Electricity and Control December 2023

ENERGY MANAGEMENT + THE INDUSTRIAL ENVIRONMENT

Kitchen waste as a cooking fuel – it works Food scraps seem an unlikely ally in any battle – except in the one we cannot afford to lose. Dr Karen Surridge of SANEDI reports how two military bases in Limpopo are demonstrating that the battle to save the planet can be won one kitchen at a time.

A man on a military base who talks and sings to what is in essence a high-tech compost heap is not an everyday sight. However, at South Africa’s most northern air force base, on the outskirts of Louis Trichardt in Limpopo, civilian military member, Morris Rathumbu has forged a close relationship with the biodigester he has made his own over the past two years. Air Force Base Makhado and the 523 Squadron (SQN) army base in the town of Louis Trichardt were selected, in partnership with the South African National Energy Development Institute (SANEDI), as the pilot sites for the Department of Defence’s biodigester project In 2021. This saw the installation of a pre-cast biodigester at each base to turn kitchen food waste, which is normally sent to landfill, into biogas that is used for cooking. The biogas plants consist of large, sealed anaerobic digesters in which waste material is decomposed to produce methane gas. The anaerobic digesters were installed underground at the bases, to make them unobtrusive and to prevent any unpleasant sights or smells around them. In addition to providing gas for cooking, the digesters produce an organic byproduct, called the digestate, which is an excellent organic fertiliser. A relatively simple solution to the twin problems of waste management and increasing energy costs makes sense. However, a critical factor that supports their success – or often scuppers biogas projects, Surridge says, is people. “Biodigesters are the most labour-intensive renewable energy technology. I liken it to having a baby, and people don’t believe that until they have a biodigester to look after and keep alive.” Literally. Biodigesting is an organic and biological process driven by the same bacteria that keep the human digestive system healthy. It is the live organisms that make every biodigester unique, with its own preferences and quirks. And that is why they require such care. At Air Force Base Makhado “I know my biodigester likes more water on Wednesdays and the time it takes me to sing Happy Birthday twice gets enough water into the digester’s daily diet,” says Rathumbu. “I love this technology because it is like having a family. Learning about it has broadened my horizons.” Surridge remembers Rathumbu being immediately interested when she introduced the project two years ago. “It is thanks to his dedication that the biodigester is such a success at the air force base,” she says. Where it usually

Karen Surridge of SANEDI, together with civilian military member, Morris Rathumbu, who takes care of the biodigester at Makhado Air Force Base. takes a biodigester six months to become fully operational, Rathumbu’s plant reached that point in only four months and in the two years since then it has run without a glitch. “I have not seen another project like this,” Surridge says, “where a biodigester is this happy.” The upshot is that the digester produces enough gas for the stove-top cooking required to provide breakfast and supper for 220 people at the base, every day. This will amount to a predicted saving of around 116 MWh of electricity over the combined system’s lifetime. Based on this performance, it will be possible to add two more biodigesters in parallel to power additional cooking burners and a water heater in the kitchen. At the 523 SQN army base The pilot project at the 523 SQN base took a more roundabout route to success. Although the base commander had also adopted his biodigester from the start, the duties of a high ranking officer limited the extent of attention he could give it. “Twice since July 2021 the biodigester had almost ‘died’ and had to be resuscitated,” Surridge says – both times when the commanding officer had been away from the base. In the first instance, it seemed that cleaning fluids used to clean the sink in which the macerator is installed had somehow reached the biodigester and had quickly killed all the bacteria. In the second instance, the system

DECEMBER 2023 Electricity + Control

13