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The emptying and transportation of sludge from pit latrines in hard to reach areas is a common dilemma as narrow pathways or other obstacles make access by larger sludge emptying vehicles impossible. There are now a few motorized vehicles or machines that have been developed for densely populated areas with difficult or limited access. Designs such as the ROM2, Vacutug, Dung Beetle, Molsta, and Kedoteng desludging units carry a small sludge tank and a pump and can negotiate narrow pathways.
However, when you are faced with ‘difficult’ or thick sludge with very high solids content (>15%), the sludge needs to be made more fluid or else it cannot be pumped. Fluidization is the process where pressurised water is sprayed at the sludge to break it down and make it easier to pump. In most cases, the amount of water used during the fluidization process should only be between 15 – 20 % of the total sludge removed and after fluidization. The quantity of water used in the fluidisation of the pit sludge has a bearing on the efficiency and cost of the operation. A bigger percentage of water used means that less sludge is pumped out of the pit resulting in less operational efficiency and higher transport costs. There are also consequences for dewatering the sludge. The presence of solid waste in the pits can cause damage to the pumps. After fluidising the sludge, solid waste can be fished out of the pit using hooks or in some cases, (for example for large rocks etc.), the solids might have to be removed by hand.
In general, fluidising can take approximately 15 mins, and fishing approximately 30 mins.
The ROM2 is a vacuum-operated machine with an integrated high-pressure pump for fluidizing sludge and an 800L holding tank and has show particularly good results in the removal and transport of thick sludge in hard to reach locations. For more information, see research here: https://www.speedkits.eu/sites/www.speedkits.eu/files/Summary%20field%20testing%20pit%20emptying%20Blantyre.pdf
For specifications of the ROM2, see here: https://www.rombv.com/rom-mobile-desludging-unit-6
A note on health and safety The use of a vacuum truck presents a significant health improvement over manual emptying and helps to maintain the collection and storage part of the sanitation chain. Still, service personnel are in contact with faecal sludge and therefore need to wear adequate personal protective equipment (PPE).
A note on costs The investment cost for a vacuum truck is high. It however it can often become a lucrative activity for private entrepreneurs. The major operational cost is fuel, which depends on the distance to the discharge point / treatment facility. Cost for spare parts may also be high, as they may not be available in the local market.
A note on social considerations Truck operators are not always well accepted by the community and may face difficulties with finding appropriate locations to discharge the collected sludge. It is thus important to formalise the service and provide adequate equipment. If putting solid waste in the pits is a prevalent practice it should be addressed as part of hygiene or behaviour change activities, and through a proper solid waste management scheme.
References & Further Reading
Compendium of different sanitation technologies appropriate for emergency situations. Compendium of Santiation Technologies in Emergencies (pending publication) German WASH Network, EAWAG, Global WASH Cluster.
Report presenting the findings of research on desludging of sometimes difficult to access pit latrines with sometimes solid sludge. WASTE and NRC (2014) Testing and developing of desludging units for emptying pit latrines and septic tanks - Summary of findings field work in Blantyre - Malawi https://www.speedkits.eu/sites/www.speedkits.eu/files/Summary%20field%20testing%20pit%20emptying%20Blantyre.pdf
Book compiling the current state of knowledge on faecal sludge management Mikhael, G., Robbins, D. M., Ramsay, J. E., Mbéguéré, M. (2014). Methods and Means for Collection and Transport of Faecal Sludge. In: Strande, L., Ronteltap, M. and Brdjanovic, D. (Eds.). Faecal Sludge Management. Systems Approach for Implementation and Operation. IWA Publishing, London, UK, pp. 67-96. Available at: www.sandec.ch/fsm_book
An overview of faecal sludge emptying and transport businesses in Africa and Asia Chowdhry, S. and Koné, D. (2012). Business Analysis of Fecal Sludge Management: Emptying and Transportation Services in Africa and Asia. Bill & Melinda Gates Foundation, Seattle, US. Available at: www.susana.org/library
Methods of Pit Latrine Emptying, including a detailed analysis of experience with the Vacutug O’Riordan, M. (2009). Investigation into Methods of Pit Latrine Emptying. Management of Sludge Accumulation in VIP Latrines. WRC Project 1745, Water Research Commission, Pretoria, ZA. Available at: www.susana.org/library
Comprehensive summary of technical components, performance with different sludge types, and maintenance Boesch, A. and Schertenleib, R. (1985). Pit Emptying on-Site Excreta Disposal Systems. Field Tests with Mechanized Equipment in Gaborone (Botswana). International Reference Centre for Waste Disposal, Dübendorf, CH. Available at: www.sandec.ch