Development countries use various forms of pit latrine. Design improvements in the past thirty years have upgraded what used to be a crude and temporary system into one that is desirable and affordable. But the chambers of “permanent improved pit latrines” have to be emptied every three years, and their sludge contents have a wide range of flow behaviours and are notorious for being difficult to handle by a conventional vacuum tanker.
Pit latrines contain very different contents from domestic sewage in developed countries. In particular, the liquid content can be as low as 50 per percent. There are usually considerable quantities of sand and stones, together with rubbish such as rags, wood pieces, bricks and bottles.
The main constraints on designing an emptying system were the viscous and abrasive nature of the sludge. In fact, in many urban housing areas with uncontrolled development, access to individual household latrines is often very limited.
To cope with this difficulty, BRE developed BREVAC, which combines a partial vacuum effect with pneumatic conveying. There was also considerable potential for use in the UK, particularly in the oil and water industries. This was for emptying settlement lagoons.
After conducting exploratory pumping trials at Garston. It soon became clear that the only feasible method was a suction system employing a flexible hose, and a specification was devised for what became known as the BREVAC suction tanker. A prototype BREVAC tanker was built by Airload Engineering Ltd and shipped to Botswana in September 1983 for extensive field trials in conjunction with the Botswana Ministry of Local Government and Lands, the World Bank and the WHO International Reference Centre for Waste Disposal.
BREVAC pumps over horizontal distances of up to 50 m and vertical lifts of at least 6 m, allowing for the depth of the chamber and the height of the vacuum tank. The storage capacity is over 5000 litres, and sludges that cannot be pressure ejected can be discharged by tipping the tank.
Trials showed that BREVAC handled compacted pit sludge effectively and hygienically. However, the high capital cost was a major drawback to its wide application in developing countries. More effective planning of pit emptying services – and urban layouts – can reduce costs. Another option was the development of very small machines such as BREVAC Limited Access. This comprises a liquid ring vacuum pump system, mounted on a Land Rover or similar suitable pick-up truck and working with a fleet of mini-tankers to collect and transport pit sludge to disposal. A liquid ring pump is a rotating positive displacement pump where the vanes are an integral part of the rotor. They churn a rotating liquid ring to form the compression chamber seal. They are an inherently low friction design, with the rotor being the only moving part.
BRE developed BREVAC, and the Permanent Improved Pit latrine, was carried out by BRE as part of its programme for the (then) Overseas Development Administration.
R F Carroll CEng MIMechE : Mechanised emptying of Pit Latrines
Overseas Building note 193 ; BRE1989
Bosch and Schertenleib; Emptying on site excreta disposal systems
IRCWD News 21/22 May 1988
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