New Main, Chlorine Drum, and a Creek
A crew has just laid 3,000 ft of new 12-inch ductile iron main. The utility's written standard is the continuous-feed method: dose to at least 25 mg/L free chlorine, hold 24 hours, verify residual, flush, then collect two consecutive sets of satisfactory bacteriological samples at least 24 hours apart before the main goes into service. The crew chief has 5 gallons of 12.5% sodium hypochlorite on the truck, wants to dose the whole main in one shot, and plans to start sampling "first thing tomorrow morning" so the line can be energized tomorrow afternoon. The far end is a fire hydrant 200 ft from a creek, and the crew intends to let the chlorinated water run into the roadside ditch that feeds it.
1. Compute the main volume and the amount of 12.5% hypochlorite needed. Is 5 gallons enough, and how do you confirm the dose actually landed?
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Volume: a 12-inch main has an area of 0.785 ft². 0.785 x 3,000 ft = 2,356 ft³. x 7.48 gal/ft³ = 17,600 gallons.
Chlorine required at 25 mg/L: 17,600 gal x 8.34 lb/gal = 146,800 lb of water; x 25 x 10^-6 = 3.7 lb of available chlorine.
Product required: 12.5% hypochlorite holds about 0.125 lb available chlorine per lb of product, so 3.7 / 0.125 = 29 lb of product. At roughly 10 lb/gal that is about 3 gallons, so 5 gallons is enough, with margin to dose a bit heavy and still stay reasonable.
Confirmation is the part crews skip: the calculation assumes the whole main volume is actually treated. Air pockets, high points, and dead ends mean the slug does not distribute evenly. Feed at a controlled rate with the main filling, bleed air at high points, and then measure free chlorine at the far end and at intermediate points. Do not accept the math as proof - accept a residual reading. The hold period starts when the whole main is at strength, not when the first gallon went in.
2. The crew wants one sample set tomorrow and to energize the line tomorrow afternoon. What is your decision, and what residual and sampling criteria govern it?
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Decision: the main does not go into service tomorrow. Two things are wrong.
Contact time: 24 hours means 24 hours from the moment the entire main is at strength. If dosing finishes at 3 p.m., the earliest flush is 3 p.m. the next day. "First thing tomorrow morning" is closer to 18 hours and the chlorine has not had its contact time against the pipe wall and any residual debris.
Sampling: the standard requires two consecutive sets of satisfactory samples collected at least 24 hours apart. One set tomorrow is half the requirement. After the 24-hour hold, check residual (a common criterion is at least 10 mg/L remaining at the far end after the hold; if it has dropped below that, the line was under-chlorinated or had demand from debris and the treatment must be repeated), then flush thoroughly and sample. Sample again 24 hours later. Both sets satisfactory, or the line stays isolated.
Also verify before anything else: the pipe was swabbed and kept clean during installation, and the trench was dewatered so the main was not sitting full of groundwater. Chlorination cannot fix a dirty main. If there is any doubt, repeat the whole procedure - one extra day against a contaminated new main is not a close call.
3. What do you require before the chlorinated flush water goes to that ditch and creek?
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Dechlorinate, don't dilute and hope. High-strength flush water at 10 to 25 mg/L will be toxic to fish at the concentrations that reach a small creek, and a fish kill is an enforcement action and a newspaper story.
Requirements:
- Neutralize with sodium thiosulfate or a dechlorination tablet/canister setup, and check with a test kit at the discharge point before releasing. Hold and re-treat any batch that tests above the discharge limit.
- Contain and control the path - hay bales or silt fencing so the water does not carry sediment and so you can stop the flow.
- Discharge at a controlled rate, not a full hydrant blast, so the receiving water has dilution and so you can react.
- Check whether a general permit or the local pretreatment/NPDES authority covers hydrostatic and disinfection water discharge; many jurisdictions explicitly regulate it and some require notification.
- If a fish kill or visible sheen occurs, stop, notify the state environmental agency immediately, and document everything.
The judgement is that the chlorination step is regulated work with a discharge at the end of it, not an open-hose operation. Plan the discharge before the first gallon goes in.