COMPARISON OF ONE STAGE AND
TWO-STAGE ANAEROBIC CO-DIGESTION
PROCESS OF FOOD WASTE AND ACTIVATED
SLUDGE
Isabella Pecorini
1, Elena Albini
2, Renato Iannelli
1and Giovanni Ferrara
21 DESTEC - Department of Energy, Systems, Territory and Construction Engineering - University of Pisa, 56122 Pisa, Italy
2 DIEF - Department of Industrial Engineering - University of Florence, via S. Marta 3, 50139 Florence, Italy
ABSTRACT: In this study, the co-digestion of food waste and activated sludge was evaluated in a two-stage anaerobic system. The two-two-stage system was composed by two reactors connected in series able to perform the fermentative and the methanogenic phases separated. Experiments were carried out in semi-continuous mode under mesophilic conditions (37 °C). Biogas production of each reactor is controlled daily and the composition analysis are carried out using gas chromatography. Volatile Fatty Acids, Total Solids, Total Volatile Solids, pH and alkalinity of incoming waste and outgoing digestate are measured too for evaluate process’ performance.
Keywords: Hydrogen; Methane; Food waste; Activated sludge; Co-digestion; Two-stage process
1. INTRODUCTION
Anaerobic digestion (AD) could be considered a rising technology for bio-fuels, bio-products and renewable energy production (Pecorini et al., 2017). Most of the conventional wastewater treatment plants (WWTPs) use AD process for the treatment of the produced sludge, but energy recovery via anaerobic digestion normally is not sufficient to cover WWTPs energy consumption due to low organic loading and low biogas yields of sludge. Despite AD is broadly used and consolidated, more studies need to be carried out in order to assess the optimal process’ conditions to maximize production yields.
Co-digestion of bio-waste and wastewater sludge (WS) is considered a strategic way for increasing the energy production (Nghiem et al., 2017; Cavinato et al., 2013; Da Ros et al., 2014; Cavinato et al., 2015). The Organic Fraction of Municipal Solid Waste (OFMSW) seems to be an optimum co-substrate for improving digestion efficiency of wastewater sludge in AD process, due to its biodegradability characteristics (Cavinato et al., 2012, Chinellato et al., 2013, Micolucci et al., 2014). The co-digestion of these two substrates could be suitable also for bio-hydrogen production during the acidogenic phase of AD called “Dark Fermentation” (DF).
The two-phase anaerobic digestion process, obtained by the union of bio-hydrogen production during the acidogenic phase and bio-methane production in methanogenic phase, can be considered the new frontier of AD process optimization (Ghimire et al., 2015). This technology is based on the split of the traditional AD process into two digestion phases made up of a fermentative reactor and a
methanogenic reactor. During the acidogenic phase, H2 and CO2 as gaseous products are produced and volatile fatty acids (VFAs) are released in the liquid solution. Into the methanogenic reactor VFAs and the residual organic biodegradable matter are converted into CH4 and CO2 (De Gioannis et al., 2013, De Gioannis et al., 2017). The combined production of bio-hydrogen and bio-methane has been demonstrated to improve the gasification yield of organic substrates compared to the single-stage AD process.
The aim of this research is investigated the performance of two-phase AD process in terms of bio-hydrogen and bio-methane production. In more depth, the process is developed in pilot scale with the aim to evaluate the performance of co-digestion of OFMSW and WS in dark fermentation process for improving methane production rates in the methanogenic stage.
2. MATERIALS AND METHODS 2.1 Substrates and Inocula
Concerning the co-digestion trials, 200 L of activated sludge (AS) were collected from the aerobic unit of the municipal WWTP of Viareggio (Italy). The sample of AS was stored in plastic tanks and kept under refrigeration at 4°C until use. Each sample of FW was manually sorted from source-separated OFMSW collected in Viareggio (Italy) by means of a kerbside collection system. As for the batch tests, the sample was immediately treated aiming at obtaining mashes with a dry matter content of 5% by weight. More specifically, it was shredded in a food processor (Problend 6, Philips, Netherlands) and diluted with tap water. Finally, the obtained mash (FW1) was stored at -20 °C until use.
As inocula for methanogenic stage was used digested sludge collected from an anaerobic reactor treating the organic fraction of municipal solid waste (OFMSW) and cattle manure. Activated sludge collected from the aerobic unit of a municipal WWTP was used in fermentative stage. Fermentative inocula were heat treated at 80°C for 30 minutes prior to the tests with the aim of selecting only HPB while inhibiting hydrogenotrophic methanogens (Alibardi and Cossu, 2016; Cappai et al., 2014). The treatment was performed in 250 mL beakers placed in a static oven (UM200, Memmert GmbH, Germany).
2.2 Semi-continuous trials
The fermentative stage (R2) was carried out using stainless steel (AISI 316) reactors of 6 L (working volume of 3 L). The methanogenic stage (R1) was performed in a similar stainless steel (AISI 316) reactor of 20 L (12 L working volume). Continuous mixing inside the reactors was ensured by a mixing blade connected to an electric gear motor (COAX MR 615 30Q 1/256, Unitec s.r.l., Italy). Temperature was constantly kept at mesophilic conditions (by a jacket where warm water heated up by a thermostat (FA90, Falc Instruments s.r.l., Italy) was continuously recycled. pH was continuously measured by pH probes (InPro4260i, Mettler Toledo, Italy). The volume of the produced gas during the tests was measured by using volumetric counters connected to the upper side of the reactors through a 3-way valve (see the figure 1). Substrates were daily fed to the reactors by means of a syringe. The digestion of FW and the co-digestion of FW and AS were characterized by two scenarios. In the first and second scenario (S1 and S2), the methanogenic reactor was run alone aiming at evaluating the traditional one-stage AD, respectively using diluted activated sluge and thickened activated sludge. Simultaneously, the fermentative reactor was also fed in order to reach steady state conditions. In the therd scenario (S3), the two digesters were connected in series aiming at evaluating the two-stage process and using
thickened activated sludge. Each scenario was performed for two HRTs of the methanogenic reactor (see figure 2).
Table 1 summarizes the operational conditions applied to the reactors during the tests. Process stability was daily monitored on digestate by means of alkalinity, pH and VFA. Aiming at determining the volatile solids removal efficiency (ηTVS), digestate was daily controlled in its volatile solids content, Anaerobic performances were evaluated in terms of ηTVS, specific gas production (SGP) and methane and hydrogen content in biogas.
Figure 1. Semi-continuous reactors used for the study and volumetric counters.
Figure 2. Design of experiment. Table 1. operational conditions.
Configuration Substrates HRT (d) OLR (kgTVSmr
-3d-1)
S1: one-stage (AD) activated sludgeFW and diluted 17 2.5
Waste layer depth (m) from - to (in meters from ground)
FW and thickened activated sludge
17 2.5
from - to activated sludgethickened
2.3 Analytical parameters
TS, TVS and pH were measured following standard procedures (APHA, 2006). Alkalinity was measured according to the methodology of Martín-González et. (2013). VFAs, including acetic, propionic, butyric, isobutyric, valeric, isovaleric and caproic acids were measured using a gas chromatograph (7890B, Agilent Technology, US) with hydrogen as gas carrier, equipped with a CPFFAP column (0.25 mm / 0.5 μm / 30 m) and with a flame ionization detector (250°C). The temperature during the analysis started from 60°C and reached 250°C with a rate of 20 °C/min. Samples were centrifuged (30 minutes, 13,500 rpm) and filtrated on a 0.45 μm membrane. 500 μL of filtrate were mixed with isoamyl alcohol (1.00179, Merck KGaA, Germany) in a volumetric ratio of 1:1, 200 μL of phosphate buffer solution (pH 2.1), sodium chloride and 10 μL of hexanoic-D11 acid solution (10.000 ppm) used as internal standard. The blend was mixed with a Mortexer™ Multi-Head vortexer (Z755613-1EA, Merck KGaA, Germany) for 10 minutes. The liquid suspension of the sample was then inserted in the gas chromatograph by means of an auto-sampler. Concerning gas quality, hydrogen, methane, carbon dioxide, nitrogen, oxygen and hydrogen sulphide contents in biogas were analysed using a gas chromatograph (3000 Micro GC, INFICON, Switzerland) equipped with a thermal conductivity detector. Carbon dioxide and hydrogen sulphide passed through a PLOTQ column (10μm/320μm/8m) using helium as gas carrier at temperature of 55°C. The other gas passed through a Molsieve column (30μm/320μm/10m) using argon as gas carrier at a temperature of 50°C.
3. RESULTS AND DISCUSSION
Analytical characterization of inocula and substrates was performed in order to provide a detailed description of the media and to assess the extent of potential inhibitory compounds for the fermentative process. Results expressed with averages and standard deviations are presented in Table 2.
Table 2. Characterization of inocula and substrates.
Substrates TS [%] TVS [%] TVS/TS pH
FW 19.53±0.51 17.10±0.45 87.55±0.15 4.43±0.06
Inoculum AD 2.49±0.02 1.53±0.02 61.33±0.20 8.36±0.02
Activated sludge 0.84±0.01 0.65±0.01 78.00±0.31 6.97±0.04
Thickened activated sludge 2.09±0.07 1.72±0.04 82.32±1.36 7.44±0.05
Semi-continuous trials were performed taking stock of batch experiments findings. Therefore activated sludge was used as inoculum to start-up the fermentative stage, pH was set to 5.5 an the OLR was set to guarantee a high daily organic load Results are firstly presented by analysing process stability through pH, alkalinity and VFAs (see figure 3). Subsequently, single-stage and two-stage processes are compared in their anaerobic performances through biogas production, biogas quality and
volatile solids removal efficiency (see figures 4 and 5).
From figures 4 and 5 it is evident that the total biogas produced is greater in the third scenario. This trend was obtained by summing the outgoing biogas in both reactors for the third scenario, while for the first and second one only the biogas produced by the R2 reactor was considered. In particular, the average of the biogas volume thus obtained for the first scenario is 13.1 Nl/d, for the second 11.71, while the average for the third scenario is 18.16 N/d. This means that in the third scenario there is an increase in biogas produced by 38.63% compared to the first scenario and by 55.08% compared to the second. As already mentioned, the greater biogas production obtained from the two-phase system is due to the optimization of the metabolism of hydrolytic and fermentative bacteria in the dark fermentation reactor and of the methane-producing bacteria in the methanigen reactor. By separating the anaerobic digestion phases it is therefore possible to manage the two reactors with the optimal control parameters for each type of bacteria.
The two-stage system (R1 + R2 in series) as a whole made it possible to treat the sludge / FORSU mixture with a lower residence time (-26%) compared to the single-stage case but the daily biogas production was on average higher (+64%). The fermentative digestate rich in VFA (see AIcalinity values of R2) has proved to be an excellent substrate for the methanigen reactor. The overall SGP was over S2 by 33% and the GPR increased by 175% (see figure 4); the combined energy recovery resulting from the combustion of the two off gases has improved by 1.4%. However, the abatement efficiency of solids, TS and TVS, decreased by 15% and 7% respectively. By setting a mass balance in the R1 reactor it has been seen that the daily TVS mass entering the system (about 30 g / d) equates the TVS mass leaving the system in the form of gas and therefore the removal of real TVS coincides with that theoretical (TVS outgoing from R1 equal to 1.36%). The same balance has always been applied to the R1 reactor using the values measured in S2 (incoming TVS equal to 29 g / d) and also in this case the real conversion approaches the theoretical one (TVS in output equal to 1.12%). Therefore it can be stated that the cause of the reduced efficiency of total removal of solids (in S3 R1 + R2) is due to an increase in the OLR compared to the case S2 and to a consequent accumulation of TVS in the digestate leaving R1.
Figure 4. Specific Gas Production and Gas Production Rate.
Figure 4. Composition of gas and solid removal.
4. CONCLUSIONS
The two-stage co-digestion of food waste and activated sludge efficiently improved the traditional single-stage process. The enhancement of the anaerobic performances in terms of biogas production, biogas quality and volatile solids removal were even higher than the two-stage digestion of the sole food waste, thus highlighting the viability of this technology also for the mixture of food waste and activated sludge. Furthermore, the co-digestion configuration observed a better process stability. In conclusion, dark fermentation are suitable pretreatments that improve the anaerobic digestion of the OFMSW and activated sludge. The adoption of such technologies can be useful the anaerobic process making the most complex organic substance easier to bacteria attack.
AKNOWLEDGEMENTS
This work was supported by the Bio2Energy project, a project funded by 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)..
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