Pollution in rapidly expanding cities poses a serious threat to public health. Lacking adequate sewerage systems and treatment plants, many cities discharge daily huge volumes of raw human wastes and industrial effluent into the environment. In slums, diarrhoea caused by contaminated drinking water is a major cause of child deaths.
Garbage is left to rot in the streets or dumped unsorted into landfills, adding to ground water contamination. Industry and traffic produce air pollution, responsible in Jakarta for a third of all respiratory illnesses. The urban poor face other environmental hazards: settlements built on marginal land are vulnerable to landslides and flash floods.
Urban and peri-urban horticulture can turn waste into a productive resource. In North America, cities routinely recycle organic waste and offer it to citizens as compost for home gardens. In Addis Ababa, a private company collects each day some 3.5 tonnes of organic waste and converts it into almost two tonnes of high-quality fertilizer. Cuba’s national programme for UPH prohibits chemical fertilizer in cities and encourages instead organic composting.
Using wastewater for horticulture is more problematic: pathogens on vegetables grown with untreated wastewater can cause gastrointestinal ailments and even cholera. But, when appropriately treated for agricultural re-use, wastewater from domestic sources can supply most of the nutrients needed to grow fruit trees, vegetables and ornamental plants.
To reduce the risk of contamination, Funds for Africa together with FAO helps train vegetable growers in the safe handling of wastewater and selection of suitable crops. In Gaza and the West Bank, it introduced low-cost treatment units that allowed residents to irrigate gardens and orchards with the greywater discharged from kitchens and showers.
As competition for urban water intensifies, wastewater recycling for horticulture needs to be incorporated in urban planning. One promising option for developing cities is shallow stabilization ponds that use algae and bacteria to eliminate pathogens while retaining nutrients.
UPH has other environmental benefits. It reduces the need to transport produce into cities from distant rural areas, generating fuel savings, fewer carbon dioxide emissions and less air pollution. It lowers city temperatures – in Cairo, rooftops planted with vegetables are 7° C cooler than those next door – and, when practised on greenbelts, improves landscapes and citizens’ quality of life.
On Peru’s arid coast, horticulture has helped to “green” many municipalities. Greenbelts also stabilize environmentally fragile land, such as hillsides and river banks, and protect them from being used for unsafe residential development. In Bogotá, Hanoi and Sao Paulo in Brazil, city gardens help maintain good soil structure and porosity, which improves aquifer recharging and reduces runoff, thus preventing landslides and flooding.