Secondary treatment check
Compare your 30-day and 7-day effluent averages and percent removal for BOD5, CBOD5 or suspended solids with the levels in 40 CFR 133.102, with the exact words of the rule.
BOD5 against 40 CFR 133.102 Our calculation
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“(1) The 30-day average shall not exceed 30 mg/l. (2) The 7-day average shall not exceed 45 mg/l. (3) The 30-day average percent removal shall not be less than 85 percent.”
How the averages are defined
“30-day average. The arithmetic mean of pollutant parameter values of samples collected in a period of 30 consecutive days.”
“7-day average. The arithmetic mean of pollutant parameter values for samples collected in a period of 7 consecutive days.”
“Percent removal. A percentage expression of the removal efficiency across a treatment plant for a given pollutant parameter, as determined from the 30-day average values of the raw wastewater influent pollutant concentrations to the facility and the 30-day average values of the effluent pollutant concentrations for a given time period.”
We work out percent removal as (influent − effluent) ÷ influent × 100 from the two 30-day averages. The rule describes percent removal in words and does not print a formula.
What these levels are
“The following paragraphs describe the minimum level of effluent quality attainable by secondary treatment in terms of the parameters—BOD5, SS and pH. All requirements for each parameter shall be achieved except as provided for in §§ 133.103 and 133.105.”
Section 133.103 allows adjustments, for example for combined sewers in wet weather:
“For such treatment works, the decision must be made on a case-by-case basis as to whether any attainable percentage removal level can be defined, and if so, what the level should be.”
A study aid, not a compliance determination. Your facility's NPDES permit sets the limits that apply to you, and it can differ from these levels (for example treatment equivalent to secondary treatment in 133.105). Your State program and your facility's permit may have their own requirements, so check them too.
A study aid, not legal or compliance advice. goraedu does not certify wastewater operators, and our questions are not a state operator certification exam. Not affiliated with or endorsed by the U.S. EPA, any state water program or any operator certification body. Spot an error? Tell us.