NC AWWA-WEA 2015 Annual Conference Monday, November 16, 2015 The Use of Chlorine Dioxide to Reduce...

NC AWWA-WEA 2015 Annual Conference Monday, November 16, 2015

The Use of Chlorine Dioxide to Reduce TOC at Davidson Water, Inc. Water Treatment Plant


Presentation Agenda

• Introduce Davidson Water, Inc. and discuss the water quality issues experienced by the system.

• Discuss - what is Chlorine Dioxide?

• What were the beneficial impacts from using Chlorine Dioxide at the DWI WTP.


Davidson Water, Inc.Davidson Water Inc. is located in Welcome NC• Member Owned Water Utility• Water system was started in 1968 with

construction of a 4 MGD WTP and distribution system

• Original WTP was expanded several times, finally to a capacity of 20 MGD in 1995

• Original treatment process included Flash Mix, Flocculation, Conventional Sedimentation Basins (later retrofitted with Tube-Settlers), and twenty – 1 MGD dual media filters


Davidson Water, Inc.DWI serves all of the unincorporated areas of Davidson County and parts of Randolph County. The system consists of:• Service to over 58,000 customers• Distribution system consisting of:

1800 miles of pipeline ranging from 36 inch to 2 inch

27 elevated and ground storage tanks with over 14 million gallons of storage

19 pump stationsThe DWI system developed utilizing USDA funding. During the early development, funding restricted line sizes which resulted in the need to have multiple parallel lines throughout the system. Water age within the system creates a water quality problem.


DWI Raw Water SourceThe raw water source is the Yadkin River• The DWI intake is located downstream from

Winston Salem. Winston Salem has a run of the river impoundment structure that can have an impact on the Yadkin during low flow periods.

• The Yadkin River can experience flashes in turbidity.

• Generally the water quality is good. The TOC in the Yadkin ranges from 2.0 to 3.0.


DWI Treatment IssuesDWI had experienced issues with finished water quality in the past including:• Discolored water in distribution system• Lead and Copper • TTHM (Especially with Stage 2)

In order to address water quality issues in the past, DWI had changed the type of coagulant utilized:• Aluminum Sulfate through June 2002• Ferric Chloride from June 2002 through September

2008• Ferric Sulfate from September 2008 to present• Utilized Sodium Permanganate to control Iron and

Manganese


DWI Treatment GoalsDWI constructed a major upgrade and expansion to the WTP including raw water intake beginning in 2011 and extending through 2013. The treatment goals for the upgrade included:• Stabilization of water quality – eliminate

discolored water in the distribution system• Reduce TTHMs in the distribution system• Keep distribution system on Free Chlorine


DWI Upgrade – Process

Before Upgrade After Upgrade


DWI Upgrade – ProcessThe WTP Upgrade and Expansion included: • Construction of New River Intake and PS• Modification of Off Stream Storage Reservoirs• Construction of New Raw Water Pump Station• 30 MGD Flash Mix• 15 MGD SuperPulsator • 15 MGD Conventional Filters with Air Scour• Granular Activated Carbon Contactors (utilize

when necessary• 5 MG Contact Chamber / Clearwell• 40 MGD Finished Water Pump Station• New Chemical Feed• New SCADA


DWI Upgrade – Process


DWI Upgrade - TreatmentDWI Upgrade included modifications to chemical feed systems including:• Added Sulfuric Acid

Used acid to control coagulation pH without having to use as much Ferric Sulfate

Lowered Ferric Sulfate dosage from 40+ ppm to 18-20 ppm

• Eliminated Sodium Permanganate• Added Chlorine Dioxide


Chlorine DioxideWhat is Chlorine Dioxide and how is it used?• Oxidant approved by EPA for Primary and

Distribution Disinfection• All Chlorine Dioxide is produced from one of two

precursors: Sodium Chlorite (NaClO2) Sodium Chlorate (NaClO3)

• Chlorine Dioxide is generated on-site Two part system using Gaseous Chlorine and

Sodium Chlorite Three part system using Sodium

Hypochlorite, Sodium Chlorite, and Muriatic Acid


Chlorine Dioxide• Typically applied to raw water prior to

coagulation but can be used at other locations within the WTP.

Pre-Filter (What about Manganese?) Can be used to achieve CT


CT Value of ClO2

CT Values for Inactivation of Various Microorganisms Temp

(oC) 4-Log

Inactivation of Viruses

(min-mg/L)(a)

3-Log Inactivation of Giardia CYSTS

(min-mg/L)(a)

2-Log Inactivation of Cryptosporidium

(min-mg/L)

Free Chlorine 5 8 (pH 6-9) 139 - 182 -

Free Chlorine 25 2 (pH 6-9) 35 - 46 7200(c) (pH 7)

Chlorine Dioxide 5 33.4 (pH 6-9) 26 858(b)

Chlorine Dioxide 25 8.4 (pH 6-9) 11 150(b)

Ozone 5 1.2 1.9 32(b)

Ozone 25 0.3 0.48 4.9(b)

Chloramines 5 1,988 2,200 (pH 6-9) -

Chloramines 25 497 750 (pH 6-9) 7200(c) (1-Log pH 7)

(a): EPA Guidance Manual - LT1ESWTR Disinfection Profiling and Benchmarking,May 2003(b): LT2ESWTR Toolbox Guidance Manual, June 2003(c): Hoff & Geldreich


Benefits from Chlorine Dioxide

What benefits are some of the benefits from Chlorine Dioxide?• Beneficial in reducing THM and HAA• Beneficial in reduction of Iron and Manganese• Reduces taste and odor issues• Effective in pH ranges from 2 - 10• Does not oxidize Bromide to Bromate• There is no reaction with Ammonia• Does not chlorinate organics


Drawbacks to Chlorine DioxideThere are typically some drawbacks to any chemical. With Chlorine Dioxide, some drawbacks include:• Cost• The MCL for Chlorine Dioxide in the system is 0.8

ppm• The use of ClO2 forms Chlorite. Chlorite is

regulated by EPA with an MCL of 1.0 ppm. The more Chlorine Dioxide you use in the

process, the more chlorite is formed Required to test for Chlorite daily

• Requires additional operations interface and maintenance


ClO2 at DWI

How did we incorporate Chlorine Dioxide into the DWI facility?• We received a lot of assistance and coordination

from the Area WTP Specialist with NC DENR• Able to install the equipment in the same building

as the chlorine storage and feed equipment• Equipment was installed in an existing storage

room approximately 22 feet X 30 feet• Equipment included ClO2 generation equipment

and a bulk sodium chlorite tank• Generation skid is capable of using a 2-part or 3-

part system• System can be monitored on SCADA


ClO2 at DWI

How did we incorporate Chlorine Dioxide into the DWI facility?• Were permitted to perform a full scale pilot study

which was then converted to a permanent installation

• Applied the Chlorine Dioxide in the raw water main as far upstream as possible

• Started feeding the chemical at a low rate and gradually increased

• Normal feed rate is approximately 0.7 ppm


ClO2 at DWI

How did we incorporate Chlorine Dioxide into the DWI facility?• New Water Treatment plant was approved and

went on-line in April 2013• Chlorine Dioxide feed system was installed and

went on-line in early summer 2013• Daily testing was one challenge that had to be

addressed


ClO2 at DWI


ClO2 at DWI


ClO2 at DWI


ClO2 at DWI

What were the benefits from Chlorine Dioxide? (How would we know it was working?)• TOC• TTHM testing• Chlorine Residual at Point of Entry and in the

distribution system• Water quality issues

Manganese – Discolored Water Taste and Odor


ClO2 at DWI

Raw and Finished TOC

0

0.5

1

1.5

2

2.5

3

3.5

Raw Water TOC Finished TOC

TO

C


ClO2 at DWI

April-12 July-12 November-12 March-13 July-13 November-13 March-14 July-1410.0

30.0

50.0

70.0

90.0

110.0

SAMPLE B01 SAMPLE B02 SAMPLE B03 SAMPLE B04 SAMPLE B05SAMPLE B06 SAMPLE B07 SAMPLE B08

TT

HM

Results from TTHM Sampling


ClO2 at DWI

Chlorine Residual at the Point of Entry

1 3 5 7 9 111.7

1.9

2.1

2.3

2.5

2.7

2.9

2012 Cl Residual 2013 Cl Residual

Month

Ch

lori

ne R

esi

du

al


SummaryWhat is the take-away from DWI’s experience in feeding ClO2?• Quantifiable improvement in testing parameters

TOC TTHM Chlorine Residual at Point of Entry and in the

distribution system• Noticeable improvement in water quality

Manganese – Discolored Water Taste and Odor

• Distribution system continues to utilize free chlorine


SummaryWhat about the cost of feeding ClO2?• Cost is based on:

Replacement of equipment Cost of Chemical Testing

• Estimated cost per 1000 gallons - $0.015

You cannot credit ClO2 for all the improvements• Operations• Optimization of coagulation chemistry• Improvements in raw water supply


One Final CommentWhatever happened to the GAC Contactors?• Sitting full of GAC but have not needed to be put

into service yet

Credits:Tom Johnson – Plant Superintendent DWI Vince Alarid – EvoquaTA Loving Construction


Questions or Comments

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NC AWWA-WEA 2015 Annual Conference Monday, November 16, 2015 The Use of Chlorine Dioxide to Reduce...

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