Getting the right mix is vital to digester performance

ADBA MEMBER PRESS RELEASE

Improving efficiency and reducing costs are key challenges in a tough economic climate. AD operators therefore need to make every aspect of their AD plant and process work as efficiently as possible. Questions to consider then are: what constitutes ‘good operation’ for a digester, how can it be measured and what can AD operators do to improve the performance of their digester and be even more competitive?

Cambridgeshire based System Mix Ltd, in conjunction with P&M Pumps have developed pumped mixing systems with their Rotamix System – introduced in 2003 – which incorporates the Vaughan Chopper Pump. Today, numerous AD operators across the UK are appreciating the benefits of System Mix’s expertise and the company is in an ideal position to provide some answers to the questions posed above.

Good operation can be measured by the quantity of gas produced per volume of feed material, ie: the best price for gas produced, multiplied by the volume, less the minimum of material to be processed at the lowest price, equals best profit.

Top priorities for operators to focus on:

• Highest gas production possible
• Maintained plant throughput at its design maximum – or higher
• Maintained working volume by minimising deposition
• Stable chemically balanced operation with no foaming problems
• Low maintenance costs combined with equipment reliability

Customer’s tend to gauge good operation by the quantity of gas being produced. However, running costs should also be taken into account, operators must remember best practice.

After commissioning and following a period of operation, operators should review throughput and performance and compare this to how the digester performed following commissioning. Markers for this include temperature profile and volumes being processed, and would provide a picture of how much capacity is lost through solids deposition in the digester.

Other things to consider…

1. The suitability and efficiency of preconditioning digester feed material should always be reviewed, ensuring that the material being fed to the digester is providing optimum digestion performance.

2. The efficiency and reliability of the equipment. Reviewing the performance of any ancillary equipment maintains the best working conditions for the life of the digester. For example effective mixing is a significant factor in overall digester performance.

System Mix have completed successful digester mixing projects over the last few years to Veolia, Shanks Waste Management, Fernbrook Bio, BioGen, Imtech, Black and Veatch, in addition to many of the UK’s waste water companies. One recent project is for Thames Water involving System Mix installations at their waste treatment sites at Becton and Crossness in Barking and Dartford.

The continuous development and understanding of the digestion process and with sludge rheology, plus a wealth of understanding of solids pumping and mixing, provides AD system operators with the confidence that they can install the Rotamix System, without fear of expensive downtime and poor performance.

Furthermore, by providing additional process performance guarantees, backed by CFD analysis (computerised, fluid dynamics) it is possible to show that over 95% of available working volume can be used ensuring that high volatile solids destruction and therefore gas production, is maintained throughout the working life of the digester.