6688
Ind. Eng. Chem. Res. 2007, 46, 6688-6695
Operation and Maintenance of Full-Scale Municipal Membrane Biological Reactors: A Detailed Overview on a Case Study Francesco Fatone,*,† Paolo Battistoni,‡ Paolo Pavan,§ and Franco Cecchi† Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15, Ca` Vignal 37134 Verona, Italy; Institute of Hydraulics and Transportation Infrastructures, Marche Polytechnic UniVersity, Via Brecce Bianche, 60100 Ancona, Italy; and Department of EnVironmental Science, UniVersity of Venice “Ca` Foscari”, Calle Larga Santa Marta, Venice, Italy
This paper deals with the long-term operation of a full-scale municipal membrane bioreactor, focusing mainly on the membrane section. The ultrafiltration chamber, equipped with 12 130 m2 of submerged hollow-fiber membranes, was installed after an existing alternating oxic/anoxic bioreactor. Attention is paid to a number of practical details, from the effectiveness of different sieves to the impact of the operating parameters on the membrane performances; finally, data of the energy consumptions and items of the operating costs are presented. Starting from the pretreatments, while the wedge wire sieve (openings ) 1 mm) involved the visible accumulation of fibers and trash in the filtration chamber, the punched holes (holes diameter ) 1.5 mm) involved less than 100 milligrams of dry trash per liter of mixed liquor, demonstrating that they suited the following membrane system. Moving on to the off-line equalization basin, a volume of one-fifth of the bioreactor was sufficient to cope with the normal fluctuations of the municipal inloadings. As for the membrane section, a net flux of 26 LMH and chemical maintenance cleaning in place once a week with only hypochlorite (around 300 mgCl L-1) was a sustainable practice to keep the long-term permeability (adjusted at 20 °C) stable in the range 220-240 LMH bar-1. Under this operating protocol, increases of specific aeration for membrane scouring from 0.12 to 0.19 Nm3 m-2 h-1 did not lead to permeability gains that might justify the increased power requirements. This evidence allows us to conclude that 0.12 Nm3 m-2 h-1 was sustainable for the normal operation of the hollow-fiber membranes. Irreversible membrane fouling (permeability decrease up to 150-160 LMH bar-1) was observed because of irregular discharges of municipal landfill leachate, which, from one side, caused a drastic deflocculation of the activated sludge (sludge volume index (SVI) increased from 110 up to 250 mL gMLSS) and, from the other side, involved a probable incoming of recalcitrant compounds that might have acted as foulants. The power requirements of the whole treatment facility were always lower than 0.6 kWh m-3 thanks to the general good utilization of the air supplied. The specific operating costs related to the energy consumptions and chemicals purchase were in the range 0.06-0.08 euro m-3. Introduction Membrane biological reactors (MBRs) are going toward widespread application, including in the field of municipal wastewater treatment.1-3 Although at the moment the majority of the municipal MBRs do not exceed 10 000 m3 d-1,4 large MBRs are becoming sustainable thanks to the rapidly declining cost of the membranes.2 In fact, to date, the world’s largest MBR (Nordkanal, Germany)5 can treat up to 1 880 m3 h-1, and larger ones are under construction or have been commissioned.2,6 In spite of the growing number of both full-scale MBRs for municipal wastewater treatment and scientific papers published on lab and pilot experimentations, the most appropriate practice to design and operate these systems is not consolidated.2 Hence, further detailed knowledge about the operation of full-scale MBRs is a subject of wide interest. As for the Italian scenario, the two largest municipal MBRs are located in Brescia (1 580 m3 h-1)7 and in Viareggio (250 m3 h-1).8 Both come from the upgrading of existing plants and adopt submerged ZeeWeed (GE-Zenon) hollow-fiber mem* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +39 071 2204911. Fax: +39 071 2204525. † University of Verona. ‡ Marche Polytechnic University. § University of Venice.
branes. The first case is widely known because is has been the largest for a few years; the second is more recent and less known, but its novelty is remarkable, mainly because its biological process is alternating, continuously fed, and automatically controlled on the basis of on-line signals. This promising coupling of process control automation and membrane separation has been described in a recent paper,8 which focused on the specific loadings successfully treated by the plant and on the overall performances of the biological process. The authors finally pointed out the good quality standard of the effluent and the benefits achieved in terms of energy savings. This paper pays further attention to the Viareggio MBR but focuses mainly on the membrane section. A detailed analysis of the parameters routinely monitored has been carried out over one whole year of operation. This allowed us to find out the long-term effects of parameters that (a) can be directly manipulated by the operators; (b) depend on the characteristics of the incoming sewage; and (c) can vary in consequence of the points mentioned above. Finally, the operation and maintenance (O&M) costs coming from energy consumptions and chemicals purchase are presented. Materials and Methods Alternating Anoxic/Oxic Process. The alternate cycles is an automatically controlled process, where the intermittent
10.1021/ie0616848 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6689
Figure 1. Simplified block flow diagram of the plant. Table 1. Main Features of the UF Membranes modules type nominal pore membrane area/line total membrane area reaction volume of the UF tank filtration cycle routine cleaning cycles a
ZeeWeed 500d hollow fibers 0.04 µm 3032 m2 12 128 m2 266 m3 suction (600 s)-relaxation (60 s) 1 mCIP/weeka
mCIP ) maintenance cleaning in place.
aeration of the continuously fed bioreactor is managed on the basis of on-line signals of dissolved oxygen (DO) and oxidation reduction potential (ORP), which are processed in real time by the control device. In particular, these signals, coming from consolidated and inexpensive probes, are able to indirectly detect the exhaustion of the ammonia, during the aerobic phases, and of the nitrates, during the anoxic phases.9-11 Therefore, the control device can establish the optimal alternation of the aerobic and anoxic phases with relation to the actual influent loadings. The Plant. The Viareggio municipal wastewater treatment plant (WWTP) was originally built more than 30 years ago and adopted primary sedimentation, Carousel tanks, and, before the disinfection contact tank, conventional secondary clarifiers for the final solid/liquid separation. Year by year, the system underwent hydraulic and mass overloading and became due for retrofitting. At first, the existing primary longitudinal clarifier was retrofitted, adopting the alternate cycles process (reaction volume ) 2200 m3). Later, the water shortage for the local floricultures suggested the plant undertake wastewater reuse as the most sustainable remedy. So, an ultrafiltration chamber was coupled to the alternating anoxic/oxic bioreactor in order to obtain the alternate cycles-membrane bioreactor (AC-MBR). In addition to the filtration chamber, adequate pretreatments (sieving and degritting) and an off-line equalization basin were built. Figure 1 shows the simplified block flow diagram of the plant. As for the sieve, first an inappropriate wedge wire geometry (openings ) 1 mm) was installed and operated for about half a year. Later, owing to the direct observation of accumulation of fibrous material in the filtration chamber, the former sieve was replaced by a punched hole one (openings ) 1.5 mm), which was demonstrated to suit the application before a membrane bioreactor. After the pretreatments, the off-line equalization volume is about one-fifth of the following bioreactor (450 m3). Although this was possible because a parallel
conventional treatment line could cope with the wet weather flows, the equalization volume is anyway rather small if compared to multizone MBRs that need larger buffer tanks.12 The membrane tank is equipped with eight Zenon cassettes (see Table 1 for the main features of the membranes and Table 2 for the gross fluxes used for the design) disposed in four parallel and separately fed trains. Basically the ultrafiltration (UF) section was engineered according to the usual practice for Zenon hollow-fiber membranes, with coarse aeration at the bottom of the modules for the membrane scouring. To avoid possible accumulation of biological foam and floating materials, the UF chamber was provided with a continuous weir in the side opposite to the feeding (see Figure 2). Monitoring Protocol. The routine protocol to monitor the treatment system did not change significantly after the installation of the MBRs, so it involved similar analyses workload. It included lab analysis to determine the main chemicalphysical parameters (carbon, nitrogen, phosphorus, suspended solids (total and volatile), and sludge volume index (SVI)) and on-line measurements, processed either in real time or later by the operators. Also, the power requirements were on-line monitored and logged by a gauge of the electrical input located in the main switchboard. Sieving Test. The sieving tests were performed with the twofold aim to find out (a) the removal efficiency of the fine sieve and (b) the trash content in the activated sludge, including also a rough estimation of the size of the particles. Hence, two types of tests were carried out: (1) “in-out”, on grab samples from the wastewater influent and effluent to the sieve; and (2) “trash-test”, on samples of activated sludge from the membrane bioreactor. Samples of 180-200 L were passed through a lab sieve having a mesh aperture of 2 000 µm (ASTM set, mesh no. 10), which was previously dried and weighed. This choice was related to the aim of evaluating mainly the fibrous materials that could be removed by the industrial sieve, avoiding the invalidation of the test caused by the packing of the smaller mesh. After the sample sieving, the sieves were dried again for 24 h at 105 °C, then weighed in order to find out the content of dry trash, both in the raw wastewater and in the activated sludge. Maintenance Cleaning in Place, in Air, and Recovery Cleaning (mCIP, mCIA, and RC). According to the routine cleaning protocol, the membranes were cleaned line by line so to avoid the interruption of the filtration. The mCIP were done usually once a week and consisted of a sequence of backwashing
Table 2. Gross Fluxes Used to Design the Membrane Section low loadings operating condition 4 lines operating (routine) 3 lines operating and 1 under cleaning
average flux LMH 17.7 23.6
high loadings peak flux LMH 23.1 30.8
average flux LMH 20.6 27.5
peak flux LMH 26.4 35.2
6690
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007
Figure 2. 3D drawing of the ultrafiltration chamber.
Figure 3. Example of TMP data screening.
with a back flux of 30 LMH and spaced out by prolonged phases of relaxation. Hypochlorite was dosed in backwashed permeate so to reach a concentration of ∼300 mgCl L-1. Each mCIP incurs around 40-50 min. The mCIA is not a routine procedure and includes the following: (1) the preliminary emptying of the tank and (2) the sequence of backwashing/relaxation, after which the hypochlorite solution (300-1000 mgCl L-1) is left percolating on the fibers. Finally, the recovery cleaning included an overnight soak of hypochlorite solution, as provided by the usual protocol for municipal MBRs equipped with ZeeWeed membranes (see also ref 2). TMP and Flux Data Processing. Since the filtration cycle was 600 s of filtration and 60 s of relaxation, the TMP and the flux data showed low and high random peaks that could affect the calculation of the actual membrane permeability. In order to standardize the data processing, a previous data screening was carried out by selecting the periods when the set net flux was kept for more than 180 s (see Figure 3 as example of screening for the TMP).
Results and Discussion Factors Influencing the Membrane Performances. According to recent reviews concerning fouling in MBRs,2,13-15 three aspects have a major influence on the filtration performances: (1) suitability of the pretreatments; (2) nature of the feed to the membranes and operating parameters for the activated sludge process; and (3) the hydrodynamic environment imposed in the filtration chamber. Further, each of the aforementioned processes depends on boundary conditions and parameters that can be manipulated. (a) Pretreatments and Trash in the Activated Sludge. When dealing with full-scale municipal membrane bioreactors, suitable pretreatments are essential in determining membrane performances.16 In this study, the performances of the installed sieve were evaluated by lab sieve tests, for which results are reported in Table 3. The retentate by the lab sieve was taken as an index of the trash into the activated sludge. The experimental value (Table 3, column 5) was in acceptable agreement with the theoretic calculation (Table 3, column 6) and can be considered not
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6691 Table 3. Results of the Sieving Test: “In-Out” and “Trash Test” lab sieve aperture (mm)
test (no.)
dry trash influent (dry mg/L) (average ( st.dev.)
dry trash effluent (dry mg/L) (average ( st.dev.)
accumulated into the sludgea (dry mgtrash/L) (average ( st.dev.)
accumulated into the sludgeb (dry mgtrash/L) (average ( st.dev.)
2
10
20 ( 15
3(2
75 ( 20
98 ( 65
a
b
Measured by sieving test. Calculated according to ref 17 considering SRT 15 days and HRT 11 h.
Table 4. Characteristics of the Feed to the AC-MBR
COD (mg L-1) NH4-N (mg L-1) TN (mg L-1) TP (mg L-1) TSS (mg L-1) COD/TN a
municipal WW-high loading average over 60 samples ((st.dev./average)
municipal WW-low loading average over 19 samples ((st.dev./average)
unsteady nature of the feeda average over 9 samples ((st.dev./average)
657 ((16%) 39 ((9%) 64 ((11%) 7.2 ((10%) 264 ((29%) 12.4 ((23%)
604 ((16%) 34 ((20%) 47 ((9%) 5 ((21%) 255 ((19%) 13.1 ((20%)
417 ((31%) 41 ((42%) 54 ((40%) 4.7 ((30%) 193 ((56%) 8.3 ((27%)
Irregular incomings of landfill leachate that involved random peaks of inloading.
Table 5. Main Operating Parameters of Importance for the Membrane Fouling parameter liquor temperature MLSSACreactor MLSSUFsection solids retention time total recycles of activated sludge (QRAS/Qin)
°C g L-1 g L-1 d
low loading
high loading
14-24 ∼8 10-11 14-21 ∼2
26-27 ∼6 7-8 13-15 ∼2
significant with respect to the operating conditions (mixed-liquor suspended solids (MLSS) concentration in the range 6-8 g L-1). As a matter of fact, in this case, the punched-hole sieve had effectiveness comparable to double-stage sieving (i.e., sieve 3 mm-degritting-sieve 0.5 mm) recommended by some membrane producers. This may confirm the actual impossibility to generalize a set of most suitable pretreatments for municipal MBRs. (b) Activated Sludge Process and Membrane Filtration: Feedwater Characteristics. The nature of the feed and the organic loading rate can influence the amount and/or the bioproduction of foulants. In this case study, the wastewater was almost domestic, with pretty high seasonal fluctuations of nutrients, in consequence of the summer tourism (Table 4, columns 1 and 2). The flow hourly peaking factors were 1.3 and 0.8 for the maximum and minimum hour value, respectively. Furthermore, a number of occasional discharges of landfill leachate into the municipal sewers over a period of 20-30 days were allowed by the local environmental authority (Table 4, column 3). As a consequence, the MBR had to face severe irregularities of the influent loadings (see standard deviations of Table 4 and compare columns 1 and 2 vs column 3). (c) Operating Parameters. Sludge filterability has been related to operating parameters that are easily determinable such as food-to-microorganisms (F/M) ratio, solids retention time (SRT), sludge volume index (SVI), mixed-liquor suspended solids (MLSS).15,17 In this case study, the operating choices (Table 5) met the present trend of the full-scale MBRs to operate MLSS under 10 g L-1 18 and SRT was as low as necessary to match the nitrification potential of the plant with the influent nitrifiable loadings. (d) Hydrodynamic Environment. Basically, the hydrodynamic environment imposed to the membranes in a submerged system can be altered by manipulating four parameters: (1) the operating flux; (2) the filtration cycle, (3) the scouring aeration; and (4) the flowrate of recycled activated sludge (RAS).
Table 6. Specific Air Demand Per Unit of Membrane Area (SADm) and Permeate (SADp) operating modality
aeration time (h/h)
SADm (Nm3 m-2 h-1)
SADp (Nm3 m-3)
A B C D
0.5 0.5 0.5 0.5
0.06 0.12 0.19 0.26
2.3-2.7 4.5-5.5 5.0-6.1 10.1-12.2
The start-up phase lasted about 50 days and began with a net flux of 21.4 LMH, which was increased up to the regime value of 26.0 LMH by 3 day long steps of 2-1.3-1.3 LMH each. The operating net flux was close to the critical flux, which is reported to be in the range 17-30 LMH at 20 °C for ZeeWeed membranes.19 As far as the aeration of the system, a couple of two-velocity blowers (reserve excluded) were installed for the membrane scouring, while the biological process relied on two further and automatically controlled blowers. The membrane system was engineered with the flexibility to work with four possible coarse bubble air flowrates (see Table 6 for the specific air demand for unit of membrane area, SADm, and for unit of permeate, SADp). Considering the numerous data from pilot MBRs reported by Judd,2 the values of Table 6 are (a) lower than those for the same commercial MBR technology (SADm ) 0.25-0.54; SADp ) 15-30), also according to the recent developments of the Zenon engineering; and (b) much lower than those of the flatsheet membranes (SADm ) 0.6-1.5; SADp ) 24-90). With further concern to hydrodynamic environment and energy savings, attention must be paid to the RAS flowrate. In fact, while MBRs are usually associated to a high number of RAS, both from the filtration chamber and between the aerobic/anoxic/ anaerobic zones, the AC process operates a low number of recycles (total recycle ratio in the range 1-2). In this case study, the total recycle ratio was ∼2 and the long-term permeability was comparable with the literature data for ZeeWeed membranes.2 This allows one to conclude that, as far as the MLSS in the filtration chamber is under the critical values,2,19 a hydrodynamic environment related to low recycle systems is not problematic for the membranes. History of Membrane Permeability. According to the parameters of Table 7, the whole operation can be divided into four pseudo-steady-state periods and two start-up phases. Furthermore, besides the recovery cleaning (RC) carried out after 9 months of operation, also two singular events (“A” and “B”) occurred and are reported.
6692
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007
Table 7. One Year Membrane History
first start-up run 1c run 2d singular eVent A1 run 3c maintenance second start-upc singular eVent B2 run 4c a
days
net flux(LMH)
51 20 50 18 99 45 10 20-30 37
21.4-23.4-24.7-26 26 26 26 26 26
Adjusted according to Mulder.23 permeability.
b
permeability @ 20 °Ca (LMH/bar)
SADm (m3/(m2 h))
F/Mb (gCOD/(gVSS d))
0.12 214 ( 12 0.12 0.18 ( 0.04 224 ( 27 0.12 0.32 ( 0.09 no chemical in the backwashed permeate during supposed mCIPe 243 ( 32 0.19 0.25 ( 0.04 extraordinary maintenance of the AC tank and RC of the membranes 0.19 irregular inloading due to the a number of incoming landfill leachates 155 ( 17 0.19 0.25 ( 0.05
MLSSU F (g/L) 11.0 ( 0.4 8.4 ( 1.0 7.6 ( 0.7
7.9 ( 0.5
To the AC tank. c Low loading. d High loading. e After the event, a mCIA event was performed to recover the
Table 7 leads to the following comments: (1) Given a cleaning protocol of 1 mCIP per week, the increase of SADm was not cost-effective; in fact, this operating choice did not give gains of permeability that can justify the increased power requirements. (2) MLSS concentration has a minor effect on membrane permeability; in fact, a MLSS decrease from 11 to 8 involved a gain of only 7 LMH bar-1. (3) The long-term variation of F/M ratio, which has been proposed as a fundamental parameter influencing fouling propensity,2,17 did not show significant effects on membrane permeability within the range observed. Membrane Permeability Decline and Singular Events. The effects of the singular events over the first year of operation are evaluated in terms of fouling rates. Given the same operating net flux (26 LMH), the TMPs plotted in Figure 4 show the behavior of the system under (1) routine operation (Table 7, run 2, which is also a sample representative for all the normal operation), (2) no maintenance cleaning (Table 7, singular event A), and (3) irregular incomings of municipal landfill leachates mixed to the municipal wastewater (Table 7, singular event B). Figure 4 shows that coupling a net flux of 26 LMH and 1 mCIP/week was demonstrated to be a sustainable O&M practice, because the TMP is almost constant. As for the singular events, case A caused a stable TMP increase around 0.30 kPa day-1. However, good permeability was reestablished (run 3) in about 20 days, using only hypochlorite solution first in a membrane cleaning in air (mCIA) and later in the routine cleaning protocol. More relevant fouling was observed when random inloading peaks were caused by irregular incomings of municipal landfill leachate. Here, the steady-state TMP increase was ∼0.74 kPa day-1. Also, the severe unsteady operation, coupled with the
possible incoming of recalcitrant compounds, caused a steady fouling rate over 15-20 days that finally led to an irreversible loss of permeability, not recoverable by the routine protocol of 1 mCIP/week (run 3 vs run 4). This gap might be due either to a recalcitrant fouling layer or to the deflocculation of the activated sludge quantified by the rapid increase of the sludge volume index (SVI), coupled to a decrease of membrane permeability (Figure 5). Power Requirements. Even adopting the more convenient submerged configuration, operating municipal MBRs still requires more energy than the conventional activated sludge plants. In this case study, the energy-saving policy was pursued by three main choices: (1) the low aeration demand for membrane scouring, suggested by the membrane producer and already discussed before; (2) the choice of the AC technology, a biological process able to save up to 20-30% energy with respect to traditional multizone processes;20 and (3) always operating the plant according to MLSS as low as possible, which allows one to optimize the oxygen transfer to the biomass.21 Moreover, frequency regulators for pumps and blowers were installed along the treatment line. Looking at the power installed over the whole AC-MBR (Figure 6), one can observe that >50% is for the blowers, equally divided among biological process and membrane scouring. So, optimizing the total exploitation of the air supplied was the successful strategy to reduce the energy consumptions. Also, the low number of recycles of activated sludge contributed to saving energy for pumping. While the consumptions of pretreatments and biological process were measured by processing the ON/OFF signals related to the electromechanics, the UF section was monitored
Figure 4. TMP increase in routine operation (data shown are representative for the whole normal operation); irregular peaks inloading; no hypochlorite maintenance cleaning in place.
Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6693
Figure 5. SVI and membrane permeability at 20 °C.
Figure 6. Power installed over the whole AC-MBR. Table 8. Energy Consumptions and Comparison to Literature Data plant Viareggio Brescia-Verziano Nordkanal-Kaarst Park Place Knautnaundorf general range
EE (kWh m-3)
