Elsevier

Biomass and Bioenergy

Volume 142, November 2020, 105790
Biomass and Bioenergy

Improving anaerobic co-digestion of different residual biomass sources readily available in Colombia by process parameters optimization

https://doi.org/10.1016/j.biombioe.2020.105790Get rights and content

Highlights

  • Anaerobic co-digestion of organic residues available in Colombia is evaluated.

  • Optimizing the experimental conditions, batch reactors and mesophilic conditions.

  • Grams of volatile solids is the strongest variable influencing the biogas production.

  • Box-Behnken and Multivariate adaptive regression splines model comparison.

Abstract

In the framework of sustainable development, there is an increasing need to assess, model, and optimize the implementation of renewable energy technologies such as the anaerobic co-digestion of different organic residues. This work studied the influence of some independent parameters on the production of biogas through anaerobic co-digestion of specific organic residues widely available in Colombia (pig manure -PM-, sewage sludge –SS–, organic fraction of municipal solid waste -OFMSW-, residues from the bottled fruit drinks industry -RBFDI-, and cocoa industry residue -CIR-). The Biochemical Methane Potential (BMP) of different mixtures was assessed through a Box-Behnken experimental design, where the parameters were: volatile solids content (0.5, 1.25 and 2 g VS), C/N ratio (25, 35 and 45), and nitrogen source (0% for only SS, 50% both residues PM and SS, and 100% if only PM). The results allowed empirical model building by using second-order polynomial and MARSplines. According to the maximization, the best mixture for biogas production is the one containing RBFDI, OFMSW, and SS, a C/N ratio of 40 and 0.5 g VS; the estimated production was around 382.17 ml CH4 g−1 VS, MARSplines demonstrated to have the best predictions values and the highest determination coefficient (94–97%). Moreover, it was evidenced that the content of g VS is a critical variable on the methane production, C/N ratio must be in average values, and both nitrogen sources are suitable to be used in the co-digestion process depending on the availability of the area of interest.

Introduction

Several ways to monetize solid waste have been widely studied during the last decades, and one of them is the anaerobic digestion of putrescible waste, it is a biological treatment to stabilize organic matter while producing biogas [1,2]. A process by which organic carbon is reduced to methane (CH4) and oxidized to carbon dioxide (CO2), that, in addition to other trace gases, makeup biogas [3,4]. One of the variants in anaerobic digestion practices is co-digestion, which consists in the use of two or more substrates in the process; under which higher yields are obtained as far as the production of biogas is concerned [5,6]. Anaerobic co-digestion is one of the biological processes with the highest potential for the valorization and treatment of putrescible waste [2]. It is considered a waste management alternative and an efficient recovery process, by which the dilution of potentially toxic compounds is achieved, the balance of nutrients is improved, and synergistic effects of microorganisms are obtained [7]. Also, anaerobic digestion provides multiple environmental benefits associated to: organic waste disposal, green energy production, environmental protection, and (Green House Gases) GHG emission reduction [8]. This process allows the valorization of different types of organic waste (animal manure, agro-industrial and municipal solid waste), which are generally chosen by their nature, availability, carbon/nitrogen ratio, potential of generation and its environmental impact [9]; in order to favor the anaerobic digestion to carry out the aforementioned activities.

There are previous researches on co-digestion of different kinds of waste: animal feces, sewage sludge, and various types of biomass. The co-digestion of animal feces is the substrate with the highest number of publications, and the co-substrates most frequently reported are agro-industrial waste and organic fraction of municipal solid waste [[10], [11], [12]]. There are also numerous publications of co-digestion of oils, various fats, and sludge with algae as co-substrate, taking advantage of the fact that these substrates can be obtained in the same wastewater treatment plant [13].

Ahn et al. [14] evaluated different mixtures for the co-digestion of sewage sludge, organic fraction of municipal solid waste and swine manure, which allowed to define synergistic effects for the optimization of the methane yield through ANOVA tests, approximately 294 ml g−1 TS of cumulative production for the evaluation of the optimal mixing ratio of the three wastes. Likewise, Rao, P. & Baral, S [15]. evaluated the biogas generation potential of two sets of mixtures. Mixtures consisted of sewage sludge (SS) with cow dung (CD) and garden waste (GW), and mixtures consisting of SS with CD and fruit juice wastewater (FJW). This study allowed to define that the mixtures containing SS and CD in major proportion with GW in minor proportion were having a synergistic effect with cumulative methane volume being highest, through augmented simplex centroid design.

It is important to remark that these studies mainly include waste from industrialized countries, which have different characteristics from those of a developing country like Colombia. For its part, Colombia has a high potential for the development of this technology due to the high amounts of organic waste production caused by the growth of its economy [16,17]. The accuracy of the optimization of methane yield through empirical models allows its use as a tool for decision making on the technologic transfer of anaerobic co-digestion to a real-scale process by recognizing synergistic effects between different substrates based on physicochemical characteristics. Therefore, research on the performance and synergy of waste from industries: cocoa, bottled juices, and wastewater treatment; adding: the organic fraction of municipal solid waste and pig manure, susceptible to biomethanization valorization process, is presented as an option in the generation of alternative energy in the country.

The bottled fruit drinks industries and the cocoa industry generate large volumes of organic waste. It is recorded that between the 25–40% of the fruit is turn as residue during transformation processes, more than 5.500 tons of raw fruits like mango, blackberry and lulo among others; are used annually for bottled fruit drinks production and the residue treatment is based on composting [18]. Likewise, it is recorded that the cocoa industry only uses 30% of the biomass present in the fruit, which generates an imbalance of the natural cycle causing considerable environmental inconveniences associated to their final disposal [19]. In Colombia, during the last decade, the cocoa production raised 62%, where for 2018 the total production was of 56,867 tons [20]. As an alternative, Rodríguez, R. et al. [21], assessed potential locations of biomass to energy plants according to the availability of cocoa residue in Santander department (Colombia).

In the same way, the statistics related to the livestock sector reported a swine population of 6,4 million in around 237,380 areas, of this total, 29.68% are located in Antioquia department, followed by Cundinamarca with 8.94% of the production [22], the amount of manure produced by each lifting pig is around 2.35 kg per day [23]. Consequently, the PM is used in low-cost anaerobic reactors to provide energy and biofertilizers. Nevertheless, not all the manure is valorized and the energetic potential of the swine sector in the country favors the inclusion of different alternatives for the energetic monetization [24,25].

In turn, sewage sludge from wastewater treatment is highly used in the production of biogas, which solves the problem of disposition of them and guarantees somehow the stabilization of the residue [26]. Bogota river basin has 31 wastewater treatment plants within 24 municipalities meant to guaranty low pollutant discharges to support the environmental services conservancy of the basin [27]; therefore, alternatives on the treatment of the sewage sludge is a task to address. The Organic Fraction of Municipal Solid Waste (OFMSW) is defined as those generated in private homes, shops, offices, and services, as well as all those that do not qualify as dangerous; it is evident that the volume and characteristics of the MSW depend directly on cultural, economic, technological aspects [28,29]. These aspects are associated with the level of development of the country, and the management - valorization of the OFMSW is related to the region in particular [30].

As a result of the current environmental impacts of diverse economic activities and international commitments, the Colombian government has established policies such as Law 1715 of 2014 that aims to regulate the integration of non-conventional renewable energy into the national energy system, taking the first step to a regulated renewable energy market in the country. Once the purpose of the same focuses on the establishment of a legal framework with the necessary instruments for the promotion of: renewable energy sources, investment, research, and development of clean technologies for energy production, energy efficiency, and demand response.

Regarding anaerobic digestion conditions, the analysis of the factors affecting the performance of the anaerobic digestion has been studied to improve process efficiency. Along the AD stages, several aspects of the physicochemical characteristics have to be considered for the optimization of the process, parameters such as: carbon/nitrogen ratio (C/N), volatile solids (VS), pH, temperature, substrate properties, and composition, and the effects of co-substrates addition; are critical parameters on the AD process [[31], [32], [33]]. The mathematical models simulation evaluates different parameters for different scenarios, one of the statistical techniques is the response surface methodology, which is feasible for the design of experiments to empirical models searching for the optimum conditions for the desirable response [[34], [35], [36], [37], [38]].

The aim of this paper is the evaluation of parameters as the C/N ratio, the grams of volatile solids (g VS), and the nitrogen source percentage in the processes of anaerobic co-digestion of organic waste available in Colombia, creating an empirical model based on an individual physicochemical characterization. These studies were carried out using an experimental design (Box - Behnken), which allows optimizing the experimental conditions, improving the quality and quantity of methane production. This work is an important step to plan a route for the valorization of the residues evaluated through anaerobic co-digestion in Colombia.

Section snippets

Substrates

The experimental design presented was meant to the evaluation of the following substrates: Residues from the Bottled Fruit Drinks Industry (RBFDI), Cocoa Industry Residue (CIR), Organic Fraction of the Municipal Solid Waste (OFMSW), Pig Manure (PM) and Sewage Sludge (SS). The RBFDI were simulated in a laboratory at Universidad Cooperativa de Colombia (UCC) based on the references related to the residual streams of this sector: mango, banana, blackberry, lulo, and passion fruit [39]. The CIR was

Physicochemical characterization

The results of the characterization were summarized in Table 3.; all the values reported were obtained according to the characterization processes standardized. These values allowed the generation of the mixtures previously exposed in the experimental design. Volatile solids (VS) and total solids (TS) are represented on the organic matter, most of the substrates have a high organic matter and moisture content indicating its potential of biodegradability [11].

High nitrogen concentrations in pig

Conclusions

According to the maximization, the best mixture for biogas production is M1 containing RBFDI and OFMSW, as the optimum values obtained for the independent variables allowed to raise the BMP around 382.17 ml CH4 g−1 VS; the values are: C/N ratio of 39.95, organic load of 0.5 and sewage sludge fixed as only nitrogen provider. Indeed, these conditions could be used in a scale-up of the anaerobic digestion process, given the accuracy of the experimental values vs. MARSplines prediction (r2 0.973),

Acknowledge

The authors acknowledge financial support from COLCIENCIAS (Administrative Department of Science, Technology, and Innovation of Colombia). Project number FP 279–2015.

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