Application of solar assisted bioreactor for biogas production from palm oil mill effluent co-digested with cattle manure

Environmental Technology & Innovation 16 (2019) 100446

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Application of solar assisted bioreactor for biogas production from palm oil mill effluent co-digested with cattle manure ∗

Zaied Bin Khalid a , Md. Nurul Islam Siddique b , , Mohd Nasrullah a , Lakhveer Singh a , Zularisam Bin Abdul Wahid a , Mohd. Fadhli Ahmad b a b

Faculty of Engineering Technology, Universiti Malaysia Pahang (UMP), 26300 Gambang, Kuantan, Pahang, Malaysia School of Ocean Engineering, Universiti Malaysia Terengganu (UMT), 21030 Kuala Nerus, Terengganu, Malaysia

highlights • • • •

Bioreactor is powered by solar energy and mixing was done semi-continuously. Optimal mixing proportion of POME and CM has been found 50:50. Mesophilic temperature (35◦ C) was best preferred for reactor operation. Solar assisted bioreactor is feasible for both economic and environmentally.

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Article history: Received 5 January 2019 Received in revised form 1 August 2019 Accepted 1 August 2019 Available online 22 August 2019 Keywords: Solar-assisted bioreactor Anaerobic co-digestion Palm oil mill effluent Cattle manure Biogas production

a b s t r a c t The key objective of this research was to investigate the potential of the co-digestion of palm oil mill effluent (POME) with cattle manure (CM) in a solar-assisted bioreactor (SABr) to produce enhanced biogas. The solar panel first converted solar radiation into electricity, which then warmed up the POME and CM mixture to maintain the required reactor temperature. The operation was conducted semi-continuously at mesophilic temperature (35◦ C). The produced energy was analyzed at 0:100, 25:75, 50:50, 75:25, and 100:0 mixing ratios of POME and CM. The mixture with equal proportions of POME and CM produced the maximum amount of biogas, i.e., 1567.00 mL, while the methane content present was 64.13%. The results revealed that this mixing ratio of POME and CM at mesophilic temperature (35◦ C) was the best arrangement of anaerobic co-digestion for storing solar energy during biogas production. The economic impact of constructing a biogas plant has been successfully analyzed and predicted as well. The proposed biogas plant seems to be economically feasible because an approximately five-year payback period on investment may be achieved if this technology is used on a large scale. Finally, the present work demonstrated that a complete solution to the application of solar-assisted bioreactor (SABr) is to integrate different features to enhance biogas production. © 2019 Elsevier B.V. All rights reserved.

1. Introduction A sustainable solution for energy-related issues is the utilization of solar energy. It is thought to be the most abundant energy source with the potential to overcome energy-related barriers. It is mostly utilized in water-heating systems ∗ Corresponding author. E-mail address: [email protected] (M.N.I. Siddique). https://doi.org/10.1016/j.eti.2019.100446 2352-1864/© 2019 Elsevier B.V. All rights reserved.

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as thermal energy. At present, it is widely used for commercialization in various countries all over the world (Zhong et al., 2015) and thus, the application of solar energy has increased immensely in countries that have large potential for harvesting solar energy. However, solar energy is hard to accumulate, change, and store because of inconsistent energy flow and low energy density. Therefore, existing technologies for a solar-assisted generation of power have inadequate applications (Richter, 2013). There is a need for innovative and applied approaches to collect and store solar energy that make it possible to overcome complications typically associated with the use of solar energy. Anaerobic digestion (AD) that is an organic transformation process that converts carbon-based wastes into biogas, and thus, it is a suitable choice to store solar energy (Hartmann et al., 2002). Anaerobic digestion decreases the potential risks associated with anaerobically digested materials and sanitizes waste materials. It is a good opportunity for technical and engineering societies to solve these problems by making noteworthy scientific contributions. The practical measures for utilizing solar radiation necessitates two elementary processes: collection of solar radiation and its storage. Firstly, solar radiation is collected; some proportion of it is converted to electricity or heat. Secondly, the means of energy storage carries any additional energy that is produced during sunny days and makes it useful by releasing it during the night time or at other times, when the sun is unavailable to provide a constant energy supply. Anaerobic co-digestion (ACoD) is an anaerobic treatment that processes multiple substrates simultaneously, which results in an improvement in system efficiency (Álvarez et al., 2010). It also leads to a higher energy production and better system stability concerning mono-digestion of the substrate. However, palm oil mill effluent (POME) is a dark liquid-like material that can easily dissolve and release suspended particles that produce odors after a chemical breakdown of organic material by bacteria. If discharged directly into the waterways, POME can pollute the environment because of its high biochemical oxygen demand and chemical oxygen demand. Moreover, feedlot farming with slaughterhouse incorporation requires careful waste management. Although the cattle are kept only for three months to make them fatter before sending them for slaughter, the amount of cow dung and effluent produced from the slaughterhouse per day is approximately 200–300 tonnes (Omar et al., 2008). The POME is acidic in nature, and its degradation can be quite tricky (Abu Bakar et al., 2018). Conversely, element analysis in CM has indicated a large amount of oxygenated compound to be present in it, perhaps because of the nature of the food that the cattle consume (Sidik et al., 2013). However, AD is achieved by microbial consortia and is subject to various factors, including pH, temperature, retention time, and microbial presence; this process is quite slow (Amani et al., 2010). In AD, the microbial growth of waste requires sodium, potassium, and other cationic elements. If there is high absorption of these ions, it can hinder microbial activity (Appels et al., 2008). Anaerobic digestion is positively affected by the presence of metal contents because of their low percentages in the overall mixture. During the ACoD process, biogas production is enhanced. However, as a special inoculum in ACoD, CM contributes prominent buffering capacity, anaerobic microorganisms, and an extensive variety of essential nutrients for optimum bacterial growth (Liu et al., 2013). Solar energy is used to run cost-effective bio-organic waste reactors. The heating system of an anaerobic bioreactor is associated with greater bioenergy production. Instead of using an electric and diesel heating system for such a bioreactor, a solar-powered reactor also produces an appreciable amount of biogas, and henceforth, the cost of bioenergy generation can be reduced. The solar-assisted bioreactor consists of a solar thermal energy accumulator, a temperature controller, a reactor, and a biogas collector (Rasi et al., 2007). Solar energy received by the solar panel is converted to electricity, then provides the required temperature for biological waste degradation and maintains the reactor temperature (Ahmad et al., 2003). The main reactor at the ideal reaction temperature produces biogas and capitalizes on net energy production (Siedlecka and Stepnowski, 2005). Previous studies have shown that mesophilic (35◦ C) and thermophilic (55◦ C) conditions provide stable temperature for biogas production process during AD. A bioreactor heating system can be easily developed from a solar energy storage system, which can then produce biogas throughout the year. In this study, a mesophilic (35◦ C) temperature was maintained to warm the organic substances under anaerobic conditions. At the end of the 1970s, the first solar-powered anaerobic treatment system was developed and enhanced by the U.S. Environmental Protection Agency (U.S. EPA) to enhance thermal competence of the system (Wu et al., 2010). Afterwards, numerous studies focused on refining the effectiveness of the thermal conditions of AD. In 1980, Hills and Stephens predicted the probability of consuming solar energy to warm up an anaerobic continuously stirred tank reactor (CSTR) under mesophilic conditions (Hills and Stephens, 1980). In 2000, Alkhamis planned and ran a laboratory-scale reactor using a horizontal solar energy plate accumulator as the radiator in Jordan (Alkhamis et al., 2000). In 2001, Axaopoulos established a scientific simulation model of a swine dung digestor furnished by solar energy (Axaopoulos et al., 2001). Nonetheless, the idea of using solar radiation to feed the energy-storing units during AD was not widely and systematically premeditated till very recently (Kocar and Eryasar, 2007). The solar-powered anaerobic reactor system will possibly offer a solution for the overwhelming hindrances posed by specific element cations, for example, the inconsistent energy flow from solar energy accumulators the and costly and multifaceted methods of energy storage in AD (Khan et al., 2009). There are many benefits of using solar-assisted bioreactors in unindustrialized countries because of their potential to aid in saving costs of power consumption, and thus, may become vital for improvement in national revenue. Therefore, warming up a reactor by means of solar energy is an exceptional method for biogas production. The solar collector panel may have a hypothetical competence of above 60.00% (Alkhamis et al., 2000). The required anaerobic reactor temperature can be attained within an hour, and then maintained. This indicates another significance of a system controlled by solar energy. Here, a financial investigation on investment in such a solar-powered reactor system revealed that the internal rate of return (IRR) was about 32.60% (Alkhamis et al., 2000). Moreover, energy savings could be amplified further by partial use of gas produced for heat in a modified hybrid system.

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Table 1 Compositions and characteristics of palm oil mill effluent and cattle manure. Parameter

pH COD BOD TS VS VFA TC TN TP C/N Ratio

POME

4.60 ± 0.40 28.34 ± 0.30 15.28 ± 0.20 39.75 ± 0.30 32.56 ± 0.30 3.20 ± 0.10 15.69 ± 0.20 0.73 ± 0.10 0.13 ± 0.02 21.64

POME:CM

CM

0:100

25:75

50:50

75:25

100:0

5.49 ± 0.50 20.82 ± 0.20 11.43 ± 0.10 12.61 ± 0.10 9.71 ± 0.10 3.07 ± 0.05 6.30 ± 0.10 0.45 ± 0.10 0.06 ± 0.02 14.15

5.44 ± 0.50 23.12 ± 0.30 12.63 ± 0.20 19.15 ± 0.10 15.19 ± 0.10 3.18 ± 0.05 8.62 ± 0.10 0.52 ± 0.10 0.08 ± 0.02 16.52

5.48 ± 0.50 24.58 ± 0.30 13.40 ± 0.20 22.79 ± 0.10 18.23 ± 0.10 3.28 ± 0.05 9.93 ± 0.10 0.57 ± 0.10 0.09 ± 0.02 17.49

5.50 ± 0.50 26.92 ± 0.30 14.62 ± 0.20 28.90 ± 0.20 23.35 ± 0.20 3.42 ± 0.05 12.11 ± 0.10 0.64 ± 0.10 0.10 ± 0.02 18.87

5.31 ± 0.50 27.85 ± 0.30 15.09 ± 0.20 32.89 ± 0.20 26.71 ± 0.20 3.41 ± 0.05 13.47 ± 0.10 0.68 ± 0.10 0.11 ± 0.02 19.82

5.40 ± 0.50 16.72 ± 0.20 9.28 ± 0.10 2.38 ± 0.10 1.17 ± 0.10 2.80 ± 0.05 2.62 ± 0.05 0.32 ± 0.10 0.03 ± 0.02 8.27

Note: All parameters are in g/L except pH.

Nowadays, co-digestion of POME and CM is facing the following impairments: prolonged start-up periods and slow reactions lead to a requirement of longer hydraulic retention times in conventional reactor designs. Furthermore, no prior mixing ratio was observed to enhance biogas production, and methanogenesis was inhibited by system stability failure due to sudden drop in pH (De Kock, 2015). Essential factors that are required to achieve the goal of successfully applying a solar-powered reactor system that co-digests POME with CM have been examined in this research; these factors include degradation by bacteria, process stability, bioenergy production with maximum biogas and methane yield, all which leads to a reduction in chemical oxygen demand and volatile solids. Such an improved solar-powered anaerobic reactor could help overcome specific drawbacks of conventional bioreactors, i.e. inconsistent energy flow from the collection of solar radiation, an exorbitant and multifaceted storage system for solar energy, and the lack of steadiness in energy in the AD system. The solar-assisted reactor with different digestion operations proposed here may enhance the anaerobic setup to ensure the best use of both the system and solar energy storage effectiveness. The economic and environmental impact has also been evaluated in this research to obtain a better view on the execution of a biogas plants on a large scale. By using solar-powered bioreactors, we not only produce bioenergy but also help the environment get rid of pollutants in carbon-based organic wastes. 2. Materials and methods 2.1. Feedstock collection and preparation A 100.00 L of the sample of POME was accumulated in the sample collection container from the anaerobic pond of the LKPP Corporation Sdn. Bhd., No.45/4, Jalan Teluk Sisek, 25000 Kuantan, Pahang, Malaysia. Approximately, 100.00 kg of partially digested CM was collected duly from the ejection of an average-sized farm in Gambang, Malaysia. The POME sample was subjected to a simple screening procedure to eliminate all coarse materials. It was then further screened over a filter media that comprised of trivial stones having an average dimension of 0.60 cm. The screened deposit was filtered over another bed consisting of a combination of trivial stones and sand (average dia 300–600 µm) 1:2. The remainder was exposed to surface filtration by Whatman No. 41 filter paper (20–25 µm). The CM sample was diluted in the water at a proportion of 1:25 and then filtered through a sieve (20 µm) to remove debris. 2.2. Characterization of substrates The physicochemical characterization of POME and CM before co-digestion are presented in Table 1. The TS and VS in POME were 39.75 g/L and 32.56 g/L, respectively. This large amount of such solid contaminants indicates that microorganisms are readily available in the substrate. Mostly, the POME holds cellulose, hemicellulose, sugars, carbohydrates, and lignin, while CM holds prominent buffering capacity, plenty of anaerobic microbes, and an extensive variety of important nutrients for optimum bacteriological rising (Ahmadi-Pirlou et al., 2017; Liu et al., 2013). The pH of POME was found to be 4.60, while the pH of the co-substrate, i.e., CM, was found to be 5.40. The COD of the POME was 28.34 g/L. The nitrogen content was more in manure livestock than in other surplus constituents. The ammonia from CM during the digestion process subsidized the steadiness of the advanced process. The carbon to nitrogen ratio is another very important parameter for the anaerobic digestion process (Ivana et al., 2016). To form new cells, carbon and nitrogen are vital nutrients. 2.3. Reactor design, fabrication, and operation The conventional solar reactor design is not appropriately proficient at maintaining pH and temperature. Therefore, pH and temperature controllers were installed to attain good control over the system. There was a two-unit battery cell

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Fig. 1. Experimental Photographs of the Solar Assisted Bioreactor (A: Battery, B: DC–AC Converter, C: Water Vaporization, D: Stirrer Motor, E: Main Reactor, F: Solar Cell).

which was meant to store solar energy from the solar panel and convert it to electrical power via a DC–AC converter. The total volume for the reactor was 5.00 L, where 3.50 L was the working volume. The reactor was made of a cylindrical global configuration system. The main reactor was made of glass and stainless steel. It was tightly closed with a steel frame topping, in combination with four nuts. The stirrer motor was fixed over the reactor. A speed control device was also installed in place of the stirrer, which had a range of 0–450 rpm. There was also water jacketing to provide the required temperature for bacterial degradation. Two feeding injectors fed the sample into the reactor. The gas that was produced was collected through a gas collection bag. The reactor was run at 35◦ C for 24 days, while it was fed with 437.50 mL of the substrates every three days until 3.50 L of its working volume was achieved for subjection to 24 days of digestion. Various volume mixing ratios were analyzed; these included 0:100, 25:75, 50:50, 75:25, and 100:0 of POME:CM. The reactors were named R1 (0% POME + 100% CM), R2 (25% POME + 75% CM), R3 (50% POME + 50% CM), R4 (75% POME + 25% CM), and R5 (100% POME + 0% CM). Mixing was achieved with the help of direct motors that were powered from solar energy; these were joined to form blades that functioned at 60 rpm. Meanwhile, the anaerobic bacteria consumed organic compounds in the sludge by treating it as a substrate and created an anaerobic environment that was suitable for the development of firm anaerobes. The properties of fed wastewater were analyzed every three days, excluding pH that was examined daily. The data recorded every three days is shown in Tables 2–6. The pH was maintained at 7.00 ± 0.10 using 1 N NaOH throughout the co-digestion time. The outcomes of the reactor operation were the consequence of co-digestion on biodegradation, biogas production, and moreover, system stability. A gas bag was also attached for the collection of the produced gas. The mixture of POME and CM was gradually increased because it took some time for the microbes to adapt in the new situation. A picture taken at the time of the experiment is shown in Fig. 1. The substrates, POME and CM, exhibited distinct characteristics regarding certain physicochemical properties. Particularly, CM had a high pH value, as well as was rich in BOD5 and alkaline content than POME, therefore, it had a superior buffering capacity (Siddique et al., 2015). According to Hashimoto et al. (1981), the carbon to nitrogen ratio in POME is generally more than that in CM (Hashimoto et al., 1981). From this result, it can be seen that the synchronous waste degradation should favor AD because the mixing of two wastes lessens the intrinsic difficulties in analysis. Although both the waste types had a steady deterioration in pH, alkalinity, and COD, their characteristics suggested that they had an optimum proportions of the substrate to produce methane (Siddique et al., 2015). By mixing substrates in optimum proportion for co-digestion, there were consequences on the subsequent circumstances: (i) maximum pH, COD, and alkalinity were recorded during the co-digestion period and (ii) approximately three times more methane production was observable. The economic and ecological feasibility studies for constructing a biogas plant were conducted as well.

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Table 2 Properties of palm oil mill effluent and cattle manure mixing (0:100) during co-digestion period. Parameter

POME:CM (0:100) 0th Day

3rd Day

6th Day

9th Day

12th Day

15th Day

18th Day

21th Day

24th Day

pH COD BOD TS VS Biogas Methane

5.49 ± 0.5 20.82 ± 0.20 11.43 ± 0.10 12.61 ± 0.10 9.71 ± 0.10 0.00 0.00

6.95 ± 0.5 20.42 ± 0.20 11.18 ± 0.10 12.43 ± 0.10 9.30 ± 0.10 0.00 0.00

6.80 ± 0.5 20.09 ± 0.20 10.96 ± 0.10 12.18 ± 0.10 8.93 ± 0.10 0.00 0.00

7.10 ± 0.5 19.64 ± 0.20 10.69 ± 0.10 12.03 ± 0.10 8.49 ± 0.10 152.40 ± 10.00 61.28 ± 12.00

7.00 ± 0.5 19.29 ± 0.20 10.46 ± 0.10 11.74 ± 0.10 8.12 ± 0.10 282.46 ± 10.00 113.58 ± 12.00

7.00 ± 0.5 18.89 ± 0.20 10.17 ± 0.10 11.54 ± 0.10 7.74 ± 0.10 387.69 ± 10.00 155.89 ± 12.00

7.10 ± 0.5 18.46 ± 0.20 9.94 ± 0.10 11.39 ± 0.10 7.32 ± 0.10 528.37 ± 10.00 212.46 ± 12.00

7.00 ± 0.5 18.07 ± 0.20 9.63 ± 0.10 11.19 ± 0.10 6.93 ± 0.10 602.55 ± 10.00 242.29 ± 12.00

7.00 ± 0.5 17.70 ± 0.20 9.38 ± 0.10 10.97 ± 0.10 6.51 ± 0.10 637.00 ± 10.00 256.14 ± 12.00

Note: Biogas & Methane are in mL. Rest of all parameters are in g/L except pH.

Table 3 Properties of palm oil mill effluent and cattle manure mixing (25:75) during co-digestion period. Parameter

pH COD BOD TS VS Biogas Methane

POME:CM (25:75) 0th Day

3rd Day

6th Day

9th Day

12th Day

15th Day

18th Day

21th Day

24th Day

5.44 ± 0.5 23.12 ± 0.30 12.63 ± 0.20 19.15 ± 0.10 15.19 ± 0.10 0.00 0.00

7.10 ± 0.5 22.29 ± 0.30 12.11 ± 0.20 18.31 ± 0.10 14.32 ± 0.10 186.60 ± 10.00 85.31 ± 15.00

7.10 ± 0.5 21.46 ± 0.30 11.64 ± 0.20 17.46 ± 0.10 13.45 ± 0.10 306.40 ± 10.00 140.09 ± 15.00

6.95 ± 0.5 20.57 ± 0.30 11.13 ± 0.20 16.58 ± 0.10 12.52 ± 0.10 415.70 ± 10.00 190.06 ± 15.00

6.90 ± 0.5 19.75 ± 0.30 10.64 ± 0.20 15.72 ± 0.10 11.61 ± 0.10 498.50 ± 10.00 227.91 ± 15.00

7.10 ± 0.5 18.96 ± 0.30 10.17 ± 0.20 14.86 ± 0.10 10.74 ± 0.10 586.4 ± 10.00 268.10 ± 15.00

6.95 ± 0.5 18.11 ± 0.30 9.74 ± 0.20 13.96 ± 0.10 9.85 ± 0.10 667.30 ± 10.00 305.09 ± 15.00

6.80 ± 0.5 17.24 ± 0.30 9.22 ± 0.20 13.14 ± 0.10 8.96 ± 0.10 744.80 ± 10.00 340.52 ± 15.00

7.00 ± 0.5 16.41 ± 0.30 8.72 ± 0.20 12.26 ± 0.10 8.05 ± 0.10 782.00 ± 10.00 357.53 ± 15.00

Note: Biogas & Methane are in mL. Rest of all parameters are in g/L except pH.

Table 4 Properties of palm oil mill effluent and cattle manure mixing (50:50) during co-digestion period. Parameter

POME:CM (50:50) 0th Day

3rd Day

6th Day

9th Day

12th Day

15th Day

18th Day

21th Day

24th Day

pH COD BOD TS VS Biogas Methane

5.48 ± 0.5 24.58 ± 0.30 13.40 ± 0.20 22.79 ± 0.10 18.23 ± 0.10 0.00 0.00

6.90 ± 0.5 22.53 ± 0.30 12.36 ± 0.20 21.05 ± 0.10 16.82 ± 0.10 317.00 ± 20.00 203.29 ± 31.00

6.80 ± 0.5 20.46 ± 0.30 11.29 ± 0.20 19.26 ± 0.10 15.34 ± 0.10 502.00 ± 20.00 321.93 ± 31.00

7.10 ± 0.5 18.32 ± 0.30 10.19 ± 0.20 17.47 ± 0.10 13.95 ± 0.10 749.00 ± 20.00 480.33 ± 31.00

6.95 ± 0.5 16.29 ± 0.30 9.13 ± 0.20 15.74 ± 0.10 12.46 ± 0.10 944.00 ± 20.00 605.39 ± 31.00

7.00 ± 0.5 14.17 ± 0.30 8.06 ± 0.20 13.98 ± 0.10 11.03 ± 0.10 1197.00 ± 20.00 767.64 ± 31.00

7.10 ± 0.5 12.07 ± 0.30 6.94 ± 0.20 12.22 ± 0.10 9.64 ± 0.10 1324.00 ± 20.00 849.08 ± 31.00

6.95 ± 0.5 9.98 ± 0.30 5.92 ± 0.20 10.45 ± 0.10 8.17 ± 0.10 1431.00 ± 20.00 917.70 ± 31.00

7.00 ± 0.5 7.87 ± 0.30 4.82 ± 0.20 8.66 ± 0.10 6.75 ± 0.10 1567.00 ± 20.00 1004.92 ± 31.00

Note: Biogas & Methane are in mL. Rest of all parameters are in g/L except pH.

Table 5 Properties of palm oil mill effluent and cattle manure mixing (75:25) during co-digestion period. Parameter

POME:CM (75:25) 0th Day

3rd Day

6th Day

9th Day

12th Day

15th Day

18th Day

21th Day

24th Day

pH COD BOD TS VS Biogas Methane

5.50 ± 0.5 26.92 ± 0.30 14.62 ± 0.20 28.90 ± 0.20 23.35 ± 0.20 0.00 0.00

6.80 ± 0.5 25.13 ± 0.30 13.72 ± 0.20 26.98 ± 0.20 21.65 ± 0.20 276.40 ± 20.00 151.61 ± 27.00

6.80 ± 0.5 23.44 ± 0.30 12.86 ± 0.20 24.97 ± 0.20 19.93 ± 0.20 474.30 ± 20.00 260.15 ± 27.00

6.95 ± 0.5 21.62 ± 0.30 11.78 ± 0.20 23.06 ± 0.20 18.15 ± 0.20 686.70 ± 20.00 376.65 ± 27.00

6.90 ± 0.5 19.96 ± 0.30 10.94 ± 0.20 21.13 ± 0.20 16.43 ± 0.20 792.00 ± 20.00 434.41 ± 27.00

7.00 ± 0.5 18.19 ± 0.30 10.07 ± 0.20 19.17 ± 0.20 14.76 ± 0.20 895.60 ± 20.00 491.24 ± 27.00

7.10 ± 0.5 16.45 ± 0.30 9.13 ± 0.20 17.24 ± 0.20 13.06 ± 0.20 994.6 ± 20.00 545.54 ± 27.00

7.05 ± 0.5 14.64 ± 0.30 8.24 ± 0.20 15.29 ± 0.20 11.32 ± 0.20 1141.60 ± 20.00 626.17 ± 27.00

7.00 ± 0.5 12.92 ± 0.30 7.31 ± 0.20 13.30 ± 0.20 9.57 ± 0.20 1346.00 ± 20.00 738.28 ± 27.00

Note: Biogas & Methane are in mL. Rest of all parameters are in g/L except pH.

Table 6 Properties of palm oil mill effluent and cattle manure mixing (100:0) during co-digestion period. Parameter

pH COD BOD TS VS Biogas Methane

POME:CM (100:0) 0th Day

3rd Day

6th Day

9th Day

12th Day

15th Day

18th Day

21th Day

24th Day

5.31 ± 0.5 27.85 ± 0.30 15.09 ± 0.20 32.89 ± 0.20 26.71 ± 0.20 0.00 0.00

7.00 ± 0.5 26.37 ± 0.30 14.32 ± 0.20 30.93 ± 0.20 25.33 ± 0.20 266.80 ± 10.00 131.72 ± 18.00

7.10 ± 0.5 24.89 ± 0.30 13.56 ± 0.20 28.95 ± 0.20 23.91 ± 0.20 458.00 ± 10.00 226.11 ± 18.00

7.00 ± 0.5 23.06 ± 0.30 12.72 ± 0.20 26.98 ± 0.20 22.64 ± 0.20 566.80 ± 10.00 279.83 ± 18.00

7.15 ± 0.5 21.53 ± 0.30 11.98 ± 0.20 25.05 ± 0.20 21.26 ± 0.20 696.00 ± 10.00 343.62 ± 18.00

6.95 ± 0.5 20.30 ± 0.30 11.17 ± 0.20 23.03 ± 0.20 19.93 ± 0.20 785.20 ± 10.00 387.65 ± 18.00

6.80 ± 0.5 18.64 ± 0.30 10.44 ± 0.20 21.09 ± 0.20 18.56 ± 0.20 894.00 ± 10.00 441.37 ± 18.00

7.10 ± 0.5 17.24 ± 0.30 9.56 ± 0.20 19.06 ± 0.20 17.14 ± 0.20 922.80 ± 10.00 455.59 ± 18.00

7.00 ± 0.5 15.59 ± 0.30 8.75 ± 0.20 17.10 ± 0.20 15.76 ± 0.20 942.00 ± 10.00 465.07 ± 18.00

Note: Biogas & Methane are in mL. Rest of all parameters are in g/L except pH.

2.4. Analytical methods 2.4.1. Feedstock composition analysis The substrates, POME and CM, were examined to determine the chemical and physical properties. We measured the COD, BOD, total solids (TS), volatile solids (VS), volatile fatty acids (VFA), pH, total carbon (TC), and total nitrogen (TN) using standard APHA methods. In the substrates, TS was determined by placing the substrate in an oven at 105◦ C for approximately 24 h, followed by the weight being measured before and after the oven-dried operation. For determining VS in the substrates, the crucibles used for determining TS were again placed in a muffle furnace for 5.50 h at 550◦ C. Afterwards, the crucibles were left to be cooled to room temperature to dissipate excess heat. The sample was then

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measured for weight, and VS was calculated. The measurements were taken three times for sample, each and the mean values were considered. 2.4.2. Biogas composition analysis The amount of Biogas produced was determined by the water displacement method under fixed temperature and pressure conditions (Wang et al., 2014). The daily and cumulative biogas amount was similarly determined by the water displacement method. Biogas production was represented as volume yield and was expressed in mL. The more the amount of biogas generated, the more water was displaced. Biogas yield and methane composition were analyzed using Agilent’s gas chromatography (GC) device. Helium was used as the carrier gas with a flow rate of about 30.00 mL/min. The temperatures of the oven, inlet, and detector were maintained at 70◦ C, 120◦ C, and 200◦ C, respectively. 2.4.3. Statistical analysis The data were analyzed for three replicates by using Microsoft Excel 2016. All necessary statistical data were derived using this software (analytical graphs and equations are given as supplementary data). Mean, standard deviation, and standard error results were calculated from replicates by OriginPro 9.1 and were applied to values in all figures and tables. 2.5. Economic analysis tools 2.5.1. Model setup elements A preliminary dataset is required for a commercial feasibility study of any biogas plant. About 100,000 m3 substrate/year was calculated to be treated, considering the quantity of POME, CM, and the dilution applied (20,000 m3 POME without dilution/year, 2000 tonnes CM/year, and for dilution 90,000 m3 water/year) (Moreno et al., 2017). The production of biogas per m3 substrate was about 13 m3 , and the methane generation per m3 substrate was about 6.85 m3 (Karthika and Narayanan, 2016). A biomethanization plant required a stirred container digester with an operating volume of about 10,600 m3 , whose height and diameter were 15 m on both sides to produce this amount of energy. The plant needed a biogas storage container whose volume was 2700 m3 . The POME treatment was performed in a stagnant vessel with a horizontal flow with a volume of 350 m3 ; it had a width of 10 m, length office 28 m, and a height of 1.25 m. The plant was constructed over a total area of 20,500 hectares, where around 25% of the land area was used for collection and storage of CM. 2.5.2. Heat and electrical energy determination To measure the heat and electrical energy, the biogas plant was predicted to work up to 4380 h per year (i.e., 12 h per day); therefore, the energy system works at 78% efficiency. The net required energy for the plant was estimated to be about 19.2 GWh/year, considering a 10% loss from the roughness of pipes. The methane’s calorific value per m3 substrate is 9.7 kWh/m3 CH4 ; thermal energy generated during the AD of these substrates was approximately 7.55 GWh/year (Karthika and Narayanan, 2016). 2.5.3. Annual charges The operational expenses were projected to be 2.5% of construction costs in a biogas plant. An annual loan payment was calculated assuming a 6% interest to be paid for biogas plant construction charges. The operational manpower cost was predicted to be 16,638 USD per year. The costs for transport of substrates in a biogas plant were considered to be 1.85 USD per km for an empty truck and 2.18 USD per km for a loaded truck (Siddique et al., 2015). 2.5.4. Annual benefits Among all the electrical energy generation from the plant, about 80% would be used in the national grid services. The biogas plant itself consumed the remaining 20%. The heat was required to sustain. The predicted results attained by ACoD biogas plants are shown in Table 9; it shows a complete breakdown of the finances for construction of an ACoD biogas plant that uses POME and CM. Differences between annual income and annual cost, as well as the annual profits, were calculated. The operation and maintenance costs were measured to be only 5% of the capital cost; it was deemed to be an insignificant difficulty because it was due to shifting inlet or outlet pipes and air leaks only. 2.5.5. Economic variables The payback time (PBT) was measured by dividing the capital cost by the annual cost. Payback period = Capital cost/Annual cost The following equation was used to determine the net present value (NPV): NPV =

n ∑

CF/(1 + k)t − I

t =1

Where, CF = Cash flow at time (t), n = number of years considered, k = interest, I = Initial investment.

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Table 7 Effect of mixing ratio on biogas production and methane composition. Reactor

POME:CM

Biogas (mL)

R2 value

Equation

R1 R2 R3 R4 R5

0:100 25:75 50:50 75:25 100:0

637.00 ± 10.00 782.00 ± 10.00 1567.00 ± 20.00 1346.00 ± 20.00 942.00 ± 10.00

0.9566 0.9707 0.9806 0.9751 0.9230

Y Y Y Y Y

= 31.37x − 88.68 = 31.64x + 85.63 = 65.01x + 112.20 = 51.28x + 118.86 = 37.92x + 159.53

To determining the IRR, it is necessary to set the NPV equal to zero and perform calculations for finding the discount rate (k). In practice, IRR cannot be measured logically, and there is a need for trial-and-error methods; software program are often necessary to compute IRR. IRR = NPV =

n ∑

CF/(1 + k)t − I = 0

t =1

2.6. Analysis of environmental benefit There are two main ecological benefits of the co-digestion of POME and CM; these are: (a) prevention of the discharge of possible impurities from nearby areas and (b) the production of waste after treatment can easily be utilized in for irrigation, while the remaining sludge can be used as a soil fertilizer (Siddique et al., 2015). However, ensuring these advantages from biogas can lead to another problem, which is the occurrence of greenhouse gases (GHGs) during the transformation of biogas into electricity by application of combustion heat and power (CHP) unit. As in biogas, the main elements are carbon dioxide and methane, therefore, it can cause immense GHG effects. The CH4 and CO2 emissions can be calculated using the following equations: (a) Methane emissions: CH4 emissions (t / year) = W × ηCH4 xρ CH4 Where, W = Generation of waste per year (m3 /year) ηCH4 = Yield of methane from ACoD study (m3 CH4 / m3 substrate) ρ CH4 = Density of methane (680 t/m3 ) (b) Carbon dioxide emissions: CO2 emissions (t / year)=W × ηCO2 xρ CO2 Where, W = Generation of waste per year (m3 /year) ηCO2 = Yield of carbon dioxide from ACoD study (m3 CO2 / m3 substrate) ρ CO2 = Density of carbon dioxide (1.87 t/m3 ) (c) Carbon dioxide emissions generated by the use of methane: CH4 + 2O2 → CO2 + 2H2 O From the chemical reaction of methane combustion, 1 mol of carbon dioxide is generated from 1 mol of methane. The emission of carbon dioxide following the consumption of methane is measured by the following equation: CO2 emission using methane energy (t / year) =

(moles CH4/ year) xMWCO2 10ˆ6

3. Results & discussions 3.1. Biogas production and methane composition The concept of co-digestion is defined as the continuous digestion of two or more wastes following their proper mixing. For anaerobic co-digestion operation, the complete characterization of the wastes is essential to evaluate if they are susceptible to treatment during the ACoD operation, which is strongly effected by their physicochemical properties. The characteristics of POME and CM are presented in Table 1 as the means of three value with standard deviation. To measure the consequence of environmental and financial features, various co-digestion mixtures with POME and CM were prepared. The mixing ratios of the samples ranged from 0% POME (i.e., 100% CM) to 100% POME (0% CM). The statistics on total biogas generation from the five reactors with different POME and CM blends are given in Fig. 2. The reactors, R2 , R3 , R4 , and R5 , produced biogas from the first day of reactor operation, while R1 did not. The total biogas generation after 24 days of reactor operation was 637.00 mL, 782.00 mL, 1567.00 mL, 1346.00 mL, and 942.00 mL from digestors R1, R2 , R3 , R4 , and R5 , respectively. Here, the reactor with the mixing ratio of 50:50 was observed to work the best as it

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Fig. 2. Total Biogas Production after 24 Days of Digestion.

was able to improve biogas production by 146%, 100%, 16%, and 43% in R1, R2 , R4 , and R5 as presented in Table 7. This outcome supports that the co-digestion of 50:50 mixing ratio possibly expands the generation of biogas by 15% to 150%, which is mostly due to functional conditions and substrate utilization. In Table 7, the R2 value is given for every mixing ratio, along with digestion days. From this, it can be inferred that 50:50 mixing ratio was the highest as the R2 value was maximum and close to 1. The graph between biogas production and digestion days is given in supplementary materials. The most important and terminal step in the ACoD process is methanogenesis that eventually produces biogas. If methanogens are present in a low number, lesser biogas production will occur. As presented in Table 7, the maximum fraction of methane (64.13%) was attained from reactor R3 ; it was 40.21% for R1 , 45.72% for R2 , 54.85% for R4 , and 49.37% for R5 . Hydrogen (H2), oxygen (O2), nitrogen (N2), carbon dioxide (CO2 ), and water (H2O) formed the remaining proportions in biogas produced. From this experiment, the mixture with 50:50 ratio of POME and CM was documented to be optimal for generation of biogas and methane. The reason for this is the high proportion of accessible decomposable constituents and reduction in the percentage of volatile solids (Wang et al., 2017). A high amount of nitrogen and moist nature of the mixed substrate usually intensify bacterial action, which then alters the transformation of biomethane from biomass (Rahman et al., 2017). R1 and R2 digesters exhibited a low methane production, 40.21% and 45.72%, respectively, as compared to the other digesters because of the absence of nitrogen and carbon in the mixture. The pH was maintained in the range of 6.50 and 7.50 in the ACoD medium during operation to test the effects of different mixing ratios of POME and CM. Optimal biogas and methane production were observed for the 50:50 mixing ratio. Any pH values that were present beyond the 6.50–7.50 range during the ACoD operation acted as a buffer for the co-digestion medium, which prevented an unexpected drop in the pH. These values continue to be the optimal choices for development of methanogenic bacteria (Lü et al., 2013). Again, another influencing factor was temperature as it induced co-digestion. According to Zhong et al. 35◦ C temperature improves the performance of ACoD by increasing biogas and methane production and by raising the temperature to 55◦ C (Zhong et al., 2015). Moreover, microorganisms under thermophilic temperatures are not much assorted than they under mesophilic temperatures (Levén et al., 2007), and the performance of anaerobic digestion under thermophilic conditions is less steady and incurs additional restrictions on microbial mixed culture circumstances (De la Rubia et al., 2013). Therefore, in view of the effectiveness of temperature and the co-digestion activity, mesophilic temperature was preferred in our experiments for POME and CM co-digestion in the SABr system. As shown in Table 2, the reactor R1 (0% POME + 100% CM) did not generate any biogas during the first six days. Two factors were considered responsible for the delay in biogas generation. Firstly, the feed given to cows is mostly crops; about 90% of the dry weight of these plants is present in the form of lignin, cellulose, and hemicellulose. The presence of lignin in lignocelluloses forms a protecting blockade which hinders plant cell degradation by microbes and fungi for biogas production, unless the feed is pretreated (Angelidaki and Ellegaard, 2003). There are numerous pretreatment methods which can change the chemical and physical assemblage in the lignocellulosic biomass, and thus, enhance hydrolysis. Secondly, it can induce VFA accretion because of little biodegradation of CM, which results in the inhibition of biogas production in the reactor. After the VFA were disbursed, the inhibition of biogas yield was reversed and production was initiated. Although the accumulation of VFA was suspected, the pH value for reactor R1 (0% POME + 100% CM) was kept in the range of 6.50 to 7.50 due to the buffering capacity of CM.

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Table 8 Effect of mixing ratio on COD removal and VS reduction. Reactor

POME:CM

Methane (%)

COD removal (%)

VS reduction (%)

R1 R2 R3 R4 R5

0:100 25:75 50:50 75:25 100:0

40.21 45.72 64.13 54.85 49.37

15 29 68 52 44

33 47 63 59 41

± ± ± ± ±

2.00 2.00 3.00 2.00 2.00

± ± ± ± ±

1.00 2.00 2.00 2.00 2.00

± ± ± ± ±

2.00 2.00 3.00 2.00 2.00

Table 9 Economical study for large scale solar bioreactor plant. Items

Description

Amount (USD)

Total Overhead Cost

Plant Construction Cost Motor & Pump Cost Solar Panel Cost Periodic Maintenance Cost Labor Cost Transport Cost Electricity Consumption Cost Heat Consumption Cost Electricity Revenue Heat Revenue Fertilizer Revenue Yearly Profits Net Present Value (NPV) Payback Time (PBT) Internal Rate of Return (IRR, %)

1,760,300.00 78,500.00 122,700.00 64,370.00 194,200.00 90,400.00 0.00 0.00 805,310.00 11,760.00 34,680.00 502,780.00 1,900,971.46 4.60 44.08

Yearly Operational & Maintenance Cost

Yearly Income

Yearly Benefits Economic Ratios

3.2. Reduction in COD and volatile solids Our results specify that the maximum biogas yield could be attained from the reactor R3 (50% POME + 50% CM), i.e., 1567.00 mL. In contrast, the lowest production of biogas was obtained in the reactor R1 (0% POME + 100% CM); it was 637 mL. This high inconsistency in production of biogas in reactor R1 was because of the low COD removal, VS reduction, and lesser amounts of methanogens present in the mixture of POME and CM. These observations indicate that biogas production increased with the reduction in COD and VS content. This can be attributed to the proper functioning of methanogens that may lead to a complete breakdown of biological substrates (El-Mashad and Zhang, 2010). This indicates that the microbes present in reactor R3 were much more efficient in biogas production than those in the other mixtures. These findings were consistent with the previously reported ones (Sidik et al., 2013), where reactors with active microorganisms produced more biogas than other reactors. The reduction efficacy for COD and VS was measured for every reactor of this study and the outcomes are presented in Table 8. Here, the reactor R3 exhibited the highest COD removal percentages, i.e. 68%, and the lowest proportion of COD removal was observed in the reactor R1 , which was 15%. The COD removal efficiency indicated that the co-digestion of SABr was moderately active. It indicated that VS play a vital role in inducing biodegradation, which highlights the metabolic position of the mist abundant microbial group within the AD system and represents the steadiness of the process. The highest VS elimination efficacy (63%) was attained from the reactor R3 . There was also another probable clarification via the carbon to nitrogen (C:N) ratio. It is recommended that C:N ratio should ideally range from 16.00 to 19.00 in a co-digestion medium (Nyns, 1986). Kivaisi and Mtila similarly recommended 16.80 to 18.00 as the perfect ratio after considering lignin (Kivaisi and Mtila, 1997). The C:N ratio in reactor R3 (50% POME + 50% CM) was found to be within the range reported by the above mentioned scientists. 3.3. Economic impact analysis Wastewater treatment is an important issue for developing nations because the governments are under a continuous pressure to handle wastewater efficiently and to allocate controlled investment. A financial assessment for construction of large-scale biogas plant is essential (Murphy and McKeogh, 2004). On the basis of the outcomes of our experimental study, a financial feasibility analysis was conducted to evaluate the suitability of POME in an ACoD process with CM as a co-substrate. In this study, the ratio (50:50) of substrates POME:CM gave the best results in which the biogas production was 447.70 L/m3 and methane composition was 64.13% after a digestion period of 24 days; this led to the electric power generation of 1.69 kWh. Considering the cost and revenue calculations, the cost comprises of investment cost, transport cost, income and development cost (manpower and operational maintenance), which were calculated based on the overall estimations and are presented in Table 9. The overall investment in construction of a biogas plant with per day production capacity of 18.65 L/m3 is 1,961,500.00 USD. The effectiveness of the system was analyzed using different parameters, such as the PBT, IRR, and NPV.

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The construction costs of a biogas plant are projected to be approximately 1,760,300 USD (Table 9). The effectiveness of the container in heating the POME is essential to make sure that thermal steadiness in the reactor at a mesophilic temperature (35◦ C) is accounted for. In the upper crust of the reactor, usually 22% of the overall losses occur, while pipes and plant insulation are deemed to account for more than 5% of the losses. The overall energy thus produced from the biogas plant is 437.00 kWh/year. It was identified that 33 pumps that were fueled by biogas were selected The nominal power of these pumps was 50.00 kW and the cost of operation was 2378.80 USD per pump, and thus, the total cost of motor pumps was 78,500.00 USD. The transportation area was estimated to be approximately 20.00 km. The plant will not need to make a payment for the electric energy because it uses solar power. A yearly profit will also come from savings in the form of energy produced by using motor pumps, where the fuels come from biogas generated in the biogas plant, instead of using electric power. Additionally, economic variables, such as IRR, NPV, and PBT, were estimated to be 1,900,971.46 USD, 44.08%, and 4.60 years, respectively. This analysis explains the magnified expenses and sizes for apparatus. Therefore, the outcome of this manuscript may be considered established. All over the world, Malaysia is considered to be the 23rd largest crude palm oil (CPO) producer, which amount to 5.52 billion barrels (PMIDA). Moreover, it is anticipated to produce 60,000 cattle head by 2015 as per the report by the 9th Malaysia Strategy (Putrajaya Insists National Cattle Project a Success, 2013). Therefore, the production of POME and CM are predicted to increase. The co-digestion of POME and CM and constructing biogas plant will make this scheme beneficial as follows: (a) proper circumventing sludge removal and extra soil compost generation will carry workability of ACoD, (b) a biogas plant planned to operate on 50:50 mix ratio of POME:CM all over the year, including CM conveyance costs, can be balanced via the greater yield of biogas by ACoD. Additionally, the solar-assisted bioreactor will help save electricity and heat, as well as maintain proper temperature requirements for co-digestion operation. It was deduced that the financial and viability evaluations of the ACoD biogas plant exhibited maximized charges and scope of operation. Intrinsically, the outcomes presented here will be deemed conservative. The total electricity production cost is a function of expenditure, principal invested, functioning and maintenance, and fuel costs (Shafie et al., 2012). Additional aspects that affect the biomass cost for energy production include energy storage capacity, the lifetime of a biogas plant, as well as electrical and heat efficiency (Carneiro and Ferreira, 2012). To overcome the drawbacks of POME- and CM-based bioenergy production costs, inventors consider various factors and attempt to attain emergent knowledge that reduces heat and electricity production. Solar energy storage and its utilization in biogas plants is one of the solutions to reduce costs. Biogas and biomethane production fluctuate depending on technical knowledge that is required in the alteration procedure. Therefore, consistent efforts are required in finding more effective biogas production methods. 3.4. Environmental impact analysis From an ecological point of view, biogas generation and its use are the best approaches to treat POME because of its reclamation creates a great amount of bioenergy. The foremost advantages of bioenergy reclamation from POME and CM may vary because of a reduction in wastewater quantity, lessened demand of land, reduction in costs of carrying wastes over long distances to landfill locations, and most importantly, a net mitigation in environmental pollution. During the transformation of biogas into electric energy by exerting combustion heat and power (CHP) units, GHGs can be produced. If the digestion method is not involved in electricity generation from biogas, then the emission of GHG is about 22,700.00 m3 per year (Thanarasu et al., 2018). If the waste-to-energy system works well, it helps diminish the GHG emission by 17,870.00 m3 annually (Akbulut, 2012). Conversely, the global warming potential (GWP) effect may be minimized by about 80.00–130.00 CO2 eq/ton, when vehicle fuel comes from biogas, because it provides better environmental benefits than conventional ignition (Pérez-Fortes and Tzimas, 2016). Because of the biological characteristics of POME, there is a surplus removal of organic compounds, along with their normal organic breakdown, produces GHGs. As the proportions of the two main gases, i.e., CH4 and CO2 , produced from ACoD operations are the same, it could be because of normal organic breakdown. The positive environmental effects were possible because of GHG reduction as a consequence of biogas plant construction (González González, 2014). Approximately 693,590 Nm3 and 614,290 Nm3 of methane and carbon dioxide per year, respectively, is expected to be produced by the co-digestion of 100,000 m3 substrate used per year. If natural degradation occurs, an equal quantity of methane and carbon dioxide would be extricated. Nitrous oxide does not have a local environmental impact, although on a global scale, it does contribute to global warming and is one of the most important GHGs. Although it is released in relatively small amounts, its GWP is 310 times more than of carbon dioxide. The ozone layer is also affected by nitrous oxide, and thus, the fortification effect against damaging ultraviolet radiations is reduced. The solid wastes leftover from the ACoD process can be used as compost because of the high proportion of inorganic nutrients (N, P, and K). Another significant objective of biogas use as a vehicle fuel is to achieve cleaner air, because diesel-powered vehicles produce 11.00 particulates/MJ of fuel, while only 0.02 particulates/MJ are produced from biogas-powered vehicles (Patterson et al., 2011). The probable usage of sludge and recovered water can be associated with their physicochemical properties and the environmental restrictions specified as rules and regulations. The redeemed sludge is evaluated for compost usage. The heavy metals concentration in the sludge is below the bound levels, and thus, it can be used as manure. However, if the recycled water does not meet the minimum standards, it is allowed to be used only for irrigation of agricultural lands that are not close to municipal and forest regions because the SS concentration must be under 35 mg/L (Moreno et al.,

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2017). Therefore, it is required that recycled water should be subjected to another suitable treatment operation that can ensure the reduction of SS within their standard limits. Furthermore, if the recycled and treated water do not meet the standards, they can only be useful for agricultural or industrial usage because recycled water cannot be used for direct consumption by humans. 4. Conclusions Currently, anaerobic bioreactors are the most extensively practiced means of treating a vast range of wastewaters. Regardless of the application of wastewater thus treated, the simplicity of this bioreactor’s installation makes it a unique technology. Despite such benefits, it is a challenge to maintain desired degradation efficiency, process stability, and methane yield. A distinct disadvantage of existing solar reactors is that they have less control over the operating temperature and pH. The addition of digital control on the operating temperature and pH is an intensification of the solar-powered reactor system. Anaerobic co-digestion of POME and CM in equal proportions in solar-powered bioreactors provides the ultimate degradation efficiency and process stability by controlling temperature and pH. This, in turn, maximizes biogas production and methane yield up to 50%–65% as compared to conventional treatment systems. It is notable that mesophilic temperature (35◦ C) was proven to be the most suitable condition to accelerate methanogenesis in a solar-powered reactor. Environmental and economic analysis highlight the advantages of using solar-driven anaerobic reactors. This technology provides an excellent opportunity to address waste treatment and removal on a larger scale. It must be emphasized that operating costs can be minimized by utilizing methane for generating electricity and heat in the plant itself. Furthermore, an SABr is capable of treating any type of wastewater and achieve satisfactory water quality at low cost. This model should be investigated further to evaluate the consequences of reactor performance with different types of substrates, inlet substrate concentrations, flow rates, and biomass. Acknowledgment The authors would like to thank the Faculty of Engineering and Technology, UMP and School of Ocean engineering, UMT for using their laboratory and for the financial support from RDU160315 and RDU190332 of Universiti Malaysia Pahang (UMP). Appendix A. Supplementary data Supplementary material related to this article can be found online at https://doi.org/10.1016/j.eti.2019.100446. References Abu Bakar, S.N.H., Abu Hasan, H., Mohammad, A.W., Sheikh Abdullah, S.R., Haan, T.Y., Ngteni, R., Yusof, K.M.M., 2018. A review of moving-bed biofilm reactor technology for palm oil mill effluent treatment. J. Cleaner Prod. 171, 1532–1545. Ahmad, A.L., Ismail, S., Ibrahim, N., Bhatia, S., 2003. Removal of suspended solids and residual oil from palm oil mill effluent. J. Chem. Technol. Biotechnol. 78 (9), 971–978. Ahmadi-Pirlou, M., Ebrahimi-Nik, M., Khojastehpour, M., Ebrahimi, S.H., 2017. Mesophilic co-digestion of municipal solid waste and sewage sludge: Effect of mixing ratio, total solids, and alkaline pretreatment. Int. Biodet. Biodeg. 125, 97–104. Akbulut, A., 2012. Techno-economic analysis of electricity and heat generation from farm-scale biogas plant: Çiçekdağı case study. Energy 44 (1), 381–390. Alkhamis, T., El-Khazali, R., Kablan, M., Alhusein, M., 2000. Heating of a biogas reactor using a solar energy system with temperature control unit. Sol. Energy 69 (3), 239–247. Álvarez, J.A., Otero, L., Lema, J.M., 2010. A methodology for optimising feed composition for anaerobic co-digestion of agro-industrial wastes. Biores. Tech. 101 (4), 1153–1158. Amani, T., Nosrati, M., Sreekrishnan, T.R., 2010. Anaerobic digestion from the viewpoint of microbiological, chemical, and operational aspects — a review. Environ. Rev. 18 (NA), 255–278. Angelidaki, I., Ellegaard, L., 2003. Codigestion of manure and organic wastes in centralized biogas plants. Appl. Biochem. Biotech. 109 (1–3), 95–105. Appels, L., Baeyens, J., Degrève, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Ener. Comb. Sci. 34 (6), 755–781. Axaopoulos, P., Panagakis, P., Tsavdaris, A., Georgakakis, D., 2001. Simulation and experimental performance of a solar-heated anaerobic digester. Sol. Energy 70 (2), 155–164. Carneiro, P., Ferreira, P., 2012. The economic, environmental and strategic value of biomass. Renew. Energy 44, 17–22. De Kock, M., 2015. Anaerobic Bioconversion of Liquid and Solid Wastes from the Winemaking Process. Stellenbosch University, Stellenbosc. De la Rubia, M., Riau, V., Raposo, F., Borja, R., 2013. Thermophilic anaerobic digestion of sewage sludge: focus on the influence of the start-up. A review. Crit. Rev. Biotechnol. 33 (4), 448–460. El-Mashad, H.M., Zhang, R., 2010. Biogas production from co-digestion of dairy manure and food waste. Bioresour. Technol. 101 (11), 4021–4028. González González, A., 2014. Viabilidad medioambiental, energética y económica de la Biometanización de Residuos provenientes de la Industria Agroalimentaria en Extremadura. Hartmann, H., Angelidaki, I., Ahring, B.K., 2002. Co-digestion of the organic fraction of municipal waste with other waste types. In: Biomethanization of the Organic Fraction of Municipal Solid Wastes. pp. 181–200. Hashimoto, A.G., Varel, V., Chen, Y., 1981. Ultimate methane yield from beef cattle manure: effect of temperature, ration constituents, antibiotics and manure age. Agric. Wastes 3 (4), 241–256. Hills, D.J., Stephens, J.R., 1980. Solar energy heating of dairy-manure anaerobic digesters. Agric. Wastes 2 (2), 103–118.

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Ivana, C., Maríaa, T., Auraa, V., Paolaa, A., Marioc, H., 2016. Anaerobic co-digestion of organic residues from different productive sectors in Colombia: biomethanation potential assessment. Chem. Eng. 49, 385–390. Karthika, P., Narayanan, D., 2016. Performance analysis of biogas production potential from banana pseudo-stem. Int. J. Res. Electr. Electron. Eng. 4 (2), 01–07. Khan, E., Wirojanagud, W., Sermsai, N., 2009. Effects of iron type in fenton reaction on mineralization and biodegradability enhancement of hazardous organic compounds. J. Hazard. Mater 161 (2–3), 1024–1034. Kivaisi, A.K., Mtila, M., 1997. Production of biogas from water hyacinth (Eichhornia crassipes) (Mart) (Solms) in a two-stage bioreactor. World J. Microbiol. Biotechnol. 14 (1), 125–131. Kocar, G., Eryasar, A., 2007. An application of solar energy storage in the gas: Solar heated biogas plants. Energy Sources A Recovery Util. Environ. Effects 29 (16), 1513–1520. Levén, L., Eriksson, A.R., Schnürer, A., 2007. Effect of process temperature on bacterial and archaeal communities in two methanogenic bioreactors treating organic household waste. FEMS Microbiol. Ecol. 59 (3), 683–693. Liu, Z., Zhang, C., Lu, Y., Wu, X., Wang, L., Wang, L., Han, B., Xing, X.-H., 2013. States and challenges for high-value biohythane production from waste biomass by dark fermentation technology. Biores. Tech. 135, 292–303. Lü, F., Hao, L., Guan, D., Qi, Y., Shao, L., He, P., 2013. Synergetic stress of acids and ammonium on the shift in the methanogenic pathways during thermophilic anaerobic digestion of organics. Water Res. 47 (7), 2297–2306. Moreno, L., González, A., Cuadros-Salcedo, F., Cuadros-Blázquez, F., 2017. Feasibility of a novel use for agroindustrial biogas. J. Cleaner Prod. 144, 48–56. Murphy, J.D., McKeogh, E., 2004. Technical, economic and environmental analysis of energy production from municipal solid waste. Renew. energy 29 (7), 1043–1057. Nyns, E.J., 1986. Biomethanation processes. In: Schonborn, W. (Ed.), Microbial Degradations. Wiley-VCH Weinheim, Berlin, pp. 207–267. Omar, R., Harun, R.M., Mohd Ghazi, T., Wan Azlina, W., Idris, A., Yunus, R., 2008. Anaerobic treatment of cattle manure for biogas production. In: Proceedings Philadelphia, Annual meeting of American Institute of Chemical Engineers, pp. 1–10. Patterson, T., Esteves, S., Dinsdale, R., Guwy, A., 2011. An evaluation of the policy and techno-economic factors affecting the potential for biogas upgrading for transport fuel use in the UK. Energy Policy 39 (3), 1806–1816. Pérez-Fortes, M., Tzimas, E., 2016. Techno-Economic and Environmental Evaluation of CO2 Utilisation for Fuel Production. JRC Science Hub: ZG Petten, The Netherlands. Rahman, M.A., Møller, H.B., Saha, C.K., Alam, M.M., Wahid, R., Feng, L., 2017. Optimal ratio for anaerobic co-digestion of poultry droppings and lignocellulosic-rich substrates for enhanced biogas production. Energy Sustain. Dev. 39, 59–66. Rasi, S., Veijanen, A., Rintala, J., 2007. Trace compounds of biogas from different biogas production plants. Energy 32 (8), 1375–1380. Richter, C., 2013. Solar thermal energy, introduction. Solar Energy 655–657. Shafie, S., Mahlia, T., Masjuki, H., Ahmad-Yazid, A., 2012. A review on electricity generation based on biomass residue in Malaysia. Renew. Sustain. Energy Rev. 16 (8), 5879–5889. Siddique, M.N.I., Abdul Munaim, M.S., Zularisam, A.W., 2015. Feasibility analysis of anaerobic co-digestion of activated manure and petrochemical wastewater in Kuantan (Malaysia). J. Cleaner Prod. 106, 380–388. Sidik, U.H., Razali, F., Alwi, S., Maigari, F., 2013. Biogas production through co-digestion of palm oil mill effluent with cow manure. Nigerian J. Basic Appl. Sci. 21 (1), 79–84. Siedlecka, E., Stepnowski, P., 2005. Phenols degradation by fenton reaction in the presence of chlorides and sulfates. Polish J. Environ. Stud. 14 (6). Thanarasu, A., Periyasamy, K., Devaraj, K., Periyaraman, P., Palaniyandi, S., Subramanian, S., 2018. Tea powder waste as a potential co-substrate for enhancing the methane production in anaerobic digestion of carbon-rich organic waste. J. Cleaner Prod. 199, 651–658. Wang, C., Hong, F., Lu, Y., Li, X., Liu, H., 2017. Improved biogas production and biodegradation of oilseed rape straw by using kitchen waste and duck droppings as co-substrates in two-phase anaerobic digestion. PLoS One 12 (8), e0182361. Wang, X., Lu, X., Li, F., Yang, G., 2014. Effects of temperature and carbon-nitrogen (C/N) ratio on the performance of anaerobic co-digestion of dairy manure, chicken manure and rice straw: focusing on ammonia inhibition. PLoS One 9 (5), e97265. Wu, X., Yao, W., Zhu, J., Miller, C., 2010. Biogas and CH4 productivity by co-digesting swine manure with three crop residues as an external carbon source. Bioresour. Technol. 101 (11), 4042–4047. Zhong, Y., Bustamante Roman, M., Zhong, Y., Archer, S., Chen, R., Deitz, L., Hochhalter, D., Balaze, K., Sperry, M., Werner, E., Kirk, D., Liao, W., 2015. Using anaerobic digestion of organic wastes to biochemically store solar thermal energy. Energy 83, 638–646.