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News & Press: Illinois Section AWWA News

Water Treatment Tip - Shared by Capt. Mike Curry

Monday, June 17, 2019   (0 Comments)
Posted by: John Dillon
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Salem, IL WTP Experience with High Turbidity and Low Alkalinity in Raw Water

(Salem does not lime soften because hardness is not high.)

June 2019

(Shared by Capt. Mike Curry)


Jason Weber at Salem does a great job in dealing with variable raw water conditions. (2.5” overnight rainfall tributary to Salem Reservoir: raw turbidity 1350 NTU … a record high; raw alkalinity 16 mg/L as CaCO3; settled water turbidity 0.26 NTU and filtered water turbidity < 0.05 NTU)


Rapid runoff (rain and/or snow) often results in low raw water alkalinity which makes coagulation difficult.  Jason normally feeds soda ash into the effluent end of the clearwell (after attainment of CT at lower pH with free Cl) to adjust pH and alkalinity of water leaving the plant.  He also has the capability to feed soda ash at the rapid mix when necessary to balance the alkalinity loss due to alum reaction … so he is able to maintain the coagulation process pH and alkalinity for effective turbidity removal.  If too much pre-soda ash is fed with alum … the pH rises too much and gets outside the zone of least solubility for alum (pH 5.5 to 6.8 … sometimes in low 7s depending on WQ parameters) … if too little is fed … then sufficient alkalinity is not present to sustain coagulation reactions.  The relationships that Jason uses in jar tests and initially making the adjustment are:


all alkalinity values as CaCO3


1 mg/L alum (dry equivalent) decreases alkalinity 0.45 mg/L        


1 mg/L soda ash (dry equivalent) increases alkalinity 0.94 mg/L


Initially in jar tests …  (  1 mg/L alum  )  (  0.94 mg/L alkalinity   )   = 1 mg/L alum for each 2.08 mg/L soda ash

                                         (0.45 mg/L alk)   (1 mg/L soda ash)


               1 mg/L hydrated lime increases alkalinity 1.35 mg/L

               1 mg/L caustic soda (100% dry equivalent) increases alkalinity 1.25 mg/L


We are sure that most Operators are aware of all this … but we wanted to share Jason’s experience … in case it might be useful to anyone else that has to deal with sudden decreases in alkalinity.


The worst raw water alkalinity situation I personally experienced was in early 1980s at Centralia … when the ground was covered with lots of snow … and a rainstorm occurred that melted to snow and rapid runoff resulted in raw water alkalinity of < 10 mg/L.  It was a mess getting the process balanced.


Jason sometimes experiences high pH and low alkalinity in the winter … due to “cold weather” algal blooms (this was a surprise to me … but Ellen Storment and Vern Snoeyink confirmed that it can happen; Mike Machesky, retired WQ expert from ISWS … told me he has seen blooms occur under ice cover!).  In that instance, Jason sets up an emergency carbon dioxide feed system (it is permitted by IEPA) … which lowers pH without impacting alkalinity … and then adds soda ash as needed … allowing unhindered increase in alkalinity because he can feed enough CO2 to keep pH in the optimum zone for coagulation.  Jim Leonard at Nashville has done the same thing to offset low alkalinity, except in their case they have used joint feed of hydrated lime and carbon dioxide.  The temporary/emergency carbon dioxide feed setup is similar to “fast food restaurants” … not ideal … but effective for infrequent emergency conditions at plants <  3 mgd flow rate.

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