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Article

Industrial Pilot for Assessment of Polymeric and Ceramic Membrane Efficiency in Treatment of Liquid Digestate from Biogas Power Plant

by
Alexandros Yfantis
1,
Nikos Yfantis
1,
Triantafyllia Angelakopoulou
1,
George Giannakakis
1,
Fabien Michelet
1,
Spyros Dokianakis
2,
Evangelia Vasilaki
2 and
Nikos Katsarakis
2,*
1
Sychem Advanced Water Technologies s.a., 716 01 Heraklion, Greece
2
Center of Materials Technology and Photonics, Hellenic Mediterranean University, 714 10 Heraklion, Greece
*
Author to whom correspondence should be addressed.
Energies 2022, 15(18), 6574; https://doi.org/10.3390/en15186574
Submission received: 17 August 2022 / Revised: 3 September 2022 / Accepted: 5 September 2022 / Published: 8 September 2022
(This article belongs to the Special Issue Advanced Wastewater Treatment Processes and Their Integration in Water Reuse Systems)
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Abstract

:
Due to the depletion of available water resources and the consistently rising environmental pollution levels, the exploitation of the digestate generated as an unfavorable by-product of the industrial wastewater treatment plants, could not only offer a readily available source of recycled water, but also an efficient agricultural fertilizer. However, the first step for the utilization of the digestate is the removal of any potentially harmful contaminants, and ultrafiltration membranes can provide successful remediation routes in this direction. This work investigates the industrial pilot-scale purification and reusability of the liquid digestate derived from the anaerobic treatment of waste mixtures of high organic content, using ultrafiltration membrane technology. Two different types of ultrafiltration membranes, polymeric and ceramic, were evaluated regarding their efficiency and long-term performance, parameters that heavily affect the overall costs of the operational unit. Our results indicate that the ceramic membranes exhibited a superior performance compared to its polymeric analogues, such as a higher flux, as well as significantly increased lifetime, signifying promising cost-effective and long-term applicability on an industrial level. In addition, the analytical physicochemical characterization of the ultrafiltration reject indicated its high nutrient value, suggesting its highly promising exploitation as an added value fertilizer, further enhancing the sustainability of the proposed approach.

1. Introduction

The ever-increasing environmental pollution and the depletion of natural resources demand an urgent implementation of stricter environmental protection policies [1,2,3]. There is a variability of processes (physical, chemical and biological) commonly used to degrade high organic load waste and remove pollutants [4,5,6]. Towards this direction, the transformation of industrially produced waste in biogas plants into useful by-products of added value constitutes a promising zero-waste strategy [7,8,9,10]. Biogas plants exploit various biological fermentation processes to generate energy from biogas. Unavoidably, the production of biogas is accompanied by the production of the digestate, with physicochemical properties that are not consistent, but depend on the biological fermentation process that is implemented and the input materials source. Most commonly, the digestate is alkaline and has high contents of water, while it comprises mainly bacteria and organic matter [11,12,13]. The main focus on the Anaerobic Digestion (AD) process is directed to enhance the biogas yield over the last years, ignoring the quality of digestate that can be produced [14]. Therefore, even though its appropriate handling can be tedious and expensive, there is a great opportunity for the reuse of both the liquid and the dry fraction of the digestate for irrigation and fertilizing purposes [15,16,17]. The additional income for a biogas plant that will use both liquid and solid fraction of digestate as a biofertilizer can reach up to 2095 EUR per day according to a recent study [18]. This value was estimated by combining data (from 8 different biogas plants) for liquid and solid digestate and comparing them with the market prices of typically used mineral fertilizers. The annual quantities for liquid and solid digestate produced in these plants were 38,129–79,706 m3 and 17,562–28,261 tonnes respectively.
However, it is clear that before the reuse of the digestate, any contaminants that could possibly pose a threat for the environment must be removed and membrane processes, and in specific ultrafiltration, have been proven industrially and scientifically effective. Pressure-driven ultrafiltration membranes can successfully purify the digestate from bacteria and viruses, colloidal particles and particulate matter via a sieve-like mechanism [19]. Currently, two different types of ultrafiltration membranes are widely employed at an industrial level: ceramic and polymeric. Polymeric ultrafiltration membranes are mainly prepared from cellulose, polyacrylonitrile (PAN), high-density polyethylene (HDPE), polyethylsulphone (PES), polysulphone (PS), polytetrafluoroethylene (PTFE) and polyvinylidine difluoride (PVDF). Such membranes are endowed with the general merits of polymeric materials, including easy processability and generally low costs [20]. However, they exhibit poor mechanical properties and low chemical and thermal resistance, as well as a high fouling due to their hydrophobic nature. These drawbacks heavily affect their sustainability, as they reduce their life duration and increase their repairing and replacement costs and frequency, thus limiting their applicability for long-term applications [21]. The use of amino-functionalized CuO and ZnO nanoparticles in PES membranes is suggested in order to improve pure water permeation and antifouling capacity in PES membranes [22]. In a recent review paper authors, have gathered useful information in order to understand the related mechanisms for fouling and present cleaning strategies that are described in literature [23]. On the other hand, ceramic membranes are mainly fabricated from Al2O3, SiC, CeO2, TiO2 or ZrO2 and are characterized by enhanced mechanical properties and high chemical and biological stability, resulting in long lifetimes and higher efficiency [24,25]. However, the industrial applicability of ceramic membranes is restricted by their high production costs. As a result, polymeric membranes, despite their significant disadvantages, are still more widely used for waste and wastewater remediation applications [26].
In this context, the application of polymeric and ceramic membranes for wastewater purification has gained momentum during the last decade, with the literature mainly focusing on membrane fouling, as this remains the most common challenge for sustainable and efficient wastewater treatment [21]. However, to the best of our knowledge, the published studies consist of short-term, bench-scale reports, using simulated or real wastewater of medium organic load. Such reports fail to take into account important parameters that are expected to heavily influence the operation efficiency and costs of applied industrial wastewater treatment, such as long-term performance, the extremely high organic loads of real industrial wastewater, as well as the seasonality in the composition and nature of such waste. In this work, we conduct an industrial pilot-scale, systematic and long-term study (>3 years) on the efficiency of two different types of ultrafiltration membranes, a ceramic and a polymeric, towards the remediation of the digestate of an industrial scale pilot unit. The studied influent wastewater comprised mixtures of waste of high organic loads resulting in a complex nature. We correlated the characteristics of the two types of membranes with their performance and their lifetime, while last but not least, we evaluated critical physicochemical properties of the concentrated UF liquid fraction that could allow its potential exploitation as a high added-value liquid fertilizer. Towards this direction, one of the most important parameters for high productivity and nitrogen utilization from plants is the amino acids concentration [27,28]. Plant development is enhanced by the presence of amino acids in the biomass, since they work as precursors of phytohormones and participate in the increase of chlorophyll levels in plants, as well as in the biosynthesis of polyamines, which is another plant growth-promoting compound [29,30,31]. We also examined the further treatment of the purified (UF filtrate) effluent with Reverse Osmosis (RO) to generate high quality water that could be potentially reused in the internal power plant water demands. The proposed methodology can be applied as an effective and green approach for the treatment of organic waste of high-strength, as well as complicated and seasonal composition on an industrial level, producing high-added value by-products, zero generated waste and a sustainable water balance.

2. Materials and Methods

2.1. Materials

The Liquid Digestate (LD) was obtained from an industrial scale AD facility installed in Heraklion Crete (Greece), which treats high-strength waste and wastewater. The composition of the AD feed consisted mainly of pig manure, food waste, cheese whey, slaughterhouse waste and olive mill wastewater. The AD feed was comprised of 50% pig manure, 30% slaughterhouse waste, while the nature of the remaining 20% was varied according to the seasonal availability of the waste mentioned above. AD facility involves using two sequential digesters of 4000 m3. Separation of dry and liquid phases of digestate was achieved by using a centrifuge. LD was added to a balance tank of 155 m3 which was separated into two compartments (25 and 130 m3) for anoxic treatment and denitrification. The next stage of the process consisted of an aeration tank of 78 m3, which is made of polypropylene. Effluent of the aeration tank was supplied to an Industrial scale Pilot Unit (PU) equipped with two types of ultrafiltration (UF) membranes for further treatment. The design of the UF system has been scaled up for a Biogas Power Plant with a production of 100 m3/d sludge from the AD.
Two different types of UF membranes were applied and compared. Wastewater was fed from the existing aeration tank and the efficient operation was investigated under different parameters such as Permeability, Flux, and Operational lifetime.
The first line of Ultrafiltration membranes was equipped with Pentair X-Flow Compact 27 Helix (Netherland), a PVDF membrane of 30 nm average pore size. The installed total membrane area was 54 m2, while the maximum operating temperature was 40 °C.
The second line of Ultrafiltration membranes was retrofitted from polymeric membranes to ceramic membranes, by TAMI industries (Nyon, France), INSIDE CeRAM™ membrane series. Due to the nature of the waste, the 8-channel monolithic element was used. The active layer of the membrane was made of ZrO2 and the nominal Molecular Weight Cut-off was 300 kg/mol. Finally, the installed total membrane area was 22 m2.
The characteristics of the membranes used in this study are presented in Table 1, as they are provided by the manufacturer.
The reverse osmosis unit employed was a small containerized commercial unit for leachate treatment, of LW2.000-IND C design and build by SYCHEM S.A. The system was fully automated and equipped with a series of treatment stages consisting of
  • Multimedia filtration stage with alumino-silicate glass from recycled green glass
  • Activated carbon filtration stage
  • Bag filtration at 5 microns
  • Two-Pass Reverse Osmosis
    ◦
    1st pass reverse Osmosis equipped with SUEZ 4″ IND RO3 4040F50 (France) membranes
    ◦
    2nd pass reverse Osmosis equipped with SUEZ 4″ AG 90LF (France) membranes
The nominal overall recovery was set to 45% due to the increased dissolved organic load.

2.2. Methods

The PU comprised the following treatment stages:
  • Pre-treatment Stage
The pretreatment stage was a secondary biological treatment with Denitrification and Nitrification stages. The inlet wastewater was fed by the supernatant of the Anaerobic Digester (AD) and the slurry was dewatered upstream the secondary biological treatment by a decanter centrifuge PIERALISI (Jesi (AN), Italy), Maior series, operating at gravitational force of 3.990 g.
  • Secondary Biological Stage
The effluent of the centrifuge was treated further at a biological treatment stage consisting of an Anaerobic Tank (50 m3), Anoxic Tank (155 m3) and an Aeration Tank (200 m3). The waste was treated to reduce the organic and nitrogen load. Additionally, the sludge was stabilized in the aeration tank to increase the quality of the produced liquid fertilizer.
  • Filtration Stage
At this step the effluent from the aeration tank was filtrated through an Ultrafiltration unit. The inlet temperature to UF was high and was maintained at a constant narrow range between 40–43 °C. A thorough investigation in the hydraulic operational values was performed to compare the UF lines. For each UF line, the following parameters were monitored: Feed pressure, Recirculation pressure, Inlet temperature, Concentrate flow, Recirculation flow and Permeate flow, all parameters that are commonly used in the literature [19,21,32].
Additionally, and only for the Ceramic UF line, the following parameters were also monitored: Ceramic membrane 1st pressure vessel (PV) inlet pressure, Ceramic membrane 2nd PV inlet pressure, Ceramic membrane 1st PV permeate pressure, Ceramic UF line total energy consumption.
The concentrate of the UF system was recycled to the Secondary Biological Treatment stage, while a fraction was used for the production and stabilization of the Liquid fertilizer.
Prior to the installation of the ceramic UF, a small industrial pilot (0.2 m2 membrane area) was tested to specify the pore size suitable for the given type of waste produced by the specific bioreactor. The test compared the different molecular weight cut-offs (MWCO), ranging from 50 kDa to 300 kDa as mentioned above, against different parameters, while for comparison reasons, the pre-existing polymeric UF line was also evaluated. UF membranes were installed externally, and operation mode was crossflow. Initially different crossflow velocities (2.0 m/s, 3.5 m/s, 4.0 m/s and 4.4 m/s), at stable Trans Membrane Pressure (TMP) 2.5 bar were tested at the small industrial pilot to verify the most stable operation.
The operation of the membranes was continuous due to the fact that the Biogas Power Plant did not allow downtime in operation. Following the installation and commissioning of the Ceramic UF line, the operation was conducted according to the needs of the plant, without any changeover for the membranes. Concerning the polymeric UF line, two (2) sets of identical membranes were used in order to fully replace the operational membranes whenever their performance was severely deteriorated and Clean-In-Place (CIP) procedure was performed. PVDF membranes were equipped with an extra set which was in standby mode and was used whenever CIP was required, in order to avoid time delays to the filtration process.
The effluent of the UF was processed downstream in a Reverse Osmosis (RO) system for pure water production to increase the recovery of water towards the internal processes of the plant and to reduce the consumption of water derived from other sources (groundwater). The RO process was studied for a fraction of the UF effluent, since the needs for reusing the industrial unit were limited. Nevertheless, data for the RO are presented in order to show the efficiency of the overall process. Application of the proposed technology (partial or whole) is dependent on the final effluent’s requirements and needs of the user regarding the type of discharge selected.
The optimum operation of the system was assessed by a holistic approach, as, in addition to the reliable operation of the UF system, the efficiency of the Aeration Tank was monitored along with the quality of the liquid fertilizer to maintain the high value of the side product. The feed pumps used in the two UF lines were operated at similar discharge flow to maintain a comparable data set. The average discharge flow on the Polymeric UF line was 152 m3/h and the Ceramic UF line was 147 m3/h. Since the feed/recirculation pumps were stable at a fixed discharge flow, the TMP was varied according to the instant quality of the waste fed to UF. The range of the TMP for polymeric line was 2.2–2.9 bar, while for the ceramic line the TMP was 2.71–3.8 bar.
For the Aeration Tank, a weekly sampling schedule was followed, and the major parameters were Chemical Oxygen Demand (COD), Mixed Liquor Suspended Solids (MLSS) and Ammonium nitrogen.
The analysis was performed in the internal laboratory following the methods that are shown in Table 2. Samples were either analyzed immediately at the laboratory or kept at the refrigerator (4 °C) until further use.
The research period of this work was almost 3 years, and it should be mentioned that the industrial unit under study is still in operation. The presented set of data is selected to reflect the efficiency of the process, due to the extensive period of the study.
Furthermore, the composition of the liquid digestate and the influent of the UF process is presented in Table 3, while various important parameters were quantified, i.e., total solids (TS), total suspended solids (TSS), volatile solids (VS) and volatile suspended solids (VSS), as proposed by Standard Methods [33]. Biochemical oxygen demand (BOD5) and pH values were determined using a WTW OXITOP 12 (Weilheim, Germany) and an Orion 3 Star Thermo Scientific (Singapore) pH meter, respectively. Total coliforms and E. coli were determined by using the IDEXX Quanti-Tray® (Maine, ME, USA) enumeration procedure with Colilert-18® (Maine, ME, USA) reagent according to the manufacturer’s guidelines. As can be observed in Table 3, the investigated digestate exhibited high values of COD, BOD, TS, and TN, suggesting high organic load waste used in AD. Moreover, the anoxic and aeration treatment contributed to the decrease of the organic load.
The operation of the UF system, which is discussed herein was, heavily affected by the TSS concentration. The analysis method for the determination of TSS was achieved by using Grade MGB Glass fiber paper filters (47 mm with 1 μm pore size). The turbidity of the permeates was determined by using 2100 Q Portable turbidimeter (Hach, Ames, IA, USA).
Regarding the liquid fertilizer, the concentration of amino acids was quantified, which constitutes a critical parameter that defines the quality and value of applied fertilizer. For the amino acids analysis liquid chromatography was used, equipped with mass spectrometry at an external private laboratory. Moreover, critical parameters were monitored, such as TN, pH, TOC, Phosphorus and Calcium, Potassium, Zinc, Magnesium as proposed by Standard Methods −3120 [33].

2.3. Statistical Analysis

All measurements were replicated in triplicate and the data are presented as mean ± SD. Statistical analysis of the data and the figures were performed using Microsoft Excel.

3. Results and Discussion

The methodology followed in this work is schematically presented in Scheme 1.
First, a preliminary, small-scale evaluation of the efficiency of the ceramic membranes with different MWCO was performed, in order to determine the most suitable pore size for the industrial scale unit and the results are presented in Table 4. The 300 kDa membrane in the small pilot system exhibited a reliable and stable performance over time, and resulted in the lowest permeate COD (1530 mg/dm3) and therefore was chosen as the most appropriate system for the larger scale application in the industrial unit.
Concerning the different crossflows tested in the initial small industrial pilot equipped with ceramic membranes, in order to verify the most stable operation, results showed that for increased crossflows the flux was also increased and the operation was stable. Only when the crossflow was 2 m/s did the system lose its stability.
Figure 1 shows the crossflow obtained in the ceramic and polymeric UF lines at similar recirculation flow for the full-scale process. It can be observed that, at similar recirculation flow, the Ceramic membranes operated at increased crossflow. During the data set logging, two (2) membrane changeovers on the polymeric UF line were performed (red dots). The changeover was carried for Clean-in-Place (CIP) reasons, to maintain the operation of the line and avoid any downtime in the industrial process. On the ceramic line, no CIP had been performed during the detailed hydraulic investigation.
The crossflow is a critical parameter for the reduction of the fouling on the membrane surface, but it should be cautiously increased, as the higher the crossflow was, the higher the energy consumption of the system became [34].
The net permeate flow was found comparable for both systems, as shown in Figure 2, although, the installed area (22 m2) of the Ceramic membranes was significantly lower compared to the polymeric ones (54 m2). As stated before, the red dot symbol indicates the membrane changeovers for CIPs. This results in a much higher flux for the Ceramic UF when compared to the Polymeric UF membranes, as can be observed in Figure 3.
Based on all the above data, it is apparent that the Polymeric UF membranes suffered mainly from significant deterioration of the membrane surface due to the increased temperature. This, in combination with the high pollutant load, resulted in excessive CIP requirements, as, at an average, the system demanded 2 CIPs per month. The average lifetime of each polymeric module was shortened to approximately 1 year. On the contrary, the ceramic UF were not affected by the parameters of temperature, and the variations on pollutant load did not affect the operation to an extent sufficient to hinder the constant operation of the Power Plant. The ceramic membranes remained in operation for over 1.5 year, without signs of decreased flux or low quality permeate, meaning that the active layer was still intact.
Following the above detailed data logging, a simpler set of data was applied for a more extended time period (392 days), where only the permeate of each line was monitored (Figure 4).
At Day 202 of the operation, a critical redesign on the Ceramic UF pipeline was performed. This redesign exploited the full operational advantages of the ceramic membranes; afterwards, an unparallel flux was achieved that ranged 7–9 times higher than the polymeric UF line. Following the upgrade on the design, the Polymeric line was switched off and the Ceramic line was operated stand-alone in order to verify the operation.
From Day 358, a series of operational problems arose, as a result of multiple destruction incidents of the membrane tubes in the Polymeric UF line, which resulted in an unstable operation of the overall process, and as a result affected the Ceramic UF line. The achieved flux in the current industrial application dwarfs the achieved flux achieved in other laboratory experiments for similar types of waste and membranes supplied from the same membrane manufacturer, an increase that can be attributed to the beneficial effect of the crossflow [19].
The operation of the UF was correlated to the aeration tank variables for each UF line by applying the Pearson’s product–moment correlation coefficient (PPMCC). The samples were collected in a weekly basis and the data collected represent a total of 881 days of operation.
According to Table 5, the Polymeric membrane was mainly affected by the concentration of the TSS and, as already mentioned, by the temperature. Specifically, as the TSS were reduced, the Flux increased. Similarly, as the temperature increased, the Flux decreased. Specifically for the temperature parameter, the correlation was proven by the real time conditions, and the explanation is the deterioration of the PVDF surface. As the temperature increased, the flux was reduced and eventually led to the end life of the membranes. The TS showed a weak positive correlation to the operation. Since the correlation is weak, no further investigation was performed on the effect of this parameter.
For the ceramic UF line, the TSS concentration exhibited a strong correlation to Flux, while on the contrary the temperature exhibited a weak correlation to flux. This can be explained by the durability of the material at high temperatures, while the weak negative correlation can be assigned to the complex mixture and a possible gelatinization on the membrane surface [35]. This requires further investigation for a better understanding of the mechanisms and reactions occurring in similar applications.
According to our study, no fouling was observed for the ceramic membranes used, during the whole experimental period, a phenomenon that indicates the economic viability of the process. It is well-known and reported that one of the main limitations for UF process treating digestates is the membrane fouling [32]. Moreover, it should be stated that the high temperature of the digestate treatment (~40 °C) was proven detrimental for the examined PVDF membranes, but no decrease in efficiency and in flux was observed in the case of their ceramic analogues. It has recently been reported that TMP presents a significant effect on the membrane fouling and resistance in a lab-scale unit that treated liquid digestate from the agricultural biogas facility with aerobic granules and ultafiltration [36]. The better performance of the ceramic membranes can be attributed to their more hydrophilic surfaces [37,38]. The results presented in our study led to an adaptation of the initial approach for the most suitable type of UF membranes, so both treatment lines of the industrial unit employed in this study will utilize Ceramic UF membranes in the future.
For the reliable and stable operation of the UF system, an optimization of all the treatment stages upstream of the Ultrafiltration unit had to be performed. In Figure 5 the monitoring data for a total of 1100 days are displayed for critical parameters relevant to biological activity.
From Figure 5 it can be observed that the biological treatment required approximately 270 days to be stable. Thus, after stabilization, and specifically on day 573, the Ceramic UF line was commissioned in parallel with the Polymeric UF. The periodical variation of the data was found to be affected by the seasonal changes in the AD feedstock. A 10% removal of TN in the aerobic tank was observed for the last 60 days of operation, comparing to the digestate from 6.86 ± 0.15 to 6.26 ± 0.27 g/dm3 respectively. This observation indicates that nitrification–denitrification was improved during that period.
The ammonium nitrogen concentration (Figure 6) exhibited a similar trend to the TN values and was also affected by the seasonal changes in the AD feedstock. From day 680, it is clear that the nitrification stage showed some enhanced operation, which was the result of internal recirculation between the anoxic to anaerobic digester that took place. The convergence observed at a later stage, a deterioration on the nitrification process, was the result of a series of technical problems in the air distribution pipeline in the aeration tank. After successful replacement, the operation was restored, and the Nitrification process delivered the expected results by removing a fraction of Ammonia. Again, as it was expected, in the last 60 days of operation, NH4-N was decreased in the aerobic tank from 5.97 ± 0.14 to 4.91 ± 0.35 g/dm3, signifying a 17.6% removal.
The efficiency of the UF and RO is presented in Table 6 for an average set of data. The overall COD, BOD, TN, NH4-N and BOD removal for the liquid digestate was above 98%. Thus, the final effluent meets the requirements for reuse according to Greek Legislation [39] and new EU regulation 2020/741 [40], which allow unlimited irrigation and industrial use (boilers) for effluents coming from treatment like the one presented in this study, meeting criteria of BOD (≤10 mg O2/dm3), TSS (≤10 mg /dm3), Escerichia coli (≤50 EC/100 mL) and turbidity (≤5 NTU).
In another study recently reported by Gienau et al. [41], pilot tests were conducted at a digestate produced in a 2.5 MWe agricultural biogas plant using two-stage solid/liquid separation unit followed by UF and a 3 stage RO, in which the supernatant from the decanter COD was 15.7 g/dm3, which was decreased after UF and RO to 4.7 g/dm3 and <15 mg/dm3 respectively. In our study, the initial value of the COD after the centrifuge was higher (23.03 g/dm3) and the UF, presenting better removal, reached up to 93%. The efficiency of the treatment of liquid digestate in another study with aerobic granules and ultrafiltration equipped with ceramic membranes (pressure TMP 0.3 MPa) was about 76% for the COD [36]. Urbanowska et al. [19] demonstrated that for the treatment of municipal waste biogas plant digestate, the best separation efficiency of organic substances using ceramic membranes, was achieved for the most compact with a cut-off 1kDa (43% decrease in COD). The evaluation of a MBR (PP/PE membranes)-RO (Polyphtalamide membranes) system for the treatment of liquid digestion produced by AD of swine wastewater was examined by Qi et al. [42], who reported more than 90% removal of TSS (UF) and 40% of COD respectively.
Concerning the removal of pollutants in UF, Świątczak et al. [36] reported that some portion of COD was adsorbed on the ceramic membranes, which reached a maximum effect for TMP of 0.4 MPa and didn’t correspond with the highest fouling, a phenomenon that indicates that the size of the adsorbed particles may be bigger than the membrane pores. It has also been reported that UF is a better method for the removal of TN when compared to the microfiltration, due to the smaller size of the pores [43,44].
In order to determine the amino acids concentration, samples from the solid fraction of the digestate and the rejection of ultrafiltration of the PU were analyzed with liquid chromatography coupled with mass spectrometry (LC-MS/MS). The resulting concentrations of amino acids are presented in Table 7.
It is obvious that the presence of amino acids in both the solid and liquid fertilizer is significant. Several studies are dealing with the determination of amino acid content in the digestates, in order to enhance the valorisation of these substances [45,46]. In one of these works, a modified airlift reactor was, used to treat liquid digestate produced from an AD unit that was fed with chicken manure, in which the total amino acids were up to 30.8% of the dry matter of biomass [45]. These results are comparable to our study, since the total amino acids concentration in the solid fertilizer was found to be 8.71% (not dried) and 1.76% in the liquid digestate, respectively. In another recent study, the determination of amino acids in the digestate certified the existence of several amino acids (arginine, histidine, isoleucine, lysine, tryptophane, threonine, valine) that also were present in our samples in similar amounts [46].
It is apparent that in the UF concentrate stream, the side product meets the specifications of a high-quality liquid fertilizer. Furthermore, an analysis of the rejects of the UF (Table 8) shows that the concentration of common chemical substances needed for cultivation is significant.
The fate of the nutrients and humic acid in a (UF-MBR) process followed by Reverse Osmosis for the treatment of liquid digestate from swine wastewater was studied recently in a lab scale bioreactor [42]. The results showed that this process is very promising for water recycling and nutrient recovery. Barzee et al. [47] reported that the ultrafiltered dairy manure biofertilizer, applied through a subsurface drip irrigation system, proved to have the highest yield of red tomatoes, 136.4 tn/ha, compared to mineral N fertilizer and ultrafiltered food waste digestate. In order to optimize the fate of amino acids especially in permeate, future experiments will validate the cut-off size of the ceramic UF membrane, so that highest possible amino acid concentration can be successfully recovered in the liquid fertilizer fraction.

4. Conclusions

Based on the long-term comparison of the effectiveness and stability of polymeric and ceramic membranes in an industrial scale unit treating liquid digestate of high organic load waste and wastewater that were pre-treated anaerobically, the following conclusions can be drawn:
  • In the proposed industrial-scale UF process, the ceramic membranes (ZrO2, Molecular Weight Cut-off of 300 kg/mol) showed a better behavior concerning the flux when compared to the polymeric (PVDF, 30 nm average pore size) analogues. A key factor for this was proven to be the increased temperature of the digestate and the high pollutant load of the treated wastewater.
  • The ceramic membranes exhibited longer lifetimes than the polymeric options for the whole duration of the project, suggesting their superiority for long-term industrial applications that could eventually result in lower replacement costs, in spite of the higher initial production costs.
  • The proposed approach exhibits great sustainability, as the purified (UF filtrate) can be potentially reused in the internal Power Plant water demands after further treatment with Reverse Osmosis.
  • Last but not least, the side product of UF was proven to be a high added value fertilizer containing many amino acids and a significant amount of chemical substances that can efficiently enhance the production yields of agricultural cultivation.

Author Contributions

Conceptualization, A.Y.; methodology, A.Y. and N.Y.; validation, A.Y., N.Y. and N.K.; formal analysis, F.M., G.G., S.D. and E.V.; funding acquisition, A.Y. and N.K.; investigation, F.M., S.D. and E.V.; resources, A.Y., T.A. and N.K.; writing—original draft preparation, G.G., S.D. and E.V.; writing—review and editing, A.Y., N.Y. and N.K.; visualization, A.Y., G.G., S.D. and N.K.; supervision, A.Y., N.Y., T.A. and N.K.; project administration, A.Y and T.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research has been co-financed by the European Regional Development Fund of the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH—CREATE—INNOVATE (project code: T1EDK-01633).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 15 06574 sch001 550
Scheme 1. Schematic representation of the proposed methodology.
Scheme 1. Schematic representation of the proposed methodology.
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Figure 1. Crossflow achieved in each UF line at similar recirculation flow. The red dots indicate the membrane changeovers on the polymeric UF line.
Figure 1. Crossflow achieved in each UF line at similar recirculation flow. The red dots indicate the membrane changeovers on the polymeric UF line.
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Figure 2. Net permeate flow of the Polymeric UF and the Ceramic UF. The red dots indicate the membrane changeovers on the polymeric UF line.
Figure 2. Net permeate flow of the Polymeric UF and the Ceramic UF. The red dots indicate the membrane changeovers on the polymeric UF line.
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Figure 3. Flux of the Polymeric UF and the Ceramic UF. The red dots indicate the membrane changeovers on the polymeric UF line.
Figure 3. Flux of the Polymeric UF and the Ceramic UF. The red dots indicate the membrane changeovers on the polymeric UF line.
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Figure 4. Flux of the Polymeric UF line and the Ceramic UF line. The monitored operational time is 392 days.
Figure 4. Flux of the Polymeric UF line and the Ceramic UF line. The monitored operational time is 392 days.
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Figure 5. Total Nitrogen concentration monitored during each stage of the pilot plant.
Figure 5. Total Nitrogen concentration monitored during each stage of the pilot plant.
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Figure 6. NH4-N concentration at each stage.
Figure 6. NH4-N concentration at each stage.
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Table
Table 1. UF membranes applied in liquid Digestate treatment.
Table 1. UF membranes applied in liquid Digestate treatment.
Compact 27 HelixCeramic 50 kDaCeramic 150 kDaCeramic 300 kDa
Cut-off (kDa)150 * (30 nm)50150300
MaterialPVDFZrO2ZrO2ZrO2
Water permeability at 25 °C (L/(h.m2.bar)1000 +/− 10%252293692
Membrane area per module (m2)270.20.20.2
Hydraulic membrane diameter (mm)8666
* Estimation.
Table
Table 2. Description of the kits that were used for the analysis of the digestate properties.
Table 2. Description of the kits that were used for the analysis of the digestate properties.
ParameterKit
CODHACH LCK514
(150–2000 mg/dm3)
Soluble Ammonium nitrogen (sNH4-N)MACHEREY-NAGEL
Tube test NANOCOLOR Ammonium 100
(4–80 mg/dm3 NH4-N)
Soluble Total Nitrogen (sTN)HACH LCK138
1–16 mg/dm3
Soluble Nitrate-N (sNO3-N)HACH LCK339
0.23–13.5 mg/dm3
Soluble Nitrite-N (sNO2-N)HACH LCK341
0.015–0.6 mg/dm3
Table
Table 3. Characteristics of liquid digestate and the influent of Ultrafiltration process.
Table 3. Characteristics of liquid digestate and the influent of Ultrafiltration process.
ParameterUnitLiquid DigestateInfluent UF
pH 8.16 ± 0.208.52 ± 0.20
TSg/dm377.13 ± 4.5779.1 ± 5.65
TSSg/dm321.05 ± 1.5025.0 ± 1.76
BOD5g O2/dm36.20 ± 0.313.49 ± 0.15
VSg/dm330.10 ± 2.2239.03 ± 2.78
VSSg/dm39.32 ± 0.4116.18 ± 1.40
CODg O2/dm359.30 ± 1.1023.03 ± 0.62
TNg/dm37.8 ± 0.77.2 ± 0.8
NH4-Ng/dm35.60 ± 0.224.25 ± 0.20
NO3-Nmg/dm3117.9 ± 9.5111.5 ± 8.2
NO2-Nmg/dm311.5 ± 0.89.4 ± 0.6
Table
Table 4. Results from different MWCO in a small industrial pilot unit.
Table 4. Results from different MWCO in a small industrial pilot unit.
MWCO (kg/mol)Ceramic
50 kDa		Ceramic
150 kDa		Ceramic
300 kDa		Polymeric
30 nm
Recirculation (m3/h)4.44.44.4-
TMP (bar)2.952.52.5-
Permeate flow (dm3/h)5.564.34.52-
Flux (LMH)27.821.522.6-
TSS inlet (mg/dm3)16,00012,75014,500-
COD permeate (mg/dm3)1634193215301796
TSS permeate (mg/dm3)3.51.342.2
Table
Table 5. PPMCC correlation table of different parameters to Flux (LMH).
Table 5. PPMCC correlation table of different parameters to Flux (LMH).
Correlation of Different Parameters to Flux (LMH)Polymeric UFCeramic UF
TSS−0.56−0.49
TS0.340.14
Temperature−0.65−0.34
Table
Table 6. Characteristics of effluent of the Ultrafitration and RO process.
Table 6. Characteristics of effluent of the Ultrafitration and RO process.
ParameterUnitUF EffluentRO Effluent
pH 8.5 ± 0.25.4 ± 0.1
TurbidityNTU14.3 ± 0.10.23 ± 0.01
TSg/dm39.2 ± 0.20.29 ± 0.09
TSSmg/dm312.0 ± 0.71.0 ± 0.1
BOD5mg O2/dm3270 ± 10<10
VSg/dm31.3 ± 0.10.03 ± 0.01
VSSmg/dm34.0 ± 0.3n.d.
CODmg O2/dm31470 ± 36<15
TNmg/dm34300 ± 14145.6 ± 2.1
NH4-Nmg/dm34093 ± 31837.9 ± 3.6
NO3-Nmg/dm382.0 ± 7.33.9 ± 0.4
NO2-Nmg/dm346.0 ± 4.12.8 ± 0.3
Total ColiformsMost Probable Number (MPN)n.d.n.d.
E. coliMost Probable Number (MPN)n.d.n.d.
n.d.: not detected.
Table
Table 7. Amino acid concentration of liquid fertilizer (UF reject) against the solid fertilizer (Decanter Centrifuge).
Table 7. Amino acid concentration of liquid fertilizer (UF reject) against the solid fertilizer (Decanter Centrifuge).
Amino AcidSolid Fertilizer (g/kg)Liquid Fertilizer (g/kg)
Lysine4.931.22
Arginine5.350.73
Histidine0.450.12
Glycine4.851.14
Serine5.480.79
Alanine7.521.72
Threonine5.230.89
Glutamic Acid10.92.05
Aspartic Acid13.12.36
Proline6.060.95
Valine6.011.12
Methionine<0.10.16
Tyrosine2.080.77
Isoleucine3.610.99
Leucine7.121.31
Phenylalanine4.441.12
Tryptophane-0.14
Total Amino Acids87.117.6
Table
Table 8. Chemical analysis of the liquid fertilizer obtained from the UF.
Table 8. Chemical analysis of the liquid fertilizer obtained from the UF.
ParameterValue
TN1.1% w/w
pH8.0–8.9
TOC2.6% w/w
Calcium0.6% w/w
Phosphorus0.1% w/w
Potassium0.17% w/w
Zinc510 mg/kg
Magnesium230 mg/kg
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Yfantis, A.; Yfantis, N.; Angelakopoulou, T.; Giannakakis, G.; Michelet, F.; Dokianakis, S.; Vasilaki, E.; Katsarakis, N. Industrial Pilot for Assessment of Polymeric and Ceramic Membrane Efficiency in Treatment of Liquid Digestate from Biogas Power Plant. Energies 2022, 15, 6574. https://doi.org/10.3390/en15186574

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Yfantis A, Yfantis N, Angelakopoulou T, Giannakakis G, Michelet F, Dokianakis S, Vasilaki E, Katsarakis N. Industrial Pilot for Assessment of Polymeric and Ceramic Membrane Efficiency in Treatment of Liquid Digestate from Biogas Power Plant. Energies. 2022; 15(18):6574. https://doi.org/10.3390/en15186574

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Yfantis, Alexandros, Nikos Yfantis, Triantafyllia Angelakopoulou, George Giannakakis, Fabien Michelet, Spyros Dokianakis, Evangelia Vasilaki, and Nikos Katsarakis. 2022. "Industrial Pilot for Assessment of Polymeric and Ceramic Membrane Efficiency in Treatment of Liquid Digestate from Biogas Power Plant" Energies 15, no. 18: 6574. https://doi.org/10.3390/en15186574

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