By Dr. Guihe Tao, Kiran Kekre, Dr. Jianjun Qin, Maung Htun Oo, Bala Viawanath and Harry Seah
Introduction
Singapore has been investing in new technologies to ensure a diversified and sustainable water supply for its people. A dual ultrafiltration (UF) or microfiltration (MF) reverse osmosis (RO) membrane process has been successfully applied to produce high-grade, ultra-clean water from treated used water (Editor’s Note: Singapore has stopped using the term wastewater). Branded NEWater in Singapore, it is primarily supplied to wafer fabrication plants for the production of ultrapure water, commercial buildings for cooling towers and other industries for process applications. This achievement is the first in the world. 
A small amount of NEWater is also blended with raw reservoir water before undergoing treatment at the waterworks. (NEWater is expected to be 30 percent of total supply by 2011; currently it is roughly one-third that amount. PUB blends the country’s reservoir waters for nationwide uniformity of product.)
Previous pilot studies have demonstrated the advantages of membrane bioreactor (MBR) technology for reclamation of residential/domestic used water in Singapore. (For over 20 years, MBRs have been successfully used to treat wastewaters for discharge and reuse applications. Submerged in each MBR are membranes that physically reject pathogens and other suspended solids. However, it is the biological process that removes contaminants such as biochemical oxygen demand (BOD), nitrogen and phosphorus. As such, MBRs are generally only one part of a system that is designed to biologically treat wastewater. )
This study was to investigate a new MBR-RO process to produce water up to drinking water standards from domestic used water. This integrated MBR-RO system could also be used in other water reclamation projects.
Methods
MBR-RO pilot plant
An MBR-RO integrated system was used for this pilot study at Bedok Water Reclamation Plant. The MBR pilot plant had a treatment capacity of 300 m3/d (Figure 1) and the RO pilot plant had a production capacity of 25m3/day. The settled domestic used water was used as MBR feed and part of the MBR permeate was used as the RO feed. A branch stream of the filtrates from two MBR plants with submerged hollow-fiber membranes of different pore sizes (0.4 µm and 0.035 µm) was fed to the RO plant separately. The RO pilot consisted of six spiral-wound elements* configured in single stage and was operated at a water recovery rate of 50 percent. The pilot system ran continuously (24-hour) during the study. Trial runs on various fluxes of the RO membrane were conducted over six months. A comparison of the new MBR-RO process to the conventional activated sludge treatment (AS) MF-RO process was also made. AS had a sludge retention time (SRT) of five days. MF is 0.1µm PVDF cross flow membrane. The operating conditions of the MBR and RO pilot plants are given in Tables 1 and 2.
Analytical and on-line monitoring methods
Liquid: COD, SCOD, BOD5, TSS, TN, TKN, NH4+-N, TP, TDS, alkalinity and pH of settled sewage were analysed twice per day in the laboratory in accordance with the Standard Methods (20th ed.). TP and TKN of MBR product and tertiary UF permeate were analyzed regularly in the laboratory. GC-ECD, HPLC, and GC-MS scans were performed for MBR product and tertiary UF permeate. The mixed liquor from membrane tanks was filtered using 0.4µm membrane filter, the filtrate was measured for TOC as supernatant TOC; TOC was analyzed**.
Sludge: MLSS were analyzed weekly in the laboratory; viscosity was analyzed twice a week***.
On-line monitoring: TOC, NH4+-N, NO3–-N and NO2–-N of MBR product were monitored on-line; **** TOC of RO permeate was on-line monitored as well. ***** The typical settled used water fed to MBR and AS is shown in Table 3.
Results and discussion
On-line TOC readings of RO permeate were similar under the same operating conditions when the feeds were from different MBR plants; however, the TOC readings of RO permeate from AS-MF-RO and MBR-RO were different. This was due to the lower TOC of MBR permeate than that of AS-MF permeate and the longer SRT of MBR than that of AS. Difficult biodegradable compounds that could not be removed in AS could be removed in MBR. Figure 2 illustrates the on-line TOC readings of RO permeate from AS-MF-RO and MBR-RO processes. The TOC level of RO permeate from AS-MF-RO process fluctuated in the range of 33-53 ppb, while the TOC level from the MBR-RO process was in the narrow range of 24-33 ppb. The results showed that MBR-RO had better product quality and consistency than AS-MF RO.
During one shock loading incident, GS-MS and GC-ECD scan results showed that organic compounds were below detection limits for volatile, semi-volatile organics and similar ranges for halogenated compounds for both RO permeates from MBR-RO and AS-MF-RO; however, analysis of organics by HPLC Scan indicated that there were more organics of small molecules with retention time between 11 and 14.5 minutes in AS-MF-RO than in the MBR-RO permeate from the pilot plant (Figures 3, 4 and 5).
Rejections of the RO membrane for most components were similar in both processes. However, MBR-RO permeate also had lower NH4+-N and NO3– besides the lower TOC (Tables 4 and 5). The better quality of RO permeate from MBR-RO could be attributed to the better performance of MBR than AS process on the reduction of the biodegradable components.
There was no clean-in-place (CIP) conducted throughout the MBR-RO pilot study when the RO pilot plant was operated at a membrane flux of 22 l/m2/h, which was 30 percent higher than AS-UF-RO at the NEWater Plant (operated at 17 l/m2/h as recommended by the RO membrane supplier for this particular application). This demonstrated low fouling on RO membranes when MBR permeate was used as the RO feed.
Conclusions
The pilot study demonstrated that the hybrid MBR process offered better synergy between membrane and activated sludge process. The new MBR-RO process could produce water of drinking water standards (such as Singapore’s NEWater) and have better resistance to shock loading than the conventional AS-MF/UF RO from domestic used water. RO membranes in the MBR-RO process could be operated 30 percent higher flux than in AS-MF/UF RO process for water reclamation and purification.
About PUB
PUB is a statutory board under the Ministry of the Environment and Water Resources in Singapore. It is the water agency that manages Singapore’s water supply, water catchment and sewerage in an integrated way. PUB won the 2007 Stockholm Industry Water Award and was named Water Agency of the Year at the Global Water Awards 2006. For more information please log onto www.pub.gov.sg.
About the Singapore International Water Week
The Singapore International Water Week is the global platform for water solutions. It will bring policymakers, industry leaders, experts and practitioners together to address challenges, showcase technologies, discover opportunities and celebrate achievements in the water world.
Comprising the Water Leaders Summit, Water Convention and Water Expo, it culminates in the presentation of the Lee Kuan Yew Water Prize, a prestigious international award to recognise outstanding contributions in solving water issues. The inaugural Singapore International Water Week will be held from 23 to 27 June 2008.
For more information, log onto www.siww.com.sg.
About the authors
Dr. Guihe Tao, Section Head, Centre for Advanced Water Technology Division of Singapore Utilities International Pte. Ltd. (CAWT/SUI), Singapore; Kiran Kekre, Dr. Jianjun Qin, Maung Htun Oo and Bala Viawanath are all with the Centre for Advanced Water Technology, Singapore Utilities International Pte. Ltd. as well. Corresponding author Harry Seah is with the Technology and Water Quality Office, PUB Singapore, Singapore.