Agricultural products from algal biomass grown in piggery wastewater: A techno-economic analysis

Abstract

The intensification of livestock activities lead to an increase in waste generation with high content of nutrients, as is the case of piggery wastewater. However, this type of residue can be used as culture media for algae cultivation in thin-layer cascade photobioreactors to reduce its environment impact and produce a valorizable algal biomass. Biostimulants were produced by enzymatic hydrolysis and ultrasonication of microalgal biomass, using membranes (Scenario 1) or centrifugation (Scenario 2) as harvesting methods. The co-production of biopesticides by solvent extraction was also evaluated using membranes (Scenario 3) or centrifugation (Scenario 4). The four scenarios were analyzed by a technoeconomic assessment estimating the total annualized equivalent cost and the production cost, i.e., the minimum selling price. Centrifugation provided biostimulants approximately 4 times more concentrated than membranes, but with higher expense due to the cost of the centrifuge (contribution of 62.2 % in scenario 2) and the electricity requirements. The biopesticide production resulted the highest contribution to investment cost in scenarios 3 and 4 (34 % and 43 % respectively). The use of membranes was also more advantageous to produce biopesticides, although it was 5 times more diluted than using centrifuge. The biostimulant production cost was 65.5 €/m3 with membranes and 342.6 €/m3 by centrifugation and the biopesticide production cost was 353.7 €/m3 in scenario 3 and 2,122.1 €/m3 in scenario 4. Comparing the treatment of 1 ha of land, the cost of the biostimulant produced in the four scenarios was lower than the commercial one (48.1 %, 22.1 %, 45.1 % and 24.2 % respectively). Finally, using membranes for biomass harvesting allowed economically viable plants with lower capacity and longer distance for biostimulant distribution (up to 300 km) than centrifuge (188 km). The algal biomass valorization for agricultural products production is an environmentally and economically feasible process with the adequate capacity of the plant and distribution distance.