Washout of slow-growing cultures
In continuous flow operation, slow-growing microorganisms for difficult fibrous compounds can be displaced from the process.
Technical solution for existing and new biogas plants
Batch anaerobic digestion in fully mixed stirred digesters
The process uses proven fully mixed stirred vessels as temporarily closed batch anaerobic digesters rather than only in continuous-flow operation. Defined minimum treatment times and retained microbial cultures improve the treatment of fibrous and heterogeneous organic feedstocks.
Initial challenge
Continuously operated stirred digesters often fail to ensure sufficient minimum treatment time
Uncertain actual retention time
Short-circuit flows can cause part of the substrate to leave the vessel much earlier than the calculated average retention time would suggest.
Unsuitable cultures for changing feedstocks
With varying feedstocks, a microbial culture permanently adapted to the respective process stage and substrate quality is often lacking.
Insufficient hygienisation
Without a defined minimum thermophilic treatment period, reliable whole-stream hygienisation of digestate is difficult to demonstrate.
Process principle
Alternating batch operation with partial emptying
At least two process vessels per stage
Hydrolysis, main digestion or post-digestion stages are equipped with at least two stirred vessels. While one vessel is treating substrate, the other can be filled or emptied.
Defined treatment phase
After complete filling, the vessel is neither fed nor emptied for 5 to 15 days. Treatment is limited to temperature control and demand-based mixing.
Partial rather than complete emptying
Before the next filling cycle, the process vessel is emptied only to the extent that at least 20%, preferably around 40%, of the vessel content remains.
Remaining substrate as inoculum
The substrate remaining in the vessel contains the microbial culture adapted to the respective process stage and acts as active inoculum for the next batch.
Controlled organic loading
Refilling is controlled so that the daily load does not exceed 15 kg of organic dry matter per cubic metre of biomass already present.
Mesophilic main and thermophilic post-digestion
Main digestion can be operated in batch mode for at least five days under mesophilic conditions. A thermophilic post-digestion period of at least ten days additionally enables whole-stream hygienisation.
Technical integration
Use of existing stirred vessels in multi-stage plants
Hydrolysis stage
Hydrolysis can be operated continuously or also in batch mode. With two vessels, defined aerobic or semi-aerobic treatment times can be achieved.
Main digestion stage
Fully mixed stirred vessels can be used in mesophilic batch operation with adapted cultures and a guaranteed minimum treatment period.
Post-digestion stage
Thermophilic batch operation supports further degradation of fibrous components, retains slow-growing cultures and enables hygienisation.
Storage tank and phase separation
A downstream storage tank decouples batch operation from continuous or demand-based phase separation of the digestate.
Client benefits
Higher process reliability and better use of existing plant equipment
Use of proven stirred vessels
Existing fully mixed digesters can be technologically upgraded through adapted operation without necessarily requiring special digester designs.
Retention of adapted cultures
The retained inoculum prevents complete removal of microorganisms adapted to difficult substrate components.
Guaranteed minimum treatment times
The batch phase ensures that the entire vessel content actually undergoes the intended treatment period.
Improved energy recovery
Spatially separated process stages and adapted cultures can improve degradation of different organic fractions and increase biogas yield.
Reduced mixing-energy demand
During the batch phase, mixing is performed only as intensively and as often as required for gas release and prevention of floating layers.
Hygienised fertiliser products
A thermophilic post-digestion period of at least ten days can support whole-stream hygienisation and improve the quality of organic NPKS fertilisers.
Technical background
The technical solution presented is based on German published patent application DE 10 2018 002 883 A1. The number of vessels, usable volumes, batch cycles, temperature control, mixing periods and organic loading must be adapted to the specific feedstocks and existing plant configuration.
Technical project assessment