Enhancing biomass-coated carrier materials in aerobic upflow bioreactor system for polishing palm oil mill effluent

Abstract

This study is aimed at diversifying the use and adding value to several selected palm oil milling by-products: palm oil clinker (POC), palm oil fuel ash (POFA), empty fruit bunch (EFB), and palm kernel shell (PKS). The industry is also lacking of a technically feasible technology, in achieving sustainable, reliable, and affordable palm oil mill effluent (POME) polishing. The study hence provided a reference on the use of materials renovated from oil palm co-products in an effective high-rate biological system to perform POME polishing treatment. Four materials were made available: (1) POCs were used as it is, after being de-oiled and crushed to the required material size, (2) zeolitization of POFA into chemically-treated POFA (CT-POFA), (3) EFB fibres were transformed into activated carbons (AC-EFBF), and (4) a commercial activated carbon (AC-PKS). Basic characterizations were carried out. Several instruments were used to acquire surface characteristics related to microbial cells adhesion. A carrier material selection matrix compiling surface information which were then qualitatively ranked was prepared. Basic material properties were classified as Tier One important selection criteria, followed by physical properties and chemical nature as Tier Two and Three, respectively. AC-PKS fulfilled seven of the selection criteria. This material has accomplished the highest BET surface area (833.56 m2 g-1) and pore volume (0.3620 cm3 g-1), a near neutral pH (7.1) and density close to water (1,401.43 kg m-3), irregularly-shaped, in addition to several benefits as revealed by spectroscopic characterizations and morphological observation. Carrier material selection is essential to evaluate these new waste-renovated materials, besides choosing the most suitable material for the operation of a bioreactor system. The process was continued by evaluating all four materials in the aerobic upflow bioreactor system (AUBS)-batch mode (expanded bed process). AC-PKS-packed bioreactor acquired the fastest start-up in five weeks, followed by AC-EFBF-packed bioreactor in six weeks. With a volumetric chemical oxygen demand (COD) loading rate of 4.71 to 5.71 kg m-3 day-1 and 24 hours HRT, AC-PKS-packed bioreactor had the best COD removal efficiency of 75.2 % and the removal rate was 4.29 kg COD m-3 day-1. AC-PKS fulfilled the selection criteria of having the best removal efficiency and rapid system start-up, it was therefore concluded as the best carrier material to be evaluated in the AUBS-continuous mode (fluidized bed process). Aerobic fluidized bed is an emerging technology yielding promising results. Operated in 30 % bed expansion (equivalent to 980 mm bed height), a quick HRT of twelve hours, and low loading rates of 0.16 to 0.34 kg BOD m-3 day-1, the highest biochemical oxygen demand (BOD) removal efficiency recorded was 79.0 % with a removal rate of 0.24 kg BOD m-3 day-1. Treated effluents consistently reached BOD values below 50 mg L-1, with the lowest value of 31.8 mg L-1. System start-up was completed in just three weeks. AC-PKS was concluded as an enhanced carrier material due to its higher biomass concentration attained in comparison with other studies (0.1721 g attached volatile solid (AVS) g-1 AC-PKS at the highest sampling location).