The Complete Guide to DMR Preparation for Small Utilities

If you operate a municipal wastewater treatment plant, Discharge Monitoring Reports are a fact of life. Every facility with an NPDES permit is required to submit DMRs—typically monthly—to document compliance with effluent limits. For small utilities, where one operator is responsible for running the plant, doing lab work, and handling paperwork, DMR preparation is often the most time-consuming administrative task on the calendar.

This guide covers the full picture: what goes into a DMR, how to read your permit, how every key calculation works, how to handle tricky situations, and how to build a process that runs reliably month after month—even when someone is on vacation or just retired.

1. What Is a DMR?

A Discharge Monitoring Report is the standardized self-monitoring form required by the Clean Water Act for any facility holding an NPDES (National Pollutant Discharge Elimination System) permit. It documents the actual results of your effluent monitoring against the limits set in your permit and gets submitted to EPA or your delegated state agency on a regular schedule—usually monthly.

The DMR captures:

• Measured sample values for each parameter your permit requires you to monitor
• Summary statistics: monthly averages, weekly averages, daily maximums
• Pollutant loads (mass per day, typically in lbs/day)
• Percent removal for BOD and TSS where required
• Number of samples taken vs. required
• Any NODI (No Data Indicator) codes where required data is absent

Once submitted, your DMR data flows into EPA's ICIS-NPDES database and becomes part of the public record in the ECHO (Enforcement and Compliance History Online) system. Environmental groups, regulators, and the public can search ECHO to see your facility's compliance history. This matters—incorrect data doesn't just create a clerical problem; it creates a documented compliance record.

2. Reading Your NPDES Permit

Your NPDES permit is the authoritative document. Everything on your DMR flows from it. Before you can prepare a DMR correctly, you need to understand the key sections of your permit.

Effluent Limitations Table

This table is the core of your permit. It lists every regulated parameter, the applicable limits (monthly average, weekly average, daily maximum), the units (mg/L, lbs/day, colony-forming units, etc.), and the sample type (composite vs. grab). The limits in effect at the end of the monitoring period are the ones that apply to that DMR—relevant when permits are renewed mid-month.

Monitoring and Reporting Requirements

This section specifies sampling frequency ("2/week," "1/month," "continuous"), the location of each monitoring point (Outfall 001, Influent, etc.), and the submission deadline. Most permits require DMR submission by the 28th of the following month, though some states set different deadlines.

Special Conditions

Seasonal limits, nutrient trading requirements, wet-weather bypass provisions, and other facility-specific conditions live here. Read this section carefully—it's where permit-specific complexity hides.

3. The Monthly Workflow

A reliable DMR process has five stages. Each stage has its own failure modes; knowing them in advance prevents the panic that comes from discovering a problem the day before the deadline.

Stage 1 — Sample Collection (throughout the month)

Samples must be collected at the right location, at the right frequency, using the right method (composite vs. grab). Composite samples are collected continuously over a period (often 24 hours) to represent average conditions. Grab samples are taken at a single point in time. Your permit specifies which is required for each parameter.

Common failure: taking more samples than required and not knowing you must report all of them. If your permit requires 2 samples per week and you take 3 to confirm an outlier result, all 3 must be included in your average. You cannot discard samples to improve your numbers—that's a reporting violation.

Stage 2 — Lab Analysis

Most small plants send samples to a contract laboratory. Turnaround time is typically 5–10 business days. You need to factor this in—if you're waiting on lab results that arrive on the 20th of the following month and your DMR is due the 28th, you have 8 days. If results are late, contact the lab immediately and document the delay.

When results arrive, check that the lab report includes: the sample date and time, the parameter name and CAS number, the result value and units, the method detection limit, the analytical method used, and the lab's chain of custody documentation. If anything is missing, request it before the deadline.

Stage 3 — Data Entry and Calculation

This is where most errors happen. You're transcribing values from a lab report, calculating summary statistics, handling non-detects, converting units, and computing loads—often across multiple data sources. See the calculations section below for the full breakdown.

Stage 4 — QA Review

Before submission, someone should review the completed DMR independently of whoever prepared it. Check: do calculated averages match raw data? Are units consistent? Are all required parameters included? Do NODI codes match actual field conditions? Are values within historical ranges—if BOD jumped from 8 to 85 mg/L, is that a real result or a transcription error?

Stage 5 — Submission

Log into NetDMR (or your state's eDMR system), enter or upload the data, resolve any hard errors, and submit. Keep a timestamped confirmation record. If you're submitting close to the deadline and the system is down, document your attempts. EPA has established procedures for system outages.

4. Key Calculations Every Operator Must Know

Get these right and your DMR data will be accurate. Get them wrong and you may be out of compliance on paper even if your plant is performing well.

Arithmetic Mean (Monthly and Weekly Averages)

Used for most parameters: BOD, TSS, ammonia, total nitrogen, total phosphorus, and most metals. Sum all values in the period and divide by the number of samples.

A weekly average uses only the samples within that calendar week. If your permit requires a "weekly average," you need at least one sample per week, and you report the average of samples in each week—not the overall monthly average.

Geometric Mean (Bacteria Parameters)

Used exclusively for fecal coliform and E. coli. The geometric mean reduces the distorting influence of a single extreme spike, which is important for bacterial counts that can vary by several orders of magnitude.

Both methods give identical results. The log method is easier in a spreadsheet. In Excel: =10^AVERAGE(LOG10(range))

Pollutant Loads (lbs/day)

Many permits include load limits in addition to concentration limits, particularly for nutrients in sensitive watersheds. The load tells regulators the total mass of pollutant you're discharging per day, not just how concentrated it is in your effluent.

The 8.34 factor converts mg/L × MGD to lbs/day (since 1 gallon of water weighs 8.34 lbs). The flow used in the load calculation should be the flow measured on the same day as the sample, not the monthly average flow. Using average flow when calculating loads from grab samples is a systematic error that understates loads on high-flow days.

Percent Removal

Secondary treatment standards under 40 CFR Part 133 require 85% removal of BOD and TSS. Percent removal compares your influent (incoming raw wastewater) to your effluent (treated discharge).

This requires simultaneous influent and effluent sampling. Your permit will specify whether to use grab or composite samples for influent. The influent sample must represent the wastewater entering your plant, not a flow-equalized or blended stream.

Daily Maximum

The highest single measured value during the reporting period. This is a grab sample result, not a calculated average. If your permit has a daily maximum limit for a parameter, check every individual sample result against it—not just your monthly average.

5. Handling Non-Detects

A non-detect (ND) occurs when your lab cannot detect a pollutant above its method detection limit (MDL). Labs report these as "<0.5 mg/L" or "ND" in their reports. You cannot use zero for non-detects in your calculations.

The Standard Approach: Half the MDL

EPA's general guidance is to use one-half the MDL as a substitute value for calculation purposes. So if the MDL is 0.5 mg/L and your result is reported as <0.5, use 0.25 mg/L in your average.

When Most Results Are Non-Detects

If you have a parameter where most or all results are below the MDL, your calculated average will be a function of whatever substitute value you use. Document your approach. If the parameter is consistently non-detect, you may be able to request reduced monitoring frequency from your state at permit renewal—but you must continue monitoring at the required frequency until the permit allows otherwise.

Zero vs. Non-Detect

A true zero would mean absolutely no detectable amount exists. In environmental monitoring, this is essentially impossible to prove. Non-detect simply means the lab's analytical method couldn't measure it above the detection threshold. Always report as ND with the MDL, never as zero, unless your permit explicitly instructs otherwise.

6. Reporting Periods and Averaging Windows

Understanding which samples belong to which averaging period is critical when permits use terms like "monthly average," "weekly average," or "30-day average."

Calendar Month vs. 30-Day Rolling Average

"Monthly average" typically means the calendar month: all samples from January 1 through January 31. A "30-day average" is a rolling window that can end on any day. These are different—check your permit language carefully. Most municipal permits use calendar-month averaging.

Calendar Week Definition

A "week" in EPA terms is Sunday through Saturday unless your permit specifies otherwise. If you collect samples Monday through Friday and the permit requires a "weekly average," your weekly average includes only the samples from that Sunday–Saturday window. A sample collected on a Saturday counts in a different week than one collected the next Monday.

What to Do When a Permit Is Reissued Mid-Month

Use the limits that were in effect at the end of the monitoring period. If your new permit takes effect January 15 with different limits, the DMR for January uses the new limits (since January 31 falls under the new permit). Document this in your records.

No Discharge During the Monitoring Period

Even if your plant has no discharge during a monitoring period (planned maintenance, dry weather bypass didn't occur, etc.), you may still be required to submit a DMR with a "C" (No Discharge) NODI code. Failure to submit because "there was nothing to report" is a reporting violation. Read your permit—it will specify what's required.

7. Submitting via NetDMR

Most NPDES-permitted facilities now submit DMRs electronically through EPA's NetDMR system. Here's what you need to know.

Account Setup

NetDMR access is managed through EPA's Central Data Exchange (CDX) portal. As of August 2024, CDX authentication uses Login.gov. Setup involves: creating a Login.gov account, creating a CDX account, requesting the NetDMR role, and completing an Electronic Signature Agreement (ESA). The ESA can be completed electronically or via paper mail.

Hard Errors vs. Soft Errors

NetDMR validates data before accepting submission. Hard errors block submission and must be resolved. Soft errors are warnings that don't block submission but should be investigated. Common hard errors include: conflicting NODI codes (entering a value and a NODI code for the same field), values in incorrect format, and invalid permit-parameter combinations.

NODI Codes

NODI (No Data Indicator) codes explain why a field has no numeric value. Common codes:

• C — No discharge occurred during the monitoring period
• I — Sample not taken (with explanation required)
• 9 — Sampling location inaccessible
• B — Permit condition not yet effective

Use NODI codes carefully. Some codes automatically generate a violation record in ICIS-NPDES; others do not. When in doubt, contact your state permitting agency before using a NODI code for a parameter that has a required monitoring frequency.

Authorized Agent Submission

You can designate a third party (such as a compliance service) to submit DMRs on your behalf. This requires a Principal-Agent Relationship Agreement and role assignment in CDX. You retain full review and approval authority—nothing is submitted until you approve it. Many small utilities use this approach when outsourcing DMR preparation.

8. Common DMR Violations and How They Happen

Understanding how violations are generated—mechanically, in the EPA's ICIS system—helps you avoid them.

Effluent Limit Exceedances (E90 violation code)

Triggered when a reported value exceeds the permit limit. The most obvious violation type, but not always the most common. An E90 is generated even if the exceedance is due to a calculation error or a transposition mistake. Once the DMR is submitted, the violation is recorded. You can submit a corrected DMR, but the original violation remains in the system.

Reporting Violations (D80/D90 violation codes)

D80 is generated when a DMR is submitted more than 31 days after its due date. D90 is generated when a required DMR is never submitted. These are automatic—the system flags them based on the submission date versus the due date. No human review triggers them. They are also the most common violation type nationally, which is why "just submit something, even incomplete" is sometimes better strategy than waiting for perfect data.

Monitoring Frequency Violations (M10 violation code)

Triggered when the number of samples reported is less than the required monitoring frequency. If your permit requires 2 samples per week for BOD (approximately 8–9 per month), submitting a DMR with 6 samples will generate an M10 violation for the missing samples.

The Extra-Sample Problem

If you collect more samples than required (say, 3 instead of 2 in a week to confirm a borderline result), all samples must be reported. You cannot discard the outlier to improve your average. Reporting only a subset of collected samples is a reporting violation, even if the omitted sample would have caused an exceedance. Report everything; let the numbers be what they are.

9. When You Have an Exceedance

An exceedance is not the end of the world, but how you respond matters as much as the exceedance itself.

Report It Accurately

Do not adjust, average around, or omit exceedance values. Submit the actual data. Falsification of monitoring data is a federal crime under the Clean Water Act, and the penalties (including criminal prosecution of individual operators) are severe. Regulators treat an honest exceedance with appropriate notification as a very different situation than a hidden or falsified one.

Check Your Permit for Notification Requirements

Many permits require you to notify your regulatory agency within 24 hours (or by the next business day) of becoming aware of an exceedance for certain parameters. Some require a written report within 5 days. Read the reporting requirements section of your permit so you know exactly what triggers a notification obligation before you have an exceedance.

Investigate Before Submitting

Before concluding you have a real exceedance, verify: Was the sample collected correctly? Is the lab result for the right parameter and unit? Did a transcription or calculation error occur? A 10-minute data review can save a compliance headache. If you confirm the exceedance is real, investigate the cause—treatment process upset, equipment failure, sampling error—and document your findings.

Resampling

If your permit allows for resampling and you resample within the monitoring period, the resampled result must also be reported. Resampling to "get a better number" is not allowable; resampling to verify a result is legitimate, but all results get reported. Some permits explicitly prohibit resample-and-average for certain parameters.

10. Record-Keeping Requirements

EPA regulations (40 CFR Part 122.41) require you to retain monitoring records for at least 3 years. Your permit may require longer. Records that must be kept include:

• The date, exact location, time, and method of each sample collection
• The name of the person who collected each sample
• All lab analysis reports, including chain of custody documentation
• All calculations used to prepare the DMR
• Copies of all submitted DMRs
• Equipment calibration records (flow meters, pH meters, DO meters)
• Any written notifications to regulatory agencies

Records should be organized such that an inspector could trace any value on a submitted DMR back to the original sample and lab analysis. This is not a hypothetical requirement—state inspectors conduct file reviews and will ask to see this chain of documentation.

11. The Small Plant Reality

The compliance requirements that apply to a facility treating 1 million gallons per day are the same as those applying to a facility treating 100 million gallons per day. The EPA doesn't scale the paperwork burden by plant size. This is the fundamental challenge for small utilities.

The Single-Operator Problem

At small plants, one person is often responsible for plant operations, maintenance, lab sampling, sample shipping, record-keeping, DMR preparation, and submission. When that person is sick, on vacation, or retires, the institutional knowledge walks out the door. The monthly DMR workflow exists in one person's head, not in a documented system.

The solution is to document your process as if you're writing instructions for your replacement. Not because you're leaving, but because the act of writing it down creates a process that's repeatable by someone else. What lab does your facility use? How are samples transported? Where are the field logs kept? What spreadsheet is used to calculate geometric means, and where is it stored?

The Succession Risk

30 to 50 percent of water and wastewater operators nationally are eligible to retire in the next 5 to 10 years. For small utilities, this represents an existential compliance risk. Municipalities that lose an experienced operator without a documented handoff process frequently find themselves in violation during the transition period—not because the new operator is incompetent, but because the process was never written down.

When to Get Help

Some situations warrant bringing in outside expertise: when a permit is reissued with significantly changed limits; when your plant processes become more complex; when you're facing staff turnover; or when you've received a Notice of Violation and need to understand what went wrong. Environmental engineering consultants and DMR compliance services can take on the calculation and submission burden, leaving operators to focus on running the plant.

12. Pre-Submission Checklist

Before hitting submit on any DMR, work through this checklist:

• All required parameters are included (cross-reference permit monitoring table)
• Sample counts match required monitoring frequency for each parameter
• Correct averaging method used for each parameter (arithmetic vs. geometric)
• Non-detects substituted with appropriate value (half MDL or state-specific)
• Load calculations use same-day flow values, not monthly average flow
• Units are consistent throughout (lab reports in same units as permit limits)
• Weekly averages calculated from correct calendar-week windows
• Daily maximums verified as highest individual sample, not calculated average
• Percent removal calculated from correct influent/effluent samples
• NODI codes are appropriate and not conflicting with numeric values
• All exceedances reviewed and notification requirements checked
• Data reviewed by a second person before submission
• Lab reports filed with this month's records
• Submission deadline confirmed (due date is before the 28th? Check your permit)