Biogas production from Ecklonia maxima using an anaerobic digestion process: investigation, analysis and optimisation

Abstract

South Africa is in the grip of an energy crisis. The SA government is unable to provide a consistent supply of energy, implementing loadshedding as a coping measure, while alternate energy sources are being investigated and electrical power units upgraded. Worldwide, there is significant research conducted on the use of land-based crops as feedstocks for biofuel production. The use of these crops are often limited by the food vs. fuel dilemma. As a result, waste products and algal products are being considered as alternative sources for feedstocks for energy production. The research presented investigates the use of brown macroalgae, Ecklonia Maxima, which is widely available in South Africa, as a potential feedstock for biogas production. The process utilised is anaerobic digestion (AD), which is able to utilise a wet feedstock, therefore removing the additional drying costs required from other processes. Four inocula were ded two synthetic inocula as well as an organic inoculum and a starter fluid from the wastewater treatment plant. Results show that at mesophilic temperatures, the synthetic inoculum, BioGanic, shows the highest biomethane yield (40.5 ml/g VS). The highest overall yield when comparing the mesophilic and thermophilic yields was obtained at thermophilic temperatures. The BioGanic/Ecklonia Maxima system was subjected to a series of pre-treatment processes to investigate if the biomethane yield could be improved upon. Mechanical, chemical and microwave pre-treatment was applied at both mesophilic and thermophilic temperatures. Size reduction of the Ecklonia Maxima was applied first. Results show that biomethane yield increased in the mesophilic range from 31.5 ml/g VS using raw seaweed to 126 ml/g VS. Biomethane yield decreased in the thermophilic range from 222.6 ml/g VS using raw seaweed to 110.6 ml/g VS using mechanically pre-treated seaweed. The increase in yield in the mesophilic range using pre-treated seaweed was less than the yield obtained for the thermophilic range of the raw seaweed. Chemical pre-treatment using 0.15M HCl showed improved biomethane yield (60.3 ml/g VS) when compared to raw seaweed at mesophilic temperatures. The thermophilic range recorded a decrease in biomethane yield when compared to the raw seaweed. Biomethane yield was 70.3 ml/g VS using 0.3M HCl at thermophilic conditions. Alkaline pre-treatment using NaOH at two different concentrations gave mixed results. The biomethane yield was slightly lower than the acid pre-treatment at 59.8 ml/g VS, and slightly higher for the thermophilic range at 72.5 ml/g VS when compared to the acid pre-treated seaweed. Microwave pre-treatment of Ecklonia Maxima saw similar results for both the mesophilic and thermophilic ranges at biomethane yields of 57.8 ml/g VS and 56 ml/g VS, respectively. Kinetic modelling indicated that the first order modified Gompertz equation was a good fit for the raw seaweed and pre-treated seaweed data, which allowed for the determination of the maximum methane yield for the batch system. It also allows for the prediction of the performance of the seaweed in biomethane production. A techno-economic study with two case studies proved that seaweed biomaterial could produce biomethane, which could then be converted to electricity using a CHP unit. The levelised cost of electricity of the seaweed was determined at R5.65/kWh. Replacing the existing feedstock with seaweed was more expensive. The cost of the seaweed feedstock is the over-riding factor in determining system feasibility.