Complete Conditioning of Digestate in Anaerobic Digestion Plants

Anaerobic digestion plants (“AD Plants”) are used to create energy from a variety of different biomasses. The problem is that roughly eighty percent of what goes in at the front end of the plant has eventally to go for disposal, usually to land, at the back end. Technologies now exist to dewater the digestate to form a cake like material which can be composted. The liquid phase from the dewatering process contains mainly water, minerals and some solids. Industrial evaporators can be used to treat the liquid phase to complete the conditioning of the digestate.

Decanter centrifuges are used extensively where continuous dewatering is required. They are widely used in waste water treatment both in municipal and industrial applications. The links on this page give more information on how the centrifuge works.

The main reason why they are not deployed more widely in AD Plants is the electrical energy required to run them. The centrifuge and the required pumps represent a parasitic load on the AD Plant whose primary function is to create energy.

Fortunately HAUS, a leading manufacturer of decanter centrifuges, has developed a new Low Energy Decanter Centrifuge that uses far less energy than in conventional designs. It works as follows:

When a decanter centrifuge is run up to operating speed from standstill it requires its full electrical load. This can be mitigated with the use of variable speed drives, inverters, which are used in all HAUS Low Energy Decanters. When the decanter is rotating at its full operating speed the excess energy produced by the motor is used to run the back drive motor. This means that there is almost no extra electrical demand to run the back drive / secondary motor. This is achieved by sophisticated software control, monitoring and feedback through the entire system. Other centrifuge manufacturers rely on conventional gearbox arrangements for differential speed control. Only HAUS has perfected this technique which is now adopted throughout the product range.

The net effect is to substantially reduce the electrical demand of the centrifuge in day to day operation which is good news for AD Plant designers.

So, the decanter centrifuge is very efficient at dewatering the digestate and produces a stackable crumb or cake as the solids phase and will typically remove up to 99% of the solids in the digestate. The liquid phase, centrate, contains all of the water, minerals and the residual solids the decanter could not remove. If there environmental constraints that prevent this going to drain then evaporation is an established solution. The evaporator effectively boils the water off as steam and this is condensed back to clean water that can safely go to drain or for re-use in the plant.  The remaining minerals and solids come off the evaporator as a thick pasty, concentrate, which can be added to the cake for composting.

The evaporator shown is a SigmaStar Rising Film Plate Evaporator from API Schmidt Bretten, a leading German manufacturer of evaporation  systems and process experts.  These are either supplied as single stage or multi-stage  evaporation systems.  These systems can also be fitted with a Thermal Vapour Recompression module to significantly reduce energy consumption.