A hybrid moving bed biofilm reactor–membrane bioreactor (hybrid MBBR-MBR) system was studied as an alternative solution to conventional activated sludge processes and membrane bioreactors. This paper shows the results obtained from three laboratory-scale wastewater treatment plants working in parallel in the start-up and steady states. The first wastewater treatment plant was a MBR, the second one was a hybrid MBBR-MBR system containing carriers both in anoxic and aerobic zones of the bioreactor (hybrid MBBR-MBRa), and the last one was a hybrid MBBR-MBR system which contained carriers only in the aerobic zone (hybrid MBBR-MBRb). The reactors operated with a hydraulic retention time of 30.40 h. A kinetic study for characterizing heterotrophic biomass was carried out and organic matter and nutrients removals were evaluated. The heterotrophic biomass of the hybrid MBBR-MBRb showed the best kinetic performance in the steady state, with yield coefficient for heterotrophic biomass = 0.30246 mg volatile suspended solids per mg chemical oxygen demand, maximum specific growth rate for heterotrophic biomass = 0.00308 h−1 and half-saturation coefficient for organic matter = 3.54908 mg O2 L−1. The removal of organic matter was supported by the kinetic study of heterotrophic biomass.

INTRODUCTION

Advanced technologies for wastewater treatment have been developed to control stricter effluent limits or upgrade existing overloaded activated sludge plants (Wang et al. 2006). Several advantages are attributed to the membrane bioreactor (MBR) such as the increase of the organic loading rate, the reduction of the required space and the improvement of the effluent quality in relation to the conventional activated sludge processes according to Rodríguez et al. (2014), although maintaining membrane permeability and preventing fouling are the main problems of this technology (Judd 2006). Conversely, the moving bed biofilm reactor (MBBR) systems have been proved to be reliable for organic matter and nutrients removal without suffering the typical problems of suspended biomass processes (Ivanovic & Leiknes 2008), although the settleability of biosolids is their largest challenge (Ødegaard 2000). In these systems, biomass grows as biofilm attached to small plastic elements called carriers which keep moving inside the bioreactor by aeration in an aerobic reactor or by a mechanical stirrer in an anaerobic or anoxic reactor.

The moving bed biofilm reactor–membrane bioreactor (MBBR-MBR) has emerged as a highly effective biological process which solves the problems of the MBR and MBBR systems (Leiknes & Ødegaard 2007) regarding the fouling and settleability, respectively. These systems combine a biofilm reactor with a membrane bioreactor. The hybrid MBBR-MBR had suspended and attached biomass.

Regarding the kinetic modeling, there are still some uncertainties concerning the kinetic behavior of hybrid MBBR-MBR as the coexistence of suspended and attached biomass could lead to a modification in the kinetics of both biomasses, compared with processes involving pure suspended or attached biomass (Di Trapani et al. 2010).

The aim of this study was the analysis and comparison of the start-up, as well as the steady state, of an MBR configuration and two hybrid MBBR-MBR systems regarding the organic matter removal through the heterotrophic kinetics and the nutrient removal.

MATERIALS AND METHODS

Three laboratory-scale urban wastewater treatment plants (WWTPs) working in parallel were fed with municipal wastewater. The length of the study was 237 days. The first wastewater treatment plant consisted of an MBR (Figure 1(a)), the second one was a hybrid MBBR-MBR system containing carriers in the anoxic and aerobic zones of the bioreactor (hybrid MBBR-MBRa) (Figure 1(b)), and the last one consisted of a hybrid MBBR-MBR system which contained carriers only in the aerobic zone (hybrid MBBR-MBRb) (Figure 1(c)).
Figure 1

Diagram of the three pilot plants of municipal wastewater treatment. (a) MBR. (b) Hybrid MBBR-MBR containing carriers in the anoxic and aerobic zones (hybrid MBBR-MBRa). (c) Hybrid MBBR-MBR containing carriers only in the aerobic zone (hybrid MBBR-MBRb). (d) Nomenclature concerning the reactor zones, membrane tank, permeate tank and some peristaltic pumps.

Figure 1

Diagram of the three pilot plants of municipal wastewater treatment. (a) MBR. (b) Hybrid MBBR-MBR containing carriers in the anoxic and aerobic zones (hybrid MBBR-MBRa). (c) Hybrid MBBR-MBR containing carriers only in the aerobic zone (hybrid MBBR-MBRb). (d) Nomenclature concerning the reactor zones, membrane tank, permeate tank and some peristaltic pumps.

Municipal wastewater was pumped into the bioreactor from the influent tank. It went through the anoxic zone and the rest of the aerobic compartments by a communicating vessel system. The anoxic chamber was in the second compartment instead of the first because recycling from the membrane tank to the first compartment could change the anoxic conditions, as the mixed liquor of the membrane tank contained a higher concentration of dissolved oxygen to prevent membrane fouling. The stirrers in the anoxic zone and the diffusers in the aerobic zone homogenized the mixed liquor and kept the carriers moving in the bioreactor. Recycling from the membrane tank to the first chamber of the bioreactor was necessary for maintaining the working mixed liquor suspended solids (MLSS) concentration and allowing the nitrogen removal. The outlet of the bioreactor was led into the membrane tank and the permeate was extracted through the membrane. The sludge was considered as digested since the sludge retention time was 91 days due to the high hydraulic retention time (HRT), 30.40 h, and temperature, 22 °C (Table 1). Therefore, it would not be necessary to carry out a subsequent digestion treatment to stabilize it; the sludge would only have to be thickened and dehydrated. The operational conditions are shown in Table 1.

Table 1

Technical data, operational conditions and stabilization concentrations of MLSS, MLVSS, attached BD and VBD of the experimental plants

  MBR
 
Hybrid MBBR-MBRa
 
Hybrid MBBR-MBRb
 
Parameter Aerobic zone Anoxic zone Aerobic zone Anoxic zone Aerobic zone Anoxic zone 
Working volume of bioreactor (L) 18 18 18 
Filling ratio with carriers (%) 35 35 35 
Working volume of membrane tank (L) 4.32 4.32 4.32 
Flow rate (L h−10.93 0.93 0.93 
Hydraulic retention time (h) 30.40 30.40 30.40 
Sludge retention time (day) 91 91 91 
Total membrane area (m20.10 0.10 0.10 
Nominal pore size (μm) 0.4 0.4 0.4 
Membrane flux (L m−2 h−19.3 9.3 9.3 
MLSS (mg L−12,691.30 ± 114.99 1,569.87 ± 82.01 1,823.99 ± 51.11 
MLVSS (mg L−12,232.14 ± 95.37 1,321.50 ± 69.03 1,552.67 ± 43.50 
BD (mg L−1− 1,228.18 ± 75.89 880.00 ± 43.01 
VBD (mg L−1− 983.44 ± 60.77 720.21 ± 35.20 
  MBR
 
Hybrid MBBR-MBRa
 
Hybrid MBBR-MBRb
 
Parameter Aerobic zone Anoxic zone Aerobic zone Anoxic zone Aerobic zone Anoxic zone 
Working volume of bioreactor (L) 18 18 18 
Filling ratio with carriers (%) 35 35 35 
Working volume of membrane tank (L) 4.32 4.32 4.32 
Flow rate (L h−10.93 0.93 0.93 
Hydraulic retention time (h) 30.40 30.40 30.40 
Sludge retention time (day) 91 91 91 
Total membrane area (m20.10 0.10 0.10 
Nominal pore size (μm) 0.4 0.4 0.4 
Membrane flux (L m−2 h−19.3 9.3 9.3 
MLSS (mg L−12,691.30 ± 114.99 1,569.87 ± 82.01 1,823.99 ± 51.11 
MLVSS (mg L−12,232.14 ± 95.37 1,321.50 ± 69.03 1,552.67 ± 43.50 
BD (mg L−1− 1,228.18 ± 75.89 880.00 ± 43.01 
VBD (mg L−1− 983.44 ± 60.77 720.21 ± 35.20 

MLSS (mixed liquor suspended solids), MLVSS (mixed liquor volatile suspended solids), BD (biofilm density), VBD (volatile biofilm density).

Samples were collected from the influent, the three effluents and the anoxic and aerobic zones of the bioreactors and the membrane tanks every day. Chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD5), total suspended solids (TSS) and total phosphorus (TP) were measured in accordance with Standard Methods (APHA 2012). The assessment of TSS on the fixed biomass carriers was executed as follows: four representative plastic elements were extracted from the bioreactor, diluted in Tween 80, sonicated, centrifuged, washed off to separate the biomass and filtered, and the TSS concentration was assessed through the total number of carriers in a liter of reactor (Zhang et al. 2014). Total nitrogen (TN) was determined by ion chromatography. The kinetic parameters, yield coefficient for heterotrophic biomass (YH), maximum specific growth rate for heterotrophic biomass (μm,H), half-saturation coefficient for organic matter (KM) and decay coefficient for total biomass (kd), were assessed by respirometric experiments which allowed heterotrophic biomass to be characterized according to the procedure described by Leyva-Díaz et al. (2013).

SPSS 20.0 for Windows was used to determine the existence of statistically significant differences between the results concerning COD, BOD5, TSS, TN and TP by Tukey's honestly significant difference (HSD) post hoc procedure under the null hypotheses of independence and homogeneity with a significance level of 5%.

RESULTS AND DISCUSSION

The evolutions of MLSS and attached biofilm density (BD) during the start-up and steady states are shown in Figure 2.
Figure 2

Evolution of the mixed liquor suspended solids (MLSS) and attached biofilm density (BD) during the start-up and steady states. (a) MLSS from the MBR. (b) MLSS and BD from the hybrid MBBR-MBRa. (c) MLSS and BD from the hybrid MBBR-MBRb.

Figure 2

Evolution of the mixed liquor suspended solids (MLSS) and attached biofilm density (BD) during the start-up and steady states. (a) MLSS from the MBR. (b) MLSS and BD from the hybrid MBBR-MBRa. (c) MLSS and BD from the hybrid MBBR-MBRb.

The total time of the start-up and steady states was 110 days and 127 days, respectively, although the steady state was reached in less time in the MBR. The biomass concentration in the three laboratory-scale WWTPs was similar as the difference between the concentrations of MLSS in the laboratory-scale WWTPs was compensated for by the attached BD on the carriers contained in the hybrid MBBR-MBR systems. Sriwiriyarat & Randall (2005) conducted their research with similar values of MLSS and BD in integrated fixed-film activated sludge wastewater treatment processes. Mixed liquor volatile suspended solids (MLVSS) and volatile biofilm density (VBD) were used for the estimation of kinetic parameters (Table 1).

The average values of COD, BOD5, TSS, TN and TP of the influent of the experimental plants and the reduction percentages of these parameters in the start-up and steady states are shown in Table 2.

Table 2

Average values of COD, BOD5, TSS, TN and TP of the influent and removal percentages of the experimental plants in the start-up and steady states

  Sampling zone   Wastewater treatment plant
 
Parameter Influent Removal percentage MBR Hybrid MBBR-MBRa Hybrid MBBR-MBRb 
Start-up state 
COD (mg O2 L−1386.01 ± 136.64 COD (%) 85.10 ± 9.14 84.11 ± 11.05 86.60 ± 10.35 
BOD5 (mg O2 L−1240.00 ± 88.85 BOD5 (%) 95.00 ± 3.10 93.66 ± 5.35 95.92 ± 2.37 
TSS (mg L−1172.63 ± 89.60 TSS (%) 95.82 ± 4.76 93.04 ± 8.79 96.07 ± 3.79 
TN (mg N L−1109.42 ± 23.18 TN (%) 48.96 ± 17.69 42.18 ± 19.84 48.53 ± 20.08 
TP (mg P L−112.68 ± 6.20 TP (%) 39.86 ± 26.20 43.15 ± 20.93 37.46 ± 29.30 
Steady state 
COD (mg O2 L−1336.08 ± 104.48 COD (%) 90.75 ± 3.30 90.83 ± 3.53 91.71 ± 2.59 
BOD5 (mg O2 L−1262.78 ± 80.78 BOD5 (%) 98.18 ± 1.01 98.18 ± 0.84 98.21 ± 0.85 
TSS (mg L−1157.56 ± 65.71 TSS (%) 95.62 ± 4.67 94.82 ± 6.33 94.28 ± 8.27 
TN (mg N L−199.17 ± 36.50 TN (%) 63.06 ± 8.42 61.80 ± 11.95 64.07 ± 8.69 
TP (mg P L−110.15 ± 4.50 TP (%) 36.16 ± 18.31 38.74 ± 16.57 41.30 ± 14.07 
  Sampling zone   Wastewater treatment plant
 
Parameter Influent Removal percentage MBR Hybrid MBBR-MBRa Hybrid MBBR-MBRb 
Start-up state 
COD (mg O2 L−1386.01 ± 136.64 COD (%) 85.10 ± 9.14 84.11 ± 11.05 86.60 ± 10.35 
BOD5 (mg O2 L−1240.00 ± 88.85 BOD5 (%) 95.00 ± 3.10 93.66 ± 5.35 95.92 ± 2.37 
TSS (mg L−1172.63 ± 89.60 TSS (%) 95.82 ± 4.76 93.04 ± 8.79 96.07 ± 3.79 
TN (mg N L−1109.42 ± 23.18 TN (%) 48.96 ± 17.69 42.18 ± 19.84 48.53 ± 20.08 
TP (mg P L−112.68 ± 6.20 TP (%) 39.86 ± 26.20 43.15 ± 20.93 37.46 ± 29.30 
Steady state 
COD (mg O2 L−1336.08 ± 104.48 COD (%) 90.75 ± 3.30 90.83 ± 3.53 91.71 ± 2.59 
BOD5 (mg O2 L−1262.78 ± 80.78 BOD5 (%) 98.18 ± 1.01 98.18 ± 0.84 98.21 ± 0.85 
TSS (mg L−1157.56 ± 65.71 TSS (%) 95.62 ± 4.67 94.82 ± 6.33 94.28 ± 8.27 
TN (mg N L−199.17 ± 36.50 TN (%) 63.06 ± 8.42 61.80 ± 11.95 64.07 ± 8.69 
TP (mg P L−110.15 ± 4.50 TP (%) 36.16 ± 18.31 38.74 ± 16.57 41.30 ± 14.07 

COD (chemical oxygen demand), BOD5 (five-day biochemical oxygen demand), TSS (total suspended solids), TN (total nitrogen), TP (total phosphorus).

The removal percentages of COD, BOD5 and TN were lower in the start-up phase than those obtained in the steady state. There were not statistically significant differences between the laboratory-scale WWTPs concerning these parameters in the start-up and steady states as the p-values obtained from the post hoc procedure, Tukey's HSD, were higher than α = 0.05. The removal percentages of BOD5 and TN were lower with an HRT of 30.40 h than those obtained with an HRT of 26.47 h by Leyva-Díaz et al. (2013), who studied higher biomass concentrations, with values of total biomass concentration ranging from 3,500 mg L−1 to 4,500 mg L−1. The values relating to TSS were very similar in the start-up and steady states, as the laboratory-scale WWTPs contained a module including hollow-fiber microfiltration membranes in the membrane tank. The removal percentages of TP were low in the laboratory-scale WWTPs as there was not a strict anaerobic zone to initialize the process of biological phosphorus removal (Kermani et al. 2009), although small anaerobic zones were created in the anoxic compartments of the bioreactor, which made phosphorus removal possible together with the physical process of the membrane separation.

Kinetic parameters for the characterization of heterotrophic biomass in the start-up and steady states are shown in Table 3.

Table 3

Kinetic parameters for the characterization of heterotrophic biomass in the start-up and steady states of the experimental plants

  Sampling zone
 
Parameter MBR Hybrid MBBR-MBRa Hybrid MBBR-MBRb 
Start-up state 
YH (mg VSS mg COD−10.40002 0.42942 0.45923 
μm,H (h−10.07006 0.01849 0.01726 
KM (mg O2L−154.87864 23.07049 20.65062 
kd (d−10.03502 0.10334 0.10317 
Steady state 
YH (mg VSS mg COD−10.27975 0.34526 0.30246 
μm,H (h−10.00284 0.00440 0.00308 
KM (mg O2 L−14.74640 10.83096 3.54908 
kd (d−10.03326 0.02304 0.02074 
  Sampling zone
 
Parameter MBR Hybrid MBBR-MBRa Hybrid MBBR-MBRb 
Start-up state 
YH (mg VSS mg COD−10.40002 0.42942 0.45923 
μm,H (h−10.07006 0.01849 0.01726 
KM (mg O2L−154.87864 23.07049 20.65062 
kd (d−10.03502 0.10334 0.10317 
Steady state 
YH (mg VSS mg COD−10.27975 0.34526 0.30246 
μm,H (h−10.00284 0.00440 0.00308 
KM (mg O2 L−14.74640 10.83096 3.54908 
kd (d−10.03326 0.02304 0.02074 

YH (yield coefficient for heterotrophic biomass), μm,H (maximum specific growth rate for heterotrophic biomass), KM (half-saturation coefficient for organic matter), kd (decay coefficient for total biomass).

The amount of heterotrophic biomass produced per substrate oxidized, measured by YH, in the bioreactors of the laboratory-scale WWTPs was higher in the start-up phase because of the biomass growth during this period of time. The heterotrophic biomass of the MBR had a better kinetic performance in the start-up phase when the substrate degradation rate, rsu, was evaluated according to Leyva-Díaz et al. (2014) depending on the kinetic parameters, biomass concentration and substrate concentration (Figure 3(a)), as the biomass required more time to grow on the carriers in the hybrid MBBR-MBR systems (hybrid MBBR-MBRa and hybrid MBBR-MBRb). It involved the steady state being reached in less time in the MBR as observed in Figure 2. The hybrid MBBR-MBRb showed the best kinetic behavior of heterotrophic biomass in the steady state when rsu was evaluated (Figure 3(b)) under the operational conditions of this study, with values of YH = 0.30246 mg VSS mg COD−1, μm,H = 0.00308 h−1 and KM = 3.54908 mg O2 L−1. Thus, the heterotrophic biomass from the hybrid MBBR-MBRb required less time for organic matter oxidation, and the maximum specific growth rate was achieved with less available substrate.
Figure 3

Substrate degradation rate (rsu) obtained in the heterotrophic kinetic study depending on the substrate concentration (S) for the different bioreactors from the laboratory-scale WWTPs. (a) Start-up phase. (b) Steady state.

Figure 3

Substrate degradation rate (rsu) obtained in the heterotrophic kinetic study depending on the substrate concentration (S) for the different bioreactors from the laboratory-scale WWTPs. (a) Start-up phase. (b) Steady state.

These results supported the percentages of organic matter removal of the hybrid MBBR-MBRb, which showed values of 91.71 ± 2.59% and 98.21 ± 0.85% for the COD and BOD5 removal in the steady state. Heterotrophic kinetics indicated that the attached biomass provided an extra contribution to the organic matter removal. Thus, an anoxic zone without carriers was necessary to oxidize the organic matter in less time and to achieve the maximum specific growth rate with less available substrate. In light of this, the nitrification and denitrification processes could be facilitated by the absence of carriers in the anoxic zone, which provided better contact between nitrate and the microorganisms (Larrea et al. 2007). The hybrid MBBR-MBRb had a performance of TN removal of 64.07 ± 8.69% (Table 2). Similar values of these heterotrophic kinetic parameters for an MBBR and fluidized bed biofilm reactor were obtained in other studies (Ferrai et al. 2010; Seifi & Fazaelipoor 2012). The decay coefficient for the biomass contained in the bioreactors was lower in the steady state of the three laboratory-scale WWTPs as the systems were stabilized. Therefore, the total quantity of biomass oxidized per day was higher in the start-up phase.

CONCLUSIONS

The preferable system to remove organic matter in stable conditions was the hybrid MBBR-MBRb, which contained carriers only in the aerobic zone of the bioreactor, as its biomass, both suspended and attached, facilitated the removal of organic substrate faster than the suspended biomass from the MBR, according to the heterotrophic kinetics. Therefore, operational costs could be optimized regarding the HRT, which may be reduced. However, the main disadvantage of the hybrid MBBR-MBRb is the associated cost of the carrier and its higher time to reach the steady state since the biomass had to be developed on the carriers.

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