Methane and energy recovery potential from swine wastewater digestion by anaerobic and microaerobic processes

Abstract

The research aimed to evaluate the use and potential of biogas production as a source of renewable energy from swine wastewater (SWW) treatment in upflow anaerobic sludge blanket reactor (UASB) and upflow microaerobic sludge blanket reactor (UMSB) reactors. The SWW was seen as a potential substrate for energy recovery due to its high complexity and high organic matter content and the large growth of pig farming in Brazil. The focus was primarily on electricity production, understanding the potential of the by-products generated in SWW digestion, and identifying the system self-sufficiency and the application of the energy generated in other sectors. A mathematical model has been developed to more accurately estimate the potential for energy recovery in UASB reactors treating wastewater from a pig farm with up to 10,000 pigs. The model was developed based on Monte Carlo simulation to estimate the energy potential of biogas in three categories of pig farms (small, medium, and large), which corresponded to different scales (250, 500, and 1,000 pigs). ln order to determine the SWW energy potential, various organic loading rates (OLR) from 9.3 to 21.1 kgCOD m-3d- 1 were analyzed. For each piggery scale, the potential for electrical energy recovery was 1.6 to 2.1 kWh kgCODrem-1 and 6.0 o 8.2 kWh msww-1. The SWW energetic potential was calculated based on the UASB reactor operational parameters, the calorific value of methane in biogas, and the daily SWW production. The linear regression between the variables SWW (L d-1) (axis x) and biogas energy potential (kWh d-1) (axis y) was y=-241.60+33.55x (r2=0.82), while for COD removed (kg d-1) (axis x) and biogas energy potential (kWh d-1) (axis y) was y=- 80.10+2.24x (r2=0.93). The model showed good accuracy, and there was an adequate fit to the monitored data, indicating that can be applied to estimate the energy potential of biogas generated in UASB reactors. It was also investigated the effects of the in-situ microaeration on the digestion performance of SWW for methane and electrical energy recovery. ln this regard, the performances of the UASB reactor and UMSB reactor were compared. Two oxygen doses (0.17 and 0.25 LO2 Lfeed-1d-1) were evaluated on methane and electrical energy recovery with the reactors operated at the Hydraulic Retention Time (HRT) of 12h. The air dose of 0.25 LO2 Lfeed-1d-1 was subsequently evaluated at HRT of 10, 8, and 6h. The highest methane yield achieved in the UMSB reactor was 228.6 NL kgcon-1 d-1, during an OLR of 12.2 kgCOD m- 3d-1 (HRT of 10h). OLR and air dose increase generated methane dilution in the biogas, which reduced the possibility of electrical energy recovery. Nevertheless, the results showed that the UMSB reactor was self-sufficient and had energy efficiency in the 12h HRT at doses of0.17 and 0.25 L02 Lfeed-1d-1 and in the lüh HRT at a dose ofü.25 L02 Lfeel1d-1 . The UMSB reactor achieved an energy retum on investment (EROI) of 11.6 to 18. 7, showing a potentially viable altemative for power generation.