Synergistic effect of anaerobic co-digestion of palm oil mill effluent (POME) with Moringa oleifera extract
Graphical abstract
Introduction
Oil palms produce fresh fruit bunches (FFB) which are harvested at regular fixed intervals to be processed into crude palm oil (CPO) at palm oil mills. During palm oil milling, a type of liquid waste - palm oil mill effluent (POME) - is generated from 3 main sources: (1) separator sludge from CPO clarification; 0.4 t/t FFB, (2) steriliser condensate from FFB sterilisation; 0.2 t/t FFB and (3) effluent from wet separation of kernel and shell; 0.07 t/t FFB [[1], [2], [3]]. The main challenge in POME treatment lies within the characteristics of POME itself which contains high concentrations of total suspended solids (TSS) and oil and grease (O&G) [4]. Typical ranges are 28,846 mg/L to 30,920 mg/L [4] and 24,267 mg/L to 25,133 mg/L for TSS and O&G, respectively. The treatment of POME via anaerobic digestion (AD) in palm oil mills from either anaerobic ponds or open digester tanks contributes to greenhouse gas (GHG) emissions as biogas containing about 60–65% methane is released into the environment [4,5]. Methane is one of the most threatening GHG as it has an estimated ability to capture heat 25 to 30 times more effective than carbon dioxide [6]. Nevertheless, it is an important energy source if it is captured and utilised. It is estimated that an average of 12.36 kg of methane is emitted from every tonne of POME treated [7,8]. Concerns on green environment and depletion of non-renewable fossil fuels have attracted the use of methane generated from POME treatment as an alternative new source of energy.
Currently, in Malaysia, new and existing palm oil mills requiring throughput expansion are mandated to install biogas trapping/methane avoidance facilities [3,4,9]. Electricity generated from biogas via POME treatment for grid connection is eligible for a Feed-in Tariff (FiT) of USD 0.096 per kWh (basic rate plus additional bonus using POME) through the approved Distribution Licensees governed under the Sustainable Energy Development Authority of Malaysia (SEDA Malaysia) [10,11]. The FiT scheme enables conversion of previously unutilised biogas into renewable energy to aid GHG emissions reduction. Though POME can be used as a sole substrate for AD, its drastic fluctuation of quantity and characteristics, inconsistent carbon to nitrogen (C/N) ratio, poor buffering capacity and low pH have great influence on biogas production [8,12,13]. In order to improve biogas productivity, co-digesting POME with other organic substances might help to complement the missing nutrients required during AD of POME [13,14].
Methanogens or methanogenic archaea belonging to the domain Archaea placed together with halophilic and thermophilic archaea are known to produce large amount of atmospheric methane. To date, there are only five orders of methanogens i.e. Methanosarcinales, Methanopyrales, Methanococcales, Methanomicrobiales, and Methanobacteriales [15]. Methanogens are non-culturable and their growth require specific laboratory settings [16]. In this study, the DNAs of methanogens from a fixed volume of POME and anaerobic co-digested POME were sequenced and quantified using real-time quantitative polymerase chain reaction (qPCR). To the authors knowledge, this will be the first reported study using complementary DNA (cDNA) as the starting material in quantification of methanogen in POME using qPCR assay as previous similar studies focused on methanogenic archaea from adult chicken [17], young broiler chicken faeces [18] and methanogenic and methanotrophic microbial communities from environmental samples [19]. The quantification via qPCR assay in this study will be more advantageous over other approaches such as radioactive oligonucleotide probe hybridization [20] and thiol biomarker coenzyme M quantification via high-performance liquid chromatography [21] which are mostly not accurate. The qPCR assay is the gold standard for absolute quantification of organisms in a sample [[22], [23]].
Moringa oleifera is commonly known as the ‘horse-radish’ tree or ‘drumstick’ tree with most of its parts useful for various applications. M. oleifera, native of the western and sub-Himalayan tracts [[24], [25], [26]] is grown wildly and widely found in India, Pakistan, Africa, Arabia, the Philippines, Cambodia, Central America, North and South America, and the Caribbean Islands [26,27]. As reported by Rahman et al. [28], M. oleifera is also native to Malaysia. In Sudan, it has been traditionally used in water purification [29]. The negative economic and environmental impacts of chemical-based alum and iron salts have sparked interest in exploiting natural and plant-based organic coagulants, especially using M. oleifera seed [[30], [31], [32], [33], [34], [35], [36], [37]]. M. oleifera extract has been shown to have large coagulating effects with suspended solid and chemical oxygen demand (COD) for at least 95% and 50% removal, respectively [35,38]. Additionally, effluent treated with M. oleifera extract produced smaller volume of sludge than those with conventional coagulant [34], which can be used as a source of fertiliser or animal feed. Constituting ~55% of carbon source, M. oleifera will be advantageous to complement POME with a considerably high nitrogen content (2.71%) for microbial digestion [32,39]. As such, co-digesting POME with high carbon content substrates is envisaged not only to support growth of microorganism responsible for AD but enhance biogas production too.
Co-digestion, as suggested by various researchers viz., Deublein and Steinhauser [40], Sidik et al. [13] and Nurliyana et al. [8], provides an intermediate breakdown of homogenous mixture to overcome inhibition effects, e.g. high nitrogen content, lack of trace elements or overload of light metal ions. To date, co-digestion with varying substrates can be used as a recipe to improve stability of AD as well as biogas production. Factors that could affect productivity and stability of AD are temperature, pH, retention time, C/N ratio, microbial population, type of feedstock, to name a few [41]. Feedstock variability as influenced by co-digestion can be manipulated to impart the C/N ratio. Additionally, the use of more than one substrate for AD is able to balance up the C/N ratio, which is vital during start-up of digester. For instance, Nurliyana et al. [8] showed that co-digestion of POME and empty fruit bunches with a C/N ratio of 45 could increase methane yield by 1.6-fold from 0.37 to 0.59 mL CH4/g volatile solids (cumulative). Nonetheless, such co-digestion with high carbon content substrates and the resulted buffer capacity are yet to be an attractive solution in optimising and improving methane production efficiency.
Many challenges are faced in operating POME-based biogas plants – the main one lies within the characteristics of POME itself which contains high concentrations of TSS and O&G [4]. High TSS concentration makes POME undesirably viscous [42,43], hence intense mixing is required to ensure maximum contact between the TSS and those microorganisms suspended in the anaerobic sludge. High O&G content tends to solidify at close contact with POME mixture in an anaerobic digester once POME temperature drops to less than 40 °C. A scum layer will then form and float on top of the liquid POME surface making it difficult to be digested by bacteria mostly suspended at the bottom of the digester. It is postulated that if the TSS and O&G can be coagulated and settled, efficient contact with anaerobic sludge can be established to enhance digestion efficiency of these components. This will then prolong the retention time of TSS and O&G in the digester for efficient conversion from complex into soluble matters. At the same time, scum formation can be minimised or eliminated. In this study, co-digestion of POME with M. Oleifera extract was exploited for the first time via AD. The study was attempted to determine (a) the optimum concentration of M. oleifera extract for co-digestion using laboratory jar test, (b) performance of mono- and co-digestion of M. oleifera extract with POME and (c) absolute quantification of methanogens responsible for biogas production. The advantage of naturally occurring organic substrates over inorganic salts (e.g. alum and iron salts) is additional carbon source provided for biogas generation avoiding metal ions overdose that might inhibit bacteria activity.
Section snippets
Sample source
Raw POME and anaerobic seed sludge were collected from a palm oil mill located in Pahang, Malaysia. In order to prevent the collected influent from undergoing biodegradation in the presence of microbial cells, it was preserved at less than 4 °C but above its freezing point. The required volume was thawed at room temperature of 28 °C before used. The matured and dried seed pods of M. oleifera seeds were obtained from Serdang, Malaysia. The seed wings and coats from the pods were removed manually
Effect of M. oleifera extract as coagulant on POME
The activity of M. oleifera extract to coagulate with POME in forming large flocs through reduction in COD, O&G and TSS of POME was examined. The COD, O&G and TSS of raw POME were 64,250 ± 884 mg/L, 24,700 ± 325 mg/L and 23,100 ± 5860 mg/L, respectively (Table 4). The COD, O&G and TSS exhibited by M. oleifera extract had negligible effects on the treated supernatant as the volume used was small in comparison to the total volume used. The effect of dosing M. oleifera extract on the pH of the
Conclusions
The laboratory-scale study had demonstrated promising coagulating effects of M. Oleifera extract for efficient co-digestion of POME. Optimum dosage of M. oleifera extract, 500 mg/L, led to the highest TSSremoval (92.24%) and O&Gremoval (91.90%) in POME. This study also showed that POME-M. oleifera co-digestion (T4, one-time feed) was able to improve the AD by 2.59% (BOD), 6.70% (COD) and 37.3% (TSS) with higher removal efficiencies ranged 52–93%, 51–76% and 61–92%, respectively compared to
Acknowledgements
The authors gratefully acknowledge the financial support from the Malaysian Palm Oil Board (MPOB) and the University of Nottingham Malaysia Campus (UNMC). The authors thank Crops for the Future Research Centre (CFFRC) of the UNMC for funding Cheau Chin Yap to conduct this research work for a PhD degree at the UNMC. Additionally, the authors would also like to express their gratitude to Havys Oil Mill for allowing the collection of samples throughout the study period.
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