3.3. Advantage and disadvantage of MBR

The best feature of MBRs is that they can easily convert existing activated sludge systems into MBR systems. This can be accomplished by placing submerged membranes in the aeration tank [46]. Membrane bioreactor is separating biological treatment of microorganisms and secondary cleaners from one site to another. The feed water is mixed with the biomass, the mixture is filtered from the membrane, and the biomass is separated from the treated water. Conventional activated sludge (CAS) units compared to the same operational conditions to provide better recovery efficiency in the MBR. Using MBR has many advantages [22, 47] (Table 3).

At higher MLSS concentrations, the ability to work at higher SRT than conventional treatments, reduced biomass yield, higher quality waste, less hydraulic residence time and lower area footprint generation are advantages of MBRs compared to CAS units [48]. This means a small reactor volume and a reduction in the initial investment cost. They are also more resistant to sudden different hydraulic and organic loads and better respond to existing sustainability criteria for municipal wastewater systems [49]. Biomass separation is independent of the ability of the activated sludge to precipitate as it is achieved by microfiltration or ultrafiltration; in other words, there is no need for final sedimentation, no sludge swelling, and sedimentation problems caused by filament growth. Due to high MLSS concentrations, excess organic loading can be done in the system. MBRs are less likely to be negatively affected by nitrification or by business problems related to the toxic effects of toxic organisms [50]. Since the sludge from the membrane system is less than the conventional system, the storage requirement is also reduced [51].

MBRs are becoming increasingly common throughout the world, despite the fact that they can reduce their investment and operating costs and produce effluent that cannot be used despite their different reuse areas. One of the biggest causes of this is the clogging of the wastes, and the transmembrane pressure (TMP) increases to provide a constant flux. Occlusions may occur at the membrane surface or within the membrane pores. Membrane clogs


Table 3. Comparison of MBR and CAS (adapted from [52]).

3.2. Design and operating parameters

MLSS, Mixed liquor suspended solids.

Items Zenon Mitsubishi

Recovery method Chemical soak Chlorine

membranes made in China and those made in other countries.

(2) MBR performance

Aeration per module (m3

48 Wastewater and Water Quality

a

a

b

mostly 10–100 LMH flux values.

amount of oxygen needed to grow the microorganism [43–45].

A number of parameters must be considered in order to activate an economically appropriate MBR system. These include membrane selection, membrane performance (permeate flow, transmembrane pressure, viscosity), biological performance of microorganisms (biomass concentration, ESS, HBS, F/M ratio), and economic factors (energy consumption, sludge treatment, and disposal cost). These parameters can influence each other, and a positive change can be observed in the other parameter by changing one parameter. For example, a high biomass concentration requires a long CIS, which in turn reduces the cost of sludge disposal and sludge disposal. On the other hand, at high sludge age, the cost of energy also increases as the sludge reaches a viscous structure, which leads to the decomposition of the organic fraction and the

Although Kubota was not found very active in China, it was still referenced here in order to compare flat-sheet

Table 2. Summary comparison of membranes used in full-scale MBRs and MBR performance (adapted from [39, 41, 42]).

Rayon

backwash

/h) 14 57–73 — 0.6/panel

MLSS (g/L) 12–30 <15 15–30 10–30

SRT (d) 10–100 <60 >40 40 Sludge yield (kg MLSS/kg BOD) 0.1–0.3 — 0.26

NH3 effluent (mg/L) <0.3 — <2 <2

BOD effluent (mg/L) <2 2–6 — 3–5

Cleaning method Back pulse and relax Relax — Relax Cleaning frequency (min/min) 0.5/15 2/12 1/60

Recovery frequency ≥3 months ≥3 months ≥6 months Recovery location Drained cell or in situ In situ In situ

Tianjin Motimo Kubota<sup>b</sup> Shanghai

Chlorine backwash Zizheng

These designed and operational parameters are used to design the reactor and to be able to differentiate in different configurations applied to the process, to give formulas which are used

The amount of liquid drained from the surface area of the membrane is called flux. MBRs are

in the general working principles of MBRs, also in the definition and calculation.

are roughly divided mechanically into two: recycled (removal of the surface gel and cake layer by aeration or physical backwash) and irreversible (removal of dissolved or colloidal substances in the adsorptive pore accumulation and clogging by chemical cleaning) [53]. MLSS, particle size distribution, soluble microbial by-products, extracellular polymeric materials, viscosity, pore size, porosity, surface energy, electrical charge, hydrophilic/hydrophobic properties parameters are affecting clogging [54]. The formation of cake, which is unavoidable on the membrane surface, is one of the factors that cause the membrane to become contaminated. In a general system, the sidestream of the MBR shows a higher tendency to pollute than the submerged MBR. The reason is that the sidestream MBR needs high pump energy to generate high flux which will cause repetition of pollution when compared to the submerged MBR [37]. Tank reduces production, increases operating and maintenance costs, and requires a special extra cleaning and backwashing. Membrane replacement is challenging. There are more than 10 years of MBR systems. On the other hand, there are many systems that change after 4 years. The main causes are often pollution problems. When contamination is combined with high transmembrane pressure, this contamination is most irreversible, and therefore the chemical cleaning frequency should be increased. This leads to an increase in operating cost by reducing membrane life [51].

charged surface of sludges) or anions (low interaction). Thus, the cationic species would be

Efficient Removal Approach of Micropollutants in Wastewater Using Membrane Bioreactor

http://dx.doi.org/10.5772/intechopen.75183

51

Wastewater temperature also plays an important role. WWTP with an average temperature of 15–20C can be better suited for micropollutants such as in cold countries, which are often below 10C in the USA. Summer and winter affect seasonal temperature changes, microdegradation, and biodegradation [60]. Sorption has been correlated inversely with temperature in the case of the hormone 17α-ethinyl estradiol (EE2), with a reduction of Kd values of

Studies have shown that compounds such as ibuprofen and antiseptic powder, methyl paraben, and galaxolide, an analgesic drug in hospital wastewater, do not have significant differences in effluent efficiency with activated sludge processes and MBR. MBR system was found to be efficient for hormones (e.g., estriol, testosterone, androstenedione) and certain pharmaceuticals (e.g., acetaminophen, ibuprofen, and caffeine) with approximately 99% removal [61, 62]. Experimental investigations show that the removal of such compounds from wastewater is 30–50% superior to that of conventional activated sludge process. In addition, the removal efficiencies of some compounds such as mefenamic acid, indomethacin, diclofenac, and gemfibrozil in MBR were 40%, 40%, 65%, and 32–42% [63, 64]. However, biodegradable erythromycin, TCEP, trimethoprim, naproxen, diclofenac, carbamazepine, and nonylphenoxyacetic acid have not been removed [47]. This is comparable to the results of previous studies which indicated very low elimination rates of diclofenac and carbamazepine in WWTP due to their recalcitrant nature processes in Germany. Hydrophilic compounds such as MBRs, acetaminophen, atenolol, iopromide, and sulfamethoxazole (calculated logP <2) (with the exception of sulfamethoxazole (> 62%) are more efficient than hydrophobic compounds. Hydrophobic compounds (calculated log P > 2) can largely be removed by active sludge biosorption in the MBR and in the middle, and longer holding scoops are formed in the bioreactor, resulting in a higher removal yield from the CAS process. However, some hydrophilic microspheres such as carbamazepine and diclofenac tend to be highly resistant to biological degradation in the treatment of CAS and MBR. The retention time of the hydrophilic and persistent micropollutants in the bioreactor is the same as the retention time of hydraulic retention (HRT), as the micropollutants can freely permeate MF and UF membranes. The duration of hydraulic retention in the MBR and the prolongation of the retention time of the sludge are dependent on the compound biosorption of some hydrophobics for the activated sludge, and it can be seen that the pollutants can improve

In the comparison between the two MBR modules used in this study (plate and frame versus hollow fiber), no difference in target compound removal was found [60, 65]. Some results can be negative efficiency. For example, González-Pérez et al. (2017) have worked on the system that has been operated with complex nitrification and ensured that the biodegradable organic material due to circulation is effectively retained [66]. By reducing the concentration increase in the diclofenac (DCF) in the aerobic bioreactor, negative removal efficiencies for DCF have been

Membrane bioreactor applications for these pollutants in different wastewaters are presented

adsorbed by van der Waals-type interactions [59].

the biodegradation [2, 65].

in detail given in Table 4.

obtained. This was not observed in the anoxic reactor.

20–25% when the temperature was increased from 10 to 30C [59].

The main contributors to energy costs in MBR are sludge transfer, permeate production, and aeration which is often exceeding 50% of total energy consumption. Energy consumption of membrane-related modules was in the range of 0.5–0.7 kWh/m3 , and specific energy consumption for membrane aeration in flat sheet was 33–37% which was higher than in a hollow fiber system. Submerged membranes in MBR reduces the pumping energy requirement to 0.007 kWh/m3 of permeate compared with sidestream membrane (3.0 kWh/m3 ). Future trend of MBR might be focused on two aspects which are reduction of energy demand and membrane fouling [55].
