Electricity and substitute natural gas generation from the conversion of wastewater treatment plant sludge

In Chile, the energy that can potentially be obtained from the digestion of sludge generated from wastewater treatment processing (WwT) was calculated using a holistic approach. The different pathways of electricity generation via the direct combustion of biogas and upgraded biogas produced as bio-substitute natural gas (Bio-SNG) for injection into the gas grid were assessed and compared. Information such as the served population, WwT technology employed and geographical distribution of the sludge sources was gathered to estimate energy potential; additionally, technical and economic information was collected from the literature. Furthermore, economic modelling was employed for the purpose of comparing the two end-use alternatives.The results were presented by using supply-cost curves and then integrated into a geographical information system (GIS), the latter of which shows the distribution of energy potential nationwide. A comparison with a reference market price of the corresponding secondary energy type, electricity or natural gas, was conducted to elucidate the economic attractiveness of the two assessed options. From the assessment, it was concluded that the economic potential for the injection of Bio-SNG into the grid is 19MMNm³y^-1 at a representative generation cost of approximately 43€MMBTU^-1, whereas biogas for the electricity generation pathway has an economic potential of 75GWh_ey^-1 at a representative generation cost of21.5ct€kWhe-1. In view of these results, it can be observed that the electricity generation pathway may offer more major economic and environmental benefits than the Bio-SNG pathway, despite both options being hardly competitive without subsidies. Additionally, it can be observed that the Bio-SNG route is not competitive in most cases, and it is economically attractive solely under restricted conditions, principally those established by plant capacity. Furthermore, steadily increasing electricity prices over time suggests that the biogas-to-electricity option will become progressively more competitive. These results can be considered as outcomes to elaborate a national macro-policy to tackle the sludge issue under a waste-to-energy approach.