Algae build-up in storage tank
Algae build-up in storage tank
(OP)
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.
(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.





RE: Algae build-up in storage tank
The application provides disinfection from the feed water source to the storage tank. The water can be re-circulated from the storage tank back through the UV equipment in order to maintain bacteria reduction.
RE: Algae build-up in storage tank
I'll have to do some water testing.
RE: Algae build-up in storage tank
Also there may be some water permeation into the FRP and this harbours algae spores. To thoroughly disinfect may require a higher dose of sodium hypochlorite and holding in the tank for 48 hours or more.
RE: Algae build-up in storage tank
Opaque tanks will prevent the algae from growing:
http://www.amprotec.net/water_tanks_1.htm
RE: Algae build-up in storage tank
I was considering returning the loop water back to the tank, but after thinking on it I really don't like to do that. It seems to me it would be a waste of DI water and make the DI tanks work harder. Also, I have heard in the past that DI water is not the best thing to expose to fiberglass storage tanks in the long run - too aggressive - From your experience have you heard or seen any truth to this?
RE: Algae build-up in storage tank
RE: Algae build-up in storage tank
I never liked the fiberglass tanks to begin with. They're bulky and a PITA to move around.
The FRP tanks I use for carbon and DI exchange tanks are FRP. All Park international tanks, maybe a couple of less known brand names tanks in the inventory as well. As for the fiberglass storage tanks, I couldn't tell you, if there is in fact a liner of some sort it's not noticeable, maybe it's a spray on application invisible to the naked eye?
I use Baliff for all my tank products now. Great people, great product at a great price, at least for me. JMO
RE: Algae build-up in storage tank
I took a PH reading from the Permeate line feeding the storage tank and the test showed the water to very acidic, and I mean very acidic.
I'm just a bit confused here. If the water feeding the storage tank is very acidic, how is it possible for the tank to grow green algae?
Also, Would the water being acidic be an indicator that the carbon pretreatment prior to the RO has a exhausted bed?
What do you guys think?
RE: Algae build-up in storage tank
No.
The water is acidic in the RO permeate because CO2 passes through the membrane and alkalinity is rejected by the membrane. The change ratio of alkalinity:CO2 after the RO unit is what causes the pH to change.
The CO2 is probably being stripped out in the tank, allowing a higher pH to occur in the tank than in the permeate.
RE: Algae build-up in storage tank
Another question.
With the water from the permeate being acidic going into the storage tank, would this have an adverse effect on the mixed bed deionizers I have in line after the storage tank? I also have a carbon tank in line after the storgae tank prior to the mixed bed deionizers, would this have an adverse effect on that?
Thanks, you're helping me here.
RE: Algae build-up in storage tank
If you have a water with 200 mg/l alkalinity, at a pH of 6, the CO2 in the water is 100 mg/l.
The RO rejects 90+% of the alkalinity and 0% of the CO2.
Therefore, after the RO, your alkalinity/CO2 ratio is now 20 mg/l/100 mg/l = 0.2 which equates to a pH of 5.6 (in the permeate.
CO2 was commonly air stripped after the RO or after a cation unit. When CO2 is air stripped, the pH will rise. That maybe what is happening in your tank.
Low pH of permeate due to CO2, CO2 is air stripping out in the tank, resulting in an increase in pH in the tank.
The low pH of the permeate is not going to have much of an effect on the MB unit or the carbon filter.
RE: Algae build-up in storage tank
If I find the PH in the tank to be high, what can I do to correct that situation?
RE: Algae build-up in storage tank
You didn't give any description of strippers in your scheme nor any dosing activities that increase pH of feed water to RO. In the absence of these activities, the likelyhood of increasing pH in the permeate water is nill. If you find the tank water to be acidic, then the algae can be acidophilic and you can control this by maintaining neutral pH. You can reduce the CO2 levels by maintaining feed water pH above 8.3 and thus converting it to corbonate, which can be rejected by RO.
Pleases note that CO2 increases ionic loading on the ion exchange system and this may effect your MB process.
RE: Algae build-up in storage tank
By strippers are you refering to pretreatment?
Prior to the RO I have one 12"x52" Turbity filter followed by a 14"x65" Carbon Unit followed by a 20" 5 micron filter.
Could it be a problem with the pretreatment? If it could, what could the possibilities be? I guess what I mean is, Since the water coming from the RO permeate is acidic, is there something that could be done on the pretreatment side to stop this?
I have also had the lights above the storage tank turned off as suggested.
RE: Algae build-up in storage tank
I have PH readings being taken from the feed source after the Pretreatment Unit and from the storage tank.
Will this give us a clearer picture of what is happening?
RE: Algae build-up in storage tank
"Since the water coming from the RO permeate is acidic, is there something that could be done on the pretreatment side to stop this?"
Not really.
Many RO systems use acid pretreatment prior to the RO to lower the pH to prevent scaling internally in the RO. Unless the water is low in TDS to start with, raising the pH to reject allow more alkalinity to be rejected (increasing the permeate pH) may be counterproductive because of the increased scaling potential of higher pH water.
"You didn't give any description of strippers in your scheme nor any dosing activities that increase pH of feed water to RO."
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
RE: Algae build-up in storage tank
Would it be possible to remove the CO2 by way of aeration?
If it is, I could re-route the pipe loop back to the storage tank. As it stands now the loop is a closed loop and water is drawn into the loop as demand is needed. The water does stand for a period of 36 hrs on the weekend.
RE: Algae build-up in storage tank
Strippers or membranes are another option to remove CO2. If the microbial load is of concern then strippers should use 0.22 micron filtration for the incoming air.
RE: Algae build-up in storage tank
Everything starts with water analysis.
Most pure water applications do not use strippers because the strippers will contaminant the water with airborne contaminants.
If you do have significant CO2 (maybe higher than 50 mg/l), you should investigate a membrane separation process. Otherwise, you MB unit will remove the CO2.
RE: Algae build-up in storage tank
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?
RE: Algae build-up in storage tank
"Are you refering to a membrane placed between the RO permeate line and the storage tank? "
http://ww
RE: Algae build-up in storage tank
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.
RE: Algae build-up in storage tank
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.
RE: Algae build-up in 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.
RE: Algae build-up in storage tank
RE: Algae build-up in storage tank
RE: Algae build-up in 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
RE: Algae build-up in storage tank
When you say yours is a closed loop system, water should return back. Why is it not?
RE: Algae build-up in storage tank
still concerned with the off day and stagnant water standing in the storage tank.