Exploring increased hydraulic retention time as a cost-efficient way of valorizing residual biogas potential.

Effective substrate utilization with low residual methane yield in the digestate is crucial for the economy and sustainability of biogas plants. The composition and residual methane potential of 29 digestate samples from plants operating at hydraulic retention times of 13-130 days were determined to evaluate the economic viability of extended digestion. Considerable contents of fermentable fractions, such as cellulose (8-23%), hemicellulose (1-18%), and protein (13-22%), were present in the digestate dry matter. The ultimate residual methane yields varied between 55 and 236 ml/g of volatile solids and correlated negatively with the logarithm of the hydraulic retention time (r = -0.64, p < 0.05). Economic analysis showed that extending the retention time in 20 days would be viable for 18 systems if methane were sold for 1.00 €/m3, with gains up to 40 €/year/m3 of newly installed reactor capacity. The results show the importance of operating at sufficient hydraulic retention time.

Effect of ultrasonic and electrokinetic post-treatments on methane yield and viscosity of agricultural digestate.

The impact of post-treatment of digestate prior to its recirculation to the digester has been evaluated with industrial-scale ultrasonication and electrokinetic treatment units. Residual methane yields of untreated digestate samples from four biogas plants varied between 99 and 134 ml/g of volatile solids (after 97 days of digestion). At the tested conditions (1.90-7.60 kWh/t for ultrasonication and 4.37-6.06 kWh/t for electrokinetic disintegration), no significant increase in methane yields were obtained. Increases in maximum methane production rates of up to 42% were observed in samples from one of the biogas plants, resulting in net energy gains between 1.06 and 7.04 kWh/t (after 26 and 66 days of digestion). Both treatments significantly reduced digestate viscosities, especially of highly viscous samples. Lower viscosities could reduce the energy demand for pumping and stirring in the digester, enabling the potential for increased organic loading and increased methane production.