- Email: [email protected]
Evaluation of biomass activity and wastewater characterization in a UCT-MBR pilot plant by means of respirometric techniques
Desalination 269 (2011) 190–197
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Evaluation of biomass activity and wastewater characterization in a UCT-MBR pilot plant by means of respirometric techniques Daniele Di Trapani ⁎, Marco Capodici, Alida Cosenza, Gaetano Di Bella, Giorgio Mannina, Michele Torregrossa, Gaspare Viviani Dipartimento di Ingegneria Civile, Ambientale ed Aerospaziale, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 26 October 2010 Accepted 26 October 2010 Keywords: Respirometric analysis Biokinetic coefficients Membrane Bioreactor (MBR) Wastewater treatment Pilot plant experiment
a b s t r a c t Over the last two decades, Membrane Bioreactors (MBRs) emerged even more for wastewater treatment, ensuring high removal efficiencies as well as very small footprint requirements. Indeed, in this kind of process, a modification in biomass activity and viability can exist compared to that of a CAS process. In this context, respirometric analysis represents a reliable tool in order to evaluate the actual biomass kinetic parameters, to insert in mathematical models in the design phase, as well as to monitor the biomass viability, especially when these processes are operated with high SRT values. The paper presents some results of respirometric techniques applied for the characterisation of wastewater and biomass activity in a pilot UCT-MBR plant for nutrient removal, operating with high SRT. In particular, the respirometric tests were specifically aimed at investigating heterotrophic and autotrophic bacterial activity. The pilot plant was built at Palermo WWTP and consisted of three reactors: anaerobic, anoxic and aerobic, followed by an aerobic compartment containing two submerged hollow fibre membrane modules with typical recycling lines. The kinetic parameters for heterotrophic bacteria resulted lower respect to the CAS; regarding the nitrifying bacteria, the kinetic constants were in the range of CAS, suggesting a good nitrification activity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, due to the increasing awareness about environmental impact of pollutant discharges, the effluent standards are becoming more and more stringent, leading to increased requirements for Wastewater Treatment Plant (WWTP). In this context, the technical and scientific community in the last years showed a growing interest in developing innovative technologies that, together with very high removal efficiencies, can lead to a very low space and volume request. A possible solution to cope with such issues is represented by membrane bioreactors (MBRs), which are combined systems including a bioreactor and a filtration unit (usually an ultrafiltration or microfiltration membrane). More specifically, such systems compared to the traditional ones, like the conventional activated sludge (CAS) processes, which require large aeration and settling tanks, have shown higher efficiency in terms of effluent concentrations as well as smaller footprint and sludge production, due to higher biomass concentration in the bioreactor [1]. Further, MBRs enable high treatment levels in terms of effluent total suspended solid (TSS) concentrations, organic matter and total nitrogen (TN). More in detail, the very high biomass concentration, compared to the available food in the system, contributes to create an
⁎ Corresponding author. E-mail address: [email protected] (D. Di Trapani). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.10.061
environment where bacteria are facing starvation condition so they are not in a physiological state for cell growth [2]. On the other hand, the high sludge retention times (SRTs), typical of MBR system, are highly advantageous for the growth of nitrifying bacteria [1]. Concerning nutrient removal, Ramphao et al. (2005) discussed about the influence of the MBR solid separation on the design of biological nutrient removal (BNR) systems which result smaller than the equivalent conventional systems [3]. On the other hand, Durante et al. and Parco et al. [4,5] have focused the attention on the influence of the size of the MBR flocs on substrates diffusive transport. However, up to now, the knowledge concerning the influence that high solid concentrations in the mixed liquor as well as the nature of the selected biomass have on the BNR process is controversial and still limited. Indeed, in such systems, a modification in the biomass biokinetic behaviour can arise, compared to that of a CAS process. In this context, respirometric techniques [6] should represent a useful tool for the characterization of the biokinetic behaviour of bacteria in a MBR process, to insert in mathematical models in the design phase, as well as to monitor the biomass viability [7,8]. Indeed, oxygen uptake rate (OUR), i.e. the oxygen consumption per unit volume per unit time, is widely recognized as an important parameter in order to monitor the biomass viability [9]. On the other hand, in the case of domestic wastewaters with contributions of industrial wastewater, and in a process operated with high SRTs, it might be of paramount interest the respirometric procedures in order to better characterise wastewater and biomass viability.
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Bearing in mind such considerations, the paper presents some results of a study aimed at evaluating the heterotrophic and autotrophic kinetic parameters, in terms of organic and nitrogen removal, as well as wastewater chemical oxygen demand (COD) fractionation in a UCTMBR pilot plant using respirometric techniques. 2. Materials and methods 2.1. Pilot plant and operating conditions description The experimental investigations were carried out on a MBR pilot plant, conceived for nutrient removal, built at the Acqua dei Corsari (Palermo) WWTP. In Fig. 1, the pilot plant layout is reported. The pilot plant scheme was an adaptation of the UCT process [10] with a final membrane filtration unit instead of the conventional secondary clarifier. It consisted of three reactors in series, anaerobic (mean volume 71.6 l), anoxic (mean volume 164.9 l) and aerobic (mean volume 327 l) respectively, followed by an aerobic tank (mean volume 52 l) where two submerged hollow fibre membrane modules (Zenon Zeeweed, ZW 10) were installed. Each membrane module was characterized by a pore size of 0.04 μm and a nominal surface of 0.93 m2. The pilot plant was continuously fed with real municipal wastewater, derived downstream the WWTP screening unit and then pumped into a load equalization basin (volume 1.5 m3) in order to secure a quite constant pollutant concentration during the day. Samples were taken three times per week from each section and analysed for total and volatile suspended solids (TSS and VSS), total and filtered COD (TCOD and FCOD), BOD5, ammonium nitrogen (NH4–N), nitrite nitrogen (NO2–N) and nitrate nitrogen (NO3–N), total nitrogen (TN), total phosphorous (TP). The main influent wastewater characteristics, referred to the period when the respirometric campaign was carried out, are reported in Table 1. The permeate was extracted using an extraction pump (for each membrane) by imposing an average flux almost equal to 45 l m−2 h−1 and a maximum depression (during the extraction period) of 0.50 bar. The membrane was periodically backwashed (every 9 min for a period of 1 min) by pumping a fraction of permeate back through the membrane modules. The pilot plant was operated for a period of 165 days: more in detail, until day 76th it was operated with complete sludge retention (except samples withdrawn for biological and chemical–physical analysis), while after the day 76th, the sludge has been regularly withdrawn, in order to maintain the sludge age near to 37 days. The
191
Table 1 Main influent wastewater characteristics. Parameter
Unit
Mean
Max
Min
TCOD NH4–N NO3–N TN TP TSS VSS T pH
mg/l mg/l mg/l mg/l mg/l mg/l mg/l °C –
326.6 15.0 1.8 45.7 3.8 282.5 177.3 20.8 7.6
525 30.5 10.7 88 12.8 736 404 26 8.4
138 3.9 0.0 14 1.7 108.0 60 18.9 7
average mixed liquor suspended solid (MLSS) concentrations ranged from 3 to 6.5 gTSS/l with an average percentage of volatile fraction of 70% and an average value of the Food/Microorganism ratio (F/M) equal to 0.13 kgTCOD∙kgSSV−1 d−1.
2.2. Description of the respirometric station Respirometric batch experiments were conducted using a “flowinggas/static-liquid” type as batch respirometer [11]. The biomass samples for COD fractionation as well as biokinetic characterization were taken from the oxidation tank of the UCT-MBR pilot plant and diluted with tap water or pilot plant effluent in order to obtain a mixed liquor volatile suspended solid (MLVSS) concentration in the range of 2000– 3000 mgVSS/l. Further, they were moved into the batch respirometer and aerated until endogenous conditions were reached before running the respirometric test. The samples were maintained at a constant temperature of 20 ± 1 °C with a thermostatic cryostat (JULABO). Agitation was provided by magnetic stirrer (FALC), and samples were intermittently aerated using aeration pumps. The dissolved oxygen concentration was measured with an oxygen probe and oximeter (WTW MULTI340i), while the aeration control and data acquisition were managed by the OURsys software. The aeration intervals were set from 3 to 6 mgO2/l. The OURsys software provided the respirograms chart, that feature the typical exogenous and endogenous respiration phases of the biomass. Starting from the obtained respirograms, the estimation of biokinetic parameters has been carried out. Referring to respirometric tests carried out on heterotrophic activity and COD fractionation, the nitrifying biomass has been inhibited by adding Allylthiourea (ATU). In Fig. 2, the respirometric set-up layout is shown.
Fig. 1. Pilot plant configuration layout.
192
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Fig. 2. Configuration of the respirometric system.
▪ the conversion factor between oxygen and ammonium (NOD: nitrogen oxygen demand) is equal to:
2.3. Heterotrophic biomass characterization Concerning the batch tests on heterotrophic biomass, the COD variations were calculated from O2 consumption, stated that oxygen variation is equivalent to COD through the following expression: ΔCOD ¼
ΔO2 1 fcv · YH
ð1Þ
where fcv is the conversion coefficient from COD to VSS (measured during the experimental campaign and equal to 1.57 mgCOD mgVSS−1 as average value) and YH the yield coefficient measured during the batch tests. On the other hand, COD consumption has been evaluated by solving the Monod-type kinetic expression with the finite difference procedure, estimating the parameters νH,max and KS, by fitting the following equation [9]: ΔS S ¼ νH;max · ·X Δt ðKS þ SÞ H
ð2Þ
where νH,max is the maximum removal rate, KS is the half-saturation coefficient for organic matter, S is the substrate concentration and XH is the biomass active fraction. Referring in particular to the biomass active fraction XH, MBR processes have been reported to have an active fraction between 4 and 7% with respect to VSS concentration in systems with SRT in the range of 35–50 days [12]. In the present work, since the average SRT of the pilot plant was almost equal to 37 days, it has been chosen a value of 7% for the heterotrophic active fraction. The estimation of the decay coefficient of the heterotrophic biomass bH was carried out according the “single batch test” used inter alia by Vanrolleghem et al. (1992) [13]. 2.4. Nitrifying biomass characterization For the estimation of the kinetic parameters of nitrifying biomass, the above mentioned procedure have been proposed, bearing in mind the following considerations: ▪ no inhibiting substance like ATU has been added; ▪ ammonium chloride (NH4Cl) has been added to evaluate the biokinetic parameters; ▪ pH values have been constantly monitored to avoid inhibition of the process;
ΔNH4 ¼
ΔO2 4:57
ð3Þ
The autotrophic specific yield coefficient has been evaluated according to Chandran and Smets [14], while, on the other hand, the estimation of the autotrophic decay rate bA, has been carried out according to the “multiple batch test” procedure, derived by Avcioglu et al. [15], originally proposed for heterotrophic bH evaluation. In particular, such procedure involved two aerated reactors in parallel, one aerobic reactor where the autotrophic biomass was kept in a starvation phase (digestion reactor), and a feed reactor put into operation at daily intervals, which was seeded with biomass withdrawn from the digesting reactor, and spiked with a certain amount of ammonium chloride, which was decreased every single day, to take into account the decrease in biomass activity. Each test had an average length of almost one week. 3. Results and discussion 3.1. Pilot plant performances As previously discussed, during the investigated period the average MLSS concentrations for the pilot-scale MBR ranged from 3 to 6.5 g/l and the average percentage of volatile fraction was almost equal to 70%. The observed yield coefficient (Yobs) was near to 0.07 gVSSgCOD−1 ox , after steady-state conditions were reached (day 76th), in good agreement with results obtained in previous experimental studies on MBR plants [16]. During the experimental period, the pilot plant reduced the average influent COD of 326 mg/l to a median effluent COD concentration of 27 mg/l (92% removal). More specifically, each specific unit contributed to the COD removal, even if the major part of influent COD was consumed in the anaerobic reactor, with a removal efficiency of 72%, according to previous results reported by other authors [17]. Regarding the TN removal, the average nitrification and denitrification efficiencies were about 95% and 55%, respectively. In particular, the unsatisfying denitrification efficiency was likely due to the too high nitrate (NO3–N) concentration in the mixed liquor recycled from the aeration reactor to the anoxic one. In particular, the NO3–N concentration in the permeate was found to be very variable, in the range of 0.7–
D. Di Trapani et al. / Desalination 269 (2011) 190–197
193
28
200
Unit
Mean
Max
Min
TCOD NH4–N NO3–N TN TP
mg/l mg/l mg/l mg/l mg/l
27.42 0.16 16.10 20.61 2.75
60 1.18 35.77 45 8.74
4 0 0.69 6.5 0.88
OUR [mgO2/(Lh)]
24 Parameter
180
OUR max OUR end SRT [d]
20
160 140 120
16
100 12
80
SRT [d]
Table 2 Main effluent wastewater characteristics.
60
8
40
35 mg/l. This circumstance indicated that denitrification process was not complete. This result was likely influenced by periodic discharges of industrial wastewater limited volumes as well as landfill leachate, that probably lead to an increase of the influent nitrogen concentration, especially the organic form. Finally, the chemical cleaning was done when the trans membrane pressure (TMP) value exceeded 0.5 bar, keeping constant the permeate flux (45 l/m2∙h). On the other hand, the effluent TSS concentrations were always below the detection limit of 2 mg/l, indicating the membrane integrity. In Table 2, the main effluent characteristics, related to the respirometric experimental campaign period, are summarized. 3.2. Heterotrophic biomass characterization Respirometric batch tests allowed to measure the biomass activity during the experimental period and to evaluate its biokinetic behaviour. In general, the obtained respirograms featured the typical exogenous and endogenous respiration phases, showing a change in biomass activity through the experimental investigation period, likely connected to the different operational conditions at the beginning and at the end of the study. Fig. 3 shows, as an example, two OUR profiles, at the beginning (a) and at the end (b) of the respirometric campaign, respectively. The obtained results showed that the biomass activity increased almost continuously during the experimental period; in particular, as previously discussed, in the start-up phase the UCT-MBR pilot plant has been operated with complete sludge retention, while after day 76th, the sludge has been regularly withdrawn, in order to maintain the sludge age near to 37 days. The respirometric campaign started when the pilot plant was still operated with complete sludge retention, and continued during the period characterized by daily sludge wastage, thus highlighting the increase of biomass activity under the different operational conditions. In Fig. 4, the OUR (exogenous and endogenous) as well as the SRT values, are reported; referring to the respirometric campaign. As depicted in Fig. 4, an increasing trend of the OUR exogenous and endogenous took place during the experimental period characterised by regular sludge
a
b
18
withdrawals. In this latter phase, the average SRT value was equal to 37 days, and, despite this high value, the biomass vitality was quite good. Such a fact basically reflected the biomass promptness to respond to substrate spiking during the batch tests.
3.2.1. Yield coefficient YH Yield coefficient for heterotrophic biomass was assessed from the integral of the exogenous uptake rate according to the methodology suggested by Vanrolleghem et al. [18]. According to such a methodology, after sodium acetate addiction, used as readily biodegradable carbonaceous substrate, oxygen is rapidly consumed and immediately after external substrate depletion, the endogenous phase is restored. The obtained YH values were well in the range of the ones proposed by the International Water Association [19] that vary between 0.38 and 0.75 mgCODcell mgCOD−1 ox , for a CAS process. Indeed, for the pilot plant under study, the YH values ranged from −1 0.57 mgCODcell mgCOD−1 ox (corresponding to 0.36 mgVSS mgCODox ) −1 up to 0.74 mgCODcell mgCODox (corresponding to 0.46 mgVSS mgCOD−1 ox ). The obtained values were in good agreement with previous YH values evaluated in respirometric tests on other MBR process configurations [20,21]. However, it is worthy to observe that the obtained values of YH differ significantly from the average biomass yield calculated in the present study on the basis of mass balances between sludge withdrawn and produced, dividing by the cumulative COD removed (Yobs), and equal to 0.07 mgVSS mgCOD−1 ox . The explanation of this discrepancy is likely related to the fact that the yield coefficient, calculated on the basis of mass balances, naturally takes into account the biomass decay (differently from a respirometric batch test, which length is limited in time), that in a MBR process is not negligible, similar to the maximum growth rate [17], leading to a
18
14
12
OUR [mgO2/(Lh)]
OUR exogenous OUR Exogenous OUR endogenous OUR
10 8 6 4
OUR endogenous
12 10 8 6 4 2
2
0 0
5
10 15 Time [h]
20
25
0 9/2/10
Fig. 4. OUR exogenous, endogenous and SRT values versus time.
OURexogenous OUR exogenous
14
OUR [mgO2/(Lh)]
20
0 12/10/09 1/11/09 21/11/09 11/12/09 31/12/09 20/1/10 Time [Day]
16
16
0
4
0
10
20
30
40
Time [h]
Fig. 3. Typical OUR profiles at the beginning (a) and at the end (b) of the experimental campaign, respectively.
50
194
D. Di Trapani et al. / Desalination 269 (2011) 190–197
3
tests. In Fig. 6, as an example, two Monod-type kinetic profiles for heterotrophic bacteria is reported, referring to the beginning (a) and to the end (b) of the experimental campaign, respectively.
µH,max [day-1]
2.5
2
1.5 1
0.5 0 14/10/09
3/11/09
23/11/09 13/12/09 2/1/10 Time [Day]
22/1/10
11/2/10
Fig. 5. Heterotrophic maximum growth rate versus time.
net growth close to zero, especially in MBRs operated with complete sludge retention. Further, a possible explanation of the above mentioned discrepancy could be found in the different biomass behaviour with real wastewater or synthetic substrate like sodium acetate. To support such statement, some parallel tests with both real wastewater and sodium acetate have been carried out. The experimental results provided values of the yield coefficient of 0.37 and 0.15 mgVSS mgCOD−1 ox for acetate and wastewater, respectively, therefore supporting the hypothesis outlined above about the different behaviour of the biomass in presence of a rapidly biodegradable substrate like sodium acetate. 3.2.2. Maximum growth rate μ H,max and half-saturation coefficient KS Referring to the maximum growth rate μ H,max, the obtained values reflected the above mentioned OUR exogenous trend, reaching at the end of the experimental phase values within the range reported for CAS systems [10]. In Fig. 5, the maximum growth rate values evaluated during the respirometric batch tests are shown. Previous studies on MBRs with different plant configurations showed higher values for μH,max [21], suggesting the importance of pilot plant configuration and operational conditions, with sludge complete retention or regular sludge withdrawal in particular, on biomass viability in a MBR process. Concerning the half-saturation coefficient KS, the obtained values were lower than the ones obtained in previous studies carried out on MBR systems [21]. A possible reason should be found in the use of a readily biodegradable substrate like sodium acetate during the batch
Several tests on autotrophic biomass were carried out on samples collected from the aerobic tank of the UCT-MBR pilot plant in order to evaluate the biokinetic coefficients. Fig. 8 shows as an example an OUR profile and a Michaelis–Menten modelization for nitrifying bacteria. The obtained results are well in the range proposed for nitrification in CAS systems [19], thus suggesting a good development for nitrification activity, despite the presence of the internal recycle from the aerobic to the anoxic tank that should reduce the development of the autotrophic biomass. Indeed the maximum OUR values for nitrifying biomass were similar to the heterotrophic bacteria ones, confirming a good nitrifying activity, comparable to the organic matter removal rate in the aerobic tank. Operating with high SRTs is essential to obtain complete nitrification, especially when treating wastewater containing a certain amount of landfill leachate and industrial discharges, like in the present study. In Table 3 the average values obtained for the kinetic coefficients referring to nitrifying bacteria are reported.
b
45 40
dS/dt [mgCOD/(Lh)]
3.3. Nitrifying biomass characterization
45 40
Experimental
35
Model
dS/dt [mgCOD/Lh]
a
3.2.3. Decay rate bH As previously mentioned, the decay rate of heterotrophic bacteria was evaluated through the “single batch test” procedure [18], by taking the samples in starvation condition for 2–3 days, until endogenous conditions were reached. By plotting the logarithmic OUR curve vs time, the decay rate has been estimated as the slope of the straight interpolation line (Fig. 7). The obtained values during the respirometric campaign were higher compared to previous studies on similar plants [21,20] as well as on CAS processes [10]. However, it has to be stressed that the decay rate showed a decreasing trend during the experimental period, from 0.56 to 0.30 d−1, thus indicating an increasing biomass viability starting from an initial situation characterised by complete sludge retention, to a steady state condition characterised by regular daily sludge withdrawals. Nevertheless, the obtained value for the decay rate was higher compared to the typical one assumed for activated sludge systems [10], even at the end of the experimental period. This observed result is in good agreement with previous studies; indeed, in a submerged MBR system, the heterotrophic decay rate can be an order of magnitude higher than the one typically observed for CAS processes [22–24].
30 25 20 15 10
35 30 25 20 Experimental Experimental Model Model
15 10 5
5
0
0 0
20
40
60 80 S [mgCOD/L]
100
120
140
0
10
20
30 40 S [mgCOD/L]
50
Fig. 6. Examples of Michaelis–Menten modelization profiles at the beginning (a) and the end (b) of the respirometric experimental campaign.
60
D. Di Trapani et al. / Desalination 269 (2011) 190–197
a 2.0
0
10
20
30
Time [h] 40
50
60
70
b
80
10
20
30
Time [h] 40 50
60
70
80
2.0 1.8
1.8 1.6
Experimental
1.4
Model
1.6
lnOUR [mg/Lh]
lnOUR [mg/Lh]
0
195
y = -0.0123x + 0.8579 2 R = 0.9526
1.2 1.0 0.8 0.6
y = -0.0165x + 1.5513 R2 = 0.9818
1.4 1.2 1.0 0.8 Experimental Experimental Model Model
0.6
0.4
0.4
0.2
0.2
0.0
0.0 Fig. 7. Examples of logarithmic OUR profiles at the beginning (a) and at the end (b) of the respirometric experimental campaign.
a
b
20 18
OUR [mgO2/(Lh)]
dN/dt [mgNH4-N/(Lh)]
OUR exogenous OUR endogenous
16 14 12 10 8 6 4 2 0
3.5 3 2.5 2 1.5
Experimental Model
1 0.5 0
0
10
20
30
40
50
0
4
8
Time [h]
12 N-NH4 [mg/L]
16
20
24
Fig. 8. Example of OUR profile (a) and Michaelis–Menten modelization profile (b) for nitrifying bacteria.
3.3.1. Yield coefficient YA As previously mentioned, the yield coefficient for the autotrophic bacteria YA was evaluated by using the expression proposed by Chandran and Smets [14]. The obtained values were in the range reported in the technical literature [19], with an average value of 0.18 mgCODcell mgNH4–N−1 ox , only a little lower than the ones reported by Lubello et al. [21] on a MBR pilot plant. 3.3.2. Maximum autotrophic growth rate μA,max and half-saturation coefficient KNH Also in this case the obtained kinetic coefficients were well within the usual range proposed for CAS systems [19]. Referring in particular to the maximum growth rate, the average value reported in Table 3 is equal to 0.27 d−1, suggesting a good development for nitrifying biomass in such process. However, the above mentioned value is lower than the typical one found for CAS systems [8], suggesting that a different selective pressure on nitrifying bacteria can arise in such systems. Previous studies on similar pilot plant layout showed the same behaviour [29]. On the other hand, the half-saturation coefficient KNH was found to be equal to 0.57 mgNH4–N/l (average value), lower than the typical value reported in the technical literature for nitrification in CAS systems [10]. 3.3.3. Decay rate bA The decay rate for autotrophic biomass, as previously mentioned, has been evaluated by using a procedure derived from Avcioglu et al. [15], originally proposed to evaluate the decay rate for heterotrophic bacteria. The obtained values were within the range proposed in the literature [10], confirming a good development of the autotrophic nitrifying biomass. An example of OUR profile for the estimation of endogenous decay coefficient bA is reported in Fig. 9.
3.4. COD fractionation The COD fractionation was periodically repeated during the experimental investigations, in order to characterize the influent wastewater fed to the pilot plant. As aforementioned, the biomass samples were brought to endogenous condition before starting the batch test, and the nitrifying bacteria were inhibited by adding allylthiourea. In Table 4 the average results are shown. SS, XS, SI, XI and XH are the soluble biodegradable, slowly biodegradable and rapidly hydrolysable, soluble inert, particulate inert and active biomass COD fractions respectively. The average values are in good agreement with previous studies on raw wastewater [19,8], except the percentage values referring to the inert matter, soluble and particulate, which was slightly higher, while the readily and slowly biodegradable matter fractions were lower; the reason of this should be addressed to the landfill leachate, contained in a certain amount in the wastewater fed to the pilot plant. These results are in good agreement with previous respirometric experiments carried out on the same wastewater [30–32]. 4. Conclusions An experimental gathering campaign on a UCT-MBR pilot plant, conceived for biological nutrient removal, was performed. One of the main aims of the study was the evaluation of the kinetic parameters of both heterotrophic and autotrophic biomass, as well as the COD fractionation, with the aid of respirometric batch tests. The experimental observations highlighted a decrease in the kinetic parameters for heterotrophic bacteria compared to that of a CAS system, even if they increased when the pilot was operated with daily sludge withdrawal, showing a higher heterotrophic biomass activity with lower SRT values. On the other hand, the investigations carried out on
196
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Table 3 Average values of kinetic parameters for nitrifying bacteria in different processes. Parameter
Units
Value
Reference
Process
Wastewater tested
YA YA YA YA μA.max μA.max μA.max μA.max μA.max KNH KNH KNH KNH νA,max bA bA bA Max. nitrif. rate
gVSS gNH4-N−1 gVSS gNH4-N−1 gVSS gNH4-N−1 gVSS gNH4-N−1 d−1 d−1 d−1 d−1 d−1 mgNH4-N/l mgNH4-N/l mgNH4-N/l mgNH4-N/l d−1 d−1 d−1 d−1 mgNH4-N/(lh)
0.18 0.24 0.10–0.15 0.34 0.27 0.1–0.2 2.02 0.38 0.2–0.9 0.57 0.85 0.5–1.0 0.1–0.15 1.5 0.074 0.08 0.05–0.15 2.30
This [21] [10] [25] This [26] [27] [21] [10] This [27] [10] [28] This This [21] [10] This
UCT-MBR MBR with pre-denitrification – ASP with denitrification UCT-MBR MBR MBR with low DO MBR with pre-denitrification – UCT-MBR MBR with low DO – MBR and CAS in parallel UCT-MBR UCT-MBR MBR with pre-denitrification – UCT-MBR
Municipal wastewater Tannery and domestic wastewater (20 °C) 0.123 typical Municipal wastewater Municipal wastewater Municipal wastewater (20 days SRT) 30 °C – Sludge digester supernatant (SRT N 650 d) Tannery and domestic wastewater (20 °C) 0.75 typical Municipal wastewater 30 °C – Sludge digester supernatant (SRT N 650 d) 0.74 typical Domestic wastewater Municipal wastewater Municipal wastewater Tannery and domestic wastewater (20 °C) 0.08 typical Municipal wastewater
experimentation
experimentation
experimentation
experimentation experimentation
experimentation
25
OUR [mgO2/(Lh)]
20 15 10 5 0 0
1
2
3 4 Time [Day]
5
6
7
Fig. 9. Example of OUR profile of a multiple batch test for autotrophic endogenous decay rate estimation.
Table 4 Average COD fraction values. SS
XS
SI + XI
XH
%
%
%
%
10.67
23.40
57.94
7.99
nitrifying bacteria showed that the obtained parameters were within the range proposed for nitrification in CAS systems, thus suggesting a good development for nitrification activity, and that operating with high SRTs is essential to obtain an almost complete nitrification, especially when treating wastewater containing a certain amount of landfill leachate. The suggestion is that operating conditions, and SRT values in particular, strictly influence the biokinetic behaviour of a MBR plant, moreover the increase of the activity for nitrifying bacteria happens in detriment of that for heterotrophic bacteria that probably is limited by the pilot-plant layout. Further, the obtained results confirmed respirometry as a suitable tool for wastewater and biomass characterisation, and that the obtained kinetic parameters should provide a useful support in MBR design and management, as well as in MBR simulations.
References [1] T. Stephenson, S. Judd, B. Jefferson, K. Brindle, Membrane Bioreactors for Wastewater Treatment, IWA Publishing, London, 2000.
[2] E.B. Muller, A.H. Stouthamer, H.W. Vanverseveld, D.H. Eikelboom, Aerobic domestic waste water treatment in a pilot plant with complete sludge retention by cross-flow filtration, Water Res. 29 (1995) 1179–1189. [3] M. Ramphao, M.C. Wentzel, R. Merritt, G.A. Ekama, T. Young, C.A. Buckley, Impact of membrane solid–liquid separation on design of biological nutrient removal activated sludge systems, Biotechnol. Bioeng. 89 (6) (2005) 630–646. [4] Durante, F. (2006). Analisi del funzionamento degli impianti MBR gestiti in condizioni di ritenzione completa di biomassa. Ph.D. Thesis. Università degli Studi di Napoli, Italy (in italian). [5] Parco, V. (2006). Bioreattori a membrana per la rimozione biologica dei nutrienti: cinetiche di processo ed efficienze. Ph.D. Thesis. Università degli Studi di Napoli, Italy (in italian). [6] J. Kappeler, W. Gujer, Estimation of heterotrophic parameters under aerobic conditions and characterization of wastewater for activated sludge modelling, Water Sci. Technol. 25 (1992) 125–139. [7] G. Mannina, G. Di Bella, G. Viviani, Uncertainty assessment of a membrane bioreactor model using the GLUE methodology, Biochem. Eng. J. 52 (2) (2010) 263–275. [8] G. Di Bella, G. Mannina, G. Viviani, An integrated model for physical-biological wastewater organic removal in a submerged membrane bioreactor: model development and parameter estimation, J. Membr. Sci. 322 (1) (2008) 1–12. [9] H. Spanjers, P.A. Vanrolleghem, G. Olsson, P. Dold, Respirometry in control of the activated sludge process: principles, Scientific and Technical Report No. 7, IWA Publishing, ISBN: 1 900222 04 3, 1998. [10] Metcalf and Eddy, Inc., Wastewater engineering treatment and reuse, McGrawHill, New York, 2003. [11] H. Spanjers, P.A. Vanrolleghem, G. Olsson, P. Dold, Respirometry in control of the activated sludge process, Water Sci. Technol. 34 (3–4) (1996) 117–126. [12] Guglielmi, G. (2002). Applicazione dei bioreattori a membrana al trattamento di reflui civili. Ph.D. Thesis, Università degli Studi di Trento, Italy (in italian). [13] Vanrolleghem, P.A., Van Impe, J.F., Vandewalle, J. And Verstraete, W. (1992). Advanced monitoring and control of the activated sludge process: on-line estimation of crucial biological variables in a structured model with RODTOX biosensor. Modeling and control of biotechnical processes. Karim M. N., Stephanopoulos, G., 355-358, Pergamon Press, Oxford. [14] K. Chandran, B.F. Smets, Estimating biomass yield coefficients for autotrophic ammonia and nitrite oxidation from batch respirograms, Water Res. 35 (13) (2001) 3153–3156. [15] E. Avcioglu, D. Orhon, S. Sözen, A new method for the assessment of heterotrophic endogenous respiration rate under aerobic and anoxic conditions, Water Sci. Technol. 38 (8–9) (1998) 95–103. [16] G. Laera, A. Pollice, D. Saturno, C. Giordano, A. Lopez, Zero net growth in a membrane bioreactor with complete sludge retention, Water Res. 39 (2005) 5241–5249. [17] H. Zhang, X. Wang, J. Xiao, F. Yang, J. Zhang, Enhanced biological nutrient removal using MUCT–MBR system, Bioresour. Technol. 100 (2009) 1048–1054. [18] P.A. Vanrolleghem, H. Spanjers, B. Petersen, P. Ginestet, I. Takacs, Estimating (combination of) Activated Sludge Model No.1 parameters and component by respirometry, Water Sci. Technol. 39 (1) (1999) 195–214. [19] M. Henze, C.P.L. Grady Jr., W. Gujer, G.V.R. Marais, T. MAtsuo, Activated sludge model No.1, IAWPRC Scientific and Technical Report No.1, IAWPRC, London, 1987. [20] G. Munz, R. Gori, L. Cammilli, C. Lubello, Characterization of tannery wastewater and biomass in a membrane bioreactor using respirometric analysis, Bioresour. Technol. 99 (2008) 8612–8618. [21] C. Lubello, S. Caffaz, R. Gori, G. Munz, A modified Activated Sludge Model to estimate solids production at low and high solids retention time, Water Res. 43 (18) (2009) 4539–4548. [22] S.G. Lu, T. Imai, M. Ukita, M. Sekine, T. Higuchi, M. Fukagawa, A model for membrane bioreactor process based on the concept of formation and degradation of soluble microbial products, Water Res. 35 (8) (2001) 2038–2048.
D. Di Trapani et al. / Desalination 269 (2011) 190–197 [23] C.S. Laspidou, B.E. Rittmann, A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass, Water Res. 36 (2002) 2711–2720. [24] N. Jang, X. Ren, J. Cho, I.S. Kim, Steady-state modelling of bio-fouling potentials with respect to the biological kinetics in the submerged membrane bioreactor (SMBR), J. Membr. Sci. 284 (2006) 352–360. [25] P. Harremoës, O. Sinkjaer, Kinetic interpretation of nitrogen removal in pilot scale experiments, Water Res. 29 (3) (1995) 899–905. [26] X.J. Fan, V. Urbain, Y. Qian, J. Manem, Nitrification and mass balance with a membrane bioreactor for municipal wastewater treatment, Water Sci. Technol. 34 (1–2) (1996) 129–136. [27] S. Wyffels, S. Van Hulle, P. Boeckx, E. Volcke, O. Van Cleemput, P. And Vanrolleghem, W. Verstraete, Modelling and simulation of oxygen-limited partial nitritation in a membrane assisted bioreactor (MBR), Biotechnol. Bioeng. 86 (5) (2003) 531–542.
197
[28] R. Manser, W. Gujer, S. Hansruedi, Consequences of mass transfer effects on the kinetics of nitrifiers, Water Res. 39 (2005) 4633–4642. [29] V. Parco, M. Wentzel, G. Ekama, Kinetics of nitrogen removal in a MBR nutrient removal activated sludge system, Desalination 199 (2006) 89–91. [30] G. Mannina, G. Viviani, Hybrid moving bed biofilm reactors: an effective solution for upgrading a large wastewater treatment plant, Water Sci. Technol. 60 (5) (2009) 1103–1116. [31] D. Di Trapani, G. Mannina, M. Torregrossa, G. Viviani, Quantification of kinetic parameters for heterotrophic bacteria via respirometry in a hybrid reactor, Water Sci. Technol. 61 (7) (2010) 1757–1766. [32] D. Di Trapani, G. Mannina, M. Torregrossa, G. Viviani, Comparison between hybrid moving bed biofilm reactor and activated sludge system: a pilot plant experiment, Water Sci. Technol. 61 (4) (2010) 891–902.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Evaluation of biomass activity and wastewater characterization in a UCT-MBR pilot plant by means of respirometric techniques Daniele Di Trapani ⁎, Marco Capodici, Alida Cosenza, Gaetano Di Bella, Giorgio Mannina, Michele Torregrossa, Gaspare Viviani Dipartimento di Ingegneria Civile, Ambientale ed Aerospaziale, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
a r t i c l e
i n f o
Article history: Received 4 August 2010 Received in revised form 26 October 2010 Accepted 26 October 2010 Keywords: Respirometric analysis Biokinetic coefficients Membrane Bioreactor (MBR) Wastewater treatment Pilot plant experiment
a b s t r a c t Over the last two decades, Membrane Bioreactors (MBRs) emerged even more for wastewater treatment, ensuring high removal efficiencies as well as very small footprint requirements. Indeed, in this kind of process, a modification in biomass activity and viability can exist compared to that of a CAS process. In this context, respirometric analysis represents a reliable tool in order to evaluate the actual biomass kinetic parameters, to insert in mathematical models in the design phase, as well as to monitor the biomass viability, especially when these processes are operated with high SRT values. The paper presents some results of respirometric techniques applied for the characterisation of wastewater and biomass activity in a pilot UCT-MBR plant for nutrient removal, operating with high SRT. In particular, the respirometric tests were specifically aimed at investigating heterotrophic and autotrophic bacterial activity. The pilot plant was built at Palermo WWTP and consisted of three reactors: anaerobic, anoxic and aerobic, followed by an aerobic compartment containing two submerged hollow fibre membrane modules with typical recycling lines. The kinetic parameters for heterotrophic bacteria resulted lower respect to the CAS; regarding the nitrifying bacteria, the kinetic constants were in the range of CAS, suggesting a good nitrification activity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, due to the increasing awareness about environmental impact of pollutant discharges, the effluent standards are becoming more and more stringent, leading to increased requirements for Wastewater Treatment Plant (WWTP). In this context, the technical and scientific community in the last years showed a growing interest in developing innovative technologies that, together with very high removal efficiencies, can lead to a very low space and volume request. A possible solution to cope with such issues is represented by membrane bioreactors (MBRs), which are combined systems including a bioreactor and a filtration unit (usually an ultrafiltration or microfiltration membrane). More specifically, such systems compared to the traditional ones, like the conventional activated sludge (CAS) processes, which require large aeration and settling tanks, have shown higher efficiency in terms of effluent concentrations as well as smaller footprint and sludge production, due to higher biomass concentration in the bioreactor [1]. Further, MBRs enable high treatment levels in terms of effluent total suspended solid (TSS) concentrations, organic matter and total nitrogen (TN). More in detail, the very high biomass concentration, compared to the available food in the system, contributes to create an
⁎ Corresponding author. E-mail address: [email protected] (D. Di Trapani). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.10.061
environment where bacteria are facing starvation condition so they are not in a physiological state for cell growth [2]. On the other hand, the high sludge retention times (SRTs), typical of MBR system, are highly advantageous for the growth of nitrifying bacteria [1]. Concerning nutrient removal, Ramphao et al. (2005) discussed about the influence of the MBR solid separation on the design of biological nutrient removal (BNR) systems which result smaller than the equivalent conventional systems [3]. On the other hand, Durante et al. and Parco et al. [4,5] have focused the attention on the influence of the size of the MBR flocs on substrates diffusive transport. However, up to now, the knowledge concerning the influence that high solid concentrations in the mixed liquor as well as the nature of the selected biomass have on the BNR process is controversial and still limited. Indeed, in such systems, a modification in the biomass biokinetic behaviour can arise, compared to that of a CAS process. In this context, respirometric techniques [6] should represent a useful tool for the characterization of the biokinetic behaviour of bacteria in a MBR process, to insert in mathematical models in the design phase, as well as to monitor the biomass viability [7,8]. Indeed, oxygen uptake rate (OUR), i.e. the oxygen consumption per unit volume per unit time, is widely recognized as an important parameter in order to monitor the biomass viability [9]. On the other hand, in the case of domestic wastewaters with contributions of industrial wastewater, and in a process operated with high SRTs, it might be of paramount interest the respirometric procedures in order to better characterise wastewater and biomass viability.
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Bearing in mind such considerations, the paper presents some results of a study aimed at evaluating the heterotrophic and autotrophic kinetic parameters, in terms of organic and nitrogen removal, as well as wastewater chemical oxygen demand (COD) fractionation in a UCTMBR pilot plant using respirometric techniques. 2. Materials and methods 2.1. Pilot plant and operating conditions description The experimental investigations were carried out on a MBR pilot plant, conceived for nutrient removal, built at the Acqua dei Corsari (Palermo) WWTP. In Fig. 1, the pilot plant layout is reported. The pilot plant scheme was an adaptation of the UCT process [10] with a final membrane filtration unit instead of the conventional secondary clarifier. It consisted of three reactors in series, anaerobic (mean volume 71.6 l), anoxic (mean volume 164.9 l) and aerobic (mean volume 327 l) respectively, followed by an aerobic tank (mean volume 52 l) where two submerged hollow fibre membrane modules (Zenon Zeeweed, ZW 10) were installed. Each membrane module was characterized by a pore size of 0.04 μm and a nominal surface of 0.93 m2. The pilot plant was continuously fed with real municipal wastewater, derived downstream the WWTP screening unit and then pumped into a load equalization basin (volume 1.5 m3) in order to secure a quite constant pollutant concentration during the day. Samples were taken three times per week from each section and analysed for total and volatile suspended solids (TSS and VSS), total and filtered COD (TCOD and FCOD), BOD5, ammonium nitrogen (NH4–N), nitrite nitrogen (NO2–N) and nitrate nitrogen (NO3–N), total nitrogen (TN), total phosphorous (TP). The main influent wastewater characteristics, referred to the period when the respirometric campaign was carried out, are reported in Table 1. The permeate was extracted using an extraction pump (for each membrane) by imposing an average flux almost equal to 45 l m−2 h−1 and a maximum depression (during the extraction period) of 0.50 bar. The membrane was periodically backwashed (every 9 min for a period of 1 min) by pumping a fraction of permeate back through the membrane modules. The pilot plant was operated for a period of 165 days: more in detail, until day 76th it was operated with complete sludge retention (except samples withdrawn for biological and chemical–physical analysis), while after the day 76th, the sludge has been regularly withdrawn, in order to maintain the sludge age near to 37 days. The
191
Table 1 Main influent wastewater characteristics. Parameter
Unit
Mean
Max
Min
TCOD NH4–N NO3–N TN TP TSS VSS T pH
mg/l mg/l mg/l mg/l mg/l mg/l mg/l °C –
326.6 15.0 1.8 45.7 3.8 282.5 177.3 20.8 7.6
525 30.5 10.7 88 12.8 736 404 26 8.4
138 3.9 0.0 14 1.7 108.0 60 18.9 7
average mixed liquor suspended solid (MLSS) concentrations ranged from 3 to 6.5 gTSS/l with an average percentage of volatile fraction of 70% and an average value of the Food/Microorganism ratio (F/M) equal to 0.13 kgTCOD∙kgSSV−1 d−1.
2.2. Description of the respirometric station Respirometric batch experiments were conducted using a “flowinggas/static-liquid” type as batch respirometer [11]. The biomass samples for COD fractionation as well as biokinetic characterization were taken from the oxidation tank of the UCT-MBR pilot plant and diluted with tap water or pilot plant effluent in order to obtain a mixed liquor volatile suspended solid (MLVSS) concentration in the range of 2000– 3000 mgVSS/l. Further, they were moved into the batch respirometer and aerated until endogenous conditions were reached before running the respirometric test. The samples were maintained at a constant temperature of 20 ± 1 °C with a thermostatic cryostat (JULABO). Agitation was provided by magnetic stirrer (FALC), and samples were intermittently aerated using aeration pumps. The dissolved oxygen concentration was measured with an oxygen probe and oximeter (WTW MULTI340i), while the aeration control and data acquisition were managed by the OURsys software. The aeration intervals were set from 3 to 6 mgO2/l. The OURsys software provided the respirograms chart, that feature the typical exogenous and endogenous respiration phases of the biomass. Starting from the obtained respirograms, the estimation of biokinetic parameters has been carried out. Referring to respirometric tests carried out on heterotrophic activity and COD fractionation, the nitrifying biomass has been inhibited by adding Allylthiourea (ATU). In Fig. 2, the respirometric set-up layout is shown.
Fig. 1. Pilot plant configuration layout.
192
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Fig. 2. Configuration of the respirometric system.
▪ the conversion factor between oxygen and ammonium (NOD: nitrogen oxygen demand) is equal to:
2.3. Heterotrophic biomass characterization Concerning the batch tests on heterotrophic biomass, the COD variations were calculated from O2 consumption, stated that oxygen variation is equivalent to COD through the following expression: ΔCOD ¼
ΔO2 1 fcv · YH
ð1Þ
where fcv is the conversion coefficient from COD to VSS (measured during the experimental campaign and equal to 1.57 mgCOD mgVSS−1 as average value) and YH the yield coefficient measured during the batch tests. On the other hand, COD consumption has been evaluated by solving the Monod-type kinetic expression with the finite difference procedure, estimating the parameters νH,max and KS, by fitting the following equation [9]: ΔS S ¼ νH;max · ·X Δt ðKS þ SÞ H
ð2Þ
where νH,max is the maximum removal rate, KS is the half-saturation coefficient for organic matter, S is the substrate concentration and XH is the biomass active fraction. Referring in particular to the biomass active fraction XH, MBR processes have been reported to have an active fraction between 4 and 7% with respect to VSS concentration in systems with SRT in the range of 35–50 days [12]. In the present work, since the average SRT of the pilot plant was almost equal to 37 days, it has been chosen a value of 7% for the heterotrophic active fraction. The estimation of the decay coefficient of the heterotrophic biomass bH was carried out according the “single batch test” used inter alia by Vanrolleghem et al. (1992) [13]. 2.4. Nitrifying biomass characterization For the estimation of the kinetic parameters of nitrifying biomass, the above mentioned procedure have been proposed, bearing in mind the following considerations: ▪ no inhibiting substance like ATU has been added; ▪ ammonium chloride (NH4Cl) has been added to evaluate the biokinetic parameters; ▪ pH values have been constantly monitored to avoid inhibition of the process;
ΔNH4 ¼
ΔO2 4:57
ð3Þ
The autotrophic specific yield coefficient has been evaluated according to Chandran and Smets [14], while, on the other hand, the estimation of the autotrophic decay rate bA, has been carried out according to the “multiple batch test” procedure, derived by Avcioglu et al. [15], originally proposed for heterotrophic bH evaluation. In particular, such procedure involved two aerated reactors in parallel, one aerobic reactor where the autotrophic biomass was kept in a starvation phase (digestion reactor), and a feed reactor put into operation at daily intervals, which was seeded with biomass withdrawn from the digesting reactor, and spiked with a certain amount of ammonium chloride, which was decreased every single day, to take into account the decrease in biomass activity. Each test had an average length of almost one week. 3. Results and discussion 3.1. Pilot plant performances As previously discussed, during the investigated period the average MLSS concentrations for the pilot-scale MBR ranged from 3 to 6.5 g/l and the average percentage of volatile fraction was almost equal to 70%. The observed yield coefficient (Yobs) was near to 0.07 gVSSgCOD−1 ox , after steady-state conditions were reached (day 76th), in good agreement with results obtained in previous experimental studies on MBR plants [16]. During the experimental period, the pilot plant reduced the average influent COD of 326 mg/l to a median effluent COD concentration of 27 mg/l (92% removal). More specifically, each specific unit contributed to the COD removal, even if the major part of influent COD was consumed in the anaerobic reactor, with a removal efficiency of 72%, according to previous results reported by other authors [17]. Regarding the TN removal, the average nitrification and denitrification efficiencies were about 95% and 55%, respectively. In particular, the unsatisfying denitrification efficiency was likely due to the too high nitrate (NO3–N) concentration in the mixed liquor recycled from the aeration reactor to the anoxic one. In particular, the NO3–N concentration in the permeate was found to be very variable, in the range of 0.7–
D. Di Trapani et al. / Desalination 269 (2011) 190–197
193
28
200
Unit
Mean
Max
Min
TCOD NH4–N NO3–N TN TP
mg/l mg/l mg/l mg/l mg/l
27.42 0.16 16.10 20.61 2.75
60 1.18 35.77 45 8.74
4 0 0.69 6.5 0.88
OUR [mgO2/(Lh)]
24 Parameter
180
OUR max OUR end SRT [d]
20
160 140 120
16
100 12
80
SRT [d]
Table 2 Main effluent wastewater characteristics.
60
8
40
35 mg/l. This circumstance indicated that denitrification process was not complete. This result was likely influenced by periodic discharges of industrial wastewater limited volumes as well as landfill leachate, that probably lead to an increase of the influent nitrogen concentration, especially the organic form. Finally, the chemical cleaning was done when the trans membrane pressure (TMP) value exceeded 0.5 bar, keeping constant the permeate flux (45 l/m2∙h). On the other hand, the effluent TSS concentrations were always below the detection limit of 2 mg/l, indicating the membrane integrity. In Table 2, the main effluent characteristics, related to the respirometric experimental campaign period, are summarized. 3.2. Heterotrophic biomass characterization Respirometric batch tests allowed to measure the biomass activity during the experimental period and to evaluate its biokinetic behaviour. In general, the obtained respirograms featured the typical exogenous and endogenous respiration phases, showing a change in biomass activity through the experimental investigation period, likely connected to the different operational conditions at the beginning and at the end of the study. Fig. 3 shows, as an example, two OUR profiles, at the beginning (a) and at the end (b) of the respirometric campaign, respectively. The obtained results showed that the biomass activity increased almost continuously during the experimental period; in particular, as previously discussed, in the start-up phase the UCT-MBR pilot plant has been operated with complete sludge retention, while after day 76th, the sludge has been regularly withdrawn, in order to maintain the sludge age near to 37 days. The respirometric campaign started when the pilot plant was still operated with complete sludge retention, and continued during the period characterized by daily sludge wastage, thus highlighting the increase of biomass activity under the different operational conditions. In Fig. 4, the OUR (exogenous and endogenous) as well as the SRT values, are reported; referring to the respirometric campaign. As depicted in Fig. 4, an increasing trend of the OUR exogenous and endogenous took place during the experimental period characterised by regular sludge
a
b
18
withdrawals. In this latter phase, the average SRT value was equal to 37 days, and, despite this high value, the biomass vitality was quite good. Such a fact basically reflected the biomass promptness to respond to substrate spiking during the batch tests.
3.2.1. Yield coefficient YH Yield coefficient for heterotrophic biomass was assessed from the integral of the exogenous uptake rate according to the methodology suggested by Vanrolleghem et al. [18]. According to such a methodology, after sodium acetate addiction, used as readily biodegradable carbonaceous substrate, oxygen is rapidly consumed and immediately after external substrate depletion, the endogenous phase is restored. The obtained YH values were well in the range of the ones proposed by the International Water Association [19] that vary between 0.38 and 0.75 mgCODcell mgCOD−1 ox , for a CAS process. Indeed, for the pilot plant under study, the YH values ranged from −1 0.57 mgCODcell mgCOD−1 ox (corresponding to 0.36 mgVSS mgCODox ) −1 up to 0.74 mgCODcell mgCODox (corresponding to 0.46 mgVSS mgCOD−1 ox ). The obtained values were in good agreement with previous YH values evaluated in respirometric tests on other MBR process configurations [20,21]. However, it is worthy to observe that the obtained values of YH differ significantly from the average biomass yield calculated in the present study on the basis of mass balances between sludge withdrawn and produced, dividing by the cumulative COD removed (Yobs), and equal to 0.07 mgVSS mgCOD−1 ox . The explanation of this discrepancy is likely related to the fact that the yield coefficient, calculated on the basis of mass balances, naturally takes into account the biomass decay (differently from a respirometric batch test, which length is limited in time), that in a MBR process is not negligible, similar to the maximum growth rate [17], leading to a
18
14
12
OUR [mgO2/(Lh)]
OUR exogenous OUR Exogenous OUR endogenous OUR
10 8 6 4
OUR endogenous
12 10 8 6 4 2
2
0 0
5
10 15 Time [h]
20
25
0 9/2/10
Fig. 4. OUR exogenous, endogenous and SRT values versus time.
OURexogenous OUR exogenous
14
OUR [mgO2/(Lh)]
20
0 12/10/09 1/11/09 21/11/09 11/12/09 31/12/09 20/1/10 Time [Day]
16
16
0
4
0
10
20
30
40
Time [h]
Fig. 3. Typical OUR profiles at the beginning (a) and at the end (b) of the experimental campaign, respectively.
50
194
D. Di Trapani et al. / Desalination 269 (2011) 190–197
3
tests. In Fig. 6, as an example, two Monod-type kinetic profiles for heterotrophic bacteria is reported, referring to the beginning (a) and to the end (b) of the experimental campaign, respectively.
µH,max [day-1]
2.5
2
1.5 1
0.5 0 14/10/09
3/11/09
23/11/09 13/12/09 2/1/10 Time [Day]
22/1/10
11/2/10
Fig. 5. Heterotrophic maximum growth rate versus time.
net growth close to zero, especially in MBRs operated with complete sludge retention. Further, a possible explanation of the above mentioned discrepancy could be found in the different biomass behaviour with real wastewater or synthetic substrate like sodium acetate. To support such statement, some parallel tests with both real wastewater and sodium acetate have been carried out. The experimental results provided values of the yield coefficient of 0.37 and 0.15 mgVSS mgCOD−1 ox for acetate and wastewater, respectively, therefore supporting the hypothesis outlined above about the different behaviour of the biomass in presence of a rapidly biodegradable substrate like sodium acetate. 3.2.2. Maximum growth rate μ H,max and half-saturation coefficient KS Referring to the maximum growth rate μ H,max, the obtained values reflected the above mentioned OUR exogenous trend, reaching at the end of the experimental phase values within the range reported for CAS systems [10]. In Fig. 5, the maximum growth rate values evaluated during the respirometric batch tests are shown. Previous studies on MBRs with different plant configurations showed higher values for μH,max [21], suggesting the importance of pilot plant configuration and operational conditions, with sludge complete retention or regular sludge withdrawal in particular, on biomass viability in a MBR process. Concerning the half-saturation coefficient KS, the obtained values were lower than the ones obtained in previous studies carried out on MBR systems [21]. A possible reason should be found in the use of a readily biodegradable substrate like sodium acetate during the batch
Several tests on autotrophic biomass were carried out on samples collected from the aerobic tank of the UCT-MBR pilot plant in order to evaluate the biokinetic coefficients. Fig. 8 shows as an example an OUR profile and a Michaelis–Menten modelization for nitrifying bacteria. The obtained results are well in the range proposed for nitrification in CAS systems [19], thus suggesting a good development for nitrification activity, despite the presence of the internal recycle from the aerobic to the anoxic tank that should reduce the development of the autotrophic biomass. Indeed the maximum OUR values for nitrifying biomass were similar to the heterotrophic bacteria ones, confirming a good nitrifying activity, comparable to the organic matter removal rate in the aerobic tank. Operating with high SRTs is essential to obtain complete nitrification, especially when treating wastewater containing a certain amount of landfill leachate and industrial discharges, like in the present study. In Table 3 the average values obtained for the kinetic coefficients referring to nitrifying bacteria are reported.
b
45 40
dS/dt [mgCOD/(Lh)]
3.3. Nitrifying biomass characterization
45 40
Experimental
35
Model
dS/dt [mgCOD/Lh]
a
3.2.3. Decay rate bH As previously mentioned, the decay rate of heterotrophic bacteria was evaluated through the “single batch test” procedure [18], by taking the samples in starvation condition for 2–3 days, until endogenous conditions were reached. By plotting the logarithmic OUR curve vs time, the decay rate has been estimated as the slope of the straight interpolation line (Fig. 7). The obtained values during the respirometric campaign were higher compared to previous studies on similar plants [21,20] as well as on CAS processes [10]. However, it has to be stressed that the decay rate showed a decreasing trend during the experimental period, from 0.56 to 0.30 d−1, thus indicating an increasing biomass viability starting from an initial situation characterised by complete sludge retention, to a steady state condition characterised by regular daily sludge withdrawals. Nevertheless, the obtained value for the decay rate was higher compared to the typical one assumed for activated sludge systems [10], even at the end of the experimental period. This observed result is in good agreement with previous studies; indeed, in a submerged MBR system, the heterotrophic decay rate can be an order of magnitude higher than the one typically observed for CAS processes [22–24].
30 25 20 15 10
35 30 25 20 Experimental Experimental Model Model
15 10 5
5
0
0 0
20
40
60 80 S [mgCOD/L]
100
120
140
0
10
20
30 40 S [mgCOD/L]
50
Fig. 6. Examples of Michaelis–Menten modelization profiles at the beginning (a) and the end (b) of the respirometric experimental campaign.
60
D. Di Trapani et al. / Desalination 269 (2011) 190–197
a 2.0
0
10
20
30
Time [h] 40
50
60
70
b
80
10
20
30
Time [h] 40 50
60
70
80
2.0 1.8
1.8 1.6
Experimental
1.4
Model
1.6
lnOUR [mg/Lh]
lnOUR [mg/Lh]
0
195
y = -0.0123x + 0.8579 2 R = 0.9526
1.2 1.0 0.8 0.6
y = -0.0165x + 1.5513 R2 = 0.9818
1.4 1.2 1.0 0.8 Experimental Experimental Model Model
0.6
0.4
0.4
0.2
0.2
0.0
0.0 Fig. 7. Examples of logarithmic OUR profiles at the beginning (a) and at the end (b) of the respirometric experimental campaign.
a
b
20 18
OUR [mgO2/(Lh)]
dN/dt [mgNH4-N/(Lh)]
OUR exogenous OUR endogenous
16 14 12 10 8 6 4 2 0
3.5 3 2.5 2 1.5
Experimental Model
1 0.5 0
0
10
20
30
40
50
0
4
8
Time [h]
12 N-NH4 [mg/L]
16
20
24
Fig. 8. Example of OUR profile (a) and Michaelis–Menten modelization profile (b) for nitrifying bacteria.
3.3.1. Yield coefficient YA As previously mentioned, the yield coefficient for the autotrophic bacteria YA was evaluated by using the expression proposed by Chandran and Smets [14]. The obtained values were in the range reported in the technical literature [19], with an average value of 0.18 mgCODcell mgNH4–N−1 ox , only a little lower than the ones reported by Lubello et al. [21] on a MBR pilot plant. 3.3.2. Maximum autotrophic growth rate μA,max and half-saturation coefficient KNH Also in this case the obtained kinetic coefficients were well within the usual range proposed for CAS systems [19]. Referring in particular to the maximum growth rate, the average value reported in Table 3 is equal to 0.27 d−1, suggesting a good development for nitrifying biomass in such process. However, the above mentioned value is lower than the typical one found for CAS systems [8], suggesting that a different selective pressure on nitrifying bacteria can arise in such systems. Previous studies on similar pilot plant layout showed the same behaviour [29]. On the other hand, the half-saturation coefficient KNH was found to be equal to 0.57 mgNH4–N/l (average value), lower than the typical value reported in the technical literature for nitrification in CAS systems [10]. 3.3.3. Decay rate bA The decay rate for autotrophic biomass, as previously mentioned, has been evaluated by using a procedure derived from Avcioglu et al. [15], originally proposed to evaluate the decay rate for heterotrophic bacteria. The obtained values were within the range proposed in the literature [10], confirming a good development of the autotrophic nitrifying biomass. An example of OUR profile for the estimation of endogenous decay coefficient bA is reported in Fig. 9.
3.4. COD fractionation The COD fractionation was periodically repeated during the experimental investigations, in order to characterize the influent wastewater fed to the pilot plant. As aforementioned, the biomass samples were brought to endogenous condition before starting the batch test, and the nitrifying bacteria were inhibited by adding allylthiourea. In Table 4 the average results are shown. SS, XS, SI, XI and XH are the soluble biodegradable, slowly biodegradable and rapidly hydrolysable, soluble inert, particulate inert and active biomass COD fractions respectively. The average values are in good agreement with previous studies on raw wastewater [19,8], except the percentage values referring to the inert matter, soluble and particulate, which was slightly higher, while the readily and slowly biodegradable matter fractions were lower; the reason of this should be addressed to the landfill leachate, contained in a certain amount in the wastewater fed to the pilot plant. These results are in good agreement with previous respirometric experiments carried out on the same wastewater [30–32]. 4. Conclusions An experimental gathering campaign on a UCT-MBR pilot plant, conceived for biological nutrient removal, was performed. One of the main aims of the study was the evaluation of the kinetic parameters of both heterotrophic and autotrophic biomass, as well as the COD fractionation, with the aid of respirometric batch tests. The experimental observations highlighted a decrease in the kinetic parameters for heterotrophic bacteria compared to that of a CAS system, even if they increased when the pilot was operated with daily sludge withdrawal, showing a higher heterotrophic biomass activity with lower SRT values. On the other hand, the investigations carried out on
196
D. Di Trapani et al. / Desalination 269 (2011) 190–197
Table 3 Average values of kinetic parameters for nitrifying bacteria in different processes. Parameter
Units
Value
Reference
Process
Wastewater tested
YA YA YA YA μA.max μA.max μA.max μA.max μA.max KNH KNH KNH KNH νA,max bA bA bA Max. nitrif. rate
gVSS gNH4-N−1 gVSS gNH4-N−1 gVSS gNH4-N−1 gVSS gNH4-N−1 d−1 d−1 d−1 d−1 d−1 mgNH4-N/l mgNH4-N/l mgNH4-N/l mgNH4-N/l d−1 d−1 d−1 d−1 mgNH4-N/(lh)
0.18 0.24 0.10–0.15 0.34 0.27 0.1–0.2 2.02 0.38 0.2–0.9 0.57 0.85 0.5–1.0 0.1–0.15 1.5 0.074 0.08 0.05–0.15 2.30
This [21] [10] [25] This [26] [27] [21] [10] This [27] [10] [28] This This [21] [10] This
UCT-MBR MBR with pre-denitrification – ASP with denitrification UCT-MBR MBR MBR with low DO MBR with pre-denitrification – UCT-MBR MBR with low DO – MBR and CAS in parallel UCT-MBR UCT-MBR MBR with pre-denitrification – UCT-MBR
Municipal wastewater Tannery and domestic wastewater (20 °C) 0.123 typical Municipal wastewater Municipal wastewater Municipal wastewater (20 days SRT) 30 °C – Sludge digester supernatant (SRT N 650 d) Tannery and domestic wastewater (20 °C) 0.75 typical Municipal wastewater 30 °C – Sludge digester supernatant (SRT N 650 d) 0.74 typical Domestic wastewater Municipal wastewater Municipal wastewater Tannery and domestic wastewater (20 °C) 0.08 typical Municipal wastewater
experimentation
experimentation
experimentation
experimentation experimentation
experimentation
25
OUR [mgO2/(Lh)]
20 15 10 5 0 0
1
2
3 4 Time [Day]
5
6
7
Fig. 9. Example of OUR profile of a multiple batch test for autotrophic endogenous decay rate estimation.
Table 4 Average COD fraction values. SS
XS
SI + XI
XH
%
%
%
%
10.67
23.40
57.94
7.99
nitrifying bacteria showed that the obtained parameters were within the range proposed for nitrification in CAS systems, thus suggesting a good development for nitrification activity, and that operating with high SRTs is essential to obtain an almost complete nitrification, especially when treating wastewater containing a certain amount of landfill leachate. The suggestion is that operating conditions, and SRT values in particular, strictly influence the biokinetic behaviour of a MBR plant, moreover the increase of the activity for nitrifying bacteria happens in detriment of that for heterotrophic bacteria that probably is limited by the pilot-plant layout. Further, the obtained results confirmed respirometry as a suitable tool for wastewater and biomass characterisation, and that the obtained kinetic parameters should provide a useful support in MBR design and management, as well as in MBR simulations.
References [1] T. Stephenson, S. Judd, B. Jefferson, K. Brindle, Membrane Bioreactors for Wastewater Treatment, IWA Publishing, London, 2000.
[2] E.B. Muller, A.H. Stouthamer, H.W. Vanverseveld, D.H. Eikelboom, Aerobic domestic waste water treatment in a pilot plant with complete sludge retention by cross-flow filtration, Water Res. 29 (1995) 1179–1189. [3] M. Ramphao, M.C. Wentzel, R. Merritt, G.A. Ekama, T. Young, C.A. Buckley, Impact of membrane solid–liquid separation on design of biological nutrient removal activated sludge systems, Biotechnol. Bioeng. 89 (6) (2005) 630–646. [4] Durante, F. (2006). Analisi del funzionamento degli impianti MBR gestiti in condizioni di ritenzione completa di biomassa. Ph.D. Thesis. Università degli Studi di Napoli, Italy (in italian). [5] Parco, V. (2006). Bioreattori a membrana per la rimozione biologica dei nutrienti: cinetiche di processo ed efficienze. Ph.D. Thesis. Università degli Studi di Napoli, Italy (in italian). [6] J. Kappeler, W. Gujer, Estimation of heterotrophic parameters under aerobic conditions and characterization of wastewater for activated sludge modelling, Water Sci. Technol. 25 (1992) 125–139. [7] G. Mannina, G. Di Bella, G. Viviani, Uncertainty assessment of a membrane bioreactor model using the GLUE methodology, Biochem. Eng. J. 52 (2) (2010) 263–275. [8] G. Di Bella, G. Mannina, G. Viviani, An integrated model for physical-biological wastewater organic removal in a submerged membrane bioreactor: model development and parameter estimation, J. Membr. Sci. 322 (1) (2008) 1–12. [9] H. Spanjers, P.A. Vanrolleghem, G. Olsson, P. Dold, Respirometry in control of the activated sludge process: principles, Scientific and Technical Report No. 7, IWA Publishing, ISBN: 1 900222 04 3, 1998. [10] Metcalf and Eddy, Inc., Wastewater engineering treatment and reuse, McGrawHill, New York, 2003. [11] H. Spanjers, P.A. Vanrolleghem, G. Olsson, P. Dold, Respirometry in control of the activated sludge process, Water Sci. Technol. 34 (3–4) (1996) 117–126. [12] Guglielmi, G. (2002). Applicazione dei bioreattori a membrana al trattamento di reflui civili. Ph.D. Thesis, Università degli Studi di Trento, Italy (in italian). [13] Vanrolleghem, P.A., Van Impe, J.F., Vandewalle, J. And Verstraete, W. (1992). Advanced monitoring and control of the activated sludge process: on-line estimation of crucial biological variables in a structured model with RODTOX biosensor. Modeling and control of biotechnical processes. Karim M. N., Stephanopoulos, G., 355-358, Pergamon Press, Oxford. [14] K. Chandran, B.F. Smets, Estimating biomass yield coefficients for autotrophic ammonia and nitrite oxidation from batch respirograms, Water Res. 35 (13) (2001) 3153–3156. [15] E. Avcioglu, D. Orhon, S. Sözen, A new method for the assessment of heterotrophic endogenous respiration rate under aerobic and anoxic conditions, Water Sci. Technol. 38 (8–9) (1998) 95–103. [16] G. Laera, A. Pollice, D. Saturno, C. Giordano, A. Lopez, Zero net growth in a membrane bioreactor with complete sludge retention, Water Res. 39 (2005) 5241–5249. [17] H. Zhang, X. Wang, J. Xiao, F. Yang, J. Zhang, Enhanced biological nutrient removal using MUCT–MBR system, Bioresour. Technol. 100 (2009) 1048–1054. [18] P.A. Vanrolleghem, H. Spanjers, B. Petersen, P. Ginestet, I. Takacs, Estimating (combination of) Activated Sludge Model No.1 parameters and component by respirometry, Water Sci. Technol. 39 (1) (1999) 195–214. [19] M. Henze, C.P.L. Grady Jr., W. Gujer, G.V.R. Marais, T. MAtsuo, Activated sludge model No.1, IAWPRC Scientific and Technical Report No.1, IAWPRC, London, 1987. [20] G. Munz, R. Gori, L. Cammilli, C. Lubello, Characterization of tannery wastewater and biomass in a membrane bioreactor using respirometric analysis, Bioresour. Technol. 99 (2008) 8612–8618. [21] C. Lubello, S. Caffaz, R. Gori, G. Munz, A modified Activated Sludge Model to estimate solids production at low and high solids retention time, Water Res. 43 (18) (2009) 4539–4548. [22] S.G. Lu, T. Imai, M. Ukita, M. Sekine, T. Higuchi, M. Fukagawa, A model for membrane bioreactor process based on the concept of formation and degradation of soluble microbial products, Water Res. 35 (8) (2001) 2038–2048.
D. Di Trapani et al. / Desalination 269 (2011) 190–197 [23] C.S. Laspidou, B.E. Rittmann, A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass, Water Res. 36 (2002) 2711–2720. [24] N. Jang, X. Ren, J. Cho, I.S. Kim, Steady-state modelling of bio-fouling potentials with respect to the biological kinetics in the submerged membrane bioreactor (SMBR), J. Membr. Sci. 284 (2006) 352–360. [25] P. Harremoës, O. Sinkjaer, Kinetic interpretation of nitrogen removal in pilot scale experiments, Water Res. 29 (3) (1995) 899–905. [26] X.J. Fan, V. Urbain, Y. Qian, J. Manem, Nitrification and mass balance with a membrane bioreactor for municipal wastewater treatment, Water Sci. Technol. 34 (1–2) (1996) 129–136. [27] S. Wyffels, S. Van Hulle, P. Boeckx, E. Volcke, O. Van Cleemput, P. And Vanrolleghem, W. Verstraete, Modelling and simulation of oxygen-limited partial nitritation in a membrane assisted bioreactor (MBR), Biotechnol. Bioeng. 86 (5) (2003) 531–542.
197
[28] R. Manser, W. Gujer, S. Hansruedi, Consequences of mass transfer effects on the kinetics of nitrifiers, Water Res. 39 (2005) 4633–4642. [29] V. Parco, M. Wentzel, G. Ekama, Kinetics of nitrogen removal in a MBR nutrient removal activated sludge system, Desalination 199 (2006) 89–91. [30] G. Mannina, G. Viviani, Hybrid moving bed biofilm reactors: an effective solution for upgrading a large wastewater treatment plant, Water Sci. Technol. 60 (5) (2009) 1103–1116. [31] D. Di Trapani, G. Mannina, M. Torregrossa, G. Viviani, Quantification of kinetic parameters for heterotrophic bacteria via respirometry in a hybrid reactor, Water Sci. Technol. 61 (7) (2010) 1757–1766. [32] D. Di Trapani, G. Mannina, M. Torregrossa, G. Viviani, Comparison between hybrid moving bed biofilm reactor and activated sludge system: a pilot plant experiment, Water Sci. Technol. 61 (4) (2010) 891–902.