Colorado’s Wastewater Discharge Permit Rules: What Communities Need to Know

Any community wastewater plant that discharges to a Colorado river, stream or groundwater needs a Colorado wastewater discharge permit from CDPHE's Water Quality Control Division. Its effluent limits stack up in three layers: statewide minimums for BOD, solids and pH, limits that protect your particular stream for ammonia, E. coli, metals and temperature, and Regulation 85's limits for phosphorus and nitrogen. Compliance trouble often starts where those limits meet cold water and low flow.

What is a Colorado wastewater discharge permit?

It is a Colorado Discharge Permit System (CDPS) permit, and no one may discharge a pollutant from a point source into state waters without one.

That rule is section 61.3(1)(a) of Regulation 61, the Colorado Discharge Permit System Regulations. The timing rules that matter to a community:

• Term. For a stream discharge, section 61.1(4) follows the federal limit, and 40 CFR 122.46 caps that permit at five years.
• Renewal. Apply at least 180 days before expiration. An application filed after expiration is treated as a new one (section 61.4(1)(d)).
• Construction. A new or expanded plant built for more than 2,000 gallons per day needs site location and design approval under Regulation 22 before construction.

Smaller systems are onsite systems, permitted by the local board of health under Regulation 43.

Where do the effluent limits in your permit come from?

Start with Regulation 62's statewide floor, then add whatever your receiving stream needs to meet its water quality standards.

Regulation 62 sets the floor: BOD5 of 30 mg/L as a 30-day average and 45 mg/L as a 7-day average, TSS of 30 and 45 mg/L, pH of 6.0 to 9.0, and 85 percent removal. Section 62.4(3) says stricter limits replace these whenever water quality standards require them.

Those standards come from Regulation 31, the basic standards for surface water, and the basin Regulations 32 through 38 that classify each stream segment. The four that drive most permits:

• Ammonia. Equations of the stream's pH and temperature, stricter where trout or sensitive early life stages of fish are present.
• E. coli. 126 per 100 mL, as a two-month geometric mean, for streams with existing primary contact recreation.
• Metals. Copper and zinc standards are equations of the stream's hardness.
• Temperature. 17.0 °C chronic from June through September where cutthroat or brook trout are expected (Cold Stream Tier I).

Dilution sets how tight the numbers get. Section 31.9(1) has CDPHE normally calculate limits at low flow: the 30-day, 1-in-3-year low flow (30E3) for chronic standards and the 1-day low flow (1E3) for acute ones. A small mountain creek leaves almost no room, which is why seasonal flow and snowmelt shape Colorado discharge permits.

What does Regulation 85 require for phosphorus and nitrogen?

A covered existing plant must meet 1.0 mg/L total phosphorus and 15 mg/L total inorganic nitrogen as rolling annual medians. A new plant must meet 0.7 and 7 mg/L.

Section 85.5(1) of Regulation 85 also sets 95th percentile limits: 2.5 and 20 mg/L for existing plants, 1.75 and 14 mg/L for new ones. Coverage depends on size and ownership:

• Excluded. Existing plants of 1.0 MGD or less, and existing plants owned by a disadvantaged community (10,000 people or fewer, plus the economic tests in section 85.7). They still monitor nutrients.
• Delayed. Existing permitted plants of 2.0 MGD or less, plants under watershed Regulations 71 through 74, and plants in listed low-priority watersheds. Their permits cannot carry the limits before December 31, 2027.
• Everyone else. Other existing plants get the existing-plant limits. A plant on a new site after May 31, 2012 is a new plant.

The dates may move. CDPHE's Regulation 85 page says the technology-based limits expire in December 2027, when stricter lake and reservoir standards would take effect. As of October 2026, the division expects to propose extending them to 2030 at a spring 2027 hearing. Plan any expansion against tighter numbers.

Why do Colorado plants miss ammonia limits in winter?

Cold water slows the bacteria that remove ammonia, and a small stream's winter low flow can leave little water to dilute what is left.

EPA's Nutrient Control Design Manual state of technology review puts typical nitrification solids retention times at 10 to 20 days at 10 °C, against 4 to 7 days at 20 °C. Dropping from 20 °C to 10 °C water roughly triples the sludge age a plant needs. The ammonia standard also moves with temperature and pH, so the limit can change by season.

Spring snowmelt that leaks into old sewers dilutes the influent, and Regulation 62 still requires 85 percent removal. Section 62.5(2)(a) offers relief only when excessive infiltration is not the cause.

Fixing ammonia usually takes more nitrifying biomass: more aeration volume, or plastic media added to existing tanks, as EPA describes for fixed-film and moving bed biofilm systems. A snowmelt problem starts with fixing inflow and infiltration.

What happens when a plant violates its permit or runs short of capacity?

You report it fast and can face daily penalties, and a plant near capacity can stop a town's growth.

Section 61.8(5) requires an oral report within 24 hours of noncompliance that may endanger health or the environment, and a written report within five working days. Under C.R.S. 25-8-608(1), each day of each violation can draw a civil penalty, up to a maximum the Commission adjusts each year in Regulation 101.

Capacity is the quieter risk. When needed, section 61.8(7) requires expansion planning at 80 percent of design capacity and construction at 95 percent. A municipality at 95 percent must start construction or stop issuing building permits that add wastewater. That makes knowing when your treatment plant needs an upgrade a growth question.

A new limit the plant cannot meet yet can come with a schedule of compliance under section 61.8(3)(n).

How can a small community get ahead of tighter limits?

Plan against the limits you will have at the next renewal, not the ones in today's permit.

Trend each limit by month. Whatever creeps closest in the coldest month is your next project. Before designing an expansion, ask CDPHE for preliminary effluent limits, since the site application review checks the process against effluent limits or planning targets (section 22.5(1)(h)). Regulation 22 also has the division encourage consolidation whenever feasible (section 22.3(1)(c)), so a neighboring plant may be an option.

For utilities and small districts, funding often sets the pace. CDPHE's water quality grants and loans program offers state grants and low-interest loans from the Water Pollution Control Revolving Fund.

Is your plant ready for its next permit renewal?

If you can answer yes to all five, you are ready to set a renewal strategy. Any no is your next task.

1. Do you know your renewal application date, 180 days before expiration?
2. Have you trended a year of effluent ammonia, nitrogen and phosphorus against current and future limits?
3. Do you know whether Regulation 85 excludes, delays or covers your plant?
4. Do you know your flow and loading as a percentage of design capacity?
5. Do you know how much snowmelt and rain reach the plant through inflow and infiltration?

Then ask your engineer: Which limit will we miss first, in which month, and what is the smallest fix that holds through the next permit term?

Scope note: This is general education on Colorado's regulations as of October 2026, not a permit determination. Confirm your actual limits with CDPHE and your engineer.

Frequently asked questions

Does a septic system in Colorado need a discharge permit?

Not at 2,000 gallons per day or less. The local board of health permits that system under Regulation 43. A larger system needs site location and design approval and a discharge permit, like any treatment plant.

What is total inorganic nitrogen?

The sum of nitrate, nitrite and ammonia, measured as nitrogen. Regulation 85 limits it, so leftover ammonia counts against you.

I run a 0.6 MGD plant for a Colorado mountain town and we keep missing our winter ammonia limit. What should we check before we design a new plant?

Check three things: your solids retention time in the coldest month against EPA's 10 to 20 day range at 10 °C, whether the limit tightened at your last renewal, and inflow and infiltration during snowmelt. If it was discharging before May 31, 2012, a 0.6 MGD plant is excluded from Regulation 85's numeric limits but still monitors nutrients.

I'm a developer planning 300 homes that would connect to a small Colorado sanitation district's plant. How do I find out if the plant can take my flow?

Ask the district for the plant's current flow and loading as a percentage of design capacity, and its plan if it is near 80 or 95 percent. Those are the Regulation 61 triggers for expansion planning and construction. Get the capacity answer in writing before you close on the land.

Related resources

• Colorado CDPHE Discharge Permit Process: Timeline, Requirements, and How It Differs from Texas
• CDPHE Site Application vs. Discharge Permit Application: The Two-Step Process Colorado Developers Often Confuse Into One
• Colorado Water Quality Control Commission Basin Specific Standards: Why the Same Discharge Limit Does Not Apply Statewide
• Antidegradation Policy in Colorado Water Permits: What It Means for New Discharge Permits

Facing tighter discharge limits at your Colorado plant?

The easiest fix is the one planned before the renewal, not after the first violation. MES is licensed in Colorado, has an office in Golden, and takes community wastewater plants from planning through permits and construction.

We specialize in wastewater treatment plant upgrades and expansions, discharge permit applications and renewals, site location and design approval submittals, collection systems and inflow and infiltration analysis, lift stations, and construction administration.

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