Effect of operating parameters and cleaning on the performance of ceramic membranes treating partially clarified sugar cane juice

The performance of ceramic membranes with pore sizes of 0.05 and 0.10 mm in purifying limed and partially clarified sugar cane juice was investigated under different operating conditions. From various operating conditions and strategies, switching off the permeate for 5 seconds for every 5 minutes (S5sT5 m) by an automated control valve provided higher flux. From the three pH experiments conducted on the 0.05 mm membrane, the best performance was observed at a pH of 7.5. Amongst the four fouling models tested, the cake filtration model fitted the performance of both membranes with higher accuracy at a transmembrane pressure of 0.5 bar. Filtering the cane juice through the membrane reduced the turbidity by 99.7%, color by 15%, and the starch concentration by 80% as well as increased the purity by 1.4%. The effective cleaning chemical composition from experimental results showed that 1% NaOH and 3000 ppm NaOCl solution performed the best but only for the experiments that were treating limed and partially clarified juice at pH 7.5.

Powdered activated carbon for fouling reduction of membrane in a pilot-scale recirculation aquaculture system

Recirculating aquaculture systems (RAS) are essential for the reduction in fresh water usage as well as the discharge of nutrients along with aquaculture effluents. A RAS consisting of an anoxic reactor, a membrane bioreactor (MBR) and a UV-disinfection unit was used to process 10,000 L/d of aquaculture effluent providing high-quality treated water for recirculation to a Barramundi fish culture. The system maintained low levels of nitrate (<20 mg/L), nitrite (<3 mg/L) and ammonia (<0.6 mg/L) in the fish tank. Permeate from the membrane that was recirculated to the fish tank contained <21 mg/L of nitrate, <2 mg/L of nitrite and 0 mg/L of ammonia. However, the rate of fouling of the membrane in the MBR was around 1.47 kPa/d, and the membrane in the MBR required cleaning due to fouling after 16 days. Cleaning of the membrane was initiated when the TMP reached around 25 to 30 kPa. In order to reduce the rate of fouling, 500 mg of powdered activated carbon (PAC) per litre of MBR volume was introduced, which decreased the rate of fouling to 0.90 kPa/d. Cleaning of membrane was needed only after 31 days of operation while maintaining the treated effluent quality. Thus the frequency of cleaning could be halved due to the introduction of PAC into the MBR.

Performance evaluation of different ultrafiltration membranes for the reclamation and reuse of secondary effluent

This study investigates and compares the performance of two different types of ultrafiltration (UF) membranes in the recovery of water from secondary treated wastewater. Filtration experiments were carried out on a pilot scale cross-flow unit using synthetic wastewater similar to the quality of secondary treated wastewater by varying the operating parameters such as transmembrane pressure (TMP), feed composition and membrane configuration. The filtration experiments demonstrated that the flux recovery through spiral polymeric UF membrane was more sensitive to the variation in TMP compared to the tubular ceramic UF membrane over the range of TMP studied. The resistance in series model was used for the evaluation of the resistance to the permeate flux. The fouling resistance, particularly irreversible resistance compared to reversible resistance plays a major role in the total resistance for the tubular ceramic membrane. In contrast clean membrane resistance is the major contributor for the total resistance of the spiral polymeric membrane. Finally, the effectiveness of the filtration treatment was determined by evaluating the rejection coefficients for various pollution indices of the wastewater. Significant differences in the performance of the membrane types were observed which are likely to impact on the selection, operation and maintenance of the membrane system.

The application of ultrasound to dairy ultrafiltration : the influence of operating conditions

Work previously presented has shown that ultrasound can be effective in enhancing both the production and cleaning cycles of dairy membrane  processes. In this present work we extend these previous results to consider the effect of ultrasonic frequency and the use of intermittent ultrasound. These results show that the use of continuous low frequency (50 kHz) ultrasound is most effective in both the fouling and cleaning cycles. The application of intermittent high frequency (1 MHz) ultrasound is less effective. At higher transmembrane pressure, high frequency pulsed sonication can indeed lead to a reduction in steady state membrane flux. The benefits of ultrasound arise from a reduction in both concentration polarization and in the resistance provided by the more labile protein deposits that are removed during a water wash. Conversely, the loss of membrane flux when high frequency pulsed sonication is used arises from a significant increase in the more tenacious ‘irreversible’ fouling deposit.

The optimisation of ultrasonic cleaning procedures for dairy fouled ultrafiltration membranes

Ultrafiltration (UF) of whey is a major membrane based process in the dairy industry. However, commercialization of this application has been limited by membrane fouling, which has a detrimental influence on the permeation rate. There are a number of different chemical and physical cleaning methods currently used for cleaning a fouled membrane. It has been suggested that the cleaning frequency and the severity of such cleaning procedures control the membrane lifetime. The development of an optimal cleaning strategy should therefore have a direct implication on the process economics. Recently, the use of ultrasound has attracted considerable interest as an alternative approach to the conventional methods. In the present study, we have studied the ultrasonic cleaning of polysulfone ultrafiltration membranes fouled with dairy whey solutions. The effects of a number of cleaning process parameters have been examined in the presence of ultrasound and results compared with the conventional operation. Experiments were conducted using a small single sheet membrane unit that was immersed totally within an ultrasonic bath. Results show that ultrasonic cleaning improves the cleaning efficiency under all experimental conditions. The ultrasonic effect is more significant in the absence of surfactant, but is less influenced by temperature and transmembrane pressure. Our results suggest that the ultrasonic energy acts primarily by increasing the turbulence within the cleaning solution.

Biodegradation of pentachlorophenol in a membrane bioreactor

Pentachlorophenol (PCP) is a toxic chemical, often used in the formulation of pesticide, herbicide, anti fungal agent, bactericide and wood preservative. This study is aimed at evaluating the potential of membrane bioreactor (MBR) to treat PCP contaminated wastewater. Synthetic wastewater with COD of 600 mg/L was fed into the MBR at varied PCP loading rate of 12–40 mg/m3/d. A PCP removal rate of 99% and a COD removal rate of 95% were achieved at a hydraulic retention time of 12 hs and a mixed liquor suspended solids (MLSS) concentration of 10,000 mg/L. When sodium pentachlorophenol (NaPCP), which has higher solubility in water, was used in the second phase of the study, at loading rates varying from 20 to 200 mg/m3·d, the removal rate of NaPCP was higher than 99% and the removal rate of COD was more than 96%. It was also found that at higher biomass concentrations, biosorption played an important role besides the biodegradation process. Batch experiments conducted in this study revealed that the sorption capacity to be 0.63 (mg PCP/g biomass) and occurred rapidly within 60 min. This phenomenon could enhance the PCP degradation through increased contact between microorganism and PCP. Further, the membrane resistance was low (trans-membrane pressure of 14 kPa) even after more than 100 ds of operation. In addition, the toxic level of PCP in the influent could have induced the microorganisms to secrete more extra-cellular polymeric substances (EPS) for their protection, which in turn must have increased the viscosity of the mixed liquor.

Biological treatment of oily wastewater from gas stations by membrane bioreactor

In many Asian countries, rapid industrialization and urbanization has led to an increased number of cars, making wastewater from gas stations an important issue of concern in urban environment. This wastewater is characterized by high concentration of oil-water emulsion, which cannot be effectively removed by a conventional gravity separator. An experimental investigation on the treatability of oily wastewater from gas stations using a membrane bioreactor (MBR) system revealed that MBR system could achieve good removal efficiency with stability against shock loading. Optimum operating conditions were found to be at a hydraulic retention time of 4 h and an oil-loading rate of 1.8 kg oil m^sup -3^.d^sup -1^. It was anticipated that adding powdered activated carbon (PAC) in the MBR could help to adsorb the oils. However, operating the MBR with only microbial flocs has an advantage over adding PAC particles into the MBR, since the former condition could provide a prolonged cycle of filtration with a relatively lesser increase in transmembrane pressure.

Application of submerged membrane bioreactor for aquaculture effluent reuse

Discharging the nutrient rich aquaculture effluents into inland water bodies and oceans is becoming a serious concern due to the adverse effect that brings in the form of eutrophication and subsequent damages to those waters. A laboratory scale biological reactor consisting of a denitrifying compartment followed by a submerged membrane bioreactor (SMBR) compartment was used to treat 40 L d−1 of aquaculture effluent with an average concentration of 74 mg L−1 nitrate (NO3 − ). Sugar was added to the aquaculture effluent in order that to enter into the denitrifying compartment at a carbon: nitrogen ratio (C:N) of 2:1 and 4:1. A hollow fibre membrane with a pore size of 0.4 μm and a filtration area of 0.20 m2 was used in the SMBR and was operated at an average flux of 0.20 m3 m−2 d−1. An intermittent suction period of 12 min followed by a relaxation period of 3 min was maintained in the SMBR throughout the experiment. Different aeration rates of 1, 3, 5 and 10 Lpm were applied to the SMBR to determine the rate of membrane fouling and 5 Lpm aeration rate was found to be optimum with respect to the rate of fouling of membrane at a C:N ratio of 4:1. The average rate of fouling at 1, 3, 5 and 10 Lpm were 1.17, 0.70, 0.48 and 0.52 kPa d−1, respectively. The increase in the rate of fouling when the aeration was increased from 5 to 10 Lpm may be due to the breakage of suspended particles into finer particles which could have increased the fouling of membrane. It was also found that increasing the C:N ratio from 2:1 to 4:1 resulted in more cake being formed on the membrane surface as well as an increase in the reduction of NO3 − from 64% to 78%. Preliminary calculations show that 2.4 to 3.2 g of suspended solids could be accumulated per square meter of membrane surface before physical cleaning of membrane is required (at a transmembrane pressure of 20 kPa).

Implications of short and long term critical flux experiments for laboratory-scale MBR operations

This paper evaluates the critical flux obtained by different techniques including tests with different flux step lengths (20 and 40 min and 7 days) and modes of operation (continuous and intermittent) under low and high MLSS concentrations. The paper also analyses a couple of long-term tests (flow rate of 40 and 20 L/day) to obtain the time required to reach the critical flux experimentally and compares those values with the results obtained numerically from a mathematical model. It was found that intermittent mode with membrane relaxation was useful in controlling the fouling of membrane and in restoring the membrane from fouling at lower MLSS.

Evidence of changes in membrane pore characteristics due to filtration of dye bath liquors

This study was carried out to investigate the treatment of various salt solutions and synthetic dye bath liquors by nanofiltration using Nanomax-50 membrane in a stirred cell with 150 mL working volume. Donnan exclusion was compared by filtering salts with monovalent and divalent cations and anions. This was done by comparing three salts including sodium chloride (NaCl), calcium chloride (CaCl2) and sodium sulphate (Na2SO4). The rejection order determined was Na2SO4>NaCl>CaCl2 which is typical of a negatively charged membrane where Donnan and steric exclusion play an important role in separation. Studies on the flux and rejection characteristics of sodium sulphate were undertaken for concentrations ranging from 10 to 40 gl−1 thereby replicating actual dye bath salt concentrations. Synthetic dye bath liquors were prepared using acidic dye (Acid Green 25) at a fixed concentration of 100 mgl−1 with 10 and 15 gl−1 of sodium sulphate solutions. While, the results showed evidence of flux decline due to increased resistance and decreased transmembrane pressure, pore enlargement occurred after the filtration experiments with sodium sulphate solutions greater than 20 gl−1. Pore enlargement was even more prominent in the two synthetic dye bath liquors filtered. Pore enlargement was determined by observing the pure water flux before and after filtering sodium sulphate solutions or dye bath liquors. An increase in pore diameter of 58 and 94 %was estimated when dye bath liquors containing 10 and 15 gl−1 of sodium sulphate, respectively were filtered through the membrane. The following equation was derived in estimating the pore enlargement, where de1 and de2 are the apparent diameter of membrane pore sizes before and after filtration of salt solutions or dye bath liquors and Rm1 and Rm2 are the membrane resistance of pure water flux before and after filtration of salt solutions or dye bath liquors. These results have important implications for the application of nanofiltration technology to textile wastewater treatment and reuse.

Comparative study on performance of yeast and bacterial membrane bioreactors for high salinity wastewater treatment

Two laboratory-scale membrane bioreactor systems were investigated to treat high saline wastewater containing 1,000 mg/L COD and 32 g/L NaCl, namely: the yeast membrane bioreactor (YMBR) and the bacterial membrane bioreactor (BMBR). COD removal of both processes was above 90% at a hydraulic retention time (HRT) of 5 hours (volumetric loading of 5 kg COD/m³.d), sludge retention time (SRT) of 50 days (the MLSS of above 14 g/L and the F/M of 0.4 d-1). Under these operating conditions, the YMBR could run at a ten-fold lower transmembrane pressure with significantly reduced membrane fouling rate compared to BMBR. This may be because of low production of adhesive extracellular polymers (ECP) and the secondary filtration layer formed from large yeast cells. ECP production of bacterial sludge was increased considerably at high salt concentrations (32 g/L and 45 g/L) and long SRTs. For the bacterial sludge, the increased salinity led to increase in ECP, whereas the ECP content of the yeast sludge was relatively small.