1. Introduction
Waste products, together with biomass, can be an important source of energy. Heavy metals (HM) concentrations can be a problem both in solid and liquid waste treatment. Wastewater treatment processes and poisonous heavy metals (HMs) lead to environmental pollution [
1,
2]. In particular, referring to the HMs contained in wastewaters, which undergo anaerobic digestion, we have to state that on the one hand, when contained in small concentrations, some heavy metals can also be beneficial and required for the growth of microorganisms; on the other hand, when they are present in concentrations higher than a certain threshold, they can exert a toxic action. Therefore, HMs toxicity and their accumulation in the industrial processing of wastes represent a serious environmental problem [
3].
HMs can be found, for example, in olive mill waste (OMW) effluent in high concentrations [
4]. The OMW anaerobic digestion (AD) process generates biogas, which is used as a source of energy [
5]. AD has been widely used for OMW with a high content of organic load (OL). In [
5], codigestion was performed to enhance substrate biodegradability. Sewage sludge was obtained from the secondary sludge of a wastewater treatment plant situated in the city of Irbid in Jordan. The most effective ratios between olive mill waste and sewage sludge were proved to be about 10% and 90% in volume. In [
6], the benefits of codigestion of waste-activated sludge and Organic Fraction of Municipal Solid Wastes (OFMSW) were confirmed at industrial scale. This is a very promising process, given that the disposal of OFMSW in landfills is not recommended in many European MSs, if not forbidden. The industrial tests showed promising results and good performances of the process. Stability was reached after more than 1 hydraulic retention time (HRT). The process proved to be also interesting from the point of view of economics, having a payback period of 3.5 years. In [
7], Biomethanation Potential (BMP) tests of source-selected OFMSW were performed in codigestion. Different source-separated organic fractions of municipal solid wastes were tested, as collected from: canteens, supermarkets, restaurants, households, fruit–vegetable markets, and bakery shops. Recently, more efficient AD processes guarantee a high removal efficiency of OL. In [
8], glycerol has been used to boost biogas production. Glycerol addition can boost biogas yields, when it is limited to 1% (
v/
v) of the feed volume (the production of methane can be doubled). In [
9], thermophilic processes are adopted; tests are performed at both pilot and full scale. It was demonstrated that the thermophilic option can bring an increase of 45–50% in the production of biogas. Metal contents are within the more stringent limit used in Europe for high quality amendment. In [
10], modeling is applied to the optimization of anaerobic digestion. Regardless, to have an optimized process, the concentration of HMs in the substrate should be within certain limits. These limits are not clear for all the possible anaerobic digestion substrates and in particular, for olive mill wastes. The aim of this paper is to assess them. These are an important parameters to control the quality of the AD process. In the AD process, metabolism and growth of microorganisms within the substrate play a paramount role in reducing OL and convert it into methane (CH
4) and carbon dioxide (CO
2), through sequential processes. In [
11], a detailed analysis of bacteria consortia is performed. In [
12], the influence of chlorides on the anaerobic digestion process is studied.
Small concentrations of HMs such as Fe, Ni, and Co are needed to improve the efficiency of biogas production in the AD process [
13]. Therefore, they can be considered to have a positive action. On the other hand, Pb, Cu, and As affect negatively the efficiency of the AD process; this is due to the toxic effect exerted on AD microorganisms [
14]. Some studies reported that certain HMs ions can inhibit enzymes that are produced by microorganisms, thus HMs like Zn, Cu, Cr, and Pb inhibit the AD process [
15].
Some studies report the most used methods to reduce HMs levels in raw wastewaters are: the use of cork as a sorbent [
16]; microbial [
17] or jute fibers [
18]. Critical studies reveal that not all metal-polluted OMW can produce biogas through AD; some strategies were performed to remove the HMs from the feedstock material such as the use of biosorbents [
19] and dewatering [
20,
21]. Paganelli et al. (2002) carried out a study on the use of OMW as an HMs sorbent material; they found that Cu was adsorbed in the range of 5–13.5 mg/g under certain operating parameters [
22], while Keskinkan et al. (2003) reported that the adsorption was about 10.37 mg/g for Cu, 15.59 mg/g for Zn, and 46.60 mg/g for Pb [
23].
According to recent studies, the AD process requires a certain concentration of HMs. The AD process requires, in fact, external electron acceptors. If aerobic respiration uses oxygen as an external electrons acceptor, anaerobic digestion needs alternative external electron acceptors (EAs) [
24]. Therefore, for the elimination of electrons released during the OL degradation process, Fe, CO
2, SO
4−2, and NO
3−1 act as external acceptors. When the concentration of heavy metals increases over a certain threshold, there would be inhibition. We have analyzed in this work, which is this threshold, based on the initial concentrations of heavy metals in the raw material; these have been increased stepwise to find where inhibition begun.
To the best of authors’ knowledge, olive pomace anaerobic digestion with changing HMs content has not sufficiently been analyzed, while other substrates, like wastewaters, swine effluents, poultry manure, and swine manure have attracted the bulk of the research efforts [
3]. The problem of wastewaters management is particularly important in China, where high concentrations of heavy metals can be measured also in soils and therefore, in agricultural production and agricultural residues. The area of cultivated land polluted by HMs accounts for 20% of the total agricultural land [
25].
4. Discussion
In this paper, different concentrations of heavy metals were used to simulate different compositions of olive mill waste, which already contain them. Therefore, the aim was to assess how the variability on heavy metals content can affect the final results of the anaerobic digestion. This was assessed based on the variations of the produced biogas, its composition, and the destruction of volatiles, organic acids load, TCOD, SCOD, and polyphenols.
The limitations of the study are that a complete analysis of the microbial community is missing, so we actually do not know the effect of the heavy metals on the single microorganism, but to perform this kind of analysis, the methods should be also changed significantly.
The results of this analysis are useful to optimize the anaerobic digestion process of olive mill waste effluents and it is one of the first contributions available in the literature in this sense. The fact that low concentrations of HMs promote the anaerobic digestion process is recognized also in a recent review [
3], in which it is stated that low doses of Cu
2+ and Cd
2+ serve as cofactors in the catalytic center of cellulase and stimulate enzyme activity. On the other hand, high contents of Cd2+ and Cu2+ inhibit enzyme activity by disrupting protein structures.
The effect of heavy metals on AD process are proposed in
Table 6.
5. Conclusions
Among the stress factors which may inhibit a proper anaerobic digestion process and limit biochemical reactions, heavy metals effects were discussed in this study. A significant decrease in the performance of the anaerobic digestion process, biogas yield, CH4 concentration, VS, TCOD, and SCOD was detected. The main quantitative results of this study show that HMs may be inhibitory, toxic, or even stimulatory to the anaerobic digestion process. These impacts depend on the HMs concentration. The effects of HM on the acetogenic and methanogenic stages were examined through CH4 concentration, VS reduction, and the organic acids load reduction. It was concluded that the toxicity of the HMs can be arranged according to the increasing order Cu > Ni> Pb> Cr >Zn > Fe, which lead to rapid poisoning of the activity of microorganisms. This study shows that the concentration of Fe, Zn, Cr, Pb, Ni, and Cu can safely improve the AD process (in terms of increasing biogas and methane production and increasing TCOD, SCOD, VS, and polyphenols removal) if they are lower than 2.9, 0.335, 1.211, 0.297, 0.082, and 1406.25 mg/L, respectively. Therefore, it is recommended to control the level of HMs in the digestion process for biogas production. Biogas production and methane content according to HMs addition were arranged ascendingly: Fe > Zn > Cr > Pb > Cu > Ni and Fe > Zn > Cr > Pb > Ni > Cu, respectively. Recommendations are focused on a more careful analysis of the substrate and the use of adsorption and retention systems to reduce the concentration of HMs in the substrate below the above reported toxic levels.