Performance evaluation of anaerobic digestion technology for energy recovery from organic fraction of municipal solid waste: A review

Journal Pre-proof Performance evaluation of anaerobic digestion technology for energy recovery from organic fraction of municipal solid waste: A review

Atul Kumar, S.R. Samadder PII:

S0360-5442(20)30360-1

DOI:

https://doi.org/10.1016/j.energy.2020.117253

Reference:

EGY 117253

To appear in:

Energy

Received Date:

06 September 2019

Accepted Date:

24 February 2020

Please cite this article as: Atul Kumar, S.R. Samadder, Performance evaluation of anaerobic digestion technology for energy recovery from organic fraction of municipal solid waste: A review, Energy (2020), https://doi.org/10.1016/j.energy.2020.117253

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Journal Pre-proof Performance evaluation of anaerobic digestion technology for energy recovery from organic fraction of municipal solid waste: A review By Atul, Kumar1 and S. R., Samadder2* 1Ph.

D Scholar, Department of Environmental Science & Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India. (Email: [email protected]). 2Associate

Professor, Department of Environmental Science & Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India. (*Corresponding author: Email: [email protected]). Abstract: Anaerobic digestion is one of the most effective and environment-friendly waste management techniques. It not only treats the organic fraction of municipal solid waste, but at the same time it can be considered as one of the potent renewable energy sources due to generation of methane during digestion process. The technology is not new and has been commercialised from early 1980s. But, the data suggests that it is not still widely applied for energy recovery from organic wastes at centralised level. The reason may be poor methane yield due to operational issues and process instability. There were numerous studies already done at the lab scale, now it is the time to replicate the outcomes of lab-scale studies to the full scale plant. Further studies are required to make the anaerobic digestion techno-economically sustainable. This paper presents a detailed review of essential process parameters and identifies gaps and solutions for effective implementation of the anaerobic digestion of organic fraction of municipal solid waste. The paper also presents the effect of co-digestion, pre-treatments and inhibition on the performance of anaerobic digestion. The paper will help the readers in understanding the process, operation and control of anaerobic digestion technology. Keywords: Anaerobic digestion; Co-digestion; Inhibition; OFMSW; Pre-treatment; Process parameters. Abbreviations: C/N Carbon to nitrogen ratio COD Chemical oxygen demand GC Gas chromatography LCA Life cycle Assessment MSW Municipal solid waste OFMSW Organic fraction of municipal solid waste

S/I SUBBOR TAN TS VFA VS

1. Introduction

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Substrate to inoculum ratio Super blue box recycling Total ammonia nitrogen Total solids Volatile fatty acids Volatile solids.

Journal Pre-proof Data shows that around one-third (~1.3 billion tonnes) of the total world food production becomes waste every year at different stages of food supply chain creating economic, environmental and social problems [1]. At present, United States generates 35 million tonnes of food waste annually, whereas China and European Union generate 82 million tonnes and 89 million tonnes every year respectively [1]. The increase in municipal solid waste (MSW) including food waste is expected to rise due to increasing urbanisation, economic development and population growth. Especially, the cities of the developing countries are experiencing a significant increase in MSW generation in recent times, creating a tremendous problem of solid waste disposal [2]. Most of the generated MSW in the developing countries meets with the traditional disposal techniques such as landfilling, open dumping and open burning [3]. These techniques are not recommended due to scarcity of suitable land for landfilling in urban areas and emission of greenhouse gases and other harmful pollutants into the natural environment.

Out of the total MSW generated across the world, approximately 40–70% contains degradable organic materials often termed as organic fraction of municipal solid waste (OFMSW), in which food waste is the major component [4], [5]. At present, cities are not only facing the problem of safe waste disposal and availability of suitable land for landfilling, but also the continuous increase in the energy demand [6]. Though, thermal treatment technologies such as pyrolysis, gasification and incineration can significantly reduce the waste volume and recover energy, but, these technologies are not always preferred as they are mostly suitable for dry carbonaceous waste, otherwise significant amount of energy is lost during initial drying [6]. For instance, gasification and pyrolysis are generally used for the energy recovery from specific type of wastes such as forest biomass, agricultural residues, plastics and tyres [7]. Incineration is used to recover energy in the form of heat and electricity from OFMSW, but it cannot recover the nutrient such as phosphorus and nitrogen present in the waste [8]. During incineration,

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Journal Pre-proof nitrogen may be lost into the atmosphere in the form of nitrogen oxides, while phosphorus trapped in the ash is generally not recycled [8]. On the other hand, anaerobic digestion has been credited with energy recovery in the form of biogas and nutrient recovery in the form of digestate which can be used for soil amendment (Fig. 1) [9]. Recently, a hybrid technology has been developed in which anaerobic digestion has been integrated with some other waste to energy technologies such as gasification [10]. The advantage of such integrated waste to energy system is that, it produces biogas from easily degradable organic waste such as food waste through anaerobic digestion and syngas from not so easily degradable organic wastes such as wood and agricultural residues through gasification. Such type of system is sustainable and more efficient for cities, as it will significantly divert the organic waste from landfilling [6]. Anaerobic digestion could also improve the efficiency of other waste to energy technologies such as incineration by diverting the OFMSW with high moisture content and low calorific values from incineration [11].

Fig. 1: Benefits of anaerobic digestion process

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Journal Pre-proof Initially, anaerobic digestion process was used for the treatment of sewage sludge, wastewater treatment plant sludge, animal manure and agricultural residue [12]. Now it is used on a large scale, especially in the developed countries for the treatment of other types of waste such as OFMSW [13]. Due to high moisture content, high volatile solids (VS) content and easy availability, OFMSW is a suitable substrate for anaerobic digestion. It has a potential to become an alternative and reliable renewable energy source which can eventually replace the traditional fossil fuels and reduce the greenhouse gas emissions [14], [15]. It renders several advantages over other technologies, such as low energy requirement for processing, stabilised sludge production and possible energy recovery from the treatment of OFMSW [16] (Fig. 2). It can significantly reduce the organic loads on landfills and reduce the use of fossil fuels in the transportation of wastes to landfill, thus making a sustainable waste management option [17]. Additionally, it can significantly reduce the environmental impacts especially global warming by trapping greenhouse gases that occurs due to uncontrolled dumping of organic wastes [18].

Fig. 2: Process flow diagram of the anaerobic digestion process

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Journal Pre-proof In spite of several benefits, anaerobic digestion process of OFMSW generally faces problem of low methane yield during its operation due to process instability, substrate quality and operating parameters. The accumulation of volatile fatty acids (VFA) in the digester is one of the most common problems reported in literature [19]. Some of the strategies reported in literature for enhancing methane yield and reducing process instability are substrate pretreatment and co-digestion of two or more substrates [20]. Apart from this, there are many other factors which affect the efficiency of anaerobic digestion process. Therefore, the identification of current challenges and potential solution for effective anaerobic digestion process is necessary. The aim of this review was to critically examine the recent research advancement in the field of anaerobic digestion technology in solving the challenges of process instability. The paper assesses the various aspects which affect methane yield during anaerobic digestion of OFMSW, covering in detail about the challenges, latest developments and future prospective. In this paper, a comprehensive review is presented, describing the need of anaerobic digestion technology, requirement of appropriate environmental and other conditions (pH, particle size, C/N ratio, temperature, etc.), effect of co-digestion & pre-treatments, and problem of process inhibition. The study also reviewed some of the commercially available anaerobic digesters worldwide along with a few novel research currently undergoing in the field of anaerobic digestion of OFMSW. This paper will help the scientific communities in solving the challenges of anaerobic digestion technology for its wider applicability.

2. Anaerobic digestion process

Anaerobic digestion is a complex microbial process which involves a series of metabolic reactions for breakdown of organic matter into biogas and organic fertilizer. The whole process is basically divided into four different stages: hydrolysis, acidogenesis, acetogenesis and

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Journal Pre-proof methanogenesis (Fig. 3) [7]. Hydrolysis is the first stage in which the complex organic matter such as carbohydrates, proteins and fats are broken down into soluble organic molecules such as, sugar, amino acids, fatty acids and other related compounds. In most of the cases, hydrolysis is the slowest or rate-limiting step due to the formation of VFA and other toxic by-products [21]. The hydrolysis step is generally accelerated by providing pre-treatment to the substrates [22]. The second stage is acidogenesis (or fermentation) in which the reduced organic compounds from hydrolysis stage further break into short-chain fatty acids along with hydrogen (H2), carbon dioxide (CO2) and other by-products. Acetogenesis is the third stage in which the organic acids formed in the acidogenesis stage gets converted into acetic acid as well as H2 and CO2. The last stage is methanogenesis, where two different groups of methanogens produce methane. The one group splits acetic acid into methane and carbon dioxide, while other group uses the intermediate products (H2 and CO2) for the formation of methane [23]. The anaerobic digestion process depends on various factors, some of which have been discussed in the subsequent sections.

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Fig. 3: Different stages of anaerobic digestion process for biogas generation

2.1. Effect of different factors on anaerobic digestion process

The anaerobic digestion process involves several steps and each step has different process parameters requirement.

2.2.1. pH

The pH indicates the stability of system during the anaerobic digestion process [5]. Methanogens (anaerobic bacteria responsible for methane formation) are very sensitive to pH [24]. The methane formation stops due to decline in the growth of methanogens at lower pH (acidic condition). A range of pH values (6.5-8.5) for anaerobic digestion has been reported in the literature, but the optimal pH for effective anaerobic digestion process is around 7 [25]. At 7

Journal Pre-proof pH less than 6.5 and more than 8.5, the methanogenesis is inhibited [24]. The pH in the anaerobic reactor varies based on the stage at which the process is occurring. It generally increases with increase in ammonia concentration due to breakdown of protein present in the substrate and decreases with increase in VFA concentration. During batch anaerobic digestion of OFMSW, the pH of the reactor may decrease to acidic range (pH 4 – 5.5) due to excess VFA formation [5]. This could inhibit the whole process. To tackle this problem, either co-digestion substrate with good buffering capacity such as cattle manure is mixed with the substrate or some alkaline reagents such as sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO3) are added to the reactor [24]. Thus, it is essential to monitor the pH of the system regularly for successful operation of anaerobic reactor.

2.1.2. Particle size

Particle size of the raw material affects the performance of the reactor, as smaller size particle shows better biogas yield because, methane producing bacteria have better contact with the degradable organic matter of the substrate [26]. Reduction of particle size might reduce the difficulties of material handlings during mixing and pumping [27]. Thus, particle size reduction is necessary before feeding the reactor to obtain more homogeneous mixture. The particle size has significant effect on the hydrolysis rate, and Zhang et al. [21] reported that maximum substrate utilisation rate was found for 1.02 mm particle size of food waste. However, the excessive reduction in particle size could over-quicken the hydrolysis of substrate, resulting into build-up of VFA and ammonia which could destabilise the reactor. Agyeman and Tao [28] reported that co-digestion of dairy manure and food waste with the reduced particle size from 8 mm to 2.5 mm, increased the overall methane production by 29%. In most of the studies, the

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Journal Pre-proof energy required for reducing the particle size is not measured and should be considered for evaluating the energy balance of the whole process [26].

2.2.3. Carbon to nitrogen (C/N) ratio

Carbon and nitrogen are the source of energy and the vital nutrients for the growth of microorganisms. C/N ratio helps in identifying the nature of wastes introduced in the anaerobic digester [29]. Most of the literature reported an optimum C/N ratio of 20-30 for an effective anaerobic digestion process [24], [30]. However, Nielfa et al. [31] indicated that wider ranges of C/N ratios are acceptable for biogas generation (e.g., sludge and cow manure, which has C/N ratio less than 20). Similarly, Zhang et al. [32] and Tsapekos et al. [33] reported that the optimal C/N ratios for co-digestion of food waste and cattle manure are 15.8 and 16.9 respectively. In another experiment, during co-digestion of corn-stover and sewage sludge, the reactor performed well with a C/N ratio of 15-18, whereas it failed at C/N ratio of 21 or higher due to rapid decrease in pH in first 7 days of operation [30]. At high C/N ratio, excessive acidification occurs due to rapid degradation of substrate during initial stage of the digestion, resulting into the process instability [29]. The excess carbon content will slow down the degradation process, as more time will be taken by the microorganisms to consume the available carbon. While low C/N ratio indicates the high concentration of ammonia nitrogen that inhibits the anaerobic process. Schnurer and Jarvis [34] reported that for proper functioning of anaerobic digester, C/N ratio should lie from 15 to 30. The C/N ratio depends on the type of feedstock and can be maintained to the desired level by changing the mixing ratio of two substrates of high and low C/N ratios.

2.2.4. Temperature

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Temperature is one of the most important parameters that affects the performance of any anaerobic reactor, specifically methanogenesis is greatly influenced by this parameter [12]. The degradability of OFMSW varies with change in temperature. Based on the operating temperature, anaerobic digestion process is divided into three different categories; psychrophilic (~20 °C), mesophilic (~35 °C) and thermophilic (~55 °C). Most of the anaerobic digestion process occurs at mesophilic and thermophilic temperature ranges, in which mesophilic process has been found to be used by more number of studies than thermophilic [35]. The reason could be that at mesophilic temperature, the process is more stable [19], less prone to VFA accumulation which inhibits the methanogenesis [36] and lesser investment and net energy requirement for equivalent biogas yield [37]. However, it has also some limitations over thermophilic process such as, poor methane yield, poor biodegradability of lignocellulosic biomass, long start-up and digestion time [24], [38]. Banks et al. [39] compared thermophilic and mesophilic digestion of food waste in a continuous reactor, in which thermophilic digestion showed better VS reduction and biogas yield as compared to mesophilic digestion, however high VFA concentration was observed which was controlled by reducing the organic loading rate. Another study reported that thermophilic digestion is the best suited option for treating food waste and sewage sludge and achieved 50% more biogas yield as compare to mesophilic digestion [40]. But, Marañón et al. [30] found that the methane yield was higher at mesophilic temperature in comparison to thermophilic temperature for the same feed mixture (70% cow manure, 10% sewage sludge and 20% food waste), possibly due to the formation of high quantity of volatile acids under thermophilic condition.

Most of the time, it is not possible to achieve required process stability, with high methane yield and effluent quality at single temperature. Mao et al. [37] reported that optimal conditions

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Journal Pre-proof can be achieved in two stage anaerobic digestion process. The first stage for hydrolysis/acidogenesis at thermophilic temperature and the second for methanogenesis at mesophilic temperature. Fernandez-Rodriguez et al. [41] conducted a study in which industrial OFMSW was treated under temperature-based two stage batch anaerobic process. In the first stage of experiment, wastes were kept at thermophilic temperature (55 °C) for few days and then transferred to the mesophilic reactor (35 °C) in the second stage for the completion of degradation process. The maximum methane yield and volatile matter removal was obtained, when the wastes were kept for 4-5 days at thermophilic temperature before transferring to the mesophilic reactor. Thus, using temperature-based two stage processes, improved hydrolysis and process stability can be achieved that will lead to more efficient anaerobic digestion process. In another experiment, Beevi et al. [42] treated high solids content of OFMSW at thermophilic temperature (50 °C), but the reactor was started at mesophilic (34 °C) and gradually increased by 2 °C per day till attaining the thermophilic condition to avoid the effect of sudden temperature rise on microorganisms.

2.2.5. Total solids content

Anaerobic digestion can be operated with a total solids (TS) content ranging from 5% to 35% [43]. Anaerobic digestion is divided into three different categories on the basis of TS content of the feedstock; wet (≤10% TS), semi-dry (10-20% TS), and dry (≥20% TS), however the percentage of TS for wet, semi-dry and dry processes varies across the literature. Both the processes (wet and dry) have their own advantages and disadvantages. In recent times, researchers are focusing more on dry (or solid state) anaerobic digestion, because it offers several advantages such as, high organic loading rate [19], low energy requirements, lesser reactor volume and easier handling of digestate [24]. Due to low moisture content, the digestate

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Journal Pre-proof has high nutrient concentration and thus can be used as soil conditioner without much amendments [7]. Furthermore, the digestate can be converted directly into pellets to produce energy through combustion [24]. Due to these reasons, dry anaerobic digestion has been more commercialised in last decade, particularly for the treatment of OFMSW. In Europe, dry process is more prevalent technology for treating OFMSW [44]. It has been reported that more than 90% of the total anaerobic digestion plants (for MSW treatment) across the world is situated in Europe, out of which more than 60% are using dry process [44]. However, some studies reported that wet anaerobic digestion plants have better economic performance and energy balance than dry anaerobic digestion plants [45], [46]. Dry anaerobic digestion has two major limitations of channelling and poor microbial contact with the substrate [46]. Moreover, dry process is more susceptible to the build-up of inhibiting components like ammonia, VFAs and heavy metals [16]. Wet process is generally used to treat OFMSW in co-digestion with wastewater/sewage sludge or manure [27]. Ahmadi-Pirlou et al. [22] reported the effects of TS on methane yield. They found that, under dry process, the digester at 20% TS yields more methane in comparison to the digesters at 25% and 30% TS. Another study reported that, biogas yield increases up to 8% TS, and then drops at 10% TS [47]. In a study reported by Zhang et al. [48] on co-digestion of food waste and horse manure, hydrolysis and acidogenesis were conducted under dry condition and methanogenesis under wet condition. The results showed an increase in methane yield by 11-23% and VS reduction by 10-15%. Therefore, it is not possible to come-up with single process (wet/dry) which can be optimally suited under all circumstances, as methane yield depends on many other variables such as, process parameters, substrate and end-requirements.

3. Effect of co-digestion in anaerobic digestion

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Journal Pre-proof Anaerobic digestion of single substrate (or mono-digestion) is often challenging due to their characteristics and composition, which results into lesser biogas yield and poor stabilised sludge [49]. Single substrate probably lacks buffering and desired nutrient content which lead to inadequate anaerobic digestion process [50]. In comparison to digestion of single substrate, co-digestion offers several advantages, such as better nutrient balance (e.g., C/N ratio) [51], good buffering capacity [32], less inhibition effects (e.g., accumulation of ammonia and VFA) [52], and improved process stability [53]. Fig. 4 shows the effect of co-digestion substrate in balancing several key parameters during anaerobic digestion [12].

Fig. 4: Balancing of parameters in anaerobic co-digestion

Recently, the energy production from co-digestion of various organic substrates has gained popularity [54]. Numerous studies have reported about the co-digestion of different biomass for enhancing the biogas yield. Depending on the substrate concentration and other process parameters, co-digestion can increase the biogas production by as much as 25 - 400% [55]. For e.g., 400% increase in biogas yield was reported by Astals et al. [56], when 4% glycerol was used as an inoculum for co-digestion of pig manure compared to mono-digestion. Kim and Oh [57] noted a high biogas generation rate (5 m3/m3/day) from co-digestion of paper and food waste. Björn et al. [58] reported four times increase in biogas yield from co-digestion of 13

Journal Pre-proof OFMSW and waste activated sewage sludge. Yang et al. [38] reported that a suitable or cosubstrate such as, sewage/wastewater treatment plant sludge and cattle manure can stabilise the whole digestion process by reducing the reactor start-up time and enhancing the rate of digestion, which will eventually lead to increase in biogas yield. Cattle manure is widely reported co-digestion for anaerobic digestion of other types of wastes such as, food waste, vegetable waste, fruit waste, MSW, forestry waste and crop waste [32], [51], [53]. Luste et al. [59] reported that cattle manure has relatively lesser methane yield (126-207 m3 CH4/tonne of VS) than sewage sludge (260 m3 CH4/tonne of VS), but acts as an excellent co-digestion substrate due to its better buffering capacity, high nutrient contents and easy availability. The rate of hydrolysis of cattle manure and food waste was measured in mono-digestion and codigestion using biomethane potential assays [60]. They found increased hydrolysis rates for codigestion of food waste and cattle manure as compared to mono-digestion, may be due to dilution of inhibitory compounds. On the other hand, the overall efficiency of a wastewater treatment plant can be improved by co-digesting the solid effluent (or sewage sludge) with OFMSW in an appropriate combination [61]. The co-digestion of sewage sludge and OFMSW is not only beneficial for methane generation, but a life cycle assessment (LCA) study confirms that it can significantly reduce the greenhouse gas emissions [61]. But, for higher methane production and to avoid inhibitions, the substrate mixtures must have well-balanced properties (such as pH, C/N ratio, etc.) for positive synergistic effects.

Acetoclastic and hydrogenotrophic are the two types of methanogens responsible for methane formation during anaerobic digestion [62]. Acetoclastic and hydrogenotrophic microorganisms synthesise the hydrolysis products (acetate and hydrogen) into methane. The enzymes involved in hydrogenotrophic methane formation and acetate oxidation require trace elements such as tungsten (W), molybdenum (Mo), selenium (Se), nickel (Ni), iron (Fe) and cobalt (Co) [63].

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Journal Pre-proof The deficiency of these elements restricts the formate and acetate oxidation, causing acid accumulation and severe drop in pH of the reactor that ultimately results in complete stop of methane formation. In another study, Tsapekos et al. [33] reported that, cattle manure can act as a source of Ca2+ and Mg2+, which are essential for fast fermentation of organic matters and additionally, limits the excess Na+ concentration in the reactor. Co-digestion substrates such as cattle manure and sewage sludge are the essential sources of nutrients for effective performance of the anaerobic reactor. Some of the studies in which either cattle manure or sewage sludge has been used as a co-digestion substrate are critically analysed and presented in Table 1. In Table 1, the methane yield values have been normalized to standard condition (1 atm pressure and 0 °C temperature) using ideal gas equation for proper comparison.

Table 1: Co-digestion of organic fraction of MSW with cattle manure and sludge Type of feedstock

Anaerobic digestion mode Batch

Tempera ture 35 ± 1 °C

Methane yield (NmL/g-VS) 344

OFMSW, pig manure and wastewater sludge

Batch

37 °C

220

OFMSW and dewatered sewage sludge

Batch

35 ± 1 °C

402

OFMSW and cattle manure

15

Observations

Ref.

 The optimum food waste to cattle manure [32] ratio was 2, at which methane production increased by 41%.  Cattle manure acted as a buffer to the system and hence no external pH control was required.  Food waste contains low concentration of trace elements, which was compensated by cattle manure and assisted the methanogenic activities in the reactor.  Maximum methane yield was observed at [64] food waste and pig manure ratio of 1: 1 and inoculum (waste water sludge) content of 50% on VS basis.  VFA was the main inhibitory compound during the digestion, which was revived by methanogens after few days.  Pre-treated (grinded and sieved) OFMSW [65] have shown significant increase in methane yield (1.8 times) from untreated OFMSW in mono-digestion itself.  The co-digestion of OFMSW and dewatered sewage sludge does not improve the methane yield from pre-treated mono-digestion. However, a ratio of 75% OFMSW and 25%

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OFMSW and cattle manure

Continuous

54 ± 1 °C



365

 Wheat straw and cattle manure

Batch

37 ± 1 °C



282

 OFMSW and cow manure

Batch

55 ± 1 °C



206



Cattle manure, OFMSW and sewage sludge

Continuous

Sewage sludge, OFMSW and glycerol

Semicontinuous

36 °C



533



23.6 ± 1.6 °C



174



OFMSW and sewage sludge

Semicontinuous

35 ± 1 °C



410

 OFMSW and biological sludge

Batch

35 ± 1 °C



196



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sludge provides a solution to the waste management. Most of the gas was produced within 7 days of operation. The overall process performance was stable throughout the experiment period. Wheat straw was initially pre-treated with 3% H2O2, which significantly increased the soluble component of wheat straw. The highest VS removal and methane yield were observed at a mixing ratio of 40:60 between wheat straw and cattle manure. Observed less relative error among the theoretical and experimental methane yield for the co-digesting substrates. Increase in the content of essential macromolecule (N and K) was observed in the digestate, making it suitable for agricultural application. The reactor was fed with 70% cattle manure, 20% food waste and 10% sewage sludge. Ultrasound pre-treatment was done on the feedstock, by which a slight increase in the methane yield was observed. However, the energy produced from extra methane production does not compensate the energy applied during pre-treatment. The C/N ratio of the used sewage sludge and food waste were very less (<10), therefore residual glycerol (which is basically a source of carbon) from biodiesel industry was used. But glycerol is also responsible for accumulation of chloride concentration inside the reactor which can inhibit the methanogenesis. Therefore, it should be added in appropriate proportion. Approximately, two fold increase in methane production was observed during co-digestion of sewage sludge and food waste as compared to mono-digestion of sewage sludge. In this study the proportion of sewage sludge was more than the food waste (7:3 on TS basis). The feedstock with 80% OFMSW and 20% biological sludge produced maximum methane yield. Less relative error between theoretical and experimental results, whenever there is an accurate information of stoichiometric composition and biodegradability of feedstock materials.

[33]

[66]

[67]

[30]

[68]

[69]

[31]

Journal Pre-proof 4. Inhibition during anaerobic digestion process

Anaerobic digestion has several benefits, however, poor operating condition and process instability still prohibits it from being widely commercialised for OFMSW. Typically, there are two types of microorganisms (acid forming and methane forming) found in the anaerobic reactor. These two types of microorganisms are distinct and vary significantly based on their physiology, growth kinetics, nutrition and environmental conditions requirement [16]. The presence of primary and formation of intermediate inhibitory compounds in the reactor are often found to be the main reason for imbalance between the two groups of microorganisms which leads to the upset or instability of the reactor [70]. A large variety of inhibitory substances have been reported which inhibit the anaerobic digestion process. Some of the inhibitory substances are ammonia, organic compounds, sulphide, light metal ions and heavy metals [16]. The two most common inhibitory substances reported in the literatures are ammonia and VFA.

4.1. Ammonia inhibition

Ammonia is one of the intermediate by-products of anaerobic digestion process and one of the major inhibitor of microbial activities inside the anaerobic reactor [71]. In anaerobic reactor, ammonia exists in two main forms ammonium ion (NH4+) and free ammonia, collectively termed as total ammonia nitrogen (TAN) [16]. At lower concentration, ammonia can act as a buffer for the system, but at higher concentration and high pH, it can be toxic to methanogens [72]. Ammonia inhibiting concentration in the anaerobic digester mainly depends on the pH, temperature, C/N ratio, type of substrate and inoculum. A threshold value of 1,500 mg/L (TAN) has been reported by Zhang and Angelidaki [73], above which inhibition of anaerobic digestion

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Journal Pre-proof process starts. Depending on the pH and temperature, 100% inhibition was reported at TAN level of 6,000–13,000 mg/L [72], [73]. Rajagopal et al. [74] reported that methanogenesis stops completely above TAN concentration of 4000 mg/L. It should be noted that TAN concentration is not only responsible for ammonia inhibition in the anaerobic digester, but free ammonia is considered as the main source of inhibition because of its penetrating ability to cell membrane of microorganisms. The concentration of free ammonia increases with the temperature, and affects the methane formation of high nitrogen containing waste at thermophilic temperature. The free ammonia concentration is generally less than 1% of the TAN concentration at mesophilic conditions (35 °C) and at neutral pH, while it can increase up to six folds at thermophilic conditions (55 °C) in comparison to mesophilic and at the same pH [74]. Furthermore, methanogenesis becomes more sensitive towards ammonia inhibition at higher pH (more than 7) due to the formation of free ammonia. Zhang and Angelidaki [73] reported that free ammonia concentration increases up to eight folds with an increase in pH from 7 to 8 at mesophilic conditions. C/N ratio is another important parameter responsible for ammonia inhibition. Wang et al. [51] reported that at higher C/N ratio (more than 25), methane generation was more at thermophilic temperature than mesophilic. During anaerobic digestion process, the ammonia inhibition can be reduced by chemical precipitation and air stripping [75]. The ammonia concentration in the reactor can be reduced by stripping with gas. The addition of trace elements (such as Se and Co) increase the microbial activity and thus stabilise the ammonia concentration in the system [76]. Nitrification is also reported to reduce the ammonia inhibition by converting the ammonia into nitrate [77].

4.2. VFA inhibition

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Journal Pre-proof During initial stage of anaerobic digestion process, soluble organic compounds of OFMSW get converted into intermediate products known as VFAs (comprised of acetic, propionic, butyric and valeric acid). Ideally, the produced VFAs should get converted into CH4 and CO2 by the active microorganisms [21]. But, due to high organic content in substrate, excessive formation of VFAs is observed at an early stage of digestion which lead to the sudden drop in pH of the system and inhibit methanogenesis [24]. In general, VFA concentration increases when the consumption rate by fatty acids consuming bacteria is less than the production. The ratio of substrate to inoculum significantly affects the VFAs accumulation potential [78]. The problem is bound to happen more in single stage batch reactor and more specifically during monodigestion. For stable performance of the anaerobic reactor, the VFA concentration lies between 50 - 250 mg/L [76]. Previous studies reported several methods for controlling the VFA inhibition. Gao et al. [79] used NaHCO3 as a buffering agent and found increase in the methane yield by 48%. Zero-valent iron was also found to be effective in reducing the VFA inhibition. Kumar et al. [80] used calcium chloride during anaerobic digestion of MSW and found that at an optimum dose of 2.5 g/L, VFAs get precipitated into calcium salts.

The monitoring of VFAs is vital for an effective performance of the anaerobic reactor. There are various methods such as high performance liquid chromatography, gas chromatography (GC) and ion-exchange are used for monitoring of VFAs. However, these methods are time consuming and are not reliable for field applications. Nowadays, more advanced methods like online monitoring based GC, titration and back-titration methods are available which can monitor the real-time performance of the reactor and thus digestion imbalance can be avoided in advance [21]. Thus, to deal with the problems of excess ammonia and VFA accumulation, several authors have suggested co-digestion of two or more substrate and/or two-stage anaerobic digestion process [16], [24], [53].

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5. Pre-treatments for improvement of the performance of anaerobic digestion Pre-treatment of substrate before anaerobic digestion often improves the decomposition rate of organic fraction that results into higher methane yield and more stabilised end products [59]. In the literature, different pre-treatment techniques have been reported, such as physical, chemical, biological, and combined for the improvement of anaerobic digestion of OFMSW [81], [82] (Fig. 5). The selection of pre-treatment techniques is decided based on the substrate characteristics, pre-treatment mechanism, and end requirements. In Table 2, critical observation of some of the pre-treatment techniques reported in the literature has been presented.

Fig. 5: Different pre-treatment techniques for improving the anaerobic digestion of OFMSW 20

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Many times, MSW gets mixed with other types of waste such as yard waste, crop residue and agricultural waste which have high energy content. But, such type of waste mix contains high lignocellulosic biomass, which restricts the microbial activity under anaerobic condition. The methane generation potential of OFMSW varies with the type of pre-treatment techniques applied [83]. Pre-treatment techniques such as, alkaline, acid, steam and biological treatment have been reported to be effective for the enhancement of anaerobic digestion of lignocellulosic biomass [84]. Chemical pre-treatment methods are most widely used as compared to other pretreatment methods, because they are simple, quick and effective in improving the biodegradability of complex organic materials [54]. The most commonly reported technique is alkaline pre-treatment using NaOH solution. Alkaline pre-treatment not only improves the biodegradability of lignocellulosic biomass but also helps in maintaining the pH of anaerobic digester to neutral condition. However, alkaline pre-treatment is found to be more effective at low moisture and ambient temperature. Pang et al. [85] conducted a study, in which corn stover was pre-treated with 6% NaOH, at 80% moisture content for 3 weeks at ambient temperature and found 48.5% increase in biogas yield. Whereas, Zhu et al. [86] reported 37% increase in biogas yield, when corn stover was treated with 5% NaOH at 53% moisture content for shorter pre-treatment time (1 day) at ambient temperature.

Sometimes, two or more pre-treatment techniques are given simultaneously, for e.g., wastes from food industry are sometimes given ultrasonic and microwave pre-treatment after the application of thermal or chemical pre-treatment [87]. Shahriari et al. [88] studied the effect of high temperature microwaves pre-treatment followed by hydrogen peroxide pre-treatment on the performance of anaerobic digestion of OFMSW.

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Journal Pre-proof There are some limitations of pre-treatments. Mata-Álvarez [12] reported that pre-treatment before anaerobic digestion inevitably results in a loss of 15-25% of VS that causes a proportional drop in methane production. It has been reported that some of the pre-treatment techniques induce toxicity to the reactor and its product [12]. For e.g., ozone and chemical pretreatment can create toxic by-products and inhibitory effects on the reactor due to excess release of ozone and cations respectively.

Table 2: Pre-treatment techniques of anaerobic digestion Treatment techniques Chemical (Lime)

Substrate

Observations

Ref.

Corn stover and smooth cordgrass

 Pre-treatment increased the overall biogas yield with high VS reduction.  Lime pre-treatment reduced the lignin content that improved the biodegradability of lignocellulosic substrate (smooth cord-grass).  NaOH dosing (with varying concentration) at different substrate to inoculum (S/I) ratio indicates that pre-treatment alone cannot improve the methane yield.  Appropriate NaOH dose is required at particular S/I ratio for improved methane yield.  In the study, maximum methane yield was observed at 3.5% NaOH loading at S/I ratio of 6.2.  Wheat straw treated with 3 % H2O2 showed 50% increase in methane yield.  The methane yield increased due to availability of increased portion of hemicellulose, cellulose, and lignin components for the digestion.  Pre-treatment using H2O2 at room temperature increased the degradability of maize cob waste and the biogas yield was found to be increased by 46%.  The results showed that pre-treatment using H2O2 at room temperature have maximum sugar solubilisation in comparison to microwave irradiation assisted pre-treatment.  Pre-treatment was carried out at two different temperatures (135 °C and 190 °C).  The methane yield was better at 190 °C than 135 °C, however treatment at 190 °C produced some obstinate soluble COD in the digester.  Different size reduction techniques to increase the effective surface area of the substrate for fermentation by microorganisms were compared.  The study found that the methane yield increased maximum (by 28%) after the bead milling pre-treatment carried out at 1000 rpm.

[89]

Chemical (NaOH)

Fallen leaves

Chemical [Hydrogen Peroxide (H2O2)]

Wheat straw

ThermoMaize cob chemical (H2O2 & microwave irradiation)

Thermal (Steam)

Waste activated sludge

Mechanical Food waste (Milling and grinding)

22

[84]

[66]

[90]

[91]

[26]

Journal Pre-proof Mechanical (Ultrasonic)

Food waste

Mechanical (Ultrasonic)

Cattle manure, food waste and sewage sludge

Biological (Enzymes)

Food waste

Thermal

Kitchen waste, vegetable/fruit waste and waste activated sludge

 Five different systems (one and two stage) were used to compare the methane yield from anaerobic digestion of food waste.  The system in which ultrasonic pre-treatment was given in the first stage showed superior results (in terms of methane yield and solids removal) due to better hydrolysis compared to other systems.  There is a slight increase in specific methane yield and the percentage of methane in the produced biogas of the pretreated samples.  However, the extra energy produced is not compensated by the extra energy required for the pre-treatment.  Different dosage of enzymes were added to the six food waste samples and incubated for 24 h.  At 0.1% (w/w food waste) of enzyme dosage, maximum VS removal was observed, i.e., after further increase in the dosage no further increase in VS removal was observed.  All the substrates were thermally pre-treated at 175 °C for 60 min.  Treatment improved the physical (viscosity and dewatering capacity) and chemical properties (solubilisation of organic compounds) of substrates.  Maximum methane yield (34.8%) was achieved for waste activated sludge, whereas for kitchen and vegetable/fruit waste, the methane yield was relatively low may be due to the low organic contents and melanoidin formation.

6. Commercially available anaerobic digester for the treatment of OFMSW

Several anaerobic digestion technologies have been developed over the years and most of them are successfully operating. The commercially available options can be classified into (i) dry and wet systems on the basis of TS content in the reactor, and (ii) single stage and double stage systems on the basis of number of stages. In dry category (20-40% TS), the three most widely used single stage process reactors are Dranco, Kompogas and Valorga [95]. Bekon is another method developed in the last decade for dry anaerobic digestion using single stage batch reactor [96]. The uniqueness of Bekon method is that, it does not require any complex pre-treatment and all the reaction takes place in a single reactor. In this, the dry organic material is fed into the reactor containing previously digested material which acts as an inoculum. The digestion 23

[92]

[30]

[93]

[94]

Journal Pre-proof occurs at mesophilic temperature and the leachate generated during the process is recirculated to the reactor to maintain the desired moisture and microbial level in the system. Dranco is one of the most widely used dry anaerobic digestion technology. But, Bekon is famous for its higher flexibility, as the production capacity can be expanded due to increase in OFMSW quantity during operating stage [97]. However, investment and operating cost is higher as compared to Dranco. In wet category (<20% TS), the two most commonly reported technologies are Waasa and BTA, mainly used for the treatment of animal manure, sewage sludge and industrial wastes [98]. These technologies (Dranco, Kompogas, Valorga, Bekon, Waasa and BTA) were developed in Europe and have several commercially operating facilities [99]. In general, both the anaerobic digestion processes (dry and wet) are proven to be effective for the treatment of OFMSW. The description of some of the commercially operating technologies has been given in Table 3.

Table 3: Analysis of some of the commercially available anaerobic digesters Process name

Stages

Temperature

Wet/dry

Dranco

Single

Thermophilic

Dry

Total solids content 30-40%

Kompogas

Single

Thermophilic

Dry

23-28%

24

Biogas yield 100-200 m3/tonne of input material

-

Process description In Dranco process, externally mixed feedstock is introduced from the top of the reactor and the digestate is removed from the bottom using auger. Some of the digestate is introduced to the reactor to inoculate fresh feedstock. The reactor does not have any mechanical mixing system, the mixing takes place from natural downward movement of feedstock. Kompogas process has horizontally oriented cylindrical reactor with internal axial rotors for mixing and transferring the feedstock from inlet to outlet. To maintain the TS content and inoculation inside the reactor, digestate and process water were mixed with incoming feedstock. Biogas yield data were not found in the literature.

Ref. [95]

[98]

Journal Pre-proof Valorga

Single

Mesophilic

Dry

25-35%

80-180 m3/tonne of input material

Biocel

Single

Mesophilic

Dry

35-40%

50 m3/tonne of input material

Waasa

Single

Thermophilic/ mesophilic

Wet

10–15%

100-150 m3/tonne of input material

BTA

Double

Mesophilic

Wet

10%

150 m3/tonne of input material

Super blue box recycling (SUBBOR)

Double

Thermophilic

Semidry/dry

15-25%

360 m3/tonne of VS

7. Present research on anaerobic digestion

25

It is a single stage, dry process originally designed for treatment of OFMSW. The whole anaerobic digestion process takes place in a single vertical cylindrical reactor. A unique feature of valorga process is that, it uses pressurised biogas for mixing of substrate, eliminating any need of separate inoculation. Biocel is a high solids batch process. In biocel process, manually sorted MSW is loaded in the reactor without size reduction and screening. Temperature in the reactor is maintained by recirculating the heated leachate. It requires 10 times more space than the continuously fed reactors, but requires 40% less capital investment. Waasa process has a vertical digester which is separated into several zones for pre-digestion. First zone of the reactor is a continuously stirred pre-chamber in which homogenised OFMSW is pumped to inoculate the feedstock and to minimise short-circuiting. BTA system is a two stage process developed for the treatment of OFMSW and mixed MSW. During the first stage, mechanical pretreatment is done to remove the contaminants and solids are reduced to 10% TS using pulper. In the second stage, biological conversion of organics takes place. SUBBOR is a sequential two-stage anaerobic digestion process. In the first stage (pre-digestion), primary recovery of non-digestible components as well as mixing of substrate and inoculum are done to prepare organic rich components. In the second stage, steam disruption is applied to the primary digestate to increase the degradation of substrate.

[100]

[98]

[101]

[99]

[102]

Journal Pre-proof Currently, biogas is mainly used for electricity production using conventional combustion system and as a fuel for heating purpose in kitchen. The advanced technologies such as proton exchange membrane fuel cells and solid oxide fuel cells can be used to generate electricity from biogas with a conversion efficiency of 56%, which is almost 20% higher than the conventional systems [103]. Although, fuel cells are used to generate electricity simultaneously during waste treatment, but it is still not commercialized for electricity generation from OFMSW. Apart from conventional techniques, some other technologies are required to convert raw biogas into electricity [104]. In the coming years, biogas is expected to be used as a transportation fuel [105]. Biogas containing high percentage of methane (~75%) is being used to produce methanol [104]. The European Commission has recommended for conversion of biogas into biomethane for improving the energy efficiency of anaerobic digestion [106]. Another important thing which needs attention is the refinement and purification of the produced biogas in a cost-effective manner [107]. Pre-treatment significantly improves the biodegradability and digestion efficiency of stubborn dry organic material such as agricultural residues and wood (lignocellulosic). But, difficulties in operation, high energy consumption and high cost limit its industrial application [108]. In such cases, other treatment options such as pyrolysis, gasification and incineration are better performing techniques economically and environmentally as well. Alternatively, the accelerant materials such as biochar, magnetite and haematite were found to be effective in enhancement of biogas production [109]. Biochar is also proven to be effective in reducing the inhibition effects due to VFAs and ammonia nitrogen during digestion [110]. Qin et al. [111] studied the effect of magnetic biochar on the performance of anaerobic digestion process of OFMSW. The study found that magnetic biochar significantly improves the methane yield by enriching the methanogens in the anaerobic reactor. Choline, a water soluble nutrient is an important methyl donor, provides a better anaerobic environment for methanogens in the reactor [109]. But, these chemical

26

Journal Pre-proof accelerants may introduce secondary pollutants in the effluent of anaerobic digesters. Therefore, some novel and environmentally friendly accelerants are required which can increase the anaerobic digestion process efficiency with better electron transfer [109]. Zhang et al. [112] developed an integrated biological and thermal waste treatment system to recover energy from waste stream containing both wet and dry organic materials. In their study, anaerobic reactor was used to recover biogas from biodegradable wet organic materials such as food waste and a gasifier was used for the dry organic material such as wood. The produced syngas from gasification process was mixed with biogas to improve the heating value of gas, and the produced heat during gasification was used to maintain the temperature of the anaerobic reactor. Korkakaki et al. [113] reported about an emerging research in which VFAs (an intermediate by-product) of the anaerobic digestion process can be converted into bioplastics. The studies are being done to develop low-cost methods using modern bio-sensors and advanced computational techniques for online monitoring of process instability [114]. Cruz et al. [115] developed a low cost electronic device for online monitoring of anaerobic digestion parameters such as pH, temperature, methane yield, pressure and volume of biogas. The studies related to inhibitory tolerant microbial consortium are being carried out, which can be supplemented to the reactor during VFA and ammonia inhibitions [43]. The digestate comes out of the anaerobic digestion process contains undigested inorganic and organic materials and micronutrients that make it suitable for land application. The digestate can also be used as a raw material for the production of new products such as bio pesticides [116]. However, more research is required for scaling up and improving the reactor design for new value chain products. Also, the digestate is an effective inoculum for other anaerobic digester, but its industrial application is still limited; thus further research is required in this regard [117].

8. Critical observations

27

Journal Pre-proof

Although anaerobic digestion has several advantages over other disposal techniques, but there is still a lot of scope for the process improvement and treatment of the residue generated from the process [118]. The complexity of the anaerobic digestion process and the risk associated for investment in new technologies are the major restrictions for its development across wider applicability. The other major challenge, the researchers are facing is the difference in the results obtained from lab scale studies to full-scale plant. The studies reported in the literature are mostly conducted at laboratory scale with small sample size, while full-scale or pilot studies are still limited [69]. It is often very difficult to replicate the results of laboratory scale studies at larger scale, because controlling of fluctuations in anaerobic digestion reactor at laboratory scale are relatively easier as compared to full-scale plants. Pre-treatments have been reported to be useful in high energy recovery from OFMSW [24]. But, pre-treatment should be avoided for the easily degradable substrates such as food waste, as it would inhibit the whole digestion process due to rapid hydrolysis and excessive VFA formation [119]. Moreover, pre-treatment of substrate before anaerobic digestion process could enhance or worsen the quality of digestate, depending on the method of pre-treatment and type of substrate [119]. Thermochemical pre-treatments can be useful in killing some harmful pathogens, reducing organic contaminants and antibiotic resistance [120]. But, sometimes it can spike the harmful elements and compounds such as furans and polyphenols in the digestate that can adversely affect the soil properties [121]. In order to check this problem, Tigini et al. [122] developed a protocol to evaluate the impact of digestate of pre-treated substrate on human health and environment. Recently, the researchers have advocated about the use of LCA techniques for the assessment of environmental impacts associated with each stage of valorisation chain from pre-treatment to the application of digestate [123], [119]. The LCA related studies are still very

28

Journal Pre-proof limited, especially in the developing countries for different stages of valorisation of OFMSW using anaerobic digestion [43].

As per the literature, anaerobic digestion of OFMSW is generally performed in co-digestion with some other substrates such as animal manure and sewage sludge for better methane yield and process stability. Therefore, availability of co-digestion substrates must be ensured before designing any anaerobic digestion plants. But, Tyagi et al. [124] reported that only 9.7% of the currently operating anaerobic digestion plant involves co-digestion. This may be due to the lack of understanding of importance of co-digestion, lack of technical expertise, poor design and operation and insufficient fund. Previous studies on anaerobic co-digestion mostly focused on two substrates. The studies related to anaerobic digestion of OFMSW with more than two co-digestion substrates are still very limited [125]. The methods reported in the literature attempted to reduce the effects of ammonia and VFA inhibition to the anaerobic digester performance. However, the study related to enhancement of methanogenesis during anaerobic digestion at high ammonia and VFA concentration is still very limited. The digestate coming from the anaerobic reactor as by-products often reported to be a good soil conditioner, but in actual it may contain high dose of minerals, heavy metals, antibiotics, salinity and pathogens which can adversely affect the soil quality [43]. Generally, the food products of developed countries contain high concentration of sodium [29]. The digestate coming from anaerobic digestion of such type of food wastes contain unusually high salinity, which can adversely affect the soil properties, if applied untreated [29]. Additionally, some of the organic contents remain with the digestate at the end of anaerobic digestion process [118]. Anaerobic digestion cannot remove the nutrients, instead it solubilizes them, thus produce digestate that sometimes contains excess nutrients, which needs further treatment [126]. As per the European Waste Framework Directive 2008/98/EC, it is necessary to treat the digestate generated from

29

Journal Pre-proof anaerobic digestion of OFMSW, if it has to be used as a fertilizer [29]. Therefore, proper treatment is required to the digestate before application as a soil conditioner, which will further increase the cost of overall anaerobic digestion process.

9. Conclusion

Anaerobic digestion is considered as one of the most effective techniques to treat the organic fraction of municipal solid waste. But, the system instability due to accumulation of toxic substances during anaerobic digestion process is the major concern reported in the literature. The popularisation of anaerobic digestion technology and the increase in the number of commercial anaerobic digestion plants in future largely depends on the process performance improvement and economic viability. Through this review, it can be concluded that (i) selection of appropriate co-digestion substrate is important, (ii) control of process inhibitions is necessary, (iii) pre-treatment can alleviate poor hydrolysis problem, (iv) understanding of the environmental requirements is essential.

Anaerobic Digestion is a capital intensive waste treatment process, where the revenues are generated from selling the digestate and electricity produced from methane. Further research is required to enhance the methane content in the produced biogas. This will facilitate its use in conjunction with natural gas and in other application such as chemical production from biomethane. More social acceptability and techno-economic sustainability related studies are required for full scale application of anaerobic digestion and thus for making it an efficient waste to energy option globally. The anaerobic digestion process efficiency can be improved by controlling some vital operational parameters. The operational parameters can be categorized into basic and key parameters. The basic parameters include pH, C/N ratio, particle

30

Journal Pre-proof size, and temperature. The control of basic parameters will ensure the optimum microbial activity. While key parameters include the type of co-digestion substrate, type of pre-treatment applied, wet or dry process, and single or multi-stage operation. The key parameters are directly related to the performance of anaerobic digestion. By providing a review of literatures on the process parameters and some other important aspects of anaerobic digestion, this study provides a foundation for further research on how to optimize anaerobic digestion of organic fraction of municipal solid waste for enhanced energy recovery.

Acknowledgements

The authors are thankful to the Department of Environmental Science & Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad for providing necessary facilities for carrying out the research work. The authors are also grateful to the anonymous reviewers for their constructive comments that helped to improve the quality of this paper to some great extent.

Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof Highlights 

Instability of reactor is mainly caused due to variation in process parameters.



Co-digestion is very important in process performance improvement.



Only 9.7% of the operating anaerobic digestion plant involves co-digestion.



Digestate from anaerobic digestion are not recommended for direct application on soil.



Pre-treatment improves digestion efficiency, but increases the cost.