Algae build-up in storage tank

[PurewaterGuy]

I'm experiencing green algae in a storage tank. This is the only system I have that has this problem. Located 40 minutes south of Boston. System is set-up as follows.

(1)- 12"x52" Turbidity Unit, (2) - 12"x65" Carbon units (6cft) (1)- 20" prefilter 5 micron, (1)- Reverse osnmosis Unit with (2) 4"x25" Thin film membranes, 2.5 GPM Permeate.

The rejection rate is 90%

RO feeds a 200 Gallon Fiberglass Storage Tank.

In March I did a complete system overhaul, rebeds, membrane change out and storage tank cleaning. The tank showed signs of algae in March. Never happened in the previous 13 yrs.

The storage tank is connected to a distribution pump. Water usage on a 6 day 24hr cycle is 385-390 Gallons per day. The only day the water sits stagnant in the tank is Sunday.

The water leaves the distribution tank and goes through (1) 3.6cft bed of carbon and (2) 3.6cft mixed-bed deionizers, (1) Ultraviolet unit, (1) 0.2 micron postfilter. The water does not recirculate back to the storage tank, it is a closed loop system.

What I propose to do. Add (1) Ultraviolet unit post Carbon pretreatment prior to RO ,,, or add (1) small recirculating pump to keep the water in the carbon beds moving (although I don't beleive channeling is a factor)or, Have the loop water recirculate back to the storage tank.

Any thoughts?

The feed water does contain chlorine from the town.

 

[PurewaterGuy]

"You have to know how much CO2 you have in the first place before doing anything else."

Can you give me a cheap economical way that I can measure the CO2? An instrument of some type?

"Removing the CO2 is a post RO treatment step. You can use a air stipping device to remove CO2 or a membrane device. The membrane device is similar to the RO unit"

Are you refering to a membrane placed between the RO permeate line and the storage tank?

 

[bimr]

If you have a water analysis and the pH, you can calculate the CO2.

"Are you refering to a membrane placed between the RO permeate line and the storage tank? "

http://www.liqui-cel.com/uploads/documents/TB19 Rev 4 10-05 Improving Outlet Water Quality2.pdf

 

[brucedaengineer]

Algae in FRP Tanks - I have seen this in a roofed FRP storage tank after MF, ozone GAC; client very unimpressed - this was definitely due to diffuse light penetration - algae require light. Of course be sure to get any micro-biolgical (or other) slime/scum/growth analysed to confirm your identification - but if it looks like algae and its green you are very likely correct.
Check the tank is sealed from any dust or water ingress (I assume it is above ground - not buried or in a sump?) - can't be air tight of course.
Painting outside of tank to make it opaque would be good.
I agree with Stanier - make the tank opaque and definitely exclude other light. This will prevent algal growth; but not bacteria or more complex "animals" - sponges, briozoan,(it's a zoo out there} etc - but with RO we assume there is no food for them.
Also watch water temperature - if water or your room is warm things grow much faster.
UV ahead of tank will have no residual effect - you can assume the algae (or other life forms) did not come through the RO membranes. Chlorine would be effective but I assume you want to exclude chlorine ahead of DI etc.
You ask why now after 15 years? Perhaps something has changed? - Tank walls become more translucent, holes/cracks in the roof ingress of light,water or dust? I have to assume you have not added any new water sources to your tank that bypass RO? After all water has gone through RO I believe there has to be a contamination path and light is being admitted to grow algae.
And I don't think acid/high pH or CO2 conditions relate to your algae issue - algae are very tolerant of pH (their respiration changes pH significantly and cyclically over 24 hours - day CO2/night 02)if there a bloom - you do not have a bloom though. The advice from bimr on CO2/acidity/alkalinity etc is correct
Sounds like a new 200 gallon BLACK HDPE tank with integral roof, sitting on a free draining floor is warranted - should be cheap solution - after 15 years the FRP tank may have had its day.

 

[PurewaterGuy]

Thanks for the helpful information here. Just an update and answers to a couple of questions since I haven't been here for a while.

We did clean the tank and pipe loop and installed fresh equipment. Everything was running well. I had them turn off the overheadlights above the tank as well. Just for information purposes, the tank is a closed lid tank with a 0.2 micron vent filter as well.

Anyway, I went there yesterday and the tank is turning green again. The lights were once again ON above the tank. I have told them they will need to replace the fiberglass tank with a Black Poly tank. Last week when I visited there someone had unplugged the UV Unit within the water loop for no apparent reason. I got the old, "we don't know why it was unplugged or who did it" routine.

 

[PurewaterGuy]

On going battle. I'm replacing the storage tank. One question though, Is there a benefit to adding a Ultraviolet unit to the RO Permeate line as the water enters the storage tank?

My theory is the water will pass through the UV Unit killing anything that would enter the storage tank ahead of time, thereby preventing the chance of any growth inside the tank.

 

[bimr]

The UV disinfection process does not have any residual killing effect, so you will still get growth in the tank.

 

[PurewaterGuy]

I'll go with the new tank first and take it from there. Thanks.

 

[PurewaterGuy]

I ordered up the new storage tank.

A little refresh info on the system and how it's currently configured.

The storage tank is connected to a distribution pump. Water usage on a 6 day 24hr cycle is 385-390 Gallons per day. The only day the water sits stagnant in the tank is Sunday.

The water leaves the 170 Gallon Storage tank and goes through (1)Distribution pump (1)3.6cft bed of carbon and (2)3.6cft mixed-bed deionizers, (1)5GPM 254nm Ultraviolet unit, (1)0.2 micron postfilter. The water does not recirculate back to the storage tank, it is a closed loop system.

I have spoken with an engineer and here are the recommendations for configuration.

Storage Tank to Distribution Pump to 5GPM 185nm Ultraviolet Unit (changed UV strength) to 2 Mixed Bed Deionizers (remove carbon Tank from loop) then to 0.2 Final filter.

States water will be a lot cleaner and TOC's will be lower.

I, however am still concerned about the stagnant water in the storage tank for the one day (Sunday),,, will it be enough time to promote some type of growth to occur in the tank?

Maybe I should configure the water loop back to the Storage tank and hit it with a nitrogen blanket?

Any thoughts?

Thanks

 

[quark]

185nm UV is generally used to reduce TOC. If microbial growth is a concern then go with 253.7nm. You should have minimum power of 16000 microwatt-s/sq.cm and 10 seconds contact time. Maximum power occurs at 40C of lamp wall temperature (which is a function of UV input wattage).

When you say yours is a closed loop system, water should return back. Why is it not?

 

[PurewaterGuy]

By closed loop I mean it does not return to the storage tank.

still concerned with the off day and stagnant water standing in the storage tank.