Ref # S2SMALL-53711 V.T. Mota1*, M. Zaiat1
1University of São Paulo, Department of Hydraulics and Sanitary Engineering, Biological Processes Laboratory. Avenida João Dagnone, 1100. São Carlos, São Paulo, Brazil. CEP:
13563-120
* Corresponding Author: vtaina@hotmail.com
Abstract: In this study, two and single-stage anaerobic reactors were assessed for treatment performance and bioenergy production from sucrose-based wastewater (5 gCOD L-1), at 30°C. In the two-stage system, a hydrogen-producing UASB reactor (HR) was used in the acidogenic phase. The methanogenic reactor (MR) of the two-stage system, and the single-stage methanogenic reactor (SMR) of the single-stage system consisted of structured fixed-bed reactors filled with polyurethane foam. The two-stage system showed superior performance, evidenced by less organic acids, chemical organic demand (COD) and volatile suspended solids (VSS) levels in the effluent, and higher biogas yields. At the end of operation, the OLR applied to the two- and single-stage systems were 6.4 and 5.2 gCOD L-1d-1, respectively. Under these conditions, effluent soluble COD levels and effluent VSS concentrations were, respectively, 165 and 92 mg L-1in the two-stage system, and 256 and 244 mg L-1 in the single-stage system. The energy yield of the two-stage system was 20,688 kJ g-1CODadded, which was 34% higher than the yield of the single-stage system, that was 15,485 kJ g-1CODadded.
Keywords: fixed-bed reactor, hydrogen; methane; two-stage anaerobic treatment, UASB, suspended solids.
INTRODUCTION
A two-stage anaerobic system consists of two reactors placed in series, wherein acidogenesis and methanogenesis should prevail in the first and second reactor, respectively, as a result of selective pressure. Despite its several advantages – greater efficiency of removing pollutants, reduction of toxic inhibition on methanogens (Ke et al. 2005), higher tolerance to shock loads (Cohen et al. 1982), and enrichment of methane content in the biogas (Yeoh, 1997) – about only 10% of installed capacity of anaerobic reactors in Europe is through two-stage anaerobic digestion (Rapport et al. 2008). As pointed out by these authors, the increasing of commercial application of this process is expected to growth as the higher complexity and construction costs are overcome by the improvements in the process. In this context, this study evaluated some potential advantages of the two-stage over single-stage anaerobic reactors, including higher bioenergy production potential through recovery of hydrogen besides methane, and improvement of effluent quality due to the lower volatile suspended solids (VSS) concentration in final effluent. Two and single-stage anaerobic reactor were run in parallel at increasing organic loading rates (OLR).
MATERIALS AND METHODS
The reactors were cylindric and made of acrylic. The two-stage system comprised an upflow anaerobic sludge blanket (UASB) reactor of 2.2 L working volume and 6.3 cm internal diameter, used for acidogenesis, followed by a structured fixed-bed reactor of 3.8 L working volume and 7.9 cm internal diameter, used for methanogenesis. The single-stage system
consisted of a structured fixed-bed reactor of 6.4 L working volume and 9.6 cm internal diameter. The medium compartment of the structured-fixed bed reactors was filled with polyurethane foam strips at a porosity about 80% (Figure 1). The reactors were named as follows: i) HR: hydrogen-producing reactor, used for acidogenesis in the two-stage system, ii) MR: methanogenic reactor, used for methanogenesis in the two-stage system, iii) SMR:
single-stage methanogenic reactor, used in the single-stage system. The reactors were inoculated with 15 gVTS L-1 of sludge from slaughterhouse wastewater treatment plants. The granules were completely disrupted with a blender before inoculation.
6.3 cm
Figure 1.Schematic diagram of the two-stage (left) and single-stage (right) systems.
The HR and SMR were fed with sucrose-based wastewater composed of demerara sugar (Native®) and a nutrient’s solution in the following concentrations (mg L-1): NH4Cl (170), CaCl2·2H2O (8), KH2PO4 (37), MgSO4·4H2O (9), FeCl3·4H2O (2), CoCl2·6H2O (2), MnCl2·4H2O (0.5), CuCl2·2H2O (0.03), ZnCl2 (0.05), H3BO3 (0.05), (NH4)6Mo7O24·4H2O (0.09), Na2SeO3·5H2O (0.1), NiCl2·6H2O (0.05), EDTA (1), HCl 36% (1 ȝL L-1); and the MR was fed with acidified wastewater from the HR. Demerara sugar concentration in raw affluent of the HR was approximately 4.45 g L-1 (4.8 gCOD L-1 on average). The methanogenic reactors were fed with diluted wastewater during the acclimation period, which lasted for 38 days. From days 0 to 17 (acclimation 1) the feeding was diluted 5-fold, and from days 18 to 38 (acclimation 2) the feeding was diluted 3.3-fold. After acclimation, the MR and SMR were fed with acidified effluent without dilution (4.4 gCOD L-1 on average) and raw wastewater (4.8 gCOD L-1 on average), respectively. NaHCO3 was added in the feeding of the MR and SMR as buffer agent, at concentrations of 1 g L-1and 1.5 g L-1during acclimation periods 1 and 2, respectively, and 5 g L-1from days 39 to 73, and 4.16 g L-1from days 74 to 178. MR and SMR feeding was also supplemented with yeast extract at concentration of 200 mg L-1, and during acclimation 1 and 2 it was diluted 5- and 3.3-fold, respectively (40 and 61 mg L-1). The temperature was maintained at 30±2 °C.
The operation was divided in two periods of acclimation, when MR and SMR affluent was diluted as described above, and seven phases of operation, when the MR and SMR flow rate was gradually increased according to their stability responses (Table 1).
Table 1.Operating conditions
Biogas flow rate was measured using Milligas counter gas meters (Ritter®). The composition, in terms of H2, CH4 and CO2, was analysed using Shimadzu GC-2010 gas chromatograph.
Sucrose (glucose and fructose) and organic acids (lactic, formic, acetic, propionic, isobutyric, butyric, isovaleric, valeric) were determined using Shimadzu System UV/DAD (210 nm) high performance liquid chromatography (HPLC). 7RWDO &2' VROXEOH &2' ILOWHUHG LQ ȝP membrane) and VSS concentration were analysed according to Standard Methods(2005). The pH was measured using a pHmeter (Hach equipment). Total volatile acids (TVA) and bicarbonate alkalinity were measured by titration using Kapp method, which is considered robust and reliable (Mota et al., 2015).
RESULTS AND DISCUSSION
From Phase 1 to Phase 7, when the operating parameters of the HR were set to 4.6 h HRT and 25.6 gCOD L-1d-1 OLR, this reactor presented great stability during operation, constant production of hydrogen and organic acids. Despite affluent pH was 6.5, effluent pH was 2.7 on average. The highly acid environment did not harm the H2 production, which was stable and in a high range, corresponded to volumetric hydrogen production rate (VHPR) of 175 mlH2 L-1h-1 and hydrogen yield (HY) of 3.4 mLH2 per mol of sucrose consumed. Sucrose was removed by 81%, and most of acidified sucrose was converted to acetate (54.9% molar ratio), followed by lactate (25.4% molar ratio). Minor amounts were found of (molar ratio):
butyrate (7.7%), propionate (7.7%), valerate (2.8%) and formate (1.4%).
Effluent pH values of MR and SMR were, respectively, 7.1 and 7.0, on average. Figure 2 shows the organic acid concentrations in MR and SMR. During Acclimation 1, SMR effluent showed lower concentrations of organic acids and higher concentrations of bicarbonate alkalinity (BA) than MR effluent. This is probably due to the lower OLR applied to the SMR compared to the OLR applied to HR+MR system (Table 1). The increased organic concentration in the feeding during Acclimation 2 caused organic acid concentration increase, especially in SMR, though the OLR did not change significantly. After acclimation, MR an SMR started to be fed with undiluted wastewater at the same OLR of the previous period; and an even greater increase in organic acid concentrations was observed during Phase 1, resulting in accumulation of acetate, propionate, butyrate and valerate. As organic acid levels were above 1000 mg L-1, which is the inhibitory threshold for the methanogenic archea (Foresti
2002), in Phase 2 the OLR was reduced by increasing the HRT. This procedure was sufficient to recover the systems. From this stage, in which the flow rates were adjusted to keep OLR equally in two- and single-stage systems (Table 1), the MR started to show a better performance, as can be observed by its lower organic acid concentrations.
Figure 2.Total volatile acids (TVA) and acid species concentrations (mg L-1) in the MR and SMR during the beginning of operation (a) and after reaching a pseudo-steady state (b)
The OLR increase in Phases 3 and 4 did not impact on reactors’ performance, revealing that the high concentrations of organic acid observed previously resulted from possible shock loading during the start-up and kinetic limitations. It is noteworthy that organic acid concentrations dropped substantially from Phase 3, achieving TVA concentrations lower than 400 mgHAc L-1. In Phase7, OLR in MR and SMR were increased by approximately 50%, wherein HR+MR system and the MR solely operated at 6.4 and 7.8 gCOD L-1d-1 OLR, respectively, and SMR operated at 5.2 gCOD L-1d-1OLR. However, the impacts on treatment performance were mild, suggesting that methanogenic community was established in the reactors. Lactate, formate and (iso)valerate were no more detected, and (iso)butyrate was detected only in the SMR at very low concentrations (<11 mg L-1). Especially in the SMR, acetate and propionate were detected in higher levels. Acetate accumulation suggests kinetic limitations of acetoclastic methanogenic archaea. Propionate degradation, in turn, can be hampered by relatively low concentrations of dissolved hydrogen and acetate, and hydrogen interspecies transfer limitations (Kim et al. 2002). It was demonstrated that during the stable periods (Phases 3 to 7), acid concentrations in MR effluent were below of those found in SMR effluent, even in the periods when the OLR applied in the two-stage system were the highest (Phases 5 to 7). These results suggest that the separation of anaerobic digestion phases reduced the problems related to kinetic, thermodynamic and mass transfer limitations, under the tested conditions.
Effluent soluble COD levels during the whole operation period showed the same pattern observed for organic acid concentrations (Figure 3). During start-up, affluent dilution was not sufficient to achieve satisfactory performance and effluent COD levels were quite high: mean of 484 mg L-1 in MR and of 184 mg L-1 in SMR. The reduction in dilution rate from 5- to 3.3-fold during the period of Acclimation 2 resulted in even lower COD removal, and soluble effluent COD levels increased twice in MR (mean of 1028 mg L-1), and nine-fold in SMR (mean of 1200 mg L-1), except from day 25 to day 27, when MR and SMR were fed with only water and nutrients, to avoid acid accumulation.
Figure 3. Soluble COD levels in MR and SMR’s effluents
The progressive OLR increase in MR by approximately 20% at each operational phase till Phase 6 did not affected its performance. The soluble and total COD removal in the two-stage system increased from 85.8% and 76.1%, during Phase 2, to 98.1% and 96.0%, during Phase 6, respectively. In these periods, soluble and total COD removal in the single-stage system were, respectively, 79.8% and 77.8% (Phase 2), and 96.8% and 89.9% (Phase 6). The higher OLR increase in Phase 7 caused a slightly reduction in COD removal; total COD removal in two-stage system decreased from 96.0% to 93.4%, and in the SMR from 89.9% to 86.6%.
The higher total COD levels in effluent from the single-stage system resulted not only from soluble COD, but especially from particulate COD, i.e. total COD minus soluble COD, which was also verified by its high effluent VSS concentrations (Figure 4).
Figure 4. Volatile suspended solids in MR and SMR’s effluents
The increased effluent VSS concentrations in the SMR (223±119 mg L-1) are likely to be attributed to fast growth rates of acidogenic bacteria and extracellular polymeric substances (EPS) release, which are partially adhered to the microbial flocs. On the other hand, effluent VSS concentrations in the MR were much lower (80±36 mg L-1), despite the high VSS
concentrations in its affluent, i.e. HR effluent (257±167 mg L-1). It is believed that acidogenic bacteria from the HR partially undergo decay when they entered into MR, as a result of the low availability of non-acidified substrate, culminating in the stabilization of those bacteria as well as of the EPS and soluble microbial products (SMP) released by them. In contrast, in the SMR, since there was a constant inflow of non-acidified substrate, the acidogenic bacteria were continuously growing and releasing SMP and EPS, resulting in higher concentrations of COD and VSS in the effluent.
The suppression of methanogenesis in the HR was successfully achieved; no CH4 was detected in HR’ biogas, which was H2-rich (60%) (Figure 5). Biogas from the MR and the SMR was composed of CH4, CO2, and, during the instability periods, traces of H2 (Figure 5).
It is notable that from Phase 2, CH4 content in biogas of the MR kept much higher than that of the SMR, suggesting a higher calorific potential of the MR biogas. These results corroborate with other studies that compared two-stage to single-stage anaerobic digestion, wherein increased CH4 content was found in the methanogenic reactor fed with acidified wastewater (Ghosh et al., 1985; Yeoh, 1997; Ghosh et al., 2000). It is assumed that the occurrence of acidogenesis in a previous stage, in the HR, resulted in lower CO2 production by fermentative activity in the MR, resulting in higher CH4 content in its biogas.
Figure 5. Composition of biogas from HR, MR and SMR
For energy production potential evaluation in the two-stage system and single-stage system, it was analysed data from Phase 4, where the same OLR of 3.5 gCOD L-1d-1was applied in both systems, and data from Phase 7, when the systems operated at their maximum OLR, corresponded to 6.4 gCOD L-1d-1 in the two-stage system and 5.2 gCOD L-1d-1 in the SMR. In an attempt to normalize the input parameters, it was considered the added COD; and to display the results in units of energy, it was considered a calorific potential of 142 kJ per g of H2 and 50 kJ per g of CH4 (Nasr et al., 2012). Under the conditions evaluated, the replacement of a single system by a two-stage system can result in energy yield increase around 25% (Phase 4) and 34% (Phase 7). Even if it is not consiedered the H2 recovery potential, the energy yield from the two-stage system was 7.7% and 20.7% higher in Phases 4 and 7, respectively, in relation to the single-stage system (Table 2). Similar results were obtained in the study by Luo et al. (2011), wherein the yield of the single-stage system was 11.8 kJ per g of VS added and in the two-stage system it was 13.1 kJ per g of VS added (12.4 kJ from CH4 and 0.7 kJ from H2).
Although the relatively low energy yield from H2 production, that was from 10.3 to 14.3% of the total energy, a possible use of the H2 is as combustion catalyst, rather than as an energy carrier. For this purpose, the biogas from acidogenic reactor can be mixed to the biogas from the methanogenic reactor, producing a mixture of CH4, CO2and H2. This mixture, sometimes called as bio-hythane, may offer advantages over conventional biogas (consisted of only CH4
and CO2), such as improved combustion properties and reduced CO2 and NOX emissions (Ghoniem 2011, Cavinato et al. 2012).
Table 2.Energy yields
kjH2kg-1CODadded 2288.7 32.0 2138.8 122.7
kjCH4kg-1CODadded 13729.5 12750.4 18549.6 15362.6
kjH2+CH4kg-1CODadded 16018.2 12782.3 20688.4 15485.4
H2/Total energy yield 14.3% 0.3% 10.3% 0.8%
CH4/ Total energy yield 85.7% 99.7% 89.7% 99.2%
CONCLUSIONS
It was noteworthy that organic acids, soluble and total COD levels in the final effluent from the two-stage system remained lower than that from single-stage system, when the systems were operated under equal OLR and even when the two-stage system was operated under higher OLR. It is suggested that the separation of the stages led to lower growth of suspended acidogenic biomass and release of EPS in the methanogenic reactor, as evidenced by its substantially reduced VSS concentration. In the end of operation, effluent total COD was 317 mg L-1 in the two-stage system, and 666 mg L-1 in the single-stage system. The two-stage system also demonstrated a higher potential for bioenergy production. Biogas from the two-stage methanogenic reactor presented 70% methane, in contrast to biogas from the single-stage system that presented 52% methane. In addition, increased energy yields up to 34%, through methane and hydrogen production, were achieved in the two-stage system. These results demonstrate that the two-stage anaerobic treatment, when properly designed, can result in lower required reactor volumes, significantly better effluent quality, and higher energy potential recovery from wastewater.
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