EFFECT OF TWO-STAGE ANAEROBIC
DIGESTION PROCESS ON DIGESTATE
STABILITY
Elena Albini
1, Isabella Pecorini
2, Renato Iannelli
2and Giovanni Ferrara
11 DIEF, Department of Industrial Engineering, University of Florence – Via Santa Marta 3, 50139, Italy 2 DESTEC, Department of Energy, Systems, Territory and Construction Engineering, University of Pisa – Largo Lucio Lazzarino, 56122, Italy
ABSTRACT: Anaerobic digestion is used to stabilize organic materials through the production of bio-fuels and bio-products, as digestate usable in agricolture as fertilizer and soil conditioner. Digestate can be considered as an indicator of treatment efficiency in anaerobic digestion plants and the evaluation of digestate stability index can be useful for determining process performance.
In this work, two different configurations of anaerobic co-digestion process were considered: a conventional one-stage reactor and its implementation in a two-stage process with a preliminary fermentative phase called dark fermentation. Process performances were determinated for each configuations relatively to outgoing digestate stability index, assessed by SOUR test. The objective of this paper is the evaluation of which anaerobic co-digestion configuration can reach the preferable performance in terms of digestate biostabilization.
The results indicate that using dark fermentation as a preliminary biological treatment for anaerobic co-digestion process can improve the outcoming digestate stability. Two-stage process allows a total SOUR reduction equal to 33.4% and 40.6% respectively for a hydraulic retention time of 1.5 and 3.0 days in the fermentative reactor. Compared to the conventional one-stage process, with a stabilization efficiency in the amount of 6.5%, two-stage configuration determines the best performances obtaining a biologically more stable digestate.
Keywords: specific oxygen uptake rate, dark fermentation, co-digestion, bio-waste, respirometric test.
1. INTRODUCTION
The European directives deter landfill disposal for any type of waste, in particular for organic waste (grater environmental impacts). Therefore, landfill disposal should be minimized and used only for specific and residual case. Based on the EU action plan for Circular Economy (EU Commission, 2015), waste production could be reduced by augmenting the value of products as much as possible through turning waste into resources for new purposes (Webster, 2013).
Based on the above background, biorefineries constitute a proven solution for residues valorization, especially for organic waste. In this respect, anaerobic digestion (AD) process represents an attractive technology due to its potential usefulness in biofuels and bio-products production (Pecorini et al., 2016; Iacovidou et al., 2012; Tambone et al., 2010, Orzi at al., 2010). In addition to renewable energy production, AD supplies the production of digestate, a residual organic matter with high fertilizing and
organic amendment properties that can be replace the synthetic fertilizers in agricolture (Tambone et al., 2017, Tambone et al., 2019). The rising interest in AD of bio-waste leads the scientific community through process optimization and biohydrogen production during AD process constitute a suitable solution (Pecorini et al., 2017; Micolucci et al., 2014; Chinellato et al., 2013; Cavinato et al., 2012). Biohydrogen flow is obtained by dividing the traditional AD technology in a two-stage process. In this configuration, the AD process is equipped with a fermentative reactor dark fermentation (DF) reactor -and a methanogenic reactor (AD) connected in series. In addition to this, co-digestion of bio-waste, as food waste (FW) that is the organic fraction of municipal solid waste (OFMSW), with wastewater sludge (WS) is considered a proven approach for improving digestion efficiency, increasing energy production and facilitating nutrient recycling (Nghiem et al., 2017; Cavinato et al., 2013; Da Ros et al., 2014; Cavinato et al., 2014).
For evaluating the capability of AD process in environmental impacts reduction, the characterization of outcoming digestate in terms of biological stability could be considered. Biodegradable organic matter content in digestate, indeed, could be considered as an index of treatment efficiency in AD plants (Orzi et al., 2010). Amongst the available respiration methods for biological stability determination, the Specific Oxygen Uptake Rate (SOUR) is considered a simple static method for the assessment of stability index (Lasaridi and Stentiford, 1998, Adani et al., 2001; Scaglia et al., 2007). This method measures the oxygen consumption during a limited experimental time in a liquid medium where solid sample are dissolved. Oxygen dispersion is assuranced by intermittent aeration and continuous agitation. Seeing as how material characteristics in the solid state, as moisture content and structure, could conditioned oxygen transfer in organic matrices, respirometric test performed in a liquid state can remove these limitations (Lasaridi and Stentiford, 1998, Giraldi and Iannelli, 2009).
The main objective of this paper is to compare one-stage with two-stage AD process efficiency referring to the outgoing digestate stability. In this study, two configurations were considered: a conventional one-stage AD process and a two-stage AD process with a preliminary DF followed by a AD reactor. Different scenarios were carried out in a pilot scale semi-continuous (CSTR) reactors using as substrate a co-digestion feedstock obtained by mixing OFMSW and sewage sludge. For the two-stage process (DF+AD), two different experimental set-ups were performed referring to hydraulic retention time (HRT) of DF reactor: HRT of 1.5 and 3.0 days were considered.
Process performances were evaluated in terms of digestate stability index analized by SOUR test.
2. MATERIALS AND METHODS 2.1 DESIGN OF EXPERIMENT
To compare the performances of a conventional one-stage AD reactor with a two-stage process in terms of digestate stability, the experimental set up was divided in two different scenarios (Run). In the first scenario (Run1), was assessed the stability of the digestate coming from a methanogenic reactor, that represents the reference scenario. In the second scenario (Run2), where two reactors were connected in series, the digestate coming from each reactor was analized for the assessment of two-stage process (DF+AD). Moreover, Run2 was also splitted in two configuration referring to HRT setting of DF. A summary of experimental runs is shown below:
R
un1: traditional one-stage AD process – reference scenario;
R
- R un2.1: HRT in DF reactor set to 1.5;
- R
un2.2: HRT in DF reactor set to 3.0 days.
In addition to digestate, also the biological stability of the input substrate was considered for investigating the overall process stabilization efficiency.
The performances of the two scenarios were compared each other referring to the stability index of the outcoming digestate, measured through the SOUR test.
2.2 Samples preparation
FW coming from a separated collection system was used as a substrate for the co-digestion process. The sample of FW was obtained from the OFMSW collected by a kerbside collection system in a Tuscan municipality (Itay). It has been considered as an affirm feedstock for anaerobic digestion process due to its great biodegradability and availability in the municipal areas (Micolucci et al., 2014; Chinellato et al., 2013; Cavinato et al., 2012). Given that in this study a co-digestion process was simulated, FW was mixed with WS for representing a co-digestion feedstock. Moreover, WS was added to FW in order to obtain a slurry with a correct solid content for a wet digestion process (Pecorini et al., 2016).
At the beginning, FW was treated in a food processor, combed through a strainer (3 mm diameter) and frozen. Subsequently, the FW sample was defrosted for 12hours and mixed with fresh WS in the right proportions. Regarding the digestate, the samples were removed from the reactors every day, centrifuged (30 minutes, 13,500 rpm) and stored at – 4°C.
Co-digestion substrate and outgoing digestate coming out from each reactor were analyzed in terms of Total Solids (TS) and Total Volatile Solids (TVS) content. These parameters were determined in according to standard methods (APHA, 2006). Ashes and moisture contents were then obtained in accordance with TS and TVS measurements.
To compare the biological index of the samples, also co-digestion feedstock was centrifuged to perform the respirometric test. Substrate and digestates characterization are shown in Table 1.
Table 1. Substrate and digestate characterization expressed in terms of average values and standard deviations.
Scenario Sample TS (% w/w) TVS (% w/dw) - Substrate 27.93 ± 0.13 82.47 ± 1.72 Run1 Output AD 15.88 ± 0.07 74.25 ± 0.21 Run2.1 Output DF 21.68 ± 0.07 83.20 ± 0.17 Output AD 14.71 ± 1.46 69.21 ± 0.81 Run2.2 Output DF 23.16 ± 0.17 85.33 ± 6.47 Output AD 16.60 ± 0.05 69.19 ± 0.33
Each sample obtained during the experimental tests were analized in terms of biological stability through the respirometric method represented by SOUR. SOUR liquid test was set-up according to Lasaridi and Stentiford, 1998 methodology with the modification proposed by Adani et al., 2003.
A sample of 2.5 gr (wet weight) was mixed for 30 seconds with 500 ml of distilled water in a food blender and then diluited in a Duran 1-liter bottle with other 500 ml of distilled water. For ensuring that nutrients or pH were not limiting, 15 ml of phosphate buffer solution (with a pH equal to 7.2), 1.1 ml of ATU (allylthiourea) and 5 ml of nutrient solutions (CaCl2, FeCl3 and MgSO4), made up according to the
standard BOD test procedures (APHA, 2006), were added to the aqueous suspension.
The flask was placed on a magnetic stirrer (Velp Scientifica, AREX Digital PRO), continuously mixed (250 rpm) and heated at 30° C for ensuring the optimal biological conditions. Microbial respiration was evaluated by measuring dissolved oxygen (DO) concentration into the solution by a DO probe (Mettler Toledo, InPro6000, Optical O2 Sensors) connected to an automatic data acquisition system (LabView,
National Instruments Corporation, Italy). The suspension was periodically aerated by a fish-tank air pump with intermitted aeration cycle. A control system provided the aeration/reading sequence characterized by 20 min aeration period followed by 15 min of DO measurement. The DO probe recorded DO concentration during the overall 20 hours’ experimental period.
The SOUR measure, that represents the maximum oxygen consumption rate, was evaluated via DO concentration drops during the reading cycle. From the slope of DO lines, that give the value of oxygen consumption over time, SOUR value was calculated according to the following equation (1) (Lasaridi and Stentiford, 1998; Adani et al., 2003):
SOUR=
|
S max|∗V
m∗TS∗TVS
(1)Where: SOUR is the Specific Oxygen Uptake Rate (mgO2*gTVS-1*h-1); |Smax| is the maximum
absolute slope of oxygen consumption (mgO2*l-1*h-1); V is the volume of the aqueous suspension (l); m
is the mass of the sample (gr, wet weight); TS and TVS are the decimal fraction of dry solids and total volatile solids (dry matter) of the sample.
SOUR test was performed for each sample in two replicates.
3. RESULTS AND DISCUSSION
For the evaluation of the conventional one-stage co-digestion process, Run1 was carried out using a methangenic reactor (AD reactor) only. Co-digestion substrate was fed every day to the reactor for ensuring an OLR equal to 2.5 kgTVS/m3*d. The same aliquot of digestate was daily getted out.
Relative to Run2, co-digestion process was splitted in two different reactor connected in series, where a fermentative reactor (DF reactor) was used as a preliminary stage for AD reactor. For Run2 two different operative conditions were performed reffering to HRT in the DF process and maintaining the same ORL in AD reactor used in Run1. Substrate was fed daily in DF reactor and digestate coming out from that reactor was used to feed AD reactor.
The outcoming digestate of each reactor were collected and substequentially analized through SOUR test for stability index determination. Biological stability index and the trend of SOUR values during the experimental test indicate process efficiency in terms of biostabilization.
tests. Moreover, process performances referring to stabilization efficiency are presented based on the measured digestate stability.
Fig.1 represents the trends of SOUR values during Run1, Run2.1 and Run2.2.
Table 2. Digestate stability for each scenario expressed by SOUR index and process stabilization efficiency. Average and standard deviations of SOUR vaues are shown.
Scenario Sample SOUR (mgO2*gTVS-1*h-1) Stabilization efficiency (%)
- Substrate 20 ± 1 -Run1 Output AD 18 ± 0 6.5 Run2.1 Output DF 28 ± 0 -Output AD 13 ± 4 33.4 Run2.2 Output DF 33 ± 8 -Output AD 12 ± 0 40.6
Figure 1. SOUR trend for each scenario.
The results obtained from the experimental tests show that Run2, that is the two-stage configuration, in general is more efficient in terms of biostabilization efficiency compared to Run1, or rather the traditional anaerobic digestion process. The outcoming digestate at the end of anaerobic co-digestion process, indeed, presents lower SOUR values when DF are used as a preliminary biological treatment.
Biostabilization efficiency in Run1 results to be equal to 6.5%, express in terms of SOUR reduction. Considering DF reactor, stability index reduction turns to be 33.4% and 40.6% respectively for Run2.1 and Run2.2.
The results show that from two-stage configuration a more stable digestate is produced in respect to one-stage process. The different operational conditions related to HRT setting in the DF reactor do not influence considerably the stability index. Digestate at the end of two-stage anaerobic digestion process presents SOUR values rather similar for Run2.1 and Run2.2.
The higher SOUR values obtained for digestate coming out from the DF reactor is related to the greater production of Volatile Fatty Acids (VFAs) in the first reactor due to the hydrolysis process (De Gioannis et al., 2017). This preliminary treatment improves the presence of available biodegradable substrate in the outgoing digestate of DF tank that reduce digestate stability measured in the intermediate stage.
The experimental tests performed for the evaluation of digestate stability allow to identify that the implementation of the conventional one-stage process can improved the biostabiliazation of the outcoming bio-products.
4. CONCLUSIONS
Two different configurations of anaerobic co-digestion process are presented in this study and compared in terms of biostabilization efficiency, related to the outcoming digestate stability index.
The analysis performed in this work shown that the best performances are obtained with two-stage digestion process. The presence of DF reactor as a preliminary biological treatment improve digestate stability at the end of the overall digestion process. SOUR values of outcoming digestate from AD reactor in two-stage scenario results to be on average 36.5% lower compared to the digestate outgoing from the conventional process. Based on that, the implemanetation of the traditional process with a preliminary reactor results to be a good solution for digesate biostabilization improvement.
A comparison of SOUR test with other respirometric tests is necessary in order to confirm the results obtained from this work and to verify biostabilization efficiency of the two-stage anaerobic co-digestion process compared to the traditional one.
AKNOWLEDGEMENTS
The research was carried within the Bio2Energy project, supported by the MIUR-Regione Toscana DGRT 1208/2012 and MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013 in sub-programme FAR-FAS 2014 (Linea d’Azione 1.1).
REFERENCES
Adani, F., Gigliotti, G., Valentini, F., Laraia, R., 2003. Respiration index determination: a comparative study of different methods. Compost Sci. Util. 11, 2144–2151. https://doi.org/10.1080/1065657X.2003.10702120. Adani, F., Lozzi, P., Genevini, P.L., 2001. Determination of biological stability by oxygen uptake on municipal solid
waste and derived products. Compost Sci. Util. 9(29), 52–65.
Association, Washington, DC.
Cavinato, C., Bolzonella, D., Pavan, P., Fatone, F., Cecchi, F., 2013. Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy 55, 260–265. http://hdl.handle.net/10278/37227.
Cavinato, C., Da Ros, C., Pavan, P., Cecchi, F.,Bolzonella, D., 2014. Treatment of waste activated sludge together with agro-wasteby anaerobic digestion: focus on effluent quality. Water Sci. Technol. 69(3), 525–531.
https://doi.org/10.2166/wst.2013.736.
Cavinato, C., Giuliano, A., Bolzonella, D., Pavan, P., Cecchi, F., 2012. Bio-hythane production from food waste by dark fermentation coupled with anaerobic digestion process: a long-term pilot scale experience. Int. J. Hydrogen Energy 37, 11549-11555. https://doi.org/10.1016/j.ijhydene.2012.03.065.
Chinellato, G., Cavinato, C., Bolzonella, D., Heaven, S., Banks, C.J., 2013. Biohydrogen production from food waste in batch and semi-continuous conditions: evaluations of a two-phase approach with digestate recirculation for pH control. Int. J. Hydrogen Energy 38, 4351-4360. https://doi.org/10.1016/j.ijhydene.2013.01.078.
Da Ros, C., Cavinato, C., Pavan, P., Bolzonella, D., 2014. Winery waste recycling through anaerobic co-digestion with waste activated sludge. Waste Manage. 34(11), 2028–2035. https://doi.org/10.1016/j.wasman.2014.07.017.
De Gioannis, G., Muntoni, A., Polettini, A., Pomi, R., Spiga, D., 2017. Energy recovery from one- and two-stage anaerobic digerstion of food waste. Waste Manage. 68, 595-602. https://doi.org/10.1016/j.wasman.2017.06.013.
European Commission, 2015. Report from the Commission to the European Parliament, the Council, the European economic and social committee and the committee of the regions, closing the loop – An EU action plan for the Circular Economy. COM 614 final, 1-21. https://www.eea.europa.eu/policy-documents/com-2015-0614-final.
Giraldi, D., Iannelli, R., 2009. Measurements of water content distribution in vertical subsurface flow constructed wetlands using a capacitance probe: benefits and limitations. Desalination 243, 182–194. https://doi.org/10.1016/j.desal.2008.05.012.
Iacovidou, E., Ohandja, D., Voulvoulis, N., 2012. Food waste co-digestion with sewage sludge – realising its potential in the UK. J. Environ. Manage. 112, 267-274. https://doi.org/10.1016/j.jenvman.2012.07.029.
Lasaridi, K.E. and Stentiford, 1998. A simple respirometric technique for assessing compost stability. Water Resour. 32(12), 3717-3723. https://msu.edu/~rogensu1/Projects_files/ENE806_Report.pdf.
Micolucci, F., Gottardo, M., Bolzonella, D., Pavan, P., 2014. Automatic process control for stable bio-hythane production in two-phase thermophilic anaerobic digestion of food waste. Int. J. Hydrogen Energy 39, 17563-17572. https://doi.org/10.1016/j.ijhydene.2014.08.136.
Nghiem, L.D., Koch, K., Bolzonella, D., Drewes, J.E., 2017. Full scale co-digestion of wastewater sludge and food waste: bottlenecks and possibilities. Renewable Sustainable Energy Rev. 72, 354-362.
http://dx.doi.org/10.1016/j.rser.2017.01.062.
Orzi, V., Cadena, E., D’Imporzano, G., Artola, A., Davoli, E., Crivelli, M., Adani, F., 2010. Potential odour emission measurement in organic fraction of municipal solid waste during anaerobic digestion: relationship with process and biological stability parameters. Bioresour. Technol. 101, 7330-7337. https://doi.org/10.1016/j.biortech.2010.04.098.
Pecorini, I., Baldi, F., Carnevale, E. A., Corti, A., 2016. Biochemical methane potential tests of different autoclaved and microwaved lignocellulosic organic fractions of municipal solid waste. Waste Manage. 56, 143-150. https://doi.org/10.1016/j.wasman.2016.07.006.
Pecorini, I., Ferrari, L., Baldi, F., Albini, E., Galoppi, G., Bacchi, D., Vizza, F., Lombardi, L., Carcasci, C., Ferrara, G., Carnevale, E.A., 2017. Energy recovery from fermentative biohydrogen production of biowaste: A case
study based analysis. 126, 605-612. https://doi.org/10.1016/j.egypro.2017.08.230.
Scaglia, B., Erriquens, F.G., Gigliotti, G., Taccari, M., Ciani, M., Genevini, P.L., Adani, F., 2007. Precision determination for the specific oxygen uptake rate (SOUR) method used for biological stability evaluation of compost and biostabilized products. Bioresour. Technol. 98, 706–713. https://doi.org/10.1016/j.biortech.2006.01.021.
Tambone F., Orzi, V., D’Imporzano, G., Adani, F., 2017. Solid and liquid fractionation of digestate: Mass balance chemical characterization, and agronomic and environmental value. Biores. Technol. 243, 1251-1256. https://doi.org/10.1016/j.biortech.2017.07.130.
Tambone, F., Orzi, V., Zilio, M., Adani, F., 2019. Measuring the organic amendment properties of the liquid fraction of digestate. Waste Manage. 88, 21-27. https://doi.org/10.1016/j.wasman.2019.03.024.
Tambone, F., Scaglia, B., D’Imporzano, G., Schievano, A., Orzi, V., Salati, S., Adani, F., 2010. Assessing amendment and fertilizing properties of digestates from anaerobic digestion through a comparative study with digested sludge and compost. Chemosphere 81, 577-583. https://doi.org/10.1016/j.chemosphere.2010.08.034. Webster, K., 2013. What might we say about a circular economy? Some temptations to avoid if possible. World
