Effect of hydraulic retention time and effluent recycle ratio on biogas production from POME using UASB-HCPB fermentor assisted with ultrafiltration membrane at mesophilic condition

Abstract

Biogas is renewable energy produced through anaerobic digestion based on palm oil mill effluent. Biogas production is overgrowing and is carried out in various bioreactors, such as the Upflow Anaerobic Sludge Blanket - Hollow Centered Packed Bed fermentor. Even though this process is considered successful in producing biogas, it has not adopted a recycling system. Therefore, the remaining sludge (effluent) after the processing process is generally not reused and is thrown into the environment, which still has the opportunity to produce biogas. The combination of the performance of the the Upflow Anaerobic Sludge Blanket - Hollow Centered Packed Bed fermentor with an ultrafiltration membrane is one of the latest innovations to reduce the volume of effluent wasted through recycling the effluent (retentate) into the feed tank. This research aims to produce biogas from liquid waste from palm oil mills using an Up-flow Anaerobic Sludge Blanket-Hollow Centered Packed Bed fermentor combined with ultrafiltration membranes under mesophilic conditions. In this research examines the effect of hydraulic retention time and the effluent recycling ratio on the pH profile, alkalinity, production, biogas composition, and kinetics of biogas production. The research began with loading up by varying the hydraulic retention time, namely 40, 25, and 10, until reaching the target hydraulic retention time of 6 days in a 5.4 L fermentor with a pH of 7 ± 0.2 under mesophilic conditions. Next, the effect of the recycling ratio was studied by varying the effluent (retentate) recycling ratio, namely 0, 15, and 25 %. The parameters analyzed are pH, M-Alkalinity, total solids, volatile solids, total suspended solids, volatile suspended solids, chemical oxygen demand, volume, and biogas composition. The organic content in the substrate is used as a kinetic parameter for biogas production using the modified Gompertz, Logistic, and Monod kinetic equation. The research results show that in mesophilic conditions, a recycling ratio of 25 % shows better results compared to ratios of 0 and 15 % where biogas production is 20×10^-5 L/mgVS.day, with a best composition of methane, carbon dioxide and hydrogen sulfide each of 88.2; 10.8; and 0.07 % (v/v), with ΔVS decomposition at 15 % and 25 % recycle ratio of 42.50 and 45.83 % (w/v). The equation for the biogas production rate constant as a function of temperature obtained is the biogas production rate constant: $M=\frac{299.8}{1+\text{exp}\left(\frac{14.28}{299.8}\left(0.39-t\right)+2\right)}$ for logistic model, $M=\frac{27.2xs}{2.46+s}$ for monod model, $M=250.1\times \text{exp}\left\{-\text{exp}\left[\frac{1.85.e}{250.1}\left(0.42-t\right)+1\right]\right\}$ for Gompertz modified kinetic model. Based on the scanning electron microscope results, it can be seen that the membrane used is an ultrafiltration membrane type, with a characteristic thickness of 7-8 µm and a pore size of 100–300 nm. The scanning electron microscope results also show that effluent contaminants deposition in the inner layer (spinger membrane) and deformation of the spinger structure are driven by forces during the effluent recycling process. The results obtained in this research show that this condition provides a clean, effective, and low-energy biogas production system that can be optimistically applied to the national palm oil industry power generation system.