Effect of trace element supplementation on the mesophilic anaerobic digestion of foodwaste in batch trials: The influence of inoculum origin

Abstract

Batch anaerobic trials using a source-separated food waste as a substrate with inoculums of different origins were carried out under mesophilic conditions. Reactions were operated both with and without trace element (Co, Mo, Ni, Se, and W) supplementation. Supplementation with trace metals had either neutral or slightly negative effects with inoculums originating from reactors with a high background level of metals, such as those for the co-digestion of biowaste and waste activated sludge. For inoculums from reactors treating food waste only, which inherently contain low levels of trace metals, supplementation with these metals increased methane production. In particular, Mo concentrations in the range of 3–12 mg/kg dry matter and Se concentrations of 10 mg/kg dry matter increased methane production to as high as 30–40%. Supplementation with a metal mixture (Co, Mo, Ni, Se and W) increased the methane production to the range 45–65% for inoculums with low background concentrations of trace metals. These findings may have an important impact in the commercial production of methane from food waste.

Introduction

The introduction of separated collection for different fractions of municipal solid waste (MSW), in addition to subsidies for renewable energy production, have been the main drivers for the implementation of the anaerobic digestion (AD) of biowaste in recent years [1]. Food/kitchen waste originating from door-to-door collections contains low levels of inert materials (plastic, glass, stones, etc.) and is a suitable substrate for AD, enabling biogas productions in the range 160–180 m³/tonne of raw waste treated [2]. Based on this figure, complete anaerobic digestion of the biowaste collected in the EU-27 area could generate approximately 35,500 MWh/day of electric power, assuming an organic material collection rate of 0.2 kg/person/day [3]. This increased biogas generation would contribute significantly to the production of renewable energy and the mitigation of greenhouse gas (GHG) emissions.

Food and kitchen waste generally contain low concentrations of trace elements, especially metals, this may lead to the failure of the anaerobic digestion process [3]. Several specific trace metals, including Co, Ni, W, Se and Mo, are essential for the enzyme cofactors involved in the biochemistry of methane formation and are needed in a balanced anaerobic digestion process [4]. Limiting the metals required by the enzymes may disturb the total process. As reported in literature, Co [5], [6], [7], Mo, [8], [9], Ni [10], [11], [12], [13], [14], Se [15], [16] and W [17], [18] are all involved in the methane production biochemical process.

The methanogenic requirement for trace elements, both for acetoclastic and hydrogenotrophic microorganisms, is not fully understood, presenting a serious impediment for the commercial applications of anaerobic digestion processes, which fundamentally require process reliability.

Recently, both Demirel and Scherer [19] and Schattauer et al. [20] reviewed the issues involved with the lack of trace elements in the anaerobic digestion process. In the large body of literature reviewed by these authors, only a few papers reported on the effects of trace element supplementation on the anaerobic digestion of food waste or similar substrates. Several of these studies [3], [21], [22], [23], [24], [25], [26], [27], [28] reported on the beneficial effects of the addition of multiple metal mixtures without delineating the supplementation effects for each individual metal, making it difficult to completely define the process characteristics. To further complicate this situation, the different trace metal additions were sometimes determined on a wet weight basis, while others were determined on a dry weight basis or on a volatile solids basis or a COD content or removal basis. Extensive studies in the literature also suggest that for digestive processes using source materials containing large amounts of metals, such as sludge or manure [20], [37], trace elements do not limit methane production.

In this study, we determined the effects of the addition of the trace metals Co, Ni, Mo, Se, and W, both as single elements and in mixed cocktails, for mesophilic anaerobic digestion batch trials using food waste as the only substrate. To assess the effects of different trace metal background levels on digestion performance, two sources of inocula were used in the experiments. The first source was from a reactor co-digesting waste-activated sludge and food waste. The second source was from a reactor treating only food waste.