How to read an NPDES effluent limits table

In most discharge permits, the limits come near the front, often in a section called Part I.A, though numbering varies by state. Expect one table per outfall, and sometimes two tables for the same outfall: interim limits that apply now and final limits that start on a later date. Check the dates at the top before you read a single number.

Every permit has to say how each parameter is monitored, including the type of sample, the interval, and the frequency (40 CFR 122.48(b)). So each row gives you everything you need to read it as a sentence:

Here’s the limits table from a made-up municipal wastewater plant. The BOD5, TSS, and pH numbers are the real federal secondary treatment standards; the others are invented, because they’d come from the stream the plant discharges to.

A limit is only half a number without its statistic. Every statistic starts from the daily discharge, which the federal rule defines:

The weekly average is the same calculation over a calendar week, and the daily maximum is simply the highest allowable daily discharge. Which ones a permit uses depends on who holds it:

Here’s a made-up June of BOD5 results for the sample plant, two 24-hour composites a week, with each day’s flow. The averages update as you change any number. Try pushing a single day higher and watch which limits move.

Concentration says how strong the discharge is; mass says how much pollutant actually reaches the stream. Permits usually limit both, and the conversion is one line:

lb/day = mg/L × flow in MGD × 8.34

The 8.34 is the weight of a gallon of water in pounds: one milligram per liter is one part per million, and a million gallons of water weighs about 8.34 million pounds. So 30 mg/L at the sample plant’s 2.0 MGD design flow is 30 × 2.0 × 8.34 = 500 pounds a day, which is exactly where its mass limit came from. For a municipal plant, the rule says limits are calculated from design flow (40 CFR 122.45(b)(1)).

That has a consequence worth knowing. When the plant runs below its design flow, a result at the concentration limit stays under the mass limit, so concentration is the one to watch. In wet weather, when flow climbs above design, the mass limit can be exceeded even while the concentration passes.

• Footnotes. They carry the statistic (“geometric mean”), the sampling point, compliance schedules, and special conditions. Read every one.
• Interim and final limits. If there are two tables for one outfall, the dates decide which one applies to this month.
• Extra samples count. Results from sampling more often than required, with approved methods, go into the calculations (40 CFR 122.41(l)(4)(ii)).
• “Report” still means report. No limit, but a blank makes the DMR incomplete.
• Non-detects. How they enter an average is set by your permit or state. And when the lab’s reporting limit is above the permit limit, a non-detect can’t show compliance either way, which is why permits require sufficiently sensitive methods (40 CFR 122.44(i)(1)(iv)).
• Units. A result in µg/L against a limit in mg/L is off by a factor of a thousand.
• Percent removal needs the influent. It’s worked from the month’s average influent and effluent, so the influent has to be sampled too.

Your permit governs. Where its wording, footnotes, or your state’s DMR instructions say something more specific than this guide, follow them.