Process evaluation of energy requirements for feed production using dairy wastewater for algal cultivation: Theoretical approach

Dairy production is an intensive agricultural practice that elicits many environmental concerns including noxious emissions, potential health risks to nearby human communities and eutrophication of aquatic ecosystems. These environmental impacts can be reduced by using the emissions and dissolved nutrients including nitrogen available in dairy wastewaters to produce algal biomass, which can be recycled for animal feed with higher protein content compared to alfalfa. However, energy demand is a major criterion for selecting an algal biomass cultivation system. Traditionally algae have been cultivated in raceways to minimize energy input, but raceways tend to have lower areal biomass productivity compared to photobioreactors. However, higher productivity may be achievable through growth in innovative designs utilizing gravity to provide reduced cultivation depths and temperature control strategies. The feasibility of animal feed production in raceways is compared to gravity fed production systems for evaluating production cost and direct energy demand. Agricultural style gravity fed production systems have an estimated cultivation energy consumption of 25 Wh m^− 2 d^− 1 compared to raceways at 15.4 Wh m^− 2 d^− 1. However, these systems can significantly increase areal biomass productivities reducing the energy required for biomass production from 2.01 kWh kg^− 1 to 1.41 kWh kg^− 1. Total cost of energy for biomass cultivation and harvesting, when sedimentation is combined with centrifugation to achieve 25% solid content, is estimated to be $176 and $256 per mton in gravity fed systems and raceways, respectively. This can be compared to $334 and $1407 per mton of biomass when only using centrifugation for processing in gravity fed systems and raceways, respectively.