Facility's MBR system supports wastewater reuse
The Formosa Wastewater Treatment Plant (WWTP) in central Taiwan treats effluent water from refinery and petrochemical plants, meeting national discharge water standards before discharging into receiving waters. The WWTP delivers different types of wastewater, according to their water quality characteristics, to different wastewater treatment systems in the area under administration. The treated water must achieve the discharge standards of chemical oxygen demand (COD) less than 100 mg/L and suspended solids (SS) less than 20 mg/L. Formosa WWTP has established three wastewater treatment systems according to the chloride ion (Cl–), COD and oil content of various wastewater sources.
Because the use of water resources is limited, the WWTP compared the water quality of various refinery wastewaters and selected low-electrical-conductivity (EC) and low-COD wastewaters, such as desalted and stripped waters. The plant used its existing membrane bioreactor (MBR) treatment system to plan and test wastewater reclamation, with the intention of success- fully reclaiming wastewater.
An analysis shows that the salt concentration of the stripped water and the desalted water is lower than general wastewater, with EC of 200 to 500 μs/cm, far below the cooling tower discharge water EC of 3,000 to 3,400 μs/cm. According to the evaluation of COD, SS and ammonia nitrogen (NH3-N) removal, they can be reused directly in cooling towers to replace industrial waters, and there is no need for reverse osmosis (RO) at the back end of the facility.
There are four trains for the Formosa WWTP’s MBR system. The capacity of each train is 5,000 cu meters per day. One MBR facility is separated out of the four lines for the treatment and recovery of low-EC wastewater, and the reclaimed wastewater is reused in applicable water facilities’ cooling water tower systems.
The designed treatment capacity for separated low-EC MBR is 5,000 tons per day, and membrane backwash water consumption is 6% (94% recovery rate). The designed inlet water quality is COD 1,350 mg/L and SS 200 mg/L, and outlet water quality is COD less than or equal to 90 mg/L and SS less than or equal to 5 mg/L. The pretreated water quality COD is 569 mg/L and SS is 23 mg/L. According to the designed COD and SS removal capacity, the treated low-EC wastewater quality can attain post-treatment goals and meet cooling tower supply water quality control standards.
This test-run program modified the piping to separate MBR for treating low-EC wastewater, thus avoiding cross-contamination with the wastewater from the original system. The main processes of modifying the MBR wastewater treatment system for desalted and stripped waters are shown in Figure 1. The low-EC MBR fully adopted the existing MBR treatment plants, and the following equipment and systems simultaneously were installed or rebuilt:
• A backwash tank;
• An effluent inspection tank;
• A new set of programmable logic controllers; • Online analyzers;
• Control and isolation valves; and
• Control system amendment distribution.
The revamping work was executed at the end of 2009, and then aeration basin cultivated sludge domestication was conducted. With domestication complete in February 2010, the test run was conducted and treated wastewater approved by water quality inspection, which was then reclaimed to the cooling tower facility as makeup water.
COD, SS & NH – N Removal
The program was tested for 10 months—from February to November 2010—during which the outlet water quality of treated wastewater was monitored, the tested treated wastewater passed water quality inspection before being supplied to the cooling water, and operations of the cooling tower were monitored. The average water quality prior to low-EC wastewater inlet treatment in the water operation was COD 569 mg/L and SS 23 mg/L, and the average outlet water quality after the test run treatment was COD 70 mg/L and SS 2 mg/L, for removal efficiencies of 87.8% and 91.6%.
Because a wastewater field uses activated sludge processes for biochemical treatment, there is no other NH3-N treatment facility. The low concentration NH3-N content does not change significantly in inlet or outlet waters, and the average value is 12.1 mg/L. Limited to the NH3-N of less than 1 mg/L of cooling water system, the maximum reclaimed water capacity is 3,180 tons per day, which is 63.6% of the designed inlet water flow.
During the first two months, the corrosion rate of the heat exchangers’ internal system increased. The cooling water was analyzed, and results showed that the water contained about 0.45 mg/L S2 - and 1 mg/L NH3-N. Hydrogen sulfide (H2S) is corrosive in water and NH3-N feeds the microorganisms in the CWT system, so an anti-corrosion program was researched.
During the anti-corrosion research processes, six anti-corrosion formulas were added to cooling water containing treated in a stirring breaker.
Each group was inserted with a carbon steel coupon. The appearance of the coupons in the test waters (cooling water sample: 0.45 mg/L H2S, 0.1 mg/L NH3-N) shows that the coupons were crossed without the bromide package (NH3-N inhibitor) and zincate program (H2S inhibitor) after 40 hours of stirring. The WWTP switched its conventional phosphonate-based cooling water treatment program into a phosphonate-bromide-zincate program for system corrosion control.
The Formosa WWTP’s inlet water design is for 5,000 tons per day, and recovery rate is 94%. The investment cost for related piping, equipment and software is estimated at $2.75 million. As calculated by depreciation and amortization, the cost of investment is $0.103 per ton, and MBR facilities’ operating costs total $1.267 per ton, for a total operational cost of $1.37 per ton. As limited to the NH3-N content (NH3-N less than 1 mg/L in a cooling water system), however, the actual water recovery is 3,180 tons per day, so the actual investment cost is increased to $0.133 per ton.