Monday, June 20, 2011

Unlocking the Potential of Anaerobic Digestion From UASB Digesters

It is widely accepted that whilst anaerobic digestion is a proven, effective and highly efficient treatment system, upsets in performance remain common. Where Anaerobic Digestion is used in  Upflow Anaerobic Sludge Blanket (UASB) digesters as a treatment process for water treatment before it is discharged into a watercourse or stream, or even into a sewer for further treatment at a sewage works, even small variations in performance can have damaging effects.

The advantages of AD are substantial. 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 solid handling and large-scale pond systems, and limits reliance on non-renewable fuels.

Digester disturbances, however, continue to represent a significant risk limiting the widespread adoption of this technology.

In South Africa, under 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 Rands 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, under-loaded and relatively expensive.

Despite decades of research into anaerobic digestion technology, a fundamental understanding of upstream effluent management, system sensitivity and basic process control continue to be highlighted as ongoing concerns, severely limiting the reputation and diversification of this technology.

One company which reckons to have got this problem under control is the South African AD design and installation company Talbot & Talbot (Pty) Ltd.

They are a solutions-based wastewater engineering company with over 20 years of effluent treatment expertise. They continue to successfully build and operate Upflow Anaerobic Sludge Blanket (UASB) digester treatment systems throughout South Africa and Africa, across a wide range of industrial sectors.

Digesters designed and built by Talbot & Talbot vary in capacity from 1Ml/day to 5Ml/day, treating between 1ton and 25tons of COD/day, under general limit and special limit licence conditions. They believe that a high performance anaerobic digester (AD) treatment system is attributed to a fully integrated approach, which begins with a systematic site water management plan and process optimisation within the clients’ core business. 

They have found that Upstream focus is essential in preventing discrepancies in effluent data which result in incorrectly designed, overloaded and poorly performing digesters, to segregate and correctly dispose of solid waste streams, and to ensure 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 (Pty) Ltd. 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 rapidly identify changes in effluent quality 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, whilst water reclamation can be incorporated via ultra filtration and reverse osmosis technology.

More recently, Talbot & Talbot have successfully designed and commissioned biogas recovery systems which capture the bi-product of digestion, methane, as a renewable, CO2 neutral energy source.

A 25ton digester with a biogas production of 6500Nm³/day can typically produce 52tons of steam per day, which supplements non-renewable energy usage by up to 15%. The realised value of a biogas recovery project (BRP) can be directly comparable to the cost of the fuel it replaces.

As transport fuel prices continue to rise the logistical cost of supplying fuel to remote locations becomes a bigger consideration , and similarly concerns about the availability of reliable electricity supplies throughout Africa will continue to mean that having your own source of supply by AD has a potential beyond mere economics, which Talbot & Talbot is increasingly unlocking for its clients.

Adding a CHP (Combined Heat and Power Plant) coupled to an existing AD, to use heat which would, have otherwise have been wasted, typically offers a buy-back period of less than 2 years and forms a reliable, constant energy source to the industry that directly offsets the cost of effluent treatment.

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

Vist the Talbot & Talbot website for more information.

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