WASTE IS NO LONGER A FORGOTTEN RESOURCE

Our specialisation is water and wastewater. Anaerobic digesters are one of the tools in our box. Interestingly, when Talbot initially installed anaerobic digesters some 20 years ago, biogas was viewed as merely a by-product.

Today, more and more of our clients are utilising biogas for electricity or heat generation, as it is much more financially feasible,” says Grahame Thompson, project director, Talbot.

Biogas is dependent on the amount of organic matter in the wastewater. “Most of our builds have been for the food and beverage industry where there is a high volume of sugars in the wastewater. There are also massive opportunities in the pulp and paper industry. But unfortunately, not all industries are suited to biogas production due to some toxic elements found in the wastewater. Interestingly, domestic wastewater typically has low organic content and may not be the best option for biogas production, which is why aerobic processes are mostly used in municipal wastewater plants,” he adds.

Feasibility

When assessing the feasibility of a potential biogas project, Talbot will look at both the economic and technical aspects. “We need to understand the drivers –it could be compliance, security of energy supply, waste management or environmental sustainability. We then look at the volume of wastewater and the organic content and understand where the wastewater is discharged.

There may be a limit on the amount of wastewater to be discharged or the tariffs may be expensive,” says Mike Smith, associate director, Talbot.

The chemical oxygen demand (COD) in wastewater determines the size of the digester. Biochemical oxygen demand (BOD) could be used to measure the organic content in wastewater, but COD is a faster test. The more COD in wastewater, the better the economies of scale. “Typically, a feasible biogas project in South Africa five years ago would require more than 5 000 mg/ℓ of COD. Today due to advances in technology and stricter regulations and higher tariffs around the discharge of wastewater, 2 000 mg/ℓ to 3 000 mg/ℓ of COD is feasible for a bigas project,” explains Smith.

The size of anaerobic digesters has also drastically decreased over the years and they therefore take up a much smaller footprint. They are also tightly sealed and therefore no longer release any odours. Depending on the wastewater and conditions, the conversion of waste to energy is significantly faster.

In addition to biogas, anaerobic digestion facilitates water recovery, reducing the amount of wastewater discharge and the use of potable water. “In South Africa, there is a huge interest in water reuse because our municipal systems are under strain and recovering water from industrial effluent limits the pressure applied to water and wastewater treatment facilities,” states Thompson.

Due to the high cost of anaerobic plants, there are various financing options. In some instances, Talbot can fund an anaerobic digestion plant, use the client’s wastewater and then charge for the plant’s output (water recovery/biogas). “The client is receiving the benefits, and paying for it on a tariff basis, instead of laying out a huge amount of capital. Smaller anaerobic digesters can be rented on a three-year or longer basis,” adds Smith.

Challenges

Thompson describes an anaerobic digestor as a living organism that constantly changes. Operators of biogas plants must constantly monitor the system and adapt feeding rates accordingly. It is important to remember that anaerobic digesters operate within certain temperature, pH and organic loading rate parameters. Biogas digesters can be overfed.

“It is important to know what has been fed into the system. For instance, dumping hydrocarbons or a biocide into an anaerobic digester can cause serious harm. That is why Talbot has a huge focus on the operational side of the business. The success of these types of systems is reliant on experts that monitor and control inputs and outputs of these digesters. The output of the digester largely depends on its input. Our experience in operating digesters across Africa is where we have developed a lot of our skill set.”

Anaerobic digestion holds significant promise for a country like South Africa with its energy and water problems. “This creation of energy from rich organic effluent has become a successful reality and is being applied by an increasing number of food and beverage manufacturers worldwide. These industries have started reaping the environmental and financial rewards and we look forward to more projects in the future,” he concludes.