The existing VIP process, which is virtually identical to UCT process except its shorter SRT, required a capacity increase by 40% without increasing footprint. In addition, no increase of pollutant discharge to the nearby lake was desired while treating more wastewater. MBR was chosen for the upgrade project and was commissioned in the summer of 2004.
The plant is designed to treat maximum monthly loads of 9,200 kg/day BOD at 32,000 m3/day. The annual average design flow rate is 27,000 m3/d and the maximum hourly design flow 64,000 m3/d. As summarized in Table 1, tighter discharge limits than the state government imposed were volunteered.
Table 1. Monthly average discharge limits
|Effluent quality||Discharge limit set by state||Voluntary limit|
A process diagram is shown in Fig. 1, where a separate membrane tank is added to the existing biological tank. Mixed liquor is recirculated between the membrane tank and the biological tank using pumps at up to 4 times of the influent flow rate (4Q). The biological tank is split to anaerobic, anoxic, and oxic (or aeraobic) compartments through which influent flows by gravity. The mixed liquor containing nitrate in oxic tank is pumped back to the front end of anoxic tank at a flow rate of 1Q-2Q for denitrification. Simultaneously, the denitrified mixed liquor in the end of the anoxic tank is pumped back to the front end of the anaerobic tank to induce phosphorus release by PAO.
Fig. 1. Process diagram of the MBR in Traverse City, Michigan, USA.
The plant consists of two identical treatment trains sharing 6 mm coarse screen, grit removal system, and primary clarifier with overall 830 m2 surface area. Each train consists of following.
- New 2 mm automatic in-channel traveling band screen rated at 38,000 m3/d. Screenings are dewatered, compacted and discharged to a dumpster for landfill disposal.
- Existing bioreactors based on VIP process (6,700 m3 each) with an additional aeration capacity of 7,500 m3/hr by adding new blowers.
- Additional pumps to recycle 28,000 m3/d (QO/AO) from oxic (aerobic) tank to anoxic tank and to recycle 28,000 m3/d (QAO/AA) from anoxic tank to anaerobic tank.
- New membrane tank with 4 equally divided cells containing 13 ZW500c membrane cassettes (GE Water) each. Total membrane tank volume is 1,700 m3. Each membrane cell has a dedicated 10,000 m3/d permeate pump.
- Mixed liquor recycle pumps providing up to 400% recirculation rate from membrane tank to oxic tank based on the annual average flow of 13,500 m3/d in each train.
- One 2,400 m3/d pump for waste activated sludge, a membrane backpulse pumping system, a 40 L/minute sodium hypochlorite pumps and a 65 L/minute citric acid pumps along with storage and feed systems for membrane cleaning.
The total membrane surface area in the eight cells dedicated to two trains is 45,760 m2 (=440m2/cassette x 13 cassettes/cell x 4 cells/train x 2 trains) assuming each ZW500c cassette holds 20 ZW500c modules with 22 m2 surface area each. Considering the design flow rates, the annual average net flux is calculated at 25 LMH, maximum daily flux 36 LMH, and maximum hourly flux 58 LMH as summarized in Table 2.
Table 2. Design flow criteria and corresponding net flux.
|Design Criteria||Flow Rate||Net flux|
The membrane system has an air blowing capacity of 8,240 m3/hr for membrane scouring using five new blowers. Membrane scouring is performed at a cyclic mode at 10 seconds on and 10 seconds off. The average specific aeration demand per membrane surface area (SADm) is calculated at 0.18 m3 air/m2 membrane/hr net (=8,240/45,760), but the instantaneous SADm is double the average SADm due to the cyclic aeration. Specific aeration demand per permeate volume (SADp) is calculated at 7.2 m3 air/m3 permeate for annual average flow rate.
Te overall performance of the process was checked during the trial performed in November 2004. To enhance phosphorus removal, 38 mg/L of ferric chloride was added before the membrane tank. The actual MBR influent (or primary effluent) and MBR effluent qualities during the trial in November 2004 are also summarized in Table 3. All the discharge limits were within the voluntarily set limits, which were much tighter than the government set limits.
Table 3. Traverse city MBR influent and effluent data from 30-day test in November 2004 (Daigger, 2010).
|Parameter||MBR Influent (primary effluent)||MBR effluent|
1) Ferric chloride dose = 38 mg/L
The process parameters during the trial are summarized in Table 4. Although it is just a short-term snapshot of the process performance that can change quite a bit depending on how the process is adjusted to handle each given flow conditions, there are a few things noticeable:
- Solids in anaerobic and anoxic tanks are quite scarce relative to those in oxic and membrane tanks. SRT in anaerobic, anoxic and oxic (inclouding membrane) are calculated at 0.18 day, 1.24 days, and 6.38 days. This appears caused by the low mixed liquor recirculation rates (QO/AO and QAO/AA are 1Q) and can be corrected somewhat by increasing the recirculation rates from 1Q to 2Q, but, it is an intrinsic disadvantage of VIP (or UCT) process.
- Since QO/AO ranges 1Q-2Q, the maximum TN removal efficiency remains around 70% assuming enough readily biodegradable COD exists in raw wastewater (see equation 1 here).
- The ratio of MLSS in oxic tank and membrane tank range 0.67 – 0.80.
Table 4. Operational parameters of one of the two MBR trains in November 2004 in Traverse City, Michigan, USA
(Raw data from Daigger, 2010).
|Incoming flow rate (m3/d)||28,000||42,000||56,000||42,000|
|Relative MLSS (actual)||0.17||0.34||0.65||1.00|
|Apparent HRT (hrs)||0.6||2.0||3.2||1.5||7.0|
|Effective HRT (hrs)||0.29||0.67||0.79||0.49||–|
|Solids in tank (kg)||1,582||3,061||5,839||9,044|
|Solids distribution (%)||2.3||15.9||47.7||34.1|
(see equation 7)
|Conditions / Assumptions :|
– Influent flow rate (Q) = 14,000 m3/d
© Seong Hoon Yoon