Maung Htun Oo

Experimental studies and modeling on concentration polarization in forward osmosis

Concentration polarization (CP) is an important issue in forward osmosis (FO) processes and it is believed that the coupled effect of dilutive internal CP (DICP) and concentrative external CP (CECP) limits FO flux. The objective of this study was to distinguish individual contribution of different types of DICP and CECP via modeling and to validate it by pilot studies. The influence of DICP/CECP on FO flux has been investigated in this study. The CP model presented in this work was derived from a previous study and evaluated by bench-scale FO experiments. Experiments were conducted with drinking water as the feed and NaCl/MgSO(4) as draw solutions at different concentrations and velocities. Modeling results indicated that DICP contributed to a flux reduction by 99.9% for 0.5 M NaCl as a draw solution although the flow pattern of both feed and draw solutions was turbulent. DICP could be improved via selection of the draw solution. The modeling results were well fit with the experimental data. It was concluded that the model could be used for selection of the draw solution and prediction of water flux under similar situation. A draw solution with greater diffusion coefficient or a thinner substrate of an asymmetric FO membrane resulted in a higher flux.

Preliminary study of osmotic membrane bioreactor: effects of draw solution on water flux and air scouring on fouling.

Preliminary study on a novel osmotic membrane bioreactor (OMBR) was explored. Objective of this study was to investigate the effects of draw solution on membrane flux and air scouring at the feed side on fouling tendency in a pilot OMBR system composing the anoxic/aerobic and forward osmosis (FO) processes. Domestic sewage was the raw feed, FO membrane from HTI and NaCl/MgSO4 draw solutions were used in the experiments. Fluxes of 3 l/m2/h (LMH) and 7.2 LMH were achieved at osmotic pressure of 5 and 22.4 atm, respectively. No significant flux decline was observed at 3 LMH over 190 h and at 7.2 LMH over 150 h when air scouring was provided at the feed side of the membrane. However, without air scouring, the flux at 22.4 atm osmotic pressure declined by 30% after 195 h and then levelled off. The potential advantages of the fouling reversibility with air scouring under the operating conditions of the pilot OMBR and better water quality in OMBR over the conventional MBR were preliminarily demonstrated.

Effect of feed pH on an integrated membrane process for the reclamation of a combined rinse water from electroless nickel plating

This study explored the effect of feed pH on an integrated membrane process of ultrafiltration/reverse osmosis (UF/RO) for the reclamation of spent rinse water from an electroless nickel-plating operation. Feed pH in the range of 2.54–6.64 was investigated. For UF pre-treatment, the experimental results showed that iron was not rejected by UF membrane when feed pH was below 3.68. However, when feed pH was raised to 3.68, iron rejection increased significantly and reached 98.7% at pH 6.64. For the RO process, it was found that for a particular wastewater containing weak acids and weak bases in this study, permeate pH was higher than the feed pH when the feed pH was low, but lower than the feed pH when the feed pH had reached 6.64. The critical point at which permeate pH would be equal to feed pH was about 6. This observation has not been reported previously. An attempt at explaining the observation was offered. The results showed that concentration of all measured contaminants in RO permeate decreased with an increase in feed pH. As a consequence, the permeate conductivity decreased with increasing feed pH. The membrane flux decreased with increasing feed pH. Total organic carbon (TOC) rejection increased with increasing feed pH due to the enhanced electrostatic repulsion between the negatively charged membrane surface and the negatively charged HOCH2CO2−. Moreover, the caustic soda consumption used for neutralization could be reduced by 39% in the integrated membrane process if the feed pH was set at 3.68 instead of 6.64. It was concluded that pH of the feed had a significant effect on the integrated membrane UF/RO process and the optimum pH of feed water for the treatment was in the range of 3.7–5.6.

A pilot study for reclamation of a combined rinse from a nickel-plating operation using a dual-membrane UF/RO process

Abstract A pilot study for reclamation of a combined rinse from a nickel-plating operation was conducted using a dual-membrane UF/RO process. The pilot plant has a product capacity of 1.5 m 3 /h. The OF unit, as a pre-treatment, was operated at 90% water recovery. The RO unit was operated with a 2:1 configuration in a feed-and-bleed mode with recirculation. Trial runs on various operating pressures and water recoveries were conducted and effect of feed pH on RO permeate quality was studied. Finding a critical pH value was explored to design the feed pH for RO process to treat this particular wastewater. A long-term run for the RO unit with an optimized 75% water recovery was later carried out to study the stability of the product and the fouling tendency of membranes. The cleaning-in-place methods were investigated for both OF and RO units. The pilot plant had successfully operated for 6 months at the time of reporting, consistently producing a high-quality product water ( 3 /h.

Impact of anti-scalant on fouling of reverse osmosis membranes in reclamation of secondary effluent

The objective of the study was to evaluate the impact of anti-scalant on fouling of reverse osmosis (RO) membranes in reclamation of secondary effluent which was produced by a conventional activated sludge process at Kranji Water Reclamation Plant with the capacity of 151,000 m3/d. The study was carried out using a RO pilot plant with the capacity of 2.4 m3/h. The RO plant was in 2:1 configuration and was operated at 75% recovery and at membrane flux of 17 l m(-2) h(-1). Pilot trials were conducted with and without anti-scalant. Compositions of feed and concentrate streams were analyzed and the pilot data were normalized. The results of the study showed that the plant operation was stable during the first few days after stopping dosage of anti-scalant but after 3-6 days of operation the membranes were fouled. The time lag effect of anti-scalant without dosage was not reported previously and could be potentially beneficial to save chemicals. The membrane fouling was more serious at the second stage due to the formation of calcium phosphate scale when the pilot plant was operated without anti-scalant. The flux of fouled membranes could be completely recovered after clean-in-place (CIP) with citric acid, indicating that scaling dominated the fouling of the RO membranes. These findings in the study could be applied to select an appropriate anti-scalant for prevention from formation of calcium phosphate scale in the RO operation.

Boron removal and zeta potential of RO membranes: impact of pH and salinity

Abstract Boron removal is a challenging task for any seawater desalination plant employing a singlepass reverse osmosis (RO) system. With a pKa value of 9.25, more than 99% boron removal is normally achieved at pH >10.5. As pKa value is theoretically lower at higher salinity, an increase of borate ion and a corresponding increase in boron removal could be expected as salinity increases. However, boron removals by CPA2 and SWC4+ membranes were found to decrease or, sometimes, unchanged at higher salinity where membrane zeta potentials shifted from negative values to positive values were observed at higher salinity at both pH 7 and 9. The decrease in boron removal by CPA2 membrane with increasing salinity at pH 9 could be attributed to the reduced charge repulsion mechanism. Similarly, boron removal by SWC4+ membrane decreased with increasing salinity. However, the removal efficiency reached its lowest value at 2000 mg/l of NaCl at pH 9. The subsequent increase in boron removal efficiency at salinity higher...

Pilot study of a submerged membrane bioreactor for water reclamation

The objective of this pilot study was to investigate the operational conditions of newly developed MBR modules for water reclamation under tropical conditions. MUDC-620A MBR modules with hollow fibre PVDF membranes from Asahi-Kasei Chemicals were used in the study. The pilot plant with capacity of 50 m(3)/d was operated continuously (24-hour) over four months on site of Ulu Pandan Water Reclamation Plant (UPWRP) in Singapore. During the study, the MLSS in membrane tank was in the range of 6,840 approximately 9,540 mg/L. Filtration operation mode of the membrane unit was 9 minutes on production and 1 minute backwash. The air scouring for the membranes was 0.18-0.30 Nm(3)/h per m(2) membrane area all of the time. Trials on different membrane fluxes were conducted to obtain the sustainable flux. The analytical results showed that COD, TOC, T-N and NH4-N of the treated water were <30 mg/L, 5-7 mg/L, <13 mg/L and <0.1 mg/L, respectively, which met the requirement of Industrial Water for reuse. TMP was in the range of 12-40 kPa and could be recovered after cleaning with 2,000 mg/L sodium hypochlorite solution. Sludge clogging could be a challenge for long-term operation with the current module design. It was concluded that it was feasible for MUDC-620A MBR to operate at a net flux of 25-29 LMH (or 0.6-0.7 m/d) for treating the municipal wastewater at UPWRP.

Integrated coagulation–ultrafiltration for enhanced removals of phosphate and organic in tertiary treatment

Abstract We have demonstrated an integrated coagulation–ultrafiltration (UF) process for enhanced removals of phosphate and dissolved organic carbon (DOC) in tertiary treatment. A lab scale system with hollow fiber UF membranes was used in the study. Dead-end operation was applied in the study since its advantages of low energy consumption and high water recovery over cross-flow operation. The results showed that removals of phosphate and DOC at alum dosage of 10 mg l−1 in the study were >99% (or phosphate <0.03 mg l−1 in product) and 25%, respectively. The coagulation time in the new integrated coagulation-UF process was reduced to 1 min with much less foot-print. The concentration of alum dose could be further optimized between 5 and 10 mg l−1.

Pilot Study on Treatment of Wastewater from an Ethylene Plant with Membrane Bioreactor Technology

Pilot studies were conducted with an anoxic/aerobic concept membrane bioreactor (MBR) technology and a hollow fiber Petro ® MBR system with capacity of 12 m 3 /d was operated continuously (24-hour) during the study. Trials on different membrane fluxes were conducted to obtain the sustainable flux while mixed liquor suspended solid (MLSS) was maintained at 9-11 g/L. The results of the MBR pilot trials showed that no obvious fouling of the membrane was found when the plant was operated at the flux of 12 L /m 2 /h (LMH) over 3 months and 15 LMH over one month during the pilot study. Design guidelines such as hydraulic retention time (HRT), sludge retention time (SRT), anoxic and aera- tion volume ratio, re-circulation flow rate and air scouring were obtained for a full-scale plant. It was concluded that treatment of wastewater from an ethylene plant without addition of any chemicals using MBR technology is feasible. The product quality consistently met the requirement for discharge and was suitable for the feed of further reverse osmosis (RO) post-treatment.