• Vol 11, No 3 (2020)
  • Chemical Engineering

Improvement of Biomethane Potential by Anaerobic Co-Digestion of Sewage Sludge and Cocoa Pod Husks

Daniela Mora-Cortés, Yeison Alberto Garcés-Gómez, Sebastian I. Pacheco

Corresponding email: ygarces@ucm.edu.co

Cite this article as:
Mora-Cortés, D., Garcés-Gómez, Y.A., Pacheco, S.I., 2020. Improvement of Biomethane Potential by Anaerobic Co-Digestion of Sewage Sludge and Cocoa Pod Husks. International Journal of Technology. Volume 11(3), pp. 482-491

Daniela Mora-Cortés Universidad Católica de Manizales, Environmental Engineering, Cra 23 No 60-63, 170001, Colombia
Yeison Alberto Garcés-Gómez Universidad Católica de Manizales, Academic Unit in Natural Sciences and Mathematics, Cra 23 No 60-63, 170001, Colombia
Sebastian I. Pacheco Universidad Católica de Manizales, Academic Unit in Natural Sciences and Mathematics, Cra 23 No 60-63, 170001, Colombia
Email to Corresponding Author


The objective of this study was to determine the efficiency and theoretical stability of the anaerobic digestion of the wastewater sludge obtained from a municipal wastewater treatment plant, using co-digestion with cocoa pod husks. The chemical structure of sewage sludge, including its high carbon and nitrogen content in carbohydrates and fats, gives it vast potential for biomethane generation. In this study, the main agri-food residues in the geographical area where the study was conducted that could be used for co-digestion were determined, and cocoa pod husks were found to be the best option based on elemental chemical analysis. The results demonstrate that the co-digestion of sewage sludge with cocoa pod husks can produce up to 555.7 L CH_4/Kg SV. In this article, we also propose a method for estimating biochemical methane potential (theoretical BMP) based on chemical equations and a systematic review of the most relevant research in BMP.

Anaerobic digestion; Biomethane potential (BMP), Cocoa pod husks; Methane production; Sewage sludge


Due to climate change and the global energy crisis, the world is searching a green and carbon-neutral energy source which could replace fossil fuels (Chin et al., 2019; Kusrini et al., 2019). The security of the energy supply, particularly through renewable energy, and the reduction of CO2 emissions have become priorities. The microbiological process of anaerobic digestion, which has long been known, is a promising and cheap method of producing biogas (Kusrini et al., 2016; Pilarska et al., 2019). Organic waste, including food waste, is increasingly being used in an attempt to solve another problem of the civilized world, namely the high production of waste. This technology is both a recipe for minimizing the harmful effect of waste on the environment and a source of methane, the biofuel of the future.

The conventional wastewater treatment plant sludge line generates a large amount of waste after decanting the solids from the primary (sedimentation) and secondary (biological) treatments. All the sludge is concentrated through different methods such as flotation, thickening, centrifugation, and dewatering. Variations in the quantity and quality of the mixed sludge are primarily defined by domestic and industrial habits, as well as by the correct functioning of the different treatment units (Alrawashdeh et al., 2017; Agabo-García et al., 2019).

Different technologies are being widely studied to increase the potential of biomethane in anaerobic digestion processes (Ariyanto et al., 2017). These studies focused mainly on increasing the biodegradability of sludge by physicochemical, biological, and/or biochemical methods, thus improving the hydrolysis stage in the overall anaerobic digestion process (Tetteh et al., 2018). All these methods have led to higher recovery volumes and biomethane yields, even on a real scale, as a result of: (i) the rupture of the cell membranes of pathogens, which prevents competitiveness with microbial anaerobic digestion consortia; and (ii) the increase in available compounds, such as proteins, sugars, ammonia compounds, or volatile fatty acids that serve as food for anaerobic digestion consortia. A review of the most relevant research is presented in Table 1, taking into account that the articles mainly rely on the testing of the biochemical potential of domestic wastewater sludge in anaerobic systems and the different types of agricultural waste substrates used for co-digestion with this type of sludge. Parameters such as the type of reactor used and the inoculum/substrate ratio were also taken into account in the review.

Table 1 Summary of literature on methane production from sewage sludge and co-digestion with other substrates

The literature review of the co-digestion of sewage sludge in co-digestion with substrates revealed the following results. The keyword co-occurrence analysis of the search can be summarized in Figure 1, in which one can see the main keywords used in the literature are comprised of anaerobic digestion, sewage sludge, methane production, and food waste; the latter is of great importance due to the potential that can be found in countries with an agro-economy, due to the amount of unusable waste that can be generated in agro-food industry companies.

The cocoa pod husks represents the largest byproduct of the chocolate industry, both in Colombia and worldwide. Currently, there has been an increase in related studies on this type of waste and its possible uses because it represents an important component of agricultural residues and global agro-industrial waste, constituting a promising source of renewable resources and energy (Ayeni, 2010). Colombia is the third-largest cocoa producer in Latin America after Brazil and Ecuador. In 2006, exports from the cocoa-chocolate chain totaled more than 56 million US dollars. In Colombia, cocoa occupies an important place among the most commercial agricultural products for the country, with a harvested area of 164,332 hectares or 2.8% of the total agricultural area. Cocoa production in the country has been characterized by its low level of technological development, in which only weed control, pruning, and harvesting are conducted. Cocoa is produced in almost all the departments of the country, but it is mainly concentrated in Santander, Nariño, Arauca, and Antioquia. Caldas,the study area, is the eighth largest producer with 4,1% of national cocoa production (Lu et al., 2018).

The present study was conducted in Victoria, a Colombian municipality located in the east of the department of Caldas, in the region known as Magdalena. The wastewater treatment plant there is operated by the department's public water treatment company. The plant stores about 25000mof sludge per month, which is partly used by agricultural companies in the area for composting in the fertilization of some crops. Since the production of sludge far exceeds the need for fertilizer, most of the sludge, which is then dried, is sent to the municipality's landfill. 


The substrate ratio and concentration levels for biogas production were determined by co-digesting municipal sewage sludge and cocoa pod husks, an agro-food waste that was previously wasted in the study region, by means of an iterative process. A C:N ratio was achieved for high biogas production. The biogas yield demonstrating that this co-digestion is an efficient alternative, partly solving two problems, namely, the use of sludge which is mostly being deposited in the municipality's landfill site and the use of waste from the municipality's cocoa production. Further consideration should be given to increasing the representative sample with various types and quantities of food waste resources for use as a co-substrate from local production. The optimal conditions with stable biogas production from co-digestion between agri-food waste and domestic wastewater developed in this study will be used to expand biogas production plants on a commercial scale in future engineering applications. The municipal council is planning to develop a biogas plant to support familiar small industries in production with the use of the green fuel results of this research.


This work was supported by the Universidad Católica de Manizales with the research groups on Technological and Environmental Development GIDTA, and Education and Educators Traning EFE.

Supplementary Material
R2-CE-4079-20200526223537.docx Supplementary figures to the development of the research that due to the size of the document cannot be added to it

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