DMR FAQs on Average Flow
NOTE: This guidance is to be used if the permit does not include site-specific requirements.
General Guidance
Calculations for 7-day, weekly, 30-day, or monthly average flows shall use only values collected during days of actual discharges divided by the days of actual discharge during a monitoring period.
Examples
- Seasonal facility opens for cleaning/startup activities on the 15th of the month and begins full operation 5 days later. Facility only discharges for 15 days out of the 30 day monitoring period. The facility's permit has a 30 day arithmetic mean flow limit, and is required to monitor flow 1/day with a sample type of "instantaneous". The facility should report its flow value as the combined flows for the 15 days divided by the 15 days of discharge.
- Same scenario as above except the facility uses a totalizer or water meter readings to monitor flow. The facility should report its flow value as the total flow for the 15 days divided by the 15 days of discharge.
- School or other type of facility that is in operation for only part of the week or monitoring period. Let's use a school for the example. School is in session for Monday through Friday, and has activities at the school on Saturday. The facility's permit has a 30-day arithmetic mean flow limit, and is required to monitor flow 1/day with a sample type of "instantaneous". The facility should report its flow value as the combined flows for the 6 days per week during the monitoring period and divide that total by the total days of actual discharge during the monitoring period.
- Same scenario as above except the school has a nine day vacation during the monitoring period. The facility should report its flow value as the combined flows only on the days it had a discharge, and divide that number by the total days of actual discharge during the monitoring period.
- Industrial facility that operates only 5 days a week and only has a discharge 5 days a week. The facility's permit has a 30-day arithmetic mean flow limit, and is required to monitor flow continuously with a sample type of "recorder". The facility should report its flow value as the combined flows for the 5 days per week during the monitoring period and divide that total by the total days of actual discharge during the monitoring period.
- Same scenario as above except the industrial facility has a lagoon system that discharges continuously. The facility would then combine flow values for all 30 days of the monitoring period and divide by 30.
