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Company Confidential
Distribution Network Water Quality Management
“Smart Tanks”: Mixing and Residual Control
Company Confidential
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
Company Confidential
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
Company Confidential
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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Company Confidential
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
Company Confidential
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
Company Confidential
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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Company Confidential
Effective mixing can come in a variety of configurations depending upon process objective and site constraints
PAX Impeller MixerPAX Jet MixerTank Shark® Eductor Mixer
Company Confidential
PAX impeller leads to breakthrough efficiency through biomimicry
© PAX Water Tech ‐ 10
Company Confidential
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)
Company Confidential
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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Company Confidential
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
Company Confidential
Deep cycling is not a mixing solution –intermittent and ineffective
Water level cycled 60%
Tank remained stratifiedUpper Level
Lower Level
Company Confidential
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
Company Confidential
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
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11/14/12
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Total Cl in m
g/l
No MixerMixer InstalledBasin Cleaned
Mixer On
Chlorine residuals show significant improvement after mixing with no additional chemical addition
Company Confidential
Without an active mixer – controlled dosing is impossible due to chemical stratification
Disinfectant addedat hatch…
High concentrationhypochlorite
Settles and doesn’t mixrest of tank
Company Confidential
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
Company Confidential
Complex Process Challenge: Phoenix, AZ – Thomas Road Reservoirs
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Company Confidential
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’
Company Confidential
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
Company Confidential
PAX PWM400 – Horizontal orientation to accommodate low water level in reservoir
1” PVC Cl feed pipe
Company Confidential
6 MGD In
6 MGD Out
~ 34 MG
PAX modeled a three mixer design to eliminate dead‐spots
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Company Confidential
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
Company Confidential
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
Company Confidential
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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Company Confidential
Disinfectant Residual Control: Starts with StrongMixing
“Smart Tanks”
Company Confidential
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
Company Confidential
Chloramine Breakpoint Curve: know where you are on the curve
Company Confidential
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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Company Confidential
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”
Company Confidential
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
Company Confidential
“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
Company Confidential
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
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Chemical dosing through venturi eductor – no dosing pumps
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Company Confidential
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
Company Confidential
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
Company Confidential
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
Company Confidential
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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Company Confidential
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
Company Confidential
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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
Company Confidential
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
Company Confidential
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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Company Confidential
2016 Water Quality Sample Review : lower chloramine levels correlate with increasing nitrite levels
.02 ppm
Company Confidential
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
Company Confidential
Monoclor® RCS controllers, water quality analyzers and pump skids in building
Mixer Controls
NH3
pumps
WQS
Cl2pumps
Company Confidential
Residual control results in nitrite/nitrate control
Nitrite level increases with water temperature
Monoclor® RCS ON
Higher residual+
Lower nitrite
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Company Confidential
Distribution Network Water Quality Management
“Smart Tanks”