Part II: Our Solution for Manganese Upheaval

Our Solution for Manganese Upheaval

   Guilty until proven innocent, it is the stark reality for a Water Operator.  Any water quality issue is assumed to be the operations errors until circumstances prove otherwise.  This is a well understood burden in the business and we are conditioned to bear it and react in a positive scientific direction in such instances.  Yellow, dingy, even orange water was reported in town by customers, industry, and wholesale purchasers.  But not all of them just sporadically located and not positively linked to high flow patterns in distribution.  To deflate my confidence we did have some higher than normal Mn issues at the treatment.  My salvation was finding that many of the customers and industry reported problems revealed ultra-high levels of Mn in distribution.  Amounts of 0.200-0.500 mg/l and even one isolated incident over 1.0 mg/l actually exonerated the treatment division from being the source of this particular problem.  We have a wealth of regular QC testing results of Mn at various stages of process as well as documented daily discharge levels into distribution.  Another source was the culprit in this instance, likely a mineral build up release at points of the heaviest deposits, or perhaps some weather related issue.  We speculated that turn-over of storage tank drinking water could be a factor but that happens naturally, seasonally, and has never been a known issue before.  Chlorine and pH levels in the distribution had remained consistent and in specification, we feed orthophosphate for corrosion control but only a minute maintenance amount to retain a bare measurable residual.  There was a temperature change of perhaps 5-6 deg. F in the water going into the distribution but could that be a factor?  Could the more dense water scour build up at locations where had allocated?  How would you ever definitively prove what happened?  Operations did not care too much anyway they just wanted it gone!

Once in the distribution there is but one solution, flushing and flow consumption.  A figuratively bitter pill to swallow and requires piping knowledge and some forethought to manage correctly without making the situation linger.  But in the end it will be effective and can be a positive maintenance procedure, just not when you had planned to do it.  Our particular upset has lasted about two weeks with periods of success interrupted by more lingering reports of hot spots that tested very high Mn levels, higher than even the source water in the river.

I will share my very successful attempts to thwart the elevated Mn levels in our treatment process.  We literally scrambled to maneuver our existing resources, even admittedly in drastic fashion, to remove the nuisance mineral while keeping our effluent product spot on consistent with what we always produce.  It is important to maintain consistent levels of pH, chlorine, for disinfection as well as preventing some chain reaction event possibly resulting in a distribution release. This could be associated with a drastic change in distribution chemistry.  Mostly this can occur with high or low pH, or possibly with an incorrect change in phosphate treatment.  This needs to be avoided or it could be the treatment division’s problem.

The most positive and powerful single change was the influent process pH change to 9.0-9.5.  Normally with an Alum coagulant this would be impossible to floc outside the narrow range of 6.3-6.8 pH.  Our design does not incorporate a “contact” basin so sedimentation would have to suffice.  Long ago we tested and refined our coagulant chemicals finding a special blend of Alum/Ferric, and possibly some type of PAC.  One of the redeeming characteristics is that it will create perfect settling floc at pH ranges from 6.0 to 10!  It takes some jar testing and management but is very successful.  The high pH will bring lots of organics out of solution to combine with forming floc and be gloriously removed in our sedimentation process.  We are now seeing 85-90% removal ratios in sedimentation.  I must add that we are also feeding a Chlorine Dioxide solution at a rate of 1.2-1.3 mg/l.  This powerful oxidizer normally handles all our Fe and Mn issues but was overtaxed much beyond 0.200 mg/l Mn levels in this particular instance. This particular Mn seemed to be flighty and have various times when it seemed to be more resistant to oxidation than normal.  Chlorine Dioxide remains a contributor in achieving the high removal ratio, how much is still being tested as opportunity allows.  It is limited by the Chlorite residual in the plant effluent, 0.8 mg/l being the MCLG and 1.0 mg/l for an MCL.  We adhere to the MCLG within 10% on both sides for a daily result.

With manganese spikes related to this unusual event reaching 0.500 mg/l even at 90% removal the remaining Mn levels will be high enough to still create dingy water color after disinfection is administered.  The EPA warns that levels of Mn around 0.050 mg/l can be an aesthetic problem for utilities.  What’s next?  We found that a pre-filter chlorination oxidized any remaining Mn in solution and the filters removed it.  A simple method, however not flow paced and manually regulated, of injecting a 12.5% bleach solution was installed in existing fixtures.  This did the trick.  We retained and regulated the final chlorine levels adjustment to allow for carry through of residual or reduction in Cl2 for the cleaner and pre-treated water.  Through trial and regulation we found that if we maintained a pre-sedimentation/process pH of 9.0-9.3 that the carry through of pH would drop to a point exactly where we always maintain.  The effluent is kept at a 7.5-7.8 pH range.  Final treatment in a common pit is acidic chlorine solution, acidic fluoride solution and a minute amount of 100% orthophosphate, also an acid.  These helped lower the carry through pH to acceptable levels.

The two layer approach of removal of the dissolved Mn was a success and did not require any permanent changes to our treatment.  When Mn levels consistently get below 0.100 mg/l we will resume normal treatment process that has previously proven to be more than adequate.   We learned a lot in this time of excitement and have decided to install a more permanent Cl2 injection point post-sedimentation/pre-filter for future emergency use.  I was concerned with DDBP formation with chlorinating this mid-point water with higher TOC.  So far tests have shown about a 50% average increase at our stage II sample points.  This could be a serious consideration for some utilities but for short term our level has risen from the 30’s to 50’s ug/l TTHM.  This is an acceptable short term trade off. Considering the huge swings in TOC through our processes associated with a big rain event.

During the early struggles we tried a process pH increase to 8.5 (from our normal 7.5 levels) this did show some improvement but the Mn was sluggish to convert and settle.  The 9.0 levels showed drastic results by comparison.  This after our sedimentation basins were completely saturated at the higher level pH.  You must allow some time for your efforts to be seen.  Another interim effort that did work noteworthy was the injection of sodium hydroxide (liquid caustic soda) at the post sedimentation/pre-filter manifold.  There we also found if we maintained the pH level there at 9.0-9.3 the Mn was brought out of solution and the filters took it out.  But the Mn levels in basin effluent at normal pH levels (7.5) were too high for the post-sedimentation treatment to be fully and adequately successful.  It was retained through the trial increase of process pH from 7.5 to 8.5 but when sedimentation effluent was brought to exceed 9 pH it was nullified.  At this point the Cl2 (bleach) was added as a secondary oxidant and slowly increased until the desired results were realized.  Operators watched the downstream Cl2 dosage and residuals, reducing as the carry through reduced the need.

We maintain or theories of unusual warm weather coupled with the heavy rains and large release of dam effluent in to our source water did something to accelerate a distribution release of debris.  An unfortunate problem that occurred the same time the treatment plant was under so much duress. This has been a valuable learning experience in the treatment division as to overcome this unique issue.  It also shows the value of maintaining a flexible operation with staff trained to seek best available options.  Even if the methods are seemingly unconventional.

*The only water quality parameter that seems to have suffered a bit is the Total Organic Carbon removal efficiency.  Based upon our particular source characteristics we are required a 35% average.  The TOC suffers from higher pH, while turbidity and other components are easily treated and removed.  A high TOC removal returns when pH drops below 8 in sedimentation.

Distribution discoloration still lingers in some distant areas but reports have greatly diminished.  We have flushed the lines in areas we felt would be fruitful and benefit the most customers.  There is always the possibility that flushing might rid the bulk of the most problematic conditions but cause a lingering lesser color issue.  We are planning this summer to pursue a more aggressive maintenance flushing program while flows are higher and residual debris will have a shorter residence time.  The distribution debris does come from trace elements in treated water but they should accumulate over a much extended period of time.  An organized flushing program should correctly maintain the piping and go a long way in preventing a buildup release due to some unfortunate conditions at a most inopportune time.  We don’t live in a perfect world but knowledge and a plan make life easier to digest.

One last note; we are investigating the possibility and benefit of a polyphosphate treatment program.  A bit odd for a surface treatment facility but it might be something to consider.  It would require a consolidated effort as we sell our water wholesale to several nearby purchasing utilities.  It would require a higher initial dosage, allowing good saturation throughout, then a very organized flushing of our entire system followed by each of our purchasing systems.  This is to clean out the pipe debris softened by the polyphosphate.  Once fully cleaned out the idea is that build up would be almost nonexistent or time greatly extended between required system flushing.  Even then, our well source systems report notable reduced flushing time required.  The cost and equipment would be very minimal as a very low maintenance dose is all that is required and the chemical itself is inexpensive.  I’m not fully understanding the effects on the minute minerals remaining that are pulled out of solution post chlorination.  Are they less likely to stick to pipes as debris?  That is the big question we will be researching.