How Often Must NC Aerobic Systems Be Sampled?

North Carolina sets effluent sampling by approval type. A residential wastewater treatment system (RWTS) certified to NSF/ANSI 40 must be “grab or 24-hour composite sampled annually” under 15A NCAC 18E .1504. Other advanced pretreatment systems follow Rule .1709: Provisional Systems quarterly, Innovative Systems annually up to 1,500 gallons per day (gpd), and twice a year above 1,500 and up to 3,000 gpd.

The Management Entity records flow before sampling, sends samples to a certified laboratory under 40 CFR 136 with chain of custody, and reports results to the owner, the local health department (LHD), the Department, and the manufacturer. A single site is compliant when the mean of three or more consecutive sampling dates stays within its Table XXV standard. A new complete data set from resampling within 30 days of receiving a non-compliant set may be substituted.

Summarizes public rules as of Oct 8, 2026; not legal advice; verify with your local health department.

What does 18E call an aerobic system?

Subchapter 18E does not use aerobic system as a category. NSF/ANSI 40 units are RWTS approved under Section .1500, and RWTS, TS-I, and TS-II systems are all “advanced pretreatment.” Rule .1302(a) applies its sampling rules “to all advanced pretreatment systems approved in accordance with Sections .1500 and .1700 of this Subchapter.”

How often is effluent sampled?

Rule .1504: “Effluent from an approved RWTS shall be grab or 24-hour composite sampled annually for all effluent standards listed in Table XXV of Rule .1201(a) of this Subchapter for NSF/ANSI 40 systems, unless adjusted sampling requirements have been requested and granted in accordance with Rules .1301 and .1709 of this Subchapter.”

Rule .1709(a) covers Provisional and Innovative Systems. “Provisional Systems shall be grab or composite sampled quarterly for all applicable influent and effluent constituents.” For Innovative Systems, “When the DDF is less than or equal to 1,500 gpd, Innovative Systems shall be grab or composite sampled annually,” and from 1,500 to 3,000 gpd they “shall be grab or composite sampled twice a year.” Systems serving vacation rentals “shall be sampled during the seasonal high use period” (Rule .1709(a)(5)).

What is sampled	Frequency	Rule
RWTS (NSF/ANSI 40) effluent, all Table XXV NSF/ANSI 40 standards	Annually, grab or 24-hour composite	.1504
Provisional System influent and effluent	Quarterly until Innovative Approval	.1709(a)(1)
Innovative System, DDF ≤ 1,500 gpd	Annually	.1709(a)(2)
Innovative System, DDF > 1,500 and ≤ 3,000 gpd	Twice a year	.1709(a)(3)
Fecal coliform, Innovative site compliant on all other constituents	Not required	.1709(a)(4)
Field parameters only (pH 5-9, turbidity ≤ 10, DO ≥ 2), where approved	At the VIP Management Entity inspection	.1709(e), Table XXXIII
Resample after a non-compliant result	Within 30 days of receiving lab results	.1709(a)(6), .1302(d)(5)(D)

What constituents are measured?

Table XXV in Rule .1201(a) sets the limits. For NSF/ANSI 40: CBOD ≤ 25 mg/L and TSS ≤ 30 mg/L. For TS-I: CBOD ≤ 15 mg/L, TSS ≤ 15 mg/L, NH3 ≤ 10 mg/L (or 80% removal if influent TKN exceeds 50 mg/L), and fecal coliform ≤ 10,000 colonies/100 mL. For TS-II: CBOD ≤ 10 mg/L, TSS ≤ 10 mg/L, NH3 ≤ 10 mg/L, TN ≤ 20 mg/L, and fecal coliform ≤ 1,000 colonies/100 mL.

For systems approved for field parameters only, Rule .1709(e) adds: “If the field parameters fall outside the range specified in the PIA Approval, an effluent sample shall be collected and analyzed for all parameters as necessary to demonstrate system compliance.”

Who collects and reports the samples?

The Management Entity. Under Rule .1302(d)(4), “the actual flow shall be recorded” before the visual inspection and “prior to any effluent sampling event.” Rule .1302(d)(5) then requires that “all samples shall be collected, preserved, transported, and analyzed in compliance with 40 CFR 136,” that “samples shall be taken to a certified laboratory, as defined in G.S. 130A-313(2), for analysis,” and that “documented chain of custody for each sample collected shall be maintained.”

Reporting: “The results of all sampling shall be reported by the Management Entity to the owner, LHD, Department, and the proprietary advanced pretreatment manufacturer” (Rule .1302(e)). Inspection reports with sampling results are due within 30 days under Rule .1304(c); see the 30-day report rule.

What counts as non-compliant?

For one site, Rule .1302(f) requires two things: the annual VIP meets the approval’s requirements, and the arithmetic mean for BOD5, TSS, TKN, and TN and the geometric mean for fecal coliform “from three or more consecutive sampling dates does not exceed the designated effluent standard in Table XXV.” Rule .1302(g) allows a mass loading calculation instead for systems with a DDF up to 3,000 gpd.

Rule .1710 is a different test. It applies to an approved system across “all data collected from all sites,” including that “no more than 20 percent of all data from all sites shall exceed the designated effluent standard.” It governs the product approval, not a single site.

When a site fails Rule .1302(f), Rule .1713(9) directs the LHD to “Issue a NOV to the owner.” Needed repairs also trigger the 48-hour LHD notice.

How does the 30-day resample work?

Rule .1709(a)(6): “Effluent may be re-sampled within 30 days of receipt of laboratory results indicating non-compliance with Table XXV of Rule .1201(a) of this Subchapter if requested by the owner, manufacturer, or manufacturer’s representative, or required in a PIA Approval.” Rule .1302(d)(5)(D) adds: “All sample results collected shall be reported.” Both the original and the resample results are reported.

Can sampling be reduced?

The owner may submit “a written request to the LHD and Department to reduce the wastewater system effluent sampling frequency, effluent sampling constituents, or Management Entity inspection frequency” (Rule .1301(g)), with documentation that the system complies with its OP and Rule .1302(f). Manufacturers can apply for product-wide reductions under Rule .1709(d). See the North Carolina overview and free NC tools.

Sources

• 15A NCAC 18E (Subchapter E rules, amended eff. June 1, 2026): http://reports.oah.state.nc.us/ncac/title%2015a%20-%20environmental%20quality/chapter%2018%20-%20environmental%20health/subchapter%20e/subchapter%20e%20rules.pdf
• NC DHHS On-Site Water Protection Branch: https://ehs.dph.ncdhhs.gov/oswp/

Last verified: 2026-10-08. Summarizes public rules as of Oct 8, 2026; not legal advice; verify with your local health department.

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