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Bioenergy generation from municipal solid waste and glycerin waste: Population dynamics Rosario Solera, Diego Sales, Jose Luis Garcia-Morales, and Soraya Zahedi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01526 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017
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Bioenergy generation from municipal solid waste and glycerin waste:
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Population dynamics
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Zahedi, S*; García-Morales, J.L.; Sales, D.; Solera, R*
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Department of Environmental Technologies. University of Cadiz. Faculty of Marine and Environmental Sciences (CASEM), Pol. Río San Pedro s/n, 11510 Puerto Real (Cádiz), Spain. (
[email protected]/
[email protected];
[email protected])
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Abstract
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This paper studies the microbial community, effluent characteristics and bioenergy
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generation (hydrogen and methane production) in a two-phase dry-thermophilic (55ºC)
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anaerobic co-digestion process treating actual municipal solid waste (20 % solid
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content) and biodiesel waste (glycerin waste, 1 % v/v). Four different hydraulic
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retention times (from 11.5 d to 4.5 d) and four organic loading rates (from 8.2 g to 21.1
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g VS/l/d) were studied to identify optimal conditions (maximum values of biogas and
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microbial activity). Optimal conditions (2.6 ± 0.3 l H2/l/d; 3.4 ± 0.3 l CH4/l/d and 109±
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9 x 10-13 l CH4/cell/d) were obtained at 5.9 d HRT. Fermentation end products yield
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propionic acid as the major product in the secondary effluent. The average values of the
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ratios of Eubacteria:Archaea and Acetogens:hydrolytic-acidogenic bacteria were
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respectively 83:17 and 30:53 in the first phase (1.5 d HRT) and 76:24 and 39:37 in the
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second phase (4.4 d HRT).
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Keywords: hydrogen, methane, glycerol, microbial activity, population dynamic.
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1. Introduction
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Anaerobic digestion (AD) is a widely used process for degrading and stabilizing
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municipal solid waste (MSW) due to its environmental and economic benefits.
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According to a recent study 1, electricity generation from biogas in OECD countries
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grew from 3.7 TWh in 1990 to 78.8 TWh in 2015, making it the third fastest-growing
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renewable electricity source after wind and solar energy. Due to the advantages of AD,
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many research studies have sought to optimize the AD of MSW, including the
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interesting option of the co-digestion process, which increases the load of biodegradable
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organic matter and produces a higher biogas yield 2–5. Studies on AD of MSW have
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shown that the C:N ratio of this waste presents average values of 10:1, below the
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optimum for anaerobic digestion (25:1) 6, while hydrogen production (HP) and methane
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production (MP) is reduced due to the washout of microorganisms, not to overloading 7–
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10
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addition of readily biodegradable organic substances, such as glycerol, a major by-
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product of biodiesel production, could constitute an ideal strategy 4,11–13. Biodiesel
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manufacturing worldwide has gained in importance due to several factors: (i) the
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unavailability of fossil fuels due to demographics and political instability; (ii) modern
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methods of biodiesel production and new catalyst formulations producing higher
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biodiesel yields; (iii) the breeding and cultivation of new varieties of oil crops with
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higher lipid yields; (iv) an increase in the cultivation of inedible oilseed plants on waste
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land; (v) new engine designs that can use biodiesel and its admixtures as fuel; and (vi)
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stringent regulations to reduce GHG emissions 14–16. Producing 100 kg of biodiesel
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yields approximately 10 kg of glycerin waste (GW) as a co-product. Numerous sectors,
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such as the pharmaceutical, cosmetics, and food processing industries, use refined
. Therefore, an increase in the loading rate employed in the AD process via the
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glycerol as a raw input material. However, the GW generated as a co-product of
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biodiesel production requires purification before being suitable for use in these
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industries. The problem is that the high costs of purifying glycerol from impure glycerol
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has reduced its demand in the market, in addition to the reduction in the cost of
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petroleum-derived glycerol to less than $20 per barrel 14,17. Thus, GW is often
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considered a waste stream instead of a co-product 18, which makes its disposal a
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fundamental environmental concern.
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Several strategies have been reported in the literature to maximize energy recovery and
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generate value added products from this type of waste. These options include direct
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combustion and gasification and AD of waste glycerol to produce biogas (hydrogen or
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methane), as well as fermentation of glycerol to produce methanol, ethanol, citric acid,
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1,3- propanediol, polyhydroxyalkanoates (PHA), and the like 13,14,17,19–21. This paper
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focuses on the joint AD of waste glycerol together with MSW to obtain both hydrogen
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and methane. Recent studies have demonstrated the effectiveness of two-phase dry-
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thermophilic AD of actual MSW to produce HP and MP 8, as well as that of GW
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supplementation (1% v/v) to improve the hydrogen production steps in thermophilic-dry
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dark fermentation of actual MSW in batch mode12. However, no previous studies have
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been published on the effect of GW addition on two-phase dry-thermophilic anaerobic
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digestion of MSW under semi-continuous feeding to produce hydrogen and methane or
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on the effect on the different microbial groups involved in the digestion process.
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This study aims to: (1) establish the optimal conditions (organic loading rate (OLR) and
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hydraulic retention time (HRT)) to maximize gas production (GP), including H2 and
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CH4; and (2) investigate the population dynamics in dry-thermophilic anaerobic co-
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digestion of MSW and GW (1% v/v). No previous studies have been published in this
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respect. With these aims in mind, the effect of four different OLRs (from 8.2 g to 21.1 g
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VS/l/d) and HRTs (from 11.5 d to 4.5 d) were tested. The effect of the varying operating
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parameters (HRT or OLR) on soluble chemical oxygen demand (SCOD), volatile fatty
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acids (VFA), volatile solids (VS), hydrogen production (HP), methane production (MP),
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GP, sulphide production (SP), specific HP (SHP), specific MP (SMP), microbial
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population and microbial activities was studied at laboratory scale. Fluorescent in situ
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hybridization (FISH) was used to determine the main groups involved in the anaerobic
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process.
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2. Methods
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2.1 Inoculum, substrate and feeding
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The seed used as the inoculum for the acidogenic and methanogenic reactors was
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collected from an H2-producing and a CH4-producing reactor, respectively. The VS in
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the acidogenic and methanogenic inoculums were 49 g/ kg and 13 g/kg, respectively.
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The tested substrate in the first phase was a mixture of MSW and GW (1 % v/v). The
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chosen supplementation in this trial was 1% GW, as this concentration had been shown
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to be highly effective in improving hydrogen production in a previous study12. The
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MSW and GW used were collected from an industrial trommel (30 mm) located at Las
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Calandrias MSW treatment plant (composting plant) (Jerez de la Frontera, Cadiz, Spain)
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and the Abengoa Bioenergy biofuel company (San Roque, Cádiz-Spain), respectively.
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The MSW was stored in 25 kg drums at -4ºC to avoid anaerobic degradation by the
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microorganisms found in the solid waste itself 10. The TS concentration of the feed for
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the first reactor was adjusted to 20% (which is characteristic of dry AD) by adding tap
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water. Subsequently, 1% (v/v) GW was added to the feed. The characterization of the
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substrate used (mixture of MSW and GW) in the tests is shown in Table 1.
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The tested substrate in the second phase was the effluent from the first phase.
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In the first reactor, NaOH 10 M was added to the substrate when the pH of the effluent
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fell below 5.3. In the second reactor, the pH was not controlled.
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2.2 Experimental equipment and operating conditions
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Two laboratory-scale continuously stirred tank reactors (CSTRs) were employed in this
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study. The first reactor, dedicated to HP (first phase), had a working volume of 5.5 l,
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while the second reactor (second phase), dedicated to MP, had a working volume of 5 l.
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Both were heated by recirculating water through a thermostatic jacket. PRECISTERM
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6000142/6000389 (SELECTA S.A.) baths, with a maximum capacity of 7 litres of
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water, were used for this purpose. The lids of the stainless steel reactors have a diameter
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of 200 mm and contain three openings, one for the biogas outlet, a feed inlet and
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another opening for the stirring system. The bottoms of the reactors have a discharge
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valve with a 40 mm i.d., used for sampling. The biogas was collected in 40 litre Tedlar
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(a polyvinyl fluoride plastic polymer) bags measuring 29.8 cm wide and 45.7 cm long.
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The stirring systems consisted of an IKA EUROSTAR Power Control visc-P4 overhead
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stirrer coupled to a stainless steel blade with scrapers that allows homogenization of the
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waste at a speed of 23 rpm. In CSTRs without recycling of solids, the solids retention
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time (SRT) and HRT were the same.
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As regards the feeding regime, each reactor was fed once a day (semi-continuous
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mode).
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Four different OLRs or runs ranging between 8.2 and 21.1 g VS/l/d were tested. The
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HRT in the acidogenic phase was 1.5 d (OLR= 63 1 g VS/l/d), as this was the optimal
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value previously detected in an acidogenic reactor fed once a day 8. Steady-state
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conditions lasting for at least three consecutive HRTs were clearly observed for all
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experimental runs (except for Run IV, as destabilization was observed). The operating
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conditions are shown in Table 2. The entire experiment lasted 131 d overall. The study
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was considered concluded when increasing the OLR (decreasing the HRT) resulted in a
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decrease in MP.
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2.3 Analytical methods
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The analytical determinations performed in this study can be grouped into two
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categories: physical-chemical analysis and microbiological analysis.
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2.3.1 Physical-chemical analysis
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The following control parameters were determined for the reactors: SCOD, alkalinity,
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ammonium, VFA and volume and composition of the biogas (H2, CH4, CO2 and H2S).
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These determinations were performed according to APHA22 and Zahedi 8,12.
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VS, SCOD and VFA percentage removal rates were calculated according to the
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following equations:
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(VSsubstrate – VSsecond phase)*100/ VSsubstrate
(1)
(SCODfirst phase – SCOD second phase)*100/SCOD first phase
(2)
(VFA first phase – VFA second phase)*100/ VFA first phase
(3)
The volume of gas produced in the reactor was measured directly using a high-precision Ritter drum-type gas meter TG-01-Series (Wet-Test). Biogas composition was determined by gas chromatography separation (SHIMADZU GC-2010). H2, CH4, CO2, O2 and N2 were analysed by means of a thermal conductivity detector (TCD) employing a Supelco Carboxen 1010 Plot column. A Supelco Supel-Q Plot column and a flame photometric detector (FPD) were used to determine H2S. Samples were taken using a 1 ml Dynatech Gastight gas syringe under the following operating conditions: split = 100;
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constant pressure in the injection port (70 kPa); 2 min at 40 ºC; ramped at 40 ºC/min until 200 ºC; 1.5 min at 200 ºC; detector temperature: 250 ºC; and injector temperature: 200 ºC. Helium was used as carrier gas (266.2 ml/min). Commercial mixtures of H2, CH4, CO2, O2, N2 and H2S (Abelló Linde S.A.) were used to calibrate the system.
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2.3.2 Microbiological analysis
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FISH was used to count the microorganisms contained in the substrate and reactors.
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These determinations were performed according to previous studies by our research
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group 8,12. The cellular concentration and percentages of Eubacteria, Archaea, butyrate
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utilizing acetogens (BUA), propionate utilizing acetogens (PUA), hydrogen utilizing
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methanogens (HUM) and acetate utilizing methanogens (AUM) were obtained by FISH.
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The total population was calculated as the sum of the relative amounts of Eubacteria
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and Archaea. Acetogens were calculated as the sum of the relative amounts of PUA and
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BUA. HAB were calculated as the difference in the relative amounts of Eubacteria and
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acetogens 8,12.
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Microbiological analyses were performed in triplicate under steady-state conditions
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(except at 3 d HRT in the second phase (Run IV); this condition was not analysed
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because of the observed destabilization).
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Methanogenic activity was considered to evaluate the effect of biochemical activity on
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the OLR. This activity was calculated as the ratio of the volume of CH4 generated and
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the number of Archaea determined inside the reactor by FISH staining 7.
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3. Results and discussion
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This section studies the effect of varying the operating parameters (HRT/OLR) on
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SCOD, VFA, VS, GP and the microbial population in two-phase dry-thermophilic AD
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of MSW and GW. All the values correspond to analytical determinations under steady-
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state conditions.
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3.1 Process stability
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pH was the parameter chosen to demonstrate the stability 8,23 of the process (Fig.1). In
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the first phase, the HRT was maintained at 1.5 d, the pH logically remaining
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approximately constant throughout the entire experiment (from Run I to Run V),
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presenting average values of 5.5 ± 0.4. These pH values were optimal for enhanced
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HAB activity 24,25and it committed to the growth of methanogen and acetogenic
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bacteria. In the second phase, steady-state operation was observed in Runs I, II and III
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with average values of 7.4 ± 0.2, 7.5 ± 0.1 and 7.6 ± 0.1, respectively. These values
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ranged between desirable values of methanogen and acetogenic populations. The
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sudden decrease in pH in Run IV (3 d HRT or 18.7 g VS/l/d OLR) revealed
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destabilization of the system, as well as a decrease in MP (Fig.2), methanogenic activity
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and organic matter removal, as well as an increase in VFA.
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3.2 Leachate quality
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In the first phase, alkalinity and ammonia remained approximately constant in all the
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tested runs, with average values of 6 ± 2 g CaCO3/l and 0.9 ± 0.1 g NH3-N/Kg,
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respectively. These values were in line with a former study 8 employing a similar HRT
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(1.5 d). Surprisingly, VFA values (18 ± 4 g acetic acid/l) were much higher in the
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present study (double). The higher amount of acid produced was not due to glycerol
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addition, however, as the same values were detected in this phase before
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supplementation with glycerol (data not shown). Furthermore, it has been established
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that the effect of adding GW on hydrogen production from industrial municipal solid
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waste produces an increase in SCOD, though not in VFA 12. These results (higher VFA
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values) may be explained by the fact that this reactor was maintained for more than 1
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year at 1.5 days. Therefore, it is likely that the adequate dynamics (“flexibility”) of the
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microbial community structure had been established and thus very young and activated
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HAB had been able to increase their acidogenic activity and hence the VFA produced
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from dark fermentation of MSW. The dominant fermentation products were butyric acid
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and acetic acid, ranging from 70-80% and 8-16%, in line with previous studies 9,10,12,26.
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SCOD values were clearly higher (more than 30%) than those measured before GW
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addition (data not shown) and in previous studies 8–10. This was due to the fact that 1%
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GW was added to the substrate 12.
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In the second phase, alkalinity ranged from 6-11 g CaCO3/l for all tested runs. These
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values were clearly higher than those obtained when GW was not added (around 4 g
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CaCO3/l) 8, in line with the higher VFA content (between 4 and 6 g/l). As to VFA
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consumption under steady-state conditions, this was around 70% (Fig. 3). Butyric acid
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was the main VFA consumed. Butyric and acetic acid removal rates ranged between 90-
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99% and 60-92%, respectively. These results are in keeping with others studies on two-
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phase AD of solid waste 8,26–28. The amounts of propionic acid produced in all the tested
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runs were high (3-4 g/l), being higher than previous values obtained during AD of
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MSW when GW was not added 8. However, this fact did not produce an inhibitory
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effect (pH and MP remained stable). Furthermore, numerous researchers have shown no
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inhibitory effect of high propionic acid levels on AD of organic matter 29,30. The reason
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that high values of propionic acid accumulate in the system is twofold. The first reason
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is related to the high and constant input of readily biodegradable substrate (butyrate and
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acetate). Öztürk 31 has shown that propionate is converted to acetate only after butyrate
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and acetate has completely degraded. In studies carried out by Zahedi 8 (in which no
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GW was added and high levels of propionic acid were not detected), the soluble OLR in
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the second phase, employing 6.6 and 4.4 d HRT were 5.5 and 8.2 g SCOD/l/d,
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respectively, whereas in the present study (in which GW was added to the MSW and
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high levels of propionic acid were detected), operating at 6.6 and 4.4 d HRT, these
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values were 10.0 and 14.9 g SCOD/l/d, respectively. The second reason is that
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fermentation end products from glycerol yield propionic acid as the major product and
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this acid accumulates in the system 32,33. The non-toxicity of the propionic acid thus
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generated and the high consumption of butyric acid could indicate that PUA and BUA
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levels were sufficient to ensure the performance of this anaerobic process under steady-
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state conditions (except in Run IV, employing a 3 d HRT in the second phase (OLR =
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18.7 g VS/l/d)). However, higher PUA activity could be desirable to reduce the
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accumulation of propionic acid in the effluent of the second phase resulting from the
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addition of GW. In Run IV, VFA accumulated in the second phase, reaching maximum
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VFA, acetic, propionic and butyric acid values of 20.5 g acetic acid/l, 3.7 g acetic acid/l,
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4.3 g propionic acid/l and 5.3 g butyric acid/l, respectively. These results are in line with
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those obtained in the substrate (acidogenic effluent). The accelerated increase in VFA
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concentration in this digester and the decrease in CH4 content and pH suggest that
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methanogen inhibition occurred at 22.4 g SCOD/l/d OLR.
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As regards ammonium, all results were between 775-930 mg NH3-N/kg. These values
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are lower than those reported by authors as causing inhibition of the dry anaerobic
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process when treating bio-wastes such as those used in this study 34–37. Moreover, the
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source of inocula used in the present study were dry-thermophilic anaerobic reactors
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with a medium-high ammonium content, so the inocula are expected to acclimate to
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medium-high levels of acid, allowing the digesters to operate at certain concentrations
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of acid without jeopardizing their safety 13.
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Organic matter removal rates (%) in each steady-state run are shown are shown in Fig.
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3.
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Decomposition of VS was in the 79-90% range. Similar values were found in previous
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studies in which glycerol was not used 8, in keeping with those obtained in the
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anaerobic digestion of organic waste 38 and the co-digestion of mixtures of
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slaughterhouse waste with MSW 39 and vegetable market waste with MSW 40.
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Decomposition of SCOD was in the 65–73% range. These results are in line with those
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obtained by Ueno 41 (79%) in two-phase thermophilic AD of organic waste for an HRT
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between 4.3-6.8 d (OLR between 12.4-16.6 g SCOD/l/d). The highest VS and SCOD
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removal rates were obtained in Run I, corresponding to an 11.5 d HRT (1.5 d in the first
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phase and 10 d in the second phase). Destabilization in the second phase, at 3 d HRT,
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resulted in an accumulation of organic matter (organic removal rate lower than
