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FEATURES

SEWAGE AS A FORESTRY MEDIUM

Across South Africa, the proverbial sewage has hit the fan. With municipalities unable to efficiently dispose of sewage from pit latrines and VIP toilets in outlying areas of each metropolitan, piles of sludge are building up beyond capacity until they (quite literally) join municipal workers in hitting the roof.

After 6 years, the average pit latrine will be filled by 1.5m3 of condensed sewage (referred to as sludge). For a large proportion of pit latrines and VIP toilets in South Africa, that 6 years is long past and sludge is escalating beyond the design capacities of the units. Compounding the problematic build-up of sludge are the limited means of disposal available to South African municipalities: pit latrine sludge may be removed by hand and buried on-site (provided the pit latrine or VIP toilet is built on a level and relatively spacious plot), or the top structure is moved and the old pit is filled in while an entirely new one is built (provided the toilet structure is not made from brick or blocks). The third option for disposal is to transport the sludge to existing wastewater treatment works..

None of these options appear to be working: in cities it is rare to find sufficient space for on-site disposal and it is economically futile to add concentrated sewage to the diluted, waterborne sewage of municipal wastewater treatment works. (The sludge from one pit latrine has a solids and nutrient loading equivalent to 500 kilolitres of normal sewage.) Further, wastewater treatment works do not have the on-site capacity to sort the human waste from plastic bags, disposable nappies and other household waste that is also thrown down pit latrines.

Added to the accumulation of pit latrine sludge is the major build-up of secondary sludge in all metropolitan and many district municipal wastewater treatment works themselves, as the on-site drying ponds and anaerobic digesters that traditionally clear out secondary sludge can no longer keep up with the sludge intake.

The result? A build up of thousands upon thousands of tonnes of sludge across South Africa - in Durban alone, 400 000 tonnes of excess sludge awaits processing. That is enough to cover the main field at ABSA King’s Park stadium to a depth of 30 metres.

Creating another more efficient form of disposal

In the past, South Africa was able to dump any excess sludge in landfill sites or pump it to sea outfalls (in the case of coastal cities). But due to financial constraints and stricter environmental regulations, both of these are fast becoming impossible. There is a clear national need for a more efficient and useful disposal of pit latrine and secondary sludge. The answer might well lie in forestry.

Partners in Development (PID), a sustainable water, sanitation and civil engineering firm based in Pietermaritzburg, have been following US projects on the use of waste water treatment works sludge as a nutrient base with which to rehabilitate land and grow trees. Following similar problems with landfill and agricultural surface applications, scientists in Maryland, USA, pioneered “a deep row entrenchment of waste water treatment works sludge”, in an effort to recycle the precious nutrients present in sludge and thereby rehabilitate barren land. More than twenty years later, the land is well rehabilitated and no adverse effects have been recorded.

Having taken note of the US success of deep row entrenchment, PID – led by Managing Director Dave Still - have taken note of the success of this project and have begun implementing a similar study in South Africa in order to investigate how the sludge entrenchment technique may be applied under local conditions and what safe working procedures (handling and transport, maximum application rates etc.) should be developed to protect the health of workers, local communities and the environment. The study is funded by South Africa’s Water Research Commission, while scientists and engineers from the University of KwaZulu Natal are making key contributions to the investigations.

The study will use both secondary and pit latrine sludge to cultivate banana trees, Eucalyptus grandis and Acacia mearnsii (black wattle) to assess the commercial opportunities for agroforestry and biofuel production amongst others. Trees are an effective crop to cultivate from sludge entrenchment techniques due to their long-term growth and the lack of possible pathological contamination that can be passed on from the sludge to end-users.


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