Abstract
Conventional petroleum-derived plastics are recalcitrant to biodegradation and can be problematic as they accumulate in the environment. In contrast, it may be possible to add novel, biodegradable bioplastics to anaerobic digesters at municipal water resource recovery facilities along with primary sludge to produce more biomethane. In this study, thermal and chemical bioplastic pretreatments were first investigated to increase the rate and extent of anaerobic digestion. Subsequently, replicate, bench-scale anaerobic co-digesters fed synthetic primary sludge with and without PHB bioplastic were maintained for over 170 days. Two polyhydroxybutyrate (PHB), one poly(3-hydroxybutyrate-co-4-hydroxybutyrate) and one polylactic acid (PLA) bioplastic were investigated. Biochemical methane potential (BMP) assays were performed using both untreated bioplastic as well as bioplastic pretreated at elevated temperature (35 to 90°C) under alkaline conditions (8<pH<12) for 3 to 48 h. PHB and PLA pretreatment increased average BMP values up to over 100 %. Average PHB lag time before methane production started decreased when pretreatment was performed. Bench-scale anaerobic co-digesters fed synthetic primary sludge with PHB bioplastic resulted in 80 to 98% conversion of two PHB bioplastics to biomethane and a 5% biomethane production increase at the organic loadings employed (sludge OLR=3.6 g COD per L of reactor volume per day [g COD/ LR-d]; bioplastic OLR = 0.75 g theoretical oxygen demand per L of reactor volume per day [ThOD/LR-d]) compared to digesters not fed bioplastics. Anaerobic digestion or co-digestion is a feasible management option for biodegradable plastics.
Highlights
Conventional plastics derived from petroleum are not biodegradable to a significant extent and result in accumulation of plastic waste in landfills or natural environments (Rostkowski et al, 2012)
Thermal alkaline pretreatment of PHB and polylactic acid (PLA) bioplastics increased anaerobic biodegradability in terms of increased Biochemical methane potential (BMP) values and reduced lag time compared to untreated controls as described below
BMP values and lag times resulting from 27 different pretreatment conditions for each bioplastic were determined and provided an initial assessment of biomethane production changes due to pretreatments for each bioplastic
Summary
Conventional plastics derived from petroleum are not biodegradable to a significant extent and result in accumulation of plastic waste in landfills or natural environments (Rostkowski et al, 2012). Bioplastics Anaerobic Co-digestion (PHB) bioplastic is biodegraded in aerobic and anaerobic engineered processes as well as natural environments; anaerobic co-digestion of PHB for the express purpose of waste management and renewable energy has not been investigated (Abou-Zeid et al, 2004; Volova et al, 2010; Gómez and Michel, 2013; Deroiné et al, 2014). PHB was shown to anaerobically biodegrade over 90% in 10 days at mesophilic conditions, whereas polylactic acid (PLA) only biodegraded 7% in 90 days even though it is considered to be industrially compostable under aerobic thermophilic conditions (Yagi et al, 2014). PLA is more readily available on the market today due to more efficient production at full scale (Kolstad et al, 2012; Gómez and Michel, 2013; Yagi et al, 2013, 2014)
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