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.
Low energy Decanter Centrifuges from HAUS
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.
Evaporators in AD Plants
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.
The combination of Low Energy Decanter Centrifuges with advanced Evaporators provide AD Designers and Operators a complete digestate conditioning package. This reduces the handling of waste by over eighty percent and the liquid phase is clean water i.e. a complete environmentally friendly alternative to existing methods.
Centri-Force is the UK distributor of Haus Centrifuges and API Schmidt Bretten Evaporators. If you would like to know more simply click on the link to send us an enquiry through the website or contact us by email and we will get back to you straight away. Alternatively, just pick up the ‘phone, call us on 0141 882 3351 and we will be pleased to talk through your issues, and of course there is no obligation to buy.
Reference. Utilisation of digestate from biogas plants as biofertiliser by Lukehurst, Frost and Al Seadi.