Potable water disinfection and flushing field guide for plumbers

Disinfection and flushing, and what they actually do

Potable water disinfection is the process of chlorinating a new or repaired water line, holding the chlorine in contact long enough to kill what is in there, flushing it clean, and then proving with a lab test that the water is safe before the line carries drinking water. Flushing is the partner step. You flush the construction dirt out before you disinfect, and you flush the heavy chlorine out after, so what reaches the tap is clean water at a normal residual, not a slug of bleach.

The reason both steps exist is what goes into a pipe during construction. New pipe sits in a trench. It collects mud, groundwater, insects, rodents, gloves, lunch wrappers, and whatever was on the bench when the fitting was glued. Every joint is an opening until it is made up. The line is full of the jobsite, and the jobsite is not sterile. Put that water in front of the public without cleaning it and you have built a contamination event with a permit on it.

This is public-health work, not a punch-list item. The line does not go into service on the plumber's say-so. It goes into service when the chlorination is done to the standard, the flush has brought the residual back to normal, and a bacteriological sample has come back clean from an approved lab. AWWA C651 is the standard that governs the method for water mains, and the water utility and the health department are the authority that accepts the result.

Why disinfect a new water line?

You disinfect a new water line because the pipe carried dirt and bacteria in from the install, and the people downstream are going to drink the water. Coliform bacteria, including E. coli, ride in on soil, groundwater, and standing water that got into the open pipe during construction. Some of those organisms are harmless. Some of them, and the pathogens that travel with them, make people sick. You cannot tell the difference by looking, which is the whole reason the test exists.

The hazard is not theoretical. Waterborne outbreaks have been traced to new and repaired mains that were put into service without proper disinfection, where contaminated construction water reached customers before anyone sampled it. A water line is a direct path from the trench to a kitchen faucet, a hospital sink, a daycare bottle-warmer. There is no second barrier behind it. The disinfection is the barrier.

It is also required, not optional. The plumbing code calls for the potable system to be disinfected before use, AWWA C651 sets the method for mains, and the local water authority and health department gate the connection on a passing bacteriological result. No AHJ lets a new main go live on trust. The acceptance is documentary: a clean sample, a chlorination record, and a sign-off. Skip the disinfection and you have not saved a step, you have created a liability that lands on whoever signed the line into service.

When in the sequence does disinfection happen?

Disinfection comes after the line is built, pressure-tested, and proven tight, and before it is connected to the live system or put into service. The order matters. You do not disinfect a line that still leaks, because a leak is an opening that can let contamination back in, and because the repair after a failed pressure test means cutting back into pipe you just chlorinated. Prove the joints first, disinfect second.

The hydrostatic pressure test is the step ahead of this one, and it has its own discipline: fill, isolate, hold, and read the gauge against thermal drift, covered in the hydrostatic pressure test guide. The line that passes that test is the line you disinfect. The line sizing that set the pipe diameters, the water supply fixture units and the pressure budget, is settled even earlier, in the water supply pipe sizing guide. By the time you are chlorinating, the pipe is in the ground, tight, and the right size. The only thing left is to make the water inside it safe.

After disinfection passes, the final connections to the existing distribution system get made under their own controlled procedure, because the moment you tie a new line into a live main you have joined the clean side to the public side and any mistake propagates. Disinfect, pass the bacteriological test, dechlorinate and dispose of the heavy chlorine, then make the tie-in. That is the sequence, and rearranging it is how contamination gets into service.

What standard governs water main disinfection?

AWWA C651, Disinfecting Water Mains, is the standard that governs the method for new and repaired potable water mains in North America. It is published by the American Water Works Association and adopted by reference in most utility specifications and many plumbing and public-works codes. C651 spells out the forms of chlorine, the three chlorination methods, the dose and contact time, the preventive measures during construction, the final flush, the bacteriological testing, redisinfection when a sample fails, and the procedures for final connections and for cutting into existing mains. When a spec says disinfect per C651, this is the document it means.

C651 is the method standard. It is not the acceptance authority. The plumbing code, the IPC or the UPC as adopted, requires that the potable water system be disinfected before use and generally points to a recognized procedure such as C651 or the water purveyor's own rules. The local water authority and the health department are the bodies that approve the result, run or accept the bacteriological sample, and release the line for service. Those two roles are different and people confuse them. C651 tells you how. The health department tells you whether you passed.

The same principles carry to interior potable piping inside a building, where the plumbing code calls for disinfection of the water distribution system before occupancy, often by a method modeled on C651 at a lower dose. Confirm which standard and which dose the AHJ enforces for the work in front of you, because a utility main, a building service, and an interior distribution system can be held to different procedures by different authorities, and the edition adopted plus any local amendments control.

What chlorine dose disinfects a water main?

The standard continuous-feed dose under AWWA C651 is not less than 25 mg/L of free chlorine throughout the line, held for at least 24 hours, with not less than 10 mg/L of free chlorine still remaining at the end of that contact period. That residual at 24 hours is the proof the dose was high enough and the demand of the pipe and its contents did not eat all the chlorine. If the line comes back below 10 mg/L at 24 hours, the chlorine got consumed by something dirty in there, and the disinfection has to be repeated.

There is a faster, stronger option for large lines. The slug method feeds a short, heavily dosed slug of chlorinated water through the main, with the dose set to at least 100 mg/L of free chlorine and held so that every part of the main stays in contact for at least 3 hours, re-applying chlorine if the residual anywhere drops below 50 mg/L. It is used on long, large-diameter mains where filling and holding the whole line at 25 mg/L for a day wastes a great deal of water and chemical. Many utility specs land on an intermediate dose, on the order of 50 to 100 mg/L held a few hours, as a middle ground between the two named methods, so confirm the dose and the contact against the project spec and the purveyor.

Dose and contact trade against each other, but neither one alone is the test. The 24-hour residual is the gate on the continuous-feed method. The clean bacteriological sample is the gate on all of them. A high dose held a short time and a moderate dose held a long time can both work, and both still have to come back showing no coliform before the line goes into service. Verify the dose, the contact, and the required residual against AWWA C651 and the adopted spec, because the figures shift by method and by edition.

The three chlorination methods

AWWA C651 gives three ways to get the chlorine into the main, and the one you pick is driven by pipe size and access, not preference. The continuous-feed method injects chlorine at a constant rate at a point no more than 10 ft downstream of the start of the new main, as potable water fills the line, so the whole length comes up to at least 25 mg/L. It is the workhorse for ordinary mains. You meter the chlorine to the fill rate, fill slow, and confirm the residual at the far end before you start the 24-hour clock.

The slug method feeds a concentrated slug of chlorinated water through the main so that every part of the line sees at least 100 mg/L of free chlorine for at least three hours of contact as the slug moves past. It pays off on long, large-diameter lines where holding the full volume at 25 mg/L for 24 hours would waste an enormous amount of treated water. It demands more attention, because the slug has to be maintained above the spec concentration the whole way through, and the operator has to track it down the line.

The tablet method places calcium hypochlorite tablets or granules inside the pipe during assembly, then fills the main with potable water to dissolve them up to the dose. C651 limits it to mains roughly 24 inches and smaller, and only where the pipe can be kept clean and dry during installation, because the tablets have to stay put and dry until the fill. It needs no injection equipment, which suits short extensions and remote work, but it forfeits the pre-installation flush, so the line had better have been kept genuinely clean. Calcium hypochlorite also gives off chlorine fumes once it meets moist air, so a main a worker has to enter is the wrong place for it.

Forms of chlorine on the job

Three forms of chlorine show up in this work, and they are not interchangeable in handling. Sodium hypochlorite is liquid bleach, in field concentrations from ordinary household strength up to roughly 12.5 percent available chlorine for the commercial product. It is the easiest to meter and the most common on small and mid-size jobs. It loses strength sitting in the sun and in the drum, so old stock is weaker than the label and you compensate by dosing to a measured residual, not to a calculated volume.

Calcium hypochlorite is the dry form, granules and tablets at roughly 65 to 70 percent available chlorine. It stores and ships well, which is why it suits the tablet method and remote sites. It is also a strong oxidizer and a real fire hazard when it contacts organics, petroleum, or moisture, so it is kept dry, away from fuels and rags, in a vented container. Liquid chlorine gas, from cylinders, is the third form, used on large utility work, and it is the most hazardous of the three by a wide margin. Gas chlorination is a trained-operator job with its own equipment, leak detection, and respiratory protection.

Whatever form you use, you dose to a measured free-chlorine residual with a test kit, not to a number on a calculator. The pipe, the water, and the dirt in the line all exert a chlorine demand that consumes part of your dose before any disinfection happens. Measure the residual at the far end of the line and confirm it meets the method's requirement. The measured residual is the truth. The calculated dose is the starting estimate.

Why flush a new water line?

You flush a new water line twice, for two different reasons, and skipping either one wrecks the job. The first flush comes before disinfection, to push the construction dirt, mud, and debris out of the line. Chlorine has a demand. Every bit of organic dirt in the pipe consumes free chlorine, so a filthy line eats your dose and the disinfection fails the 24-hour residual no matter how much you fed in. Flush the solids out first and the chlorine you add goes to disinfecting instead of to dissolving mud.

The second flush comes after the contact time, to scour the heavy chlorine back out until the residual drops to the level the rest of the distribution system runs at. Nobody is supposed to drink water at 25 mg/L, let alone the slug dose. The final flush brings the line down to a normal residual, on the order of the system's working residual, so the water that reaches the customer is safe to drink and tastes like the rest of the system. It also carries out any remaining loosened material the disinfection knocked off the pipe wall.

Velocity is what makes a flush actually clean the pipe instead of just running water through it. AWWA C651 calls for a scour flush at a velocity high enough to lift and carry the debris out, which the standard raised to 3.0 ft per second in recent editions based on Water Research Foundation testing, up from the older 2.5 ft per second, for a minimum of about three pipe volumes and continued until the water runs clear and the residual is back to system level. Below the scour velocity you are rinsing, not scouring, and the grit stays on the bottom of the pipe waiting for the first customer.

The pre-flush and the final flush

The pre-flush and the final flush bracket the disinfection, and each has a clear endpoint. The pre-flush runs until the water leaving the line is visibly clean, with the construction dirt carried out, before any chlorine goes in. The final flush runs after the contact period until two things are true at once: the water runs clear, and the free-chlorine residual has dropped to the level the surrounding distribution system maintains. You confirm the residual with a test kit at the flush point, not by eye, because clear water can still carry a heavy chlorine load.

Reaching the scour velocity is a flow problem, and on a large main a single hydrant or a small blowoff cannot move enough water to hit it. You size the flush outlet, or open more than one, so the flow develops the target velocity in the pipe. The math is the pipe area times the velocity: a 6-inch main needs roughly 220 gallons per minute to reach about 2.5 ft per second and more to hit 3.0, and a 12-inch main needs four times that. If the available outlet cannot pass that flow, the flush will not scour, and you arrange a larger outlet or a different discharge before you start.

Both flushes send a lot of water somewhere, and where it goes is not your choice to make casually. The pre-flush is ordinary construction water and goes to an approved discharge. The final flush carries the heavy chlorine and cannot go to a stream, a storm drain, or any waterway without dechlorination first, covered in its own section. Plan the discharge for both flushes before you open the valve, because a flush with nowhere legal to go is a flush that stops halfway and a line that never gets clean.

What is a bacteriological test?

A bacteriological test, the bac-t, is a lab test of a water sample drawn from the disinfected and flushed line that checks for coliform bacteria, including E. coli, as the indicator that the water is microbiologically safe. Coliforms are the flag. They are common in soil and gut and easy to detect, so their absence is the accepted sign that the disinfection cleared the contamination the construction brought in. A clean bac-t, no coliform detected, is what releases the line for service. A positive result, any coliform, fails it.

You draw the sample after the final flush has brought the residual to system level, from a sample point on the line, using sterile technique and a sterile bottle, and the lab has to be one the health department accepts, often a state-certified drinking-water lab. AWWA C651 calls for more than a single sample. The common requirement is two sets of samples taken on the line, and editions differ on the timing: older practice was two sets collected about 24 hours apart, and newer editions add options such as samples 16 hours apart or a pair taken 15 minutes apart after a 16-hour rest. Confirm the sampling protocol, the number of samples, and the spacing against the adopted C651 edition and the health department, because that protocol is what you pass against.

When a bac-t comes back positive, the line is not safe and the fix is to flush, redisinfect, and resample until it passes, not to argue the result. C651 is direct about it: failure to pass requires that the flushing or disinfection be repeated. A positive sample on a new main usually means a missed flush, a low dose, a dead leg that never saw chlorine, or a sampling error, and you work that list. The line stays out of service until two clean sets come back. There is no partial credit on drinking water.

How do you dispose of the chlorinated water?

The heavily chlorinated water from disinfection and the final flush cannot be dumped to a stream, a wetland, a storm drain, or any surface water without being dechlorinated first, because free chlorine is acutely toxic to fish and aquatic life at concentrations far below what is in that line. A slug of 25 mg/L water, let alone slug-method water, into a creek is a fish kill and a reportable environmental violation. The discharge falls under EPA water rules and usually a state NPDES permit or a local discharge authorization, and the receiving water has a chlorine limit measured in fractions of a milligram per liter.

Dechlorination is the step that makes the discharge legal. AWWA C655, Field Dechlorination, is the standard that covers it, and it splits releases into high chlorine, above about 4 mg/L, which is what disinfection water is, and low chlorine below that. High-chlorine water generally needs chemical treatment with a reducing agent, sodium bisulfite and related sulfite compounds being the usual choices, dosed and mixed into the flow until the residual drops to the permitted limit before it reaches the receiving water. You measure the residual at the discharge, not upstream, because the point is what actually leaves the pipe.

Plan the disposal before you chlorinate, the same as the flush. Sometimes the answer is to discharge to the sanitary sewer with the wastewater utility's permission, where the treatment plant handles the chlorine, but that needs their authorization and a check on flow volume so you do not overload a line. Sometimes it is dechlorination at the flush point with a metered reducing agent or a dechlorination diffuser on the hydrant. What it is never is the storm drain at full strength. Know the discharge route and have the dechlorination on hand before the chlorine goes in the pipe.

Handling chlorine safely

Chlorine in every form used here is a respiratory and skin hazard, and the dry and gas forms add a fire and a poison-gas risk on top of that. Liquid sodium hypochlorite burns skin and eyes and ruins clothing, so the baseline is splash goggles or a face shield, chemical gloves, and an apron, with an eyewash within reach. You do not pour it in a way that lets it splash back, and you do not work in a closed space with it without ventilation.

The hard rule that kills people is this: never mix hypochlorite with acid or with ammonia. Hypochlorite plus acid releases chlorine gas, hypochlorite plus ammonia releases chloramine vapor, and both are the kind of exposure that sends a crew to the hospital from a confined trench or a pump pit. Keep the chlorine away from acidic cleaners, from muriatic acid used on other work, and from anything ammonia-based. Calcium hypochlorite adds its own hazard, because it is a strong oxidizer that can ignite when it contacts petroleum, oil-soaked rags, or organic debris, so it is stored dry, isolated from fuels, and never swept up with shop trash.

Gas chlorine from cylinders is a different tier of hazard and a trained-operator job, with leak detection, emergency procedures, and respiratory protection appropriate to the cylinder. Working inside a large main during tablet or granule disinfection means confined-space rules, because chlorine fumes collect in the pipe and oxygen does not. Read the safety data sheet for the product you actually have on the truck, ventilate, and treat the residual in a freshly disinfected line as a chemical hazard until the final flush has brought it down. The line is not water you handle casually until the residual is back to system level.

Protecting the source during the fill

Filling and chlorinating a new line means connecting it to a source of potable water, and that temporary connection is a cross-connection hazard that can push contamination back into the live system. The new line is dirty and is about to be heavily chlorinated. If a pressure drop on the supply side pulls backward through the fill connection, the dirty or chlorine-laden water in the new line can siphon into the public main feeding everyone else. The fill hose lying in the trench, in the standing water, is exactly the path that does it.

AWWA C651 requires backflow protection on the temporary supply connection, and the protection has to suit the hazard, which here is high. An air gap, physically separating the supply outlet from the fill water with an open break, is the surest method and needs no testing. Where a direct connection is used instead, it gets a reduced-pressure-principle backflow assembly, an RPZ, sized and tested, between the public main and the new line. The fill hose end is kept up out of the trench water and off the ground, not dropped in the puddle it is filling from.

This is the same backflow discipline that protects any potable system from a cross-connection, the subject covered in depth in the backflow guides, applied to a temporary construction connection that crews treat as throwaway and therefore neglect. The connection is temporary. The contamination it can cause is not. Protect the source before you open the fill valve, and break the connection cleanly when the fill is done so a forgotten hose does not sit as a permanent cross-connection.

Do you disinfect a repaired main the same way?

A repaired main and a new main both get disinfected, but a repair under emergency conditions runs a different, faster procedure under AWWA C651 because the line cannot sit out of service for a 24-hour hold while customers have no water. On a main break, the standard's repair procedures lean on keeping the trench and the pipe interior as clean as possible, swabbing or spraying the new pipe and fittings with a strong chlorine solution, often on the order of a 1 percent, or roughly 10,000 mg/L, hypochlorite solution applied to the interior surfaces before assembly, and then flushing and chlorinating the repaired section as conditions allow.

The reason for the difference is the situation, not a lower standard of safety. A planned new main is built clean, tested, then disinfected on a schedule. A main break is an open pipe in a flooded trench with raw sewage potentially nearby and a town waiting on water, so the procedure shifts to aggressive local disinfection, heavy flushing, and a bacteriological sample after the line is back up, with a boil-water notice in place until the sample clears if the contamination risk was high. The health department drives whether a notice is required and when it lifts.

The judgment call is how dirty the break was. A clean break on a pressurized main that never lost positive pressure and stayed out of the floodwater is a lower risk than a depressurized line sitting open in a trench full of groundwater and sewage. C651 treats those differently, and so should you. When in doubt, treat the repair as contaminated, disinfect aggressively, sample, and hold service or issue the notice until the bac-t confirms the water is safe. The same final connections discipline applies when the repaired section ties back into the live main.

Storage tanks and reservoirs

Storage tanks and reservoirs are disinfected to a different AWWA standard, C652, Disinfection of Water-Storage Facilities, because the geometry and the access are nothing like a pipe. A tank has surfaces a person walks and touches, a large volume, and interior coatings that the disinfection cannot damage. C652 gives several chlorination methods, and you generally pick one. The common ones are filling the full tank to a free-chlorine residual on the order of 10 mg/L and holding it, or spraying and painting every water-contact surface with a stronger solution, around 200 mg/L available chlorine, and letting it act on the surfaces directly.

The chlorine sources are the same family as the mains work, liquid chlorine gas, sodium hypochlorite, and calcium hypochlorite, with the same handling hazards and the same confined-space concern, sharper here because a tank is a textbook confined space. A worker spraying chlorine solution inside a closed tank is in a chlorine-laden confined space, which means the full confined-space program, ventilation, atmospheric monitoring, and a rescue plan, not just a respirator and good intentions.

The acceptance is the same logic as a main: disinfect to the method, drain and refill or flush to a safe residual, and pass a bacteriological sample before the tank goes into service. Confirm the method, the residual, and the contact against AWWA C652 and the health department, because a tank coating, a tank size, and a utility's own rules can steer which of the C652 methods is allowed and how the result is sampled and accepted.

Large systems: campuses, data centers, and big mains

A large campus, a data center, or a hospital water system is the same disinfection problem multiplied, and the multiplication is where it goes wrong. The volumes are large enough that the slug method earns its keep on the big-diameter mains, the runs are long enough that dead legs and isolated branches hide from the chlorine, and the system is segmented enough that you have to plan which section gets disinfected when, and how each one is isolated, filled, dosed, held, flushed, and sampled without contaminating the section next to it.

Sequencing is the work. You disinfect and sample by section, valve off the proven sections from the unproven ones, and track which length is at full dose, which is in contact, which is flushed, and which has a clean bac-t in hand. A system with redundant feeds, a data center especially, has loops and standby branches that never see flow in normal operation, and those are exactly where chlorine fails to reach and where water later stagnates. Map the dead legs and make sure the flush and the chlorine actually move through every one of them, because the branch you forgot is the one that grows a biofilm and trips a sample months later.

The discharge volume on a large system is also a planning problem of its own. Flushing several large-diameter mains to scour velocity moves a great deal of heavily chlorinated water that all has to be dechlorinated and disposed of legally, and the receiving sewer or the dechlorination setup has to handle the rate, not just the total. On work this size the disinfection plan is a document with a sequence, an isolation scheme, a sampling map, and a discharge plan, agreed with the utility and the health department before the first section is filled.

Field example: an 8-inch service main

An 8-inch ductile-iron service main, about 600 ft long, was installed, pressure-tested, and passed tight. The crew disinfected it by continuous feed. They pre-flushed first through a hydrant at the far end until the water ran clean of the trench mud, then fed sodium hypochlorite at the upstream end as they refilled, metering to hold the line at no less than 25 mg/L. Before starting the clock they tested the residual at the far hydrant and read 28 mg/L, confirming the dose had carried the full length.

They held it 24 hours. At the end they retested the far end and read 14 mg/L free chlorine, above the 10 mg/L minimum, which told them the pipe demand had not eaten the dose and the line had stayed disinfected the whole period. Then the final flush. They opened the hydrant and developed enough flow to scour the 8-inch pipe, ran it more than three pipe volumes, and watched the residual fall until it matched the roughly 1 mg/L the surrounding distribution system carries. The flush water went to the sanitary sewer under the utility's authorization with the rate checked, because the high-chlorine slug could not go to the storm drain.

Two sets of bacteriological samples were drawn from the line after the flush, in sterile bottles, and run by the state-certified lab the health department accepts. Both came back with no coliform detected. The chlorination record, the residual readings at 0 and 24 hours, the flush data, and the two clean bac-t results went to the water authority, the line was approved, and only then was the final connection to the live main made. Start to service, the disinfection alone ran the better part of three days, which is the schedule the line needs and the one the estimate had better have carried.

What to document

The disinfection is not done when the sample passes. It is done when the record proving it passed is in the hands of the water authority and the health department, because the acceptance is documentary and the connection is gated on the paperwork, not on the plumber's word. A clean line with no record is a line that gets disinfected again, and a contamination question a year out is answered by the record or it is not answered at all.

Capture the section disinfected, the method and the form of chlorine used, the dose fed and the residual measured at the far end at the start, the contact time, the residual at the end of the contact period, the final-flush data and the residual it was flushed down to, the dechlorination and where the water was discharged, the bacteriological lab, the sample sets and their results, and the date the authority accepted the line. If a sample failed and the line was redisinfected, record that and that fresh samples passed. Tie it to AWWA C651 and the edition, because a dose and a contact time only mean something against the standard that called for them.

Common mistakes

• Putting a new or repaired line into service with no disinfection and no bacteriological sample, on the assumption the pipe was clean.
• Skipping the pre-flush, so construction dirt eats the chlorine demand and the line fails the 24-hour residual.
• Dosing or holding below the standard, then starting the clock without confirming the residual carried to the far end.
• Dumping the heavily chlorinated flush water to a stream, a storm drain, or a waterway without dechlorination.
• Leaving a dead leg or an isolated branch out of the flush and the chlorine, then chasing the positive sample it causes.
• Mixing hypochlorite with an acid or ammonia-based product on site and releasing chlorine or chloramine gas.
• Filling from the potable source with no backflow protection, risking siphoning dirty or chlorinated water into the live main.
• Connecting the new line to the live system before the bacteriological samples come back clean.
• Not sending the chlorination log and the bac-t results to the health department, so the line has no accepted record.

Field checklist

Run the disinfection in order, and do not start a step until the one before it is done and proven. The line is built, the pressure test has passed, and the discharge route and dechlorination are arranged before any chlorine goes in the pipe.

Standards and references

The method standard is AWWA C651, Disinfecting Water Mains, published by the American Water Works Association. It sets the forms of chlorine, the continuous-feed, slug, and tablet methods, the not-less-than 25 mg/L continuous-feed dose held at least 24 hours with not less than 10 mg/L remaining, the final flush, the scour velocity, the bacteriological testing and redisinfection on failure, and the procedures for final connections and for cutting into existing mains. Storage tanks and reservoirs go to AWWA C652, Disinfection of Water-Storage Facilities. Field dechlorination of the discharge goes to AWWA C655. The exact figures and the sampling protocol shift by edition, so confirm the dose, the contact, the velocity, and the sampling against the C651 edition the jurisdiction has adopted.

The plumbing code, the IPC published by the ICC or the UPC published by IAPMO as adopted, requires that the potable water system be disinfected before use and generally points to a recognized method such as AWWA C651 or the water purveyor's rules. The local water authority and the health department are the acceptance bodies. They run or accept the bacteriological sample, they decide whether a boil-water notice is needed after a repair, and they release the line for service. Federal drinking-water regulations under the Safe Drinking Water Act set the chlorine maximum residual disinfectant level for water in service, up to about 4 mg/L as an annual average, which is the level the final flush brings the line down toward.

Discharge of the chlorinated water is governed by EPA water rules, commonly through a state NPDES permit or a local discharge authorization, and free chlorine is limited in the receiving water to fractions of a milligram per liter because of its toxicity to aquatic life. The standard that controls any given piece of this work is the one the AHJ has adopted and enforces. The model standards are the starting point. The adopted editions, the local amendments, the purveyor's specifications, and the health department's acceptance are what govern the line going into service.

Units and terms

Disinfection carries vocabulary from the AWWA standards, from water-treatment chemistry, and from the field, and the same idea reads differently across a chlorination log, a standard, and a lab report. The terms below travel across the whole job.

Two notes on the numbers. Chlorine dose and residual are both given in mg/L, which is the same as parts per million for water, and free chlorine is what the test kit reads and what the standards specify, distinct from total chlorine which includes combined forms. Velocity in the flush is feet per second, and the scour figure is a threshold: above it the flow carries debris out, below it the grit stays put.

Disinfection

Chlorinating a line to kill the bacteria the install brought in, then proving it clean before service

Free chlorine residual

The active disinfecting chlorine left in the water, read in mg/L by a test kit; the figure the standards specify

mg/L

Milligrams per liter, equal to parts per million in water; the unit for dose and residual

Continuous feed

Metering chlorine into the fill so the whole line reaches at least 25 mg/L, held at least 24 hours

Slug method

Passing a short, heavily dosed slug through a large main for a short contact time

Scour velocity

The flush velocity, about 3.0 ft/s in recent C651 editions, that lifts and carries debris out instead of rinsing

Bacteriological test (bac-t)

A lab test of a line sample for coliform and E. coli; no coliform is the pass that releases service

Coliform

Indicator bacteria common in soil and gut; their absence is the accepted sign the line is microbiologically safe

Dechlorination

Reducing the chlorine in the flush water to a permitted level before discharge, per AWWA C655

Cross-connection

Any path that can let non-potable or contaminated water back into the potable system, including a fill connection

AHJ

Authority having jurisdiction, the water authority and health department that accept the result and release the line

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