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Symposium 2011

Proposal - Why treat sewage as waste?

The Challenge

The United Nations estimates that the number of megacities with a population of more than 10 million will triple from 20 in 2003 to 61 in 2015. It is estimated that more than nine tenths of urban grow ...

The United Nations estimates that the number of megacities with a population of more than 10 million will triple from 20 in 2003 to 61 in 2015. It is estimated that more than nine tenths of urban growth will occur in developing nations, with four fifths of urban growth occurring in Asia and Africa.

In most developing countries partially treated and untreated sewage is simply put back into rivers and water bodies. This causes significant pollution, eutrophication, algal blooms, anoxia and consequent death of aquatic ecosystems, not to mention the health risks for those using the water for drinking and domestic purposes downstream. Many peri-urban farmers recognize that urban sewage discharges are a reliable source of water and nutrients for vegetable production. As a consequence almost 1 billion people depend on vegetables grown in this manner and about 200 million farmers make a living from this activity. However, health risks to farmers and consumers are considerable because of viruses, bacteria, helminthes and heavy metal contamination.

At the same time, many developing countries are reeling under the cost of fertilizer subsidies and high costs of treating contaminated water for drinking. Many African soils, in particular, after years of use and limited fertilizer applications are depleted of key nutrients and becoming less productive.

Western models of sewage treatment are usually too expensive and even when installed often break down through lack of maintenance.

The time has come for a new paradigm that views sewage as a valuable resource rather than unwanted waste. This will require innovative low cost engineering solutions that can provide limited primary treatment for the sewage and then use the waste water and separated solids in agriculture. Solids can also be used to produce biogas initially, before being utilized as fertilizer. The question confronting us all is can we develop cost effective systems to do this given both the health risks, construction costs and transport costs to get the fertilizer to where it is needed. I suspect the answer is yes, particularly if we take into account the externalities associated with untreated sewage discharge, the growing cost of artificial fertilizers and the growing demand for food. Some societies including China, successfully exploited sewage at village and small town level to sustain agricultural production for centuries. However, growing cities and towns mean that we need to look at a range of potential options to make better use of water and solid wastes. Fortunately, a number of agencies including the Gates Foundation, the WHO, and IWMI are starting to look at this issue from biophysical, engineering and economic and social viewpoints. This is work in progress, but what is needed in particular is that planners and policy makers realize the potential to do this and put their support behind innovation in this sector including the development of effective business models.

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