Techno-economic analysis of a micro-scale biogas plant integrated with microalgae cultivation for the treatment of organic municipal waste

The deployment of small-scale anaerobic digestion (AD) plants for the treatment of the organic fraction of municipal solid waste (OFMSW) and associated production of renewable energy at a local level has recently attracted attention as opposed to full-scale centralized plants. In this work, we evaluate the technical and economic feasibility of an integrated process comprising small-scale AD of OFMSW followed by microalgae cultivation, to valorise the waste streams of the former (off-gases produced by combined heat and power generation and liquid digestate) for the production of valuable biomass to be destined to either the biofertilizers or the biostimulants markets, in agreement with a circular economy perspective. The material and energy balances were obtained by means of process simulation for a plant located in Northern Italy, taken as a reference. The analysis revealed that, although the high turbidity of the digestate strongly hinders biomass production, the process is profitable from a techno-economic perspective, with discounted payback time ranging from 3.3 to 7.1 years, and profitability indexes greater than 1. The low market price of biofertilizers makes it the less profitable choice, even compared to the AD process alone, while selling the biomass for biostimulants applications could strongly increase the process profitability.