Regular articleEffect of trace element supplementation on the mesophilic anaerobic digestion of foodwaste in batch trials: The influence of inoculum origin
Highlights
► The lack of trace elements may upset the anaerobic digestion process. ► The supplementation of Co, Mo, Ni, Se, W showed a process improvement. ► Mo showed to be the most beneficial element added.
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.
Section snippets
Experimental design
The effect of trace metals addition on methane production was studied using biochemical methane potential (BMP) trials. The substrate to inoculum ratio was kept at levels of approximately 0.3–0.4 on a VS basis, according to the guidelines given in Angelidaki et al. [29]. The tests were performed under mesophilic (37 °C) conditions. At the end of the incubation period, pH, total and volatile solids, COD, total and partial alkalinity, ammonia and TKN measurements were performed according to the
Inoculums and substrate characterisation
The food waste and the two inoculums used in this experimentation were characterised for both macro parameters and trace metal concentrations. Table 2 shows the representative characteristics of the inoculums. Inoculum A originated from an anaerobic reactor treating food waste and waste-activated sludge. Inoculum B originated from an anaerobic reactor treating food waste only.
The measurements for the macro-parameters, including total and volatile solids, COD and nutrients, are representative of
Conclusions
For the mesophilic anaerobic digestion of food waste, the addition of trace metals (Co, Mo, Ni, Se and W) demonstrated that in the batch trials used in our experiments, these metals could improve the methane production. Two inoculums, isolated from two sources containing different levels of metal concentrations, were studied to characterise the effects of the supplementation on reactor performance. For the inoculum from an anaerobic reactor treating food waste as the sole substrate, any level
Acknowledgements
This work was carried out with the financial support of the EU FP7 VALORGAS Project (ENERGY.2009.3.2.2).
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