Optimization of Hydraulic Retention Time in an Adsorption–Anaerobic–Aerobic Bioreactor to Enhance Methane Yield from Palm Oil Mill Effluent

Abstract

Biogas, with methane (CH4) as its primary component, is a renewable energy source produced through the anaerobic degradation of organic matter. Among various organic waste streams, palm oil mill effluent (POME) has a high potential for methane production but also requires effective treatment to comply with effluent discharge standards. The research aimed to optimize the hydraulic retention time (HRT) of an integrated oil palm empty fruit bunch (OPEFB) adsorption–Upflow Anaerobic Sludge Blanket (UASB)–modified Rotating Biological Contactor (RBC) system to enhance methane production and to develop an integrated Anaerobic Digestion Model No. 1 (ADM1)–Activated Sludge Model No. 1 (ASM1) for process prediction and optimization. The reactors were operated at HRTs of 2, 3, and 4 d, with each operating condition maintained for 30 d. The evaluated parameters included chemical oxygen demand (COD), TSS, O&G, methane production, and biofilm thickness. The integrated ADM1–ASM1 model was estimated, validated using experimental data, and subjected to sensitivity analysis to identify the parameters with the greatest influence on spesific methane production (SMP). The results demonstrated that HRT significantly affected treatment efficiency, biofilm development, and methane production. The 3-d HRT was identified as the best overall operating compromise, achieving the highest SCOD removal (80.91%) while maintaining high TSS (97.00%) and O&G (90.58%) removal. Although the 4-d HRT produced the highest methane production (2.70 L), average daily methane production (0.090 L d?¹), SMP (0.20 L CH4 g?¹ COD removed), and biofilm thickness (405.10 ± 199.97 µm), the 3-d HRT provided the most balanced overall treatment performance. The microbial community in the UASB reactor was dominated by fermentative and syntrophic bacteria, including Clostridium sensu stricto 1 and Lentimicrobium, which play important roles in hydrolysis, methane precursor formation, and methanogenesis. In contrast, the modified RBC was dominated by aerobic bacteria such as Bacillus, Azonexus, and Mesorhizobium, which promoted residual organic matter degradation, nitrogen transformation, and biofilm formation, thereby improving the final effluent quality. The integrated ADM1–ASM1 model reproduced the general trend of the experimental data with moderate agreement. The ADM1 model achieved r = 0.874, R² = 0.765, RMSE = 7.914 mgCOD L?¹, and nRMSE 11.29% while the estimated ASM1 model achieved r = 0.866, R² = 0.749, RMSE of 1,176 mgCOD L?¹, and nRMSE = 24.61%. Further validation using an independent dataset is recommended before design-scale application. Sensitivity analysis identified acetoclastic biomass (Xac) and the maximum acetate uptake rate (km,ac) as the most influential parameters governing methane production.

Biogas dengan komponen utama metana (CH4) merupakan sumber energi terbarukan yang dihasilkan dari degradasi anaerobik bahan organik. Di antara berbagai limbah organik, limbah cair pabrik kelapa sawit (POME) memiliki potensi tinggi untuk produksi metana, namun juga memerlukan pengolahan yang efektif agar memenuhi baku mutu efluen. Penelitian ini bertujuan mengoptimalkan waktu tinggal hidraulik (HRT) pada sistem terintegrasi adsorpsi tandan kosong kelapa sawit (OPEFB)–Upflow Anaerobic Sludge Blanket (UASB)–modified Rotating Biological Contactor (RBC) untuk meningkatkan produksi metana serta mengembangkan model terintegrasi Anaerobic Digestion Model No. 1 (ADM1) Activated Sludge Model No. 1 (ASM1) untuk prediksi dan optimasi proses. Reaktor dioperasikan pada HRT 2, 3, dan 4 hari, dengan setiap kondisi operasi dipertahankan selama 30 hari. Parameter yang dievaluasi meliputi kebutuhan oksigen kimia (COD), TSS, O&G, produksi metana, dan ketebalan biofilm. Model terintegrasi ADM1–ASM1 dikalibrasi, divalidasi menggunakan data eksperimen, serta dianalisis sensitivitasnya untuk mengidentifikasi parameter yang paling berpengaruh terhadap produksi metana. Hasil penelitian menunjukkan HRT berpengaruh signifikan terhadap efisiensi pengolahan, perkembangan biofilm, dan spesifik produksi metana (SMP). HRT 3 hari dipilih sebagai kondisi operasi dengan kompromi terbaik karena menghasilkan penyisihan COD tertinggi (80,91%) serta mempertahankan efisiensi penyisihan TSS (97,00%) dan O&G (90,58%) yang tinggi. Sebaliknya, HRT 4 hari menghasilkan produksi metana (2,70 L), laju produksi metana (0,090 L hari?¹), SMP (0,20 L CH4 g?¹ COD tersisihkan), dan ketebalan biofilm (405,10 ± 199,97 µm) tertinggi. Reaktor UASB didominasi oleh bakteri fermentatif dan sintrofik (Clostridium sensu stricto 1 dan Lentimicrobium), sedangkan RBC termodifikasi didominasi oleh bakteri aerob (Bacillus, Azonexus, dan Mesorhizobium) yang mendukung degradasi bahan organik, transformasi nitrogen, dan pembentukan biofilm. Model terintegrasi ADM1–ASM1 mampu merepresentasikan tren data eksperimen dengan tingkat kesesuaian sedang. Model ADM1 menghasilkan koefisien korelasi Pearson (r) sebesar 0,874, koefisien determinasi (R²) sebesar 0,765, RMSE sebesar 7,914 mgCOD L?¹, dan nRMSE sebesar 11,29%. Sementara itu, model ASM1 hasil kalibrasi mencapai nilai r sebesar 0,866, R² sebesar 0,749, RMSE sebesar 1.176 mgCOD L?¹, dan nRMSE sebesar 24,61%. Validasi lebih lanjut menggunakan dataset independen masih diperlukan sebelum model diterapkan pada skala desain. Analisis sensitivitas menunjukkan bahwa acetoclastic biomass (Xac) dan laju maksimum pengambilan asetat (km,ac) merupakan parameter yang paling berpengaruh terhadap produksi metana.