+---
+title: Disinfection of Domestic Water Systems
+category: Plumbing
+toc_depth: 3
+description: >
+ When to use: Chlorination, flushing, and bacteriological clearance testing of newly constructed, extended, repaired, or temporarily opened domestic potable water piping and inline appurtenances — building service mains, cold- and hot-water distribution piping, on-premise storage tanks, and any piping filled with potable water for the first time — on commercial, institutional, healthcare, and multi-family projects where the plumbing code requires bacteriological clearance before the system is placed in service. Covers selection of disinfecting agent and method, contact-time and concentration combinations, the flushing protocol and dechlorination of flush water, field chlorine-residual testing, and collection and laboratory analysis of bacteriological samples.
+
+ Not intended for: Domestic water pipe materials, joints, sizing, and pressure testing (see [[sync/domestic-water-piping]]); water softening, filtration, and treatment equipment (see [[sync/domestic-water-softeners-and-filtration]]); backflow preventer installation and certification (see [[sync/backflow-prevention]]); plumbing pump commissioning (see [[sync/plumbing-pumps]]); disinfection of public distribution mains, storage reservoirs, standpipes, and elevated tanks on the utility side of the service connection (governed by the water purveyor under AWWA C651/C652); disinfection of water wells and groundwater supply (AWWA C654); fire suppression piping flushing (separate commissioning scope); fuel-gas piping purging (see [[sync/fuel-gas-piping]]); and thermal disinfection for Legionella control in hot-water systems (a distinct procedure, not chlorination).
+---
+
+# Scope {toc}
+
+## This standard covers the disinfection of new, extended, repaired, or temporarily opened domestic potable water systems by chlorination, followed by flushing, field chlorine-residual verification, and bacteriological clearance testing, before the system is placed in service. {note}
+
+## The scope includes building service mains downstream of the water service entrance, domestic cold-water and hot-water distribution piping, on-premise potable water storage tanks and hydropneumatic tanks, and all inline appurtenances filled with potable water for the first time. {note}
+
+## Disinfection is a commissioning activity, not a construction activity: the system must already be installed, pressure-tested, and proven leak-free under [[sync/domestic-water-piping]] before disinfection begins. {note}
+
+## The most important boundary in this standard is the line between premise plumbing and the public distribution system. {note}
+## Premise plumbing — everything downstream of the building water service entrance — is governed by the adopted plumbing code (IPC Section 610 or UPC Chapter 6) under the plumbing inspector's oversight, and is the plumbing contractor's scope. {note}
+## Public distribution mains, reservoirs, standpipes, and elevated tanks upstream of the service entrance are governed by the water purveyor under AWWA C651, C652, and C654, are witnessed and accepted by the purveyor, and are outside the plumbing contractor's scope. {note}
+## On a project that includes both a site main extension and building plumbing, both regimes apply to their respective segments, each to its own. {note}
+
+## On a project that includes both a site main extension and building plumbing, the contract documents shall define the handoff point at which water purveyor scope ends and premise plumbing scope begins.
+
+## The Contractor shall disinfect every potable water system, or portion of a system, that is new, has been extended, has been repaired, or has been opened to the atmosphere, before that system or portion is placed in service.
+
+## Disinfection shall not begin until the system has passed the hydrostatic or pressure test required by [[sync/domestic-water-piping]] and all visible debris, flux, and construction residue have been removed by preliminary flushing.
+
+## Thermal disinfection of hot-water systems for Legionella control is a distinct procedure with its own temperature and dwell requirements and is not a substitute for the chlorination and bacteriological clearance required by this standard. {note}
+
+## Coordination {toc}
+
+### Coordinate piping materials, isolation valves, hose-connection points, and the pressure test with [[sync/domestic-water-piping]].
+
+### Coordinate isolation and bypass of chlorine-sensitive equipment — water softeners, carbon filters, reverse-osmosis membranes, and ultraviolet units — with [[sync/domestic-water-softeners-and-filtration]] so that high-concentration chlorine never contacts that equipment during the contact period.
+
+### Coordinate the position and test access of backflow prevention assemblies with [[sync/backflow-prevention]], and confirm that the assembly serving the disinfected segment is in place before the system is returned to service.
+
+### Coordinate the disinfection sequence with booster and recirculation pump commissioning under [[sync/plumbing-pumps]].
+
+### Coordinate with the water purveyor and the AHJ whether sampling must be witnessed and whether dechlorinated flush water may be discharged to the sanitary sewer, the storm system, or only to an approved disposal point. {note}
+
+## Principles {toc}
+
+### Disinfection serves two distinct purposes: it inactivates microorganisms introduced into the piping during fabrication and installation, and it provides documented evidence — through a measured chlorine residual and a clean bacteriological result — that the system is safe to place in service. A system can be physically clean and still fail clearance, and it can carry a high residual and still fail if a dead leg was never reached; both the chemistry and the coverage have to be right. {note}
+
+### Free chlorine is the active disinfectant. It is consumed by reaction with organic matter, metals, and fresh joint materials — this consumption is the chlorine demand of the system. The concentration that survives demand and remains available to disinfect is the free-chlorine residual, and it is the residual, not the dose, that this standard controls. {note}
+
+### The contact time is the period during which the system is held full of chlorinated water at or above the target concentration. Disinfection efficacy is a product of concentration and time: a high concentration achieves clearance in a few hours, while a lower concentration requires a full day. The two code regimes (50 mg/L for 24 hours and 200 mg/L for 3 hours) are the two practical points on that concentration-versus-time curve. {note}
+
+### Bacteriological clearance is the acceptance gate. The chlorine residual proves the disinfectant was present; the bacteriological sample, taken after the residual has been flushed back down to normal, proves the water leaving the system is potable. Both are required — neither alone clears the system for service. {note}
+
+# Referenced Standards {toc}
+
+## Disinfecting chemicals, methods, contact times, flushing, sampling, and disposal shall comply with the latest adopted edition of the following standards and codes unless a specific edition is cited.
+
+| Standard | Title |
+|----------|-------|
+| IPC | International Plumbing Code, Section 610 — Disinfection of Potable Water System |
+| UPC | Uniform Plumbing Code, Chapter 6 — Water Supply and Distribution |
+| ANSI/AWWA C651 | Disinfecting Water Mains |
+| ANSI/AWWA C652 | Disinfection of Water Storage Facilities |
+| ANSI/AWWA C654 | Disinfection of Wells |
+| AWWA Manual M56 | Fundamentals and Control of Nitrification in Chloraminated Drinking Water Distribution Systems |
+| NSF/ANSI 60 | Drinking Water Treatment Chemicals — Health Effects |
+| NSF/ANSI 61 | Drinking Water System Components — Health Effects |
+| Standard Methods | Standard Methods for the Examination of Water and Wastewater, Methods 9222 and 9223 (coliform analysis) |
+| ASTM E1052 | Practice to Assess the Activity of Microbicides Against Viruses in Suspension (high-risk healthcare applications) |
+| EPA NPDES | National Pollutant Discharge Elimination System requirements for chlorinated water discharge |
+
+## The applicable plumbing code (IPC Section 610 or UPC Chapter 6 as adopted by the jurisdiction) shall govern on any matter it directly addresses.
+
+## Where the contract documents, the AHJ, the water purveyor, or a referenced standard impose conflicting requirements, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+## AWWA C651, C652, and C654 are written for utility-side mains, storage facilities, and wells; they are referenced here for procedure, chemistry, and sampling practice, but on premise plumbing the code (IPC 610 / UPC Chapter 6) is the controlling authority. {note}
+
+# Submittals {toc}
+
+## Action Submittals {toc}
+
+### The Contractor shall submit the following for the Engineer's review before disinfection begins:
+
+- A written disinfection plan identifying each system or segment to be disinfected, the disinfecting agent and method selected for each, the target chlorine concentration and contact time, the dosing calculation, the injection or application point, and the planned flush and disposal route
+- Product data and NSF/ANSI 60 certification for the disinfecting chemical (sodium hypochlorite solution, calcium hypochlorite granules or tablets), confirming available-chlorine content and that the product is listed for potable water treatment
+- NSF/ANSI 61 certification for any temporary materials, hoses, and inline chlorinator components that contact potable water during disinfection
+- Field test-equipment data for the chlorine-residual analyzer (DPD colorimetric or amperometric), including range, resolution, and calibration record
+- The name and accreditation of the testing laboratory that will perform bacteriological analysis, the test method to be used, the sample holding time, and the laboratory's hours of sample acceptance
+- A dechlorination and disposal plan identifying the neutralizing agent, the dosing basis, the discharge point, and any discharge permit or purveyor authorization required
+
+```datasheet
+label: Action Submittals Required
+type: checkbox
+options:
+ - "Written disinfection plan (agent, method, concentration, contact time, dosing)"
+ - "Chemical product data with NSF/ANSI 60 certification"
+ - "NSF/ANSI 61 certification for temporary wetted materials"
+ - "Chlorine-residual test equipment data and calibration record"
+ - "Testing laboratory accreditation, method, and holding-time confirmation"
+ - "Dechlorination and disposal plan with discharge authorization"
+default: "Written disinfection plan (agent, method, concentration, contact time, dosing)"
+```
+
+### Disinfection shall not proceed until the disinfection plan has been reviewed and returned.
+
+## Closeout Submittals {toc}
+
+### The Contractor shall provide the following before the system is accepted:
+
+- A completed disinfection report for each segment, recording the agent and method used, the measured initial chlorine concentration, the start and end times of the contact period, the residual measured at the remote end at the end of the contact period, and the flush start and end times
+- The flush-endpoint record showing the final free-chlorine residual at each discharge point relative to the normal supply residual
+- The dechlorination record showing the residual of the discharged flush water and the disposal point
+- The laboratory bacteriological test report for each sample, identifying the sample location, collection date and time, receipt time at the laboratory, test method, and the total coliform and E. coli results
+- A statement of clearance confirming that all samples returned zero total coliform and zero E. coli and that the system is approved for service, signed as required by the AHJ or water purveyor
+
+```datasheet
+label: Closeout Submittals Required
+type: checkbox
+options:
+ - "Disinfection report per segment (concentration, contact times, residuals)"
+ - "Flush-endpoint residual record"
+ - "Dechlorination and disposal record"
+ - "Laboratory bacteriological test reports"
+ - "Statement of clearance for service"
+default: "Laboratory bacteriological test reports"
+```
+
+# Quality Assurance {toc}
+
+## Personnel Qualifications {toc}
+
+### Disinfection shall be performed by personnel experienced in potable water chlorination who are familiar with the handling and hazards of the disinfecting chemical selected.
+
+### Chlorine-residual measurements shall be taken with a calibrated analyzer by personnel trained in its use.
+
+### Visual chlorine test strips shall not be used to establish the residual that controls the contact-period clock.
+
+### Bacteriological analysis shall be performed by a laboratory accredited for drinking-water microbiology under the applicable state program or equivalent.
+
+## Sequence and Protection {toc}
+
+### Chlorine-sensitive equipment — water softeners, carbon filters, reverse-osmosis membranes, ultraviolet disinfection units, and instrumentation with wetted elastomers not rated for chlorine — shall be isolated by valve and bypassed, or physically disconnected, before the chlorinated water is introduced.
+
+### Equipment that was isolated during disinfection shall be returned to service only after the system has been flushed to the chlorine-residual endpoint, so that high-concentration chlorine never reaches that equipment.
+
+### The entire system, including all branch lines, dead legs, riser stubs, and hose-bibb connections, shall be confirmed full of chlorinated water at the target concentration before the contact period begins.
+
+### Dead legs and seldom-used outlets commonly retain untreated water and are the most frequent cause of a positive bacteriological result on the first test. {note}
+
+### Every terminal outlet shall be opened during dosing until chlorinated water is confirmed present at that outlet, then closed for the hold.
+
+## Chemical Handling and Safety {toc}
+
+### Concentrated chlorine sources are hazardous: sodium hypochlorite solution is corrosive and reacts with acids to release chlorine gas, and dry calcium hypochlorite is an oxidizer that can ignite on contact with organic material or moisture. {note}
+
+### Disinfecting chemicals shall be stored, mixed, and handled per the manufacturer's safety data sheet.
+
+### Personnel handling disinfecting chemicals shall use the eye, skin, and respiratory protection identified on the chemical's safety data sheet.
+
+### Calcium hypochlorite shall be kept dry and separated from acids, organic materials, and other oxidizers until it is dissolved for use.
+
+### Sodium hypochlorite solution and acid-based products (descalers, cleaners) shall never be combined, because the reaction releases chlorine gas.
+
+### Chlorine solutions shall be prepared and metered in a ventilated area.
+
+### Chlorine solutions shall not be prepared in a confined space without ventilation and monitoring.
+
+# Disinfecting Agent and Method {toc}
+
+## Agent Selection {toc}
+
+### The disinfecting agent shall be a chlorine source listed to NSF/ANSI 60 for potable water treatment: liquid chlorine gas, sodium hypochlorite solution, calcium hypochlorite granules, or pre-formed calcium hypochlorite tablets.
+
+### Sodium hypochlorite solution at 12.5% trade concentration (approximately 125,000 mg/L available chlorine) is the default for building plumbing because it is widely available, NSF/ANSI 60 listed, and dosed with a simple metering pump. {note}
+
+### Calcium hypochlorite is supplied as granules or tablets at roughly 65% to 70% available chlorine and is useful where liquid storage is restricted or where tablets are placed in pipe segments before joining. {note}
+
+### Liquid chlorine gas is reserved for large-diameter or long-run mains where on-site gas handling is established practice; it is rarely appropriate for building plumbing because of its handling hazard. {note}
+
+### Household laundry bleach carries variable concentration and may contain added fragrances or stabilizers that can contaminate the potable system and fail NSF/ANSI 61. {note}
+
+### Household laundry bleach and any chlorine source not listed to NSF/ANSI 60 shall not be used.
+
+```datasheet
+label: Disinfecting Agent
+type: radio
+options:
+ - "Sodium hypochlorite solution, 12.5% trade (NSF/ANSI 60)"
+ - "Calcium hypochlorite granules, 65-70% available chlorine (NSF/ANSI 60)"
+ - "Calcium hypochlorite tablets, 65-68% available chlorine (NSF/ANSI 60)"
+ - "Liquid chlorine gas (large mains only)"
+default: "Sodium hypochlorite solution, 12.5% trade (NSF/ANSI 60)"
+```
+
+## Application Method {toc}
+
+### The application method shall be selected to suit the pipe diameter, run length, and segment configuration.
+
+### The method choice is mostly about how to get a uniform target concentration to every part of the system: small premise plumbing fills and equalizes quickly, so continuous feed works; long large-diameter mains stratify and dilute, so a concentrated slug or in-line tablets are used to guarantee local concentration. {note}
+
+### Continuous-feed injection — metering a chlorine solution into the system inlet as the system is filled, holding a uniform concentration throughout — is the default method for small-diameter building plumbing. {note}
+
+### The slug method — moving a short length of heavily chlorinated water through a long or large-diameter run so each section sees the high concentration for the required local contact time — suits long mains and large risers. {note}
+
+### The tablet or granule method places calcium hypochlorite inside pipe segments or in an inlet chlorinator basket before the system is filled. {note}
+
+### The tablet or granule method shall be used only where the AHJ permits and only on systems that can tolerate undissolved chemical contact.
+
+### Direct application means pouring or placing granular calcium hypochlorite into an opened pipe segment. {note}
+
+### Direct application shall be limited to short repaired sections and individual joints.
+
+```datasheet
+label: Disinfection Method
+type: radio
+options:
+ - "Continuous-feed injection (metering pump at inlet)"
+ - "Slug method (large-diameter / long-run)"
+ - "Tablet or granule method (segments / inlet chlorinator)"
+ - "Direct application (repairs and short segments)"
+default: "Continuous-feed injection (metering pump at inlet)"
+```
+
+### The injection or application point shall be located so that chlorinated water reaches the hydraulically remote end of the system and all branches at the target concentration.
+### Injection or application point location is [[drawing: at the service entry or as shown on the disinfection plan]].
+
+## Injection Equipment {toc}
+
+### A chemical metering pump lets the dose rate be set and held against the fill rate so the system reaches a uniform target concentration as it fills. {note}
+
+### Continuous-feed injection shall be performed with a chemical metering pump discharging through a calibrated injection port at the system inlet.
+
+### Where a metering pump is not used, a portable pressurized chlorine-solution tank connected by hose to the system inlet, or a temporary inline tablet chlorinator housing installed at the service entry for the duration of commissioning, may be used.
+
+### All temporary injection equipment, hoses, and chlorinator components that contact potable water shall be listed to NSF/ANSI 61.
+
+### Temporary injection equipment shall be removed and the connection points restored after disinfection is complete and clearance is achieved.
+
+```datasheet
+label: Injection Equipment
+type: select
+options:
+ - "Chemical metering pump with calibrated injection port"
+ - "Portable pressurized chlorine-solution tank with hose connection"
+ - "Temporary inline tablet chlorinator housing at service entry"
+ - "Direct application (no equipment; repairs and short segments)"
+default: "Chemical metering pump with calibrated injection port"
+```
+
+## Dosing {toc}
+
+### The volume of chlorine solution required shall be calculated from the system volume and the available-chlorine content of the agent before dosing begins, and recorded in the disinfection report.
+
+### For 12.5% trade sodium hypochlorite, the solution volume in gallons equals the target concentration in mg/L multiplied by the system volume in gallons, divided by 125,000. {note}
+
+### New copper, galvanized steel, fresh solder joints, and new elastomeric gaskets exert a high initial chlorine demand and consume free chlorine rapidly. {note}
+
+### The residual shall be re-measured within the first two hours of the contact period, and the system shall be re-dosed if the residual has fallen below the minimum.
+
+### Below 50°F (10°C), chlorine demand rises and disinfection efficacy falls. {note}
+
+### For cold-weather commissioning with pipe-water temperature below 50°F (10°C), the contact time shall be extended or the concentration increased to compensate for reduced chlorine efficacy.
+
+```datasheet
+label: Disinfecting Chemical Available-Chlorine Content
+type: range
+unit: "% available Cl"
+min: 5
+max: 100
+step: 0.5
+setpoints:
+ - 12.5
+ - 65
+ - 70
+```
+
+```datasheet
+label: System Volume (Basis for Dosing)
+type: range
+unit: "gal"
+min: 10
+max: 50000
+step: 10
+drawing_ref: "from the pipe schedule and tank capacities"
+default: deferred
+```
+
+# Preliminary Flushing {toc}
+
+## Preliminary flushing removes the dirt, flux, joint compound, cutting oil, and other construction residue left in the piping before any chlorine is introduced. {note}
+
+## Disinfectant is consumed by the organic matter and debris it has to react with; a system that is flushed clean first reaches and holds the target residual with far less chemical and far fewer re-dosing cycles. {note}
+
+## The system shall be flushed with potable water until the discharge runs clear and free of visible debris before disinfection begins.
+
+## Strainers, aerators, and removable fixture outlet devices shall be removed or protected during preliminary flushing so that dislodged debris does not lodge in them.
+
+# Concentration and Contact Time {toc}
+
+## Disinfection Regime {toc}
+
+### The system shall be dosed and held at one of the two code-recognized concentration and contact-time combinations and the result shall meet the residual and bacteriological criteria of this section.
+
+### Method A — 50 mg/L minimum free chlorine held for a minimum of 24 hours — is the default for building plumbing where the schedule allows a full-day hold (IPC Section 610.1). {note}
+
+### Method B — 200 mg/L minimum free chlorine held for a minimum of 3 hours — is used when the schedule cannot accommodate a 24-hour hold (IPC Section 610.1). {note}
+
+### The two methods are a schedule-versus-chemistry trade: Method A uses a modest dose but ties up the system for a full day, while Method B clears the system in an afternoon at the cost of a much higher chlorine concentration that demands tighter handling, more chemical, and more careful dechlorination of the flush water. Most building plumbing defaults to Method A. {note}
+
+### Where the jurisdiction or water purveyor specifies a different concentration or contact time, the more stringent of the code value and the specified value shall govern.
+
+```datasheet
+label: Disinfection Regime
+type: radio
+options:
+ - "Method A: 50 mg/L free chlorine, 24-hour minimum hold"
+ - "Method B: 200 mg/L free chlorine, 3-hour minimum hold"
+default: "Method A: 50 mg/L free chlorine, 24-hour minimum hold"
+```
+
+```datasheet
+label: Target Initial Free Chlorine Concentration
+type: range
+unit: "mg/L"
+min: 25
+max: 300
+step: 5
+setpoints:
+ - 50
+ - 200
+default: 50
+```
+
+```datasheet
+label: Minimum Contact Time
+type: range
+unit: "hours"
+min: 3
+max: 48
+step: 1
+setpoints:
+ - 3
+ - 24
+default: 24
+```
+
+## Residual During the Hold {toc}
+
+### The minimum end-of-hold residual is what proves the dose survived the system's chlorine demand for the whole contact period; a residual that has decayed below the minimum means demand outran the dose and part of the contact time was spent at an ineffective concentration, which is why the clock restarts. {note}
+
+### A free-chlorine residual of at least 25 mg/L shall remain at the hydraulically remote end of the system at the end of the contact period (AWWA C651).
+
+### The contact-period clock shall start only after the target concentration is confirmed at the remote end of the system, not when dosing begins.
+
+### Starting the clock at dosing rather than at confirmed remote-end concentration is a common error: the far end of the system may not reach the target for some time after injection begins, and counting that fill time as contact time shortchanges the actual hold. {note}
+
+### If the residual falls below the minimum at any point before the contact period expires, the system shall be re-dosed to the target concentration and the contact-period clock shall be restarted.
+
+## Field Residual Testing {toc}
+
+### Field chlorine-residual measurement is the instrument that controls this entire procedure: it confirms the system reached the target, governs when the contact clock starts, signals when re-dosing is needed, and verifies the flush and discharge endpoints. The readings, not the calculated dose, are the record of compliance. {note}
+
+### Free-chlorine residual shall be measured by a DPD colorimetric or amperometric method with a range adequate for disinfection-strength concentrations.
+
+### Readings at disinfection strength may exceed the analyzer's direct range and require sample dilution per the analyzer's instructions. {note}
+
+### Residual shall be measured at the hydraulically remote end of the system to confirm the target concentration before the contact-period clock starts, again during the hold to confirm the residual is maintained, and again at the end of the contact period.
+
+### Each residual measurement that controls a decision — start of contact, re-dose, end of contact, flush endpoint, discharge limit — shall be recorded with its time and location.
+
+```datasheet
+label: Minimum Free Chlorine Residual at End of Contact Period
+type: range
+unit: "mg/L"
+min: 10
+max: 50
+step: 5
+setpoints:
+ - 25
+```
+
+# Flushing and Dechlorination {toc}
+
+## Flushing {toc}
+
+### Final flushing does two things at once: it carries the disinfection-strength chlorine out of the system so the water is drinkable, and the velocity scours loosened scale and biofilm out of the pipe before the bacteriological sample is taken. Both depend on moving enough water fast enough. {note}
+
+### After the contact period is complete and the end-of-hold residual is confirmed, the system shall be flushed until the chlorine residual is reduced to the flush endpoint.
+
+### A scour velocity of at least 2.5 ft/s lifts and carries sediment that a gentle flush would leave behind; in small-bore premise plumbing, where a single outlet cannot reach that velocity, opening many outlets at once raises the system flow enough to approximate it. {note}
+
+### Flushing shall achieve a scour velocity of at least 2.5 ft/s (0.76 m/s) in each pipe section; in small-bore premise plumbing this is approximated by opening all available outlets simultaneously.
+
+### The flush endpoint is reached when the free-chlorine residual at every discharge point is no more than 1.0 mg/L above the normal supply residual, or no more than 0.5 mg/L where the supply residual is not measurable.
+
+### Every terminal outlet, branch, and dead leg shall be flushed individually after the simultaneous flush so that no segment retains high-concentration chlorinated water.
+
+```datasheet
+label: Minimum Flush Scour Velocity
+type: range
+unit: "ft/s"
+min: 1.0
+max: 5.0
+step: 0.1
+setpoints:
+ - 2.5
+default: 2.5
+```
+
+```datasheet
+label: Flush Endpoint (Free Chlorine Above Supply Residual)
+type: range
+unit: "mg/L"
+min: 0.1
+max: 1.0
+step: 0.1
+setpoints:
+ - 0.5
+ - 1.0
+```
+
+## Dechlorination and Disposal {toc}
+
+### Discharging disinfection-strength chlorinated water to a storm drain, watercourse, or other receiving water is an environmental violation under most municipal and NPDES permits because chlorine is acutely toxic to aquatic life. {note}
+
+### Chlorinated flush water shall be neutralized to a free-chlorine residual below 0.1 mg/L before discharge to a storm drain, watercourse, or other receiving water.
+
+### Neutralization shall use sodium thiosulfate or sodium bisulfite.
+
+### Approximately 7.4 mg/L of sodium thiosulfate neutralizes 1.0 mg/L of residual chlorine, which sets the dosing basis for the flush volume. {note}
+
+### The discharge point and any required discharge permit or purveyor authorization shall be confirmed before flushing begins.
+
+```datasheet
+label: Dechlorination Agent
+type: radio
+options:
+ - "Sodium thiosulfate"
+ - "Sodium bisulfite"
+default: "Sodium thiosulfate"
+```
+
+```datasheet
+label: Maximum Residual Chlorine at Discharge
+type: range
+unit: "mg/L"
+min: 0.0
+max: 0.5
+step: 0.05
+setpoints:
+ - 0.1
+```
+
+```datasheet
+label: Flush Water Discharge Point
+type: select
+options:
+ - "Sanitary sewer (with purveyor authorization)"
+ - "Storm system (dechlorinated, with permit)"
+ - "Approved holding or disposal point"
+drawing_ref: true
+default: deferred
+```
+
+# Bacteriological Clearance Testing {toc}
+
+## Sampling {toc}
+
+### Sampling has to wait until the flush is complete, because a high chlorine residual suppresses bacterial growth in the sample and can mask contamination; the sample is meant to represent the water the building will actually receive, not the disinfectant. {note}
+
+### Two samples taken at least 24 hours apart, rather than one, guard against a single fluke result and confirm the system stays clean over time rather than at one instant; this is the minimum that most health departments and AWWA C651 accept. {note}
+
+### After flushing to the chlorine endpoint, bacteriological samples shall be collected and analyzed before the system is placed in service.
+
+### A minimum of two separate samples shall be collected from each isolated system segment, taken at least 24 hours apart, or as the local health department requires, whichever is more stringent.
+
+### Samples shall be collected from representative points throughout the system, including the hydraulically remote end and any zone isolated during disinfection.
+
+### A single sample shall not be used to clear a multi-zone or large-volume system.
+
+### Samples shall be collected in sterile containers by the method required by the testing laboratory and delivered to the laboratory within its accepted holding time.
+
+### Sample holding time and the laboratory's hours of acceptance shall be coordinated in advance.
+
+### Weekend and holiday gaps in laboratory acceptance are a common cause of expired samples and re-sampling. {note}
+
+```datasheet
+label: Minimum Bacteriological Samples per Segment
+type: range
+unit: "samples"
+min: 1
+max: 6
+step: 1
+setpoints:
+ - 2
+```
+
+```datasheet
+label: Bacteriological Test Method
+type: radio
+options:
+ - "Presence-absence (Standard Methods 9223)"
+ - "Total coliform, membrane filtration (Standard Methods 9222 B)"
+ - "Most probable number (MPN)"
+default: "Presence-absence (Standard Methods 9223)"
+```
+
+## Acceptance Criteria {toc}
+
+### Total coliform bacteria are used as the indicator organism because they are easy to detect and their absence is strong evidence that disease-causing organisms are also absent; E. coli specifically indicates fecal contamination. A zero result for both is the bright-line pass criterion. {note}
+
+### The system passes bacteriological clearance only when every sample returns zero total coliform and zero E. coli.
+
+### A measurable chlorine residual consistent with the normal supply shall be present at the sample point at the time of collection.
+
+### A sample taken from water with no residual, where the supply normally carries one, shall be investigated before acceptance.
+
+## Re-Disinfection {toc}
+
+### A positive bacteriological result, a residual that collapsed during the hold, or any opening of the cleared system breaks the chain of evidence that the water is safe; the only reliable response is to repeat the whole procedure rather than to patch or re-sample alone, because the cause is usually an untreated pocket somewhere in the system. {note}
+
+### Naming the re-disinfection triggers in advance avoids the schedule dispute that follows a first-test failure, when the question of who pays for the repeat and how it is performed is otherwise unresolved. {note}
+
+### The full disinfection procedure — re-flush, re-chlorinate to the target concentration and contact time, re-flush, and re-sample — shall be repeated whenever any of the conditions in this section occurs.
+
+### Any bacteriological sample returning a positive total coliform or E. coli result shall trigger re-disinfection of the affected segment.
+
+### Loss of the minimum chlorine residual before the contact period expires shall trigger re-dosing and a restart of the contact-period clock, and if the cause cannot be confirmed, re-disinfection of the segment.
+
+### Any physical disturbance, opening, or contamination of the system after disinfection and before it is placed in service shall trigger re-disinfection of the affected portion.
+
+# On-Premise Storage Tanks {toc}
+
+## Potable water storage tanks and hydropneumatic tanks that are part of the building system shall be disinfected before being placed in service, following the chlorination, holding, flushing, and bacteriological clearance sequence of this standard.
+
+## Tank interior surfaces shall be cleaned of construction debris before chlorinated water is introduced.
+
+## Tank disinfection shall achieve full wetting of all interior surfaces at the target chlorine concentration for the contact period, by spray-and-hold or fill-and-hold, consistent with the approach of AWWA C652.
+
+## AWWA C652 describes three methods for storage facilities: fill the tank with chlorinated water at the disinfection concentration and hold, spray or coat all interior surfaces with a strong chlorine solution and hold, or fill a fraction of the tank with a high-strength solution and then dilute to operating level. {note}
+
+## The tank disinfection method shall be selected to suit the tank size and access.
+
+## Where a coating, liner, or fitting inside the tank is not rated for the disinfection concentration, the tank manufacturer shall be consulted and a compatible method and concentration shall be used.
+
+## Tank contents shall be flushed and dechlorinated to the discharge limit before the tank is connected to service.
+
+## The tank shall be cleared bacteriologically before the tank is connected to service.
+
+```datasheet
+label: Storage Tank Disinfection Method (per AWWA C652)
+type: radio
+options:
+ - "Fill-and-hold at disinfection concentration"
+ - "Spray or coat interior surfaces and hold"
+ - "High-strength fraction then dilute to operating level"
+default: "Fill-and-hold at disinfection concentration"
+```
+
+# Hot-Water Systems {toc}
+
+## The disinfection plan shall define whether the hot-water distribution piping and storage water heaters are included in the chemical disinfection scope or handled separately.
+
+## Circulating chlorinated water through an energized water heater rapidly destroys the chlorine residual at elevated temperature and leaves those sections undertreated. {note}
+
+## During chemical disinfection of the hot-water piping, the water heater shall be de-energized, or isolated and disinfected separately.
+
+## Where the hot-water piping is included in chemical disinfection, the residual within the hot-water loop shall be verified at the recirculation return so that the loop is not left undertreated.
+
+## Thermal disinfection for Legionella control is a distinct procedure with its own temperature and dwell requirements and does not substitute for the chlorination and bacteriological clearance required by this standard. {note}
+
+## Where thermal disinfection for Legionella control is required by the project, it shall be performed as a separate procedure in addition to the chlorination and bacteriological clearance of this standard.
+
+## Hot-Water Side Treatment {toc}
+
+```datasheet
+label: Hot-Water Piping Disinfection Approach
+type: radio
+options:
+ - "Included in whole-system chemical disinfection (heater de-energized)"
+ - "Isolated and chemically disinfected separately"
+ - "Cold-water side only; hot-water side handled under separate scope"
+default: "Included in whole-system chemical disinfection (heater de-energized)"
+```
+
+# Environmental and Service Conditions {toc}
+
+## Water Temperature {toc}
+
+### Chlorine efficacy is temperature-dependent: cold water reacts more slowly, so disinfection at the same concentration takes longer to achieve the same kill, and new-pipe chlorine demand consumes residual faster relative to the slower disinfection. {note}
+
+### Where disinfection is performed with pipe-water temperature below 50°F (10°C), the disinfection plan shall state the increased contact time or increased concentration used to compensate for reduced chlorine efficacy.
+
+### In cold conditions the chlorine residual shall be verified more frequently, because demand is higher and the contact-period clock restarts on any drop below the minimum.
+
+## Supply Water Chemistry {toc}
+
+### The normal residual carried by the supply must be known before the flush endpoint and the bacteriological sampling residual can be evaluated, because both criteria are expressed relative to the normal supply residual. {note}
+
+### Where the finished water from the purveyor is chloraminated rather than free-chlorinated, the normal supply carries a combined-chlorine residual rather than a free-chlorine one, which shifts the baseline against which the flush endpoint is read (AWWA Manual M56). {note}
+
+### Where the supply is chloraminated, the free-chlorine readings taken during disinfection shall be distinguished from the background combined-chlorine residual when interpreting the flush endpoint.
+
+### The Contractor shall determine and record the normal supply residual at the project before establishing the flush endpoint.
+
+```datasheet
+label: Supply Water Disinfectant Type
+type: radio
+options:
+ - "Free chlorine"
+ - "Chloramine (combined chlorine)"
+default: "Free chlorine"
+```
+
+## High-Risk Applications {toc}
+
+### In healthcare and other high-risk occupancies, the AHJ or the project's infection-control requirements may impose additional verification — for example viral-inactivation confirmation per ASTM E1052 or supplementary sampling — beyond the bacteriological clearance required by this standard. {note}
+
+### Where the project imposes high-risk verification requirements, those requirements shall be carried out in addition to, not in place of, the chlorination and bacteriological clearance of this standard.
+
+# Return to Service {toc}
+
+## The system shall not be placed in service or connected to occupied-building demand until bacteriological clearance has been achieved and accepted.
+
+## The system shall be returned to service only after the chlorine residual has been flushed to the endpoint, all samples have cleared, equipment isolated for protection has been reconnected, and removed strainers, aerators, and outlet devices have been reinstalled.
+
+## The backflow prevention assembly serving the disinfected segment shall be confirmed in place and operable before the segment is returned to service, in coordination with [[sync/backflow-prevention]].
+
+## A long idle period between clearance and occupancy can allow regrowth that invalidates the earlier clearance. {note}
+
+## Where a disinfected system will stand idle between clearance and occupancy, the disinfection plan shall state how the system will be kept from stagnating.
+
+# Records {toc}
+
+## The Contractor shall maintain field records of the disinfection of each segment, including all residual measurements, dosing, contact-period start and end times, flush endpoint, dechlorination, and disposal.
+
+## The Contractor shall provide the field records in the closeout submittals.
+
+## Records shall identify each disinfected segment so that, in the event of a positive bacteriological result, the affected segment and its re-disinfection can be traced.
+
+## The disinfection records, laboratory reports, and statement of clearance shall be incorporated into the project commissioning record and the operation and maintenance documentation.