Yap, Cheau Chin
(2021)
Biogas production from Palm Oil Mill Effluent (POME) using mono- and co-digestion with pilot-scale Integrated Anaerobic-Aerobic Bioreactor (IAAB).
PhD thesis, University of Nottingham Malaysia.
Abstract
In the preliminary stage, start-up, long-term performance evaluation [organic loading rate (OLR) 10.5 gCOD/L.day to 32.5 gCOD/L.day] and kinetic analysis of the pilot-scale IAAB were conducted at mesophilic condition (28°C). Based on the performance studies of the pilot-scale IAAB at different OLRs, the maximum applicable OLRan at which mesophilic system could maintain steady state operation was at OLRan 30.0 gCOD/L.day. The quality of the final treated effluent remained stable with high percent of compliance with the discharge limit of 100 mg/L. Modified Stover Kincannon, Grau second-order and Monod kinetic models were applied to the pilot-scale IAAB at OLRan 10.5 - 32.0 gCOD/L.day. It was concluded that Stover Kincannon model is the most appropriate model to estimate the performance of the anaerobic (R2 of 0.9982) and aerobic systems of the pilot-scale IAAB (R2 of 0.9998).
The pilot-scale IAAB was then operated at OLRan 30.0 gCOD/L.day for a period of 118 days to produce a set of reliable data that can be used to convince the palm oil millers to accept the new approach of POME treatment technology. The average overall BODremoval, CODremoval, and TSSremoval achieved by the pilot-scale IAAB at OLRan 30.0 gCOD/L.day (corresponding to average OLRa 2.86 ± 0.37 gCOD/L.day) were more than 99%. This had also resulted in the achievement of BOD <20 mg/L and TSS <400 mg/L throughout the study period with 70% and 100% of compliance, respectively. Overall, operating the pilot-scale IAAB at OLRan 30.0 gCOD/L.day was selected as the most suitable prototype to treat large volume of POME due to its ability to operate at lower HRT, higher overall treatment removal efficiencies (more than 99%), methane yield (0.0980 LCH4/gCODremoved) and purity of methane (60.65%). In order to boost up the biogas generation and achieve high quality of final treated effluent with BOD <20 mg/L, anaerobic co-digestion of Moringa Oleifera, a natural coagulant with POME was first conducted at laboratory-scale. Co-digestion of POME with the one-time fed M. oleifera filtrate showed an improved 92% TSSremoval, 94% BODremoval and methane yield of 24.3 mL CH4/gCODremoved. Methanogen bacterial DNAs isolated from POME, granular sludge and anaerobically-treated POME was identified as Methanoculleus spp., Methanolinea spp. and Methanoculleus spp., respectively.
Quantitative polymerase chain reaction (qPCR) conducted confirmed the highest concentration of methanogen DNA in the POME-M. oleifera co-digestion (one-time feed). Therefore, this co-digestion system was feasible and efficient in enhancing methane yield (69%) and TSSremoval (59.5%), respectively compared to those of mono-digestion. The pilot-scale IAAB was then operated in co-digestion mode for a period of 35 days at OLRan 30.0 gCOD/L.day (corresponding to average OLRa 3.25 ± 0.30 gCOD/L.day). Throughout the study period, the BOD of the final treated effluent produced during the operation of the pilot-scale IAAB in co-digestion mode exhibited high percentage (74%) of compliance towards the discharge limit of BOD <20 mg/L.
Significant findings were obtained by comparing between mono- and co-digestion studies conducted using pilot-scale IAAB under the same operating condition of OLRan 30.0 gCOD/L.day. M. oleifera filtrate provided an improved BODremoval (94-95%), CODremoval (93-94%) and TSSremoval (89-97%) as compared to mono-digestion (BODremoval: 93-94%; CODremoval: 92-93%; TSSremoval: 86-90%). The average methane yield and methane purity obtained during co-digestion (0.1353 ± 0.01 LCH4/gCODremoved and 64 ± 0.44%, respectively) were higher than the mono-digestion (0.0980 ± 0.01 LCH4/gCODremoved and 61 ± 2.63%, respectively). The incorporation of M. oleifera filtrate had increased the biogas production by 72%. Such significant result was due to the protein content in the mixture of the POME and M. oleifera filtrate and the coagulant effect exhibited by the shelled of M. oleifera filtrate.
Besides, this was also contributed by the balanced carbon-to-nitrogen ratio (C/N) composition exhibited by the mixture of POME and M. oleifera filtrate. Most importantly, the addition of M. oleifera filtrate into the digestion of POME resulted in the production of final treated effluent with BOD of less than 20 mg/L at a much lower bacteria concentration and higher F/Ma of 18,012 - 20,000 mg/L and 0.185 - 0.187 gCOD/gMLVSS.day, respectively as compared to the mono-digestion. These advantages ascertain the superiority of the pilot-scale IAAB than the laboratory-scale IAAB in terms of POME treatment or other high strength wastewater at a higher OLRan and shorter HRT for the production of final treated effluent with BOD of less than 20 mg/L. Future work will focus on up-scaling the pilot-plant IAAB into commercial plant. This is aimed to evaluate the performance of the commercial plant on the constant fluctuation of the characteristics of POME and at high loading rate. Hence, the data produced can be compared with the existing POME treatment, which will act an alternative for palm oil millers to adopt such technology for POME treatment.
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