Physical and economical evaluation of laboratory-scale membrane bioreactor by long-term relative cost – bene ﬁ t analysis

Two laboratory-scale single-stage submerged membrane bioreactors (MBRs) were operated in parallel to examine the effect of different ﬂ ux conditions and several fouling mitigation methods. After control operation ( ﬁ ltration only), three fouling control methods (relaxation, standard backwash and chemical backwash) at 27 LMH ﬂ ux and four different ﬂ ux conditions (54, 36, 27 and 18 LMH) with standard backwash were applied. Physical performance of MBRs was evaluated based on the operational duration toreachmaximumtransmembranepressureandthevolumeofpermeateproducedduringtheoperational duration. Then relative cost – bene ﬁ t analysis was carried out. Results showed that the combination of chemical backwash and standard backwash was the most effective for fouling mitigation in terms of physicalimprovementof MBR performance. However, thecombinationprovedlesseconomical(400% þ α relative cost) than standard backwash alone (343% relative cost), because of the additional cost for pumps andchemical.Italsoshowedthatlower ﬂ ux(18LMH)isdesirableasitshowedbetterphysicalperformance (1,770% improvement as compared to the highest ﬂ ux, 54 LMH) and proved more economical than higher ﬂ ux con ﬁ guration. Therefore, it is concluded that the operation with standard backwash at the lowest possible ﬂ ux is the best combination to improve MBR performance as well as long-term cost – bene ﬁ t. Chemical backwash supported by standard backwash showed the highest improvement (874%) in physical performance as compared to control operation. However, the results of the relative cost – bene ﬁ t analysis showed that the chemical together with standard backwash operation was less economical than standard backwash operation, because it showed higher long-term cost (400% þ α ) than that of standard backwash operation (343%).


INTRODUCTION
Membrane bioreactor (MBR) technology has been adopted for wastewater treatment and studied extensively worldwide since the advancement of the immersed configuration in the 1990s (Lorhemen et al. ). In accordance with international progress in wastewater treatment technology, interest in MBR is now rising in the last several years in Pakistan as well to alleviate water-related problems like water scarcity and environmental water degradation. Despite many advantages of MBR (i.e. smaller footprint, better effluent quality, less sludge production, easy operation, etc.), one hindrance of biofouling remains as an unsolved challenge (Guo et al. ; Kim et al. ; Nahm et al. ). To tackle the challenge, many studies were conducted to mitigate the fouling with various approaches. These approaches include physical cleaning methods, such as back flushing (termed in this paper as standard backwash) in which permeate is reversed in the opposite direction, to remove reversible fouling (Krzeminski et al. ).
Another physical cleaning method is air scouring, in which coarse bubble aeration is provided by air pumps, to achieve a significant drop in the transmembrane pressure (TMP) (DeCarolis et al. ). Relaxation is another physical cleaning technique in which the filtration is stopped for some time and the system relaxes, which can help to prolong the operation for some time (Lade et al. ). In addition to the physical methods, there are also some chemical cleaning methods which are used for the removal of reversible fouling in MBR.
One of these methods is known as chemically enhanced backwash (CEB) in which a mild fouling cleaning chemical, such as sodium hypochlorite, is added to the backwash water (permeate water) (Kimura & Uchida ). Other than these, an advanced biological method in combination with physical and chemical cleaning methods is also used these days for decreasing fouling, i.e. Quorum Quenching (QQ) (Weerasekara et al. ).
QQ technique has been used to mitigate biofouling as QQ bacteria are introduced in the reactor either in the form of culture or in the form of immobilized beads that disturb quorum sensing (QS) signals (Waheed et al. ). It opens a new path and proves to be a new favourable alternative to delay time reaching the maximum TMP (Maqbool et al. ).
Including all these approaches, physical cleaning methods, chemical cleaning methods (use of various cleaning agents, such as NaOCl, acids, etc.), physico-chemical methods (combination of aforementioned two) and biological methods (cleaning mixtures based on microbial cultures or enzymes) (Cinar et al. ), an introduction of new fouling control techniques in MBR operation may further increase its overall performance. However, the addition of specific cost is required for the application of the new technique. Hence, performance improvement of MBR should be backed up by economic justification through a cost-benefit analysis (Xiao et al. ). Therefore, this study was designed with two distinct aims. Firstly, to find out the best combi-

METHODOLOGY MBR setup, operation and analysis
The two series of laboratory-scale MBR plants were constructed and operated in parallel to treat synthetic wastewater that simulates medium strength domestic wastewater. It was prepared by 10 times dilution with tap water from a stock wastewater solution that was 10 times stronger than the intended concentration. The stock wastewater was made of Glucose 1,200 mg/L, Peptone 900 mg/L, Yeast extract 120 mg/L, (NH 4 ) 2 SO 4 960 mg/L, KH 2 PO 4 170 mg/L, NaHCO 3 3,000 mg/L, CaCl 2 ·2H 2 O 24 mg/L, MgSO 4 ·7H 2 O 240 mg/L, MnSO 4 ·5H 2 O 21.6 mg/L and FeCl 3 ·6H 2 O 1.2 mg/L as per composition described in another study (Weerasekara et al. ). It was stored at 4 C in a refrigerator and used as per requirement. Actual values of BOD, COD and NH þ 4 -N of the influent of MBR were 178 ± 12 mg/L, 223 ± 15 mg/L and 28 ± 2.5 mg/L, respectively.
The seed sludge was obtained from a pilot-scale MBR plant at National University of Science and Technology, Islamabad, Pakistan. The MLSS was grown from around 2,000 mg/L in the beginning to 8,000 mg/L during the stabilization phase (3 months). It was maintained at 8,000 mg/L by withdrawing excess sludge whenever the concentration became higher. Two reactors were used in parallel to operate MBRs with different conditions. MBRs were operated with a constant flow mode until they reached a maximum TMP value of 4.8 psi as recommended by the membrane manufacturer. Whenever the TMP of the MBR reached the maximum limit, it was considered that the membrane module got fouled. All the operational parameters including F/M ratio, aeration flow rates, influent, effluent (i.e. permeate) and backflush were maintained within the range shown in Table 1

Research variables
In order to improve MBR performance and lifespan of membranes, many approaches have been developed by researchers. However, in this research, the effect of flux on membrane fouling and effect of three fouling control techniques, among the approaches already developed, were

Relative cost-benefit analysis
For construction and operation of MBR plant, the major cost of concern is for initial membrane installation, replacement after its lifespan finishes and initial pump installation and energy cost for pump operation. Energy consumption for aeration is one of the major concerns

RESULTS AND DISCUSSION
Removal efficiency of COD, BOD and NH þ 4 -N Average removal efficiencies of COD, BOD and NH þ 4 -N were fairly good to be 91.7, 97.2 and 98.9%, respectively, throughout this research period (Table 3). There were minor differences between operations which are statistically insignificant. P-values of One-way ANOVA tests for all the

TMP profiles of different fouling control methods
It was observed that the 1st operation (filtration only; a con-

Comparison of production of treated water
Permeate production per cycle of each operation was calculated by flow rate and operational duration with due consideration to the relaxation or backwash time and the volume of water used for backwash. Backwash flow rate was double the permeate flux.
As shown in Figure 4

Relative cost-benefit analysis
For the comparison of fouling control techniques, flux was kept constant. Membrane surface area is the same for all operations, which means the initial membrane installation cost is the same. So the initial cost for membrane installation is insignificant in relative cost. replacement of membranes. Therefore, the relative cost of membrane replacement is inversely proportional to relative production. Relative cost caused by quicker replacement was calculated to be 143, 854 and 974% for standard backwash, relaxation and filtration only operation respectively, as compared to the longest operation (chemical þ standard backwash). When an MBR plant runs for long-term, total relative costs for chemical þ standard backwash, standard backwash, relaxation and filtration only operations are then 400% þ α (α: additional cost for chemical), 343%, 954% and 1,074% respectively.
Although the operational duration and production of chemical þ standard backwash were the greatest, it was found to be less economical than that of standard backwash in long-term operation because of the higher initial cost for chemical pump and additional cost for chemicals. Therefore, standard backwash operation was found to be the most economical for long-term operation as its total relative cost is shown to be the least (circle in Figure 5(a)). As shown in Figure 5(

CONCLUSION
From the result of performance evaluation of fouling control methods, two times of chemical backwash per day supported by standard backwash can be considered as the best anti-fouling technique as it showed the highest improvement (874%) as compared to control operation (filtration only). However, it was found through relative cost-benefit analysis that standard backwash operation was the most economical in long-term operation, although its performance improvement (580%) was not as high as a chemical with standard backwash, because the relative long-term cost for standard backwash operation (343%) was smaller than that of chemical together with standard backwash operation (400% þ α). The lowest possible flux is recommended to improve MBR performance and to reduce long-term cost as 18 LMH operation showed the greatest improvement (1,770%) compared to 54 LMH operation and the smallest relative long-term cost. Therefore, it is concluded that operation with standard backwash as a fouling control method at the lowest possible flux is the best combination to improve MBR performance in terms of physical improvement as well as long-term cost. It is also recommended that any new fouling control method should be justified not only by physical performance improvement but also by cost-benefit analysis. Although this study does not indicate any scaling effect for readily usable cost-benefit analysis to full-scale plants, this approach may be useful to justify any new fouling technique in the future of MBR research.