JSDEWES: Enhancing Bioenergy Recovery from Agro-food Biowastes as a Strategy to Promote Circular Bioeconomy

An increasing world population has brought concerns over securing the availability of food, energy, water and bioresources; on another hand the increased greenhouse gases emissions and environmental pollution stressed the importance of addressing food, energy, water and climate as a complex system, considering the interactions between those factors, the so-called Food-Energy-Water nexus [1].

Implementing energy savings, renewable energy and more efficient conversion technologies can have positive socio-economic effects, create employment and if externalities such as health effects are included, even more benefits can be expected [2]. The high demand for fossil fuels replacements, has motivated a significant biogas growth, particularly in the European Union. In the year 2017, the global installed capacity for biogas totalled 16915MW, with more than70% of it being installed in the European Union [3]. In fact, to be able to meet the demand of future consumption and to guarantee sustainable structural changes on the energy supply-chains, promoted the search of alternative energy sources [4]. Innovative approaches within the sustainable development of society will lead to the definition of new systems among the energy supply-chains. One of the possible approaches is to focus on a value chain and establish synergistic interactions between actors, improving productivity and reducing losses and environmental impacts [5]. The food supply chain produces large amounts of waste and therefore is one of the targets of the Commission’s Circular Economy Package [6] to stimulate transition towards a circular economy. To tackle this problem, it is important to face the different stages of the supply chain, including primary production, processing, distribution and consumption [7]. This fact can be an opportunity for R&I addressing methodological developments aiming new valorisation strategies encompassing the Food-Energy-Water challenge [8].

As generally agro-food industries include energy intensive processes the possibility to recover energy from waste generated is a valorisation route that is worth exploring. Being a mature technology, anaerobic digestion (AD) can be used for decentralised energy production, removing barriers for market uptake of this solution. One of the possible bottlenecks regarding AD deployment is the process yield that in some cases is not enough to guarantee its viability [9].

Shifting from AD using one mono-substrate as feedstock to anaerobic co-digestion (AcoD) leads to synergetic effects of multiple substrates increasing processes sustainability [10].

Several works have addressed biogas production from agro-food waste as a sustainable option for waste management and underlined the advantages of AcoD [11]; per example, the anaerobic co-digestion of sewage sludge from wastewater treatment plant (WWTP) and agro-food biowastes (AFW) improved digester stability, methane yield and lead to shorter hydraulic retention time [12]. Many authors have highlighted that co-digestion focused on co-substrates compositions is the key factor, where fat-rich material as co-substrates, can, theoretically, produce more methane from lipids (1,014 L/kg) than from proteins (496 L/kg) or carbohydrates (415 L/kg) [13].

The strategy to promote AcoD at rural/urban level should start by establishing the energy-system boundaries, using criteria that include the availability of substrates/feedstocks for AD and AcoD processes and the options for the use of biogas and digestate, rich in nutrients.

At a rural context, most common substrates are livestock effluents [14] whereas co-substrates include for example agro-food biowastes (AFW). AFW have a wide range of physical-chemical characteristics, but in general show a high level of non-fibre carbohydrates and fat content that could lead to fast acidification during the conversion process. To overcome this drawback, AFW is used as co-substrate in mixtures prepared to maximize synergisms, attenuating the inhibition of harmful compounds and not disrupting digestate quality. Multiple feedstocks used as co-substrates can integrate, at urban level, AD plants using sewage sludge as main substrate to enhance biogas production [15].

Another option to enhance bioconversion process is to pre-treat the substrate and/or co-substrate [16]. Pre-treatments weaken cell wall and structure, allowing for enzymes and methanogens to consume organic compounds inside the cell and facilitating solubilization, through different mechanisms [17]. Pre-treatments may be physical, chemical or biological; or a combination of them [18]. Generally, the pre-treatments classification into various categories, include: Mechanical and Physical (I) [19]; Thermal Hydrolysis or Chemical Alkaline + Thermal Hydrolysis (II); and Filtration (III) to promote the recovery of the liquid phase (AcoD liquor) [20]. The main objective of these pre-treatments is to increase the cellulosic materials hydrolysis, especially AFW containing vegetable and lignocellulosic substrates [21].

This paper presents a systematic analysis of results of AcoD obtained by the authors along a period of more than 5 years. A broad variety of AFW were used as co-substrate either with livestock manure (LM) or with mixed sewage sludge (SS). Whenever found necessary co-substrates were pre-treated to increase soluble organic matter content [22]. Comparing the two studied scenarios, the mono-digestion (reference) and co-digestion trials based on an energy performance indicator (EPI), it was possible to show the percentage of increase in the specific methane yield (SMY) with feeding regime shift applied. This kind of analysis could contribute to set a management strategy for different biowastes flows, improving the sustainability of the energy supply for rural and urban sectors.

The method used in this study included the systematic analysis of 12 scientific articles on bioenergy recovery from AFW at rural/urban level, published by the author’s research group. From each article, data on key operational parameters were compiled accordingly to the main substrate used (LM or SS) to compare AD and AcoD trials. These parameters included organic loading rate (OLR, g VS_added/L·d), potential of hydrogen (pH), relation between soluble chemical oxygen demand and chemical oxygen demand (SCOD/COD), and the ratio between total carbon and total nitrogen (C/N). The SMY indicates the potential bioenergy production for the different AFW studied and to close the circular bioeconomy approach, a simple indicator (EPI) was created to link the conventional and innovative tendencies for the AD and AcoD biotechnology. The characteristics of LM and SS used in the studies are described below.

Livestock manure. The studies report two types of LM: pig slurry (PS) obtained one site system farm, including two types of production systems, closed cycle (CC) and fattening/finishing (FF). The FF slurry collected directly from pit ditches simulate the effluent produced at a fattening/finishing farm unit. The liquid dairy cattle manure (LCM) came from a single dairy farm. The monitoring plan, in the different farm units, was adapted to the improvements implemented in farm units for livestock manure at real scale, taking in account water use efficiency, along eight years period (2012-2019). The pig farms selected, covering a range between 500 and 1,000 breeding sows, reflected the concentrated animal feeding operation (CAFO) in the region [23]. The treatment systems implemented include a storage tank; mechanical solid-liquid separation followed by a lagoon system, where the manure liquid fraction is stored for further utilization, being the most common best available technique (BAT) the agronomic valorisation. All the trials presented were carried out on lab-scale, except for the farm 5 [marked (*) in Table 1]. The mobile pilot unit was installed, at the pig farm unit, in order to demonstrate the biogas production optimization in comparison with the AD reactor implemented in the facility. Table 1 exhibits the identification of livestock farms according to size, number of sows lodged, location and sample collection period.

Farms used to collect the livestock manure samples

Mixed sewage sludge. The sampling of SS was performed in three Portuguese urban WWTP located in the Lisbon Region (Portugal) covering a range of treatment capacities (13,000 to 60,000 m³/d). The treatment process includes activated sludge system coupled with anaerobic sludge digestion. The water line comprises primary, secondary and tertiary treatments, with biological process for nitrogen removal, according with the legal requirements limits to allow the effluent to be discharge in the water body (Tagus River); the sludge line integrates the following unit processes: pre-thickening, anaerobic digestion, post-thickening and mechanical dewatering by centrifugation.

Table 2 exhibits the classification of plants according to size, inhabitant’s equivalent, location and sample collection period.

Wastewater treatment plants selected for sampling mixed sewage sludge

The different types of AFW, for the AcoD experimental trials, were selected, taking in account the synergetic effect with the substrate, mainly their chemical composition to balance the feed mixtures and the Bio-CH₄ enhancement. To simplify the discussion of the data obtained, from the studies reviewed and compared according to the AFW bioenergy potential, the five broad categories selected are: (a) fruit and vegetable biowaste (FVW), (b) fish canning industry (FCI), (c) horse manure (HM), (d) coffee biowastes (CW), and (e) non-edible crops (NEC). The first category, fruit and vegetable biowaste (FVW) includes: non-marketable pears, pear biowaste (PW), orange peel biowaste (OP) and carrots peel biowaste (CP), and pineapple peel biowaste (PPW); category b) includes waste sardine oil (WSO); category c), horse manure (HM); category d), coffee biowastes (CW) includes: coffee grounds (CG) and exhausted coffee biowaste (ECB) resulting from the water-pressured extraction process to produce soluble coffee and coffee drinks; and category e), non-edible crops (NEC), that includes elephant grass "Pennisetum purpureum" (EG) and duckweed biomass (DB).

Considering the characteristics of the five AFW categories, different pre-treatment’s combinations were employed to enhance energy availability, improving biodegradability and maximizing liquor recovery. Biodegradability of co-substrates improved by reducing the macromolecule dimensions, breaking bacteria cell walls with release of intracellular organic matter led to a higher SCOD/COD and C/N ratio, contributing for the pH adjustment for the most suitable values recommended for AD process.

Figure 1 presents a chart diagram (up-to-bottom), where the different steps applied to AFW illustrate the routes followed to obtain the final feed material to be mixed with the substrates. After defining the five categories, already described, the following step was the pre-treatment selection, attending to the synergetic effects with the substrates (LM and SS) from the different categories. The pre-treatments applied, alone or in combination were: I-"Mechanical and Physical"; II-"Thermal Hydrolysis" or "Chemical Alkaline + Thermal Hydrolysis"; III-"Filtration". The final step shows the liquor obtained for AcoD trials designed, respectively by: PLF (pear liquid fraction); SOL (sieved orange peel liquor); LCP (liquid carrot peel); PPL: (pineapple peel liquor); WSO (waste sardine oil); SHM: sieved horse manure; LCG: liquor coffee ground; LECB: liquid exhausted coffee biowaste; EGL: elephant grass liquor; DB: duckweed biomass.

The main results achieved in the studies performed for both scenarios (mono and co-digestion) are presented below, highlighting that the EPI is a key factor to set a management strategy for different biowaste flows, improving the sustainability of the energy supply for rural and urban sectors.

This study provided a simple basis to design feasible solutions for bioenergy production from livestock manure, mixed sewage sludge and agro-food biowastes. Considering the energy performance indicator (EPI) proposed in this paper, co-digestion significantly improves process performance leading to considerably higher bioenergy recovery.

A stable process operation was observed for co-digestion trials with livestock manure, at an organic loading rate of up to 1.30±0.03 gVS/L_reactor·d, with the highest EPI of 250% considering a hydraulic retention time of 14 days. Regarding co-digestion trials with mixed sewage sludge, at an organic loading rate of up to 1.38±0.3 gVS/L_reactor·d, the highest EPI was 539% for a hydraulic retention time of 15 days. It is important to highlight the positive synergetic effect of the addition of two co-substrates (LCP+LCG) to improve biodegradation feedstock, resulting in a higher methane yield when compared with sewage sludge as mono-substrate.

Agro-food biowastes are a promising co-digestion substrate what contributes to the valorisation of those biowastes for bioenergy recovery (Bio-CH₄) in the framework of bioeconomy perspectives.

The Linking Landscape, Environment, Agriculture and Food (LEAF) research unit (UID/AGR/04129/2013) supported this work.