santa ana river basin section biosolids seminar … dealing with struvite.pdfat many wastewater...
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SANTA ANA RIVER BASIN SECTIONSANTA ANA RIVER BASIN SECTIONBIOSOLIDS SEMINARSEPTEMBER 13, 2011
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CALIFORNIA WEASe
DEALING WITH STRUVITEDEALING WITH STRUVITE
DAN BUHRMASTERKEN ABRAHAMKEN ABRAHAM
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r 13, 201
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CALIFORNIA WEASe
OUTLINEOUTLINE
• Struvite formation basics• Struvite formation basics
• Locations for struvite formation
• Potential impacts and maintenance requirementsPotential impacts and maintenance requirements
• Monitoring for struvite
• Design techniques to minimize struvite impactsDesign techniques to minimize struvite impacts
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WHAT IS STRUVITE?WHAT IS STRUVITE?
• Crystalline substance• Crystalline substance
• MgNH4PO4.6H2O
• Magnesium ammonium phosphate hexahydrateMagnesium ammonium phosphate hexahydrate
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WHERE DOES STRUVITE FORM?WHERE DOES STRUVITE FORM?• Anaerobic digesters
Di t d l d i i l d i• Digested sludge piping, valves, pumps, and mixers
• Dewatering equipment
Filt t d t t i i l d• Filtrate and centrate piping, valves and pumps
• Rough surfaces preferred
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CONDITIONS THAT PROMOTE STRUVITECONDITIONS THAT PROMOTE STRUVITE FORMATION
• Alkaline and higher pH conditions• Alkaline and higher pH conditions
• Minimum solubility is approximately pH = 10.3
• Elevated temperaturesElevated temperatures
• Availability of constituents (magnesium, ammonia, and phosphorus)
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Struvite Solubility Product with pH
1234567
Ksp of Struvite
Solubility decreases with increasing pH
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01
3 4 5 6 7 8 9 10 11 12 13 14
pH
WHAT IS VIVIANTE?WHAT IS VIVIANTE?
• Crystalline substance• Crystalline substance
• Fe3(PO4)2.8(H2O)
• Ferric phosphateFerric phosphate
• Color is typically blue, green, or gray‐black.
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CONDITIONS THAT LEAD TO VIVIANTECONDITIONS THAT LEAD TO VIVIANTE FORMATION?
• Excess iron concentrations• Excess iron concentrations
• Acidic conditions (lower pH)
• Elevated temperaturesElevated temperatures
Vivianite Solubility Product
35 90
36.10
36.30
36.50
f Vivianite
y
SolubilityDecreases
35.50
35.70
35.90
0 20 40 60 80 100 120
Ksp of
T t (C l i )
As Temp Increases
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Temperature (Celsius)
WHERE DOES VIVIANTE FORM?WHERE DOES VIVIANTE FORM?
• Heat exchanger sludge piping• Heat exchanger sludge piping
• Sludge conveyance systems with excess iron concentrations and lower pH
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IMPACTS OF STRUVITE (OR VIVIANITE)IMPACTS OF STRUVITE (OR VIVIANITE) FORMATION• Increased O&M costs.
• Increased energy costs.
• Equipment downtime.q p
• Need for redundant equipment and piping.
• Impacts to process control.p p
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STRUVITE CLEANINGSTRUVITE CLEANING
• Chemical cleaning (acid or g (dispersant)
• High pressure flushing
• Pigging
• Removal and replacement of conveyance system componentsconveyance system components
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MONITORING FOR STRUVITE WITH THERMALMONITORING FOR STRUVITE WITH THERMAL IMAGING• Rapid and nondestructive approach to
assess the extent of scaling in pipingassess the extent of scaling in piping systems
• Struvite decreases heat transfer and temperature variations can be detected
th f f i i ton the surface of piping systems
• Thermal imagery used to map the temperature profile for the piping systems – colder locations indicate
i f istruvite formation
• Avoids the need to remove and inspect extensive conveyance systems
M ll ti i t• May allow preemptive maintenance before process problems occur
• Dallas Water Utilities and Carollo have proven the utility of this method at their S h id WWTPSouthside WWTP
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METHOD FOR MONITORING VIVIANITE & STRUVITE FORMATION AT HEAT EXCHANGERSSTRUVITE FORMATION AT HEAT EXCHANGERS
• Struvite and vivianite reduce heat transfer.
Portable Infra red probe for monitoring surface• Portable Infra‐red probe for monitoring surface temperatures of HX and associated piping.
• On‐line heat transfer coefficient measurement and trending using temperatures and flow of hot stream and cold streams with alarms.
• Algorithm initiates acid cleaning alarm• Algorithm initiates acid cleaning alarm.
• Proven beneficial at the Truckee Meadows WWTP.Sludge Spiral HX
TE 4
TE2TE1
TE3FE
Hot WaterOut
Hot WaterIn FE
Heat Transfer Q = U x Area x LMTDU = Heat transfer Coefficient (varies with scaling)Area = Surface Area of HXLMTD = Log Mean Temperature DifferenceLMTD = {(TE1 – TE4) – (TE2 – TE3)}/LN{(TE1 – TE4) – (TE2 –
SludgeIn
SludgeOut
FELMTD = {(TE1 – TE4) – (TE2 – TE3)}/LN{(TE1 – TE4) – (TE2 –TE3)}
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CALCULATING POTENTIAL FOR STRUVITECALCULATING POTENTIAL FOR STRUVITE
• Sacramento State – Office of Water Programsg
• Software tool for calculating struvite precipitation potential
• Conduct sampling and laboratory testing, then input water quality parameters into software
• Input parameters can be changed to capture various• Input parameters can be changed to capture various operating conditions
• CD‐ROM can be ordered for $75
• http://www.owp.csus.edu/courses/additional/struvite‐precipitation‐potential‐calculation‐tool.php
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CONVEYANCE DESIGN TO MINIMIZE STRUVITECONVEYANCE DESIGN TO MINIMIZE STRUVITE
• Pipe materials – glass‐lined DIP, PVDF, or other smooth lined pipesmooth lined pipe
• Pipe sizing for optimal velocity
• Valve selection to minimize turbulenceValve selection to minimize turbulence
• Short piping runs
• Dual piping system to allow for one system to beDual piping system to allow for one system to be taken out of service
• Access to piping for inspection and cleaning
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PROCESS & MECHANICAL DESIGN TO MINIMIZEPROCESS & MECHANICAL DESIGN TO MINIMIZE STRUVITE• Potential for dilution to reduce formation conditions
( l bi thi k i t t d d t i(example – combine thickening centrate and dewatering centrate)
• Chemical addition• Ferric or aluminum salt addition (precipitation and pH• Ferric or aluminum salt addition (precipitation and pH
reduction)• Dispersing agents• pH adjustment (carbon dioxide)p j ( )
• Heat exchangers• Lower the temperature delta and oversize the heat
exchanger• Add ability to acid clean sludge lines – requires
redundant unit• Phosphorus recovery (OSTARA)
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CHEMICAL PHOSPHORUS PRECIPITATION TOCHEMICAL PHOSPHORUS PRECIPITATION TO PREVENT STRUVITE FORMATION• Goal: chemically bind soluble
phosphorus for removal as sludge or cake solids
• Ferric chloride is the most common h i l dchemical used
• Adding iron forms vivianite, iron hydroxide, and iron sulfate
• Iron preferentially reacts with hydrogen sulfide (H2S); therefore, must add sufficient iron to account f lti l tifor multiple reactions
• Actual iron dosage should be determined by bench‐scale or full‐scale testingscale testing
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FERRIC CHLORIDE ADDITION TO PREVENTFERRIC CHLORIDE ADDITION TO PREVENT STRUVITE FORMATION OR ACCUMULATION• Typical ferric chloride feed pointsyp p
• Primary clarifiers• Upstream of anaerobic
digesters
Cost Impacts Additional
chemical costs Additionaldigesters
• Upstream of dewatering equipment
Additional solids handling
• Typical dose rates• Prevention of struvite
accumulations 10 35 poundsaccumulations = 10 – 35 pounds iron per dry tons solids (lb/dt)
• Prevention of struvite formation = up to 75 lb/dt
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PH ADJUSTMENTPH ADJUSTMENT
• Carbon dioxide has been used to lower pH to a level that prevents struvite formation
• MWRDGC ‐ Stickney WRP 10
Struvite Solubility Product with pH(Chicago)
• Feed CO2 to digested sludge upstream of dewatering
if 3456789
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f Struvite
with pH
centrifuges• Target pH Centrifuge feed = 6.5
0123
3 4 5 6 7 8 9 10 11 12 13 14
Ksp of
pHg Centrate = 7.2
• Reported lower operating costs than iron additionthan iron addition
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USE OF DISPERSING AGENTSUSE OF DISPERSING AGENTS• Proprietary products have been developed.
• Prevents struvite deposition by interfering with• Prevents struvite deposition by interfering with crystal formation.
• Multiple vendors offer products. p p
Product Product Dose per Name
pL of Sludge Flow Vendor Link
Flosperse 20 – 60 mg/L
SNF INC. (previously Polydyne)
http://www.snf‐group.com/IMG/pdf/Dispersants‐
anti_Struvite.pdfSchaners
PolyGone Lines 62 mg/L
Schaners Wastewater Products
http://www.struvite.com/products.html#polygone_lines
BASF http://worldaccount.pu.basf.eu/wa/NAFTA~en_US/Catalog/WaterSolutio
Antiprex® Chemicals ns/pi/BASF
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USE OF DISPERSING AGENTS (CONT )USE OF DISPERSING AGENTS (CONT.)• Flosperse dose = 0.3 gallons per million gallons of wastewater treated. g
• Typical feed points are dewatering feed or centrate/filtrate.
• Can be used alone or in combination with ferric chloride to prevent struvite formation.
• Costs are typically similar to ferric chloride• Costs are typically similar to ferric chloride addition ($9 to $14 per ton dry solids)
• Use of chemical can also dissolve or soften existing deposits.
• Can be used effectively in pipelines along with high pressure flushing to clean outwith high pressure flushing to clean out deposits.
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HEAT EXCHANGER DESIGNHEAT EXCHANGER DESIGN• Limit hot water temperature at heat exchanger to approximately 150 degrees F Thi l t l d “b ki ”F. This also prevents sludge “baking” onto the piping.
• Delta temperature between water and l d i th li it d t h l i i isludge is then limited to help minimize struvite or vivianite formation.
• Increase the size of the heat exchanger to d t th ll d ltaccommodate the smaller delta
temperature.
• Acid phase digester heat exchangers are i ll t i i it f tiespecially prone to vivianite formation
due to low pH and high temperature conditions.
C id d d t h t h t• Consider redundant heat exchanger to allow for regular inspection and cleaning.
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PHOSPHORUS RECOVERY ALTERNATIVES FORPHOSPHORUS RECOVERY ALTERNATIVES FOR STRUVITE CONTROL
• pH control and/or magnesium addition.pH control and/or magnesium addition.
• Proprietary fluidized bed processes have been developed in Japan – not available in the USA.
• OSTARA is a patented phosphorus harvesting process for centrate, using magnesium chloride addition and pH adjustment to create struvite in aaddition and pH adjustment to create struvite in a pellet form (pearls).
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OSTARA PROCESS DESCRIPTION
• Equalized centrate laden with Mg, P and NH3‐N pumped into reactor
• MgCl2 is added in stoichiometric excess
• pH is adjusted to slightly alkaline
• Struvite crystals grow in crystallizer on previously formed crystal stockon previously formed crystal stock
• Crystals are harvested and screened for size. Large product crushed and recycled, small product returned for more growing
• Crystals are dried and baggedCrystals are dried and bagged.
• Centrate recycled to plant influent24
OSTARA PROCESS SCHEMATIC
Courtesy M. Kuzma, Ostara
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SUMMARYSUMMARY• Bad news:
• Struvite (and vivianite) formation potential existsStruvite (and vivianite) formation potential exists at many wastewater treatment facilities.
• This causes increases to capital and O&M costs.d b d• Process conditions can be impacted.
• Good news:• There has been significant progress in developing• There has been significant progress in developing monitoring techniques.
• Chemical feed or phosphorus recovery systems i i i i f ican minimize struvite formation.
• Proper design of conveyance systems and heat exchanger facilities can lower O&M costs.g
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CONTACT INFORMATION:CONTACT INFORMATION:Dan BuhrmasterBlack & Veatch CorporationBlack & Veatch Corporation15615 Alton Parkway, Suite 300Irvine, CA 92618
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