China American Petrochemical, Co., located in Taiwan, is the largest purified terephthalic acid (PTA) producer in the world for a single site. The plant consumes approximately 40,000 m3/d of fresh water for the manufacturing processes and utilities system. It also generates approximately 20,000 m3/d of wastewater from the manufacturing processes, cooling tower blowdown, demineralizer regeneration, etc. The wastewater is discharged to a local wastewater treatment center for ocean discharge with the approval quota of 22,000 m3/d.
Because the plant is located in a water-limited area with recurring seasonal droughts, water rationing during droughts has caused operational difficulties and production cutbacks. In addition, the total dissolved solids (TDS) concentration of the water supply has been increasing because of salt water intrusion to feedwater sources, resulting in a shorter production cycle for the plant’s ion-exchange-based water demineralization (DI) system.
To solve the water shortage during droughts and prepare for future expansion, the plant conducted wastewater reclamation studies with an integrated membrane-based treatment system, including ultrafiltration/reverse osmosis (UF/RO). After extensive pilot tests with 110 m3/d capacities from July 1995 to August 1996, a commercial reclamation system with the capacity of 6,600 m3/d of pure reclaimed water was commissioned in April 2000. This case history is based on a technical presentation reviewing three years of RO performance as a key process of the integrated membrane-based reclamation system. The RO system uses ILMTECTM BW30-400 elements.
Final Process Design
The dual media filter (DMF) consists of three towers packed with green sand and anthracite coal as media with particle size of 0.55±0.5 mm and 1.1±0.05 mm, respectively. Flow rates were designed with 148 m3/h/set.
The hollow fiber UF membrane (PM100, 6-by-48 in., Koch Membrane Systems) made of polysulfone has an anisotropic pore structure with nominal molecular weight cutoff of 100,000 Daltons. The UF system consists of five skids of 109 cartridges each with a total of 545 UF cartridges.
The UF system is operated at a recovery rate of 92%. The design flux (permeate) rate is 85.9 L/m2/h (50.6 gfd). The UF membrane cartridges are frequently cleaned by backwash and fast-flush of each skid of the cartridges with UF permeate water. A typical operation of backwash/fast-flush occurs every 60 min. for 3.83 min. The sequence is executed by a programmable logic controller (PLC) automatically. The flow rate is 342.9 m3/h. The influent quality is typically less than 500 counts/100 mL total coliforms (TC), and the effluent quality is less than 50 counts/100 mL TC.
An RO train configuration was designed with two stages (five vessels for the first stage and three vessels for the second stage). Train capacity is 39 m3/h with 80% recovery. The plant selected FILMTECTM BW30-400 elements because of their high productivity and high removal capability for salt and organics. Based on the expected better water quality, the averaged permeate flux was set at the relatively higher number of 22 L/m2/h (13 gfd). However, because of the high scaling potential of the feedwater, both acid and antiscalant dosing were recommended.
RO Plant Start-up
The entire reclamation unit started full operation in April 2000. The final water quality met the guaranteed water quality in terms of manganese, cobalt, chemical oxygen demand (COD) and conductivity. About 40% to 50% percent of COD was removed by the combination of DMF, granular-activated carbon (GAC) and UF. Still, more than 50% of smaller molecule organics passed though the UF membrane. Since major COD components are estimated to consist of small molecular weight compounds, including p-xylene, benzoic acid, p-toluic acid, acetic acid and terephthalic acid (TA), the rejection characteristics are reasonable. The RO process was proved to be essential to obtaining high-quality, reclaimed water.
RO Plant Performance
In May 2001, the elements of five out of eight trains were replaced with new elements because of a problem with chlorine oxidation, which has been corrected. Since then, the RO train has been operated with roughly 30 m3/h capacity with two trains on standby mode. Operating pressure has been maintained around 10–13 kg/cm2 compared to an estimated pressure of 18 kg/cm2 for the designed capacity.
Because three trains are still operating with old elements, the overall conductivity is slightly higher compared to the trains with the new elements. However, overall product water quality has met the guarantee level of 240 ?S/cm under the conditions of 3850 ?S/cm feed conductivity for nearly three years.
Conclusions
Through extensive pilot tests, an RO pretreatment system was designed to be suitable for petrochemical wastewater characterized as high COD. Under the appropriate pretreatment, the thin-film composite RO membrane (BW30) had no flux decline and no rejection losses for nearly two years of operation. Furthermore, the concept that an integrated, membrane-based reclamation system (UF/RO) could remove impurities from the wastewater was proven. The reclaimed water quality is better than that of municipal water, resulting in an extension of the regeneration frequency of the ion exchange demineralizer system. Even though the plant initially encountered several difficulties, including scaling and membrane oxidation, after taking corrective measures, the plant could operate the system without any major problems.
