Controlling Bacteria in Your Closed-Loop Water System

Sep 14, 2026

A practical guide to odor, sludge, and sanitizer use in countertop fabrication shops.

Applies to systems running liquid coagulant or GreatWhite.

The short version

Bacteria in a recirculating water system are normal. Every shop has them. They only become a problem when water sits and sludge is allowed to build up which is why almost all odor complaints trace back to a weekend shutdown and a trench that has not been flushed.

The fix is mostly mechanical, not chemical: keep water moving, flush your trenches and pits on schedule, and clean the system out completely every 6 to 12 months.

A low, steady level of bromine helps. A high level of bromine or chlorine can accelerate corrosion on steel surfaces. More is not better, this is the single most common and most costly mistake we see.

Why Bacteria Show Up, And Why That Is Normal

Your closed-loop system is doing exactly what it was designed to do: capture stone dust and slurry, drop the solids out, and send clean water back to your saws and polishers. That process is very good at recycling water. It is also, unavoidably, very good at creating conditions bacteria can thrive in.

Bacteria are everywhere, in your makeup water, in the air, on the stone, on every tool that touches the water. You are not going to sterilize an open industrial water system, and you should not try. The goal is not zero bacteria. The goal is keeping the population low enough that it never causes odor, never damages equipment, and never becomes a concern for your crew.

What your system offers them

Ingredient	Where it comes from in your shop
Water	Obviously. Recirculated continuously and rarely fully replaced.
Warmth	Shop temperature, plus heat from pumps and cutting.
Food	Cutting and polishing lubricants, tool oils, hydraulic drips, cardboard and paper dust, and anything else that finds its way into a floor trench.
Minerals	Sulfate, calcium, and iron from the stone itself and from your makeup water. Sulfate is what feeds the bacteria that cause rotten-egg odor.
Shelter	Settled sludge. Once solids pile up, the inside of that pile is protected from oxygen, from water movement, and from sanitizer. This is the key one.
Time	A 60-hour weekend with the pumps off.

The one thing that matters most

Of everything on that list, the two you control are shelter and time, sludge and stagnation.

You cannot remove the water, the warmth, the minerals, or realistically all the food. But you can keep water moving and you can keep sludge from accumulating. Shops that do those two things rarely have odor problems, regardless of which coagulant they run.

The Weekend Problem

Almost every odor complaint we investigate follows the same pattern. The shop runs fine all week. Friday afternoon the pumps go off. Monday morning someone opens the door and the shop smells like rotten eggs.

What happens between Friday and Monday

When water is circulating, it carries dissolved oxygen. Oxygen keeps the helpful, odorless bacteria in charge, they outcompete everything else and nothing objectionable gets a foothold.

When the pumps stop, that changes fast. Solids settle. Within a few hours the oxygen in the settled sludge is used up, and a few millimeters below the surface of that sludge the conditions turn anaerobic to no oxygen at all.

A different group of bacteria takes over in that zone. The most important ones are sulfate-reducing bacteria. They breathe sulfate instead of oxygen, and the byproduct is hydrogen sulfide gas, the rotten-egg smell. Your stone, your slurry, and your makeup water all supply plenty of sulfate, so there is never a shortage of fuel.

By Monday morning, 60-plus hours of that has produced enough gas to fill the headspace above your trenches and pits. Starting the pumps stir it up and releases it all at once. That is why the smell often gets worse for the first few minutes after startup, not better.

Why this is worse in some shops than others

Factor	Effect
Length of shutdown	A two-day weekend is the common case. A three-day holiday weekend is noticeably worse. A shop shut down for vacation can come back to a serious problem.
Depth of settled sludge	The single biggest variable. A trench with a half inch of sludge behaves very differently from one with six inches.
Shop temperature	CBacterial activity roughly doubles for every 18°F increase. A warm summer weekend produces far more gas than a cold one.
Sulfate in makeup water	Well water in much of the Midwest is high in sulfate. More sulfate means more fuel for the odor-producing bacteria.
Lubricant and oil load	The more organic material entering the loop, the more food is available and the faster oxygen is consumed.
Trench design and slope	Dead legs, flat runs, and settled slope leave water and solids sitting still even during production.

The practical takeaway

The weekend is not the cause. The sludge is the cause — the weekend just gives it time to work.

A clean trench sitting idle for three days smells like almost nothing. A trench with several inches of settled solids sitting idle for two days can clear a room. That is why flushing before a shutdown is worth more than any chemical you can add.

Sludge Is the Real Driver

If there is one idea to take from this document, it is that odor is a sludge problem wearing a chemistry costume. Sanitizer cannot penetrate a settled sludge bed. The bacteria producing the smell live inside that bed, protected from the water above it. You can hold a perfect residual in the water column and still have an active odor source three inches below it.

Sludge means the system is working

It is worth being clear about this, because sludge gets reported to us as a defect more often than anything else. If you have clean water going back to your equipment, you have sludge somewhere. That is the whole point — the solids came out of the water. The question is never whether sludge forms. The question is whether it leaves on a schedule or accumulates until it becomes a problem.

A note on coagulant conversions

If your shop recently changed treatment products and noticed a difference: some liquid coagulant chemistries carry incidental antibacterial properties that were never their purpose and were never on the label. When a shop switches products, that incidental effect goes away, and a sludge problem that was always there can become noticeable for the first time. We are actively studying this. If your shop’s experience changed after a conversion, tell your FCW representative — it is useful information and it may change what we recommend for your system.

Where to look

Sludge does not accumulate evenly. Check these locations specifically, because they are where problems start:

Dead legs and blind ends of trenches, where flow does not reach
Corners and direction changes
Low spots and any section where the slope has settled over time
Under grating that rarely gets lifted
The far end of the pit from the pump suction
Behind or beneath equipment that has been added since the trench was poured

What good looks like

Flush trenches and pits on a defined schedule — weekly is typical, and before any extended shutdown is important.

Flush before a holiday weekend, not after. Removing the sludge on Friday is far easier and far more effective than dealing with the odor on Tuesday.

If flushing is not clearing the accumulation, do not increase your sanitizer to compensate. Sanitizer does not dissolve sludge. Call us and we will look at flow, slope, and trench design.

Keep sludge depth under control between full cleanouts. If you are consistently seeing several inches build up between flushes, something upstream needs adjustment.

Warning Signs

Bacteria give you plenty of notice. Here is what to watch, smell, and look for.

What you notice	What it usually means	What to do
Rotten egg or sewer smell, worst on Monday	Sulfate-reducing bacteria in settled sludge. The most common complaint we see.	Flush trenches and pits. Review weekend circulation. Check sanitizer residual.
Black staining or black sludge layers	Iron sulfide — a direct byproduct of the same bacteria. Confirms the diagnosis.	Same as above. Plan a full cleanout if widespread.
Sour, rancid, or vomit-like smell	Fermenting bacteria feeding on lubricant carryover.	Look at how much lubricant and oil is entering the loop. Flush and clean.
Fishy or ammonia-like smell	Breakdown of amine-containing additives, or protein-type decay.	Note which lubricants and additives are in use and tell your FCW rep.
Musty or earthy smell	Generally nuisance-only. Common in older water.	Usually resolved by a cleanout and fresh water.
Slimy film on surfaces or in lines	Biofilm. It shelters bacteria and consumes sanitizer.	Mechanical scrubbing at next cleanout. Chemicals alone will not remove it.
Sanitizer residual that will not hold	High demand from accumulated organics — a cleaning signal, not a dosing signal.	Flush and clean. Do not increase the dose.
Water darkening or turning gray-black	Sulfide chemistry and iron in solution.	Partial or full water replacement plus cleanout.
Rising makeup water use	May indicate a leak, and tells you dissolved solids are concentrating.	Track the volume monthly. Schedule partial replacement.

Treat every pit as a confined space

Hydrogen sulfide is heavier than air and collects at the bottom of pits and trenches. At low concentrations it smells like rotten eggs. At high concentrations it deadens your sense of smell within minutes — which means the warning disappears exactly when the danger is greatest.

Never enter a pit or below-grade trench without following confined-space procedures: ventilate first, monitor the atmosphere, use an attendant, and never work alone.

This applies to your own crew and to any contractor you bring in to clean.

The Four Controls, in Priority Order

These are listed in the order that actually matters. Shops that get the first two right rarely need to think much about the others. Shops that skip the first two and try to solve the problem with chemicals end up spending more and getting worse results.

1. Keep water moving

Circulation puts oxygen in the water, and oxygen keeps odor-producing bacteria from taking over. This is the cheapest and most effective control available to you.

Run a recirculation pump on a timer during extended shutdowns — 15 minutes every one to two hours is a reasonable starting point
Address dead legs and low spots where water sits still even during production
Before a long holiday shutdown, flush first and then set circulation to run
Ask your FCW representative about low-level pit aeration if your shop shuts down regularly — a small air pump running through the weekend is often the whole fix

2. Flush trenches and pits on schedule

Sludge that leaves the building cannot generate odor. Regular flushing prevents accumulation from ever reaching the depth where anaerobic conditions develop.

Establish a written flush interval and hold to it — weekly is typical for most shops
Always flush before an extended shutdown
Pay attention to the spots that flow does not naturally reach
Track sludge depth. If it climbs between flushes, increase frequency before it becomes a problem

3. Maintain a low, steady sanitizer residual

A consistent low level of bromine makes the water less hospitable and slows growth between cleanings. It is a supplement to the first two controls, not a substitute for them. A small weekly dose of a non-oxidizing biocide can be added alongside it for shops that shut down regularly. Both are covered in the sections that follow.

4. Full system cleanout every 6 to 12 months

Even with good flushing, material accumulates over time in places routine flushing does not reach. A complete cleanout resets the system.

Interval	Appropriate when
Every 6 months	High production volume · engineered stone with heavy fines · warm shop · well water high in sulfate · recurring odor history · long or complex trench runs
Every 12 months	Moderate production · consistent weekly flushing · good weekend circulation · no odor history · municipal makeup water

What a full cleanout should include

Complete removal of settled solids from all trenches, pits, and tanks — including under grating and in dead legs
Mechanical scrubbing of surfaces to remove biofilm. Chemicals alone will not do this
Inspection and cleaning of pump suctions, screens, and strainers
Full water replacement, not a partial dilution
Sanitizer feed equipment inspected and calibrated
Proper disposal of removed solids per your local requirements
Confined-space procedures observed throughout

A note on water replacement: in a closed loop, evaporation concentrates everything dissolved in the water — sulfate, chloride, hardness, and sanitizer byproducts. Over months, those levels climb well above what your makeup water started at. High chloride in particular attacks stainless steel. Periodic partial water replacement, not just topping off what evaporates, keeps that in check.

Bromine — Why We Recommend It, and How Much

Why bromine rather than chlorine

For most fabrication shops we recommend bromine over chlorine. There are three practical reasons.

Reason	Explanation
It works at your pH	This is the big one. Chlorine loses most of its effectiveness above about pH 8. Stone slurry water is naturally alkaline — often pH 8.5 — which is exactly where chlorine performs worst. Bromine stays effective across a much wider pH range, so it keeps working in water where chlorine has largely stopped.
It tolerates ammonia better	When chlorine meets ammonia it forms chloramines, which are weak sanitizers and cause their own odor. Bromine forms bromamines, which remain effective. In a system with lubricant additives and other nitrogen sources, this matters.
It adds less chloride	Chlorine-based sanitizers add chloride to your loop with every dose, and chloride accumulates as water evaporates. Elevated chloride pits and corrodes stainless steel. Bromine avoids adding to that particular problem.

Bromine also works normally alongside both liquid coagulants and GreatWhite. Some other classes of biocide do not — certain products are chemically bound up by bentonite clay and never reach the bacteria at all. Bromine is not affected that way.

How much

Target: a low, consistent residual

For most fabrication systems, a target of roughly 1 to 3 ppm total bromine is appropriate — enough to slow bacterial growth, low enough to avoid damage.

Confirm the right target for your specific system with your FCW representative, and always follow the product label. Labels are legally binding and take precedence over any general guidance in this document.

Consistency beats concentration. A steady 2 ppm all week does far more good than a large slug dose on Friday that is gone by Saturday morning.

How to feed it

Feed continuously or on a regular schedule rather than in occasional large doses
Introduce sanitizer where water is moving, so it distributes through the system
Test regularly — two to three times a week when establishing a routine, then weekly once you know how your system behaves
Record your readings. A trend tells you far more than a single number

If your bromine keeps disappearing, do not chase it

This is the most important sentence in this document. If you dose to 3 ppm and it reads zero a few hours later, the sanitizer is being consumed by accumulated organics and sludge. That is called demand.

The instinct is to add more. Do not. You will never win — the demand will absorb whatever you add, and along the way you will corrode equipment, and you may reach genuinely hazardous concentrations while still reading zero on your test kit.

High demand is a cleaning signal, not a dosing signal. Flush your trenches and pits, clean the system, replace the water, and then re-establish your residual. It will hold at a normal dose once the demand is gone.

The Real Danger of Overfeeding Oxidizers

Bromine and chlorine are oxidizers. Used correctly at low levels, they are a useful part of a maintenance program. Used at high levels — or dumped in as a shock dose to fix a smell — they cause damage that costs far more than the odor ever did, and they create hazards that put your crew at risk.

We want to be direct about this, because overfeeding is common and it is almost always well-intentioned. A shop notices a smell, adds sanitizer, the smell does not improve, so they add more. That cycle causes more harm than the bacteria did.

What overfeeding does to your equipment

Damage	How it happens
Stainless steel pitting and corrosion	Oxidizers raise the corrosion potential of the water. Combined with the chloride that accumulates in every closed loop, this drives pitting in stainless components — tanks, pumps, fasteners, fittings. Pitting is localized and can perforate a wall without any general thinning to warn you.
Seal, gasket, and hose degradation	Elastomers harden, crack, and fail early. You see it as unexplained leaks at fittings and pump seals.
Pump and mechanical damage	Seal faces, impellers, and wear components degrade faster in aggressively oxidizing water.
Coagulant and flocculant performance loss	Oxidizers attack the polymer chemistry that makes your treatment work. Overfeeding can measurably reduce solids capture — so the water gets cloudier, and the natural response is to add more treatment chemical, which costs more and does not address the cause.
Instrument and sensor damage	pH probes, level sensors, and control components have limited tolerance for strong oxidizers.
Damage to your own product	Aggressive water contacts saw blades, polishing pads, and tooling. It is not doing them any favors either.

What overfeeding does to your people

These are the ones that can hurt someone

NEVER mix chlorine or bromine products with acids. This includes acidic cleaners, descalers, and pH-down products. The reaction releases chlorine gas, which can cause serious lung injury or death. In an enclosed shop this can happen quickly and with very little warning.

NEVER mix chlorine or bromine products with ammonia or ammonia-containing cleaners. This releases chloramine gas, which is also seriously toxic.

NEVER mix different sanitizer products with each other, even two products that both say chlorine or both say bromine on the label. Different formulations can react violently. Use one product, and use it as labeled.

NEVER shock-dose a system that already smells like rotten eggs without talking to us first. Hydrogen sulfide and strong oxidizers can react in ways you do not want happening in an occupied shop.

Overfed water off-gasses. Elevated bromine or chlorine in an open trench or pit releases vapor into the shop air, where your crew breathes it all day. Eye, throat, and respiratory irritation from oxidizer vapor is common in overfed systems and is often blamed on stone dust instead.

Store oxidizer products away from fuels, oils, solvents, rags, and any other combustible material, in a dry area, in their original labeled containers. These are oxidizers — they will feed a fire.

Basic handling rules

Read the label and the Safety Data Sheet before first use, and keep the SDS accessible to everyone who handles the product
Wear appropriate eye protection and chemical-resistant gloves when handling concentrated product
Ensure ventilation where sanitizer is stored and where it is added to the system
Add sanitizer to water — never add water to concentrated product
Keep original labels intact. Never transfer product to an unlabeled container
Train everyone who might handle it, not just the person who normally does

One more time, because it is the point of this whole section: if your sanitizer is disappearing faster than it should, the answer is to clean the system, not to add more chemical. Overfeeding does not fix a sludge problem. It just adds a second problem on top of the first.

Non-Oxidizing Biocides — A Weekly Supplement

Bromine is an oxidizer, and it works on the water column. There is a second class of product — non-oxidizing biocides — that works differently, and used correctly it is a useful addition to a maintenance program. The one we most often point shops toward for fabrication loops is DBNPA.

What DBNPA is

DBNPA is short for 2,2-dibromo-3-nitrilopropionamide. Shops often hear it shortened to “DBPA” or just “dibromo” — DBNPA is the correct name and the one you will see on the label and the SDS. It is a fast-acting, broad-spectrum, non-oxidizing biocide used widely in industrial recirculating water systems.

Its defining characteristic is speed. DBNPA works within about an hour of contact, then breaks down into low-toxicity byproducts rather than persisting in the loop. That short life is a feature, not a shortcoming — and it is specifically why DBNPA is the right non-oxidizer for a system that also carries a bromine residual.

Why it complements bromine rather than replacing it

Property	What it means for your loop
Different mode of action	DBNPA does not kill by oxidation. Bacteria that tolerate a low oxidizer residual are still susceptible to it. Using two unrelated chemistries makes it much harder for a tolerant population to establish itself.
Fast kill, short life	This is the key compatibility point. Slower non-oxidizers such as isothiazolinones are chemically attacked and destroyed by a bromine residual before they can do their work, so you pay for product that never performs. DBNPA acts within an hour, which is faster than the bromine can degrade it.
Works with your treatment chemistry	A slug dose is appropriate in systems running either liquid coagulant or GreatWhite. Some other biocide classes are bound up by bentonite clay and never reach the bacteria at all.
Non-corrosive at label dose	Used as directed it does not raise the corrosion potential of the water the way an oxidizer overfeed does, and it adds no chloride to a loop that is already concentrating chloride through evaporation.
Aimed at the right window	Dosed Friday, it delivers a kill cycle exactly when the pumps stop and oxygen begins to disappear — the first hours of a shutdown, which is when the odor problem is actually created.

How to use it

A small weekly dose, not a continuous feed

DBNPA is added as a slug — one measured dose — rather than fed continuously the way bromine is. Weekly is the typical starting point for a fabrication loop. A shop with light production, consistent flushing, and good weekend circulation may do fine every other week.

Dose late on Friday, near the end of the last shift, with the pumps still running so the product distributes before the system goes idle. Let circulation continue for at least 30 to 60 minutes after dosing where you can.

Slug concentrations for industrial recirculating systems commonly land in the range of 25 to 30 ppm as dosed for weekend protection. Your FCW representative will convert that to an actual volume for your loop and the specific product you buy — the concentration of the formulation matters, and no two labels are identical.

Where practical, ease back the bromine feed for a few hours around the dose. Not strictly required given how fast DBNPA acts, but it gets you more out of each dose.

What determines your dose

Total loop volume — pits, trenches, tanks, and everything in circulation, not just the tank
The concentration of the product you purchase, which varies considerably between suppliers
Length of the shutdown — a three-day holiday weekend warrants more attention than a normal weekend
How much organic load is entering the loop from lubricants and oils
Your current sludge condition — a heavily loaded system will consume product without solving anything

What it will not do

DBNPA is a supplement to the four controls, not a fifth control that replaces any of them. It treats the water. It does not penetrate a settled sludge bed any better than bromine does, it will not dissolve accumulated solids, and it will not compensate for a trench that has not been flushed or a system that sits dead still all weekend.

If a weekly dose is not producing a noticeable improvement, the answer is not a larger dose. It means the odor source is in the sludge rather than in the water, and the fix is mechanical. Call us before escalating the chemistry.

Handling — it is a registered pesticide, treat it like one

Read the label and the SDS before first use. The label is legally binding and takes precedence over anything in this document. Keep the SDS where everyone who handles the product can reach it.
Wear chemical-resistant gloves and eye protection when handling concentrate. Add product to water, never water to product.
Do not premix DBNPA with any other chemical, and do not store it with your bromine or chlorine products. Oxidizers and non-oxidizers are stored separately, in original labeled containers, in a cool dry area.
Do not dose into a stagnant pit and walk away. Without circulation the product stays where you poured it, and you get neither distribution nor kill.
Do not shock-dose the system with an oxidizer right before or right after a DBNPA dose. Space them out.
Train everyone who might handle the product, not only the person who normally does.

Where this fits

Think of a weekly non-oxidizing dose as an extension of Control 3 rather than a new item on the list. Circulation and flushing still do most of the work. A steady low bromine residual still handles day-to-day maintenance. DBNPA covers the gap the other three leave open — the weekend, when the water stops moving and the bromine residual is being consumed with nothing replenishing it. Shops with a recurring Monday odor problem, a long shutdown schedule, or a history of high sanitizer demand are the ones that benefit most.

Talk to your FCW representative before adding any biocide to your program. We will confirm the product is appropriate for your system materials, set the dose for your loop volume, and make sure it does not conflict with anything already in use.

Maintenance Schedule — Quick Reference

Post this near your system. Adjust intervals to match your production volume and your FCW representative’s recommendation for your specific installation.

Frequency	Task	Why
Daily	Visual check of water clarity and trench condition. Note any new odor.	Catches changes early, while they are still easy to correct.
Daily	Confirm sanitizer feed equipment is running and has product.	An empty feeder is the most common cause of a lost residual.
2–3× weekly (initially)	Test and record bromine residual.	Establishes how your system behaves. Move to weekly once the pattern is known.
Weekly	Flush trenches and pits.	Prevents sludge from reaching the depth where odor develops.
Weekly	Check and record sludge depth at a few fixed spots.	A rising trend is your early warning.
Weekly	Inspect dead legs, corners, and low spots specifically.	These are where problems start, and routine flushing often misses them.
Weekly (optional)	Slug-dose non-oxidizing biocide (DBNPA) late Friday, pumps still running.	Covers the shutdown window, when the bromine residual is being consumed with nothing replenishing it.
Before any shutdown of 3+ days	Flush thoroughly. Verify sanitizer residual. Set up circulation if available.	Prevention is dramatically easier than remediation.
Monthly	Inspect pump screens and strainers. Clean as needed.	Restricted flow creates the stagnant zones you are trying to avoid.
Monthly	Check makeup water volume being added.	A rising trend can indicate a leak; it also tells you how fast dissolved solids are concentrating.
Quarterly	Partial water replacement.	Controls the buildup of chloride, sulfate, and hardness that evaporation concentrates.
Every 6–12 months	Full system cleanout and water replacement.	Resets accumulation that routine flushing cannot reach.
Annually	Review the whole program with your FCW representative.	Production volume, materials, and water chemistry change. The program should follow.

Common Mistakes

Instead of this	Do this
Adding more sanitizer when the smell gets worse	Flush and clean first. Odor from settled sludge is a mechanical problem. Sanitizer cannot penetrate a settled sludge bed, so adding more only damages equipment.
Shock-dosing on Friday to get through the weekend	Maintain a steady low residual all week and flush before shutdown. A shock dose is largely consumed within hours and is gone long before the odor develops.
Topping off only what evaporates	Replace a portion of the water periodically. Topping off alone concentrates dissolved solids indefinitely.
Mixing sanitizer products or switching brands mid-drum	Use one labeled product at a time. Different formulations can react dangerously with each other.
Treating sludge accumulation as a product defect	Sludge means the system is capturing solids as designed. Flush it on schedule.
Entering a pit to clean it without preparation	Treat every pit and below-grade trench as a confined space. Ventilate, monitor, and never work alone.
Waiting until the smell is bad to act	Track sludge depth and sanitizer residual weekly. Both give you weeks of warning before odor appears.
Ignoring where lubricants and oils enter the loop	Reducing what goes in reduces what the bacteria eat. Often the cheapest fix available.

When to Call Us

Some situations are worth a phone call before you act. Call your FCW representative or our support line if any of the following apply:

Odor persists after a thorough flush and a full water replacement
Sanitizer residual will not hold even after a cleanout
Sludge accumulates faster than weekly flushing can keep up with
You are seeing corrosion, pitting, or premature seal failures
Your shop’s odor or sludge behavior changed noticeably after a treatment product conversion
You are considering a shock dose, a new biocide, or any chemical not currently in your program
Anyone in the shop reports eye, throat, or respiratory irritation they associate with the water system

FCW360 Support

Support line: 320-529-4035

Have your system model, approximate loop volume, treatment product, and recent sanitizer readings ready when you call. A few numbers from your log will get you to an answer much faster than a description of the smell.