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Wastewater resource recovery facilities produce wastewater solids that offer potential for energy recovery. This opinion article provides a perspective on state-of-the-art technologies to recover energy from sludge (unstabilized wastewater residual solids) and biosolids (stabilized wastewater solids meeting criteria for application on land). The production of biodiesel fuel is an emerging technology for energy recovery from sludge, whereas advancements in pretreatment technologies have improved energy recovery from anaerobic digestion of sludge. Incineration is an established technology to recover energy from sludge or biosolids. Gasification, and to a greater extent, pyrolysis are emerging technologies well-suited for energy recovery from biosolids. While gasification produces high-energy gases, pyrolysis has the benefit of producing biochar in addition to pyrolysis gas. Research on the use of pyrolysis liquids, however, must proceed to advance pyrolysis implementation efforts. Future research on improvements to dewatering and drying of sewage sludge and biosolids will help advance all technologies reviewed.

Pyrolysis can convert wastewater solids into useful byproducts such as pyrolysis gas (py-gas), bio-oil and biochar. However, pyrolysis also yields organic-rich aqueous pyrolysis liquid (APL), which presently has no beneficial use. Autocatalytic pyrolysis can beneficially increase py-gas production and eliminate bio-oil, however, APL is still generated. This study aimed to utilize APLs derived from conventional and autocatalytic wastewater solids pyrolysis as co-digestates to produce biomethane. Results showed that digester performance was not reduced when conventional APL was co-digested. Despite having a lower phenolics concentration, catalyzed APL inhibited methane production more than conventional APL and microbial community analysis revealed a concomitant reduction in acetoclastic Methanosaeta. Long-term (over 500-day) co-digestion of conventional APL with synthetic primary sludge was performed at different APL organic loading rates (OLRs). Acclimation resulted in a doubling of biomass tolerance to APL toxicity. However, at OLRs higher than 0.10 gCOD/Lrd (COD = chemical oxygen demand, Lr = liter of reactor), methane production was inhibited. In conclusion, conventional APL COD was stoichiometrically converted to methane in quasi steady state, semi-continuous fed co-digesters at OLR &le, 0.10 gCOD/Lrd. Undetected organic compounds in the catalyzed APL ostensibly inhibited anaerobic digestion. Strategies such as use of specific acclimated inoculum, addition of biochar to the digester and pretreatment to remove toxicants may improve future APL digestion efforts.

Past plastic management practices have resulted in pollution. An improved management scenario may involve adding used bioplastic to anaerobic digesters to increase methane for renewable energy. In this work, effects of polyhydroxybutyrate (PHB) bioplastic anaerobic co-digestion with synthetic primary sludge on operation and microbial communities were investigated. Co-digesters treating sludge were co-fed 20% untreated or pretreated (55 °C, pH 12) PHB. Pretreament resulted in shorter lag (5 d shorter) before methane production increased after co-digestion. At steady-state, co-digesters converted 86% and 91% of untreated and pretreated PHB to methane, respectively. Bacterial communities were different before and after bioplastic co-digestion, whereas no archaeal community change was observed. Relative abundance of 30 significant bacteria correlated with methane production and lag following PHB addition. No previously known PHB degraders were detected following PHB co-digestion. Microbial communities in anaerobic digesters treating synthetic primary sludge are sufficiently capable of co-digesting PHB to produce additional methane.