Funded by the Horizon 2020 Framework Programme of the European Union

# |eBook **Trace element supplementation as a management tool for anaerobic digester operation: benefits and risks**

## |**Introduction**


### | How to (choose to read)

## | Do I need to add trace elements to my digester?

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Improved understanding of the AD process, as well as ongoing technology development, has heightened awareness of the potential of AD biotechnologies for valorisation of sewage sludge, agro-industrial by-products and organic wastes to bioenergy. Process monitoring is fundamental in maintaining optimal AD conditions and in ensuring process stability. This usually involves measurements of key parameters, such as pH, biogas production and temperature.

**Optimal conditions for AD plants are achieved when the effective biogas production is close to theoretical production, taking into account the feedstock composition and volumetric throughput**. In reality, few anaerobic digesters ever manage to reach this target due to multiple other, interdependent factors limiting digester capacity (Figure 1).

**Figure 1**. Physico-chemical and biological factors affecting digester capacity.



**Figure 2**: Liebig's law of the minimum illustrating the effect of trace elements deficiency on digester performance

demonstrated that deprivation of elements, such as cobalt or nickel, resulted in impaired substrate conversion to methane, VFA accumulation, and digester acidification and failure (Takashima *et al.* 1990; Osuna *et al.* 2002; Zandvoort *et al.* 2002a; Zandvoort *et al.* 2002b). In fact, in some cases digester performance was recovered simply by supplementing TE and thus stimulating methanogenic activity.


## | What are the benefits of adding trace elements to a digester?

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### **Various studies at the laboratory scale have focused on:**


The available research provides a wealth of knowledge of value when assessing individual digester situations. However, it also underscores the complexity of metal supplementation and the specificity of each digester scenario. The various experiments described in the scientific literature were done under controlled, laboratory conditions and using bioreactors of varying sizes from 100 mL to 20 L, and may not be representative of process behaviour in full-scale anaerobic digesters. **In recent years, TE supplementation trials have also been conducted using full-scale AD – in part, due to expanded application of AD in Europe and generally increased awareness of the potential of TE supplementation**.

Lindorfer *et al.* (2012) sampled over 1,500 anaerobic digesters to measure TE compositions and concentrations, and digester performance. They supplemented 60 digesters with TE and observed the impact over a period of four months. Their work demonstrated that, at full-scale, TE supplementation resulted in lower VFA concentrations, more microbial biomass, and enhanced digester performance (Figure 3). They demonstrated improved digester performance within the first few days following TE supplementation, and stable performance for at least three months after TE addition.

**Figure 3**. Effect of TE dosing on (**A**) VFA (acetate and propionate) accumulation in a digester working on manure and energy crops (mainly maize), and (**B**) methane yield and plant capacity in a digester working on energy crops only (Reimlingen. Germany). Reproduced from H. Lindorfer, D. Ramhold and B. Frauz 2012 Water Science & Technology 66(9) 1923- 1929, with permission from the copyright holders, IWA Publishing.

Finally, one of the main concerns in considering TE supplementation of anerobic digesters is that the optimum, 'bioavailable' concentration lies between two zones of inhibition (i.e. *deficiency and toxicity*). Over-dosing of at least one TE may result in toxic inhibition of microbial activity, and this risk should be avoided by developing appropriate dosing strategies.

**Take-Away: Improved digester performance due to TE supplementation has been demonstrated at laboratory, pilot and full scale. Reduced VFA accumulation and higher biogas output are the main benefits reported.** 

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As with the human body or with animals, microbial individuals and communities also require optimal conditions of food (i.e. substrate, or feedstock), vitamins, macronutrients (P, N, K) and TE for efficient growth and metabolic activity. TE are essential to microorganisms at low concentrations. Each TE typically plays a unique, and key, role in specific enzymatic functions and cannot be replaced by another TE (except, to some extent in some cases, by elements of the same group). **A list of the main TE and their roles in microbial activity is shown in Table 1**. As examples, cobalt is used as a 'co-factor' (i.e. as vitamin B12) during the last step of the methylotrophic pathway; nickel is used as a 'co-enzyme' (F430) during methyltransferase, the essential step before the production of methane.


**Table 1**. Selected TE in enzymes of microbial conversions.

The concentration of each TE required for the welfare of an anaerobic digester is difficult to determine as it depends on the compositions and the metabolic activity of the microbial community in the digester, as well as the chemical speciation (and, thus, the 'bioavailability') of the TE. Determining the total TE concentrations is, of course, a good start in quantifying key TE in a digester, but it is not sufficient to allow a determination of the 'bioavailable fraction'. The behaviour of TE is governed by biochemical reactions and TE may be present as free ions, bound complexes or as precipitates, depending on pH, inorganic ligands, or 8

organic macromolecules. The result of those reactions creates a pseudo-chemical equilibrium defining a speciation for each TE and determines the bioavailable fraction.


**Figure 4**: Conceptual simplified representation of TE bioavailability in anaerobic digesters (adapted from NRC (2003)). A, B and C are related to bioavailability processes: TE interactions between phases, transport of TE to microorganisms and bio-uptake of TE through the biological membrane, respectively. D represents the biological response (i.e. methane production yield) as a function of the bioavailable TE intracellular concentration.

**Several studies have reported a wide diversity of concentrations of different TE in various anaerobic digesters (Table 2).** As with other nutrients, TE are supplemented into the digester along with the feedstock and so the TE concentration will solely depend on the input values. TE concentrations in feedstock are based on a wide range of factors, including the type of material or feedstock, absorption capacity, or even location. Sewage sludge, for example, is composed of a mixture of household, human and industrial wastes, which increases TE diversity and concentrations. Sole agricultural wastestreams, on the other hand, such as maize or wheat, may be severely deficient in one or several TE – especially elements rarely available in soil (Evranos & Demirel 2015). Food waste or food-processing waste are often deficient in iron and selenium (Ariunbaatar *et al.* 2016). Finally, manures, or animal wastes, may present the risk of having very high concentration of specific TE, such as copper or zinc, if animals have been fed with food supplements containing those elements.


**Table 2** TE concentration measured or calculated in AD (modified from Schattauer *et al.* (2011).

a (Sahm 1981), b (Takashima *et al.* 1990), c (Kloss 1986), d (Weiland 2006), e (Seyfried *et al.* 1990), f (Mudrack & Kunst 2003), g (Pobeheim *et al.* 2010), h (Bischofsberger 2005), I (Schattauer *et al.* 2011).

### **Take-Away: Co, Ni, Fe and Se are usual suspects, and should always be taken into account. However, other essential TE, such as Zn, Cu or W, should also be considered.**

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## | Strategy for trace elements supplementation

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To decide whether TE supplementation of AD is required – and how to approach the dosing process – the decision-support diagram in Figure 5 may be used. The first (green) area considers evaluation of the performance of the digester. **In the case of an underperforming digester and the reason is not clearly found (first steps in Figure 5), then TE limitation might be case.**


## | Risks linked with TE supplementation


## | COST action ES 1302

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COST action ES 1302: Ecological function of trace metals in anaerobic biotechnologies



If you have any queries, ideas or interest in the TE supplementation work please contact the COST ES1302 representatives in your country:

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## |**References**

Ariunbaatar J., Esposito G., Yeh D. H. and Lens P. N. L. (2016). Enhanced Anaerobic Digestion of Food Waste by Supplementing Trace Elements: Role of Selenium (VI) and Iron (II). *Frontiers in Environmental Science* **4**(8).

Bischofsberger W. (2005). *Anaerobtechnik*.

Callander I. J. and Barford J. P. (1983a). Precipitation, chelation, and the availability of metals as nutrients in anaerobic digestion. I. Methodology *Biotechnology and Bioengineering* **25**(8), 1947-57.

Callander I. J. and Barford J. P. (1983b). Precipitation, chelation, and the availability of metals as nutrients in anaerobic digestion. II. Applications *Biotechnology and Bioengineering* **25**(8), 1959-72.

Demirel B. and Scherer P. (2011). Trace element requirements of agricultural biogas digesters during biological conversion of renewable biomass to methane. *Biomass and Bioenergy* **35**(3), 992-8.

Evranos B. and Demirel B. (2015). The impact of Ni, Co and Mo supplementation on methane yield from anaerobic mono-digestion of

maize silage. *Environmental Technology* **36**(9-12), 1556-62.



Co-published by IWA Publishing, Alliance House, 12 Caxton Street, London SW1H 0QS, UK Tel. +44 (0) 20 7654 5500, Fax +44 (0) 20 7654 5555 publications@iwap.co.uk www.iwapublishing.com

ISBN: 9781780409429 (eBook) DOI: 10.2166/9781780409429

This eBook was made Open Access in March 2018

©2018 The Author(s)

This is an Open Access book distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).

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