Investigating the potential of deep row entrenchment of pit latrine and waste water treatment works sludge for forestry and land rehabilitation purposes

Funded by: Water Research Commission
April 2008 – March 2011

Partner organisations: Partners in Development; eThekwini Water and Sanitation 

Project description
In 2008, the WRC commissioned Partners in Development (PID) and the Pollution Research Group (PRG) to investigate the application of the deep row entrenchment method for both faecal and wastewater sludges under South African conditions. Deep row entrenchment is a method that was developed in the Washington, D.C./Baltimore, Maryland region of the USA, in which sludge is buried in deep rows, covered with soil, and then planted with trees. The research team sought to evaluate the extent to which a similar entrenchment method would adequately treat sludges in a manner that is more economic, efficient, sustainable and safe. In addition, the study aimed to develop guidelines for the optimal and safe implementation of this sludge management option, including handling and transport and maximum application rates. 

In South Africa, processing sludge from pit latrines has become an increasing problem as treatment plants are often overloaded and the number of facilities to accept sludge is diminishing. The concentrated nature of pit sludge limits the quantity that can be processed at a wastewater treatment works (WWTW) before COD and solids overload of the system causes process failure. The accumulation of secondary sludge in existing WWTWs is also a major problem in some metropolitan municipalities and many of the district municipalities of South Africa. Past research indicates that 33% of waste water treatment works were stockpiling sludge. Furthermore, the number of landfills prepared to accept sludge is shrinking and disposal costs at landfills are exorbitant.

Urgent interventions are required to deal with the escalating accumulation of sludges. South Africa’s new Guidelines for the Utilisation and Disposal of Wastewater Sludge encourage sludge management options that include recovering energy, recycling the nutrients or synthesising commercial products from the sludge. The potential benefits of the nutrients (nitrogen, phosphorus and potassium as well as micro nutrients) and the high organic carbon content of sludge have been well demonstrated and have led some agronomists to suggest that sludge is a more complete fertiliser than inorganic fertilisers. In South Africa, introducing organic carbon present in sludge to cultivated soils could yield a number of benefits, including reducing reliance on expensive fertilisers derived from scare phosphorus reserves and increasing food security when applied at the subsistence level. The utilisation of sludge must, however, be managed using stringent safety measures in order to prevent the uptake of harmful chemicals by humans or animals, contamination of the environment or the potential of exposure of humans to the pathogens found in sludge. Surface application of sludge on land for agriculture therefore requires that the sludge is first treated to destroy pathogens and achieve stability. 

To address these issues about treatment of sludges, this research project conducted entrenchment trails in three separate test grounds at which banana, Eucalyptus grandis and black wattle were grown. The study monitored changes in the sludge contents of covered trenches; monitored the movement of solutes and changes taking place in the surrounding soil and groundwater; and investigated the management, logistic, health and politico-legal aspects of transporting sludge, excavating trenches and planting trees/ vegetation. 

Data from the study reveal that the entrenchment method is a viable solution for sludge disposal in South Africa. Entrenchment of sludge enables nutrients to be retained and accessed by plant growth over time. Across the different trial sites, trees that were planted on sludge showed greater growth than trees grown exclusively on native soil. Lateral growth of tree roots enables trees to access sludge whether entrenched under or next to the trees, allowing for a variety of entrenchment methods and schemes. In addition, the study results indicate that entrenchment produces a more complete degradation of sludge in a three year period than can be achieved in traditional pits over a longer period of years. Sludge appears to dewater and degrade rapidly during the first year followed by a slow decrease in volatile solids, COD and moisture. Nutrient concentrations in groundwater did not exceed limits for drinking water even where unstable VIP sludge was entrenched in very sandy soils.

For municipalities in South Africa, the deep row entrenchment method opens a much needed avenue for the beneficial disposal of sludge. While further refinement of entrenchment techniques may be necessary in order for the option to be cost effective for forestry companies, a range of options could be implemented in which the municipality manages the entrenchment and monitoring of sludge, and a forestry company is contracted to manage a timber crop. Further work is needed to develop models which municipalities can use to implement this method cost effectively across a range of conditions. 

A Guideline has been prepared in consultation with local experts and is available to encourage implementation of the deep row method with three applications for wastewater or VIP sludge based on South African legislation and regulations, the recommendations of studies conducted in the US over the past 40 years and the knowledge gained from this research. The economics of implementing deep row entrenchment as a disposal method for municipalities was explored with Sappi and with Umgeni Water, which manages wastewater for Umgeni Municipality.

Publications and reports
  • Bakare, B.F., Foxon, K.M, Brouchaert, C.J., and Buckley, C.A. 2012. Variation in VIP latrine sludge contents. Water SA. 38, 4. 479-486. (Available on Water research Commission website; http://www.wrc.org.za/Pages/KH_WaterSA.aspx?dt=5&L0=1&L1=4)
  • Still, D., Salisbury, H., Lorentz, S., and Foxon, K.M. 2012. Beneficial Use of Faecal Sludge through deep row entrenchment. Sanitation Matters. Issue 4, March. 8-9