MOCKUP · STUDY SETUP← all screens
Millbrook WRF 2026 permit cycle update Setup
Evaluation due · 122 days Continue to sample data

Study setup

Plant flows, receiving-water conditions, and the biosolids pathway feed every derivation downstream. Change a value here and every limit re-derives — so setup is versioned with the study, not with the facility.

Plant & flows
Use the flow regime that produces the most restrictive limits
Design flows
MGD
Annual average for this scenario.
MGD
Residential and commercial contribution.
MGD
Sum of permitted industrial user flows.
MGD
Derived by subtraction.
Critical condition
MGD
August. For most plants the lowest-flow month, not the annual average, produces the most restrictive limits — run both as separate scenarios and compare before you commit.
Industrial flow is 20% of plant flow. That share is what your allocation divides.
Receiving water & dilution
Water-quality pass-through criteria are computed against these conditions
Named receiving stream from your NPDES permit.
Selects the aquatic life criteria set applied to every pollutant.
mg/L
As CaCO₃, between 25 and 400. Hardness-dependent metals criteria are computed from this.
Dilution factors
Take these from your NPDES permit fact sheet, dated 2026-03-01. If you enter values your permit does not support, the derived limits will not survive review.
Defaults to the chronic factor where not separately determined.
Biosolids pathway
Disposal route decides which sludge criteria constrain the limits
ton/d
US dry tons per day to the land-application program.
MGD
Anaerobic digestion.
Class A land application currently controls 7 of 17 draft limits — those cells carry the amber limit line in the workspace.
Pollutant selection
Screen in, derive, then justify anything you drop
17 selected

Screen in anything your industrial users could plausibly discharge, anything your permit limits, and anything you have detected in influent or biosolids. Dropping a pollutant is a decision you will be asked to justify.

Conventional pollutants (BOD, TSS, ammonia) are limited by plant capacity rather than by criteria, so they run through a separate loading tool.

Criteria pinned to library v6 · published 2026-01-15. Changing the pin re-derives every limit.
Derived constants
What this setup hands to every derivation
Design flow
5.0MGD
Industrial share
20%
Critical low-flow month
3.2MGD
Dilution — acute
×15
Dilution — chronic
×37
Dilution — human health
×48
Hardness
25mg/L
Biosolids to land
6.25dry ton/d
Pollutants under study
17
Setup is versioned into the study record. When this study is approved, these values are what your submittal defends — the numbers stay reproducible even as the plant changes. When conditions move, you decide when the study moves; it never shifts underneath you.
Sources

EPA 833-R-04-002A
Local Limits Development Guidance, July 2004. The MAHL method every derivation follows.

40 CFR 403.5
National prohibited-discharge standards; local limits implement them as specific, enforceable numbers.

40 CFR 503.13
Pollutant limits for land-applied biosolids — the source of the Class A pathway constraint above.