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State of the art and perspectives of ultrasound application for sewage sludge processing
Gianico A., Gallipoli A., Braguglia C.M., Mininni G.
Cnr – Istituto di Ricerca Sulle Acque – Area della Ricerca di Roma 1 - via Salaria km 29.3 – 00015 Monterotondo (Roma)
(e-mail: [email protected]; [email protected]; [email protected]; [email protected])
Abstract The application of ultrasound to sludge is an efficient method for sludge disintegration in order to accelerate the anaerobic digestion process, but energy balance on the basis of lab tests is disadvantaged due to the inefficiency at this scale. In this paper the energy balance is discussed for five full scale WWTPs equipped with different devices with the aim to investigate the cost-effectiveness of the ultrasonic pre-treatment before digestion. These five experiences show positive energy gains operating at 20 kHz: best performances are attained at short sonication time and high sonication power. Moreover, sludge sonication at 200 kHz is presented as an alternative pretreatment to improve the anaerobic digestibility and to remove potentially organic micropollutants. Lab experiment results of this treatment indicated quite good disintegration effect, in terms of substrate solubilization, with consequent acceleration of the hydrolysis phase and increase of biogas production. Keywords: anaerobic digestion, energy balance, frequency, ultrasounds.
INTRODUCTION Wastewater from household and industry represents a significant constraint on water management, and municipal wastewater treatment plants (WWTPs) are considered as an end-of-pipe treatment just before discharge, with the aim of preventing eutrophication and hygienic health hazard in surface water. The scope of sewage treatment is rapidly changing. In addition to the traditional ones on the organic load and nutrient removal (the latter one for the sensitive areas) today increasing attention is paid to micropollutants and pathogens removal to avoid toxic and eco-toxic effects and hygienic risks on the receiving superficial waters. Moreover, the global demographic trends and climate change strictly force towards water recycling and residual sludge recovery by sustainable processes. These objectives will be achieved by optimal integration of several treatment steps and by minimizing the amounts of sludge to be disposed off. Innovative intensive stabilization processes applied on sewage sludge are required in order to minimize the total amount of produced sludge and to achieve a better sludge quality (class A biosolids) thus increasing its agricultural recovery on land. Anaerobic digestion is worldwide used to stabilize sewage sludge, especially in large size wastewater treatment plants. Hydrolysis of the organic matter is the rate-limiting step of biological degradation of sludge in anaerobic conditions (Lafitte et al. 2002) making the process very slow and involving therefore big digesters volumes. A significant gain in solids degradation and energy recovery could be possible by applying an appropriate sludge pre-treatment that leads to the breakage of flocs, and cell walls accelerating the hydrolysis step (Carrère et al., 2010). The pre-treatment is most suited to streams containing large quantities of refractory material and/or cellular matter such as waste activated sludge (Pérez-Elvira et al. 2006; Khanal et al. 2007). Among the mechanical pretreatments, the application of ultrasound on sludge has gained considerable interest. In particular, low-frequency (∼20 kHz) high-powered ultrasounds are demonstrated to generate the cavitation necessary for producing high hydromechanical shear forces and thus an efficient sludge disintegration. Furthermore, it was figured out that the disintegration of sludge is most effective at low ultrasonic frequencies because of the formation of powerful hydrodynamic shear forces(Tiehm et al., 2001). High frequencies, on the contrary, are more suitable
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for sonochemical degradation reactions of organic micropollutants in environmental water matrices (Lifka, 2003). The efficacy of the ultrasound sludge pre-treatment at 20 kHz for improving the performances of the anaerobic digestion process has been extensively studied in laboratory, pilot and full-scale (Müller et al., 2003; Bougrier et al., 2005; Tiehm et al., 2001; Odegaard, 2004; Braguglia et al., 2006; Khanal et al., 2007; Eder and Guenthert, 2002; Groenroos et al. 2005; Nickel and Neis, 2007; Pilli et al., 2010). In the last years, advances in ultrasound equipment have generated renewed interest in this technology for hydrolysis of sewage sludge. The technology provides an easy retrofit option for existing wastewater treatment plants, with lower substantial capital costs with respect to other pretreatments as for example thermal hydrolysis (Carrère et al., 2010) In this paper a review of the principal full scale applications of ultrasound pretreatment is reported and the energetic balance is calculated on the basis of the available data. Moreover, batch-scale results of the sludge pretreatment at a “new” frequency of 200 kHz on the sludge solubilization and digestion are presented and compared with the “classic” 20 kHz pretreatment. The choice of 200 kHz as working frequency is based on the possibility of combining the positive effect of ultrasound pretreatment in enhancing the anaerobic sludge digestibility with the decontamination of sludge due to pollutants oxidation generated by radical formation. a) Full scale application of ultrasounds Ultrasound equipment has already been implemented in full-scale on a number of WWTPs in several countries and generally part-stream sonication is performed (Barber, 2003), which consists of treating only a fraction of the sludge stream, with the aim to reduce costs and enhance final sludge dewaterability (Rooksby et al., 2002). Table 1 shows the operative conditions of the sonication technology application on five full-scale municipal wastewater treatment plants; all plants perform a 20 kHz sonication on a fraction of waste activated sludge (WAS), except for Ulu Pandan WWTP (Singapore) where a mixture of primary (1/3) and thickened WAS (2/3) is ultrasonically treated (Xie et al., 2007).
Table 1. Full scale applications of ultrasounds, operative conditions.
Parameter Measure Darmstadt/Eberstadt
Welsberg Monguelfo Freising Bamberg Ulu Pandan -
Singapore WWTP capacity p.e. 35,000 30,000 65,000 330,000 ~700,000 Number of probes n 5 4 15 10 5
Total operating power kW 6 4 Not available 10 30
Sonication time min 120 300 Not available 1.12 0.025
% of sludge treated with ultrasounds % 30-40 50-60 30-40% from 30% (2002)
to 80% (2006) 100%
Sludge flow rate to sonication m3/d 18 7 12 75 200
Total solids (TS) g/L 55 55 50-60 58 16.3 VS/TS*100 % 73% 80% 71% 78% 74%
Sonication energy input kJ/kg TS 1,080 1,260 396 162 324
It is important also to note that the specific energy applied for full scale application (form 160 to 1260 kJ/kgTS) is much lower with respect to the ones investigated on pilot and lab scale (form 1000 to 200000 kJ/kg TS) worldwide and reported extensively in literature. An energetic balance, based on the available data regarding full scale WWTPs equipped with ultrasounds devices, was calculated to assess the effective advantages of the pre-treatment on the
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combined process ultrasound/anaerobic digestion of sewage sludge. The daily total power consumed by ultrasonic reactor was compared to the surplus energy production due to the gain in biogas production thanks to sonication. The Daily Energy Gain (DEG, kWh/d) has been calculated using the values of the Daily Gain of Biogas Production (DGBP, Nm3/d), the methane content of biogas (%CH4), and the inferior calorific power of methane (ICP ~ 10 kWh/Nm3), assuming an electrical efficiency of 40% (ηel), typical of a cogeneration plant :
elICPCHDGBPDEG η×××= 4%
The energy balance reported in Table 2 is the difference between the daily energy gain and the daily sonication energy spent.
Table 2. Energy balance from application of ultrasound on full scale WWTPs.
Parameter Measure Darmstadt/Eberstadt Welsberg Monguelfo Freising Bamberg
Ulu Pandan - Singapore
Sonication energy input
kWh/kg TS 0.3 0.35 0.11 0.05 0.09
Daily sonication energy input kWh/d 297 139 72 194 288
Gain in specific biogas production (L/Kg VSalim)
% 24 28 11 not available 45
Gain in VS destruction % 14 12 5 32 30
Daily Energy Gain (DEG) kWh/d 381 281 190 2,815 627
Energy Balance kWh/d 84 142 118 2,621 339
All the full scale installations evaluated show a positive energy balance, highlighting the feasibility of the ultrasonic pre-treatment as a cost-effective technology to enhance the anaerobic digestion performances. The sonication energy input applied at the 5 WWTPs varies from 0.05 to 0.3 kWh/kg TS; the ultrasonic equipments used in these facilities are characterized by different operational designs, different number of probes, different sonication times, different probe powers. As example at Welsberg/Monguelfo WWTP the ultrasonic apparatus is equipped with 4 probes (power = 950 W each probe) working for a sonication time of about 300 min, while at Ulu Pandan WWTP the ultrasonic equipment is made of 5 probes (power = 6 kWh each probe) working for a sonication time of only 0.025 min. In terms of VS destruction and biogas production, the plants operating with long sonication times and low power (as in Darmstadt/Eberstadt, Welsberg Monguelfo, Freising) present worst performances with respect to Bamberg and Ulu Pandan, where the sonication apparatus operates with short times and high power, reaching very high energy gains. The daily energy gain is linearly related to the gain in VS removal (Figure 1), except for Bamberg WWTP where the DEG was excessively high. This relation indicates the strict relationship between the conversion of the organics removed and the methane production during the anaerobic digestion process, independently on the digestion operating conditions. Moreover, by increasing the daily sonication input in the range 50-200 kWh/d the gain in VS removal increases significantly. It results, however, a threshold value of ∼200 kWh/d beyond which a worsening of the digestion performances is evident. It is interesting to note that the daily sonication input takes into account both sonication energy and sludge flow to sonication, that is highly variable from WWTP to another (Table 1).
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Figure 1. Relationships between sonication and digestion performances on full scale plants. Lab scale tests were carried out at the Roma Nord WWTP in order to assess the effect of sonication on the anaerobic digestion performances of sole WAS. Semi-continuous anaerobic digestion tests have been performed on sludge sonicated with energy input ≤ 1.7 kWh/kg TS (Mininni et al., 2007, Braguglia et al., 2008). On the basis of the produced biogas and the heat demand, generally resulted that the pretreatment was indispensable to self-sustain the mesophilic range of temperature. On the contrary, in spite of relatively low sonication energy, the surplus of energy produced in digestion of sonicated sludge covers only 50-60% of the energy requirement for sonication. This gap may be overtaken by introducing a pre-thickening step to concentrate the diluted waste activated sludge deriving from the recycle stream. Nevertheless, it is important to remark that the energy supplied by real equipments is smaller compared with the laboratory devices, therefore the energy balance is generally positive for full-scale but negative for laboratory-scale (Perez-Elvira et al. 2009) equipments. Sonication at 200 kHz is presented as an alternative pretreatment to improve the stabilization and the quality of the final sludge (Gallipoli and Braguglia, 2010). The efficacy of this “new” frequency has been assessed as regards sludge disintegration and batch digestion performances. b) Batch digesters performance: effect of frequency Methods The anaerobic digestion experiments described in this paper were performed using waste activate sludge, untreated and sonicated at 20 and 200 kHz. Sludge was sampled from recycle stream before thickener at the municipal “Roma Nord” wastewater treatment plant, one of the four wastewater plants of the city of Rome. It was gravity thickened for 24 h at 4°C before carrying out the sonication and digestion experiments. Anaerobic sludge sampled from the “Roma Nord” digesters was used as inoculum for the digestion tests. Matter analysis (volatile solids, total solids, soluble COD) procedure is reported elsewhere (Braguglia et al., 2009) The disintegration by low frequency ultrasound was performed with an horn ultrasonic processor UP400S (dr. Hielscher, Germany) operating at 300 W and 20 kHz. Sonication was performed for 4 min on 500 mL of waste-activated sludge placed in 1 L beaker with the probe allocated at 3 cm above the beaker bottom. The ultrasound pre-treatment at 200 kHz was applied for 40 min on 300
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ml of waste activated sludge. The ultrasound apparatus is constituted of an amplifier T&C Power Conversion and a sonoreactor Elac Nautik USW 51-051, operating at 200 kHz and at an average power of 90-100 W. The transducer is made up of a piezo-ceramic element with an active area of approx. 25 cm2. The ultrasound treatment was carried out immediately before starting the anaerobic digestion experiment. The degree of sludge disintegration was calculated as the ratio of the soluble COD increase by sonication to the maximum possible soluble COD increase (Braguglia et al., 2006). Anaerobic digestion tests were carried out on bench scale anaerobic reactors of 0.4 L that were operated in batch mode, immersed in a temperature controlled, agitated water bath at 37°C. The reactor were fed with a mixture of inoculum and raw sludge, either untreated or disintegrated. The food/inoculum ratio (F/I) was calculated as follows:
( ) inoculumVSsludgeVreactorVsludgeVSsludgeV
IF
)(
)(
×−
×=
The produced biogas was collected in calibrated 1 L eudiometer tube placed on the digestion bottle via a ground-glass connection. The biogas was measured daily. Results and discussion In this section the results of the digestion experiments with sludge pre-treated at 20 and 200 kHz have been compared. In order to understand the role of the anaerobic biomass on the digestion performances, the digestion experiments were performed at two different F/I ratios, 0.5 and 1. For the ultrasound pre-treatment at 20 kHz, the specific energy delivered to the sludge was in the range 4000-4300 kJ/kgTS with a DDCOD of about 13%, while for the sonication treatment at 200 kHz the specific energy was higher, ∼23000 kJ/kgTS, reaching a disintegration degree of about 7%. Contrary to what stated in literature (Tiehm et al., 2001), also the ultrasound treatment with 200 kHz gave good disintegration results. For specific energy > 20000 kJ/kgTS, ultrasound pretreatment at 200 kHz permits to reach higher DDCOD with respect to the low frequency sonication. The maximum DDCOD obtained in the sonication experiments at 200 kHz was 65%, indicating also cell lyses besides the release of extracellular organic matter. This organic substance solubilisation due to the ultrasound pre-treatment at 20 kHz and at 200 kHz enhanced the first hydrolysis phase of the digestion, disrupting the sludge flocs and causing the release of the organic substances in the surrounding liquid phase. This was highlighted by the considerable increase of soluble COD after the ultrasound pre-treatment. The values of the soluble COD in the reactors at the beginning and at the end of the digestion are reported in Table 3. It is evident that the initial soluble COD for the sonicated mixture is always noticeably higher with respect to the “control” one, and this difference is ascribed to the efficiency of the pretreatment. During the anaerobic digestion this soluble organic matter was rapidly removed; in particular, the removal of the high soluble COD content in the reactors with sonicated sludge was so intensive to reach comparable values as those measured in the digesters fed with untreated sludge (Table 3). In the case of the 20 kHz sonication the removal of the soluble COD was 73% at F/I=0.5 (while for 200 kHz was 53%) and 57% at F/I=1 (33% for 200 kHz), in spite of a slight lower disintegration degree of the 200 kHz sonicated samples. On the contrary, the soluble COD during the digestion of the untreated sludge increased due to the hydrolysis phase.
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Table 3. Soluble COD pattern.
Sonication at 20 kHz Untreated Sonicated F/I CODs (t=0) CODs (t=12d) CODs (t=0) CODs (t=12d)
0.5 104 204 981 254 1 177 408 1605 686
Sonication at 200 kHz Untreated Sonicated F/I CODs (t=0) CODs (t=20d) CODs (t=0) CODs (t=20d)
0.5 215 132 682 318 1 122 459 867 582
In Table 4 the improvement as concerns the organic solids degradation due to the sonication (at 20 and 200 kHz) of sludge are reported. These gains are calculated considering the difference in the volatile solids (VS) removals of the same sludge untreated and sonicated, digested in the same conditions. The ultrasound treatment at higher frequency provided good gains in VS removals, and surprisingly at F/I=1 the improvement was significantly higher than the one obtained at the conventional 20 kHz.
Table 4. VS degradation gain of the sonicated sludge at different frequency VS DEGRADATION EFFICIENCY (%)
F/I US 20 kHz US 200 kHz
0.5 +10 +13
1 +14 +35
The volatile solids removed during the anaerobic digestion are converted into biogas. Comparing the biogas gain obtained from the different digestion tests (Figure 2), the results are very interesting. In fact, while in the case of sonication at the “classic” 20 kHz the gain seemed not strictly dependent on the F/I, in the case of 200 kHz sonication the biogas improvement was very high (about 50%) at F/I=0.5 whereas at F/I=1 was around 20%, comparable with the biogas gained pre-treating the sludge at 20 kHz.
Biogas gain
0%
10%
20%
30%
40%
50%
60%
F/I=0.5 F/I=1
biog
as g
ain
(%)
20 kHz 200 kHz
Figure 2. Biogas gain of the 20 and 200 kHz pre-treated sludge at F/I =0.5 and 1. The digestion test at F/I=1 resulted ideal to increase the stabilization of the sludge (higher VS removals), but was not convenient from an energetic point of view because of the low biogas
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productions. Future work is needed to understand the mechanisms and the biomass dynamics in order to optimize this process, also as regards the energy gain. CONCLUSIONS The application of low-frequency (20 kHz) high powered ultrasound pretreatment is demonstrated to be effective on sludge disintegration and consequently in enhancing the anaerobic sludge digestibility, in terms of VS reduction and biogas production. In this study five full scale WWTPs equipped with different ultrasonic devices (Darmstadt/Eberstadt, Welsberg/Monguelfo, Freising, Bamberg and Ulu Pandan) are considered and the energetic balance calculated on the basis of the available data was found to be positive, demonstrating the feasibility of the ultrasonic pretreatment as a cost-effective technique to improve performance of sludge anaerobic digestion. The operating conditions of the WWTPs ultrasonic facilities play a considerable role: the highest energy gains, VS reductions and biogas productions are achieved with short sonication time and high sonication power (Bamberg and Ulu Pandan). Lab scale sonication at 200 kHz was proposed as an alternative pretreatment in order to enhance the anaerobic digestion performances and to exploit the removal of organic micropollutants from sludge. Batch anaerobic digestion tests with pretreated sludge at 20 and 200 kHz showed that both pretreatments accelerate the initial hydrolysis phase of the digestion because of the significant disintegration of sludge floc and organics release due to ultrasound. Afterwards, the organic matter was rapidly removed during the digestion of the sonicated sludge. In particular, the removal of the high soluble COD content in the reactors with sonicated sludge was so intensive to reach comparable values as those obtained with anaerobic digestion of untreated sludge. Moreover, the digestion of the 200 kHz pretreated sludge showed a higher improvement of volatile solids removal and of biogas production than sludge pretreated at 20 kHz. These gains seemed to be dependent on the F/I ratio. Future work is needed to understand the mechanisms in order to optimize this process, also as regards the energy gain. REFERENCES Barber, W.P. (2003). Full-scale studies of part-stream ultrasound to improve sludge treatment.
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