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Article

Changes in the Composition of Digestate Liquid Fraction after Ozone and Ultrasonic Post-Treatment

by
Aleksandra Chuda
1,*,
Konrad Jastrząbek
2 and
Krzysztof Ziemiński
1
1
Department of Environmental Biotechnology, Lodz University of Technology, Wolczanska 171/173 Street, 90-530 Lodz, Poland
2
Institute of Natural Products and Cosmetics, Lodz University of Technology, Stefanowskiego 2/22 Street, 90-537 Lodz, Poland
*
Author to whom correspondence should be addressed.
Energies 2022, 15(23), 9183; https://doi.org/10.3390/en15239183
Submission received: 7 November 2022 / Revised: 27 November 2022 / Accepted: 1 December 2022 / Published: 3 December 2022
(This article belongs to the Section B: Energy and Environment)
Browse Figures
Versions Notes

Abstract

:
There is a growing concern about environmental pollution with digestate, which is produced in significant amounts in the anaerobic digestion process. The inappropriate application of digestate in agriculture may lead to ammonia emission to the atmosphere, nutrients infiltration to groundwater and surface waters eutrophication. There is a great interest in the development of efficient downstream technologies that will help better handle digestate. This study assessed the effect of three different disintegration methods (ozonation, ultrasonication, combination of ozonation and ultrasound) on solids content, nutrient concentration and biodegradability of the liquid fraction of sugar beet pulp digestate. The influence of initial pH (7, 8, 9), ozone dose (0.05–0.45 g O3/g TS), specific ultrasound energy (10,381–51,903 kJ/kg TS) and vibration amplitude (50, 100%) on the performance of digestate liquid fraction treatment was investigated. The highest removal efficiencies of organic substances, total and ammonia nitrogen averaging at 13.81, 20.04 and 28.70%, respectively, in separate ozonation and ultrasonication processes, was obtained at ozone dose of 0.25 g O3/g TS, specific energy of 41,522 kJ/kg TS and amplitude of 100%. The application of combined processes, first ultrasonication and then ozonation, resulted in an increase in the above-mentioned removal efficiencies to 58.16, 36.60 and 48.71%, respectively.

Graphical Abstract

1. Introduction

Biogas production in the anaerobic digestion process is growing in Europe as a result of activities aimed at mitigating global warming, as well as the sustainable management of organic waste. The increased production of biogas is associated with the formation of a large volume of digestate containing high concentrations of nutrients, the proper management of which is obligatory [1,2,3,4]. A biogas plant with an installed capacity of 1 MW generates about 30,000 m3 of digestate per year. As a result, around 180 million tonnes of digestate are produced annually in Europe [5,6]. The oversupply of digestate constitutes the major obstacle to its sustainable recycling in regions of Europe with large numbers of biogas plants [7,8]. The common method of digestate utilization is application as a fertilizer and returns the nutrients directly to the soil. However, agricultural management in the case of a large volume of digestate is associated with transport and logistic problems, as well as high costs exceeding the fertilizing value of the digestate. A biogas plant producing about 30,000 m3 of digestate per year must pay over EUR130,000 for its agricultural use [8,9,10]. Moreover, due to nitrogen and phosphorus fertilization limits, which amounted to 170 kg N. ha−1 y−1 and 250 kg P2O5 ha−1 y−1, respectively, digestate can be applied as fertilizer only during the growing season or vegetative growth [10]. Inappropriate application of digestate can lead to methane, ammonia and nitrous oxide emission to the atmosphere, ammonia nitrogen leakage to the surface water, infiltration of nutrients to the groundwater, pathogens transfer, thus to global warming, soil acidification and eutrophication of surface waters [3,10,11]. Ammonia is also emitted from digestate during its storage before field application. Taking into account the logistic problems, high costs and negative environmental impact of agricultural digestate management, there is a great interest in the development of efficient downstream technologies that will help better handle digestate [8].
The first stage of digestate management is usually mechanical separation into a solid fraction with a high dry matter content, rich in organic matter and phosphorus, and a low dry matter liquid fraction containing a large amount of nitrogen. The solid fraction can be used as fertilizer and also composted, incinerated, dried and pelletized [1,3,8,9,10,12]. In turn, in order to decompensate the recalcitrant form of nitrogen in the digestate liquid fraction and increase its biodegradability, thus digestate disintegration, advanced oxidation processes such as ozonation and ultrasonication/sonolysis are investigated [13,14,15,16,17,18]. Ozone, which is a strong oxidant with a potential equal to 2.07 eV, inactivates biomass by breaking down the bacterial cell wall, lyse polysaccharides, proteins and lipids, solubilizes particles and oxidizes organic matter to simple molecules by two pathways: a direct, but relatively slow and selective reaction by molecular ozone at low pH (<4) and an indirect and faster reaction by hydroxyl radicals (•OH) at high pH (>7). However, during sonolysis, ultrasound at a frequency of 20–500 kHz causes the growth and collapse of microbubbles in a process defined as cavitation [13,17,19,20,21,22]. This generates transient high pressures and temperatures, as well as free, highly reactive radicals (•H, •OH) that contribute to the solid particles disintegration, microbial cells destruction, complex molecules disruption and the release of more easily decomposing monomers. As a consequence of exposure to ozone and ultrasound, the substrate becomes more biodegradable [13,15,17,20,21,22]. The efficiency of ozonation strictly depends on the composition of the supplied substrate and the applied ozone dose [23]. The factors influencing the ultrasound performance are the operating conditions (power, treatment duration and ultrasound frequency), the substrate physico-chemical properties and the reactor configuration [15]. Ozonation and ultrasonication are commonly used to oxidize wastewater [24], reduce the production of activated sludge in wastewater treatment plants [20,25,26] and pretreat substrate prior to anaerobic digestion [13]. However, their application downstream of anaerobic digestion is a relatively new concept that may open up new perspectives in the field of anaerobic digestion [13,15,16,17,27]. The research on ozonation and ultrasound of the digestate described so far in the literature focuses mainly on improving its biodegradability, returning it to the digester and, as a result, increasing the biogas production [15,16,28,29,30]. Due to the fact that digestate products vary in terms of composition, depending on the feedstock quality, the technology used for biogas production or the operating parameters of the biogas plant, each digestate should be treated as a unique research object, and therefore study on the effect of ozone and ultrasound should be carried out for any type of digestate [31]. In addition, it is also worth noting that despite the fact that mechanical separation is currently a widely used method of pre-treatment of digestate, there are few studies in which the digestate liquid fraction was subjected to the ozonation, as well as ultrasonication process [4,17]. Therefore, it is still a remarkable research question that should be answered, especially since the composition of the liquid fraction differs significantly from the digestate. The combination of mechanical separation with ozone or ultrasound treatment could lead to a reduction in the costs of digestate storage and transport, as well as a more effective agricultural management of the digestate. According to Somers et al. [17] and Lindner et al. [32], the ozonation and ultrasound used in digestate processing contribute to valorizing a residual organic content, adjusting the nutrient concentration and changing digestate physicochemical properties, and as a result, increase its agronomic value.
The study investigated the application of ozonation and ultrasound individually or in combination as post-treatment of the liquid fraction of digestate from the anaerobic digestion of sugar beet pulp obtained after mechanical separation in a decanter centrifuge. The influence of selected parameters, such as ozone dose, ultrasound energy and pH of digestate on the nutrients concentration, solids content and biodegradability of the digestate liquid fraction was determined. This will enable the expansion of knowledge in the field of processing the digestate liquid fraction through the advanced oxidation processes.

2. Materials and Methods

2.1. Substrates Characteristics

The digestate was taken from a biogas plant located in the sugar factory, which was continuously fed with sugar beet pulp (SBP). The anaerobic digester was operated under mesophilic conditions (35 °C), with a hydraulic retention time of ca. 40 days and reactor load with the organic matter of 5.45 kg VS/(d×m3). The liquid fraction of digestate obtained after its centrifugation with the addition of cationic polyacrylamide flocculant in the dose of 11 g/kg DM carried out in the UCD 305–00–32 decanter centrifuge (GEA, Warsaw, Poland) was used as a substrate for the study. During the separation, the centrifuge bowl rotational speed amounted to ca. 4000 rpm, and the centrifugal force was ca. 3400xG. The main characteristics of digestate and its liquid fraction are reported in Table 1.

2.2. Experimental Set-Up

This study investigated different techniques of digestate liquid fraction treatment: ozonation (O3), ultrasonication (US) and a combination of ozonation and ultrasound. In the ozonation and ultrasonication processes, three series of tests were carried out, marked as O3:A, O3:B, O3:C and US:A, US:B, US:C, respectively. The series differed in pH of the digestate liquid fraction in order to determine the impact of this indicator on the performance of its treatment processes. The pH value was corrected with a solution of 0.1 M H2S and 0.1 M NaOH. In series O3:A and US:A, the pH value of the digestate liquid fraction defined as substrate A was 7.05 ± 0.05. In series O3:B and US:B, the pH value of the digestate liquid fraction defined as substrate B was 8.08 ± 0.15. In series O3:C and US:C, the pH value of the digestate liquid fraction defined as substrate C was 8.97 ± 0.10.

2.2.1. Ozonation—Experiment A

The ozonation experiments were carried out at room temperature (20 ± 2 °C) in a lab-scale glass reactor consisting of a bubble column with an active volume of 2 dm3, which was connected to an ozone generator (Anseros, Tübingen, Germany). The ozone generator was fed with pure oxygen gas (Air Liquide, Schelle, Belgium). The gas stream, leaving the generator, was introduced from the bottom into the reactor containing the sample. The ozone concentration in the gas stream amounted to 150 g O3/Nm3, and the gas flow rate was 0.1 dm3/min. The sample volume was 0.2 dm3. The off-gas (exhaust gas) left the reactor in its upper part. The ozone concentration in the off-gas was monitored with a BMT 964 ozone analyzer (BMT MESSTECHNIK GMBH, Berlin, Germany). In the experiment in series marked as O3:A, O3:B and O3:C, ozone doses ranging from 0.05 to 0.45 g O3/g TS were applied. The ozone doses were estimated based on Equation (1).

2.2.2. Ultrasonication—Experiment B

The ultrasonication was performed using a UP400St ultrasonic processor with a frequency of 24 kHz, a maximum power of 400 W and a maximum amplitude of 200 µm, containing an S24d14D sonotrode with a 14 mm titanium tip (both from Hielscher, Germany). During the experiments, the digestate liquid fraction to be treated was put in a cylindrical glass beaker with the sonotrode placed in the center at a depth of 4.5 cm. The sample volume was 0.4 dm3, and the initial sample temperature was 20 °C. The ultrasonic processor power amounted to 160 W. The sonication density was 400 W/dm3. In series marked as US:A, US:B and US:C, the digestate liquid fraction was sonicated at 100% (99 µm) amplitude and specific energy (SE) ranging from 10,381 to 51,903 kJ/kg TS. This was to verify the impact of SE and the initial pH of digestate liquid fraction on the ultrasonication process efficiency. The specific energy was calculated according to Equation (2). In series marked as US:B50, in order to determine the effect of vibration amplitude on the ultrasonication process, the digestate liquid fraction with pH equal to 8.08 (substrate B) was sonicated using the specific energies listed above, but with the amplitude of 50% (49.5 µm).

2.2.3. Combination of Ozonation and Ultrasonication—Experiment C

In the study, the combined ozonation and ultrasonication processes were also carried out in order to check whether it is possible to obtain higher treatment performance of the digestate liquid fraction. Two combinations of processes were used. In the series marked as O3:B + US:B, ozonation was performed first, and then ultrasonication, while in US:B + O3:B series, the reverse sequence of processes were applied. The experiment used the digestate liquid fraction with a pH equal to 8.08. The ozonation and ultrasonication processes of the digestate liquid fraction were performed in accordance with the methodology described in Section 2.2.1 and Section 2.2.2, respectively. Treatment with ozone was carried out at the dose of 0.25 g O3/g TS, while ultrasound at the specific energy of 41,522 kJ/kg TS and the amplitude of 100%. These parameters were selected as optimal in the described research.

2.3. Analytical Methods

The digestate, liquid fractions of digestate and effluents after disintegration processes were characterized for pH and content of total solids (TS), volatile solids (VS), total suspended solids (TSS) and volatile suspended solids (VSS). These parameters were determined according to the standard methods [33]. The pH was measured using a CPI–505 pH meter (ELMENTRON, Zabrze, Poland). The concentrations of total chemical oxygen demand (tCOD), soluble chemical oxygen demand (sCOD), total nitrogen (TN), ammonia nitrogen (NH4–N), nitrate nitrogen (NO3–N), nitrite nitrogen (NO2–N) and total phosphorus (TP) were measured with Hach Lange test kits (Hach–Lange, DR 6000 UV–VIS Spectrophotometer). The analytical procedures adopted by Hach Lange GmbH (Düsseldorf, Germany) followed the standard methods [33]. The concentration of biochemical oxygen demand (BOD5) was measured using the OxiTop system, WTW. Samples for analysis were taken in triplicate; thus, the composition of influents and effluents in all experiments was the mean of the obtained results ± standard deviation.

2.4. Calculations

The ozone dose was determined based on the Equation [20]:
Ozone dose (mg O3/mg SS) = Mass O3/(Vsludge × SSi),
where Mass O3 (mg), Vsludge (dm3) is sludge working volume and SSi (mg/dm3) is the initial concentration of suspended solid.
The specific energy (SE) used during the ultrasonication of the digestate liquid fraction was estimated according to the Equation [16,27]:
SE (kJ/kg TS) = (P × t)/(V × TS),
where t (s) is the reaction time, P (W) is the ultrasound power, V (dm3) is the sample volume and TS (%) is the total solid concentration of the digestate liquid fraction treated.
The degree of disintegration (DD) used to evaluate the disintegration techniques’ efficiency was estimated according to the following Equation [16]:
SDDCOD (%) = (sCODtreatment − sCOD0)/(tCOD − sCOD0) × 100%,
where sCODtreatment is the soluble COD of effluent after treatment, sCOD0 is the soluble COD of the original sample (digestate liquid fraction) and tCOD is the total COD of the digestate liquid fraction.
The removal efficiency (RE) was calculated from the Equation [34]:
RE (%) = (C0 − Ct)/C0 × 100%,
where C0 and Ct are the concentrations of a given indicator in the digestate liquid fraction before treatment and the effluent after the treatment process, respectively.

2.5. Statistics

Analysis of variance (ANOVA test) followed by Tukey post hoc was used to determine if the disintegration techniques had a statistically significant influence on the composition of digestate liquid fraction. All tests were performed in triplicate, and differences were significant only when p < 0.05. All statistical analyses were performed using Statistica 12 program (StatSoft, Kraków, Poland).

3. Results and Discussion

3.1. Ozonation (O3)—Experiment A

In the conducted research, the ozonation process was applied to post-treatment of the liquid fraction of digestate produced in the biogas plant fed with sugar beet pulp. The effect of ozone dose and pH of the liquid fraction on the solids content, the degree of disintegration and the concentration of organic substances and nutrients were determined. The results are shown in Figure 1 and Figure 2.
In the study, the TS and TSS concentrations in the digestate liquid fraction after the mechanical separation process amounted to 11.56 and 2.75 g/dm3, respectively. The proportion of VS and VSS in TS and TSS was at the level of 55.87 and 46.91%, respectively. By analyzing the results in Figure 1a,b, it was found that the higher ozone dose applied in the ozonation process, the lower TS and TSS concentrations in the digestate liquid fraction and the greater proportion of organic parts (VS and VSS, respectively).
It was also noticed that the lowest concentrations of solids were obtained in O3:C series, in which the pH of the liquid fraction amounted to 9. This was probably related to the production of more hydroxyl radicals (OH•) under alkaline conditions, which have the ability to disintegrate the sludge faster [17,20,22]. As shown in Figure 1a,b, the removal efficiency of TS and TSS amounted to 49.20 and 93.45% (O3:C series), 45.82 and 92.36% (O3:B series), 40.03 and 90.18% (O3:A series) at the ozone dose of 0.45 g O3/g TS, respectively. The decreasing TS concentration in the digestate liquid fraction indicated the solubilization of particles by ozone [19,21]. The VS and VSS proportions averaged at 72.26 and 77.78%, 73.36 and 76.19%, 69.20 and 74.07%, respectively. The particle degradation in the ozonation process influenced changes in the COD distribution in the digestate liquid fraction (Figure 1c,d,e). By analyzing the results in Figure 1c, it was noted that the removal efficiency of tCOD intensively increased up to the dose of 0.25 g O3/g TS, at which it reached the level of 12.45% (O3:C series), 10.67% (O3:B series) and 10.25% (O3:A series). Rising the ozone amount to 0.45 g O3/g TS did not significantly affect a further decrease in the tCOD concentration (Figure 1c). The decrease in TS, VS and tCOD concentrations in the digestate liquid fraction after ozonation was probably the result of the mineralization [35]. Cesaro et al. [29] also reported that during the ozonation of digestate from the organic fraction of municipal solid waste, slight mineralization occurred with a reduction in VS content up to 10% for the ozone dose of 0.16 g O3/g TS. In turn, Chacana et al. [35], during ozonation of anaerobically digested sludge, determined an 18% decrease in the total COD at the ozone dose of 192 mg O3/g COD. It was also noticed that with increasing ozone dose, the soluble COD (sCOD) percentage in tCOD increased, whereas the particulate COD (pCOD) decreased (Figure 1d). The sCOD proportion reached 65.94% in O3:C series, 63.35% in O3:B series and 61.13% in O3:A series at the ozone amount of 0.25 g O3/g TS. These values were over 2-fold higher than that determined in the digestate liquid fraction only after the mechanical separation (26.99%). The degree of disintegration averaged at 42.12% (O3:C series), 40.54% (O3:B series) and 38.18% (O3:A series) after treatment with ozone of 0.25 g O3/g TS (Figure 1e). The application of an ozone dose equal to 0.45 g O3/g TS resulted in an increase in sCOD proportion and DD values by approximately 4 and 2%, respectively. Somers et al. [16] noted lower DD amounting to 4% and 2–20% during the treatment of different types of digestates by ozone at a dose of 0.001 and 0.01 g O3/g TS, respectively. The exposure to ozone resulted in a higher biodegradability of the digestate liquid fraction, as evidenced by the greater BOD/tCOD ratio (Figure 1e). This parameter amounted to 0.19 in the liquid fraction of digestate after mechanical separation, while it averaged at 0.58 (O3:C series), 0.55 (O3:B series) and 0.53 (O3:A series) after ozone treatment at the dose of 0.25 g O3/g TS, and 0.61, 0.58 and 0.55 at 0.45 g O3/g TS, respectively. Cesaro and Belgiorno [13], during the preliminary treatment of organic solid waste with ozone at a similar dose of 0.4 g O3/g TS determined a higher BOD/tCOD ratio amounting to 0.66 and a slightly lower percentage of sCOD equal to 47%. The BOD/tCOD ratio at an ozone dose of 0.16 g O3/g TS was on average 0.80 and was almost 2-fold higher than that obtained in our study with a dose of 0.15 g O3/g TS. Cesaro et al. [29], conducting ozonation of the digestate from the organic fraction of municipal solid waste at an ozone dose of 0.11 and 0.16 g O3/g TS, also obtained a greater BOD/COD ratio of 0.69 and 0.66, respectively. Moreover, the increase in sCOD averaged at 20 and 23%, respectively. In turn, Bougrier et al. [25] determined the sCOD concentration higher by 4–49% in waste activated sludge after ozone treatment at a dose of 0.015–0.18 g O3/g TS. By analyzing the results presented in Figure 2b, it was found that the ozone dose and the pH value of the substrate also had a significant (p < 0.05) effect on the NH4–N removal.
The NH4–N concentration in the digestate liquid fraction decreased with increasing ozone dose until 0.25 g O3/g TS, then there was no significant change to 0.45 g O3/g TS. It was also noticed that the higher of initial pH of the digestate liquid fraction, the smaller the NH4–N content. The NH4–N concentration in O3:C series at the ozone dose of 0.25 g O3/g TS amounted to 1195.15 mg/dm3, whereas in series with a lower initial pH of the digestate liquid fraction, it was 1278.54 mg/dm3 (O3:B series) and 1314.08 mg/dm3 (O3:A series). The removal efficiencies of NH4–N were 37.76, 33.44 and 31.56%, respectively. As shown in Figure 2d, the NO3−N concentration in the digestate liquid fraction increased with the progressive ammonia nitrogen degradation, and at the ozone dose of 0.25 g O3/g TS, it reached 325.42 mg/dm3 (O3:C series), 222.34 mg/dm3 (O3:B series) and 214.65 mg/dm3 (O3:A series). The NO2−N content in all series was below 1 mg/dm3 (therefore not included in the figure), probably due to the instability of nitrite, which is easily oxidized to nitrate [34]. The changes in the NH4–N decomposition performance probably were related to the pH-dependent mechanism of the ammonia-ozone reaction. Under moderately alkaline conditions, at a pH of around 7, the ammonia nitrogen in the wastewater occurs mainly in the form of NH4+, and its oxidation takes place with the participation of molecular ozone. While high pH (more than 7) accelerates the O3 decomposition and induces the formation of hydroxyl radicals (•OH), which have a strong oxidative ability, and also increase the amount of free ammonia (NH3) in wastewater [34,36,37]. Due to the fact that under alkaline conditions, the generation of hydroxyl radicals is more intensive, the performance of ammonia oxidation is faster and more effective, which was observed in O3:C series. The NH4–N decomposition resulted in a decrease in TN content in the digestate liquid fraction (Figure 2a). The removal efficiency of TN at the ozone dose of 0.25 g O3/g TS amounted to 20.21% in O3:C series, 18.40% in O3:B series and 16.36% in O3:A series, whereas the TN concentration averaged at 1765.23, 1805.42 and 1850.12 mg/dm3, respectively. These values at the dose of 0.45 g O3/g TS were not significantly (p > 0.05) higher. As shown in Figure 2c, the ozonation process also influenced the concentration of phosphorus in the digestate liquid fraction. The TP content in the digestate liquid fraction decreased from 19.21 to 17.82 mg/dm3 (O3:C series), 17.03 mg/dm3 (O3:B series) and 16.51 mg/dm3 (O3:A series) after treatment with ozone at the dose of 0.05 g O3/g TS, then it increased and remained at the initial level. This was due to the release of phosphorus from disintegrated solids. By analyzing the results presented in Figure 2d, it was observed that the pH value of the digestate liquid fraction kept reducing with the increasing ozone dose. The reason for the decrease in pH could be the generation of H+ ions in the ammonia oxidation process and the consumption of OH– ions by molecular ozone O3, as well as the sludge cell destruction resulting in the release of the deoxyribonucleic acid (DNA), and the oxidation of organic matter into more oxygenated molecules, such as carboxylic acids [20,25,34]. As shown in Figure 2d, after increasing the ozone dose to 0.45 g O3/g TS, pH dropped to 7.92 in O3:C series (the initial pH of 9), 6.64 in O3:B series (the initial pH of 8) and 5.57 in O3:A series (the initial pH of 7). Bougrier et al. [25] also noticed a decrease in the pH value during the ozone pre-treatment of excess activated sludge. This parameter dropped from 6.7 to 5.1 with increasing ozone doses from 0.015 to 0.18 g O3/g TS. In turn, Sarif et al. [20] observed a reduction in pH by 1.24 during the ozonation process of return-activated sludge.

3.2. Ultrasonication (US)—Experiment B

The research conducted an analysis of the effect of ultrasound disintegration on changes in the solids content, the concentration of organic substances and nutrients in the digestate liquid fraction. The relationship between the initial pH of the digestate liquid fraction, the applied specific US energy, the ultrasonic vibration amplitude and the process performance is shown in Figure 3 and Figure 4.
By analyzing the results in Figure 3a,b, it was found that the initial pH value influenced the solids content in the digestate liquid fraction after the ultrasonication process.
The lowest TS and TSS concentrations were obtained in US:C series in which initial pH averaged at 9. It was also noticed that the TS content in the digestate liquid fraction significantly (p < 0.05) reduced in all series at the applied SE input of 10,381 kJ/kg TS and remained at a similar level despite the use of a higher SE (Figure 3a). The TS removal efficiency was equal to 30.60% (US:C series), 29.86% (US:B series) and 29.20% (US:A series) at the SE of 10,381 kJ/kg TS. Moreover, the VS proportion averaged at 73.06, 74.06 and 79.21%, respectively. As shown in Figure 3b, the TSS content in the digestate liquid fraction reduced with the increase in SE. The removal efficiency of TSS reached the level of 64.83% (US:C series), 60.34% (US:B series) and 52.41% (US:A series) at the SE of 51,903 kJ/kg TS. With this SE value, the VSS proportion in TSS averaged at 84.31, 82.61 and 81.88%, respectively. The decrease in TS and VS contents in the digestate liquid fraction during ultrasonication suggests that mineralization or volatilization of organic matter occurred under the tested conditions. The most intense mineralization effect was observed at the lowest applied energy, equal to 10,381 kJ/kg TS. Elbeshbishy et al. [38] applied ultrasound as a pre-treatment of food waste and also observed a slight reduction in TS and VS concentrations (12% and 14%, respectively) at a specific energy of 15,000 kJ/kg TS. Moreover, in studies conducted by Boni et al. [15], Romio et al. [21] and Cesaro et al. [39] concerning the ultrasonication of lignocellulosic digestate, agricultural digestate and solid organic substrates, the concentrations of TS and VS remained unchanged after the process, indicating the absence of significant mineralization. The application of ultrasound also influenced the COD distribution in the digestate liquid fraction. When analyzing the results presented in Figure 3c, it was noticed that with increasing the SE input, the sCOD percentage in the total COD increased, whereas the pCOD decreased. This is because the greater SEs generate more transient bubbles that can exert stronger cavitation forces, which in turn promote the breakdown of complex molecules [40]. The most significant(p < 0.05) increase in sCOD amount was observed after applying the SE equal to 41,522 kJ/kg TS. Under these conditions, the sCOD percentage reached 86.22% (US:C series), 82.26% (US:B series) and 77.53% (US:A series). These values were around 3-fold higher than that determined in the digestate liquid fraction only after the mechanical separation (26.99%). Theoretically, the specific energy should also be positively correlated with the degree of disintegration [30]. This effect was observed in our study during an increase in the SE input to 41,522 kJ/kg TS. With this SE value, the DD of the digestate liquid fraction was 62.68% (US:C series), 60.56% (US:B series) and 59.15% (US:A series). While the BOD/tCOD ratio informing about the biodegradability of the digestate liquid fraction averaged at 0.80, 0.77 and 0.73, respectively (Figure 3d). The further increase in the SE did not significantly (p > 0.05) influence the values of DD and BOD/tCOD ratio. As shown in Figure 3e, the use of specific US energy of 41,522 kJ/kg TS resulted in a reduction in tCOD concentration in the digestate liquid fraction by 18.24% (US:C series), 16.94% (US:B series) and 14.15% (US:A series). The results concerning the sCOD amount and the degree of disintegration noted in our study during the ultrasonication of the digestate liquid fraction were significantly (p < 0.05) higher than those obtained by other authors. Somers et al. [16] reported a 5–15% increase in sCOD in dairy manure digestate treated with a SE of 3000–15,000 kJ/kg TS. The DD determined by the authors amounted to 14% at 9000 kJ/kg TS and 24–25% at 15,000 kJ/kg TS. By comparison, the DD of the digestate liquid fraction in our study averaged at 32.32 and 46.10% in all series at the SE of 10,381 and 20,761 kJ/kg TS, respectively. Azman et al. [30] determined the sCOD concentration higher by 17% in dairy manure digestate after ultrasonication at a SE of 3000 kJ/kg TS. Cesaro and Belgiorno [13], during the pre-treatment of organic solid waste by ultrasound at a SE of 15,000, 25,000 and 40,000 kJ/kg TS, noted the sCOD percentage in COD equal to 9.1, 9.5 and 17.6%, respectively. By analyzing the results presented in Figure 4, it was noted that the specific energy, the ultrasonic vibration amplitude and the pH value of the digestate liquid fraction also had a significant impact on the nitrogen transformation in the digestate liquid fraction.
It was found that the higher the initial pH, the smaller the nitrogen concentration in the digestate liquid fraction. The TN and NH4–N concentrations in the digestate liquid fraction decreased with increasing the applied SE input (Figure 4a,b). A similar relationship was observed in the case of changes in the COD concentrations, i.e., the largest decrease in TN and NH4–N amount in the digestate liquid fraction occurred at the SE of 41,522 kJ/kg TS. With this SE value, the TN and NH4–N removal efficiencies averaged at 25.54 and 27.58% (US:C series), 21.68 and 23.97% (US:B series), 20.70 and 21.13% (US:A series), respectively. There were no significant changes in these values at the SE of 51,904 kJ/kg TS. The concentration of NO3–N in the digestate liquid fraction slightly increased during the ultrasonication and averaged at 10.19 mg/dm3 in US:C series, 10.96 mg/dm3 in US:B series and 11.74 mg/dm3 in US:A series. By analyzing the results in Figure 4c, it was noticed that the phosphorus content in the digestate liquid fraction decreased to 16.54 mg/dm3 (US:C series), 15.52 mg/dm3 (US:B series) and 14.81 mg/dm3 (US:A series) after ultrasonication at the SE of 20,761kJ/kg TS, then it increased and remained at a slightly lower level than before treatment. Due to the fact that it was the result of phosphorus release from disintegrated solids, the lowest TP concentrations were obtained in US:A series with the lowest DD values. As shown in Figure 4e,f, during the ultrasonication process, the temperature and pH values increased with the increase in the specific energy input. The temperature increased to about 90 °C in all series at the SE of 51,901 kJ/kg TS, while the pH of the digestate liquid fraction reached 9.46 in US:C series, 8.55 in US:B series and 7.92 in US:A series. The temperature increase was associated with the generation and collapse of microbubbles in a cavitation process [13,31]. Moreover, the increase in pH was likely related to the ability of ultrasound to exhaust dissolved gases from liquids that occur through thermal mechanisms (i.e., a decrease in CO2 solubility with higher temperature) and athermal (i.e., microconvections causing the gas-filled cavity to migrate towards the surface). The dissolved CO2 released from the liquid shifts the dissolved (bi-)carbonates balance, resulting in higher pH values [41].
By taking into account the results presented in Figure 3 and Figure 4, it was noted that the application of the ultrasonic vibration amplitude equal to 50% in US:B50 series resulted in significantly (p < 0.05) higher concentrations of TS, TSS, tCOD, TN and NH4–N in the digestate liquid fraction, thus a lower disintegration efficiency compared to that obtained in US:B series, in which the initial pH of liquid fraction was the same, but the amplitude was 100%. It was probably related to the mechanism of the ultrasonication process. At high amplitudes, the cavitation bubbles reach a critical size faster and implode, resulting in faster production of reactive radicals that break down solid particles and, thus, a more efficient disintegration process [16,21,42]. In US:B50 series, due to the lower ultrasound amplitude, and thus slower ultrasonic energy transmission in the liquid, the temperature only increased to 75 °C.

3.3. Combination of Ozonation and Ultrasonication—Experiment C

Hybrid ozonation-ultrasonication is often analyzed together with other advanced oxidation processes, whereas the combination of ozone and ultrasonic alone is rarely studied. Therefore, there is still a knowledge gap about this new technology [43]. In this part of the research, the effect of combined ozonation and ultrasound processes on changes in the composition of the digestate liquid fraction was determined. The study used the optimal ozonation and ultrasound parameters, which contributed to obtaining high disintegration performance in the experiments described in Section 2.2.1 and 2.2.2 of the article, i.e., the ozone dose of 0.25 g O3/g TS, the specific US energy of 41,522 kJ/kg TS and pH of the digestate liquid fraction equal to 8.08.
By analyzing the results in Figure 5, it was noticed that the application of combined ozonation and ultrasonication processes contributed to obtaining a greater treatment performance of the digestate liquid fraction compared to the single processes.
The mean efficiencies of tCOD, TN, NH4–N and TP removal from the digestate liquid fraction obtained in the hybrid processes were higher by 44.70, 15.40, 11.67 and 14.84%, respectively, compared to the values determined during ozonation and by 38.43, 12.12, 21.13 and 13.27% than with the ultrasonic treatment. The average temperature and pH obtained in the combined processes were significantly greater in comparison to those in the ozonation and similar to those determined during the ultrasonication. It was also found that the sequence of the combined processes influenced the performance of the digestate liquid fraction treatment. The best effects were obtained using ultrasonication first, followed by ozonation (US:B + O3:B series). The digestate liquid fraction first subjected to ultrasound, and then ozone contained approximately 30, 12, 8 and 13% lower concentrations of TSS, tCOD, TN and NH4–N, respectively, compared to those determined in O3:B + US:B series (Figure 5a,b,d). The TP concentration in the liquid fraction did not differ significantly (p > 0.05) and thus was not dependent on the sequence of the processes (Figure 5f). The proportion of sCOD in tCOD, the percentage of VS and VSS in TS and TSS, the DD and the BOD/tCOD ratio amounted to 95.56%, 58.52%, 76.92%, 18.17% and 0.78 in US:B + O3:B series, respectively (Figure 5a–c). This indicated that the digestate liquid fraction after treatment with ultrasound and then with ozone became more biodegradable than in the reverse order of the processes, and therefore, ultrasound enhanced the ozonation process. This was probably the result of synergy between the effects of individual processes. The cavitation that occurs in a liquid solution during ultrasonic treatment leads to the creation of bubbles, which by collapsing, induce the formation of hot spots with extremely high pressures (>500 bar) and temperatures (>4000 K), as well as free, highly reactive radicals. These radicals can oxidize resistant organic compounds and solubilize organic matter [13,43,44]. Ultrasound is not selective and thus directs energy at both easily and non-biodegradable matter [41]. Ozone is very selective and cannot oxidize some organic compounds (e.g., natural organic compounds), as well as low molecular organic acids [20]. The selectivity and relatively poor ozone mass transfer limit the success of using ozonation in industrial conditions [43]. An improvement in the efficiency of destroying complex molecules can be obtained in the hybrid processes in which the generation of oxidizing radicals is intensified [45]. Therefore, the use of ultrasonication prior to ozonation in US:B + O3:B series made the organic matter more susceptible to oxidation and mineralization during subsequent ozonation, which resulted in better treatment performance.

4. Conclusions

In the study, the effect of ozonation and ultrasonication applied individually or in combination on the digestate liquid fraction composition was assessed under various treatment conditions. It was shown that ozone and ultrasound could be used to post-treatment of digestate liquid fraction to increase its biodegradability and thus more effective agricultural management of the digestate. The application of ozone at the dose of 0.25 g O3/g TS and ultrasound at the specific energy of 41,522 kJ/kg TS and the vibration amplitude of 100% positively affected nutrients concentration, solids content and biodegradability of the digestate liquid fraction. It was noticed that the higher the initial pH of the digestate liquid fraction, the better its disintegration efficiency. Based on the obtained results, it was also found that ultrasonication was more effective in the digestate liquid fraction treatment compared to ozonation. This was related to the lack of selectivity and directing energy towards easily and poorly biodegradable compounds. The application of combined ozonation and ultrasonication, due to the synergy between their effects, resulted in a greater treatment performance of the digestate liquid fraction compared to the single processes. The sequence of the combined processes influenced the treatment efficiency. The best effects of the digestate liquid fraction treatment, thus higher solubilization and biodegradability, were obtained using first the ultrasound and then ozonation.

Author Contributions

Conceptualization, A.C. and K.Z.; methodology, A.C. and K.J.; software, K.Z.; validation, A.C. and K.Z.; formal analysis, A.C. and K.J.; investigation, A.C. and K.J.; resources, A.C. and K.Z.; data curation, A.C. and K.Z.; writing—original draft preparation, A.C.; writing—review and editing, A.C. and K.Z.; visualization, A.C.; supervision, K.Z.; project administration, A.C. and K.Z.; funding acquisition, K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Lodz University of Technology.

Data Availability Statement

Not applicable.

Acknowledgments

We would like to express our great acknowledgements to Lodz University of Technology for financial support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Effect of ozone dose and initial pH of digestate liquid fraction on concentration of TS and VS (a), TSS and VSS (b), tCOD (c); removal efficiency of tCOD (c); percentage of sCOD and pCOD in tCOD (d); degree of disintegration and BOD/tCOD ratio (e) during ozonation of digestate liquid fraction.
Figure 1. Effect of ozone dose and initial pH of digestate liquid fraction on concentration of TS and VS (a), TSS and VSS (b), tCOD (c); removal efficiency of tCOD (c); percentage of sCOD and pCOD in tCOD (d); degree of disintegration and BOD/tCOD ratio (e) during ozonation of digestate liquid fraction.
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Figure 2. Removal efficiency of TN (a); concentration of TN (a), NH4–N and NO3–N (b), TP (c); pH value (d) depending on ozone dose and initial pH during ozonation of digestate liquid fraction.
Figure 2. Removal efficiency of TN (a); concentration of TN (a), NH4–N and NO3–N (b), TP (c); pH value (d) depending on ozone dose and initial pH during ozonation of digestate liquid fraction.
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Figure 3. Effect of specific energy and initial pH of digestate liquid fraction on concentration of TS and VS (a), TSS and VSS (b), sCOD and pCOD (c); degree of disintegration and BOD/tCOD ratio (d); removal efficiency of tCOD (e) during ultrasonication of digestate liquid fraction.
Figure 3. Effect of specific energy and initial pH of digestate liquid fraction on concentration of TS and VS (a), TSS and VSS (b), sCOD and pCOD (c); degree of disintegration and BOD/tCOD ratio (d); removal efficiency of tCOD (e) during ultrasonication of digestate liquid fraction.
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Figure 4. Removal efficiency of TN (a); concentration of TN (a), NH4–N (b), TP (c) and NO3–N (d); pH value (e); temperature (f) depending on specific energy and initial pH during ultrasonication of digestate liquid fraction.
Figure 4. Removal efficiency of TN (a); concentration of TN (a), NH4–N (b), TP (c) and NO3–N (d); pH value (e); temperature (f) depending on specific energy and initial pH during ultrasonication of digestate liquid fraction.
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Figure 5. Effect of ozonation, ultrasonication and combination of these processes on the digestate liquid fraction composition: concentrations of TS, VS, TSS and VSS (a); tCOD concentration, percentage of sCOD and pCOD in tCOD (b); degree of disintegration and BOD/tCOD ratio (c); concentrations of TN, NH4–N and NO3–N (d); removal efficiency of tCOD, TN and NH4–N (e); TP concentration (f); pH and temperature (g).
Figure 5. Effect of ozonation, ultrasonication and combination of these processes on the digestate liquid fraction composition: concentrations of TS, VS, TSS and VSS (a); tCOD concentration, percentage of sCOD and pCOD in tCOD (b); degree of disintegration and BOD/tCOD ratio (c); concentrations of TN, NH4–N and NO3–N (d); removal efficiency of tCOD, TN and NH4–N (e); TP concentration (f); pH and temperature (g).
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Table
Table 1. Characteristics of substrates.
Table 1. Characteristics of substrates.
IndicatorDigestateLiquid Fraction of Digestate
COD [g O2/dm3]14.91 ± 0.459.51 ± 0.11
BOD5 [g O2/dm3]1.47 ± 0.061.86 ± 0.05
sCOD [g O2/dm3]2.15 ± 0.052.58 ± 0.03
TN [g/dm3]2.84 ± 0.032.22 ± 0.09
NH4–N [g/dm3]2.53 ± 0.041.93 ± 0.05
TP [mg/dm3]35.84 ± 0.4519.17 ± 0.23
pH7.67 ± 0.218.08 ± 0.15
TS [g/dm3]40.92 ± 0.7811.55 ± 0.39
VS [% TS]74.93 ± 0.6855.73 ± 0.28
TSS [g/dm3]15.12 ± 0.822.73 ± 0.22
VSS [% TSS]52.58 ± 0.8747.08 ± 0.56
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Chuda, A.; Jastrząbek, K.; Ziemiński, K. Changes in the Composition of Digestate Liquid Fraction after Ozone and Ultrasonic Post-Treatment. Energies 2022, 15, 9183. https://doi.org/10.3390/en15239183

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Chuda A, Jastrząbek K, Ziemiński K. Changes in the Composition of Digestate Liquid Fraction after Ozone and Ultrasonic Post-Treatment. Energies. 2022; 15(23):9183. https://doi.org/10.3390/en15239183

Chicago/Turabian Style

Chuda, Aleksandra, Konrad Jastrząbek, and Krzysztof Ziemiński. 2022. "Changes in the Composition of Digestate Liquid Fraction after Ozone and Ultrasonic Post-Treatment" Energies 15, no. 23: 9183. https://doi.org/10.3390/en15239183

APA Style

Chuda, A., Jastrząbek, K., & Ziemiński, K. (2022). Changes in the Composition of Digestate Liquid Fraction after Ozone and Ultrasonic Post-Treatment. Energies, 15(23), 9183. https://doi.org/10.3390/en15239183

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