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Nomographs for operating biological iron and manganese filtration process at Annapolis WaterTreatment Plant2014 Tri-Association Conference, Maryland, USA
Atkins Lectures
City of Annapolis Water Treatment Plant
(WTP) in Maryland, USA
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● Original construction was in 1931, new plant currently being built
● Existing conventional treatment involves
– Chemical pretreatment – alum and lime
– Coagulation
– Sedimentation
– Filtration
– Disinfection.
Raw water quality
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● There are eight wells in three aquifers
● There are varying flow rates and quality
● The raw water quality is generally good
– Iron (Fe) - above the USEPA SMCL of 0.3 mg/L
– Manganese (Mn) – 2 to 10 times USEPA SMCL of 0.05 mg/L
– Low alkalinity and low pH (potential treatability issues).
Design/build process of the new WTP
● City of Annapolis is the owner of the plant
● Heery/Atkins/HDR is the owner’s representative
● CDM Smith/Haskell joint venture are designing and
building the new water treatment plant
● Process will include
– Proposed biological filtration alternative
– A pilot scale investigation
– Blueleaf Inc. conducted the pilot tests.
Biological iron and manganese removal
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● Ferazur and Mangazur biological filtration system by Infilco Degremont is an alternative to the existing water treatment at the plant
● 120 plants worldwide use the system (24 in US and Canada)
● There is potential for lower chemical costs
● There would be a smaller footprint
● Very sensitive to optimal pH and oxidation-reduction potential (ORP).
Pilot testing protocols/goals for biological
iron and manganese removal
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● Three individual wells (one from each aquifer)
● Five raw water blends - long term composite testing
● Finished water testing criteria (city’s pass/fail limits)
– Fe – 0.15 mg/L, 95% (average 0.02 mg/L)
– Mn – 0.025 mg/L, 95% (average 0.01 mg/L)
● Nomograph development
● Cost comparison with physical/chemical treatment.
Critical electrochemical parameters for the
pilot
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● pH
– acid solution – tendency low
– alkaline solution – tendency high
● ORP - (pE) (tendency for oxidation)
● Dissolved oxygen (DO)
● Temperature
● Pressures through the filters
● Turbidity
● Flow.
Process performance (use of nomographs)
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● Need conditions of pH and Eh that favor bacterial activity
● Need conditions that can lead to total precipitation of Fe and Mn
● Stability zone of values for pH – Eh
● What is the confidence level that we are at optimal band?
● What is the optimal operating envelope?
Empirical biological oxidation conditions
of iron (IDI)
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● Iron uptake occurs under conditions where physical/chemical oxidation is not possible
● DO = 0.2 to 0.5 mg/L
● pH = 6.3
● EH - Redox potential = 100 mV
● rH - > 14 but less than 20 (calculated value)
● rH = (ORP +200) + (2 x pH)
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[Clark-Nerst equation]
Iron and manganese removal
stability diagrams
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● IDI developed empirical stability diagram
● Upper and lower limits of activity zones
● Avoid areas of competition between biological and physical/chemical oxidation.
Process performance
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● What is the operating envelope for your plant?
● How can we have more stable conditions?
● Are the operating pH and ORP values within the required range?
● What can be done to improve the conditions for successful composite testing?
– pH?
– ORP?
– Carbonate equilibria?
– Other water chemistry?
Defining the optimum treatability band
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● Difficult when pH and alkalinity are very low
– Not affected by high Fe or Mn concentrations
– Very low pH and alkalinity
– Low hardness
● FZ (limit averages)
– pH = 5.2, ORP = 130
– pH = 5.2, ORP = 193
● MZ (limit averages)
– pH = 7.7, ORP = 304
– pH = 8.4 & ORP =224
Pilot study results and challenges
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● 115 Ferazur runs, 20 Mangazur runs (less backwash)
● Well #10
– required multiple DO and pH changes
– alkalinity adjustments
● Lime not a good option – switched to caustic (↑$)
● Instrumentation – probe cleaning, alternate control strategies, aeration relocations
● Automated aeration control needed
● Acclimation issues.
Life-cycle cost evaluation
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Overall result – stay with existing conventional treatment at new plant.
Nomograph findings
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● Knowledge of pH, ORP, DO and temperature data helps to define the envelope of biological activity zone
● Need to confirm the biological and physical chemical activity zones
● Biological Fe and Mn removal is best at a very narrow pH and ORP
● Nomographs can provide a valuable tool to obtain optimum operational conditions
● Boundary between physical/chemical and biological iron removal is difficult to determine.
For more information, contact:Leita Bennett
AcknowledgementsThe City of Annapolis
Thora Burkhardt
James FitzGerald
David Jarrell
Lily Openshaw
CDM Smith/Haskell JV
Ersin Kasirga, PhD, PE
Infilco Degremont
Blueleaf Inc.