Modeling growth kinetics and community interactions in microalgal monocultures and co-cultures for bioremediation of anaerobically digested swine wastewater

Abstract

The pork production industry generates high annual volumes of swine wastewater that increase proportionally to the global demand for pork meat. Incorrect handling and discharge of swine wastewater causes the eutrophication of water bodies as well as soil and air pollution. Microalgal-based wastewater treatment has been proposed as a cost-effective alternative to traditional treatment methods. It also possess several environmental benefits and offers the opportunity to harvest valuable biomass, thus making wastewater treatment a biocircular economy process. Additionally, microalgae can be used in either primary or secondary wastewater treatments, as they allow for the simultaneous removal of nutrients (TN - total nitrogen and TP - total phosphorus) and organic matter. Most of the existing research is based on laboratory cultures under highly controlled conditions and with previous modification of the substrate, either by applying dilution or sterilization. However, these practices make full-scale implementation complex and cost-elevated. Three microalgal strains, Chlorella vulgaris, Scenedesmus acutus and Arthrospira maxima, were monocultured and co-cultured in raw (undiluted, non-sterelized) anaerobically digested swine wastewater. An overall performance index showed that all of the treatments that included C. vulgaris were the most efficient in terms of biomass production, along with COD and nutrient removal. The co-culture of C. vulgaris and S. acutus achieved the highest OPI of 0.68, displaying 9 biomass folds, a production of 2.97 ± 0.36 gL-1, as well as 89%, 56% and 67% removal efficiencies for COD, TN and TP, respectively. Additionally, three mathematical models were used to calculate relevant growth kinetic parameters, including the specific growth rate, lag-phase duration, interspecific interaction, affinity constant and biomass productivity. C. vulgaris monoculture kinetics were adjusted using a double Gompertz model, showing a maximum growth rate (µ2) of 0.89 days-1 and a lag phase (λ2) of 9.69 days. The Lotka-Volterra model was used to assess interactions between both strains in the co-culture, showing a commensalistic relationship between C. vulgaris and S. acutus, as denoted by the interspecific paramaters βcs = 1.99 ± 0.92 and βsc = -0.007 ± 0.008. Finally, the growth kinetics as a function of the three substrates (COD, TN and TP) were adjusted to the monod model, and the resulting parameters were used in a dynamics simulation of the inside of a continuously stirred reactor tank. A SOWT (strenghts, opportunities, weaknesses and threats) analysis was developed to assess the feasibility of MbWT implementation at industrial level. MbWT is an efficient solution for the treatment of SWW, however, further research in pilot or full-scale systems is still required to move towards full-scale industry implementation. Therefore, the results of the present work presented herein should be applied to eventually make MbWT a viable circular bioeconomy solution to SWW management in Mexico and other developing countries around the world.