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Zero net growth in a membrane bioreactor with complete sludge retention
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Water Research 39 (2005) 5241–5249 www.elsevier.com/locate/watres
Zero net growth in a membrane bioreactor with complete sludge retention G. Laera, A. Pollice, D. Saturno, C. Giordano, A. Lopez CNR IRSA, Via F. De Blasio 5, 70123 Bari, Italy Received 9 February 2005; received in revised form 10 October 2005; accepted 10 October 2005
Abstract A bench-scale membrane bioreactor was operated with complete sludge retention in order to evaluate biological processes and biomass characteristics over the long term. The investigation was carried out by feeding a bench-scale 1 plant with real sewage under constant volumetric loading rate (VLR ¼ 1.2 gCOD L1 react h ). Biological processes were monitored by measuring substrate removal efficiencies and biomass-related parameters. The latter included bacterial activity as determined through respirometric tests specifically aimed at investigating long term heterotrophic and nitrifying activity. After about 180 days under the imposed operating conditions, the system reached equilibrium conditions with constant VSS concentration of 16–18 g L1, organic loading rate (OLR) below 0.1 gCOD gVSS1 d1 and specific respiration rates of 2–3 mgO2 gVSS1 h1. These conditions were maintained for more than 150 days, confirming that an equilibrium had been achieved between biomass growth, endogenous metabolism, and solubilization of inorganic materials. r 2005 Elsevier Ltd. All rights reserved. Keywords: Membrane bioreactors; Biomass activity; Complete sludge retention; Municipal wastewater
1. Introduction The main advantages of membrane bioreactors (MBR) with respect to traditional activated sludge systems were summarized as (i) longer sludge retention times (SRT) independent of the hydraulic retention time (HRT), (ii) smaller footprint, (iii) complete removal of solids and nearly complete removal of effluent microorganisms, (iv) high removal ratios for most contaminants, (v) reduced sludge production, and (vi) rapid start-up of biological processes (Stephenson et al., 2000; Visvanathan et al., 2000). All these aspects are currently under investigation, and a debate is still Corresponding author. Tel.: +39 080 5020511; fax: +39 080 5313365. E-mail address: alfi[email protected] (A. Pollice).
open on the most appropriate operating procedures to maximize treatment performance with respect to operational costs. The latter mainly relate to membrane cleaning, sludge wastage, and aeration, and these are all affected by the operational concentration and features of the sludge within MBR. A possible strategy for operational cost limitation is reduction of sludge withdrawal, despite increased aeration costs (Yoon et al., 2004). Theoretical investigations have evidenced that biomass production can be limited in MBR by appropriate operational strategies (Lu et al., 2001; Xing et al., 2003). Some authors suggested that these systems could be operated at high sludge concentrations (15–25 gSS/L) by limiting biomass withdrawal, thus minimizing bacterial growth. The main drawbacks of this operating procedure were indicated as oxygen transfer limitations and
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.10.010
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increased membrane fouling tendency (Muller et al., 1995; Lu¨bbeke et al., 1995; Coˆte´ et al., 1998; Rosenberger et al., 2000). Besides operational effects, long SRT were also investigated for their effects on the biomass, both in terms of accumulation rates and activity. Lower enzymatic activity, and smaller yield and decay coefficients were reported for the higher sludge ages, suggesting lower flexibility to xenobiotic compounds due to slower biomass renovation (Chaize and Huyard, 1991; Huang et al., 2001). The possibility of operating MBR without sludge withdrawal was explored by several researchers, some of whom worked on relatively short timeframes (Yamamoto et al., 1989; Benı` tez et al., 1995; Pollice et al., 2004). Others provided results of longterm experimental tests mainly focused on removal efficiencies and operational aspects (Chiemchaisri et al., 1992; Muller et al., 1995; Wagner and Rosenwinkel, 2000; Rosenberger et al., 2002). All these authors stated the biological applicability of complete sludge retention, reporting high and stable degradation rates and very limited sludge production or zero net growth. However, equilibrium conditions are experimentally difficult to reach in systems having high or indefinite SRT, and most of these findings were obtained under non-steady state conditions, with variable loads and sometimes with synthetic wastewater. Only few works were focused on biomass-related aspects such as microbiology and bacterial activities, and maintenance metabolism was often related to complete sludge retention (Witzig et al., 2002; Wagner and Rosenwinkel, 2000). Furthermore, starvation was recently addressed as a means to maintain the biomass in a state not allowing cell division, but still able to participate in the degradation processes to satisfy maintenance energy requirements (Lobos et al., 2005). Finally, reported results are not always consistent with respect to the accumulation of organic inerts and inorganic material within the biomass under complete sludge retention (Muller et al., 1995; Lu et al., 2001; Rosenberger et al., 2002; Pollice et al., 2004). These last two aspects of sludge biology and fate of inorganics under equilibrium conditions are especially relevant for long-term MBR practical application and modelling. Purpose of this experimental work was to verify that in a MBR with complete sludge retention fed on real municipal wastewater:
Long-term operation is achievable without decrease of treatment performance,
processes can reach equilibrium conditions, accumulation of inorganics does not occur, biomass viability is maintained.
2. Materials and methods The bench-scale submerged MBR had 6 L operating volume, and a Zenon hollow fiber membrane module having a surface of 0.047 m2 was sunk in the reactor and operated out-in. A scheme of the bench-scale experimental plant was reported elsewhere (Pollice et al., 2004). The permeate was extracted by imposing on the membrane a negative pressure that never exceeded 0.5 bar. Operational cycles lasted 6 min and included extraction of the permeate (5.5 min) and backwash (0.5 min). An air pump provided a constant air flow through the membrane module to limit fouling and cake formation. A second air pump was regulated to maintain residual dissolved oxygen (DO) concentrations between 1 and 3 mg L1 to favour COD removal and nitrification. The experimental plant was continuously monitored for DO, temperature, pH, and transmembrane pressure (TMP). Biological processes were operated by maintaining a constant volumetric loading rate (VLR), defined as the amount of COD daily fed per liter of reactor’s volume. This was obtained by keeping a constant permeate flux and influent concentration. The system was continuously fed on pre-settled municipal wastewater screened at 1 mm, stored at 4 1C and stirred. The feed concentration was maintained within a constant range by diluting the real sewage with tap water when needed. The plant was started up without any biomass inoculum, and during the whole experimental period no sludge was intentionally removed from the reactor except for measurements of suspended solids. Biomass samples were withdrawn from the reactor once or twice per week and analysed for total suspended solids (TSS) and volatile suspended solids (VSS) according to standard methods (Standard Methods, 1995). Very limited amounts of sludge were lost during on-site membrane cleaning and these were accounted for in the evaluation of biomass growth and the other sludgerelated parameters. As a general rule of this investigation, biomass samples were always returned to the reactor after non-destructive determinations. The limited amount of biomass that tended to stick to those parts of the reactor close to the surface level was daily removed and returned to the sludge bulk. Sludge management practices during the experiment are summarized in Table 1. The volume of sludge removed and not returned to the reactor was estimated to be about 35 mL/week on average, corresponding to 0.83 gTSS/week. Average values of the biomass yield were calculated over periods of 3 weeks by taking into account the sludge losses. In these periods the yield was obtained by adding the amount of biomass lost to the discrete integration of the area below the growth curve, and
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Table 1 Sludge extracted from the reactor during the experiment Event
Unit
Amount
Frequency
Returned to plant
TSS+VSS determination Sludge characterizationa Membrane cleaning Respirometry
mL mL g mL
20 250 1.8 300
Weekly Weekly —b Weekly
No Yes No Yes
a
Non-destructive viscosity and filterability tests were regularly performed on sludge samples. On-site module rinsing was performed 7 times during the whole experimental period, with a frequency that depended on the patterns of pressure growth (see following paragraph). b
dividing by the cumulative COD removed in the same period. Respirometry was performed after aerating the sludge samples overnight to obtain endogenous respiration conditions. The respirometer adopted was a ‘‘static gas–static liquid’’ system operated by measurement of the liquid phase DO concentrations, and its description was provided elsewhere (Spanjers et al., 1998; Pollice et al., 2004). The influent municipal wastewater was sampled three times per week and analysed for TSS and VSS, total COD, N-NH4, TKN, N-NO2, and N-NO3. The permeate was sampled daily and analysed for the same parameters. All analyses were performed according to standard methods (Standard Methods, 1995). The ion chromatographic method for the determination of nitrate and nitrite was modified adopting an UV detector (at 220 nm) instead of the conductivity detector.
3. Results 3.1. Plant performance The bench-scale MBR was operated for 336 days with 1 a constant volumetric loading rate of 1.2 gCOD L1 react d and process performance, sludge accumulation, and biomass features were monitored over time. The main operational parameters are provided in Table 2, and the average characteristics of the influent wastewater and the permeate are reported in Table 3. The filtration performance of the membrane module is described in Fig. 1, where the evolution over time of flux and TMP are reported. The normal cleaning procedure of the module was on-site jet rinsing of the fibers with moderately pressurized tap water and was performed when the TMP approached 500 mbar. This procedure was effective in restoring good permeability, and the membrane was chemically cleaned only when the positive effect of rinsing tended to last too short. This happened just twice in 1 year of operation (days 63 and 296, Fig. 1), and a good efficiency of chemical cleaning
Table 2 Operational parameters of the bench-scale complete retention MBR Parameter
Unit
Average
Std. Dev.
Min
Max
Flux TMP DO Temperature pH
L m2 h1 mbar mgO2 L1 1C
16.4 154 2.4 20.9 6.8
1.2 106 1.1 4.5 0.3
9.4 15 0.4 12.1 5.4
17.6 500 7.5 29.6 7.7
in restoring the membrane’s permeability was observed. A sub-critical flux behaviour is also evidenced, with a critical TMP value of 150 mbar for this system that was independent of the sludge concentration, and above which the fouling rate rapidly increased. Similar patterns were observed in other research works, although their interpretation with respect to flux sustainability is still debated (Ognier et al., 2001; Cho and Fane, 2002; Guglielmi, 2002; Le Clech et al., 2003; Pollice et al., 2005). 3.2. Biomass accumulation Fig. 2 shows biomass accumulation in the reactor under constant VLR, and compares these data with previous results obtained in short-term tests (Pollice et al., 2004). This comparison evidenced similar sludge accumulation patterns for three different VLRs, with higher initial growth rates for higher VLR. Moreover, sludge accumulation was very limited during the last 150 days of operation and the system reached an equilibrium sludge concentration of 17.371.1 gVSS L1. The VSS/ TSS ratio was rather stable over the whole experimental period, and an average value of 7572% was maintained after the initial 50 days. Further evidence that the equilibrium was reached in the long-term was provided by the calculation of the organic loading rate (OLR, gCOD gVSS1 d1). Fig. 3
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Table 3 Main average characteristics of the investigated influent primary sewage and permeate Parameter
TSS VSS COD NH+ 4 NO 3 TKN
Unit
Influent
mg L1 mg L1 mg L1 mgN L1 mgN L1 mgN L1
Permeate
Efficiency
Avg
Std. Dev.
Avg
Std. Dev.
200 160 400 36.6 0.7 49.3
70 50 106 7.6 1.7 9.5
n.d. n.d. 57 0.2 40.6 0.8
n.d. n.d. 34 0.2 16.9 0.3
499.9% 499.9% 86% 99% — 98%
n.d. ¼ below the detection limit.
600
18
TMP (mbar)
400
12 TMP Flux
200
6
0 0
30
60
90
Flux (L m-2 h-1)
chemical cleaning
0 120 150 180 210 240 270 300 330 360 time (days)
Fig. 1. TMP and flux variations during the experiment (336 days).
sludge concentration (gVSS L-1)
20 16 12 8
0.8 gCOD / Lh 1.2 gCOD / Lh
4
1.7 gCOD / Lh 0 0
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
Fig. 2. Sludge accumulation in the reactor (K) as compared with previous results of short term tests ( and n) (Pollice et al., 2004).
shows that during the last 150 days of the experiment a constant OLR was maintained (0.0770.01 gCOD gVSS1 d1). The figure also shows that these data well agree with previous short-term results (Pollice et al., 2004).
3.3. Respiration rates The long-term effects of complete sludge retention on biomass activity were investigated by periodically measuring the oxygen uptake rate (OUR) of heterotrophic
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OLR (gCOD gVSS-1 d-1)
0.8 0.8 gCOD / Lh
0.7
1.2 gCOD / Lh
0.6
1.7 gCOD / Lh
0.5 0.4 0.3 0.2 0.1 0.0 0
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
specific OUR (mg O2 gVSS-1 h-1)
Fig. 3. Organic loading rate over time as compared with previous data from short-term tests (Pollice et al., 2004).
specific OUR (mg O2 gVSS-1 h-1)
(a)
24 20 16 12 8 4 0
Acetate Endogenous
24 20 16 12 8 4 0
Ammonium Nitrite
0 (b)
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
Fig. 4. Maximum specific respiration rates of heterotrophic (a) and autotrophic (b) populations.
and autotrophic populations with respirometric tests. The two bacterial groups were examined by adopting sodium acetate, sodium nitrite, and ammonium chloride as synthetic substrates, and measuring the respiration rates after dosing concentrations of these substrates in excess with respect to the metabolic needs. The specific maximum respiration rates with respect to the VSS (i.e. specific OUR, mgO2 gVSS1 h1) are reported in Fig. 4, where well known differences can be observed in the behaviour of the two bacterial groups. Autotrophic nitrifiers are slower than the heterotrophs in reaching their maximum specific activity. However, after about 100 days of operation the specific respiration rates were not very different and both tended to decrease in the longer term. Stability was reached after about 180 days of operation.
This tendency of the specific activity to decrease towards a constant value was counterbalanced by the corresponding increase of the microrganism concentration towards a constant value over the long term. The combination of these two effects resulted in the maintenance of the overall biomass activity, with consequent constancy of the respiration rates (i.e. OUR, mgO2 h1) over the long term. Endogenous and maximum OUR of both microbial populations showed similar trends, i.e. after having reached a maximum between 30 and 90 days of operation they slowly declined towards an equilibrium value between 20% and 50% of the maximum. During the last 150 days of operation, the average endogenous OUR was 1373 mgO2 h1, and the average maximum respiration rates recorded for acetate, ammonium and
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nitrite were 2279, 46717 and 673 mgO2 h1, respectively. An occasional drop of the autotrophic activity was observed around day 260, when a higher effluent ammonium concentration was also observed (4.0 mgN1 1 NH+ against an average 0.2 mgN-NH+ 4 L 4 L ), suggesting the occurrence of an inhibitor in the influent wastewater. After this, the biomass promptly recovered its previous activity.
4. Discussion The experimental results reported above show that the system had reached an equilibrium after about 180 days of operation with negligible sludge withdrawal. This equilibrium was maintained over the long term. Operation of the plant with complete sludge retention does not allow for the definition of the system’s sludge age. However, if the limited volumes of sludge sampled and not returned to the reactor are taken into account, a sludge age of about 1200 days can be estimated. Sludge accumulation in the reactor showed the same pattern observed in short-term tests (Pollice et al., 2004), i.e. the initially high sludge growth rate strongly decreased after about 30 days, and then slowly declined resulting in stabilization of the average biomass concentration during the last 150 days (Fig. 2). Also other relevant process parameters such as the VSS/TSS ratio, the OLR and the specific biomass activities reached constant values in the same period (Figs. 3 and 4). This tendency of the OLR and the specific activities towards constant (minimized) values and the stabilization of the VSS and the VSS/TSS ratio suggest that the system had reached and maintained equilibrium conditions. Fig. 3 also shows that in MBRs operated under complete sludge retention the OLR evolves towards a minimized value that is independent of the VLR. This suggests that under equilibrium conditions the biomass concentration is proportional to the volumetric loading rate. The absence of biomass net growth observed during the final 150 days can be explained with the occurrence of maintenance metabolism. Moreover, the stabilization of the VSS and the specific activities means that no accumulation of ‘‘inert’’ material occurred. This is consistent with a recently proposed interpretation stating that accumulation of inerts inversely depends on the system’s SRT, and that they can be degraded by slow growing bacteria (van Loosdrecht and Henze, 1999). The MBR showed good removal efficiencies of suspended solids, COD, ammonia and total nitrogen (Table 3). These performances suggested that the biomass viability was maintained over a long period.
Respirometric tests also confirmed the persistence of biomass viability and no tendency towards the mineralization of biosolids, even over long timeframes. During the last 150 days of operation the OUR values were below their maximum values, but respiration rates ranging between 20% and 50% of the maximum were maintained until the end of the experiment. Moreover, an occasional drop of the autotrophic activity was promptly recovered and the OUR values measured before the inhibition were immediately restored, showing good flexibility of the biomass towards possible changes occurring in the influent. In order to estimate biomass growth and decay rates over the long term, the active fraction needs to be quantified. Available literature data on this ‘‘classical’’ issue of biological wastewater treatments are extremely limited and case-specific, and mostly refer to conventional-activated sludge. MBRs were reported to have an active fraction between 4% and 7% with respect to the VSS in systems with SRT around 35–50 days (Guglielmi, 2002). This fraction refers to steady state conditions, and its application to complete retention MBR has to take into account the evolution over time of the biomass (and of the active fraction). In the present study, constant OLR, VSS, and endogenous respiration rate after 180 days of operation suggested that the concentration of active biomass was approximately constant. Exact determination of this fraction would have required sludge sampling that, at the experimental scale adopted, was not compatible with the assumption of complete sludge retention. However, some indications on the growth and decay of the biomass can also be obtained without information on the active fraction. Biomass build-up in biological systems is commonly represented by the Monod model where a decay term is introduced (Henze et al., 1987): dX S ¼ mX bX ¼ mmax X bX , dt kS þ S where X is the active fraction of the biomass, m is the growth rate, b the decay rate, S the substrate concentration, kS the half-saturation constant. According to this model, absence of biomass growth can be expresses as follows: dX m ¼ 0 ) m ¼ b3 ¼ 1. dt b Oxygen consumption during biological processes depends on the utilization external substrate and the endogenous biomass decay. The OUR can be defined accordingly, by separately considering these two contributions 1 Y0 X þ bX , OUR ¼ OURext þ OURend ¼ m Y0
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where Y 0 is the yield of substrate utilization for biomass production. The maximum OUR is obtained for maximum growth rate (i.e. when SbkS) and can be expressed as follows: 1 Y0 X þ bX . OURmax ¼ mmax Y0 When all the available external substrate has been consumed, the OUR is represented by the decay term bX (i.e. the oxygen requirement for endogenous respiration) OURend ¼ bX . The ratio between OURmax and OURend leads to the following expression: OURmax Y0 m ¼ max 1 0 OURend 1Y b According to this expression, the ratio between mmax and b is independent of the active fraction X . Therefore, when OURmax and OURend have comparable values, the maximum growth is similar to the biomass decay. The experimental ratio OURmax/OURend at the equilibrium was calculated to be 1.770.3. The adoption of a standard yield value of 0.67 mgCODbiom mgCOD1 sub (Henze et al., 1987) resulted in mmax/b ¼ 1.570.6. Since the actual growth rate m is smaller than the mmax , it could be assumed that the ratio m=b approaches one. The above considerations explain the absence of net biomass growth experimentally observed and confirm previous model predictions (Wintgens et al., 2003). The plot of the observed sludge yield as a function of the OLR over the whole experimental period evidenced a tendency of the yield to decrease for decreasing OLRs (Fig. 5). The experimental data showed that minimized OLR resulted in extremely limited yields, and that zero biomass yield could be reached, as previously observed under non-steady state conditions (van Houten and Eikelboom, 1997; Rosenberger et al., 2002). Although extremely low values of OLR are likely to take very long
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time to be reached, negligible yield can already be assumed for the operational loading rates around 0.07 gCOD gVSS1 d1 experimentally maintained, also taking into account the limited sludge losses occurred during the experiment. Absence of sludge accumulation means zero net biomass growth and also no accumulation of inorganic material. The amount of inorganics accumulated in the reactor, determined as the difference between the TSS and the VSS in the sludge, was compared with the cumulative inorganic material in the influent wastewater (Fig. 6). The ratio between these two non-volatile fractions was found to be rather constant (about 30%) for the initial 100 days, in accordance with previous findings on conventional activated sludge and nonsteady state MBR (Wentzel et al., 2002; Wagner and Rosenwinkel, 2000). Then the accumulation rate was observed to decrease, and finally resulted in a constant concentration of inorganics in the reactor over the long term. Also in this case, as for the other process parameters, the accumulation of inorganic material ceased after about 180 days, confirming that the studied system had reached equilibrium conditions. Constancy of the VSS/TSS ratio together with zero net growth provides an experimental evidence of previously proposed theories predicting the absence of excess sludge production under sufficiently long sludge retention times (Xing et al., 2003). No discernible accumulation of inorganics was already suggested only based on the observation of constant VSS/TSS ratio, although never experimentally demonstrated under steady state conditions (Muller et al., 1995). At the end of the experiment, the reactor was emptied to check for possible localized accumulation of solids due to mixing problems. None of such deposits was observed, confirming that the system was completely stirred and that the constant concentration of inorganics was not caused by ‘‘hidden’’ materials as in previous experiments (Rosenberger et al., 2002). Therefore, when
sludge yield (gVSS gCOD-1)
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.00
0.02
0.04
0.06 0.08 0.10 OLR (gCOD gVSS-1 d-1)
0.12
Fig. 5. Observed sludge yield vs. organic loading rate.
0.14
0.16
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81
240
cumulative influent inorganics
72
210
sludge inorganics
63
180
54
150
45
120
36
90
27
60
18
30
9
0 0
30
60
90
sludge inorganics (g)
influent inorganics (g)
270
0 120 150 180 210 240 270 300 330 360 time (days)
Fig. 6. Inorganic fractions in the influent wastewater and accumulated in the reactor.
high sludge concentrations were maintained, the results showed a limit value above which no accumulation of inorganic materials occurred in the reactor. This observation can be explained by considering that at sludge concentrations above 15 gVSS L1 the influent wastewater contributes for about 5–10% to the total amount of inorganics in the reactor, and this fraction could be involved in processes leading to solubilization.
5. Conclusions Long-term experiments with a complete retention MBR fed on real sewage at constant volumeric loading rate showed that equilibrium conditions were reached after about 180 days of operation and maintained for more than 150 days. Equilibrium was characterized by constancy of three key process parameters, i.e. biomass (VSS) and inorganic material concentrations and the specific bacterial activity. As a consequence, the organic loading rate reached values around 0.07 gCOD gVSS1 d1 in the long term, resulting in sludge yield values close to zero. This was confirmed by the results of the activity tests, that showed comparable values for growth and decay rates (mmax/b ¼ 1.570.6). No accumulation of inorganic material within the reactor was observed once the equilibrium was reached. Complete nitrification and good COD removal efficiencies were maintained over the whole experimental period. Moreover, adopting a typical flux and a simple membrane-cleaning procedure, the initial biomass growth and long-term operations at high sludge concentrations caused no significant variations of membrane permeability and cleaning needs. The experimental work showed the feasibility of operating MBR with complete sludge retention (or with SRT41000 d) from the standpoint of biological processes. Moreover, the reported data show that the
biomass evolves towards a minimized maintenance OLR, resulting in proportionality between the VLR and the equilibrium biomass concentration. When designing new plants, this feature allows for defining the biomass concentration based on the VLR. Larger scale studies are needed to evaluate all costrelated issues, including aeration, membrane cleaning and sludge treatability.
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ARTICLE IN PRESS G. Laera et al. / Water Research 39 (2005) 5241–5249 Le Clech, P., Jefferson, B., Chang, I.S., Judd, S., 2003. Critical flux determination by the flux-step method in a submerged membrane bioreactor. J. Membr. Sci. 227, 81–93. Lobos, J., Wisniewski, C., Heran, M., Gramsick, A., 2005. Effects of starvation conditions on biomass behaviour for minimization of sludge production in membrane bioreactors. Water Sci. Technol. 51 (6–7), 35–44. Lu, S.G., Imai, T., Ukita, M., Sekine, M., Higuchi, T., Fugakawa, M., 2001. A model for membrane bioreactor process based on the concept of formation and degradation of soluble microbial products. Water Res. 35 (8), 2038–2048. Lu¨bbeke, S., Vogelpohl, A., Dewjanin, W., 1995. Wastewater treatment in a biological high-performance system with high biomass concentration. Water Res. 29 (3), 793–802. Muller, E.B., Stouthamer, A.H., van Verseveld, H.W., Eikelboom, D.H., 1995. Aerobic domestic waste water treatment in a pilot plant with complete sludge retention by cross-flow filtration. Water Res. 29 (4), 1179–1189. Ognier, S., Wisniewski, C., Grasmick, A., 2001. Biofouling in MBRs: phenomenon analysis and modeling. Proceedings of MBR3 Third International Meeting on Membrane Bioreactors for Wastewater Treatment, Cranfield University, 16th May 2001, Cranfield, UK. Pollice, A., Laera, G., Blonda, M., 2004. Biomass growth and activity in a membrane bioreactor with complete sludge retention. Water Res. 38 (7), 1799–1808. Pollice, A., Brookes, A., Jefferson, B., Judd, S., 2005. Subcritical flux fouling in membrane bioreactors—a review of recent literature. Desalination 174 (3), 221–230. Rosenberger, S., Witzig, R., Manz, W., Szewzyk, U., Kraume, M., 2000. Operation of different membrane bioreactors: experimental results and physiological state of the microorganisms. Water Sci. Technol. 41 (10–11), 269–277. Rosenberger, S., Kru¨ger, U., Witzig, R., Manz, W., Szewzyk, U., Kraume, M., 2002. Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water. Water Res. 36 (2), 413–420. Spanjers, H., Vanrolleghem, P.A., Olsson, G., Dold, P.L., 1998. Respirometry in control of the activated sludge process: principles. IAWQ Scientific and Technical Report no. 7, IAWQ Publishing, London, UK. Standard Methods, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public
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Health Association/American Water Works Association/ Water Environment Federation, Washington DC. Stephenson, T., Judd, S., Jefferson, B., Brindle, K., 2000. Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London, UK. van Houten, R.T., Eikelboom, D.H., 1997. High performance membrane bioreactors: a physiological approach. Proceedings of MBR1–First International Meeting on Membrane Bioreactors for Wastewater Treatment. Cranfield University, 5th March 1997, Cranfield, UK. van Loosdrecht, M., Henze, M., 1999. Maintenance, endogenous respiration, lysis, decay and predation. Water Sci. Technol. 39 (1), 107–117. Visvanathan, C., Ben Aim, R., Parmeshwaran, K., 2000. Membrane separation bioreactors for wastewater treatment. Crit. Rev. Environ. Sci. Technol. 30 (1), 1–48. Wagner, J., Rosenwinkel, K.-H., 2000. Sludge production in membrane bioreactors under different conditions. Water Sci. Technol. 41 (10–11), 251–258. Wentzel, M.C., Ubisi, M.F., Lakay, M.T., Ekama, G.A., 2002. Incorporation of inorganic material in anoxic/aerobic activated sludge system mixed liquor. Water Res. 36 (20), 5074–5082. Wintgens, T., Rosen, J., Melin, T., Brepols, C., Drensla, K., Engelhardt, N., 2003. Modelling of a membrane bioreactor for municipal wastewater treatment. J. Membr. Sci. 216, 55–65. Witzig, R., Manz, W., Rosenberger, S., Kruger, U., Kraume, M., Szewzyk, U., 2002. Microbiological aspects of a bioreactor with submerged membranes for aerobic treatment of municipal wastewater. Water Res. 36 (2), 394–402. Xing, C.-H., Wu, W.-Z., Quian, Y., Tardieu, E., 2003. Excess sludge production in membrane bioreactors: a theoretical investigation. J. Environ. Eng. ASCE 129 (4), 291–297. Yamamoto, K., Hiasa, M., Mahmood, T., Matsuo, T., 1989. Direct solid–liquid separation using hollow fiber membrane in an activated sludge aeration tank. Water Sci. Technol. 21 (Brighton), 43–54. Yoon, T.I., Lee, H.S., Kim, C.G., 2004. Comparison of pilot scale performances between membrane bioreactor and hybrid conventional wastewater treatment system. J. Membr. Sci. 242, 5–12.
Water Research 39 (2005) 5241–5249 www.elsevier.com/locate/watres
Zero net growth in a membrane bioreactor with complete sludge retention G. Laera, A. Pollice, D. Saturno, C. Giordano, A. Lopez CNR IRSA, Via F. De Blasio 5, 70123 Bari, Italy Received 9 February 2005; received in revised form 10 October 2005; accepted 10 October 2005
Abstract A bench-scale membrane bioreactor was operated with complete sludge retention in order to evaluate biological processes and biomass characteristics over the long term. The investigation was carried out by feeding a bench-scale 1 plant with real sewage under constant volumetric loading rate (VLR ¼ 1.2 gCOD L1 react h ). Biological processes were monitored by measuring substrate removal efficiencies and biomass-related parameters. The latter included bacterial activity as determined through respirometric tests specifically aimed at investigating long term heterotrophic and nitrifying activity. After about 180 days under the imposed operating conditions, the system reached equilibrium conditions with constant VSS concentration of 16–18 g L1, organic loading rate (OLR) below 0.1 gCOD gVSS1 d1 and specific respiration rates of 2–3 mgO2 gVSS1 h1. These conditions were maintained for more than 150 days, confirming that an equilibrium had been achieved between biomass growth, endogenous metabolism, and solubilization of inorganic materials. r 2005 Elsevier Ltd. All rights reserved. Keywords: Membrane bioreactors; Biomass activity; Complete sludge retention; Municipal wastewater
1. Introduction The main advantages of membrane bioreactors (MBR) with respect to traditional activated sludge systems were summarized as (i) longer sludge retention times (SRT) independent of the hydraulic retention time (HRT), (ii) smaller footprint, (iii) complete removal of solids and nearly complete removal of effluent microorganisms, (iv) high removal ratios for most contaminants, (v) reduced sludge production, and (vi) rapid start-up of biological processes (Stephenson et al., 2000; Visvanathan et al., 2000). All these aspects are currently under investigation, and a debate is still Corresponding author. Tel.: +39 080 5020511; fax: +39 080 5313365. E-mail address: alfi[email protected] (A. Pollice).
open on the most appropriate operating procedures to maximize treatment performance with respect to operational costs. The latter mainly relate to membrane cleaning, sludge wastage, and aeration, and these are all affected by the operational concentration and features of the sludge within MBR. A possible strategy for operational cost limitation is reduction of sludge withdrawal, despite increased aeration costs (Yoon et al., 2004). Theoretical investigations have evidenced that biomass production can be limited in MBR by appropriate operational strategies (Lu et al., 2001; Xing et al., 2003). Some authors suggested that these systems could be operated at high sludge concentrations (15–25 gSS/L) by limiting biomass withdrawal, thus minimizing bacterial growth. The main drawbacks of this operating procedure were indicated as oxygen transfer limitations and
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.10.010
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increased membrane fouling tendency (Muller et al., 1995; Lu¨bbeke et al., 1995; Coˆte´ et al., 1998; Rosenberger et al., 2000). Besides operational effects, long SRT were also investigated for their effects on the biomass, both in terms of accumulation rates and activity. Lower enzymatic activity, and smaller yield and decay coefficients were reported for the higher sludge ages, suggesting lower flexibility to xenobiotic compounds due to slower biomass renovation (Chaize and Huyard, 1991; Huang et al., 2001). The possibility of operating MBR without sludge withdrawal was explored by several researchers, some of whom worked on relatively short timeframes (Yamamoto et al., 1989; Benı` tez et al., 1995; Pollice et al., 2004). Others provided results of longterm experimental tests mainly focused on removal efficiencies and operational aspects (Chiemchaisri et al., 1992; Muller et al., 1995; Wagner and Rosenwinkel, 2000; Rosenberger et al., 2002). All these authors stated the biological applicability of complete sludge retention, reporting high and stable degradation rates and very limited sludge production or zero net growth. However, equilibrium conditions are experimentally difficult to reach in systems having high or indefinite SRT, and most of these findings were obtained under non-steady state conditions, with variable loads and sometimes with synthetic wastewater. Only few works were focused on biomass-related aspects such as microbiology and bacterial activities, and maintenance metabolism was often related to complete sludge retention (Witzig et al., 2002; Wagner and Rosenwinkel, 2000). Furthermore, starvation was recently addressed as a means to maintain the biomass in a state not allowing cell division, but still able to participate in the degradation processes to satisfy maintenance energy requirements (Lobos et al., 2005). Finally, reported results are not always consistent with respect to the accumulation of organic inerts and inorganic material within the biomass under complete sludge retention (Muller et al., 1995; Lu et al., 2001; Rosenberger et al., 2002; Pollice et al., 2004). These last two aspects of sludge biology and fate of inorganics under equilibrium conditions are especially relevant for long-term MBR practical application and modelling. Purpose of this experimental work was to verify that in a MBR with complete sludge retention fed on real municipal wastewater:
Long-term operation is achievable without decrease of treatment performance,
processes can reach equilibrium conditions, accumulation of inorganics does not occur, biomass viability is maintained.
2. Materials and methods The bench-scale submerged MBR had 6 L operating volume, and a Zenon hollow fiber membrane module having a surface of 0.047 m2 was sunk in the reactor and operated out-in. A scheme of the bench-scale experimental plant was reported elsewhere (Pollice et al., 2004). The permeate was extracted by imposing on the membrane a negative pressure that never exceeded 0.5 bar. Operational cycles lasted 6 min and included extraction of the permeate (5.5 min) and backwash (0.5 min). An air pump provided a constant air flow through the membrane module to limit fouling and cake formation. A second air pump was regulated to maintain residual dissolved oxygen (DO) concentrations between 1 and 3 mg L1 to favour COD removal and nitrification. The experimental plant was continuously monitored for DO, temperature, pH, and transmembrane pressure (TMP). Biological processes were operated by maintaining a constant volumetric loading rate (VLR), defined as the amount of COD daily fed per liter of reactor’s volume. This was obtained by keeping a constant permeate flux and influent concentration. The system was continuously fed on pre-settled municipal wastewater screened at 1 mm, stored at 4 1C and stirred. The feed concentration was maintained within a constant range by diluting the real sewage with tap water when needed. The plant was started up without any biomass inoculum, and during the whole experimental period no sludge was intentionally removed from the reactor except for measurements of suspended solids. Biomass samples were withdrawn from the reactor once or twice per week and analysed for total suspended solids (TSS) and volatile suspended solids (VSS) according to standard methods (Standard Methods, 1995). Very limited amounts of sludge were lost during on-site membrane cleaning and these were accounted for in the evaluation of biomass growth and the other sludgerelated parameters. As a general rule of this investigation, biomass samples were always returned to the reactor after non-destructive determinations. The limited amount of biomass that tended to stick to those parts of the reactor close to the surface level was daily removed and returned to the sludge bulk. Sludge management practices during the experiment are summarized in Table 1. The volume of sludge removed and not returned to the reactor was estimated to be about 35 mL/week on average, corresponding to 0.83 gTSS/week. Average values of the biomass yield were calculated over periods of 3 weeks by taking into account the sludge losses. In these periods the yield was obtained by adding the amount of biomass lost to the discrete integration of the area below the growth curve, and
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Table 1 Sludge extracted from the reactor during the experiment Event
Unit
Amount
Frequency
Returned to plant
TSS+VSS determination Sludge characterizationa Membrane cleaning Respirometry
mL mL g mL
20 250 1.8 300
Weekly Weekly —b Weekly
No Yes No Yes
a
Non-destructive viscosity and filterability tests were regularly performed on sludge samples. On-site module rinsing was performed 7 times during the whole experimental period, with a frequency that depended on the patterns of pressure growth (see following paragraph). b
dividing by the cumulative COD removed in the same period. Respirometry was performed after aerating the sludge samples overnight to obtain endogenous respiration conditions. The respirometer adopted was a ‘‘static gas–static liquid’’ system operated by measurement of the liquid phase DO concentrations, and its description was provided elsewhere (Spanjers et al., 1998; Pollice et al., 2004). The influent municipal wastewater was sampled three times per week and analysed for TSS and VSS, total COD, N-NH4, TKN, N-NO2, and N-NO3. The permeate was sampled daily and analysed for the same parameters. All analyses were performed according to standard methods (Standard Methods, 1995). The ion chromatographic method for the determination of nitrate and nitrite was modified adopting an UV detector (at 220 nm) instead of the conductivity detector.
3. Results 3.1. Plant performance The bench-scale MBR was operated for 336 days with 1 a constant volumetric loading rate of 1.2 gCOD L1 react d and process performance, sludge accumulation, and biomass features were monitored over time. The main operational parameters are provided in Table 2, and the average characteristics of the influent wastewater and the permeate are reported in Table 3. The filtration performance of the membrane module is described in Fig. 1, where the evolution over time of flux and TMP are reported. The normal cleaning procedure of the module was on-site jet rinsing of the fibers with moderately pressurized tap water and was performed when the TMP approached 500 mbar. This procedure was effective in restoring good permeability, and the membrane was chemically cleaned only when the positive effect of rinsing tended to last too short. This happened just twice in 1 year of operation (days 63 and 296, Fig. 1), and a good efficiency of chemical cleaning
Table 2 Operational parameters of the bench-scale complete retention MBR Parameter
Unit
Average
Std. Dev.
Min
Max
Flux TMP DO Temperature pH
L m2 h1 mbar mgO2 L1 1C
16.4 154 2.4 20.9 6.8
1.2 106 1.1 4.5 0.3
9.4 15 0.4 12.1 5.4
17.6 500 7.5 29.6 7.7
in restoring the membrane’s permeability was observed. A sub-critical flux behaviour is also evidenced, with a critical TMP value of 150 mbar for this system that was independent of the sludge concentration, and above which the fouling rate rapidly increased. Similar patterns were observed in other research works, although their interpretation with respect to flux sustainability is still debated (Ognier et al., 2001; Cho and Fane, 2002; Guglielmi, 2002; Le Clech et al., 2003; Pollice et al., 2005). 3.2. Biomass accumulation Fig. 2 shows biomass accumulation in the reactor under constant VLR, and compares these data with previous results obtained in short-term tests (Pollice et al., 2004). This comparison evidenced similar sludge accumulation patterns for three different VLRs, with higher initial growth rates for higher VLR. Moreover, sludge accumulation was very limited during the last 150 days of operation and the system reached an equilibrium sludge concentration of 17.371.1 gVSS L1. The VSS/ TSS ratio was rather stable over the whole experimental period, and an average value of 7572% was maintained after the initial 50 days. Further evidence that the equilibrium was reached in the long-term was provided by the calculation of the organic loading rate (OLR, gCOD gVSS1 d1). Fig. 3
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Table 3 Main average characteristics of the investigated influent primary sewage and permeate Parameter
TSS VSS COD NH+ 4 NO 3 TKN
Unit
Influent
mg L1 mg L1 mg L1 mgN L1 mgN L1 mgN L1
Permeate
Efficiency
Avg
Std. Dev.
Avg
Std. Dev.
200 160 400 36.6 0.7 49.3
70 50 106 7.6 1.7 9.5
n.d. n.d. 57 0.2 40.6 0.8
n.d. n.d. 34 0.2 16.9 0.3
499.9% 499.9% 86% 99% — 98%
n.d. ¼ below the detection limit.
600
18
TMP (mbar)
400
12 TMP Flux
200
6
0 0
30
60
90
Flux (L m-2 h-1)
chemical cleaning
0 120 150 180 210 240 270 300 330 360 time (days)
Fig. 1. TMP and flux variations during the experiment (336 days).
sludge concentration (gVSS L-1)
20 16 12 8
0.8 gCOD / Lh 1.2 gCOD / Lh
4
1.7 gCOD / Lh 0 0
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
Fig. 2. Sludge accumulation in the reactor (K) as compared with previous results of short term tests ( and n) (Pollice et al., 2004).
shows that during the last 150 days of the experiment a constant OLR was maintained (0.0770.01 gCOD gVSS1 d1). The figure also shows that these data well agree with previous short-term results (Pollice et al., 2004).
3.3. Respiration rates The long-term effects of complete sludge retention on biomass activity were investigated by periodically measuring the oxygen uptake rate (OUR) of heterotrophic
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OLR (gCOD gVSS-1 d-1)
0.8 0.8 gCOD / Lh
0.7
1.2 gCOD / Lh
0.6
1.7 gCOD / Lh
0.5 0.4 0.3 0.2 0.1 0.0 0
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
specific OUR (mg O2 gVSS-1 h-1)
Fig. 3. Organic loading rate over time as compared with previous data from short-term tests (Pollice et al., 2004).
specific OUR (mg O2 gVSS-1 h-1)
(a)
24 20 16 12 8 4 0
Acetate Endogenous
24 20 16 12 8 4 0
Ammonium Nitrite
0 (b)
30
60
90
120
150 180 210 time (days)
240
270
300
330
360
Fig. 4. Maximum specific respiration rates of heterotrophic (a) and autotrophic (b) populations.
and autotrophic populations with respirometric tests. The two bacterial groups were examined by adopting sodium acetate, sodium nitrite, and ammonium chloride as synthetic substrates, and measuring the respiration rates after dosing concentrations of these substrates in excess with respect to the metabolic needs. The specific maximum respiration rates with respect to the VSS (i.e. specific OUR, mgO2 gVSS1 h1) are reported in Fig. 4, where well known differences can be observed in the behaviour of the two bacterial groups. Autotrophic nitrifiers are slower than the heterotrophs in reaching their maximum specific activity. However, after about 100 days of operation the specific respiration rates were not very different and both tended to decrease in the longer term. Stability was reached after about 180 days of operation.
This tendency of the specific activity to decrease towards a constant value was counterbalanced by the corresponding increase of the microrganism concentration towards a constant value over the long term. The combination of these two effects resulted in the maintenance of the overall biomass activity, with consequent constancy of the respiration rates (i.e. OUR, mgO2 h1) over the long term. Endogenous and maximum OUR of both microbial populations showed similar trends, i.e. after having reached a maximum between 30 and 90 days of operation they slowly declined towards an equilibrium value between 20% and 50% of the maximum. During the last 150 days of operation, the average endogenous OUR was 1373 mgO2 h1, and the average maximum respiration rates recorded for acetate, ammonium and
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nitrite were 2279, 46717 and 673 mgO2 h1, respectively. An occasional drop of the autotrophic activity was observed around day 260, when a higher effluent ammonium concentration was also observed (4.0 mgN1 1 NH+ against an average 0.2 mgN-NH+ 4 L 4 L ), suggesting the occurrence of an inhibitor in the influent wastewater. After this, the biomass promptly recovered its previous activity.
4. Discussion The experimental results reported above show that the system had reached an equilibrium after about 180 days of operation with negligible sludge withdrawal. This equilibrium was maintained over the long term. Operation of the plant with complete sludge retention does not allow for the definition of the system’s sludge age. However, if the limited volumes of sludge sampled and not returned to the reactor are taken into account, a sludge age of about 1200 days can be estimated. Sludge accumulation in the reactor showed the same pattern observed in short-term tests (Pollice et al., 2004), i.e. the initially high sludge growth rate strongly decreased after about 30 days, and then slowly declined resulting in stabilization of the average biomass concentration during the last 150 days (Fig. 2). Also other relevant process parameters such as the VSS/TSS ratio, the OLR and the specific biomass activities reached constant values in the same period (Figs. 3 and 4). This tendency of the OLR and the specific activities towards constant (minimized) values and the stabilization of the VSS and the VSS/TSS ratio suggest that the system had reached and maintained equilibrium conditions. Fig. 3 also shows that in MBRs operated under complete sludge retention the OLR evolves towards a minimized value that is independent of the VLR. This suggests that under equilibrium conditions the biomass concentration is proportional to the volumetric loading rate. The absence of biomass net growth observed during the final 150 days can be explained with the occurrence of maintenance metabolism. Moreover, the stabilization of the VSS and the specific activities means that no accumulation of ‘‘inert’’ material occurred. This is consistent with a recently proposed interpretation stating that accumulation of inerts inversely depends on the system’s SRT, and that they can be degraded by slow growing bacteria (van Loosdrecht and Henze, 1999). The MBR showed good removal efficiencies of suspended solids, COD, ammonia and total nitrogen (Table 3). These performances suggested that the biomass viability was maintained over a long period.
Respirometric tests also confirmed the persistence of biomass viability and no tendency towards the mineralization of biosolids, even over long timeframes. During the last 150 days of operation the OUR values were below their maximum values, but respiration rates ranging between 20% and 50% of the maximum were maintained until the end of the experiment. Moreover, an occasional drop of the autotrophic activity was promptly recovered and the OUR values measured before the inhibition were immediately restored, showing good flexibility of the biomass towards possible changes occurring in the influent. In order to estimate biomass growth and decay rates over the long term, the active fraction needs to be quantified. Available literature data on this ‘‘classical’’ issue of biological wastewater treatments are extremely limited and case-specific, and mostly refer to conventional-activated sludge. MBRs were reported to have an active fraction between 4% and 7% with respect to the VSS in systems with SRT around 35–50 days (Guglielmi, 2002). This fraction refers to steady state conditions, and its application to complete retention MBR has to take into account the evolution over time of the biomass (and of the active fraction). In the present study, constant OLR, VSS, and endogenous respiration rate after 180 days of operation suggested that the concentration of active biomass was approximately constant. Exact determination of this fraction would have required sludge sampling that, at the experimental scale adopted, was not compatible with the assumption of complete sludge retention. However, some indications on the growth and decay of the biomass can also be obtained without information on the active fraction. Biomass build-up in biological systems is commonly represented by the Monod model where a decay term is introduced (Henze et al., 1987): dX S ¼ mX bX ¼ mmax X bX , dt kS þ S where X is the active fraction of the biomass, m is the growth rate, b the decay rate, S the substrate concentration, kS the half-saturation constant. According to this model, absence of biomass growth can be expresses as follows: dX m ¼ 0 ) m ¼ b3 ¼ 1. dt b Oxygen consumption during biological processes depends on the utilization external substrate and the endogenous biomass decay. The OUR can be defined accordingly, by separately considering these two contributions 1 Y0 X þ bX , OUR ¼ OURext þ OURend ¼ m Y0
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where Y 0 is the yield of substrate utilization for biomass production. The maximum OUR is obtained for maximum growth rate (i.e. when SbkS) and can be expressed as follows: 1 Y0 X þ bX . OURmax ¼ mmax Y0 When all the available external substrate has been consumed, the OUR is represented by the decay term bX (i.e. the oxygen requirement for endogenous respiration) OURend ¼ bX . The ratio between OURmax and OURend leads to the following expression: OURmax Y0 m ¼ max 1 0 OURend 1Y b According to this expression, the ratio between mmax and b is independent of the active fraction X . Therefore, when OURmax and OURend have comparable values, the maximum growth is similar to the biomass decay. The experimental ratio OURmax/OURend at the equilibrium was calculated to be 1.770.3. The adoption of a standard yield value of 0.67 mgCODbiom mgCOD1 sub (Henze et al., 1987) resulted in mmax/b ¼ 1.570.6. Since the actual growth rate m is smaller than the mmax , it could be assumed that the ratio m=b approaches one. The above considerations explain the absence of net biomass growth experimentally observed and confirm previous model predictions (Wintgens et al., 2003). The plot of the observed sludge yield as a function of the OLR over the whole experimental period evidenced a tendency of the yield to decrease for decreasing OLRs (Fig. 5). The experimental data showed that minimized OLR resulted in extremely limited yields, and that zero biomass yield could be reached, as previously observed under non-steady state conditions (van Houten and Eikelboom, 1997; Rosenberger et al., 2002). Although extremely low values of OLR are likely to take very long
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time to be reached, negligible yield can already be assumed for the operational loading rates around 0.07 gCOD gVSS1 d1 experimentally maintained, also taking into account the limited sludge losses occurred during the experiment. Absence of sludge accumulation means zero net biomass growth and also no accumulation of inorganic material. The amount of inorganics accumulated in the reactor, determined as the difference between the TSS and the VSS in the sludge, was compared with the cumulative inorganic material in the influent wastewater (Fig. 6). The ratio between these two non-volatile fractions was found to be rather constant (about 30%) for the initial 100 days, in accordance with previous findings on conventional activated sludge and nonsteady state MBR (Wentzel et al., 2002; Wagner and Rosenwinkel, 2000). Then the accumulation rate was observed to decrease, and finally resulted in a constant concentration of inorganics in the reactor over the long term. Also in this case, as for the other process parameters, the accumulation of inorganic material ceased after about 180 days, confirming that the studied system had reached equilibrium conditions. Constancy of the VSS/TSS ratio together with zero net growth provides an experimental evidence of previously proposed theories predicting the absence of excess sludge production under sufficiently long sludge retention times (Xing et al., 2003). No discernible accumulation of inorganics was already suggested only based on the observation of constant VSS/TSS ratio, although never experimentally demonstrated under steady state conditions (Muller et al., 1995). At the end of the experiment, the reactor was emptied to check for possible localized accumulation of solids due to mixing problems. None of such deposits was observed, confirming that the system was completely stirred and that the constant concentration of inorganics was not caused by ‘‘hidden’’ materials as in previous experiments (Rosenberger et al., 2002). Therefore, when
sludge yield (gVSS gCOD-1)
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.00
0.02
0.04
0.06 0.08 0.10 OLR (gCOD gVSS-1 d-1)
0.12
Fig. 5. Observed sludge yield vs. organic loading rate.
0.14
0.16
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81
240
cumulative influent inorganics
72
210
sludge inorganics
63
180
54
150
45
120
36
90
27
60
18
30
9
0 0
30
60
90
sludge inorganics (g)
influent inorganics (g)
270
0 120 150 180 210 240 270 300 330 360 time (days)
Fig. 6. Inorganic fractions in the influent wastewater and accumulated in the reactor.
high sludge concentrations were maintained, the results showed a limit value above which no accumulation of inorganic materials occurred in the reactor. This observation can be explained by considering that at sludge concentrations above 15 gVSS L1 the influent wastewater contributes for about 5–10% to the total amount of inorganics in the reactor, and this fraction could be involved in processes leading to solubilization.
5. Conclusions Long-term experiments with a complete retention MBR fed on real sewage at constant volumeric loading rate showed that equilibrium conditions were reached after about 180 days of operation and maintained for more than 150 days. Equilibrium was characterized by constancy of three key process parameters, i.e. biomass (VSS) and inorganic material concentrations and the specific bacterial activity. As a consequence, the organic loading rate reached values around 0.07 gCOD gVSS1 d1 in the long term, resulting in sludge yield values close to zero. This was confirmed by the results of the activity tests, that showed comparable values for growth and decay rates (mmax/b ¼ 1.570.6). No accumulation of inorganic material within the reactor was observed once the equilibrium was reached. Complete nitrification and good COD removal efficiencies were maintained over the whole experimental period. Moreover, adopting a typical flux and a simple membrane-cleaning procedure, the initial biomass growth and long-term operations at high sludge concentrations caused no significant variations of membrane permeability and cleaning needs. The experimental work showed the feasibility of operating MBR with complete sludge retention (or with SRT41000 d) from the standpoint of biological processes. Moreover, the reported data show that the
biomass evolves towards a minimized maintenance OLR, resulting in proportionality between the VLR and the equilibrium biomass concentration. When designing new plants, this feature allows for defining the biomass concentration based on the VLR. Larger scale studies are needed to evaluate all costrelated issues, including aeration, membrane cleaning and sludge treatability.
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