Efficient mixing is one of the most important stages in maximising the anaerobic digestion (AD) process and the ROTAMIX system from solids pumping and mixing specialists SYSTEM MIX Ltd (System Mix is the specialist mixing system design and supply subsidiary of P&M Pumps Ltd) is playing a vital role at many of the UK’s AD installations. A good example of this is at Southern Water (SW) where Rotamix systems are currently mixing in excess of 95% (85,000m³) of the total volume of sludge generated at Southern Water sites.

Good mixing is a pre-requisite when it comes to retaining digester volume, maximising solids reduction and ultimately increasing gas production with Southern Water in particular showing the foresight over a decade ago with their investment in the ROTAMIX concept. These systems are operating successfully on 14 of Southern’ s STC’s (Sludge Treatment Centres) which are more specific and focused on the sludge process than traditional wastewater treatment works digester volumes ranging from less than 1000m³ and up to 4500m³.

The first Rotamix systems were installed into Southern Water’s digesters back in 2004 and have been operating successfully for well over 10 years. Jason Curtis, P&M’s Southern and Thames Water Area Manager commented, “Southern Water have taken a very proactive approach to dealing with the issues around sludge treatment to ensure the assets they select are fit for purpose and offer efficient operation with minimal downtime. We welcome this opportunity to continue our relationship with Southern Water and its contracting partners”.

Commenting on the performance of the Rotamix Systems a Senior Sludge Operations Manager at Southern Water said, “It is the responsibility of maintenance management to continually seek ways to increase production or reduce costs.


We are delighted with the performance of the Rotamix Systems which help us to deliver cost benefits which we can pass onto our customers.”

The 38 digesters situated at the STC sites across the south of the UK include some tanks being both above and below ground and thus each mixing system is a bespoke design and often accompanied with a CFD (Computational Fluid Dynamics) analysis ensuring optimum mixing patterns and efficiency prior to even the first bolt being tightened on the pipe work at installation. The unique Vaughan SP range of auto self-priming chopper pumps are utilised for an above ground mixing solution to a below ground or submerged tank, something System Mix believe cannot be offered with any other pumped tank mixing design.

All Rotamix systems feature a pre-determined internal jet nozzle layout, the number, size and orientation of which depend on the tank size, geometry and contents being mixed. This is always the case for the Vaughan Chopper Pump which is always at the ‘heart’ of each design ensuring suitable liquid velocity and pressure within the closed loop system is always maintained without the risk of cogging the pump or nozzle barrels.

This means fibrous solids (and other similar challenging materials) that are prone to re-weaving and growing in size are constantly reconditioned within the tank, while remaining in suspension and eliminating settlement of solids on the tank floor. The two-tier rotation of the sludge or DUAL ZONE mixing as it’s more commonly known, is what maintains maximum tank volume for the anaerobic process to achieve optimum gas production.

However, this mixing concept is not exclusive to AD digesters. Southern Water alone have over 80 other tank systems mixing imported sludge, RAS (Return Activated Sludge), SAS (Surplus Activated Sludge), blended, thickened and stored sludge’s, to name but a few.

The UK government has set ambitious targets for zero food and packaging to landfill in the future and this will in part be significantly affected by access to technology that contributes to reducing a reliance on carbon based fuels and reducing emissions. System Mix are therefore excited about the potential for solving problems associated with disposing of organic waste materials for energy generation by the process of anaerobic digestion.

Furthermore, having worked closely with most of the UK’s other water companies on numerous AD mixing projects to the extent that several major works have become energy self-sufficient by the use of their own excess gas production with the help of reliable and efficient mixing.

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