The efficiency of shredded and briquetted wheat straw in anaerobic co-digestion with dairy cattle manure

• Straw shredding and briquetting were evaluated in co-digestion with cattle manure.
• The use of straw as co-substrate increased cellulose and hemicellulose content.
• Shredded and briquetted straws increased specific and volumetric methane yields.
• Shredding and briquetting for co-digestion resulted in a positive energy balance.
• Briquetting technology can be suitable in decentralised biogas plants.

Anaerobic co-digestion of cattle manure (CM) with shredded or briquetted wheat straw (SS and BS, respectively) was evaluated in thermophilic continuously stirred tank reactors (CSTR) in two experiments (lab and full-scale). Three lab-scale CSTR (15 l) were used with 20 days hydraulic retention time (HRT); one was fed with CM and the other two with mixtures of CM (95% of fresh matter, FM) and SS or BS (5% FM). In the second experiment, two full-scale CSTR (30 m3) were operated with 25 days HRT; one reactor was fed with CM and the other with CM + BS (9% FM). Ultimate CH4 yield was analysed from each substrate. Biochemical CH4 potential at 21 days for CM, SS and BS were 128; 187 and 200 lSTP [CH4] kg−1 [VS]. Anaerobic digestion of CM, CM + SS and CM + BS in lab-scale reactors yielded 165; 214 and 217 lSTP [CH4] kg−1 [VS]. In full scale-reactors, CM and CM + BS yielded 264 and 351 lSTP [CH4] kg−1 [VS]. Increments of 31 and 33% on CH4 yield were achieved in CM + BS compared to CM in lab and full-scale reactors, respectively. Regarding the energy balance, the energy yields were the same for both reactors using straw as co-substrate (CM + SS and CM + BS) after subtracting the energy consumption of the pretreatment, corresponding to 1100 kWh of net energy output. However, briquetting technology could be advantageous for biogas plants where the straw might be transported over longer distances, due to reduction of the transportation costs.