Comparison of Variable and Constant Loading for Mesophilic Food Waste Digestion in a Long-Term Experiment

He Song,Yue Zhang,Sigrid Kusch-Brandt,Charles J Banks

doi:10.3390/en13051279

Abstract

Operators of commercial anaerobic digestion (AD) plants frequently note the challenge of transferring research results to an industrial setting, especially in matching well-controlled laboratory studies at a constant organic loading rate (OLR) with full-scale digesters subject to day-to-day variation in loadings. This study compared the performance of food waste digesters at fluctuating and constant OLR. In a long-term experiment over nearly three years, variable daily OLR with a range as wide as 0 to 10.0 g VS L−1 day−1 (weekly average 5.0 g VS L−1 day−1) were applied to one laboratory-scale digester, while a pair of control digesters was operated at a constant daily loading of 5.0 g VS L−1 day−1. Different schemes of trace elements (TE) supplementation were also tested to examine how they contributed to process stability. Variable loading had no adverse impact on biogas production or operational stability when 11 TE species were dosed. When TE addition was limited to cobalt and selenium, the stability of the variable-load digester was well maintained for nearly 300 days before the experiment was terminated, while the control digesters required re-supplementation with other TE species to reverse an accumulation of volatile fatty acids. This work demonstrated that variation in daily OLR across quite a wide range of applied loadings is possible with no adverse effects on methane production or stability of food waste digestion, giving confidence in the transferability of research findings. The positive effect of variable OLR on TE requirement requires further investigation considering its practical significance for AD industry.

Highlights

Food waste is a ubiquitous and energy-rich organic material with a high potential to be used in anaerobic digestion (AD) for biogas production [1,2,3], but it can pose challenges with regard to maintaining process stability and favourable long-term performance of the digester [4,5]
In single-stage continuous stirred tank reactor (CSTR) digestion, which is the most common type of AD plant operated in practice, mono-digestion of food waste without the addition of stabilising agents is limited to an organic loading rate (OLR) of up to 2.0 g volatile solids (VS) L−1 d−1 [6,7,8], while even lower OLRs may be necessary to maintain a stable process [9]
volatile fatty acids (VFAs) with inhibitory impact, in particular propionic, iso-butyric and iso-valeric acid [9,15], were found only in low concentrations less than 500 mg L−1. This indicates that there was no risk of process instability and confirms that with appropriate supplementation of trace elements, mono-digestion of food waste is a stable process in long-term operation

Summary

Introduction

Food waste is a ubiquitous and energy-rich organic material with a high potential to be used in anaerobic digestion (AD) for biogas production [1,2,3], but it can pose challenges with regard to maintaining process stability and favourable long-term performance of the digester [4,5]. Energies 2020, 13, 1279 and process failure, for two interlinked reasons: (1) disturbance or breakdown of the delicate balance between production of volatile fatty acids (VFAs) through hydrolysis, acidogenesis and acetogenesis on one hand and consumption of VFAs through methanogenesis on the other hand [10]; and (2) washing out of slow-growing anaerobic microbial biomass when hydraulic retention time is reduced as a result of overloading [11]. In food waste AD, deficiency of trace elements (TEs) was identified as a major factor adversely affecting process stability [15,16,17]

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