The 90-Day Rule: What California Industrial Users Must Do Before Discharging to a Sewer

Who does the 90-day rule apply to?

Categorical industrial users: facilities in industries with a federal categorical pretreatment standard. Metal finishing, electroplating and pharmaceutical manufacturing are common examples. The rule comes from the General Pretreatment Regulations at 40 CFR Part 403.

In California, the city or sanitation district that runs the treatment plant administers them as the control authority when its pretreatment program is approved. The Regional Water Board oversees those programs as the approval authority, and acts as the control authority where a POTW has no approved program.

A significant industrial user (SIU) with no categorical standard does not file these two reports. It still gets a permit and reports at least every six months, as covered below.

What is due at least 90 days before discharge?

The baseline monitoring report, for a new source. Under 40 CFR 403.12(b), new sources, and facilities that become categorical users after their standard is issued, file it at least 90 days before they start discharging. Existing sources file within 180 days after the standard takes effect or a final category determination.

The report covers identifying information, permits, a description of operations, flow, pollutant measurements, a certification and, where needed, a compliance schedule. A new source does not have production data yet, so it gives estimates and describes the pretreatment it plans to use.

That 90-day lead is a floor. The POTW also needs your questionnaire, may want sampling of representative water, and has to issue your permit. Start the conversation well before the 90-day mark.

What is due within 90 days after discharge starts?

The 90-day compliance report. Under 40 CFR 403.12(d), a new source files it within 90 days after it begins discharging; an existing source files it within 90 days after its final compliance date. It reports measured flow, sampling results for the regulated pollutants, and a signed statement on whether the standards are being met consistently and, if not, what added treatment or operation is needed.

It is a performance report backed by data, not a construction completion notice. If the facility started discharging before its baseline was settled, this is where any gap shows up.

Does pretreatment have to be running before the first discharge?

Yes, for a new source. Under 40 CFR 403.6(b), a new source must install, put in operating condition and start up all pollution control equipment needed to meet its standards before it begins to discharge. It must then meet all applicable standards within the shortest feasible time, not more than 90 days.

So the second 90-day window is not a grace period for building equipment. It is time to stabilize and prove a system that is already running.

How do all the federal dates fit together?

One table holds the federal defaults. Your control authority's ordinance and permit can make any of them earlier or more frequent.

Federal pretreatment deadlines as of October 2026

When	What is due	Who	Rule
At least 90 days before discharge	Baseline monitoring report, with estimates and planned pretreatment	New sources	403.12(b)
Before discharge	Pretreatment installed and started up	New sources	403.6(b)
Within 180 days after the standard or category determination	Baseline monitoring report	Existing sources	403.12(b)
Within 90 days after discharge starts, or after the final compliance date	Compliance report with certified statement	Categorical users	403.12(d)
Within 14 days after each schedule date	Progress report	Users on a compliance schedule	403.12(c)
Each June and December, unless the control authority sets other months	Periodic compliance report	Categorical users	403.12(e)
At least every six months	Periodic report	SIUs with no categorical standard	403.12(h)

A very small categorical user that the control authority finds non-significant certifies once a year instead of filing the June and December reports, under 403.12(q).

Who decides whether you are a categorical user?

The control authority, not you. It decides based on your questionnaire and characterization data. Under 40 CFR 403.3(v), every categorical user is also an SIU, unless the control authority finds it non-significant. A non-categorical facility is an SIU if it discharges an average of 25,000 gallons per day or more of process wastewater, makes up 5 percent or more of the plant's average dry weather hydraulic or organic capacity, or is designated by the control authority.

If the category is in doubt, the facility or the POTW can ask for a written category determination under 40 CFR 403.6(a). For a new source, ask before discharge. Assuming you are a non-categorical SIU when your process falls under a standard is how a facility misses its baseline report entirely.

What happens if a report is late?

A late report is a violation on its own, and a very late one is significant noncompliance. Under 40 CFR 403.8(f)(2)(viii), failing to provide a required report, such as a baseline monitoring report or a 90-day compliance report, within 45 days after its due date puts a user in significant noncompliance. The POTW must publish the names of users in significant noncompliance at least once a year.

POTWs must also have authority to seek penalties of at least $1,000 a day for each violation. Beyond the penalty, a late or missing report leaves a gap in your compliance record that is hard to close later.

Limits of this article: these are the federal default rules as of October 2026. Under 40 CFR 403.4, state and local rules can be stricter. Your control authority's ordinance and permit control your actual dates, so get them in writing.

Frequently asked questions

Does the 90-day rule apply to every industrial user?

No. The baseline monitoring report and the 90-day compliance report apply to categorical industrial users. Other SIUs have permit and six-month reporting duties, and smaller users follow the POTW's local rules.

Is the 90-day compliance report due 90 days after the permit is issued?

No. For a new source it is due within 90 days after discharge begins. For an existing source it is due within 90 days after the final compliance date for its standard.

I'm the operations manager for a new metal finishing line in California, starting up next spring. When should we contact the POTW?

As soon as the process and expected wastewater are defined. Your baseline monitoring report is due at least 90 days before discharge, and the POTW has to review your questionnaire and issue your permit before that. Working back from the start date, that usually means talking to the POTW during design.

Our California plant is adding a new process to an existing permitted discharge. Do the 90-day deadlines start over?

Possibly. You must notify the control authority in advance of any substantial change in the volume or character of your discharge, under 40 CFR 403.12(j). If the new process brings a categorical standard into play, ask the control authority in writing which baseline and compliance reports it will require and when.

Is pretreatment always required before discharging to a sewer in California?

Only when your untreated wastewater would not meet its categorical standard, the POTW's local limits, or the prohibitions in 40 CFR 403.5 that apply to every user. A new categorical source that needs it must have it installed and started up before discharge.

Related resources

• California Industrial Discharge Permits: What the Sewer Connection Process Really Looks Like
• Local Sewer Connection vs. State Permit: When a California SIU Permit Is the Faster Path
• Self-Monitoring and Reporting Requirements for Texas Significant Industrial Users
• Enforcement Response Plans: How Texas POTWs Escalate Pretreatment Violations

Approaching startup on a California industrial discharge?

We help facilities lay out the pretreatment design, the POTW filings and the startup schedule so the reports are on time and backed by real data. Tell us about your startup date.

We specialize in:

• Industrial wastewater and pretreatment system design
• Wastewater characterization and sampling plans
• POTW permit applications and coordination
• Wastewater engineering in California
• Sewer connection and utility design
• Construction administration

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com