Membrane Bioreactor plate failures

[kbak47]

We replaced the plates in a two cassette MBR with 100 plates each in March 2018. Within the last 3 months we have had to tie off multiple plates due to failures.
The TSS is kept between 11000 and 13000 mg/l. The daily flows are between 20000 and 30000 gals. We have the system set at about 25 gpm on a
3 minute permeate cycle and between a 60 to 180 second air scour depending on flows. This is at a mining camp and it is regular sewage being processed.
All products used that end up in the system are regulated so there are no harsh chemicals being put down the drain.
Any ideas about how to figure out the issues that would cause these failures.

 

[ashtree]

A couple of questions.

What is the flux rate? Flux rate is a measure of flow rate divided by the membrane area.
How often do you do a back pulse? What back pulse flow rate are you using?
What is your typical operating Transmembrane pressure during the permeate cycle? How does this compare with the max product TMP specs?
Do you monitor routinely the TMP , to kbnow how much it has risen over time , are back pulse and clean in place operations successful etc.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[kbak47]

The flow rate is about 25 gpm and the total surface area of both cassettes is 280m2. There is no back pulse on this system,
but continuous air flow up through the plates.
It has a permeate cycle then an air scour cycle with the permeates off. It has been running a maximum of 3 minutes permeate, and between 1 and 2 minutes
air scour depending on flows.
The psid is running about .80 at the 25 gpm. the resting psid is 1.24. Recommended module TMP is no more than 20.
Book says continuous average filtration flux (m3/m2/d) 0.75 or less.
Also, there is only a rotary screen (.06 slots)at the headworks into the equalization tank, there is no screening on the return sludge lines into
the MBR tank. There is a bit of fiber braiding on the modules.
After a few months the permeate tubes started turning black inside.
The unit is a Toray TMR140-100s x2


K. Bias

 

[ashtree]

Breakages are normally are caused by:
Excessive TMP
Excessive back pulse flow/frequency/pressure
Mecahnical damage from turbulence (often caused by excessive airflow)
Incorrect cleaning

A couple of questions.

By your profile you are in the US. So can i assume that the 25gpm is US Gallons?

Is that flowrate measured during the permeate phase or is this an average measured over an hour or two taking into account time not permeating etc?

The two pressures you have given do not make much sense. Are they measured on the suction side of the permeate pump? Are those numbers psi or kpa? The max TMP of 20 is 20kpa not 20psi!

Have you done any membrane cleans?
If yes how often and what drives that cleaning decision?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[kbak47]

Yes 25 gpm U.S. during permeate cycle. And yes psi on the suction side.
The manufacturer recommended a cleaning every 6 months but then we were told that we should only do a cleaning when
the pressure differential was a -5 or so. The only CIP, since the new plates were installed in March 2018, was done last month (4800 ppm sodium hypochlorite) and the pressures
at the time were about the same as they are now. Didn't seem to make any difference in flows or pressures afterwards.

 

[kbak47]

A CIP was also done in October 2018

 

[kbak47]

How about if the plates are a bit loose in the frame? As in there is some motion as the air is being pushed through them.

 

[ashtree]

No, being loose in the frame probably won't make any difference.
There is a problem with your pressure readings though.
For a start the Toray units suck from the top. Therefore values measured in the pump suction will be negative as they are a vacuum. Secondly when operating they should be a number that is more negative and when shut down should be about zero. I would be checking your gauges and the reading and perhaps if this is logged electronically or manually the trends on this data.
Five psi differential pressure is more than Toray's maximum of 20kpa, so i would be thinking they may have been suggesting -5kpa. It is far better to clean when there has been a modest increase rather than waiting for the maximum pressure to occur.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"