It is widely accepted that while anaerobic digestion is a proven, highly effective treatment system, upsets in performance remain common.

A correctly run digester will efficiently convert up to 95% of organic material into a low-odour stabilised slurry and produce a renewable resource in the form of biogas that can be flared or utilised on site. This eliminates the need for additional solids handling and large-scale pond systems and limits reliance on non-renewable fuels. However, digester disturbances continue to represent a significant risk, preventing the widespread adoption of this technology.

Under the Department of Water Affairs (DWA) water use licence conditions, the discharge of untreated effluent into water bodies following reactor failure can attract non-compliance penalties of millions of rand and, under special limit conditions, force full production shutdown of operations for lengthy reseed periods of the digester.

Consequently, new applications are often over-engineered, underloaded and relatively expensive. Despite decades of research into anaerobic digestion technology, the lack of a fundamental understanding of upstream effluent management, system sensitivity and basic process control continues to be highlighted as an ongoing concern, severely limiting the reputation and diversification of this technology.

Talbot & Talbot continues to build and operate upflow anaerobic sludge blanket (UASB) digester treatment systems successfully throughout South Africa and Africa, across a wide range of industrial sectors. Digesters vary in capacity from 1 to 5 Mℓ/d, treating between 1 and 25 t/d of COD under general limit and special limit licence conditions.

The company believes that a high-performance anaerobic digester (AD) treatment system is the result of a fully integrated approach, which begins with a systematic site water management plan and process optimisation within the clients’ core business. Upstream focus is essential in preventing discrepancies in effluent data, which result in incorrectly designed, overloaded and poorly performing digesters for segregating and correctly disposing of solid waste streams and ensuring that potentially harmful contaminants are identified and isolated from the effluent system.

Plant design and construction is offered on a full turnkey basis, in addition to a full aftercare service through Talbot Operations, a business division of Talbot & Talbot. This includes operator training, performance review and compliance monitoring on a support basis, and a dedicated team of competent operations and maintenance personnel on a fully outsourced basis.

In addition, effluent sampling schedules are implemented via Talbot Laboratories to identify changes in effluent quality rapidly and monitor final discharge compliance. This guarantees the long-term treatment potential of UASB technology and a full commitment to the industry, which cannot be achieved on a ‘build only’ basis.

A well-managed, high-performance AD system provides clients with the opportunity to recover water and energy resources from their effluent. Secondary treatment systems in the form of activated sludge (AS), sequential batch reactor (SBR) and biofilter technology provide exceptional effluent treatment options, while water reclamation can be incorporated via ultrafiltration and reverse osmosis technology.

More recently, Talbot & Talbot has successfully designed and commissioned biogas recovery systems that capture the by-product of digestion, methane, as a renewable, CO2-neutral energy source. A 25 t digester with a biogas production of 6 500 Nm³/d can typically produce 52 t/d of steam, which supplements non-renewable energy use by up to 15%.

The realised value of a biogas recovery project (BRP) is directly comparable to the cost of the fuel it replaces, the logistical cost of supplying fuel to remote locations and the availability of electricity throughout Africa. This system, coupled to an existing AD, typically offers a buy-back period of less than two years and forms a reliable, constant energy source for the industry that directly offsets the cost of effluent treatment.

Despite these benefits, Talbot & Talbot states that the real value of BRPs is the renewed interest in AD technology, a deeper understanding of good effluent handling practices and an ongoing commitment to ensuring anaerobic digesters reach their full treatment potential.

by Claire Lipsett Pr.Sci.Nat., process specialist at Talbot & Talbot