Substrate Quantification, Experimental Procedure & Proximate Analysis

Renewable energy potential of anaerobic mono- and co-digestion of chicken manure, goat manure, potato peels and maize pap in South Africa

Substrate Quantification

Food waste was collected from the cafeteria and animal manure samples were collected from the Department of Agriculture farm of the Mangosuthu University of Technology (KwaZulu-Natal Province, South Africa). Samples were collected into 5-L plastic containers, closed airtight, and stored at 4 °C in the refrigerator. The samples were labelled as chicken manure, goat manure, potato peels, maize pap, and inoculum from the cow manure. To pre-treat the samples and minimise the maturing effect, as well as to limit the microbial activities, the feedstock was dried in the oven at 60 °C, reduced to a 2-mm size diameter, and stored in the fridge at 4 °C until utilised. The inoculum was prepared by digesting cow dung samples until no biogas was produced. This was done in order to ensure that the microorganisms digest all the substrate in cow dung completely before adding new substrate to ensure that biogas produced was from new substrate and not from substrate in the cow dung. The retention time for the inoculum preparation was 14 days.

Experimental Procedure

The biomethane production rate was determined by feeding the substrates and the inoculum into a batch digester. Chicken manure, goat manure, potato peels, maize pap, and inoculum of cow manure were fed as a mono-substrate for mono-digestion. Chicken manure and potato peels, chicken manure and pap, goat manure and potato peels, goat manure and maize pap, maize pap and potato peels, and a mix of chicken manure, potato peels, goat manure and maize pap were codigested in the ratio of 1:1. The substrates were fed with the control. The conditions were set at mesophilic temperatures of 37 °C and the working pH of 6.5–7.5 was adjusted using sodium hydroxide and sulfuric acid. A bioprocess controller (AMPTS II) by Dürr Systems, Inc. (De Pere, WI, USA) was used to perform the biomethane potential test. The AMPTS II consisted of an automated digester, CO2 fixing unit, and biomethane collection unit. Bioreactors with a volume of 500 mL and a headspace of 100 mL were used. Sodium hydroxide was used to remove CO2 from the production of biogas to biomethane. A 3-M sodium hydroxide solution was used to remove CO2 and H2 S. A pH indicator solution with 0.4% thymolphthalein was applied to the sodium hydroxide solution where the pH indicator was used as a scrubber. Before preparing the substrates and feeding into the digesters, the prepared NaOH with pH indicator was used to determine the saturation point for the cleaning solution to be replaced. Thus, the digesters were purged with nitrogen gas to create an anaerobic state by discharging the oxygen. The gas that exited the CO2 fixing unit was sent to the stream cell (gas collection unit) and assessed on a daily basis using water downward displacement technique until the retention time was completed. Figure 3 shows the bioprocess controller (AMPTS II).

Analytical techniques: Proximate analysis

The chemical and physical compositions, moisture content, total solids (TS), volatile solids (VS), and hydrogen potential (pH) were calculated using standard methods (APHA 1995).42 Ultimate analysis A Flash 2000 CHNS-O element analyser (Thermo Fisher Scientific Inc., Waltham, MA, USA) fitted with an autosampler was used to analyse the elemental (C, H, N, S) composition of the substrate. In replicates per sample, a dry mass of 1 mg of each substratum was weighed into a tin capsule. CHNS was then determined by an autosampler by placing the samples in an electrolyte-filled quartz reactor and then inserting them into the reactor cell. After combustion in an oxygen-rich environment, the gases given off were carried by a helium flow past a copper-filled layer, through a gas chromatography column where the combustion gases were separated and detected by a detector (Thermal Conductivity Detector, Waltham, MA, USA) with a column oven temperature of 65 °C detection.

Article TitleRenewable energy potential of anaerobic mono- and co-digestion of chicken manure, goat manure, potato peels and maize pap in South Africa

Abstract

The energy sector is an essential part of a country’s economy – it drives innovation and advances in industrialisation. Coal is the primary source of energy in South Africa. Coal contributes 95% of energy production; coal-fired power also contributes to greenhouse gas emissions, and is thus a hazard to human health and the environment. This calls for an energy mix that is renewable, sustainable, and affordable and that is carbon neutral (climate action). We investigated the potential of anaerobic mono-and co-digestion of goat manure, chicken manure, potato peels, maize pap, and cow manure inoculum for mesophilic recovery of renewable energy using the biomethane potential test. The substrates were characterised through proximate and ultimate analyses to determine the composition preferable for mono- and co-digestion. The key considerations in the determination of both the yield and production rate of methane from digestion of biomass are the substrate composition and characterisation. A high percentage of volatile solids favoured optimum biomethane production as highly volatile components provide microbes with balanced nutrients that enhance metabolic processes to produce biomethane. The mono-digestion process produced lower biomethane than did co-digestion. Higher production of biomethane by co-digestion was due to the balance of the micronutrients and macronutrients that favoured microbial metabolism and regulation of pH.


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