PNNL

Abstract

Algal-bacteria biomass generated in high-rate algal ponds during wastewater treatment represents an inexpensive and potentially sustainable resource for bioenergy production through anaerobic digestion (AD). However, reduced biodegradability of wastewater algae can limit methane production. While various pretreatment methods can improve biodegradability of recalcitrant biomass, the required energy input may exceed the energy gain due to enhanced methane production. This study assesses production of algal-bacteria biomass in pilot-scale wastewater treatment ponds and evaluates the impact of mechanical and hydrothermal pretreatments on biomass disintegration, methane production kinetics and the AD system energy balance. Biomass bead beating had limited effects on particle size distribution and methane production. In contrast, hydrothermal pretreatment caused substantial particle size reduction and increased the fraction of solubilized organic matter up to 3.5-fold. Thus, the methane ultimate yield and production rate increased by 20-55% and 20-85%, respectively, with highest values corresponding to pretreatment at 121?C for 60 min. While the 1st order rate and pseudo-parallel 1st-order rate kinetic models demonstrated better fit of the methane production from untreated biomass (R2 > 0.993) the modified Gompertz kinetic model provided superior fit for digestion of hydrothermally pretreated algae (R2 ? 0.99). Importantly, the AD system energy balance analysis showed that hydrothermal pretreatment increased the total energy output by 25-40% with highest values for volume-specific and mass-specific total energy outputs reaching 0.23 kW per m3 of digester and 2.3 MW per ton of biomass volatile solids. Similarly, the net energy recovery (energy output per biomass HHV) was improved from 20% for untreated algae to up to 32-34% for hydrothermally pretreated algae, which corresponded to a net energy ratio and net energy efficiency of 2.14 and 68%, respectively.