Methanogenic potential and microbial community of anaerobic batch reactors at different ethylamine/sulfate ratios

BIOPROCESS ENGINEERING

Methanogenic potential and microbial community of anaerobic batch reactors at different ethylamine/sulfate ratios

D. V. Vich^I,*; M. L. Garcia^II; M. B. A. Varesche^I

^ILaboratório de Processos Biológicos, (LPB), Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos, (EESC), Phone: + (55) (16) 3373-8358, Fax: + (55) (16) 3373-9550, Universidade de São Paulo, (USP), Engenharia Ambiental-Bloco 4-F, Av. João Dagnone 1100, Santa Angelina, CEP: 13.563-120, C. P. 359, São Carlos - SP, Brasil. E-mail: [e-mail], ^*[e-mail]

^IIDepartamento de Petrologia e Metalogenia, Universidade Estadual Paulista Julio de Mesquita Filho, (UNESP), Phone: + (55) (19) 3526-2829, Avenida 24-A, 1515, Bela Vista, CEP: 13506-900, C. P. 178, Rio Claro - SP, Brasil. E-mail: [e-mail]

ABSTRACT

Methylamine and sulfate are compounds commonly found in wastewaters. This study aimed to determine the methanogenic potential of anaerobic reactors containing these compounds and to correlate it with their microbial communities. Batch experiments were performed at different methylamine/sulfate ratios of 0.71, 1.26 and 2.18 (with respect to mass concentration). Control and experimental runs were inoculated with fragmented granular sludge. The maximum specific methane formation rates were approximately 2.3 mmol CH₄ L^-1 g TVS^-1 day^-1 for all conditions containing methylamine, regardless of sulfate addition. At the end of the experiment, total ammonium-N and methane formation were proportional to the initial concentrations of methylamine. In the presence of methylamine and sulfate, Firmicutes (46%), Deferribacteres (13%) and Proteobacteria (12%) were the predominant phyla of the Bacteria domain, while Spirochaetes (40%), Deferribacteres (17%) and Bacteroidetes (16%) predominated in the presence of methylamine only. There was no competition for methylamine between sulfate-reducing bacteria and methanogenic archaea.

Keywords: Anaerobic process; Methylamine; Sulfate reduction; Methane formation; Microbial community.

INTRODUCTION

Methylamine is an organic compound with a molecular weight of 31.1 g.mol^-1, boiling point of -6,32ºC (760 mmHg) and characteristic unpleasant odor at concentrations up to 100 ppm. It is introduced into the environment through several industrialized processes. It is mainly used as an intermediate in organic synthesis, water gel explosives, accelerators, pharmaceuticals, insecticides, herbicides, plasticizers, N-methylpyrrolidine, methylalkanolamines, surface active agents, and fungicides; in tanning; as a component of photographic developers and paint removers; as a fuel additive; in the production of dyes; as a polymerization inhibitor; and as a rocket propellant (Yaws, 2001). Complex methylamine-derived organic molecules and others such as animal proteins can be broken down biologically to give methylamine as an end product, either by aerobic microorganisms in the presence of external electron acceptors or mainly by anaerobic microorganisms (Thauer, 1998). Hence, methylamine is a key compound to be studied, representing a variety of chemical classes.

Methanogenesis from methylamine yields carbon dioxide (CO₂), methane (CH₄) and ammonia (NH₃). In pure cultures, the ratio of product formation is 1:3 for CO₂ and CH₄, respectively, as indicated by reaction 1. Unionized free ammonia is also produced in the conversion of methylamine to methane (Sossa et al., 2004), which can be an important element when representing mass balances of the anaerobic process.

4CH₃NH₂ + 2H₂O → 3CH₄ + 1CO₂ + 4NH₃ (Reaction 1)

Methylotrophy, the metabolism of microorganisms that consume organic compounds containing no carbon-carbon bonds (C1 compounds), such as methane, methanol and methylated amines, is an important part of the global carbon cycle on Earth (Kalyuzhnaya et al., 2008). With respect to the Bacteria domain, methylotrophic ability is especially widespread within the Proteobacteria phylum, encompassing the alpha, beta and gamma subdivisions (Hanson and Hanson, 1996, Kalyuzhnaya et al., 2006). Within the methanogenic archaea, the order Methanosarcinales contains species with the most versatile substrate spectrum, including microorganisms that are able to grow on methylotrophic substrates such as methanol or methylamines as their sole carbon and energy sources (Deppenmeier et al., 1999).

To our knowledge, there are few studies of methylamine removal through biological processes. Because methylamine removal efficiencies can be extremely high and energy recovery can be practiced, anaerobic digestion can be an advantageous alternative. The process stability might be negatively affected in the presence of sulfate, which is often found in wastewaters. This work aimed to determine the influence of sulfate at different concentrations on the specific methane production rate and the microbial community structure of anaerobic batch reactors containing methylamine.

MATERIALS AND METHODS

Granular sludge from an Upflow Anaerobic Sludge Blanket (UASB) reactor treating poultry wastes (Dacar, Tietê, SP, Brazil) was used as an inoculum in batch reactors containing [mg L^-1]: NH₄Cl [500], MgCl₂^.6H₂O [400], KH₂PO₄ [100], CaCl₂^.2H₂O [50], Tritriplex [45], FeSO₄^.7H₂O [5.56], MnSO₄^.H₂O [0.86], CoCl₂^.6H₂O [1.7], ZnSO₄^.7H₂O [2.1], H₃BO₃ [1.9], NiCl₂ [0.2], Na₂MoO₄ [0.1], NaHCO₃ [1000], biotin [0.02], folic acid [0.02], thiamine HCl [0.05], riboflavin [0.05], niacin [0.05], calcium pantothenate [0.05], pyridoxine HCl [0.1], vitamin B₁₂ [0.001], Na₂S^.9H₂O [500] and methylamine [1550] (Touzel and Albagnac, 1983; Zinder et al., 1984). The reactors were mounted in 2000 mL bottles, with 1000 mL of reaction volume and 1000 mL of headspace, which was initially filled with helium gas (He, 100%). The experiments were performed in triplicate. Control conditions were set up as follows: I. no sulfate and no methylamine; II. no methylamine and sulfate addition; III. no sulfate and methylamine addition. Methylamine/sulfate experiments (IV) were designed with the following mass concentration ratios: 0.71 (a), 1.26 (b) and 2.18 (c) (Table 1).

Before inoculating the reactors, the sludge was previously washed, fragmented and kept under nitrogen (N₂, 100%) flow. Throughout the experiments, the gas pressure in the headspace of the experimental bottles was not released. Incubation was performed by using a shaker at a stirring speed of 150 rpm and a heater at the temperature of 30±1ºC.

The composition of the biogas generated through anaerobic degradation of methylamine was monitored by gas chromatography using a Gow-Mac® chromatograph with a thermal conductivity detector (TCD) and Porapak® Q (2m×1/4" – 80 a 100 mesh) column. The biogas samples were collected in a glass syringe. Internal pressure was not equalized to atmospheric pressure to obtain total mass values of formed methane (headspace pressures were unknown; concentration of methane was determined, not the percentage of methane in the biogas). The point of maximum methane production rate was obtained by fitting a sigmoidal Boltzmann function to the experimental data (concentration of CH₄ as a function of time). The peak was then determined by numerical differentiation of the adjusted equation. The fitting procedure was carried out by using the software Microcal® Origin 6.1 (Levenberg-Marquardt and Savitzky-Golay smoothing algorithm). The maximum specific methane production rate was determined from the average mass of biomass in each reactor (g of total volatile solids – TVS).

Total ammonium-N (NH₄⁺-N), sulfate (SO₄^2-), sulfide (S^2-) and TVS concentrations were quantified according to the Standard Methods for the Examination of Water and Wastewater (2005).

The microbial community present in the anaerobic batch reactors was analyzed by partial sequencing of the 16 subunit ribosomal RNA gene (16S rRNA). The total genomic DNA was extracted according to Melvin and Hobson (1994). For the Polymerase Chain Reaction (PCR), universal primers for Bacteria domain 27F and 1100R were used (Lane, 1991). The PCR products were purified with Illustra® GFX® kit (GE Healthcare®). Cloning was performed with pGEM® Easy Vector System I, according to the manufacturer's specifications. Samples were sequenced in the automatic sequencer ABI Prism® 310 (Applied Biosystem®). The sequences obtained were verified with the software DNAstar® (Lasergene® Sequence Analysis), aligned within the Greengenes NAST aligner (http://greengenes.lbl.gov/cgi-bin/nph-NAST_align.cgi) and phylogenetically analyzed with the software ARB (Ludwig et al., 2004).

RESULTS AND DISCUSSION

With respect to the control experiments, the methane concentrations were higher in the methylamine- and sulfate-free tests (~6 mmol CH₄ L^-1; Figure 1(a)) compared to the methylamine-free and sulfate-containing reactors (~2 mmol CH₄ L^-1; Figure 1(b)). This was anticipated because it is well established that sulfate, when present in anaerobic media, creates a competition for substrate by acting as the final electron acceptor in the anaerobic respiration reaction.

The highest headspace concentrations of CH₄ and CO₂ were of the same order of magnitude in the reactors that received methylamine as carbon source, regardless of the presence of sulfate and its concentration. The concentrations of CH₄ and CO₂ were approximately 36 and 8 mmol L^-1, respectively (Figure 2 (a), 2 (b), 2 (c) and 2 (d)) in the reactors containing methylamine (with or without sulfate). Unlike the control cases (Figures 1 (a) and 1 (b)), the addition of sulfate in the process did not affect the highest produced methane concentration when methylamine was used as the sole carbon source. This indicates that methane-producing organisms and sulfate-utilizing organisms do not compete for methylamine, which is quite interesting because it makes anaerobic digestion of methylamine even more robust.

The maximum methane production rates were 13.16±0.61, 13.41±1.80, 11.65±0.44 and 14.24±0.028 mmol CH₄ L^-1 day^-1 for the control (with methylamine and without sulfate) and for methylamine/sulfate ratios of 0.71, 1.26 and 2.18, respectively (Table 2). For these ratios, maximum specific methane production rates were 2.14±0.31, 2.41±0.27, 2.26±0.25 and 2.54±0.10 mmol CH₄ L^-1 g TVS ^-1 day ^-1, respectively (Table 2). These results were not statistically significantly different from each other (ANOVA: p = 0.46, F = 0.98; Fcrit = 4.76).

The pH decreased significantly during the tests at the different methylamines/sulfate ratios (Table 3). Due to the addition of methylamine (electron donor), the CO₂ concentrations, ~8 mmol L^-1 (Figures 2 (a), 2 (b), 2 (c) and 2 (d)), were higher than the CO₂ concentrations in the control, ~2 mmol L^-1 (without addition of methylamine) (Figures 1 (a) and 1 (b)). The accumulation of CO₂ in the tests was responsible for the further decrease of pH. Because the production of biogas in the control reactor was lower, the final pH value was not as low as the values obtained in the other reactors (Table 3). The addition of sulfate contributed to the maintenance of pH, since it is biologically converted to S^-2 and, consequently, other forms such as HS^- and H₂S are formed. A simplified analysis, considering only sulfide aquatic chemistry, can be made to predict the theoretical pH in the reactors containing methylamine and sulfate (data: TOT-S^-2=[H₂S]+[HS^-] +[S^2-]; pK_a1 = 6.99 and pK_a2 = 12.92; H_H2S = 0.4 L/L; TOT-S^-2 = 0.00469 mol L^-1 [Figure 3 (b)]; [H₂S]_aq = 0.0025 mol L^-1 [Figure 2 (b), 2 (c), and 2 (d)]). The theoretical pH was found to be 6.94, while the measured pH was ~7.14 (Table 3). Even though the theoretical and measured pH values are similar, the biological phenomena involved in determining the final measured pH are too complex. If we consider CO₂ formation and its aqueous chemistry, the theoretical pH could be even lower than the predicted one. This is balanced out by the added buffer compounds and others, such as ammonia, that are produced during the biological reactions.

Total ammonium-N (Figure 4) and sulfide (Figure 3 (b)) were produced during the experiment. The final concentration of total ammonium-N was around 1100 mg NH₄⁺-N L^-1 for all methylamine/sulfate ratios, substantiating the process represented by reaction 1. Control reactors (without methylamine as a source of C and N) had a concentration of ammonium of ~300 mg NH₄⁺-N L^-1.

Sulfide concentrations values were about 150 mg L^-1 in the reactors subjected to different sulfate concentrations, regardless of the methylamine/sulfate ratio. Control reactors (without methylamine and with sulfate) achieved an average sulfide concentration of 241 mg L^-1 (Figure 3 (b)). According to these results, it is possible to infer that methanogenesis was favored by the addition of methylamine and that a negative effect on sulfidogenesis occurred, possibly due to methylamine inhibition of sulfate-consuming microorganisms and/or scavenging of free protons (H⁺) to form ammonium and methane by acetoclastic methanogens.

The control reactors with sulfate (without methylamine) consumed 71.9% of the added sulfate (1269 mg L^-1). The reactors with methylamine/sulfate ratios of 0.71, 1.26 and 2.18 consumed 41.3%, 61.6% and 83.2% of all the sulfate added, for sulfate concentrations of 2183, 1230 and 711 mg L^-1, respectively (Figure 3 (a)). These percentages reveal greater efficiencies of consumption of sulfate when it is present at lower concentrations.

The microscopic analysis confirmed the similarity of the morphologies that are expected to be present in the reactors containing methylamine (Vich, 2006). Microorganisms similar to those belonging to the Methanosarcinales family were the representative ones for the Archaea domain (Figure 5(c)). The Bacteria domain was represented by rods and cocci-rods (Figure 5 (b)), cocci (Figure 5(a)), and filaments (Figure 5 (d)).

For the control reactors fed with methylamine and without addition of sulfate, the microbial community structure related to the Bacteria domain was mainly represented by the phyla Spirochaetes (40%), Deferribacteres (17%) and Bacteroidetes (16%) (Figure 6 (a)). The phyla Firmicutes (46%), Deferribacteres (13%) and Proteobacteria (12%) were predominant in the reactor containing methylamine and sulfate at the ratio of 1.26 (Figure 6 (b)). Despite this shift of populations in the bacterial community that resulted from the addition of sulfate, methane production did not change in the tests with addition of sulfate. It should be pointed out that the phylum Proteobacteria was enriched by sulfate addition (from 5 to 12%), which is in agreement with the literature (Hanson and Hanson, 1996; Kalyuzhnaya et al., 2006); the Proteobacteria phylum contains species capable of converting methylotrophic substrates.

Sarti (2007) analysed the granular sludge used in our work by the Fluorescent in situ Hybridization (FISH) technique. The sludge was sampled from an Upflow Anaerobic Sludge Blanket (UASB) reactor treating poultry wastes (Dacar, Tietê, SP, Brazil), which contained sulfate (there is no information on the presence of methylamine in this digester). The microbial population distribution within a biomass sample was 49.8%, 56.7% and 27.9% for the FISH probes EUB338 (Bacteria), ARC915 (Archaea) and BRS385 (sulfate-consuming bacteria), respectively. These percentages represent a sludge with high methanogenic potential and it might have facilitated methylamine removal from the media, as the methane production seems to be driven predominantly by methanogenic activity because: i. the bacterial community changed, but the methane production did not; ii. total ammonium-N and methane formation obeyed methanogenic stoichiometric levels (reaction 1). For instance, the theoretical methane and total ammonium-N concentrations formed from methylamine consumption at a concentration of 1550 mg L^-1 (1-L reaction and headspace volumes) are approximately 37 mmol L^-1 and 700 mg N L^-1, respectively. We found methane and total ammonium-N production values of ~36 mmol/L (Figures 2 (a), 2 (b), 2 (c), and 2 (d)) and ~800 mg N L^-1 (Figure 4) (difference between experimental conditions II and IV [(a), (b), and (c),]), respectively. If methylamine-consuming bacterial activity were high, which might have been the case if a much longer adaptation time had been allowed, it is likely that methane formation would decrease with increasing sulfate concentrations.

CONCLUSIONS

The addition of sulfate inhibited the production of methane only in the control reactors without addition of methylamine. In the presence of methylamine, the addition of different concentrations of sulfate did not influence the production of methane, even though the bacterial community structures were altered, indicating that methanogenic archaea and sulfate-reducing bacteria did not compete for this source of carbon and that bacterial methylamine degradation does not occur to a significant extent.

(Submitted: April 9, 2010; Revised: June 13, 2010 ; Accepted: June 29, 2010)

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To whom correspondence should be addressed

This is an extended version of the manuscript presented at the SINAFERM 2009 - 17th National Symposium on Bioprocesses 2009, Natal, Brazil.

Publication Dates

• Publication in this collection
  15 Mar 2011
• Date of issue
  Mar 2011

History

• Reviewed
  13 June 2010
• Received
  09 Apr 2010
• Accepted
  29 June 2010