A novel process, inclined-plates hydrolytic tank (IHT) and membrane bioreactor (MBR), was used to treat domestic sewage continuously. In this study, the effects of carriers' addition on operational performances of IHT-MBR were studied at the hydraulic retention time of 5.4 h and the recycling rate of 200%. Experimental results indicated the removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus reached 86.8%, 82.9% and 89.6%, respectively, corresponding trans-membrane pressure decreased to 1.50 kPa/d at a packing ratio of 20%. Simultaneously, the scanning electron microscope and soluble microbial products analysis demonstrated that high nutrient removal and low membrane fouling were attributed to the attached growth of microorganisms on carriers. The bioattachment and adsorption of carriers not only decreased the soluble proteins and polysaccharide in MBR, but also provided good living environments for denitrifying bacteria and phosphorus-accumulating bacteria.

INTRODUCTION

The discharge of wastewater with high nitrogen and phosphorus is the main reason for water eutrophication. In this way, biological nutrient removal (BNR) processes have been widely used in existing wastewater treatment plants (WWTPs) to protect receiving waters (Lee et al. 2015). However, the discharge standards of WWTPs in China become more and more rigorous, and the demands for novel and advanced technologies for wastewater treatment are more urgent.

Membrane bioreactor (MBR) is considered as a perfect integration of a biological process and membrane separation process, which is widely used in municipal and industry wastewater treatment (Jensen et al. 2015; Leyva-Díaz et al. 2016). Recently, some biological processes such as moving bed biofilm reactor, inclined-plates and anaerobic-anoxic-oxic configuration are combined with MBR, enhancing the simultaneous organic and nutrient removal of municipal wastewater (Falahti-Marvast & Karimi-Jashni 2015; Reboleiro-Rivas et al. 2015). Xing et al. (2006) discovered that the application of inclined-plates effectively controlled the concentration of mixed liquor suspended solids (MLSS) in MBR, and the average removals of chemical oxygen demand (COD) and total nitrogen (TN) reached 92.1% and 71.7%, respectively.

MBR has some advantages such as an excellent effluent, process control and small footprint (Drews 2010). However, membrane fouling increases the operational costs (high energy and chemical costs) and shortens the life of the membrane, and as a result, it becomes a key obstacle that restricts its widespread application in wastewater treatment (Le-Clech et al. 2006). In general, membrane fouling is correlated with cake layer caused by sludge particle deposition on the membrane surface, the gel layer caused by adhesion of macromolecules to the membrane surface, and pore clogging caused by small molecules (Huang et al. 2008). Some studies demonstrated that the cake layer was the main contributor to membrane fouling in MBR (Lee et al. 2003), and the lower MLSS resulted in less membrane fouling (Xing et al. 2006).

Therefore, a number of methods have been undertaken to control membrane fouling, among which, adding suspended carrier with specific characteristics into MBR is a promising technology (Wei et al. 2006; Jin et al. 2013). The dissolved organic substances considered as important contributors to membrane fouling can be adsorbed to the carriers, leading to the reduction of suspended solid concentration in MBR (Leiknes & Ødegaard 2007). It was also reported that effective carrier dose played a very important role in mitigating membrane fouling in MBR (Huang et al. 2008). However, the impacts of carriers on both membrane fouling and BNR in inclined-plates hydrolytic tank (IHT) and MBR has not been discussed.

The objectives of this study were to investigate the effects of carriers packing ratios on the nutrient removal performance and the membrane fouling in IHT-MBR process. The COD, TN, and total phosphorus (TP) removal performances in IHT and MBR were investigated, respectively. Also, the trans-membrane pressure (TMP) was applied to analyze membrane fouling at different carriers packing ratios. Especially, the scanning electron microscope (SEM) and soluble microbial products (SMP) analysis were used to explore the mechanism for good nutrient removal and membrane fouling mitigation.

EXPERIMENTAL MATERIALS AND METHODS

Experimental setup and operation

The IHT-MBR process used in this study was shown in Figure 1. The effective volumes and the hydraulic retention time (HRT) of IHT and MBR were 9 L and 5.4 h, respectively. Nine inclined-plates were set at the upper part of the IHT with a horizontal angle of 60 degrees. The polyvinylidene fluoride hollow fiber membranes with nominal pore size 0.2 μm and surface area 0.5 m2 (Beijing Origin Water, China) was immersed vertically into the MBR. The dissolved oxygen (DO) in MBR was controlled at 2.5–4 mg/L. The influent was fed continuously with flow of 40 L/d, and the effluent was withdrawn from MBR using a suction pump (Masterflex, Cole-Parmer) in intermittent mode with on/off ratio of 8 min:2 min. The recycling rate was controlled at 200%.
Figure 1

Schematic diagram of inclined-plates and MBR process. 1-influent tank, 2-peristaltic pump, 3-hydrolytic tank, 4-stirrer, 5-sample viewing ports, 6-electromagnetic valve, 7-aerobic MBR, 8-pressure gauge, 9-aeration pump, 10-air flow meter, 11-automatic control cabinet.

Figure 1

Schematic diagram of inclined-plates and MBR process. 1-influent tank, 2-peristaltic pump, 3-hydrolytic tank, 4-stirrer, 5-sample viewing ports, 6-electromagnetic valve, 7-aerobic MBR, 8-pressure gauge, 9-aeration pump, 10-air flow meter, 11-automatic control cabinet.

The carriers used in this study were made of polyether and polyester with a cubic shape (16 mm × 16 mm × 16 mm). Carriers with packing ratios of 20% and 40% were respectively added into MBR to investigate nutrient removal performances and membrane fouling. The IHT-MBR process at different packing ratio continuously operated for 30 days at room temperature.

Water quantity

The domestic sewage was obtained from the residential area of Harbin Institute of Technology. The influent qualities of IHT-MBR were summarized as follows: COD 212.42–322.28 mg/L, ammonia nitrogen (NH4+-N) 35.68–47.78 mg/L, TN 40.67–55.71 mg/L, TP 3.22–5.99 mg/L, pH 6.95–7.40, and alkalinity 211.2–389.7 mgCaCO3/L.

Analytical methods

Influent and effluent samples of IHT and MBR were analyzed every other day. The COD, NH4+-N, TN, TP and MLSS concentrations were determined according to Standard Methods (APHA 2005). DO was monitored using WTW Handheld Dissolved Oxygen Analyzer (JPB-607, Shanghai, China). SMP were collected and extracted based on the reference (Zhang et al. 2015), and the concentrations of proteins (PN) and polysaccharide (PS) in SMP were analyzed, respectively. The PN concentrations were measured by the Lowry method with the bovine serum albumin as the standard (Lowry et al. 1951), and the PS concentrations were analyzed by the phenol-sulfuric method with the glucose as the standard (Herbert et al. 1971).

RESULTS AND DISCUSSION

Overall removal performances

The removal performances of COD, TN and TP at different packing ratios are described in Figure 2. It can be seen that the effluent COD concentrations were below 50 mg/L with influent COD ranging from 212.6 to 322.3 mg/L, and corresponding average COD removal efficiencies were over 85%. Also, obvious changes of TN and TP removal efficiencies were observed as a result of adding carriers into MBR. For example, the TN and TP removal efficiencies enhanced to 82.9% and 89.6% from 64.7% and 46.2%, respectively, when the carrier packing ratio increased to 20%. The high TN and TP removal at a packing ratio of 20% should be attributed to the matured biofilm in carriers (in Figure 3), which might promote the retention of nitrifying bacteria and phosphate accumulation organisms (PAOs) in MBR. Notably, the TN and TP removal efficiencies remained relatively steady with carrier packing ratio continuously increased to 40% from 20%. It is assumed that excess carriers could not be suspended and scattered by hydraulic action, which might restrain the contacts between microorganisms and organics, and the mass transfer among oxygen, liquid and solids (Ferraris et al. 2009).
Figure 2

Changes of (a) COD, (b) TN, and (c) TP at different carriers packing ratios.

Figure 2

Changes of (a) COD, (b) TN, and (c) TP at different carriers packing ratios.

Figure 3

SEM images of internal surface of (a) new and (b) matured carriers.

Figure 3

SEM images of internal surface of (a) new and (b) matured carriers.

TN and TP removal in IHT and MBR

In order to explore the TN and TP removal reason in a further step, the changes of TN and TP removal in IHT and MBR were separately investigated. As is shown in Figure 4(a), the TN removal in IHT were over 30 mg/L at any carrier packing ratios, which were higher than that in MBR (<5 mg/L). It might be because the anoxic environment in the IHT was beneficial for the growth of denitrifying bacteria (Guo et al. 2013). Meantime, the TN removal in MBR increased from 0.4 to 4.9 mg/L as the carriers packing ratios increased from 0% to 20%. It can be seen from Figure 4(b), the TP removal was accomplished in both IHT and MBR. For instance, the TP removal in IHT and MBR enhanced to 2.18 and 1.82 mg/L from 0.86 and 1.3 mg/L, respectively, when the packing ratio increased from 0% to 20%. As it is reported, a decreased oxygen gradient from exterior to interior in biofilms provided aerobic-anoxic-anaerobic microenvironment for denitrifying bacteria and PAOs (Yang et al. 2012). In this study, adding carriers into MBR was beneficial for the growth of PAOs and denitrifying bacteria.
Figure 4

(a) TN and (b) TP removal performances in hydrolytic tank and MBR.

Figure 4

(a) TN and (b) TP removal performances in hydrolytic tank and MBR.

Membrane fouling

The TMP at different carrier packing ratios were also described in Figure 5. It was obvious that the TMP changed with the increase of carriers packing ratio. For example, when the packing ratio increased from 0% to 20%, the gradient of TMP decreased from 2.16 to 1.50 kPa/d, while the gradient of TMP suddenly increased to 4.18 kPa/d with the packing ratio continuously increasing to 40%. As we know, the TMP was positively related to suspended solid (Praneeth et al. 2014). In this study, more activated sludge attached to the surface of carriers at a packing ratio of 20%, making the MLSS in MBR decrease to 3,441 from 9,686 mg/L (without carriers addition). It is a remarkable fact that the rub between carriers and membrane module was enhanced at a packing ratio of 40%, which led to biofilms detachment and MLSS increase.
Figure 5

Changes of TMP at different carries packing ratios.

Figure 5

Changes of TMP at different carries packing ratios.

SMP analysis

It is well known that PN and PS are the main compositions of SMP, which may attach to and then accumulate on the membrane surface to form a cake or block the pore (Tansel et al. 2006; Zhang et al. 2015). Therefore, the changes of PN and PS in MBR were analyzed in Figure 6. The PN and PS contents were significantly changed by adding carriers. For example, the PN and PS contents with no carrier addition were 5.67 and 1.96 mg/L, respectively, and corresponding mass ratio of PN and PS (PN/PS) was 2.89. After adding carriers into MBR, a large number of sludge flocs attached to the surfaces of carriers (Figure 3), and the minimum contents of PN and PS were 1.45 and 1.33 mg/L at the packing ratio of 20%. The PN and PS contents increased as the packing ratio increased to 40%, and the corresponding PN/PS decreased to 0.45. The decrease of PN contents might be due to two aspects. For one thing, since PN content in the cell is more than PS, the attached growth of microorganisms decreased the PN in the supernatant. For another, the carriers made of hydrophobic polyether and polyester might be easier to absorb PN than PS. The similar conclusion was drawn in a biofilm membrane bioreactor (BF-MBR) (Ivanovic & Leiknes 2012).
Figure 6

Changes of PN and PSs in MBR at different carrier packing ratios.

Figure 6

Changes of PN and PSs in MBR at different carrier packing ratios.

CONCLUSIONS

A combined process of IHT-MBR was applied to treat domestic sewage. The nutrients were effectively removed, and the membrane fouling was remitted by adding carriers into MBR at the HRT of 5.4 h and the recycling rate of 200%. The optimal packing ratio was determined to be 20%, which provided a good living environment for denitrifying bacteria and PAOs. Simultaneously, the SEM analysis indicated carriers were beneficial for attached growth of micrograms, which decreased the MLSS in the supernatant of MBR. Also, more PN were absorbed on the surfaces of carriers contributing to the mitigation of membrane fouling.

ACKNOWLEDGEMENTS

This work was financially supported by the National High Technology Research and Development Program of China (863 Program) (No. 2012AA063503), and Natural Science Foundation of Jiangsu Province (No. BK20160937).

REFERENCES

APHA
2005
Standard Methods for the Examination of Water and Wastewater
.
American Public Health Association (APHA)
,
Washington, DC
,
USA
.
Guo
H.
Chen
J.
Li
Y.
Feng
T.
Zhang
S.
2013
Nitrogen and phosphorus removal in an airlift intermittent circulation membrane bioreactor
.
J. Environ. Sci.
25
(
Supplement 1
),
S146
S150
.
Herbert
D.
Philipps
P.
Strange
R.
1971
Carbohydrate analysis
.
Methods Enzymol.
B 5
,
265
277
.
Ivanovic
I.
Leiknes
T. O.
2012
The biofilm membrane bioreactor (BF-MBR)-a review
.
Desalin. Water Treat.
37
,
288
295
.
Jensen
P. D.
Yap
S. D.
Boyle-Gotla
A.
Janoschka
J.
Carney
C.
Pidou
M.
Batstone
D. J.
2015
Anaerobic membrane bioreactors enable high rate treatment of slaughterhouse wastewater
.
Biochem. Eng. J.
97
,
132
141
.
Le-Clech
P.
Chen
V.
Fane
T. A. G.
2006
Fouling in membrane bioreactors used in wastewater treatment
.
J. Membrane Sci.
284
,
17
53
.
Leiknes
T.
Ødegaard
H.
2007
The development of a biofilm membrane bioreactor
.
Desalination
202
,
135
143
.
Lowry
O. H.
Rosebrough
N. J.
Farr
A. L.
Randall
R. J.
1951
Protein measurement with the Folin phenol reagent
.
J. Biol. Chem.
193
,
265
275
.
Praneeth
K.
Moulik
S.
Vadthya
P.
Bhargava
S. K.
Tardio
J.
Sridhar
S.
2014
Performance assessment and hydrodynamic analysis of a submerged membrane bioreactor for treating dairy industrial effluent
.
J. Hazard. Mater.
274
,
300
313
.
Reboleiro-Rivas
P.
Martín-Pascual
J.
Juárez-Jiménez
B.
Poyatos
J. M.
Vílchez-Vargas
R.
Vlaeminck
S. E.
Rodelas
B.
González-López
J.
2015
Nitrogen removal in a moving bed membrane bioreactor for municipal sewage treatment: community differentiation in attached biofilm and suspended biomass
.
Chem. Eng. J.
277
,
209
218
.
Wei
C. H.
Huang
X.
Wang
C. W.
Wen
X. H.
2006
Effect of a suspended carrier on membrane fouling in a submerged membrane bioreactor
.
Water Sci. Technol.
53
,
211
220
.
Yang
F.
Wang
Y.
Bick
A.
Gilron
J.
Brenner
A.
Gillerman
L.
Herzberg
M.
Oron
G.
2012
Performance of different configurations of hybrid growth membrane bioreactor (HG-MBR) for treatment of mixed wastewater
.
Desalination
284
,
261
268
.