Home-grown sewage-treatment process scores All Africa Games gold medal. The large numbers of people attending the All Africa Games in Maputo meant that organisers needed sewage-treatment infrastructure that really worked.

In a continent where water is scarce and infrastructure is lacking, a showpiece event can pose significant challenges for organisers. That was certainly the case when Maputo staged the 10th All Africa Games on 3-18 September 2011. A key challenge was providing the infrastructure to deal with the sewage from the athletes’ village and the stadium itself—this in a country where such infrastructure is distinctly lacking.

The organisers clearly wanted a system that could be installed quickly, would be easy and inexpensive to maintain, and that would meet health and environmental regulations. The solution proved to be the Biozone Nokak system, which was developed here in South Africa for use in many scenarios common in Africa in which water must be wisely used, money is short and skills for keeping the system operative are lacking.

The Biozone system uses naturally occurring bacteria in the effluent (“black water”) to produce clean water that can be used for irrigation, drinking for domesticated animals and game, and for household/office use in flushing and fire-fighting. To make the water potable further treatment is necessary. The system is modular and can be designed according the specific requirements of each use. Common applications include new townships, golf estates, wildlife estates and mines.

Five-step natural process

Raw sewage passes into a tank, from which material that is in solution is drawn off into the first of a series of sealed tanks or bio-reactors. Solids remain in the tank so the system does not block, and gradually break down into suspension in the water to enter the process. This stage is anaerobic.

The second stage, Nitrification, is aerobic as the right concentrations of dissolved oxygen (1 milligram per litre or more) are added by pumps located on the side of the tank. Under these conditions, Nitrosomanas bacteria that occur naturally in human waste begin to convert ammonia and ammonium into nitrates. The process is completed by Nitrobacter bacteria. Both types of bacteria are grown on a submerged medium that forms part of the system.

In the third stage, Denitrification, nitrate is converted to nitrogen gas by Heterotrophic bacteria. During this process, the oxygen concentrations are reduced, forcing the bacteria to obtain their oxygen from the nitrate, thus producing nitrous oxide and ultimately nitrogen. To set this process in motion, oxygen should not exceed 0.5 milligrams per litre, with fewer than 0.2 milligrams preferable. Nitrogen is not very soluble in water and the resulting gas escapes into the atmosphere. As nitrogen is the major component of our atmosphere, these nitrogen emissions have no negative environmental consequences.

The effluent is now taken to the clarification tank via gravity feed where the remaining separated solids sink to the bottom to form a sludge, leaving a clear, odourless liquid on the top. The sludge is pumped back into the collection tank to pass through the process again.

The clear liquid, however, passes to the fifth stage where it is sterilised using ozone. The ozone is produced by passing a weak 0.6 kW current through air—it oxidises any particles within the water, effectively purifying it for any of the uses noted above.

To make this water potable for humans means further purification—we tend to use reverse osmosis using a brak membrane.

Putting the system to the test in Maputo

The Maputo installation is the largest single installation of this technology that our company has completed to date. It can cope with around 2 250 people daily, and produces 500 000 litres of clear water a day. It processes the sewage from the stadium, the athletes’ village and a small neighbouring township.

The system was commissioned by 18 August, beating its 25 August deadline by a comfortable margin. It coped admirably with the needs of the athletes and spectators at the Games, and we are very happy to report that the Mozambiquan authorities are negotiating to put in a system to make the water produced potable. Such is the quality of the water that it will be cheaper to treat it than to treat the water from a nearby borehole!

Some conclusions

Based on this case study, it is abundantly clear that the Biozone process can work successfully at scale. The process is based on the natural process of decomposition for waste material—it just makes it more efficient and so quicker. And because it takes place in a totally sealed system, it is odour free and very easy to maintain. In fact, during the life of the system, which is determined by the 30-year lifespan of the sealed units, only the external pumps would need servicing and replacing as needed. Doing so does not require specialist skills, and the parts are readily available.

It’s worth also pointing out that the system uses relatively little power—the pumps use only 0.75 kW and, as mentioned already, the ozone generator uses only 0.6 kW.

This kind of approach offers a practical solution to Africa’s challenge to provide sewage infrastructure that does not place additional strain on overburdened infrastructure or that would require new infrastructure. It also means that water used for sewage becomes a resource that can be used to cut down on the need to use expensive and scarce potable water for irrigation and livestock use. It’s an African solution designed for African conditions.