A critical review on anaerobic co-digestion achievements between 2010 and 2013
Introduction
Anaerobic digestion (AD) is a biological treatment performed in the absence of oxygen to stabilize organic matter while producing biogas, a mixture formed mainly of methane and carbon dioxide. The oldest and more widespread application of AD is the treatment of sewage sludge (SS). AD experienced an important growth after the first energy crisis in the 1970s, especially with the appearance of immobilized biomass systems to treat soluble effluents, and now it can be considered a mature technology [1], [2]. Nonetheless, AD of single substrates (mono-digestion) presents some drawbacks linked to substrate properties. For instance, (i) SS is characterized by low organic loads, (ii) animal manures have low organic loads and high N concentrations, that may inhibit methanogens, (iii) the organic fraction of municipal solid waste (OFMSW) has improper materials as well as a relatively high concentration of heavy metals, (iv) crops and agro-industrial wastes are seasonal substrates, which might lack N, and (v) slaughterhouse wastes (SHW) include risks associated with the high concentration of N and/or long chain fatty acids (LCFA), both potential inhibitors of the methanogenic activity. Most of these problems can be solved by the addition of a co-substrate in what has been recently called anaerobic co-digestion (AcoD).
AcoD, the simultaneous AD of two or more substrates, is a feasible option to overcome the drawbacks of mono-digestion and to improve the economic viability of AD plants due to higher methane production [1]. Initially, because of the research perspective, AcoD focused on mixing substrates which favor positive interactions, i.e. macro- and micronutrient equilibrium, moisture balance and/or dilute inhibitory or toxic compounds [3]. Under these circumstances, 1+1>2 may be achieved, that means, co-digestion is producing more methane than the addition of the methane produced in both single digestions. However, nowadays, because of the industrial outlook and since the improvement of methane production is mainly a consequence of the increase in the organic loading rate (OLR) rather than synergisms, all kinds of mixtures are considered and used. Actually, the transport cost of the co-substrate from the generation point to the AD plant is the first selection criteria. Despite this fact, it is still important to choose the best co-substrate and blend ratio with the aim of favoring synergisms, dilute harmful compounds, optimize methane production and not disrupt digestate quality.
As illustrated in Fig. 1, AcoD publications have experienced a dramatic increase in the last years. In fact, at the present time, AcoD can be considered the most relevant topic within anaerobic digestion research. As can be seen, 50% of the overall papers have been published between 2012 and 2013, whereas 75% of them have been published in the period 2009–2013.
Examining the papers published between 2010 and 2013, it appears that the most frequent main substrates are animal manures (54%), SS (22%) and the OFMSW (11%). At the same time, the most used co-substrates are industrial waste (41%), agricultural waste (23%) and municipal waste (20%) (Fig. 2).
The aim of this paper is to present a review of the achievements and perspectives of anaerobic co-digestion within the period 2010–2013 (up to the 13th World Congress on Anaerobic Digestion), which represents a continuation of the previous review made by the authors [3]. The use of manures, sewage sludge, and organic fraction of municipal solid waste as main substrates, either full-scale or research experiences, is discussed throughout the paper. Furthermore, the review also pays attention to other aspects like pretreatments, digestate quality, microbial community dynamics, and modeling.
Section snippets
Animal manures as a main substrate
In the rural sector, AD has been established as an important economical alternative, specifically as a renewable energy source; hence animal manures have become an important raw material [3]. Nonetheless, manures are often associated with poor methane yields. AcoD of manures with other substrates has been applied as a cost-effective alternative to improve process efficiency and consequently make plants economically feasible [4], [5], [6]. Two main models can be chosen for the implementation of
Sewage sludge as a main substrate
Sewage sludge ranks as the second main substrate for AcoD. Historically, AcoD between SS and OFMSW is the most reported co-digestion research. However, between 2010 and 2013 the publications dealing with fats, oils and greases (FOG) as SS co-substrate have steeply increased, amounting to 26% of the publications. Within the same period, other co-substrates such as fruit and vegetable waste (FVW), SHW, GLY, and algae have also been broadly reported.
The low organic load of the SS together with the
Biowaste as a main substrate
Biowaste, largely OFMSW (all qualities), as a main substrate has not been as studied as manures or SS (Fig. 2). That may be a consequence of several factors: the high biogas potential of this kind of waste, the lower number of plants and their location, but above all the presence of undesired compounds. The later makes that the digestates, followed or not by composting, able to be used in restricted applications (e.g., land reclamation, landfill daily cover, etc.) or even dumped in landfills.
Pre-treatments as option to improve AcoD performance
Pre-treatments applied to AcoD were rarely described in literature until 2011, when the production of papers related to this topic steeply increased. This boom is partly because, in some regions, easy biodegradable co-substrates are already being used or not available and consequently the utilization of more complex organic sources as co-substrates have been considered. Most of the research has been devoted to mechanical pre-treatments (including ultrasound) with a 33% of occurrence, followed
Microbial dynamics
During later years, the development of molecular techniques based on 16S rRNA sequences, such as FISH, PCR, DGGE or T-RFLP, have led to an increase of the number of papers dealing with the study of the microbial community. This is mainly due to the reduction of the investment (equipment) and operating (fungible and technician) costs, the increase of the methods reproducibility and the lower time spent for each analysis. These improvements have made possible the evaluation and/or quantification
Anaerobic co-digestion modeling
In later years (2010–2013), not much attention has been paid to AcoD modeling. It is indeed very surprising that only few articles among the more than two hundreds AcoD articles published within this period were devoted to this topic. It should be pointed out that reliable AcoD modeling is required to predict, in a clear and quantifiable manner, the effect of mixing two or more wastes in a digester and remove potentially negative impacts from mixing based on random or heuristic decisions [3],
Conclusions
Anaerobic co-digestion has been widely implemented in the rural sector because of the need to improve digesters biogas production. Agro-industrial waste and the organic fraction of the municipal solid waste are the most reported co-substrate for manure-based digesters. However, easily degradable substrates are currently limited. Therefore, more complex waste have been considered, which raised the interest on pre-treatments. Even though pre-treatments need to be further developed, because of the
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