been quoted as problematic. But this is becoming less of a problem because cars no longer run on leaded fuels in the UK, and in Bristol de-industrialisation has led to a great improvement in the quality of Bristol’s sewage sludge.

Another important aspect of sustainable urban development is the creation of new kinds of eco-efficient housing estates. This concept is now flourishing across Europe. In South London, a pioneering project was completed in 2002 – the Beddington Zero Energy Development. This is a housing and workshop project for some 200 people, created by the Peabody Trust and the Bioregional Development Group. All buildings have south-facing facades and 30 centimetres of insula­tion in walls, floor and ceilings. The apartments require only 10 per cent of conventional heating energy and this is provided by a small, wood-chip fired combined heat and power plant. There are solar electric panels installed on all the south-facing facades and these will supply electricity to a small fleet of electric cars. All apartments have their own small roof gardens that can be used for recreation and/or vegetable growing. The estate’s wastewater is treated in a ‘living machine’ which uses plants and zooplankton for extracting surplus nutrients. The water from the treatment plant is used for irrigating gardens.

The Beddington project shows how ideas for making cities eco-efficient can be turned into practical reality. We need to turn the linear throughput of resources through our city into circular systems, where minimal inputs into the city result also in minimal waste outputs. Energy efficiency, resource productivity – these are the key themes in this context.

Good urban design in the twenty-first century should start by mimicking natural eco-systems. Above all else, we would be well served to learn from the metabolism of natural, closed-loop sys­tems in which all wastes are recycled into resources for future growth. This is an issue for policymakers, but also for the general public which needs to exert pressure on local and central governments, and on developers, to adopt forward looking practises.

We are certainly seeing a lot of interest in these ideas now in cities all over the world. But a major push is still needed to reduce the wasteful resource consumption and the vast, sprawling ecological footprints of cities that we have at present. We need to move towards much more localised, efficient, cir­cular urban systems, and this scenario certainly includes the use of land within and on the edge of cities for food production. Ideas for creating sus­tainable cities have been around for some time, but implementation on an adequate scale has hardly begun.

Updated: October 4, 2015 — 2:30 pm