Presentation on theme: "Presentation for Wyoming DEQ Seminar, February 28, 2012 by Joel Bleth, President, SolarBee, Inc. Pro-active Management Tips for Potable Tanks and Hydraulic."— Presentation transcript:
Presentation for Wyoming DEQ Seminar, February 28, 2012 by Joel Bleth, President, SolarBee, Inc. Pro-active Management Tips for Potable Tanks and Hydraulic Considerations Pertaining to Mixing and THM problems
Water Quality Challenges in Potable Tanks AWWA estimates 65% of all potable tanks have water quality problems: - Temperature stratification - Stagnation - Dead zones - Short-circuiting - Biofilm build up - Loss of residual - Nitrification events - Risk of ice damage (some)
Chlorine Systems, main problems: – Loss of residual, warm weather, bacteria growth 2X/10C – THM’s and HAA’s if source water was surface water Chloramine Systems, main problems: – Loss of residual, can occur very rapidly, 1-2 days, caused by chemistry – Formation of Nitrite and Nitrate – Also can have THM’s if source water was surface water, if chlorine used to achieve CT at the plant In both systems, there are huge benefits to continuous active tank mixing: – Consistent testing, consistent water age, consistent water quality
There are 3 ways to gauge whether you are apt to have water age problems. Usually all 3 use temperature as a surrogate predicting chlorine residual and other problems, and range from: a) Complex but not-so-accurate tool, to b) Accurate but more costly tool, to c) One accurate, practical, quick, low-cost tool
5 MG Tank, No MIxer Computational Fluid Dynamics Modeling for Tank Mixing Commonly used, but there are so many variables and assumptions made it is generally not precise enough to predict problems 5 MG Tank, with Mixer Strong direct flow leaving the broad side of the mixer Model shows tank completely mixed by the GS-12.
Data Logger or “Hobo” Strings Give great temperature results if you have enough sensors (1 every foot), should go top to bottom, with depth sensor too
The indispensable tool for tank management !! AquaCal ClineFinder from Cabela’s $ gives you a simple but reliable tool to test your tanks and obtain water column temperature profiles
Have a "Plan A” for each tank: Try to prevent major problems – Periodic cleaning of reservoir – Install a good mixer for constant mixing, 24/7, to the boundary walls – Reduce water age by cycling the tank as much as possible without causing problems for other departments, such as fire department or Factory Mutual insurance underwriters – Do more-frequent testing in warm weather (>55F water): Test temperature stratification, and chlorine residual With chloramines, test free ammonia and nitrite The problems are worse in the summer, but can occur year-round in warm states
Know how you will deal with a major water quality crisis – Make sure your mixing system works when tank is also off-line Otherwise you may be lowering boats into the tank – Make sure you have a chlorine boosting system in place. – Try Fast Response Early Boost (FREB) first gallons of boost, early, may be all that is needed – These steps avoid the need for crisis management meetings, dramatic drawdowns, fire protection problems, wasting of water, taking tank offline – As a last resort, be prepared to breakpoint chlorinate the tank Again, needs a very good mixing and boosting system Have a "Plan B",Too, For Each Tank:
At the first sign of trouble in a tank, within 1-3 days, boosting the tank with a small dose of chlorine will usually correct the problem. Example: To boost 500,000 gallon tank by 1mg/l of chlorine requires only 3.3 gallons of sodium hypochlorite 12.5% solution Small periodic dosing can reduce overall chlorine usage by 50 to 80% A continuous mixer made for boosting is generally required Make sure your license allows boosting [[[[Note: The purchase of a mixer is not an absolute cure-all; a mixer can’t manufacture chlorine or cure a water chemistry problem. The first month of mixing the tank can depress residual, some chlorine is used up as the biofilm is being killed; boosting is usually not required that first month]]]] Fast Response, Early Boost Program
Portable Disinfectant Boosting System Air-Operated Diaphram Pump, Secondary Containment Chlorine or chloramine Compact, contained, portable design Self-priming configuration Corrosion-resistant components Flow and liquid level indication Flush line with both water and air after boosting
Keep spare portable mixer around for use in any tank?
Hydraulic Considerations for Tank Mixing
Is the water coming in faster than the mixer can pick it up ?? (Data logger or Clinefinder will show this, test 6 inches off bottom compared to upper water) Inflow, 1000 gpm Old water New water builds up at bottom of tank during fill cycle Mixer, 300 gpm
If inflow is greater than mixer flow : Example: If new fill water comes in at 1000 gpm x 4 hours, it will take a 300 gpm mixer over 12 hours to get the new chlorine mixed throughout tank. During those 12 hours, some high-chlorine new water will leave the tank, so won’t be available for “boosting” residual in rest of tank Results: match the size of the mixer to the problem you are trying to solve, and with consideration of the incoming flow rate to the tank
Hydraulic Considerations for THM removal
THMs: Where/how to address them? 1st Choice: Try to prevent THM formation at the front end of the plant, before residual disinfectant is added. Reduce NOM? Sometimes circulation in lake will reduce NOM. Reduce Chlorine Dose used for the CT calculation? To lower “C”, you have to increase “T” “T” is calculated in one basin or in several, depending on the plant design. If there is a significantly sized reservoir in the CT calc, consider intense mixing there to increase “T”. Prove an improved baffle factor with a tracer study.
2nd Choice: If possible, go to a neighborhood with high THMs and strip them from the water storage tank for that region. – Much smaller system needed to treat a neighborhood than for treating entire WTP flow through the clearwell – But is limited to neighborhoods where most of the water actually goes into the tank and then back out of the tank. – More often than not this WON”T work well because there is no guarantee the water will ever go into the tank, may go right by the tank to consumers. Need to determine now much water actually goes into the tank. – However, sometime it does work; next slides pertain to THM stripping in tanks
THM’s generally trend upward after water leaves the plant until full formation potential is fully achieved (based on NOM and CL), then downward as they volatize over time. - Literature talks about formation for XXX hours after treatment, we have found it is more about mixing than hours. - After THM formation potential is exhausted, and THM’s are dropping in a tank, whether from stripping system or general mixing, the first step is “solution by dilution”..
THM “solution by dilution” for tanks: Sometimes mixing alone, with a large enough mixer, can solve THM compliance problems Inflow, 1000 gpm, high THMs Old water lower in THMs, naturally or else due to THM stripping system New water Mixer, 1100 gpm Mix the new high-THM water into the low-THM water, as fast as the new water enters the tank.
If THMs are still in the “forming” stage in a tank, or not dropping at all, there are at least 4 ways to strip THMs from the tank water: – Air bubbles system Work in all reservoirs, can be deep (for stripping and mixing) or shallow (strip only, mix with something else) – Spray Droplets Nozzles or else splasher machines May have headspace issue, may need air exchange too – Thin water film Often sidestream systems, counterflow media tank – Vacuum based systems
THM Removal, Air Bubbles, Separate Mixer + Shallow Diffusers.
Las Vegas Elkhorn tank:12 MG tank, 2 MGD flow-through, 12,000 gpm inflow rate, 15,000 gpm being lifted off bottom to handle hydraulic problem of keeping THM’s from flowing out before treated
Tank with THM system showed reduction Downstream tank declined too, presumably due to intense mixing in upstream tank taking all formation reaction to the end Two years data, THM removal, Shallow Diffusers
TTHM Removal, Spray Nozzles + Tank Mixer System
3rd Choice, last resort: Many cities with THM problems will have to treat the entire plant flow in the clearwell – Problem: THM’s aren’t fully formed yet in clearwell THM’s are harder to get out when low to start with THM’s may rise downstream throughout town – But intense mixing, to react as much NOM as possible with Chlorine, with simultaneous stripping, can reduce the downstream increases. – Physical constraints are often greater Limited availability for footprint, 1-3 ft headspace – New designs for stripping THMs in clearwells are being developed, there will be more choices in the future
Miscellaneous Hyd. Considerations for THM removal Chloroform has specific gravity of 1.6 in water, but 4.6 in air, so you have to pay attention to air flow to prevent re-condensing Reducing THMs from 100 ug/l to 70 ug/l, is equivalent to there being 33 “bad guys” in the entire US population, and you finding and eliminating 9. Will the THM volatize if not exposed on the side of droplet? If not against the air bubble as it forms? Probably not. Most THM removal testing that resulted in published papers was done in a small fish-aquarium-sized tank. It’s hard to scale the mixing and stratification factors to match reality in a larger tank. Most THM testing is expensive, and not very accurate ( +- 20ug/l or 15%). We have a lot to learn but this makes it a slow process!