Characteristics of bio-diatomite dynamic membrane process for municipal wastewater treatment

Journal of Membrane Science 325 (2008) 271–276

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Characteristics of bio-diatomite dynamic membrane process for municipal wastewater treatment Hua-qiang Chu a , Da-wen Cao b , Wei Jin a , Bing-zhi Dong c,∗ a

School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China National Engineering Research Centre for Urban Pollution Control, Shanghai 200092, PR China c State Key Laboratory of Pollution Control and Resource Reuse, Shanghai 200092, PR China b

a r t i c l e

i n f o

Article history: Received 16 March 2008 Received in revised form 20 July 2008 Accepted 23 July 2008 Available online 31 July 2008 Keywords: Bio-diatomite dynamic membrane Filtration Municipal wastewater Treatment

a b s t r a c t The characteristics of bio-diatomite dynamic membrane (BDDM) process in municipal wastewater treatment were investigated with a laboratory-scale continuous-flow device. Experimental results indicate that the BDDM reactor was highly effective in removal of COD, NH4 -N and TN, and exhibited the advantages of good retention capacity for suspended solids (SS), short precoating time, high filtration flux, and easy backwash. In the precoating stage, a retention time of 25 min could reduce the effluent SS to nondetectable level. The filtration resistance of BDDM was composed of thickness-increase resistance and compaction resistance. At a low flux, microbial adhesion occurred on the interface between the BDDM and the stainless steel support mesh, which negatively impacted the system operation and backwash. However, microbial adhesion could be effectively minimized by increasing the filtration flux. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Membrane bioreactor (MBR) has attracted great attention in municipal and industrial wastewater treatment and reclamation in recent years [1–4]. Compared with the conventional activated sludge processes, MBR is advantageous in high effluent quality, high sludge concentration and small plant footprint. However, the popularization of MBR still encounters several problems such as high cost of membrane module, membrane fouling and high energy consumption [5]. Dynamic membrane technology may be a promising approach to resolve these problems. Dynamic membrane is formed on the underlying support mesh when filtering a solution containing fine particles, thus is also called secondary membrane [6]. The Oak Ridge National Laboratory first utilized a ZrOCl2 dynamic membrane for reverse osmosis in 1965 [7]. The dynamic membrane formed on microfiltration and ultrafiltration membrane had high anti-fouling characteristics [8–10]. Additionally, the dynamic membrane formed on the big pore mesh could increase the intrinsic membrane retention capacity [11–13]. Many researchers have focused on the variations of effluent SS and flux of the dynamic membrane. The dynamic membrane could achieve a high solid-liquor separation efficiency (thus a low effluent SS) [14–16], and a high filtration flux [17]. The cleaning of dynamic

∗ Corresponding author. Tel.: +86 21 65982691; fax: +86 21 65982691. E-mail addresses: [email protected], [email protected] (B.-z. Dong). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.040

membrane is convenient. Tap water backwash, air backwash, or brushing can readily clean the dynamic membrane without using any chemical reagents. Fan and Huang [18] reported that bottom aeration (120 m3 /m2 h) was adequate for removing all clogs of the dynamic membrane. It was also observed by Ye et al. [19] that the flux of a fouled dynamic membrane could be totally recovered after brushing. Al-Malack and Anderson [20] studied the backwash methods for a MnO2 dynamic membrane formed on the multifilament woven polyester mesh for domestic wastewater treatment. Diatomite, which consists primarily of amorphous SiO2 , is the sedimentary fossils of ancient diatom and has the characteristics of high porosity, good hydrophilicity and high chemical stability [21]. Diatomite particles have been used as carriers for microorganisms. The microbial colonies on diatomite can form zoogloeas through microbial capsules and surface mucus, which are called bio-diatomite. The bio-diatomite combined with anoxic and/or aerobic processes, namely bio-diatomite reactor, has become a new wastewater treatment technology [22,23]. The bio-diatomite concentration in the reactor is usually above 10,000 mg/L. High concentrations of microorganisms and species diversities can be obtained through continuous quantitative addition of the diatomite and proper control of the residual sludge. The bio-diatomite reactor generally yields high and stable treatment efficiency and good effluent quality if used for municipal wastewater treatment. It was also reported that diatomite could form a dynamic membrane on the big pore mesh used for long-term water treatment [24,25]. The diatomite dynamic membrane could not be

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compressed easily, and could achieve high flux, good effluent quality and easy backwash. In this work, a bio-diatomite dynamic membrane (BDDM) reactor was designed, which combined the advantages of both bio-diatomite reactor and dynamic membrane, and used to treat municipal wastewater. The characteristics of the BDDM reactor including precoating, flux variation and backwash were correlatively investigated with wastewater treatment efficiency. 2. Materials and methods 2.1. Reaction system The BDDM reactor with a total effective volume of 0.035 m3 consisted of three parts, i.e., an anoxic tank, an aerobic tank and a dynamic membrane filter (DMF) in sequence (Fig. 1). The configuration of the BDDM support module is shown in Fig. 2. The support

Fig. 2. Configuration of BDDM support module.

layer used a stainless steel mesh with an equivalent aperture of 74 ␮m. The support module was fixed in submerged mode with a double-sided effective filtration area of 0.084 m2 (28 cm × 15 cm). 2.2. Experimental methods The experiments were carried out from June to December of 2007. The bio-diatomite was cultivated with activated sludge. The inoculation sludge (about 5400 mg/L MLSS) and raw municipal wastewater were collected from a local municipal wastewater treatment plant in Shanghai. The characteristics of the raw municipal wastewater were summarized in Table 1. The diatomite was added continuously into the bioreactor during the inoculation period to reach a concentration of about 7000 mg/L. The bioreactor was continuously operated through feeding the raw municipal wastewater with a peristaltic pump and withdrawing the effluent with another peristaltic pump. When the bio-diatomite matured, the MLSS and MLVSS in the BDDM reactor were maintained at about 11,000 mg/L and 4500–5000 mg/L, respectively. The mixed liquor in the DMF was recycled to the anoxic tank at a flow rate two times the influent flow. The feeding rate of diatomite was properly adjusted according to the surplus sludge quantity and the actual MLSS and MLVSS concentrations in the bioreactor. A high-precision vacuum pressure gauge was connected to the effluent pipe to measure the suction pressure (operation pressure). The operation pressure was used to calculate the transmembrane pressure and the filtration resistance of the BDDM. The operation period of BDDM involved three stages, i.e., precoating, filtration and backwash (Fig. 1). In the precoating stage, the bio-diatomite mixed liquor was recirculated and the effluent from the inner recycle was withdrawn for SS analysis at fixed intervals. In the filtration stage, a constant flux was applied and the thickness of BDDM showed a continuous increase which resulted in the rising of filtration resistance. The filtration was stopped once the operation pressure reached 40 kPa and then backwash was started. To improve the membrane recovery, on-line backwash was performed with air through the bottom outlet of the membrane module using an electromagnetic air pump (Model ACO-006, SunSun Industry Corp., Zhejiang, China). The operation parameters of the BDDM reactor are shown in Table 2. Table 1 Characteristics of raw municipal wastewater Water quality parameters

Fig. 1. Diagrams of different operation stages of the BDDM reactor: (a) precoating stage; (b) filtration stage; (c) backwash stage.

COD (mg/L) NH4 -N (mg/L) SS (mg/L) Water temperature (◦ C) pH

Concentration 113.4–582.7 5.6–11.2 20.5–360.5 15.0–33.5 6.5–7.4

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Table 2 Operation parameters of the BDDM reactor Operation parameters

Value

MLSS (mg/L) Design flux (L/m2 h) SRT (d) HRT (h) Water temperature (◦ C) DO in aerobic tank (mg/L) DO in anoxic tank (mg/L)

11,000 8.6–130.0 87 7a 16–33 3–4 <0.15

a HRT was calculated with the design flux of 60 L/m2 h that lasted for 2 months to examine pollutant removal efficiency in the BDDM reactor.

Fig. 3. Flux variation with time in the precoating stage.

2.3. Analytical methods The turbidity, SS, pH, COD, NH4 -N and TN of both influent and effluent and the MLSS and MLVSS in the bio-diatomite reactor were analyzed according to Chinese NEPA standard methods [26]. The samples were taken simultaneously each time. Since the effluent SS concentration was lower than the detection limit of the measuring device, turbidity was measured instead with a turbidity meter (Model 2100N, HACH, USA). The dissolved oxygen (DO) in the bioreactor was measured with a DO meter (Model HQ10, HACH, USA). The membrane flux was measured with volumetric method. 3. Results and discussion 3.1. Performance of BDDM reactor Table 3 shows that the BDDM reactor was very effective in removing COD, NH4 -N, TN and SS. The most significant advantage of the dynamic membrane is its good solid–liquid separation efficiency [27]. The following test was carried out to compare the contributions of the BDDM and the bio-diatomite reactor to the removal of soluble COD and NH4 -N. The whole dynamic membrane module was carefully taken out from the bioreactor, and then put into a tank filled with the raw municipal wastewater to examine the removal efficiency of soluble COD and NH4 -N by BDDM independently. Results show that BDDM alone could achieve little NH4 -N removal. The removal rate of COD was below 40%, which was primarily due to the retaining of suspended COD. Therefore, the removal of soluble COD and NH4 -N was mainly ascribed to the microbial degradation in the bio-diatomite reactor. Since the bioreactor could maintain a high concentration of MLSS, biodegradation of pollutants was enhanced. Both nitrification in the aerobic tank and denitrification in the anoxic tank occurred at a high level, reaching 6.09 mg NH4 -N/(gVSS h) and 5.05 mg NO3 N/(gVSS h), respectively. Therefore, a high removal efficiency of NH4 -N and TN was achieved, which was also reported by Jin et al. [23] and Xu et al. [28]. The long sludge age in the bioreactor was beneficial for the reproduction of nitrifying bacteria, and a DO concentration of 3–4 mg/L was also adequate for their growth [29,30]. Meanwhile, denitrification could effectively proceed at a low DO concentration of <0.15 mg/L in the anoxic tank. All these factors exerted positive effects on pollutant removal.

3.2. Precoating of BDDM The precoating of BDDM was monitored in real-time when the support module was placed into the bio-diatomite reactor. The flux change in the precoating stage is shown in Fig. 3 with the water head maintained constant at 0.6 m. It can be seen that the flux dropped rapidly from about 500 L/m2 h to 174.4 L/m2 h in the first 5 min and then decreased slowly. The inflection point on the curve (i.e., at 5 min) indicates the completion of dynamic membrane formation and thus is called conventional precoating time. Before the inflection point, the thickness of the dynamic membrane increased continuously and the filtration resistance increased quickly, which led to a severe flux decline. Once the dynamic membrane was formed, the filtration resistance increased slowly and the flux declined also slowly. In fact, the precoating stage in this study continued far beyond 5 min until no effluent SS was detected. The change of the effluent SS during the precoating stage is shown in Fig. 4. Each data point represents an arithmetic mean calculated from seven operation periods measured at the same time interval. The effluent SS was determined to be 272.5 mg/L at the conventional precoating time because the mixed liquor permeating into the module had not been completely discharged at the time of measurement. As the precoating time was extended, the effluent SS dropped slowly to below the detection limit. Therefore, the precoating time, which is defined as the interval between the time of startup and the time when no effluent SS is detected, is about 25 min in this study (Fig. 4). 3.3. Relationship between design flux and filtration time The time from the BDDM formation to the operation pressure reaching 40 kPa is defined as the filtration time. The relationship between the design flux and the filtration time was studied by means of increasing the operation pressure and can usually be described with an exponential equation. The experimental results are described in Table 4 with data obtained from multiple well-

Table 3 Characteristics of treated municipal wastewater Water quality parameters

Concentration

COD (mg/L) NH4 -N (mg/L) TN (mg/L) SS (mg/L) Turbidity (NTU)

8.1–28.1 0.08–0.53 6.18–14.90 0 0.392–0.726

Fig. 4. Effluent SS variation with time in the precoating stage.

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Table 4 Different design flux versus filtration time Design flux, J (L/m2 h)

Filtration time, tf (h)

8.6 17 22 30 35 40 45 50 60 70 80 90 100 110 120 130

248.00 79.83 48.00 35.50 23.73 17.00 10.60 7.45 4.92 3.55 3.05 2.17 1.55 1.05 0.62 0.43

Fig. 6. Filtration resistance as a function of filtration time.

reproduced repetitions. Based on these data, the following equation can be derived with a correlation coefficient of R2 = 0.972: J = 114.23tf−0.4169

(1) (L/m2

h) and the filtration where J and tf represent the design flux time (h), respectively. The change of operation pressure with filtration time at different design fluxes is shown in Fig. 5. The relationship between the operation pressure, which equals the transmembrane pressure in this study, and filtration resistance can be readily calculated using Eq. (2) based on Darcy’s law: R=

P J

(2)

where R = filtration resistance (m−1 ), P = transmembrane pressure (Pa), J = design flux (m/s), and  = filtrate viscosity (Pa s). The variation of the calculated filtration resistance with the filtration time is shown in Fig. 6. The filtration resistance of the stainless steel mesh can be ignored here. Due to the weak cross-flow effect and the high concentration of bio-diatomite in the mixed liquor, the thickness of BDDM tends to grow along with the operation time, resulting in an increased resistance of BDDM which is called thickness-increase resistance. Microorganisms start to adhere and reproduce on the surface of diatomite, which can be compressed easily [31,32]. Although the diatomite is almost incompressible, it is inevitable that the BDDM will become partially compressible. So the permeation space is gradually reduced with the increase of thickness-increase resistance. The resistance caused by compaction is called compaction resistance. Both the thickness-increase resistance and the compaction resistance contribute to the filtration resistance of BDDM. At a high design flux, the thickness of BDDM grows rapidly and the effect of compaction resistance also becomes important. Fig. 6

Fig. 5. Operation pressure at different design fluxes as a function of filtration time.

shows that the filtration resistance increased rapidly at a high flux, and the time for the operation pressure to reach 40 kPa was substantially reduced (Fig. 5). In contrast, during the early stage of a low flux the thickness of BDDM grew slowly, thus exhibiting a low resistance. During the intermediate and final stages, the compaction resistance started to rise quickly as the thickness resistance increased. This is clearly reflected by the fact that the filtration resistance increased with increasing slopes as a function of the filtration time. It is difficult to identify the balance point at which the filtration resistance stops growing due to the cross-flow effect although it exists theoretically. 3.4. Fluxes of different operation stages The operation period of BDDM included three stages such as precoating, filtration and backwash. There was no effluent discharge during the precoating and backwash stages. Thus the total amount of wastewater treated in the operation period was equal to the amount discharged during the filtration stage. The average design flux of effluent discharged by a unit filtration area of BDDM over a complete operation period can be calculated using the following equation: Jt =

tf J tf J = Tt tp + tf + tb

(3)

where Jt = average design flux (L/m2 h), J = design flux (L/m2 h), Tt = operation period (h), tp = precoating time (h), tf = filtration time (h), and tb = backwash time (h). Based on the data shown in Table 4 and also taking the precoating time (25 min) and backwash time (2 min) into account, the average design flux as a function of operation period is plotted in Fig. 7. Results indicate that there was a maximal average design flux of 77.5 L/m2 h at 2 h, which was derived from a design flux of 100 L/m2 h. Eq. (4) was used to fit the experimental data in the close region of the maximal point and showed a high correlation

Fig. 7. Average design flux as a function of operation period.

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Table 5 Backwash parameters of Case A and Case B Case

Design flux (L/m2 h)

Filtration time (h)

Operation pressure (kPa)

Backwash flux (m3 /m2 h)

Backwash pressure (kPa)

Backwash time (min)

A B

22 40

48 9.3

40 20

18.58 18.58

10 10

3 2

coefficient (R2 = 0.973): Jt = −11.76Tt 2 + 46.804Tt + 32.237

(4)

where Jt is the average design flux (L/m2 h) and Tt is the operation period (h). The maximal average design flux can be used to select the design flux for effective operation of BDDM. 3.5. Backwash of BDDM Our preliminary experiments showed that air backwash applied at the end of the filtration stage was effective in cleaning BDDM. Compared with water backwash, air backwash has the following advantages: (1) air is relatively easy to permeate the support mesh pores; (2) pressure loss is small; (3) clogging materials can be easily sloughed off; (4) operation process is simplified with reduced energy consumption; and (5) the recovery rate of BDDM is significantly increased.

The on-line air backwash has the merits of low operation pressure and low backwash flux, thus suitable for BDDM cleaning. Table 5 shows the backwash parameters of two typical work conditions of BDDM. The residual layers on the support mesh surfaces right post-backwash are illustrated in Fig. 8. Results indicate that at the same backwash pressure and flux, Case A was much less effective than Case B. The residual biodiatomite occupied more than 30% of the supporting mesh surface in Case A though a relatively longer backwash time of 3 min was applied. Case B showed a higher cleaning efficiency with little residual bio-diatomite retained on the support mesh surface after being backwashed for 2 min. This difference can be explained by the microbial activities on the surface of BDDM. Although the organic substrate was effectively utilized by microorganisms, as demonstrated by a very low organic concentration in the effluent, microorganisms still can grow and adhere at the interface between BDDM and the support mesh. At a low design flux, microorganisms had a long time to reproduce and form membrane-mesh adhesion (Case A). In contrast, the membrane-mesh adhesion occurred less frequently at a high design flux because the support mesh surface was strongly flushed and a short backwash interval also reduced the time for microorganism growth (Case B). It is seen that the occurrence of membrane-mesh adhesion can negatively impact the subsequent filtration and backwash. Thus, a reasonably high design flux is preferred for effective operation of BDDM. 4. Conclusions The removal of COD, NH4 -N, TN and SS was very effective with the BDDM reactor that had the advantages of short precoating time, high filtration flux, and easy backwash. Once the BDDM was formed, the SS concentration in the effluent dropped continuously to nondetectable level within 25 min. However, it had little effect on the removal of soluble organic matters. The filtration resistance of BDDM was composed of thicknessincrease resistance and compaction resistance. When the maximal operation pressure was set at 40 kPa, the filtration resistance increased rapidly at a high design flux, resulting in a short filtration time. In contrast, at a low design flux the filtration resistance increased slowly, resulting in a long filtration time. The filtration resistance could not reach the balance point where the resistance increase was offset by the cross-flow effect under the condition of a weak cross-flow. When the BDDM reactor was operated at a low flux, microbial adhesion was formed easily at the interface between BDDM and the stainless steel support mesh. The adhesion exerted a significantly negative effect on the subsequent filtration and backwash. However, this problem is readily resolved by using a high flux (or equivalently a short filtration time). Acknowledgements

Fig. 8. Residual layer on support mesh surface after backwash.

This research was financially supported by the National Key Technologies R & D Program “Drinking Water Safety Ensuring Technologies and instruments Development for Small Cities and Towns” (2006BAJ08B02) and by the Foundation of The State Key Laboratory of Pollution Control and Resource Reuse, China (PCRRK08006).

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Nomenclature J Jt P R tb tf tp Tt

design flux (L/m2 h) average design flux (L/m2 h) transmembrane pressure (Pa) filtration resistance (m−1 ) backwash time (h) filtration time (h) precoating time (h) operation period (h)

Greek letter  filtrate viscosity (Pa s)

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