tanks”: mixing and … · • 28+ storage tanks case study #3 company confidential walnut valley...

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Company Confidential

Distribution Network Water Quality Management

“Smart Tanks”: Mixing and Residual Control


For the last several decades, water utilities have focused on bringing state‐of‐the‐art technologies to treatment plants in an effort to improve system water quality ‐ the next several decades will focus on the distribution network itself

• Today, control of disinfectant residual and THM production in a drinking water plant is manageable

• Tank systems are designed for storage and system hydraulics – not water quality management

• Utilization of water storage tanks and reservoirs to improve delivered water quality starts with mixing

– THM reduction

– Residual control


Water storage tanks often suffer from a lack of maintenance which exacerbates a loss of water quality

Biofilm Formation

Inferior Corrosion Protection

Infrequent Inspection

Sediment Accumulation

• Biofilm formation is indicative of a lack of disinfectant residual and can result in furthering coating failure as well as a source of AOB/NOB

• Compromised venting is common

• Sediment can increase disinfectant load and harbor colonies

• Infrequent maintenance reduces tank life


Operators of both chlorine and chloramine systems struggle to maintain residuals against a host of “real‐world” issues:

• Introduction of ammonia can lead to nitrification as it is a nutrient and feeds AOB’s / NOB’s – leading to nitrification

• Over‐chlorination can create chloramine variants which lead to taste and odor problems in drinking water (DBP’s, dichloramine and trichloramine)

• Low residual levels can also lead to costly and wasteful mitigation efforts such as:– Chlorine burns– Line flushing– Water wasting or tank dumping– Corrosion control measures– Tank cycling

Ammonia Oxidizing Bacteria (AOB)

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Nevertheless, a “Smart Tank” is the right place for water quality intervention in a distribution network

Reservoir water quality management requires:

• Energetic mixing to de‐stratify aging water and ensure tank homogeneity

• Effective monitoring to ensure real‐time water quality understanding

• Accurate dosing of chemicals at the right time in the right amount

• Strong process control with feedback for optimization of consumable use


Un‐mixed tanks suffer from temperature and chemical stratification which creates a cascade of issues

Effective tank mixing:

• Better distribution of disinfectant throughout the tank that can reduce biofilm risk and ensure consistent effluent residual

• Lower overall water temperature that is favorable for residual longevity

• Decreased sediment accumulation in tank

• Prevention of destructive ice formation


Properly sized active tank mixing eliminates tank stratification

Fill Cycles

Company Confidential 8

Increasing awareness of the benefits of tank mixing have forced evolution in technology

Nozzles “Passive” Draft Tube “Active”

Only mixesduring fill cycle

Continuous mixing

1990 2000

Today

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Effective mixing can come in a variety of configurations depending upon process objective and site constraints

PAX Impeller MixerPAX Jet MixerTank Shark® Eductor Mixer


PAX impeller leads to breakthrough efficiency through biomimicry

© PAX Water Tech ‐ 10


Active mixers always out‐perform static mixers that rely on tank cycling for mixing activity

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Tracer Study (red is tracer introduction)


Cl2 Cl2 Cl2 Cl2 Cl2Cl2Cl2

Plenty of “turnover”‐but upper layer isnot mixed

Separate Inlet Separate Outlet

Dead spotsDead spots

Separate inlets and outlets is not an effective mixing solution

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Separate Inlet

Separate Outlet

Cl2Cl2Cl2Cl2Cl2Cl2Cl2Cl2Cl2Cl2Cl2

Large dead spots remain in tank

Colder, denser inlet water tends to sink (and notmix)

Cl2Cl2

Cl2 Cl2 Cl2

Cl2

Separate inlets and outlets plus elevation differences is not an improvement – tough to fight density


Deep cycling is not a mixing solution –intermittent and ineffective

Water level cycled 60%

Tank remained stratifiedUpper Level

Lower Level


Courtesy of Prof. Phillip RobertsGeorgia Institute of Technology

warmer

colder

Scaled model of passive mixingGeorgia Institute of Technology

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Passive mixing systems are not effective ‐ field research and modeling demonstrates very short‐lived mixing results –density differences are powerful

Cold Water Introduction


Central Highlands Water: one million gallon reservoir with low chlorine residual

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Flow

1 MG Enfield Reservoir

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Company Confidential 17

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

11/14/12

11/21/12

11/28/12

12/5/12

12/12/12

12/19/12

12/26/12

1/2/13

1/9/13

1/16/13

1/23/13

1/30/13

2/6/13

Total Cl in m

g/l

No MixerMixer InstalledBasin Cleaned

Mixer On

Chlorine residuals show significant improvement after mixing with no additional chemical addition


Without an active mixer – controlled dosing is impossible due to chemical stratification

Disinfectant addedat hatch…

High concentrationhypochlorite

Settles and doesn’t mixrest of tank


Basic Process Challenge: de-icing – mixers can prevent ice formation in some tanks – Old Town, ME

Tank #1 – Mixed by PAX

Tank #2 – Not mixed

© PAX Water Tech ‐ 19


Complex Process Challenge: Phoenix, AZ – Thomas Road Reservoirs

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Thomas Road – 34 MG Reservoir

Length 546’ [165m]

Diameter (floor) 411’ [124m]

Depth 20’ [7m]

Column spacing 45’

Column size (square)

16”x 16”

Outlet (48” @ 6 MGD)

Inlet (48” @ 6 MGD)

Actual water level in this reservoir: 8’‐12’


Despite 6 MG daily flow, large dead‐spots exist through‐out reservoir

6 MGD

6 MGD

~ 30MG

Biggest problem here –this gyre is isolated from the outlet

Inlet jet is strong but…

Volume of water that exits the reservoir


PAX PWM400 – Horizontal orientation to accommodate low water level in reservoir

1” PVC Cl feed pipe


6 MGD In

6 MGD Out

~ 34 MG

PAX modeled a three mixer design to eliminate dead‐spots

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Initial CFD model still resulted in a gyre or dead‐spot in the center

6 MGD

~ 30MG

6 MGD

Gyre size is greatly reduced,velocity is increased (more mixing)

This gyre is close to outlet – will be reduced during outflow

Company Confidential© 2016 PAX Water Technologies, Inc. CONFIDENTIAL 26

A vertical mixer was added to the center and the gyre was eliminated

Vertical mixer will send water up and outwards from gyre – increasing velocity in the center of the gyre

Company Confidential© 2016 PAX Water Technologies, Inc. CONFIDENTIAL 27 Company Confidential© 2016 PAX Water Technologies, Inc. CONFIDENTIAL 28

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Company Confidential© 2016 PAX Water Technologies, Inc. CONFIDENTIAL 29 Company Confidential

Process objective and tank geometry dictate important aspects of mixer choice

• Depends on Process Goal– Maintaining well-mixed conditions

– De-stratification (thermal load)

– Ice prevention (may need additional heat)

– Blending chemical dose (chlorine/ammonia)

– Aeration for THM reduction

• Depends on shape and size of tank– Small, flat tanks

– Large flat tanks, square tanks

– Wide Standpipes

– Narrow Standpipes – Draft tube needed

– Massive tanks (10 MG+)

LessPower

MorePower

HardEasy

Easy

Hard


Bare minimum for mixing – blend time versus cycle time

• Mixing must be faster than the rate at which water enters and leaves the tank

– Mixer must achieve blended condition within the cycle time of the tank

– Faster cycle times requires more powerful mixing solutions

• Mixing slower than the cycle time for the tank is not mixing

• For “miscible blending” literature says > 6 tank turn‐overs

Size of Tank

GPM Pumping

(24 ‐Hour cycle)

GPM Pumping

(4‐Hour Cycle)

300,000 Gallons 1,250 GPM 7,500 GPM

500,000 Gallons 2,085 GPM 12,500 GPM

1 MG 4,166 GPM 25,000 GPM

4 MG 16,667 GPM Challenging


Mixer choice depends on process objective with consideration of available power, turnover, geometry, climate and dosing needs

• Mixer must achieve mixed tank within cycle‐time of tank (mixing has to be faster than rate water enters and leaves the tank)

• De‐stratification, ice‐prevention, chemical dosing, THM aeration all require different mixer capacity and power

For Example:

Turnover/Cycling

Process Objective

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Disinfectant Residual Control: Starts with StrongMixing

“Smart Tanks”


In a “real world” water system, distribution disinfectant residual levels are challenged :

• Temperature stratification in reservoirs and tanks

• Chemical stratification in reservoirs and tanks

• Imported or mixed water compatibility

• Poor condition of distribution pipelines

• Water aging in pipelines and reservoirs

• Changes in ammonia levels over time and as pipeline conditions change over transmission distance


Chloramine Breakpoint Curve: know where you are on the curve


Four criteria must be met for proper chloramine control in reservoirs:

1. Proper mixing to ensure a homogenous water body that will not stratify

2. Accurate dosing of ammonia and chlorine to ensure proper ratio given the position on the breakpoint curve

3. High energy mixing that ensures instantaneous reaction of introduced chemicals

4. Real‐time monitoring and control logic to maintain or achieve equilibrium by responding to dynamic reservoir conditions

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Effective disinfectant residual control completely eliminates the threat of nitrification

Before: August‐October (4) tank nitrificationevents as water temperature remained high

After: August‐October next year with residualcontrol – steady 3.3ppm residual through“nitrification season”


A “Smart Tank” with residual control automatically adjusts disinfectant residual to a pre‐determined set‐point and maintains that set‐point with beneficial water quality impact to the zones it serves


“Smart Tanks” efficiently dose chlorine and/or ammonia accurately and when needed to ensure a reservoir can maintain a disinfectant residual set‐point

PAX Smart Controller

PAX Chemical Feed Skids

Chloramine Residual Management


The ChemLockerTM is an example of a smart boosting station that safely offers customers a water quality monitoring and disinfectant boosting complete with mixing and on‐line monitoring

40

Chemical dosing through venturi eductor – no dosing pumps

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Residual control systems can be fully automated with automatic dosing of chlorine and/or ammonia based on the real‐time determination of where the tank chemistry is on the break‐point curve


San Jose Water Company – chloramine residual control trailer

Tank Size: 1 MGTurnover: 4 daysProblem: Chronically low

residual (<0.2 mg/l)Solution: Monoclor® RCS –automated residual control

Case Study #1


Monoclor® RCS Chloramine Management System Results

Imported water introduced in high quantities throughout the trial caused momentary and intermittent concentration changes followed by quick recovery

60 Days

Introduction of “challenge water” volumes


Installing a Boosting System in Miguelito Reservoirs Results in Achieving Target Residual Levels in Downstream Alum Rock and Crothers Tanks

Crothers

Alum Rock

Miguelito

44

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Ultimately, optimal disinfectant residual control involves a number of mitigation steps throughout a distribution network – analysis, mixing and controlled boosting

Residual ControlSystem

Disinfectant Level

Treatment plant

No Intervention ResidualIntervention Residual

MixerMixer


1

1.5

2

2.5

3

3.5

July 1 ‐ December 18, 2014

Residual

East Bay Municipal Utility District (EBMUD) – residual control trailer at 10 MG reservoir

Case Study #2


Walnut Valley, CA

• Water source:– Metropolitan Water

District (MWD) – water wholesaler

– Chloramine secondary disinfectant

• Population served:– 113,000

– 26,500 connections

• 28+ storage tanks

Case Study #3


Walnut Valley operations staff invest significant labor and time as well as power to manage distribution water quality

• Deep cycling for all tanks

– Very little water stored in their tank • Maintain their storage at 6 feet despite having 30 feet tall

tanks

– Fire protection is not a concern, they have capacity (pumping and storage)

• Tanks regularly put out of service

– Manual breakpoint chlorination

– Cleaning• Pine trees and heavy rain caused a lot of debris

inside the tank each year. Requires frequent cleaning

Energyintensive

Time& labor intensive

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2016 Water Quality Sample Review : lower chloramine levels correlate with increasing nitrite levels

.02 ppm


Maintaining disinfectant residuals > 2.3 ppm effectively controls nitrite levels

Total chlorine > 2.3 mg/Lto avoid nitrification risk

Elevated nitrite levels whentotal chlorine residual is low


Monoclor® RCS controllers, water quality analyzers and pump skids in building

Mixer Controls

NH3

pumps

WQS

Cl2pumps


Residual control results in nitrite/nitrate control

Nitrite level increases with water temperature

Monoclor® RCS ON

Higher residual+

Lower nitrite

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Distribution Network Water Quality Management

“Smart Tanks”

tanks”: mixing and … · • 28+ storage tanks case study #3 company confidential walnut valley...

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