Potential for domestic biogas as household energy supply in South Africa

Abstract

Domestic biogas technology is a clean, renewable form of energy that is accessible to lowincome households through anaerobic digestion of readily available organic waste. The objective of this desktop study was to determine the amount of biogas required for substitution of conventional domestic fuels (fuelwood, paraffin and coal), to quantify the health benefits from reduced indoor air pollution due to such a substitution and to evaluate the availability of feedstock for adoption of domestic biogas technology in South Africa. The energy demand by low?income South African households for cooking with fuelwood was calculated to be 27 MJ/day and the total energy demand to be 68 MJ/day. Approximately 80% of the total energy is used for cooking, water heating and space heating and approximately 20% is used for lighting. To meet the energy demand for cooking (27 MJ/day) with fuelwood with a thermal efficiency of 13%, it was calculated that 2 500 L/day/household of biogas is required which is in line with studies conducted in India and China. In order to meet the total energy demand of 68 MJ/day by low?income South African households, it was calculated that biogas of approximately 6 250 L/day/household is required of which 5 000 L/day/household is used for cooking, water heating and space heating and also 1 250 L/day/household for lighting. A photovoltaic (PV) solar home system is recommended for lighting in rural households instead of using the inefficient biogas lamps which often pose a safety risk to the household members. Complete substitution of fuelwood used for cooking with 2 500 L of biogas per day results in cost savings of R904 per household per annum which is 4.3% savings of the average household income and translates to a gross national annual cost savings of approximately R 1.5 billion. Complete substitution of fuelwood as a source of energy results in cost savings of R1 808 per household per annum which is 8.6% of the household income and translates to a gross national annual cost savings of R4 ? 5 billion. In terms of burden of disease and mortalities, it was determined that fuelwood use in South African households results in 702 790 and 22 365 attributable disability?adjusted life years (DALYs) lost and mortalities respectively. It was also determined that 50% of the attributable DALYs lost and mortalities from solid fuel use can be avoided by substitution of fuelwood used for cooking with 2 500 L of biogas per day per household whereas complete substitution of fuelwood with biogas can result in the avoidance of approximately 85.4% of total DALYs lost and mortalities from solid fuel use. It terms of feedstock availability, it was determined that there is potential for domestic biogas technology utilising cattle and pigs waste as feedstock. Due to access to sufficient cattle dung, it was determined that approximately 613 662 households can potentially benefit from 2 500 L/day capacity biogas digester installations fed with cattle dung. Approximately 131 392 households can potentially benefit from 5 000 or 6 250 L/day capacity biogas digester installations fed with cattle dung. The number of households that have access to sufficient pigs waste to benefit from installations of 2 500 or 5 000 or 6 250 L/day capacity biogas digesters fed with pig waste are 12 089. Due to the number of chickens required and the average number of chickens kept by South African households, it can be deduced that it is not feasible to operate a biogas digester fed solely with chicken waste. It was also determined that South African households do not generate sufficient human excreta and food waste to feed a biogas digester of a sufficiently large size. It is therefore recommended that community digesters in peri?urban areas/informal settlements be co?fed with 1:1 mixture of sewage and food waste. It is also recommended that the households interchangeably utilize the biogas from the community digester for cooking purposes. Non?sewered households with access to on?site water supply generate sufficient greywater for feeding a domestic biogas digester. This is therefore recommended over drinking water. Nonsewered households with access to off?site water supply generate insufficient greywater for feeding biogas digesters of 5 000 L/day and 6 250 L/day capacity. It is therefore recommended that in non?sewered households with access to off?site water supply greywater be augmented with harvested storm water or water from nearby rivers, dams and streams. Since the present work is a desktop study, it is recommended that a pilot scale study be launched to confirm the findings of this study regarding the quantity of biogas required to substitute conventional domestic fuels as well as the feasibility of domestic biogas technology in low?income South African households or at community level.