A membrane bioreactor (MBR) was used for treating biological aerated filter effluent in a municipal wastewater plant, and chemical phosphorus removal was accomplished in the MBR. The results showed that ferric chloride of 20 mg/L and aluminum sulfate of 30 mg/L were the optimal dosages for total phosphorus (TP) removal, and the TP removal efficiency was over 80%. In long-term continuous operations, both ferric chloride and aluminum sulfate effectively mitigated membrane fouling, with the corresponding growth rate of transmembrane pressure decreased to 0.08 and 0.067 kPa/d, respectively. Sludge particle sizes analysis demonstrated that the decrease of particle sizes lower than 50 μm was the main reason for membrane fouling control. Simultaneously, the proteins and polysaccharide (PS) concentrations in the MBR supernatant were analyzed, and the PS concentration significantly decreased to 2.02 mg/L at aluminum sulfate of 30 mg/L, indicating the flocculation of aluminum sulfate on PS was the main reason for mitigation of membrane fouling.

INTRODUCTION

The reuse of municipal wastewater is the goal of modern and sustainable wastewater treatment systems (Verstraete et al. 2009). However, the majority of municipal wastewater treatment plants (WWTPs) in China are designed on the basis of the activated sludge process, with effluent total phosphorus (TP) of 2–4 mg/L. Even in an enhanced biological phosphorus removal process, the effluent TP concentrations are usually in the range of 0.5–1.0 mg/L (Piekema 2004), which does not meet the requirements of reuse in China.

Recently, membrane bioreactor (MBR) technology has been used for wastewater reuse due to its ability to improve the removal of refractory organic matters whilst possessing a small footprint (Ferraris et al. 2009). To prevent eutrophication, some chemical agents such as iron and aluminum salts are added into MBRs to achieve a sufficient and consistent degree of phosphorus removal (Yeoman et al. 1988; Gutierrez et al. 2010). Iron salt, in either ferrous or ferric form, is commonly used in municipal wastewater treatment as an aid to phosphorus removal, and the influent TP concentrations of 10 mg/L are consistently reduced to effluent concentrations of 0.03–0.04 mg/L at Fe/P (both ferric and ferrous) molar ratio of 2.0 (Zhang et al. 2015). Simultaneously, the addition of ferrous salt has been demonstrated to be a good alternative to ferric salt for phosphorus removal in a pilot-scale MBR (Wang et al. 2014). It was also reported that 30 mg/L of alum was effective to remove 3 mg/L of phosphorous in feed for the MBR process (Song et al. 2008). Results suggested that ferric chloride has more advantages in phosphorus removal than aluminum chloride.

However, the wide use of MBRs in wastewater treatment is still limited by membrane fouling, which decreases permeability and, in turn, increases energy consumption (Wang et al. 2014). Several experimental studies indicated that the mixed liquor colloidal and soluble microbial products (SMP) were mainly responsible for membrane fouling (Zhang et al. 2011; Wu et al. 2012). The SMP are the metabolic products of microorganisms consisting of proteins (PN) and polysaccharides (PS), which may attach to and then accumulate on the membrane surface to form a biocake layer (Tansel et al. 2006; Zhang et al. 2015). Recently, more strategies for fouling control have focused on the chemical coagulation/flocculation process to remove suspended solids (SS), colloidal particles, as well as other soluble materials in MBRs (Wu et al. 2006; Gkotsis et al. in press). The surface negative charges of microbial flocs change to almost neutral along with cationic coagulants added into mixed liquid, which is beneficial for producing larger flocs, and greatly reducing transmembrane pressure (TMP) (Praneeth et al. 2014). Additionally, it has been found that a low coagulants dosage is sufficient for mitigating membrane fouling, and that ferric salts are more effective than aluminum salts in membrane pollution control (Mishima & Nakajima 2009).

A comprehensive review of the literature reveals that MBRs have been commonly used for sewage or municipal wastewater treatment, but there is little published data available with regard to comparison of ferric chloride and aluminum sulfate on phosphorus removal in MBRs for municipal wastewater advanced treatment. Therefore, in this study, MBR technology was used to treat biological aerated filter (BAF) effluent in a municipal wastewater plant. The effects of different ferric chloride and aluminum sulfate dosages on TP removal in the MBR were investigated, and the appropriate dosages for these two coagulants were determined. Simultaneously, the TP removal and TMP at optimal ferric and aluminum salts were respectively studied by long-term continuous operational experiment. Also, the particle size and SMP analysis were used to explain the mechanism for membrane fouling control.

MATERIALS AND METHODS

MBRs

Two identical MBRs with effective volume of 5 L were operated at room temperature. These two MBRs were controlled at the same conditions except two different coagulants, ferric chloride and aluminum sulfate, were continuously added into the MBRs, respectively. Two identical U-shaped bundle membrane modules of polyvinylidene fluoride hollow fiber membranes with nominal pore size 0.2 μm and total surface area 0.3 m2 (Beijing Origin Water, China) were immersed vertically into the MBR with coarse bubble aeration used to limit membrane fouling. The influent was fed continuously, and the effluent was withdrawn from the membrane modules using a suction pump (Masterflex, Cole-Parmer) in intermittent mode with on/off ratio of 8 min:2 min. The gas–water ratio and flux were controlled at 10:1 and 10–15 L/(m2·h), respectively. The sludge retention time was 40 d, and the hydraulic retention time was 2.4 h.

Feedwater characteristics

The feedwater used in this study was from a BAF reactor of a municipal WWTP in Guangdong. The influent characteristics of the MBRs including pH, chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN), TP and SS are summarized in Table 1.

Table 1

The feedwater characteristics of MBRs

Item Feedwater characteristics 
pH 6.5–8 
COD (mg/L) 20–50 
NH4+-N (mg/L) 5–15 
TN (mg/L) 10–17 
TP (mg/L) 1.5–2 
SS (mg/L) ≤15 
Item Feedwater characteristics 
pH 6.5–8 
COD (mg/L) 20–50 
NH4+-N (mg/L) 5–15 
TN (mg/L) 10–17 
TP (mg/L) 1.5–2 
SS (mg/L) ≤15 

Ferric chloride and aluminum sulfate dosing

The TP removal and the membrane fouling after adding ferric chloride and aluminum sulfate solutions into the MBRs were assessed. The ferric and aluminum concentrated feeds were separately prepared by dissolving 8 g ferric chloride heptahydrate and 8 g aluminum sulfate octadecahydrate in 2 L tap water. Then, the same tap water was used for diluting the concentrated feed to be ferric and aluminum solutions of 0, 5, 10, 15, 20, 25 and 30 mg/L, respectively. Then, the MBRs at ferric and aluminum salts dosage of 20 and 30 mg/L were continuously operated for 100 days to compare the TP removal and membrane fouling control, respectively. Simultaneously, a MBR reactor without the addition of coagulants was set as the blank test. In this study, the pH values in reactors were adjusted and maintained at 6.5–7.5 using 2 M hydrochloric acid (HCl) or 2 M sodium hydroxide (NaOH) solutions.

Analytical methods

Influent and effluent samples of MBRs were analyzed every other day. The COD, NH4+-N, TN and TP concentrations were determined according to Standard Methods (APHA 2005). Dissolved oxygen was analyzed using WTW Handheld Multi-parameter Instruments (pH/Oxi 340i, WTW, Germany). pH, and oxidation reduction potential were monitored using on-line detection (pHs-8D, Dongrun, China). The SMP were collected and extracted based on the reference (Zhang et al. 2015), and the concentrations of PN and PS in SMP were analyzed, respectively. The PN concentrations were measured by the Lowry method with bovine serum albumin (BSA) as standard (Lowry et al. 1951), and the PS concentrations were analyzed by the phenol-sulfuric method with glucose as the standard (Herbert et al. 1971). The SMP analysis was performed in triplicate, and the average value was calculated.

RESULTS AND DISCUSSION

Effects of coagulant dosages on TP removal

Changes of TP concentrations and removal efficiencies with different ferric chloride and aluminum sulfate addition are shown in Figure 1. It was obvious that TP removal efficiencies first rapidly increased with the increase of ferric chlorine dosages, but gradually leveled off at ferric chlorine over 20 mg/L. For instance, TP removal efficiencies increased from an average 20.3% to 84.2% with ferric chloride increasing from 0 to 20 mg/L, corresponding effluent TP concentrations decreased from an average 1.42 to 0.29 mg/L. As shown in Figure 1(b), the TP removal efficiencies obviously increased from 16.4% to 80.3% when aluminum sulfate dosages increased from 0 to 30 mg/L, corresponding effluent TP concentrations decreased from an average 1.61 to 0.33 mg/L. Additionally, no significant increase of TP removal efficiency was observed when aluminum sulfate continuously increased to 35 mg/L. Results suggested that both ferric chloride and aluminum sulfate effectively reduced the TP concentrations, and the optimal dosages for ferric chloride and aluminum sulfate were 20 and 30 mg/L, respectively. The decrease of TP is due to the high surface area particulate oxyhydroxides formed after ferric or aluminum salt addition, which reduced ξ potential, increased the flocculating ability of colloidal materials, and further enhanced P scavenging ability (Caravelli et al. 2010).
Figure 1

Effects of (a) ferric chloride and (b) aluminum sulfate dosages on TP removal.

Figure 1

Effects of (a) ferric chloride and (b) aluminum sulfate dosages on TP removal.

Effects of continuous dosing on MBR operation

Figure 2 shows the TP removal performances and membrane fouling during the complete running period. In Figure 2(a), the average TP removal efficiency was 83.9% and 80.2% at ferric chlorine of 20 mg/L and aluminum sulfate of 30 mg/L, respectively. The TP removal efficiencies with ferric or aluminum addition was 4–5 times higher than that in the blank test, indicating that ferric chlorine and aluminum sulfate significantly enhanced the TP removal. Simultaneously, the TMP was decreased obviously by ferric chloride and aluminum sulfate (Figure 2(b)). The growth rates of TMP with ferric chloride of 20 mg/L and aluminum sulfate of 30 mg/L were 0.08 and 0.067 kPa/d, respectively, less than that in the blank test (0.105 kPa/d). Although ferric chlorine showed its advantage in TP removal, the aluminum sulfate had greater potential in controlling membrane fouling. Similar results have been reported in previous research (Holbrook et al. 2004; Park et al. 2006). The decrease of TMP might be attributed to the good treatment effect of BAF which led to a small quantity of microorganisms growing in the MBR, and therefore, not enough SMP were linked to the coagulants (Mishima & Nakajima 2009).
Figure 2

Effects of ferric and aluminum salts continuous dosing on TP removal and TMP.

Figure 2

Effects of ferric and aluminum salts continuous dosing on TP removal and TMP.

Comparison of ferric chloride and aluminum sulfate on sludge particle sizes

The changes of sludge particle sizes after ferric and aluminum addition are shown in Figure 3. In the blank test, the sludge particle sizes in the MBR were in the range of 0 to 100 μm, and the average particle size was 53.9 μm. The particle sizes with ferric and aluminum salts addition varied from 0 to 300 μm, and the average particle sizes were 127.6 and 148.1 μm, respectively. In addition, the cumulative percentage of particle sizes lower than 50 μm with ferric and aluminum salts addition were 13.5% and 7.2%, while that in the blank test was approximately 50%.
Figure 3

Changes of sludge particle sizes (a) in blank test, (b) with ferric salt addition, and (c) with aluminum salt addition.

Figure 3

Changes of sludge particle sizes (a) in blank test, (b) with ferric salt addition, and (c) with aluminum salt addition.

It can be found that the small particle sizes in the MBR were flocculated to be large flocs due to the addition of ferric and aluminum salts, resulting in the decrease of membrane fouling (Song et al. 2008).

Comparison of ferric chloride and aluminum sulfate on PN and PS

It is well known that PN and PS are the major hydrophilic substances in SMP which can cause membrane fouling (Zhang et al. 2015). Thus, it is necessary to analyze the changes of PN and PS with ferric chloride and aluminum sulfate addition. In Figure 4, the PN concentrations were changed slightly, while the PS concentrations were decreased significantly with ferric chloride or aluminum sulfate addition. For example, the PN and PS concentrations in the blank test were 3.93 and 3.71 mg/L, and the PN and PS decreased to 3.34 and 2.02 mg/L after aluminum sulfate of 30 mg/L was continuously added into the MBR, respectively. It was also apparent that the PN/PS were 1.25 and 1.73 with ferric chloride of 20 mg/L and aluminum sulfate of 30 mg/L. Results indicated that the decrease of PS is the main reason for the mitigation of membrane fouling, the PS are more readily attached to aluminum sulfate to decrease the organics in the supernatant and form large flocs (Praneeth et al. 2014).
Figure 4

Changes of PN, PS and PN/PS (a) in blank test, (b) with ferric salt addition, and (c) with aluminum salt addition.

Figure 4

Changes of PN, PS and PN/PS (a) in blank test, (b) with ferric salt addition, and (c) with aluminum salt addition.

CONCLUSIONS

The impacts of ferric chloride and aluminum sulfate on TP removal and membrane fouling in a MBR treating BAF effluent of municipal wastewater were investigated. The optimal ferric chloride and aluminum sulfate dosages for TP removal were determined by a short-term test. Simultaneously, long-term continuous operational results suggested that aluminum sulfate of 30 mg/L was more effective than ferric chloride of 20 mg/L in decreasing TMP. The sludge particle sizes lower than 50 μm and the PS contents were decreased in this study, which might be the main reason for membrane fouling mitigation.

ACKNOWLEDGEMENTS

This work was financially supported by the National High Technology Research and Development Program of China (863 Program) (No. 2012AA063603-02), Science and Technology Planning Project of Guangdong Province (2012A061600010), and Nanhai District Environmental Protection Industry Innovation and Development Special Project (20120306).

REFERENCES

REFERENCES
APHA
2005
Standard Methods for the Examination of Water and Wastewater
.
American Public Health Association (APHA)
,
Washington, DC
,
USA
.
Caravelli
A. H.
Contreras
E. M.
Zaritzky
N. E.
2010
Phosphorous removal in batch systems using ferric chloride in the presence of activated sludges
.
J. Hazard. Mater.
177
(
1
),
199
208
.
Ferraris
M.
Innella
C.
Spagni
A.
2009
Start-up of a pilot-scale membrane bioreactor to treat municipal wastewater
.
Desalination
237
(
1–3
),
190
200
.
Gkotsis
P. K.
Batsari
E. L.
Peleka
E. N.
Tolkou
A. K.
Zouboulis
A. I.
Fouling control in a lab-scale MBR system: comparison of several commercially applied coagulants
.
J. Environ. Manage
.
https://doi.org/10.1016/j.jenvman.2016.03.003
.
Herbert
D.
Philipps
P.
Strange
R.
1971
Carbohydrate analysis
.
Methods Enzymol. B
5
,
265
277
.
Holbrook
R. D.
Higgins
M. J.
Murthy
S. N.
Fonseca
A. D.
Fleischer
E. J.
Daigger
G. T.
Grizzard
T. J.
Love
N. G.
Novak
J. T.
2004
Effect of alum addition on the performance of submerged membranes for wastewater treatment
.
Water Environ. Res.
76
(
7
),
2699
2702
.
Lowry
O. H.
Rosebrough
N. J.
Farr
A. L.
Randall
R. J.
1951
Protein measurement with the Folin phenol reagent
.
J. Biol. Chem.
193
(
1
),
265
275
.
Piekema
P.
2004
The case study of a phosphorus recovery sewage treatment plant at Geestmerambacht, Holland-design and operation
. In:
Phosphorus in Environmental Technologies: Principles and Applications
(
Valsami-Jones
E.
, ed.).
IWA Publishing
,
London
, pp.
19
27
.
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
.
Verstraete
W.
Van de Caveye
P.
Diamantis
V.
2009
Maximum use of resources present in domestic ‘used water’
.
Bioresource. Technol.
100
(
23
),
5537
5545
.
Wu
J.
Chen
F.
Huang
X.
Geng
W.
Wen
X.
2006
Using inorganic coagulants to control membrane fouling in a submerged membrane bioreactor
.
Desalination
197
(
1–3
),
124
136
.
Wu
B.
Kitade
T.
Chong
T. H.
Uemura
T.
Fane
A. G.
2012
Role of initially formed cake layers on limiting membrane fouling in membrane bioreactors
.
Bioresource Technol.
118
,
589
593
.
Yeoman
S.
Stephenson
T.
Lester
J.
Perry
R.
1988
The removal of phosphorus during wastewater treatment: a review
.
Environ. Pollut.
49
(
3
),
183
233
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY-NC-ND 4.0), which permits copying and redistribution for non-commercial purposes with no derivatives, provided the original work is properly cited (http://creativecommons.org/licenses/by-nc-nd/4.0/).