Using “Forensics” to Reach Your
Full Zeta Potential
Changing Primary Coagulants at UVWTP
Michael Rau
Central Utah Water
Conservancy District
Water Treatment Forensics
• Look at clues to better understand treatment process
• Increased understanding enables optimization
UVWTP History
• Achieved Partnership for Safe Water Phase 4 in 2003
UVWTP History
• Continued optimization of treatment processes
UVWTP History
UVWTP History
Direct Filtration Chemical Strategy
• Aluminum Sulfate 6-9 mg/L
• Cationic Polymer 3-3.5 mg/L
Conventional Treatment Chemical Strategy
• Ferric Chloride 6-9 mg/L
• Cationic Polymer 3-4 mg/L
PRIMARY COAGULANT SWITCH
Alum vs. Ferric
Aluminum Sulfate
Ferric Chloride
Zeta Potential
Zeta Potential
UVWTP Conventional w/ Ferric
• Switch to Ferric and conventional was seamless
• Noticed 2 small differences
• slightly higher filter effluent turbidity despite
excellent settled turbidity
• Sed NTU ~ 0.25
• Filter Eff NTU ~ 0.035 vs. 0.025
• more negative ZP than expected after chemical
addition
Zeta Potential
Jar Testing
Replicate Full Scale Plant
• Ferric 7.5
• PEC 3.75
ZP -6.4 mV
• PEC 3.75
ZP -2.5 mV
• PEC 3.75
wait
• Ferric 7.5
ZP -5.5 mV
• Ferric 7.5
wait
• PEC 3.75
ZP -1.8 mV
mixed
• Ferric 7.5
• PEC 3.75
ZP -6.9 mV
Full Scale Implementation
PEC
Ferric
Full Scale Implementation
• PEC injection moved 12 feet downstream of Ferric
injection
• 6 seconds between Ferric and PEC
Full Scale Implementation
Within minutes…
Full Scale Implementation
A few hours later… the filters rejoiced!
Settled Turbidity was unchanged.
Full Scale Implementation
About one month later…
•
Coagulated ZP was back to ~ -6
•
Filter Effluent NTU back to 0.035
Full Scale Implementation
• Plant flow had almost doubled.
• 6 seconds had become 3 seconds
• Decided to move the PEC
injection further downstream.
Full Scale Implementation
This required some creative plumbing…
Full Scale Implementation
Mission critical garden hose.
Full Scale Implementation
• Plant chems were 10 mg/L Ferric and 4 mg/L PEC
• PEC application point was the only change
• Zeta Potential changed from -6.5 to 0 mV
• We were actually overdosing
• Cut PEC dose to 3.75, ZP = -2.6 mV
Full Scale Implementation
And the filters rejoiced!
Significance
What does this mean?
Is it anecdotal?
Could it be site/matrix specific?
What is the cause of the chemical interference?
Similar Results at Other Treatment Plants
Ashley Valley Water Treatment Plant – Vernal, UT
• 3.5 mg/L FeCl3
ZP was -3.8 mV
and 2.5 mg/L of PEC, resulting
• Moved PEC injection 11 ft. (~12 sec.)
downstream of FeCl3
• Same chem doses, resulting ZP was +4.0 mV
• Reduced PEC dose to 2.0 mg/L, resulting ZP of
-3.1 mV
Similar Results at Other Treatment Plants
Perdue Water Treatment Plant – San Diego, CA
• Jar tests showed ZP change from -7.6 mV to
-3.2 mV by feeding PEC 10 seconds after FeCl3
Similar Results at Other Treatment Plants
Perdue Water Treatment Plant – San Diego, CA
• Decreased PEC dose by 1 mg/L – Achieving same
ZP and WQ as before chemical injection change
• PEC adjustment = $21,000.00 per year savings
Wrap Up
• Moving PEC downstream eliminated Ferric interference
• Floc/Sed was unaffected by change
• Achieved better ZP with less chemical
• Filter performance improved as result of charge
• We don’t know cause of chemical interference
• Results verified at 3 different treatment plants
• Water Treatment Forensics – Look at all clues to
understand what is really happening
Questions?