Nomographs for operating biological iron and manganese filtration process at Annapolis WaterTreatment Plant2014 Tri-Association Conference, Maryland, USA

Atkins Lectures

Leita Bennett, senior project manager,

North America

28 August 2014

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).

Annapolis WTP historical information

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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.

Pilot plant overview

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Air

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.

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Pilot study results

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

leita.bennett@atkinsglobal.com

AcknowledgementsThe City of Annapolis

Thora Burkhardt

James FitzGerald

David Jarrell

Lily Openshaw

CDM Smith/Haskell JV

Ersin Kasirga, PhD, PE

Infilco Degremont

Blueleaf Inc.