biological iron and manganese filtration process at Annapolis Water - - PowerPoint PPT Presentation

Nomographs for operating biological iron and manganese filtration process at Annapolis Water Treatment Plant

2014 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

• f 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)

30 [Clark-Nerst equation]

Iron and manganese removal stability diagrams

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• IDI developed empirical

stability diagram

• Upper and lower limits
• f activity zones
• Avoid areas of

competition between biological and physical/chemical

• xidation.

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
• ptimum operational conditions
• Boundary between physical/chemical and biological iron

removal is difficult to determine.

For more information, contact:

Leita Bennett leita.bennett@atkinsglobal.com

Acknowledgements

The City of Annapolis Thora Burkhardt James FitzGerald David Jarrell Lily Openshaw CDM Smith/Haskell JV Ersin Kasirga, PhD, PE Infilco Degremont Blueleaf Inc.