1.2 DEQ Wastewater System Classification and Supervision Rules
Key Takeaways
Treatment systems are classed I-IV by treatment type and design average dry weather flow (ADWF) in Table A; the highest-rated process sets the class, and DEQ may move a plant up or down for complexity.
Collection systems are Class I up to 1,500 design population, II from 1,501 to 15,000, III from 15,001 to 50,000 and IV above 50,000.
A Small Wastewater System is a Class I treatment type with under 500 design population or under 150 connections, and it may be supervised solely by an SWWS-certified operator.
Systems with ADWF of 0.075 MGD or more need full-time supervision (at least 30 hours a week); smaller systems may use part-time supervision under a written contract.
Shift supervisors may be one grade below the system class, owners must report supervisor changes within 30 days, and a system may not go more than 30 days without its required operator.
Why Classification Comes First
Every certification question about "who may run this plant" starts with the system classification. DEQ classifies each domestic wastewater treatment system and collection system, tells the owner the class in writing, and the owner must then have the system supervised by an operator holding a certificate of the same type (treatment or collection) at a grade equal to or greater than the class (OAR 340-049-0015 and -0020).
Treatment System Classes: Table A (OAR 340-049-0025)
DEQ does not use a point-rating score for wastewater plants. Table A assigns the class from the treatment type and the design average dry weather flow (ADWF) in million gallons per day (MGD):
| Treatment type | ADWF up to 1 MGD | over 1 to 5 | over 5 to 10 | over 10 |
|---|---|---|---|---|
| Lagoon, non-aerated | I | I | I | I |
| Lagoon, aerated | I | II | II | II |
| Biofiltration | II | II | III | III |
| Extended aeration | II | II | III | III |
| Conventional activated sludge | II | II | III | IV |
| Membrane bioreactor | II | II | III | IV |
| Activated sludge with separate fixed film; SBR, MBBR, integrated fixed film, oxidation ditch or other suspended growth media | III | III | IV | IV |
| Activated sludge with tertiary filtration (includes chemical phosphorus removal) | III | III | IV | IV |
| Activated sludge with biological nutrient removal | III | IV | IV | IV |
Rules that travel with the table:
- If more than one treatment type is used, the highest-rated type sets the class.
- DEQ may classify a plant higher or lower than Table A if it is more or less complex than similar plants, if receiving-water conditions demand tighter control, or if it uses an approved process that is not listed. DEQ may also reclassify a collection system higher than its population would indicate.
- An owner may appeal a classification through the variance procedure in OAR 340-049-0075.
Worked classification examples
- A 3.2 MGD conventional activated sludge plant: the "over 1 to 5" column gives Class II.
- The same process at 12 MGD: Class IV.
- A 2.0 MGD plant with biological nutrient removal: Class IV, even though its flow is modest.
- A 0.6 MGD aerated lagoon followed by a small UV system: the lagoon line sets Class I.
Collection System Classes (OAR 340-049-0020)
| Collection class | Design population served |
|---|---|
| Class I | 1,500 or less |
| Class II | 1,501 to 15,000 |
| Class III | 15,001 to 50,000 |
| Class IV | 50,001 or more |
Design population can be an equivalent population. The rules base it on 70 gallons per person per day of ADWF or 0.17 lb BOD5 per person per day, whichever is greater (OAR 340-049-0010).
Small Wastewater Systems
A Small Wastewater System (SWWS) is a Class I treatment type serving fewer than 500 design population or fewer than 150 connections. It is designated in the permit, and an operator holding the single SWWS certificate may supervise it alone. DEQ's entry-level fact sheet notes that a qualifying small system can ask its permit compliance contact to be reclassified as an SWWS.
Supervision Requirements (OAR 340-049-0015)
| Rule | Requirement |
|---|---|
| ADWF of 0.075 MGD (75,000 gpd) or more | Supervised full-time by one or more operators certified for the system type at or above its class. Full-time means at least 30 hours per week for an owner or city employee. |
| ADWF under 0.075 MGD | Supervised part-time or full-time by a properly certified operator. A part-time contract must meet OAR 340-049-0070. |
| More than one daily shift | A shift supervisor, if used, may be certified no more than one grade lower than the system class. |
| Availability | The system supervisor need not be on site at all times but must be available and able to respond on site immediately. |
| Grade I Provisional | May be designated to supervise a Class I system of the matching type. |
| Notification | The owner reports supervisors to DEQ in writing (certificate type, grade, expiration) and reports replacement or re-designation within 30 days. |
| Vacancy | A system may not be without its required operator for more than 30 days; meanwhile an operator no more than one grade lower, authorized by the owner, must run it. |
| Time extension | On written request the Director may extend the deadline. Extensions over 120 days need extraordinary circumstances or the class-increase criteria, and none may exceed 365 days. |
The class-increase extension applies when DEQ raises a plant's class and the current supervisor has worked there at least one year, holds a certificate one grade below the new class, and the plant has had no Class I permit violations. The owner may then ask for 365 days so the operator can upgrade.
Part-time supervision contracts (OAR 340-049-0070)
A contract for part-time supervision of a small system must list the parties and their contact information; each supervising operator's certificate type, grade and expiration date; the contract start and end dates; the minimum number of site visits; and each party's duties. It must also make a certified operator available to respond on site. The owner keeps the contract on file for DEQ review.
Who else must be certified?
Only the designated supervisor and any shift supervisor must hold certificates. Other staff working under a properly certified supervisor may certify voluntarily, and many employers require or reward it. DEQ's rules apply equally to engineers: an engineer designated as the system supervisor must be certified. Owners of permitted systems also pay an annual operator certification program support fee scaled to design ADWF (Table C). For example, the fee effective July 1, 2026 is $100 for systems under 0.075 MGD and $457 for 1.0-1.999 MGD.
Exam Traps
- Table A uses design ADWF, not peak wet weather flow, and treatment type, not a point score.
- A plant's class is driven by its most complex process. Adding biological nutrient removal can raise a small plant to Class III or IV.
- The one-grade-lower allowance is for shift supervisors and for the 30-day vacancy period. It is never for the designated system supervisor.
Using DEQ Table A, what class is a 7.5 MGD (design ADWF) conventional activated sludge plant with no nutrient removal?
Class I
Class IV
Class III
Class II
A wastewater plant with a design ADWF of 0.05 MGD wants to use a certified operator who visits twice a week instead of hiring full-time staff. What does OAR 340-049 require?
Part-time supervision is allowed below 0.075 MGD under a written contract listing the operator, visits and duties
Part-time supervision is allowed only when the contract operator holds a Grade IV wastewater treatment certificate
Part-time supervision is never allowed; every permitted plant needs a full-time certified supervisor on site
The plant may operate without any certified supervisor as long as it files its monthly DMRs on time
A Class III treatment plant runs three daily shifts. What is the minimum certificate a designated shift supervisor may hold?
Grade IV Treatment
Grade I Treatment
Grade III Treatment
Grade II Treatment
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