Two-Phase Anaerobic Reactor Coupled with Microalgae Photobioreactor for Slaughterhouse Wastewater Treatment and Bioenergy Production

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Date

2023-10

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Addis Ababa University

Abstract

In Ethiopia, the transformation from agricultural to agro-industrial based development contributed to an increase in the number of agro-processing industries such as slaughterhouses, which discharge its process wastewater to the environment with partial or no treatment. Uncontrolled and regular disposal of partially treated or untreated wastewater into the water body has significant impacts on the environment and humans. The objective of this study was therefore to evaluate the pollutant removal efficiency and bioenergy (biogas and biodiesel) production potential of an integrated biological treatment system composed of two-phase anaerobic reactor and microalgae photobioreactor treating slaughterhouse wastewater. The experiment was conducted using two consecutively connected forty-liter galvanized metal anaerobic batch bioreactors (hydrolytic-acidogenic and methanogenesis reactors) integrated with a 9000-cm3 microalgae photobioreactors. From the results of the hydrolytic-acidogenic phase, a hydraulic retention time (HRT) of three days and an organic loading rate (ORL) of 1788.81 mg COD/L*day offered optimal reactor stability and performance. Thus, highest degree of hydrolysis of 63.92%, degree of acidification of 53.26%, soluble chemical demand (3430.2 ± 80.44mg/L), and total volatile fatty acids (1176.50 ± 81.66 mg/L) were achieved for hydrolytic-acidogenic reactor operating at indicated condition. The NH4+-N concentration varied from 278.67 to 369.46 mg/L, which is not in a range that would affect the reactor’s stability. The results from methanogenesis phase also revealed improved stability and performance of the system with peak operating parameters: total volatile fatty acid (TVFA) = 520 ± 19 mg/L; Total Alkalinity (TotA) = 1424 ± 10 mg/L; TVFA: TotA. Ratio = 0.36; salinity = 1172 mg/L, pH = 6.92) were found at a HRT of six days and an OLR of 298, mg COD/L*day. Additionally, highest removal efficiency for COD = 81%; volatile solid (VS) = 95%; biogas production = 185 ± 4 mL; and methane yield = 0.03 per mg COD consumed were achieved at HRT of 6 days and OLR of 298 mg COD//L*day. Moreover, the system showed good performance in terms of methane yield (67%) and methane production rate (123 mL/day). The results from the two-phase anaerobic digestion operated at optimal hydrolysis and methanogenesis reactors operating conditions showed substantial organic matter (TCOD (82.87%), TDS (93.32%), turbidity (98.0%6)) but low nutrient (TN (57–63%) and TP (27.57%)) removal efficiency. At the polishing step using microalgae photobioreactor, removal efficiencies above 85% were found for COD, TN, NO3-N, NH4+-N, TP, and PO4-3–P. The average crude lipid content of Chlorella, Scenedesmus, and co-culture microalgae biomass was 15.70 ± 1.27%, 17.75 ± 1.08%, and 21.65 ± 1.03%, with 72-79%, 67-78%, and 71-81% biodiesel content, respectively. The variation in lipid and biodiesel content was significant (p ≤ 0.05) between the treatments. In general, the integrated two-phase AD and microalgae photobioreactor showed removal efficiencies of greater than 90% and 95% for nutrients (TN, NH4+-N, NO3-N, TP, PO4-3–P, and SO4-2) and organic matter (BOD, SCOD, and TCOD), respectively. The bioenergy produced from the integrated system in terms of methane and biodiesel were 128.40 mL/day and 67-81%, respectively. Furthermore, a two-phased AD system integrated with Chlorella, Scenedesmus species, and the co-culture exhibited final effluent concentrations below the permissible discharge limit for slaughterhouse effluent standards of Ethiopia for all pollutants. Therefore, it can be concluded that the two-phase AD system integrated with microalgae photobioreactor used to treat slaughterhouse wastewater in this study has implications for circular economy approach by converting this biowaste into valuable products. The treated final effluent can also be reused for non-potable domestic purposes such as floor cleaning, irrigation, and greenery purposes, which in turn guarantee sustainable practices.

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Keywords

Biogas Yield, Microalgae Co-Culture, Microalgae-Based Wastewater Treatment, Slaughterhouse Wastewater, Two-Phase Anaerobic Digestion

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