Types Of Membrane Modules Biology Essay

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Membrane basically, can be defined as a barrier, which separates two phases and restricts transport of various chemical in selective manner. It can be also defined as a material that separates particles and molecules from liquids and gaseous. The membrane separation process is based on the presence of semi permeable membrane. The principle is quite simple: the membrane acts as a very specific filter that allows water to flow through, while it catches suspended solids and other substances. There are two factors that determine the effectiveness of a membrane filtration process; selectivity and productivity. Selectivity is expressed as a parameter called retention or separation factor, while productivity is expressed as a parameter called flux (Abdullah Ali Al Amri, 2010).

2.2 Types of Membrane Modules

There are many types of membrane modules used in MBR system according to the design and pore size. Membrane types according to the design are tubular, plate and frame, rotary disk and hollow fibre. Otherwise, there are four main types according to the pore sizes, which are Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF) and Microfiltration (MF).

http://ars.els-cdn.com/content/image/1-s2.0-S0043135498002127-gr1.gif

Figure 2.1 Madaeni S.S. The application of membrane technology for water disinfection, Water Research Volume 33, Issue 2, February 1999, Pages 301-308.

Membrane materials can be organics (polyethylene, polyethersulfone, polysulfone, polyolefin, etc), inorganic (ceramic) or metallic and they should be inert and non-biodegradable (Abdullah Ali Al Amri, 2010).

2.3 Membrane Bioreactor (MBR) Definition

MBRs can be broadly defined as systems integrating biological degradation of waste products with membrane filtration. Membrane bioreactors are composed of two primary parts, the biological unit responsible for the biodegradation of the waste compounds and the membrane module for the physical separation of the treated water from mixed liquor (Cicek, N. 2003).

2.4 Membrane Bioreactor Technology Development

Ultrafiltration as a replacement for sedimentation in the activated sludge process was first described by Smith et al. (1969). In the 1970s the technology first entered the Japanese market through a license agreement between Dorr-Oliver and Sanki Engineering Co. Ltd. By 1993, 39 of these external membrane bioreactor systems had been reported for use in sanitary and industrial applications. In the late 1980s to early 1990s Zenon Environmental continued the early development of Dorr-Oliver in developing systems for industrial wastewater treatment, resulting in two successful patent applications. Zenon's commercial system, ZenoGem®, was subsequently introduced in 1982 (Francesco Fatone, 2007).

In the late 1980s, Japanese researchers began to explore application of the MBR technology where the membranes were mounted directly in the biological reactor and the membrane permeate or biosystem effluent was withdrawn through the membranes by the use of a suction pump. This development ultimately led to the introduction of various commercial, internal membrane MBR systems such as Zenon Environmental's ZeeWeed ® ZenoGem ® system and the Kubota Submerged Membrane Unit (Enegess, D. et al., undated).

Significant advances were made with respect to the efficiency and performance of crossflow membranes through the 1980s, and in 1991 the first large-scale external membrane MBR system for treatment of industrial wastewater in the US was constructed. It began operation at a General Motors' (GM) plant in Mansfield, Ohio. The first large-scale internal membrane system for treatment of industrial wastewater in the US was installed in 1998, and began operation in early 1999 at a food ingredients manufacturing plant in the Northeast (Enegess, D. et al., undated).

In 2004, the largest MBR plant in the world was commissioned in Kaarst (Germany). It was designed by VA Tech Wabag Germany to serve a population of 80,000 p.e., and is equipped with Zenon modules. In March 2005, Zenon announced the contract award for an MBR plant to treat 144,000 m3/d in Washington. This is very representative of the quick development and application pace of the MBR technology, with sizes of constructed plant growing from few thousands to hundred of thousand population equivalent in few years only (MEDINA Project, 2007).

2.5 MBR Types

MBR systems can be classified into two major groups according to their configuration: internal (submerged) and external (sidestream or recirculated). MBR systems can also be classified into two groups: aerobic and anaerobic MBR.

2.5.1 Internal and External MBRS

The first group, commonly known as the integrated (Submerged) MBR, involves outer skin membranes that are internal to the bioreactor. The driving force across the membrane is achieved by pressurizing the bioreactor or creating negative pressure on the permeate side. Cleaning of the membrane is achieved through frequent permeate back-pulsing and occasional chemical backwashing. A diffuser is usually placed directly beneath the membrane module to facilitate scouring of the filtration surface. Aeration and mixing are also achieved by the same unit (Cicek, N. 2003).

In the external system (side-stream) the membrane unit works independently of the reactor. Influent enters the bioreactor and is degraded by microorganisms. The bioreactor effluent is then conducted into a membrane filtration unit, which generally works in a cross-flow mode, where a pump provides the velocity and transmembrane pressure to enhance flux and prevent fouling during filtration. The membrane permeate is the treated product and the retentate is continuously returned to the bioreactor (B. Lew et. al; 2008). Figure 2.2 (Paul et al., 2006), simply shows the two types of MBR.

Figure 2.2 Simplified schematics of MBR configurations. a) internal MBR configuration and b) external MBR configuration (Paul et al., 2006).

The choice between operating options is dependent upon the application, as both systems have advantages and disadvantages. Till (2001) lists advantages and disadvantages of MBR configurations:

Submerged MBR:

• Aeration costs high (~ 90 %).

• Very low liquid pumping costs (higher if suction pump used ~ 28 %).

• Lower flux (higher footprint).

• Less frequent cleaning required.

• Lower operating costs.

• Higher capital costs.

Side stream MBR:

• Aeration costs low (~ 20 %).

• High pumping costs (60 - 80 %).

• Higher flux (smaller footprint).

• More frequent cleaning required.

• Higher operating costs.

• Lower capital costs.

2.5.2 Aerobic and Anaerobic MBRS

In the anaerobic MBR, the biodegradation process of the microorganisms occurs in the absence of the oxygen, but in the aerobic MBR it occurs and depends on the oxygen supplying through the aeration system (Abdullah Ali Al Amri, 2010).

2.5.2.1 Aerobic MBRS

The aeration used in MBR systems has three major roles: providing oxygen to the biomass, maintaining the activated sludge and mitigating fouling by constant scouring of membrane surface.The aerobic MBR has been applied quite widely to domestic, municipal wastewater treatment instead of the conventional activated sludge system (Gander et al., 2000; Jefferson et al., 2000; Ueda and Hata, 1999 and Murakami et al., 2000). Darren et al. (2005) reported that, their laboratory-scale aerobic MBR system managed to remove 98% of the suspended solid and achieving a remarkable COD removal efficiency of 96% in treating high strength synthetic wastewater. However, phosphorus removal in MBR varied from 15% (Cote et al., 1997) to 74% (Ueda and Hata, 1999). The concentration of the MLSS is reported to be 10 g/l and up to 50 g/l in some studies (Muller et al., 1995 and Scholz and Fuchs, 2000).

Aerobic MBR has been applied to treat a wide range of industrial wastewater, such as oily (Scholz and Fuch, 2000 and Seo et al., 1997) and tannery wastewaters (Yamanoto and Win, 1991). Despite the high strength of the industrial wastewater, many studies have reported high COD removal efficiencies at high organic loading rate (Scholz and Fuch, 2000; Yamanoto and Win, 1991; Kurian and Nakhla, 2006 and Rozich and Bordacs, 2002). Aerobic biological process operated at high temperatures. It has a ability to integrate the advantages of conventional aerobic and anaerobic processes that include rapid biodegradation kinetics and low biological solids production respectively in treating high strength industrial wastewaters (Rozich and Bordacs, 2002). The low yield of 0.03 g VSS/g COD, observed by Kurian and Nakhla (2006) reveals that the aerobic MBR is a potential solution to difficulties related to high sludge generation in conventional systems treating high strength wastewaters (Abdullah Ali Al Amri, 2010).

2.5.2.2 Anaerobic MBRS

In the absence of oxygen as an electron acceptor, anaerobic microbial systems discard the electrons into methane instead of using them to grow more microorganisms, leading to low biomass production. Less biomass production is an advantage due to reduced sludge treatment cost. However, the slow growth rates of the methanogenic organisms and the microbial complexity of the systems make the operation of anaerobic systems difficult. Biomass retention becomes a critical factor to keep sufficient biomass within the reactor (Visvanathan and Abeynayaka, 2011).

The AnMBR process can be basically defined as a biological treatment process operated without oxygen and using a membrane to provide complete solid-liquid separation. AnMBR were first introduced in the 1980s in South Africa and till it has less investigated compared to aerobic MBR systems. However, today there is a growing interest in the field of AnMBR as shown in numerous and still increasing number of studies going on. Because MBRs could operate independently in relation to the retention times, it enables to go for high organic loading rates. Therefore this became an attractive solution for low (i.e., municipal wastewater) to high strength industrial wastewater treatment with simultaneous energy recovery and less excess sludge production (Visvanathan and Abeynayaka, 2011).

The membrane may be operated under pressure or it may be operated under a vacuum. In the first approach, the membrane is separated from the bioreactor and a pump is required to push the bioreactor effluent into the membrane unit which makes permeate to come through the membrane. This configuration is often called as an external cross-flow membrane bioreactor (Figure 2.3.a). The cross-flow velocity of the liquid across the surface of the membrane serves as the principle mechanism to disrupt cake formation on the membrane. When the membrane is immersed into the bioreactor and operated under a vacuum (Figure 2.3.b), instead of under direct pressure, the configuration is called submerged membrane bioreactor due to the location of the membrane. In this configuration, a pump or gravity flow due to elevation difference is used to withdraw permeate through the membrane. Because the velocity of the liquid across the membrane cannot be controlled, cake formation can be disrupted by vigorously bubbling gas across the membrane surface. The vacuum driven immersed membrane approach may be used in two configuration. The applications submerged MBRs for anaerobic wastewater treatment are still limited. The observation, investigation and maintenance difficulties of membranes inside a closed anaerobic reactor made the external membrane operation favorable. Membrane fouling in AnMBRs, which is major drawback. To reduce cake formation on the membranes in submerged AnMBRs, the produced biogas is recirculated and used instead of air bubbling in aerobic submerged MBRs by Lin et al (Visvanathan and Abeynayaka, 2011).

The membrane may be immersed directly into the bioreactor or immersed in a separate chamber (Figure 2.3.c). The latter configuration now looks like an external membrane, and will likely require a pump to return the retentate to the bioreactor. However, unlike the external cross-flow membrane, the membrane here is operated under a vacuum instead of under pressure. The external chamber configuration is used for full-scale aerobic wastewater treatment plants because it provides for easier cleaning of the fouled membranes, because the chambers can be isolated instead of the membranes being physically removed. More studies are conducted in order to enhance the performance of AnMBR. I.e., the configuration (d) in Figure 2.3, the system is operating intermittently under semi dead-end mode to reduce the continuous pumping cost and to minimize the harmful effects, such as biomass activity reduction, of sludge pumping (Visvanathan and Abeynayaka, 2011).

Figure 2.3 Different configuration of AnMBRs

Another AnMBR reactor configuration is the two-stage reactor configuration the reactions of hydrolysis, acetogenesis and acidogenesis occur within the first reactor which is named as the hydrolytic (or acidogenic) reactor, followed by methanogenic reactor where the methanogenic process take place (Figure 2.4). The methanogenic reactor which facilitates for the methanogens operates in a strictly defined optimum pH range for the growth of the microorganisms. In a single-stage reactor, where both of the processes take place inside. The biological reactions of the different species in a single stage reactor can be in direct competition with each other. In a two-stage treatment system two reactors are operating with the optimized conditions of the respective bacteria to bring maximum control of the bacterial communities living in the reactor. Acidogenic bacteria produce organic acids. They grow fast with higher biomass yield than methanogens. In addition methanogenic bacteria require stable pH and temperatures in order to optimize their performance. In the past, operation of two-stage anaerobic system was hindered by difficulties in solid-liquid separation and the maintenance of separate and distinct biomass populations in each reactor (Anderson et al., 1986). Yet the membrane coupled bioreactors provides the applicability of the two-stage anaerobic degradation both with excellent separation and high biomass retention (Visvanathan and Abeynayaka, 2011).

Figure 2.4 Single and two stage AnMBR configurations.

It is obvious that anaerobic wastewater treatment is specially suitable for high strength wastewaters and could operate in higher loading rates. Furthermore, with the advantages of biomass retention, membrane coupled anaerobic membrane bioreactors are able to operate at higher loadings conditions.

AnMBRs have operated in wide range in terms of different feed concentrations, loading rates, reactor types in mesophilic as well as in thermophilic conditions. Most AnMBRs studies conducted in CSTR configuration with pressure driven mode reactors have achieved good COD removal efficiencies (Bailey et al., 1994, Fakhru'l -Razi, 1994, Saddoud et al, 2007 ). In addition to that Lew et al. (2009) have studied on external configuration under gravity flow instead of having pressure pump and have achieved 88% of COD removal for domestic wastewater. This could opens new research directions in order to achieve high biomass activity and less fouling in external cross flow AnMBR applications. Early stage, high rate AnMBR studies were conducted under external membrane configuration reactors (Bailey et al., 1994, Jeison and Lier 2006, Jeison et al, 2009 and Yejian et al, 2008). Successful performances of CSTR applications would lead to the simple reactor construction and easy maintenance and operation over the complex high rate reactors in the wastewater treatment sector (Visvanathan and Abeynayaka, 2011).

Most of the studies have worked on synthetic wastewaters at the initial due to the easiness in process control. The feed solutions used in these studies were: VFA, sucrose, glucose, simulated domestic wastewater, as well as simulated high salinity wastewaters. Among those studies almost all the studies have achieved good removal efficiencies such as more than 90% (Visvanathan and Abeynayaka, 2011).

The use of industrial or other types of high strength real wastewater was also studied and achieved very good removal efficiencies as well. For an example, Saddoud et al. (2007) studied with cheese whey effluent with influent COD in the range of 12-80 kg/m3 and COD loading of 3-20 kgCOD/m3.d. Interestingly, the study was able to achieve 98.5% of COD removal efficiency. Cheese whey contains high levels of fats which make difficulties in anaerobic high rate reactors due to biomass wash out and less activity by making a coat over the biomass. However, this performed well in terms of COD removal as well as biogas yield (0.38 m3 CH4/kg COD) with the two stage CSTRs. Most of the reported studies are conducted in mesophilic range. However, thermophilic anaerobic treatment is one of the areas that have higher potential of AnMBR application and it urges more researches to optimize the performances. In addition, studies of Lier and Jeison, (2006) provided a comparison on AnMBR operation in thermophilic and mesophilic conditions, clearly indicating the ability of achieving higher OLRs with smaller HRT in thermophilic AnMBR (Visvanathan and Abeynayaka, 2011).

Studies on AnMBR conducted with low to medium strength wastewater have generally achieved more than 90% removal efficiencies. Brockmann and Seyfried, (1996) have illustrated the disadvantage of cross flow operation associated with biomass activity reduction in AnMBR. Further, the authors recommended the recirculation velocity in cross flow would not exceed 5 m/s to minimize the biomass activity reduction. In addition, most of the studies have used biogas bubbling as a fouling reduction strategy, achieved successful performances (Stuckey and Akram, 2008, Lier and Jeison, 2007). Furthermore OLRs achieved so far were not very high yet the study of Fakhru'l- Razi (1994) has achieved a high OLR (20 kg/m3.d) for high strength wastewater (46-84 kg/m3) obtaining about 98% COD removal for industrial wastewater under mesophilic condition. In addition, Lier and Jeison, (2007) achieved a similar range of OLR but for medium concentration wastewater while Choo and Lee (1996) obtained about 84% COD removal efficiency with high strength wastewater with low OLR. In addition Stukey and Akram, (2008a) have achieved 98% of COD removal for OLR of 16 kg/m3.d. Interestingly the HRT of this study is only 6 h (Visvanathan and Abeynayaka, 2011).

Methane yield is an important parameter which reflects the performances of the anaerobic wastewater treatment systems. Since the studies on AnMBR are still under development phase, the studies on biogas yield optimization have not gained much attention. However the studies which reported the methane yield indicate around 0.27-0.36 m3CH4/kgCOD (Lin et al. 2011a, Wijekoon et al. 2011, Fakhru'l-Razi 1994) with other high rate anaerobic reactors (Visvanathan and Abeynayaka, 2011).

2.6 MBR and Conventional Treatment Process Comparisons

A comparison of the organic loading rates and removal efficiencies of varying unit treatment processes is presented in Table 2.1. It is seen that MBRs offer a system that competes very effectively with conventional treatment processes. The organic loading rates are generally higher than trickling filters (TF), sequencing batch and conventional activated sludge process (ASP), due to shorter HRT, but lower than biological aerated filter BAFs, complete-mix and high rate ASP (Till, 2001).

Table 2.1 Organic loading rates for treatment processes (Gander et al., 2000)

Reactor

Organic loading rate (kgBOD5.m-3.day-1)

HRT(h)

Percentage removal

BAF

Downflow

Upflow

Downflow

1.5

4

7.5

1.3

-

-

93

>93

75

TF

Low rate

Intermediate

High rate

0.08-0.4

0.24-0.48

0.48-0.96

-

-

-

80-90

50-70

65-85

ASP

Sequencing

Conventional

Complete-mix

High-rate aeration

0.08-0.24

0.32-0.64

0.8-1.92

1.6-16

12-50

4-8

3-5

2-4

85-95

85-95

85-95

75-90

MBR

Sub, P+F (Kubota)

Sub, HF (Tech-Sep)

Sub, P+F

Sub, HF

SS ceramic

SS P+F

0.39-0.7

0.03-0.06

0.005-0.11

1.5 (COD)

0.18

0.45-1.5 (COD)

7.6

1

8

0.5

24

8

99

98-99

98

87-95

>95

88-95

Every type of MBR processes has resulted in advantages over conventional biological treatment particularly in term of small footprint, process intensification, modular, and retrofit potential ( Alia Chaturapruek, 2003). One of the main features of MBR technology is the ability of the membrane to remove pathogenic organisms, providing disinfection of the effluent (Till, 2001). Galil (2003) declares that the MBR ability to develop and maintain a concentration of over 11,000 mg per litre of mixed liquor volatile suspended solids in the MBR bioreactor enabled an intensive bioprocess at relatively high cell residence time. Galil also declares that biosolids, which had to be removed as excess sludge were characterised by a relatively low volatile to total suspended solids ratio - around 0.78. This could facilitate and lower the cost of biosolids treatment and handling. Since MBR has produced highly effluent quality and less sludge production, it has usually been used to replace conventional biological treatment. MBR can be operated at MLSS of up to 20,000 mg/L and as sludge settling is not required, submerged MBR can be up to 5 times smaller than a conventional ASP. The high biomass concentration in the MBR tank allows complete breakdown of carbonaceous material and nitrification of municipal wastewater to be achieved within an average retention time of 3 hours (Alia Chaturapruek, 2003).

Sludge production is significantly reduced, compared to conventional ASP, as longer sludge ages are achievable (Till, 2001). A comparison between the sludge production of various processes is given below.

Table 2.2 Sludge Production for Various Wastewater Treatment Processes (Till, 2001)

Sludge production for various wastewater treatment processes

Treatment processes

Sludge production (kg/kgBOD)

Submerged MBR

Structured media biological aerated filter (BAF)

Trickling filter

Conventional activated sludge

Granular media BAF

0.0-0.3

0.15-0.25

0.3-0.5

0.6

0.63-1.06

The MBR system does not require flocs to be formed to remove the solids by settlement and therefore the biomass can operate at very high levels of MLSS, generally in order of 10,000 - 18,000 mg per litre. This high concentration enables a low tank volume and a long sludge age to be utilised, which reduces sludge production and allows for a small plant footprint. It allows for a 50 % reduction in aeration tank volume.

The long sludge age process produces 35 % less sludge than conventional treatment process. Hence, less sludge handling and disposal cost. Also, the sludge is highly stabilized.

Bacteria and most viruses can be removed by the process, dependant upon the pore size. Good disinfection capability, with significant bacterial and viral reductions achievable using UF and MF membranes. High and reliable quality of treated water is achieved. Consequently, the treated water can be reused for flush water for toilets and sprinkling water. Turbidity of the effluent is less than 0.2 NTU and suspended solids are less than 3 mg per litre (Aquatec Maxcon product literature).

A paper by Galil (2003) summarises the results obtained in a study based on a pilot plant, which compared a membrane biological reactor (MBR) process to the conventional activated sludge (ASP) process in the aerobic treatment of the effluent obtained from an anaerobic reactor. During the pilot operation period (about 90 days after achieving steady state) the MBR system provided steady operation performance, while the activated sludge produced effluent, which was characterised by oscillatory values (John Coppen, 2004). The results are given below.

Table 2.3 Average Results Comparison (Galil, 2003)

Activated Sludge

MBR

Suspended solids (mg/L)

COD (mg/L)

BOD (mg/L)

37

204

83

2.5

129

7.1

The results were based on average values and indicated much lower levels of suspended solids in the MBR effluent, 2.5 mg/L, as compared to 37 mg/L in the activated sludge. As a result, the total organic matter content was also substantially lower in the MBR effluent, 129 vs 204 mg/L as COD, and 7.1 vs 83 mg/L as BOD. The results of comparative study indicate that in the case of MBR there will be no need for further treatment, while after activated sludge additional filtration will be required (Galil et al., 2003).

Another comparison is provided by Stephenson et al (2000), which has been taken from Cicek et al (1999), on the performance of an activated sludge plant with a sidestream MBR. The comparison is shown in Table 2.4 below. The flocs in the MBR were shown to be significantly smaller and more active with a higher volatile fraction in the mixed liquor and a greater diversity of species especially in terms of free swimming bacteria. Enzyme activity was also seen to be higher in the MBR and this was attributed to washout in the activated sludge system.

Table 2.4 Performance Comparison (Stephenson at al, 2000)

Parameter

Activated Sludge

MBR

Sludge age (days)

COD removal (%)

DOC removal (%)

TSS removal (%)

Ammonical N removal (%)

Total P removal (%)

Sludge production (kgVSS/COD/day)

Mean floc size (mm)

20

94.5

92.7

60.9

98.9

88.5

0.22

20

30

99

96.9

99.9

99.2

96.6

0.27

3.5

Stephenson et al (2000) qualify their statements by saying that the fundamental differences in the biology of an MBR compared to an activated sludge process are not yet clear, since a limited amount of information is available on the way in which descriptive variables such as the floc structure, respiration rate, species and off gas production are affected by the changes in operation (John Coppen, 2004).

2.7 Limitations of MBRs

Membrane bioreactors are now widely used in wastewater treatment, and their advantages are well recognized. However, membrane fouling remains one of main technical problems in full-scale MBR plants, both for submerged and external MBR operators (Kim et al., Cho and Fane, 2002; Nuengjamnong et al., 2005; Ji and Zhou, 2006).

The main drawback of the membrane bioreactor technology stil remains the capital and operation costs due to use of the membrane filtration components (first sets and replacements), and the high energy requirement resulting from module aeration. It is also a "high-tech system" requiring qualified and committed staff, clear operational guidelines, and quick reaction in case of any process or system disturbance (MEDINA Project, 2007). Non-economic advantages and disadvantages of the MBR is given below.

Table 2.5 Advantages and Disadvantages of the MBR (MEDINA Project, 2007)

Advantages

Disadvantages

Small footprint.

Extremely high quality effluent;

State-of-the art treatment.

Achieves nitrogen removal.

Combines clarification and filtration with oxidation process.

High MLSS provides resistance to loading shocks.

Significantly reduces disinfection requirements.

Provides pretreatment for TDS removal by reverse osmosis.

Requires fine screening.

Limited equipment manufactures.

Relatively "new" process.

Requires dedicated sludge handling facilities.

2.8 MBRs Economics

The high cost connected with MBR is often mentioned in discussions about applicability of MBR. However, it is not easy to make a general economical comparison between MBR and CAS system. First of all, the reference system should not simply be an activated sludge system, but a system that produces an effluent of the same quality. Morever, an MBR is a modular system, that is, easily expandable, which is often mentioned as an advantage of the system. However, this makes the system less competitive with conventional systems, since these become relatively less expensive per population equivalent (p.e.) at larger scale. It should be noted that although the equipment and energy costs of an MBR are higher than systems used in conventional treatment, total water costs can be competitive due to the lower footprint and installation costs (Pearce, 2008b; Lesjean et al., 2004; Cote et al., 2004; De Wilde et al., 2003). MBR costs have declined sharply since the early 1990s, falling typically by a factor of 10 in 15 years. As MBR technology has become accepted, and the scale of installations has increased, there has been steady downward trend in membrane, which is still continuing. The uptake of the membrane technology for municipal applications had the affect of downward pressure on price (F I.Hai, K Yamamoto, 2011).

Studies show that depending on the design and specific factors the total water cost associated with MBR may be less or higher than the CAS-UF/MF option. For example, a cost comparison by the US consultant HDR in 2007 showed that MBR was 15% more expensive on a 15 million liters a day (MLD) case study, whereas a study by Zenon in 2003 gave MBR 5% lower costs (Pearce, 2008a). The differences were due to the design fluxes assumed and the capital charge rate for the Project. Neither study allocated a cost advantage from the reduced footprint, which could typically translate to a treated water cost saving of up to 5% ( F I.Hai, K Yamamoto, 2011).

Davies et al. (1998) made a cost comparison for two wastewater treatment plants (WWTPs), with capacities of 2,350 and 37,500 p.e. With the assumptions they made (e.g., a membrane lifetime of 7 years) they conclude that depending on the design capacity (i.e., 2 times DWF to be treated) MBR is competitive with conventional treatment up to a treatment capacity of 12,000 m3.d-1 (Table 2.6).

Table 2.6 Capital and operating cost ratios of MBR and conventional activated sludge (CAS) process assuming a capacity of 2 *(dry weather flow) (Davies WJ, Le MS, and Heath CR, 1998)

Parameter

Cost ratio (MBR:ASP)

Capital cost

2350 p.e

37 500 p.e

Operating costs per year

2350 p.e

37500 p.e

0.63

2.00

1.34

2.27

Adham et al. (2001) made a cost comparison between MBR and oxidation ditch followed by membrane filtration. They concluded that MBR is competitive with the other treatment systems (Table 2.7).

Table 2.7 Capital and total cost ratios of MBR and tertiary MF following alternative biological processes (Adham S, Mirlo R, and Gagliardo P, 2000)

Alternatives

Cost ratio (MBR:alternative)

Capital

Total per year

Oxidation ditch-MF

CAS-MF

0.91 0.89

0.85 0.9

Van Der Roest et al. (2002a) described a cost comparison between an MBR installation and a CAS system with tertiary sand filtration. The calculations were caried out for two new WWTPs with the aim of producing effluent with low concentrations of nitrogen and phosphorus. Almost the same investment costs and 10-20% higher operating costs, depending on the capacity of the plant, for MBR were estimated (Table 2.8). Cost differences between an MBR and a traditional WWTP concerning manpower, chemical consumption, and sludge tretment were noted to be minimal.

Table 2.8 Capital and total cost ratios of MBR and tertiary sand filtration following CAS (Van Der Roest HF, Lawrence DP, and Van Bentem AG, 2002a)

Parameter

Cost ratio

Capital cost

10 000 p.e

50 000 p.e

Operatinig costs per year

10 000 p.e

50 000 p.e

0.92

1.01

1.09

1.21

McInnis (2005) reported a detailed comparative cost analysis of two membrane-based tertiary treatment options: (1) MBR and, (2) CAS process followed by MF (CAS/MF). According to that study, irrespective of design flow rate, the MBR entails slightly higher unit capital costs as compared to CAS/MF process, while, depending on the design flow rate, the operation and maintenance costs (O&M) of the former are higher than or comparable to that of the latter. Comparative (O&M) cost breakdown revealed that MBR entails less labor cost, considerably higher power and chemical consumptions and slightly higher membrane cost, other costs remaining virtually the same. In the CAS/MF process, labor cost induces the highest cost, while in case of the MBR process, labor and electrical power-consumption costs are almost similar. Overall, the MBR imposes slightly higher capital and operating/maintenance cost over that of CAS/MF ( F I.Hai, K Yamamoto, 2011).

MBR provied an equivalant treatment level to CAS-UF/MF, but at the expense of higher energy cost since the efficiency of air usage in MBR is relatively low. The MBR process uses more air, and hence higher energy than conventional treatment. This is because aeration is required for both the biological process and the membrane cleaning, and the type, volume, and location of air required for the two process are not matched ( F I.Hai, K Yamamoto, 2011).

K Yamamoto and F I Hai (2011) gave a comparative typical energy consumption. Table 2.9 lists typical energy-use rates of different biological-based treatment combinations.

Table 2.9 Comparative typical energy consumption by different treatment options

Treatment option

Energy use (kWh-1m-3)

CAS

CAS-BAF

CAS-MF/UF

MBR

0.15

0.25

0.35-0.5

0.75-1.5*

*Power consumption range for large-to smaller-scale plants.

2.9 MBR Fouling

The term fouling is used to describe the potential deposition and accumulation of constituents in the feed stream on the membrane. Membrane fouling is an important consideration in the design and operation of membrane system as it affects pretreatment needs, cleaning requirements, operating conditions, cost, and performance. Constituents in wastewater that bring about membrane fouling. Fouling of the membrane can occur in three general forms: (1) a buildup of the constituents in the feedwater on the membrane surface, (2) the formation of chemical precipitates due to the chemistry of the feedwater, and (3) damage to the membrane due to the presence of chemical substances that react with the membrane or biological agents that can colonize the membrane.

Three accepted mechanisms resulting in resistance to flow due to the accumulation of material within a lumen are (1) pore narrowing, (2) pore plugging, and (3) gel/cake formation caused by concentration polarization (Ahn et al., 1998). Gel/cake formation, caused by concentration polarization, occurs when the majority of the solid matter in the feed is larger than the pore size sor molecular weight cutoff of the membrane. Concentration polarization can be described as the buildup of matter close to or on the membrane surface that causes an increase in resistance to solvent transport across the membrane. Some degree of concentration polarization will always occur in the operation of a membrane system. The formation of a gel or cake layer, however, it is an extreme case of concentration polarization where a large amount of matter has actually accumulated on the membrane surface, forming a gel or cake layer. The mechanisms of pore plugging and pore narrowing will occur only when the solid matter in the feedwater is smaller than the pore size or the molecular weight cutoff. As the name describes, pore plugging occurs when particles the size of the pores become stuck in the pores of the membrane. Pore narrowing consists of solid material attaching to the interior surface of the pores, which results in a narrowing of the pores. It has been hypothesized that once the pore size is reduced, concentration polarization is amplified further, causing an increase in fouling (Metcalf&Eddy, 2003).

2.9.1 Fouling Reduction

Fouling reduction methods has a major concern in membrane bioreactor since the

membrane performances are directly effect by the fouling. There are two main approaches are incorporate in fouling reduction such as wastewater pretreatment and

membrane cleaning.

• Pretreatment

It is important to have pretreatment process before the reactor as in the conventional process with microscreen, primary sedimentation tank to remove debris, fibers, oil, silt, sand and other suspended solids to avoid those coming to the membrane.

• Cleaning

Next fouling reduction method is membrane cleaning which has two main types such as chemical cleaning and physical cleaning further this could done as insitu or exsitu cleaning operations.

Physical cleaning is the physical removal of suspended solids from the membrane

material. This can be done by aeration, water back flushing, air back flushing, relaxation, increase cross flow velocity, low flux operation and/or as combination.

Chemical cleaning is essentially a chemical reaction between cleaning chemical and foulant. Cleaning chemical use is based on the manufacture specification and mostly acids are used. To have a better cleaning and better membrane performances chemical cleaning in certain frequency is required (Kaushalya C.Wijekoon, 2010).

2.10 Applications of Anaerobic Membrane Bioreactors

2.10.1 Synthetic Wastewaters

Synthetic wastewaters are typically used to test new concepts such as the AnMBR. The results of a number of studies are summarized in Table 2.10. The substrates used included volatile fatty acids, starch, glucose, molasses, peptone, yeast, and cellulose. Although the COD removal was generally >95%, only a few studies had an OLR of >10 kg COD/m3/d and only two had a maximum OLR of 20 kg COD/m3/d or higher. These generally low OLRs for such readily biodegradable substrates are surprising, considering that OLRs for high-rate anaerobic reactors are typically in the range of 10-20 kg COD/m3/d. In addition, none of the reported studies achieved high COD removal at HTRs <10 h. For example, Cadi et al. observed a large drop in COD removal efficiency form 91% to 78% when the HRT was decreased from 11 to 6 h at a constant OLR of 2 kg COD/m3/d (Liao et al., 2006).

Table 2.10 Summary of AnMBR Performance for Treatment of Synthetic Wastewaters

Type of wastewater

Scale

Type of reactor

Reactor volume (m3)

Temp. (ËšC)

HRT (d)

SRT (d)

OLR (kgCOD.m-3.d-1)

MLSS (gL-1)

Feed COD (gL-1)

Effluent COD (gL-1)

COD removal efficiency

Acetate

L

CSTR

0.007

35

1.0

30

8.5

10

8.5

<0.4

>95%

Acetate,lactate,propionate,butyrate

P

CSTR

0.24

33

3.9

100

17

12

67

0.7

99%

Glucose

L

2 phase

CSTR+M/

CSTR+M

0.003/0.01

35

1.5/7.7

-/-

36/12

-/-

53/41

1.5

97%

Glucose,peptone,yeast extract,acetate

L

CSTR

0.007

30

0.5

-

20

22

9.7

<1

>90%

Starch

L

CSTR

0.0075

35

0.5

45

2.0

-

0.93

0.09

90%

Molasses

L

UASB

0.005

20

-

-

0.3-1.3

-

-

-

-

Synthetice

L

UASB

0.009

30

0.6

-

8.3

-

5

0.05

99%

Skim milk and cellulose

L

CSTR

0.01

35

2.0

-

2.5

15

5

<0.08

>98%

L= Laboratory/bench scale, P= pilot scale

CSTR= completely stirred tank reactor, PB= packed bed, UASB= upflow anaerobic sludge blanket, M

designates the location of the membrane (no M indicates the membrane produced the final effluent)

-= Indicates value not reported

d = Units are TOC instead of COD

e = Composition not reported

f = Units are cellulose instead of COD

2.10.2 Food Processing Wastewaters

Food processing wastewaters are characterized by high organic strengths (1000-85,000 mg COD/L) with a wide range of suspended solids concentrations (50-17,000 mg/L) (Table 2.11). Food processing wastewaters are readily biodegradable, so anaerobic treatment is well established; approximately 76% of all the anaerobic reactor installations worldwide are for the food and related industries.

Many AnMBR studies have assessed food-processing wastewaters (Table 2.11). Both pilot and full-scale AnMBR system have been used to facilitate the retention of biomass and improve effluent quality. AnMBRs have been used for the treatment of effluents from field crop processing (sauerkraut, wheat, maize, soybean, palm oil), the dairy industry (whey), and the beverage industry (winery, brewery, distillery). High COD removal efficiencies (usually >90%) were achieved, but the organic loading rates, generally in the large 2-15 kg COD/m3/d, were low in comparison to existing high-rate anaerobic systems, which can achieve OLRs of 5-40 kg/m3/d. Most studies used completely mixed reactors with external cross-flow membranes, though several studies investigated the use of two-phase systems with and without a membrane after the acidification reactor. This compares with traditional high-rate anaerobic treatment of food-processing wastes, which predominantly uses the UASB reactor configuration (Liao et al., 2006).

Table 2.11 Summary of AnMBR Performance for Treatment of Food Processing Wastewaters

Type of wastewater

Scale

Type of reactor

Reactor volume (m3)

Temp. (ËšC)

HRT (d)

SRT (d)

OLR (kgCOD.m-3.d-1)

MLSS (gL-1)

Feed COD (gL-1)

Feed TSS (gL-1)

Effluent COD (gL-1)

COD removal efficiency

Acid whey permeate

P

CSTR

0.3

35

5.7

27

9.6

37

52

0.1

0.5

99%

Sauerkraut brine

L

CSTR

0.007

30

6.1

-

8.6

55

52.7

0.5

0.5

99%

Wheat starch

P

2 phase

UFAF+M/

UASB+M

-

-

-

-

-

-

36

17

8.8

76%

Maize

F

CSTR

2610

35

5.2

-

2.9

21

15

-

0.4

97%

Soybean

P

2 phase

UFAF+M/

UFAF

1.0/2.0

30

3.5/7.0

-

3.0

-

1.3/0.9

0.5/0

0.1

92%

Soybean

P

2 phase

CSTR+M/

FB+M

-

-

-

-

-

-

10

4.3

0.9

91%

Alcohol distillery

L

CSTR

0.004

54

13

-

3.3

2.0

38

0

3.8

90%

Brewery

P

CSTR

0.12

35

4.0

59

19.7

38

84

-

3.1

96%

L= Laboratory/bench scale, P= pilot scale, F= full scale

CSTR= completely stirred tank reactor, FB= fluidized bed, UFAF= upflow anaerobic filter, UASB= upflow anaerobic sludge blanket, M

designates the location of the membrane (no M indicates the membrane produced the final effluent)

-= Indicates value not reported

2.10.3 Industrial Wastewaters

Non-food-processing industrial wastewaters include effluents from the pulp and paper, chemical, pharmaceutical, petroleum, and textile industries. The characteristics of industrial wastewaters are sector specific, they have a high organic strength and contain synthetic and natural chemicals that may be slowly degradable or nonbiodegradable anaerobically, and/or toxic. Traditionally, industrial wastewaters are treated by a combination of physical, chemical and biological processes bacause no single method can achieve complete treatment. An important concern associated with the biological treatment of such wastewaters is toxicity to the microorganisms. However, wastewaters containing toxic compounds can still be anaerobically degraded provided that approriate precautions are taken. This can include pretreatment to remove the inhibitors prior to anaerobic treatment, acclimation of the biomass by gradual increase of inhibitor concentration, and provision of a sufficiently over other anaerobic systems because the biomass can be retained even if an inhibitor upsets the treatment system. Because toxics rarely cause cell death, treatment would only be temporarily impaired (Liao et al., 2006).

As yet, AnMBRs have only been applied to pulp and paper and textile wastewaters (Table 2.12). Anaerobic treatment of pulp and paper wastewaters has become more common; approximately 9% of all anaerobic installations are for the pulp and paper industry. The use of AnMBRs for pulp and paper wastewaters has been reported five times, as summarized in Table 2.12. The use of an AnMBR to treat segregated kraft bleach plant wastewater provided a modest increase in the adsorbable organic halogen removal efficiency, from 48% to 61% in comparison to a UASB without membrane. Economic analyses indicated that the total cost of AnMBR treatment of kraft mill effluent was significantly lower than for aerobic treatment and only slightly higher than for high-rate anaerobic treatment, although the AnMBR had higher effluent quality (Liao et al., 2006).

An AnMBR treating wool-scouring wastewater achieved a 50% COD removal at an OLR of 15 kg/m3/d. The addition of membrane filtration approximately doubled the biomass concentration and increased the total organic carbon (TOC) and grease removal efficiencies from 45 to 90% and from 33 to 99%, respectively (Liao et al., 2006).

Table 2.12 Summary of AnMBR Performance for Treatment of Industrial Wastewaters

Type of wastewater

Scale

Type of reactor

Reactor volume (m3)

Temp. (ËšC)

HRT (d)

OLR (kgCOD.m-3.d-1)

MLSS (gL-1)

Feed COD (gL-1)

Feed TSS (gL-1)

Effluent COD (gL-1)

COD removal efficiency

Kraft bleach plant effluent

L

CSTR

0.015

35

1.0

0.04e

7.6-15.7

0.04e

-

0.016e

61%e

Kraft pulp effluent

P

UFAF

5

-

0.5

35d

9.4

19.2d

-

1.5d

93%d

Pulp and paper effluent

P

FB

7

-

-

-

-

28

15

1.1

96%

Evaporator condensate (methanol)

P

UFAF

5

53

0.5

35.5d

7.6

17.8d

<0.003

1.2d

93%d

Wool scouring

P

UFAF

4.5

37

6.8

15

-

102.4

30.5

51

50%

L= Laboratory/bench scale, P= pilot scale

CSTR= completely stirred tank reactor, FB= fluidized bed, UFAF= upflow anaerobic filter, M designates the location of the membrane (no M indicates the membrane produced the final effluent)

-= Indicates value not reported

d =Units are BOD instead of COD

e = Units are AOX (adsorbable organic halogen)

2.10.4 High-Solid-Content Waste Streams

Waste streams that contain a high proportion of particulates include wastewater treatment plant sludges, the organic fraction of municipal solid waste, animal processing plant effluents, and manures. Anaerobic digestion is a common technology for treating such high-solids waste streams, as discussed in recent reviews and books. Digestion is usually performed in completely mixed reactors at low organic loadings of 1-3 kg COD/m3/d. The slow hydrolysis/solubilization of particulates is often rate limiting. Therefore, in completely mixed reactors that do not decouple SRT from hydraulic retention time (HRT), the long retention time required for hydrolysis leads to large reactor volumes and lower OLRs (Liao et al., 2006).

In recent years, the AnMBR technology has been successfully tested in both pilot-and-full-scale plants for treatment of high solids wastes, as summarized in Table 2.13. AnMBRs have been tested with wastewater treatment plant sludges, pig manure, and chicken slaughterhouse effluent. Relatively high OLRs of 3-5 kg COD/m3/d were achieved with high COD removals (80% or higher) for the manure and slaughterhouse wastewaters as compared with the usual loadings of 1-3 kg COD/m3/d for high-solids wastes. The COD removals were generally lower for sludge treatment possibly because of a larger portion of inert solids (Liao et al., 2006).

All the studies considered here used a CSTR reactor configuration. Without a membrane, the SRT would have been equal to the HRT. In membrane sludge digesters, however, the complete retention of solids in the reactor decoupled the SRT from the HRT. Pillay et al. were able to increase the reactor solids concentration from 2.6% to 5.5% using a membrane and as a result decreased the HRT by almost half (to 14 d) while the SRT was maintained at 26 d. Pierkiel and Lanting used HRTs of 1-3 d with SRTs of 8-12 d (Liao et al., 2006).

Another proposed advantage of complete retention of particulates and biomass is an increase in the rate of hydrolysis/solubilization of solids (Liao et al., 2006).

Based on the results from pilot-scale trials, Pillay et al. conducted an economic evaluation of a full-scale anaerobic digester for sludge treatment with and without membrane filtration. The results indicated that the AnMBR process was technically and economically feasible, and offered significant advantages over the conventional anaerobic digestion process. Compared to the conventional anaerobic digester, the capital and total project cost saving for AnMBR using their low-cost membrane were 27% and 12%, respectively (Liao et al., 2006).

Table 2.13 Summary of AnMBR Performance for Treatment of High Solids Content Wastes

Type of wastewater

Scale

Type of reactor

Reactor volume (m3)

Temp. (ËšC)

HRT (d)

SRT (d)

OLR (kgCOD.m-3.d-1)

MLSS (gL-1)

Feed COD (gL-1)

Feed TS (gL-1)

Effluent COD (gL-1)

COD removal efficiency

Primary sludge

P

Upflow mixed

0.12

35

20

-

1.06

22-35

40.2

44.4

18

54%

Coagulated raw sludge

L

VFA fermenter CSTR

0.076

35

0.5

10

4.6e

34

2.3e

6.8

1.3e

43%e

Screened sludge

P

Semi continuous CSTR

1.8

-

14

26

-

55

-

-

-

-

Sewage sludge

L

CSTR

0.004

25-40

6.7-20

-

0.17-1.35d

20-40

-

-

<0.3

-

Pig manure

F

CSTR

200

35

10

-

3

-

30

20

2.4

92%

Pig manure

P

2phase CSTR+M/Hybrid

3/3

20/25

1-2/1-2

-/-

2.8-5.5/-

-/-

5.5

0.6

1.1

80%

Chicken slaughterhouse

L

CSTR

0.007

30

1.2

-

4.3

22

5.2

2.4-4.7

<0.5

90%

L= Laboratory/bench scale, P= pilot scale, F= full scale

CSTR= completely stirred tank reactor, Hybrid= UASB with anaerobic filter instead of a solids/liquid/gas separator, M

designates the location of the membrane (no M indicates the membrane produced the final effluent)

-= Indicates value not reported

d= Units are VSS instead of COD

e= Units are TOC instead of COD

2.10.5 Municipal Wastewater

Municipal wastewater is characterized by low organic strength (250-800 mgCOD/L) and low suspended solids concentrations (120-400 mg/L). The aerobic activated sludge process is the dominant technology for treating municipal wastewater and in recent years. Furthermore, the aerobic membrane bioreactor has been widely used. Anaerobic treatment of sewage is not widespread traditionally being performed in UASB reactors in warm climate regions, but it is technically feasible even for temperature climates are discussed in recent reviews. Conventional UASB sewage treatment usually has an HRT of 0.25-0.33 d and results in a BOD removal efficiency of 80%, effluent COD of 100-220 mg/L, and effluent total suspended solids (TSS) of 30-70 mg/L. For example, Elmitwalli et al. found the anaerobic biodegradability of domestic sewage to be 71-74% at 30 ËšC whereas most studies for food processing wastewaters (Table 2.11) had COD removals of >90% (Liao et al., 2006).

Table 2.13 summarizes the studies on the use of AnMBRs for sewage treatment. In general, AnMBR sewage treatment had lower effluent COD (<100 mg/L) and suspended solids concentrations compared to conventional UASB treatment. This is expected because the membrane can provide approximately 100% removal of suspended solids. In addition, the COD or BOD removal efficiency was compared to UASB treatment and very high SRTs could be maintained (e.g., 150 days). AnMBRs also provided high COD removals for the treatment of night soil and sludge heat treat liqour (Liao et al., 2006).

A comparison between conventional activated sludge, aerobic MBR, UASB, and AnMBR for municipal wastewater treatment is shown in Table 2.14. For both aerobic and anaerobic systems, a membrane dramatically improves TSS removal, although the treated effluent quality in terms of COD is better from the aerobic systems than the anaerobic systems. The anaerobic HRT appears to be generally longer than 8 h, compared to 4-8 h for aerobic (Liao et al., 2006).

On the other hand, the anaerobic processes had lower energy requirements than their aerobic counterparts. The electricity use of the UASB is the lowest, well below that of activated sludge. The electricity use of the AnMBR is slightly lower but nevertheles near that of the aerobic MBR when both systems used immersed membranes. However, the electrivity use of both anaerobic systems can be offset by use of the produced methane, so the net energy consumption by both anaerobic systems should be less than the aerobic ones. In all cases electricity consumption will be higher if external cross-flow membranes are used instead of immersed membranes, because a suction pump operates at lower pressure and less water is pumped (Liao et al., 2006).

Table 2.14 Summary of AnMBR Performance for Treatment of Municipal Wastewaters

Type of wastewater

Scale

Type of reactor

Reactor volume (m3)

Temp. (ËšC)

HRT (d)

SRT (d)

OLR (kgCOD.m-3.d-1)

MLSS (gL-1)

Feed COD (gL-1)

Feed TSS (gL-1)

Effluent COD (gL-1)

COD removal efficiency

Night soil (heat-treated and hydrolyzed)

P

UASB

0.4

-

-

-

-

-

25.5

2.6

2.0

92%

Heat-treat liquor

L

CSTR

0.2

37

0.6

-

15.4

21.4

10.3

0.3

2.0

81%

Primary effluent

L

CSTR

0.01

3

0.5

217

1.6

7

0.08

0.12

0.02

68%

Sewage

P

Hydrol CSTR/UASB+M

-/5.4

-

-

-

-

-

1.1

0.5

0.07

94%

Sewage

P

Hydrol CSTR+M/FB+M

2.0/5.4

35/-

3/0.27

-

5.7

7/40

0.49

0.3

0.08

83%

Domestic wastewater

L

Hybrid

0.018

20

0.25

150

0.4-10

16

0.1-2.6

0.1-0.8

<0.03

>92%

L= Laboratory/bench scale, P= pilot scale

CSTR= completely stirred tank reactor, FB= fluidized bed, Hybrid= UASB with anaerobic filter instead of a solids/liquid/gas separator, Hydrol= side-stream suspended solids hydrolysis reactor plus methanogenic reactor for combined hydrolysate and primary clarifier effluent, UASB= upflow anaerobic sludge blanket, M designates the location of the membrane (no M indicates the membrane produced the final effluent)

-= Indicates value not reported

d =Units are BOD instead of COD

e = Units are VSS instead of COD

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