System Mix explains how the Mini-Drop and Rotamix systems, both of which incorporate Vaughan Chopper Pumps, are helping to avoid      equipment failure caused by blockages in sewage sludge mixing and anaerobic digestion applications respectively

Many sewage sludge mixing applications utilise submersible, guide-rail type mounted mixers on the basis that they are easy-to-install and remove, thanks to the guide rail system. However, these systems are vulnerable to failure usually due to rag-accumulation on their rotating parts (Picture 1) which results in expensive repair or replacement. In many cases the build-up of rag becomes so severe that the lifting davits exceed safe working limits and this can lead to possible safety implications. Also, process throughput and the associated equipment reliability, can also be affected.

System Mix, a division of P&M Pumps, the UK distributor of Vaughan Chopper Pumps and Rotamix Tank Mixing Systems, takes a versatile approach to sludge mixing, resulting in a number of successful installations across the UK. These include Peacehaven (Southern Water), Howden (Northumbrian Water), Jaywick & Clacton (Anglian Water) and Mogden (Thames Water).

Retrofit system

A recent requirement has been to develop a ‘Mini Drop’ retrofit mixing system that can be installed into a tank without having to empty and clean it. There was also the need to utilise existing guide rails and lifting equipment, previously used for unsuitable submersible mixers which failed due to rag accumulation. This type of system provides effective mixing for tanks where a suitable suction connection is available for re-circulation through the guide rail mounted vertical nozzle manifold header.

It is not uncommon for users of submersible mixers where fibrous material causes frequent and expensive breakdowns (Picture 2) to have to tolerate the situation because of the cost-implications associated with emptying and cleaning sludge tanks. However, the Mini-Drop system reduces these costs by eliminating the need to empty and clean the tank, as it utilises existing guide rail and lifting davit arrangements and in some situations the existing tank openings can also be utilised. The associated Vaughan Chopper Pump required for the system is competitively priced when compared against the frequent submersible mixer repairs and can also be supplied on a hire basis if required.

The System Mix Mini-Drop system is in operation at several sites and is proving to be a reliable and cost-effective alternative to submersible mixers where capital projects are unlikely to be allocated.

The main features and benefits are: utilisation of the powerful and effective external Vaughan Chopper Pump (Picture 3); tank entry not required to install or remove the system; orientation of the nozzle can be adjusted as required; the pump conditions as it mixes, all submersible parts in 316 stainless steel; quick installation, even into full tanks; full design service available if required.

In addition to finding success in  sewage sludge mixing applications, System Mix is also helping anaerobic digestion plant owners meet their objectives through the use of the Rotamix System incorporating the Vaughan chopper pump. 

While maximising the benefits of an anaerobic digestion system involves achieving a higher gas yield, reducing foaming, and eliminating the potential for maintenance problems, many owners are also seeking to maximise the sustainability benefits of their AD process plant, which means each part of the process is coming under even closer scrutiny.


Mixing is a crucial part of the AD process. Andy Parr, director of System Mix, said: “It is crucial to the AD process that suitable pre-conditioning of solids is carried-out prior to digestion and our system has been proven to be one of the most effective in achieving this.

He continued: “The Vaughan pump prevents re-accumulation of fibrous and fatty material in the digester and this means that material continues to pass through the nozzles. This in turn ensures that digestate is adequately conditioned and actually benefits all post-digestion equipment.

“As many AD operators are now looking to maximise the sustainable credentials of their plant and equipment even further, we are confident that the benefits of our mixing systems are becoming even more important,” he concluded.

Over the last few years System Mix has been involved with ‘retrofit’ projects at existing AD plants where existing mixing systems were contributing to operational and biological issues.

For example, high grit deposition was reducing the active volume of digesters and threatening to reduce output outside compliance requirements. Also, compressors were requiring high levels of maintenance and were costly to keep running. Furthermore, as deposition increased, the maintenance issues escalated as  the rag mass reweaving within the digester impaired their efficiency, and caused blockage problems in the heat exchangers and further downstream pumps and systems.   

System Mix

T: 01487 830123