6.1 Water Distribution Hydraulics, Storage Facilities & Water Quality Maintenance

6.1 Water Distribution Hydraulics, Storage Facilities & Water Quality Maintenance

1. Distribution Network Configurations & Layouts

A public water distribution system conveys finished, potable drinking water from treatment facilities and storage reservoirs directly to residential, commercial, industrial, and institutional consumers, while maintaining adequate capacity and pressure for emergency fire suppression. The geometric layout and hydraulic topology of the piping network directly dictate reliability, pressure stability, and finished water quality.

1. GRID SYSTEM (Interconnected Loops):
   [Source] ──┬─────────────┬─────────────┬──►
              │             │             │
              ├─────────────┼─────────────┤
              │             │             │
              ┴─────────────┴─────────────┴──►
   - Multi-directional flow paths
   - Excellent pressure stabilization & fire flow delivery
   - Minimal dead ends; isolated shutdowns do not cut service

2. BRANCHING / TREE SYSTEM:
   [Source] ──► [Main Trunk] ──┬──► [Sub-Main] ──► [Dead End 1 (Blow-off)]
                               └──► [Sub-Main] ──► [Dead End 2 (Blow-off)]
   - Unidirectional flow paths; severe head loss at extremities
   - Rapid water aging, stagnant zones, and sediment accumulation
   - Single main break isolates all downstream consumers

Primary Network Topologies

• Grid Systems: The optimal configuration for municipal distribution networks. Mains are laid out in an interconnected grid pattern where pipes crisscross and interconnect at every street intersection. Water flows toward any withdrawal point from multiple directions simultaneously. This multi-directional flow minimizes friction head loss, stabilizes residual pressures during localized peak demands (such as fire emergencies), and ensures that an isolated main break can be valved off with minimal customer disruption.
• Loop Systems: Major transmission mains (arterials) form complete circular or rectangular loops encircling high-demand neighborhoods, industrial parks, or pressure zones. Secondary distribution mains branch inward from the perimeter loop. Like grid systems, loop configurations prevent dead-end stagnation and provide two parallel supply paths to any point along the primary arterial.
• Branching / Tree Systems: Characterized by a large central transmission main that progressively subdivides into smaller sub-mains, branch lines, and terminal laterals without interconnecting loops. Common in rural water districts, rugged mountainous topography, or linear suburban developments. Branching networks suffer severe operational liabilities:
  • Water moves in a strictly unidirectional path, resulting in high water age and rapid disinfectant residual decay at the extremities.
  • Any pipe rupture or maintenance shutdown along the central trunk isolates all downstream customers.
  • During high-demand events (e.g., firefighting), heavy friction losses occur because flow cannot be drawn from parallel paths.

Dead Ends and Blow-Off Hydrants

Dead ends are unavoidable terminal terminations in cul-de-sacs, phase-developed subdivisions, and political boundary limits. Because water consumption at the end of a terminal line is low, velocity drops to near zero, creating a stagnant zone where:

1. Suspended inorganic particulates (iron, manganese, silt) and chemical precipitates settle onto the pipe invert.
2. Chemical disinfectant residual (free chlorine or chloramines) decays rapidly through reactions with pipe walls and sediment.
3. Biofilm proliferation, taste-and-odor complaints, red/black water episodes, and bacterial regrowth accelerate.

Under 15A NCAC 18C .0903, where installation of dead-end water mains cannot be avoided, a hydrant or a valve of adequate size for flushing shall be installed at the terminal end of the line, and flush valves must have an above-ground discharge protected from contamination. (18C .0906 is a different rule — it governs the separation of water mains from sewers.) Operators must establish a scheduled flushing program to periodically purge stagnant volume from these extremities until fresh disinfectant residual is restored.

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2. Hydraulics of Distribution Piping & Material Properties

Distribution Pipe Materials

Water utilities select piping materials based on structural beam strength, internal operating pressure ratings, external overburden loads, soil corrosivity, and fluid friction characteristics.

Hazen-Williams Friction Loss & C-Factor Mechanics

In pressurized municipal drinking water distribution systems, friction head loss in pipes flowing full is modeled using the empirical Hazen-Williams Equation:

$h_f = \frac{10.44 \cdot L \cdot Q^{1.852}}{C^{1.852} \cdot D^{4.87}}$

Where:

• $h_f$ = Friction head loss (feet of water)
• $L$ = Length of pipe (feet)
• $Q$ = Volumetric flow rate (gallons per minute, gpm)
• $D$ = Inside pipe diameter (inches)
• $C$ = Hazen-Williams roughness coefficient ($C$-factor)

[!IMPORTANT] The Significance of the $C$-Factor: The Hazen-Williams $C$-factor represents the internal smoothness of the pipe wall. Higher $C$-factors denote smoother internal surfaces, which produce lower friction head loss. As shown in the formula, head loss is inversely proportional to $C^{1.852}$.

• New PVC and HDPE: $C = 140 \text{ to } 150$
• New Cement-Mortar Lined Ductile Iron: $C = 130 \text{ to } 140$
• Aged, Unlined Cast Iron (Moderate Tuberculation): $C = 80 \text{ to } 100$
• Severely Corroded / Heavily Tuberculated Mains: $C = 50 \text{ to } 60$

When tuberculation (mounds of oxidized iron corrosion products) forms inside unlined cast iron mains, the effective inside diameter ($D$) decreases while surface roughness increases, dropping the $C$-factor. An operator pumping water through an aged unlined main with $C=70$ must expend more than three times the electrical pumping energy required to move the exact same flow rate through a new cement-lined or PVC main with $C=140$.

Flow Velocity Criteria

Distribution networks must be hydraulically balanced to maintain velocities within strict design windows:

• Normal Peak Domestic Demand: 2.0 to 5.0 ft/sec (0.6 to 1.5 m/sec). Velocities below 2.0 ft/sec promote solids deposition and water age accumulation. Velocities above 5.0 ft/sec cause excessive dynamic friction head loss, reducing downstream delivery pressure.
• Fire Flow Conditions: Maximum allowable velocity of 8.0 to 10.0 ft/sec (2.4 to 3.0 m/sec). During emergency fire suppression, higher velocities and transient pressure drops are accepted temporarily.
• Flushing Scour Velocity: Minimum 5.0 to 6.0 ft/sec during unidirectional flushing (UDF) to generate the boundary shear stress necessary to strip biofilms, loosen tuberculation scales, and scour settled silts.

$\text{Velocity } (V, \text{ ft/sec}) = \frac{Q (\text{cfs})}{A (\text{sq ft})} = \frac{Q (\text{gpm})}{2.448 \times (D \text{ in inches})^2}$

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3. Water Hammer Dynamics & Surge Suppression

Physical Cause and Hydraulic Shock Waves

Water hammer (transient hydraulic surge) is a rapid, destructive pressure wave created when the velocity of a moving liquid column inside a closed conduit is abruptly altered. It is governed by the Joukowsky Equation:

$\Delta P = \rho \cdot a \cdot \Delta v$

Where $\Delta P$ is the transient pressure surge, $\rho$ is fluid density, $a$ is the acoustic wave speed in the pipe (typically 3,000 to 4,000 ft/sec), and $\Delta v$ is the instantaneous change in velocity.

Common operational causes include:

1. Rapid, instantaneous closing of a quarter-turn valve (butterfly, ball) or quick-acting solenoid valve.
2. Sudden tripping or shutdown of a high-service booster pump due to power failure.
3. Rapid closure of a fire hydrant by untrained personnel.

When a moving water column is instantly halted, kinetic energy converts into elastic potential energy, generating an explosive positive pressure wave that travels back and forth through the piping network. This positive shock wave can split pipe barrels, shatter valve casings, blow out mechanical joint gaskets, and shear service connections. Conversely, when the pressure wave rebounds, it creates an intense negative pressure (sub-atmospheric vacuum) wave. Sub-atmospheric pressures can structurally collapse thin-walled pipes (especially large-diameter steel or HDPE) and induce severe backsiphonage, pulling contaminated groundwater, pesticides, and bacterial pathogens through leaking pipe joints and cracked fittings directly into the potable supply.

Surge Suppression Devices

Utilities mitigate water hammer by installing engineered surge control equipment:

• Hydropneumatic Surge Tanks (Bladder Tanks): Pressure vessels connected to the transmission main containing a pressurized bladder of air or nitrogen. When a pump trips or a valve slams shut, the expanding pressure surge compresses the air cushion, absorbing the hydraulic shock; during the subsequent negative pressure wave, the tank discharges water back into the line, preventing vacuum formation and column separation.
• Air Release and Vacuum Relief Valves (Air/Vac Valves): Installed at high points along the pipeline profile where air pocket accumulation and column separation occur. During filling, they vent large volumes of air; under positive pressure, they release small pockets of entrained air; during a pump trip or line break, they open instantly to intake massive volumes of atmospheric air, breaking the vacuum and preventing pipeline collapse.
• Slow-Closing Check Valves & Controlled Actuators: High-service pumps are equipped with cushioned swing check valves, electric motorized ball valves, or hydraulically operated pump control valves programmed to open slowly on startup and close gradually over several minutes prior to pump de-energization, safely dissipating hydraulic momentum.

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4. Operating Pressures & State Regulatory Standards (15A NCAC 18C)

In North Carolina, drinking water distribution system design and operational pressures are strictly codified under Title 15A, Subchapter 18C, Section .0900 of the North Carolina Administrative Code (Rules Governing Public Water Systems).

+-----------------------------------------------------------------------------+
|              15A NCAC 18C .0901 AND .0902 — WHAT THE RULE SAYS              |
+-----------------------------------------------------------------------------+
| Minimum pressure at all points during peak demand (fire flow):  20 psi      |
| Systems NOT designed for fire flows, during peak flow:          30 psi      |
| Minimum main size:                          2-inch nominal diameter         |
| Fire hydrants:   not on mains smaller than 6 inches, and not on systems     |
|                  not designed to carry fire protection flows                |
| 2-inch main:     max 20 residences (40 if looped); max 1,000 ft in length   |
| Cover over mains (18C .0904):  below the frost line or 30 inches, whichever |
|                                is greater; 12 inches clearance to other     |
|                                utilities                                    |
+-----------------------------------------------------------------------------+
| Industry practice (not the rule): 35-80 psi normal operating band; PRV      |
| stations where static pressure runs high enough to stress plumbing.         |
+-----------------------------------------------------------------------------+

1. Minimum pressure: 20 psi — or 30 psi without fire flows

Under 15A NCAC 18C .0901, water distribution mains "shall be sized to provide a minimum pressure at all points within the distribution system of not less than 20 pounds per square inch (gauge) during periods of peak demand (fire flow)," and in no case shall mains be less than two-inch nominal diameter. The same rule adds a requirement operators often miss: systems not designed for fire flows shall have the capacity to maintain a pressure of at least 30 psi throughout the system during periods of peak flow. Fire hydrants may not be installed on mains smaller than six inches or on systems not designed to carry fire protection flows.

• Public Health Rationale: Maintaining a continuous positive pressure of at least 20 psi ensures an outward hydraulic gradient. If pressure drops below 20 psi, the risk of backsiphonage through cross-connections and contamination infiltration through loose slip joints, cracked mains, and service taps escalates exponentially. A pressure drop below 20 psi generally triggers a mandatory Boil Water Advisory (BWA) under North Carolina Public Water Supply rules.

2. Normal Operating Pressure Range: 35 to 80 psi

Under standard non-emergency domestic demand conditions, utilities maintain pressures between 35 psi and 80 psi (241 to 552 kPa):

• Pressures below 35 psi generate customer complaints regarding inadequate flow in multi-story structures, lawn irrigation failure, and insufficient pressure for commercial appliances.
• Pressures between 45 and 65 psi represent the ideal operational sweet spot for domestic comfort and structural longevity.

3. Maximum Operating Pressures & Pressure Reducing Valves (PRVs)

When static water pressures exceed 80 to 100 psi (552 to 689 kPa) (frequently occurring in valley bottoms, low elevations near booster stations, or downhill legs of high pressure zones), utilities must install Pressure Reducing Valve (PRV) stations or require customer-side pressure regulators:

• Pressures exceeding 80–100 psi accelerate distribution leakage, stress aging pipes, increase the frequency of main breaks, damage residential water heaters, blow out washing machine hoses, and void fixture manufacturer warranties.
• Pilot-operated diaphragm PRVs continuously throttle flow to maintain a stable, dialed-in downstream pressure regardless of upstream pressure fluctuations or changing flow rates.

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5. Finished Water Storage Facilities & Hydraulic Grade Line (HGL)

Distribution storage facilities decouple water treatment production rates from variable customer demand, provide critical reserves for emergency firefighting, and maintain hydraulic pressure across the distribution grid.

                    HYDRAULIC GRADE LINE (HGL) PROFILE

 Elevation (ft)
     ▲
     │             Elevated Tank High Water Level
     │             [============================] ◄── Static HGL
     │                           ╲
     │                            ╲  Dynamic HGL (Friction Slope)
     │                             ╲
     │                              ▼  
     │                                [Consumer Demand Node]
     │                                (Pressure Head = HGL Elev - Ground Elev)
     │
  ───┴───────────────────────────────────────────────────────────────────► Distance

Types of Distribution Storage Facilities

1. Elevated Storage Tanks (Towers):
   • Spheroids, composite concrete-steel pedestals, and multi-column legged tanks elevated on structural supports high above the terrain.
   • Operational Mechanism: Gravity provides immediate, reliable pressure head. The elevation of the water surface inside the elevated tank directly establishes the Hydraulic Grade Line (HGL) for that pressure zone. No electrical pumping energy is required to deliver water into the network during peak demands or power outages.
2. Standpipes:
   • Ground-supported cylindrical steel or concrete tanks whose vertical height is greater than its diameter.
   • Operational Consideration: Only the water volume stored in the upper section of the standpipe (the volume situated above the system's required HGL elevation) provides usable gravity pressure to the distribution system. The large volume stored in the lower portion of the standpipe provides emergency storage, but cannot supply adequate pressure to upper-elevation customers without booster pumps.
3. Ground-Level Storage Reservoirs & Clearwells:
   • Reinforced concrete or welded steel tanks installed at or slightly below ground level. Commonly used as finished water clearwells at treatment plants or regional bulk transfer reservoirs.
   • Possess massive volumetric storage capacities at lower construction cost per gallon than elevated tanks, but require continuously operated high-service booster pumps to pressurize the distribution grid.
4. Hydropneumatic (Pressure) Tanks:
   • Enclosed, sealed steel pressure vessels containing an air cushion pressurized over water (typically maintained at approximately 1/3 air and 2/3 water volume, or fitted with a flexible rubber bladder).
   • Application & Limitations: Used primarily in small groundwater systems, mobile home parks, and booster pump stations serving fewer than 50 to 100 homes.
   • Exam Trap: Hydropneumatic tanks do not provide fire reserve or emergency equalization storage; their sole mechanical function is to maintain pressure within a narrow pressure switch band (e.g., 40 to 60 psi) and prevent pump short-cycling (chattering) during minor customer demands.

The Four Core Functions of Storage

$\text{Total Required Storage Volume} = V_{\text{Equalization}} + V_{\text{Fire Reserve}} + V_{\text{Emergency}} + V_{\text{Dead Storage}}$

1. Equalization (Operating) Storage: Compensates for hour-to-hour diurnal demand swings. Treatment plants operate most efficiently at a steady, uniform 24-hour rate. During midday peak demand hours, elevated tanks discharge water into the system to supplement treatment production; during nocturnal minimum hours, excess plant output refills the tanks.
2. Fire Reserve Storage: A dedicated volume maintained exclusively to supply high-rate fire flows (e.g., 1,000 to 3,500+ gpm) for a duration of 2 to 4+ hours as established by the Insurance Services Office (ISO) and local fire codes.
3. Emergency Storage: Volume reserved to supply critical domestic demand during unforeseen supply disruptions, such as transmission main ruptures, raw water intake contamination, or prolonged electrical blackout.
4. Pressure Maintenance: Maintains the height of the water column to ensure steady pressures between 35 and 80 psi across all service connections.

The Hydraulic Grade Line (HGL)

The Hydraulic Grade Line (HGL) represents the potential energy available to the water, defined as the sum of elevation head ($z$) and pressure head ($P/\gamma$):

$\text{HGL} = \text{Elevation} + \frac{\text{Pressure (psi)} \times 2.31 \text{ ft/psi}}{\text{Specific Gravity}}$

• Static HGL: When no water is moving, the HGL is perfectly flat and horizontal across the entire pressure zone, equal to the water surface elevation in the elevated storage tank.
• Dynamic HGL: When water flows through the piping network, friction causes the HGL to slope downward in the direction of flow. The vertical distance between the dynamic HGL and the physical ground elevation represents the available operating pressure head at that point.

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6. Distribution Water Quality Deterioration & Nitrification

Maintaining biologically safe, chemically stable drinking water does not end at the treatment plant clearwell. Potable water continuously deteriorates as it traverses miles of distribution mains and resides inside storage tanks.

Water Age and Storage Tank Dynamics

Water age is the cumulative elapsed time from when water exits the treatment plant clearwell to when it emerges from a consumer's tap. High water age (> 5 to 7 days) is the single greatest driver of distribution water quality degradation.

• Thermal Stratification in Storage Tanks: In warm weather, solar radiation heats the upper layers of water in elevated tanks and standpipes. The warm water becomes less dense, floating on top of the cooler, denser influent water. This creates an unmixed, stagnant upper zone where water age can exceed weeks or months, completely exhausting disinfectant residuals.
• Turnover Guidelines: Operators must actively cycle storage tanks to exchange 30% to 50% of the tank's total volume daily, ensuring that average water age within the facility does not exceed 3 to 5 days.

Disinfectant Residual Decay: Free Chlorine vs. Chloramines

Biological Nitrification in Chloraminated Systems

Utilities that dose ammonia to form chloramines must tightly manage the chemical dosing ratio of chlorine to ammonia-nitrogen (optimum weight ratio of 4.5:1 to 5:1 $Cl_2 : NH_3\text{-}N$ to form pure monochloramine, $NH_2Cl$). If excess free ammonia exists, or if chloramines decay, biological nitrification is triggered by autotrophic bacteria in a two-stage microbiological cascade:

STAGE 1: Ammonia Oxidation (AOB - Nitrosomonas)
         Free Ammonia (NH3) + O2 ──► Nitrite (NO2-) + H+ + H2O

STAGE 2: Nitrite Oxidation (NOB - Nitrobacter)
         Nitrite (NO2-) + O2 ──► Nitrate (NO3-) + Energy

The Nitrification Vicious Cycle

1. Nitrite Reaction with Chloramines: Nitrite ($NO_2^-$) chemically reduces and destroys monochloramine ($NH_2Cl$).
2. Release of Free Ammonia: As chloramines are destroyed, more free ammonia is liberated into the water, providing additional food substrate for the Ammonia-Oxidizing Bacteria (AOB).
3. Autocatalytic Acceleration: The bacteria multiply, consuming more ammonia and generating more nitrite, which destroys the remaining disinfectant residual.

Key Chemical Indicators of Active Nitrification

Operators must recognize the early warning signs of nitrification on water quality logs:

• Rapid decline in total chlorine residual that cannot be explained by seasonal water temperature changes.
• Spike in nitrite ($NO_2^-$) concentration (levels exceeding 0.05 mg/L as N indicate active nitrification; baseline is typically < 0.01 mg/L).
• Increase in nitrate ($NO_3^-$) concentration.
• Drop in pH and total alkalinity (ammonia oxidation releases hydrogen ions, $H^+$).
• Increase in Heterotrophic Plate Count (HPC) bacteria counts (> 500 CFU/mL).

Remediation of Nitrification

• Deep Tank Cycling: Lower storage tank operating levels to force aggressive turnover and purge stagnant water.
• Unidirectional Flushing (UDF): Flush affected distribution branches to eliminate nitrifying biofilms.
• Free Chlorine Burnout: The definitive corrective action. The utility temporarily ceases ammonia feed at the treatment plant, feeding solely free chlorine throughout the entire distribution network for 3 to 6 weeks. Free chlorine inactivates nitrifying bacteria, penetrates and destroys protective biofilms, and oxidizes accumulated nitrites, resetting the distribution system.

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7. Distribution Flushing Protocols & AWWA C651 Disinfection

Unidirectional Flushing (UDF) vs. Conventional Flushing

CONVENTIONAL FLUSHING:                  UNIDIRECTIONAL FLUSHING (UDF):
(Random Hydrant Opening)                 (Engineered Path & Closed Valves)

    Main 1 ──► [Hydrant] ◄── Main 2          [Clean Water] ──► [Closed Valve] ──► [Hydrant]
           ▲         ▲                                              │
           └── Dirty ┘                                              ▼ (Scour > 5.0 ft/sec)
  - Low velocities (< 3 ft/sec)            - High scour velocity (5.0 to 6.0 ft/sec)
  - Pulls dirty water from all directions   - Moves strictly from clean source outward
  - Wastes high volumes of water            - Strips biofilm, scale, and sediments

• Conventional Flushing: Operating hydrants at random locations without isolating upstream lines. Water flows toward the open hydrant from multiple directions simultaneously. Consequently, linear velocities inside individual feeder pipes remain low (often < 2 to 3 ft/sec), which is insufficient to scour pipe walls. Worse, conventional flushing can draw dirty, stagnant water into previously clean zones.
• Unidirectional Flushing (UDF): A meticulously planned, engineered procedure where distribution isolation valves are closed to force clean water to flow along a single, predetermined path from the treatment plant outward toward the system periphery.
  • Valves are sequenced so that water always flows from "clean to dirty" mains.
  • Flow is concentrated through a single pipe segment, achieving high scouring velocities of 5.0 to 6.0 ft/sec (1.5 to 1.8 m/sec).
  • The high boundary shear stress dislodges adhered biofilms, strips loose mineral scale (tubercles), removes heavy inorganic sediments, minimizes water waste, and restores disinfectant residuals.

Disinfection of New & Repaired Water Mains (AWWA C651)

Under 15A NCAC 18C .1001, all interior surfaces of new potable water supply systems — wells, filters, storage tanks, and distribution lines — must be thoroughly disinfected with hypochlorite or chlorine solutions, after which bacteriological samples are collected, and the supply shall not be placed into service until the results from an approved laboratory are satisfactory. AWWA Standard C651 supplies the methods below. (Wells are disinfected under ANSI/AWWA C654-13 per 18C .1002, with records kept three years.)

The Three Primary Chlorination Methods

1. Continuous Feed Method (Most Common):
   • Potable water is fed into the new pipe while chlorine solution (sodium hypochlorite or calcium hypochlorite) is continuously injected at the upstream entrance.
   • Chemical feed is proportioned to achieve an initial free chlorine concentration of at least 25 mg/L throughout the entire length of the pipe.
   • The heavily chlorinated water is held in the pipe for a minimum contact time of 24 hours.
   • At the conclusion of the 24-hour retention period, the free chlorine residual must be tested and confirmed to be not less than 10 mg/L at all sampling points.
2. Tablet / Granular Method:
   • Calcium hypochlorite granules and 5-gram tablets ($65% \text{ available } Cl_2$) are adhered to the top inside crown of each pipe length using food-grade adhesive during pipe laying.
   • The main is slowly filled with water at a velocity under 1.0 ft/sec to prevent washing the tablets to the end of the line.
   • Water is retained for 24 hours, achieving an initial dose of $\ge 25 \text{ mg/L}$ and a 24-hour residual of $\ge 10 \text{ mg/L}$.
   • Restriction: Allowed only for clean, dry pipe installations; strictly prohibited if trench water, dirt, or mud enters the pipeline during installation.
3. Slug Method:
   • A continuous slug of heavily chlorinated water—dosed to achieve a minimum free chlorine concentration of 100 mg/L—is moved slowly along the pipeline.
   • The flow velocity is regulated so that every interior pipe surface is exposed to the 100 mg/L chlorine slug for a minimum contact time of at least 3 hours.
   • Preferred for large-diameter transmission mains where filling the entire volume with 25 mg/L solution would be economically or hydraulically prohibitive.

Dechlorination and Bacteriological Clearance

• Neutralizing Waste Water: Disinfection water containing high chlorine residuals (10 to 100+ mg/L) must never be discharged directly into storm sewers, ditches, or receiving streams, as it is lethal to aquatic life. Operators must apply neutralizing chemical agents (sodium thiosulfate, sodium bisulfite, ascorbic acid) through a diffuser to achieve 0.0 mg/L chlorine residual prior to environmental release.
• Bacteriological Testing Protocol:
  1. After final flushing with finished drinking water until the residual matches normal distribution levels, two independent sets of coliform samples must be collected.
  2. Under AWWA C651, new mains require two consecutive sets of acceptable samples collected at least 24 hours apart; the specific protocol for repairs and the number of sets are set by the utility's specification and the State.
  3. All samples must show Total Coliform Absent (negative for coliform bacteria) before NC DEQ authorizes the pipeline to be placed into service.