REPORTING OF CONCENTRATION
The example Daily Operations Log shows that four individual BOD5 grab samples were obtained during the month as follows:
March 3 – 22.00 mg/L
March 10 – 29.00 mg/L
March 17 – 18.00 mg/L
March 24 – 10.00 mg/L
The daily average concentration is calculated by adding the four values obtained and dividing by the number of samples taken during the month. The calculated BOD5 daily average is 19.75 mg/L.
The highest BOD5 concentration was obtained on March 10. This value is reported as the maximum BOD5 individual grab for the reporting period.
REPORTING OF LOADINGS
Some parameters in the permit are limited in terms of pounds per day (lbs/day). Although all of these parameters are measured initially in milligrams per liter (mg/L), conversion to lbs/day can be achieved by using the following formula. Always be sure to use the flow measurement determined on the day when sampling was done.
March 3 – (.19 MGD) (22.00 mg/L) (8.34 lbs/gal) = 34.86 lbs/day
March 10 – (.20 MGD) (29.00 mg/L) (8.34 lbs/gal) = 48.37 lbs/day
March 17 – (.18 MGD) (18.00 mg/L) (8.34 lbs/gal) = 27.02 lbs/day
March 24 – (.19 MGD) (10.00 mg/L) (8.34 lbs/gal) = 15.85 lbs/day
Using the four BOD5 concentrations and the flow measurements obtained on the days of sampling, the individual daily loadings are calculated. The daily average loading (lbs/day) is calculated by adding the individual daily loading values together and dividing by the number of samples taken during the month. The calculated BOD5 daily average loading is 31.53 lbs/day.
FLOW-WEIGHTED AVERAGES
When four samples are not available in a calendar month, the daily average concentration should be calculated using the four most recent measurements or the arithmetic average (weighted by flow) of all values taken during the month. The example Daily Operations Log shows that TSS grab samples were taken only three times during the month. The values obtained and the flows on the days of sampling are as shown:
TSS (mg/L)
March 3 30.00, TSS 0.19 MGD
March 10 23.00 TSS 0.20 MGD
March 17 16.00 TSS 0.18 MGD
Total flow on sample days 0.57 MGD
To calculate the flow-weighted concentration for each sample, the following equation must be used. The flow-weighted average concentration is then determined by adding the flow-weighted concentrations for the individual samples together.
(Flow on Day of Sampling, MGD) / (Total Flow on Days of Samling, MGD) = Flow-weighted conc.
Sum of Flow-weighted Concentrations = Flow-weighted Daily Average Concentration
March 3 30.00 mg/L x .19/.57 = 10.00
March 10 23.00 mg/L x .20/.57 = 8.07
March 17 16.00 mg/L x .18/.57 = 5.05
Flow-weighted Daily Average TSS Concentration = 23.12 mg/L
To calculate the daily average flow-weighted loading, the equation illustrated in Figure A4.6 must be used:
Daily avg flow-weighted conc X Avg of flows on sampling days X 8.34 lbs/gal
pH
A review of the example Daily Operations Log indicates four pH values were obtained during the reporting period ranging from a minimum value of 6.80 s.u. on March 24 to a maximum value of 7.50 s.u. on March 3. The highest pH value obtained on March 3 is reported as the maximum pH for the reporting period. The lowest pH value obtained on March 24 is reported as the minimum pH for the reporting period. (Note that pH is not subject to averaging.)
CL2 RESIDUAL
A review of the example Daily Operations Log shows Cl2 residual values ranging from a minimum value of 0.00 mg/ L on March 16 to a maximum value of 2.20 mg/L on March 30 were obtained during the reporting period. The highest Cl2 value obtained on March 30 is reported as the maximum Cl2 residual for the reporting period. The lowest Cl2 value obtained on March 16 is reported as the minimum CL2 residual for the reporting period. (Note that chlorine residual is not subject to averaging.)
FLOW
Daily Average Flow and Daily Maximum Flow
The average daily flow is calculated by adding the individual daily flow measurements together and dividing by the number of days on which flow measurements were taken during the month. The average daily flow calculated from the example Daily Operations Log is 0.183 MGD. The daily maximum flow is the highest daily flow value obtained during the reporting period. On the example Daily Operations Log, the daily maximum flow value is 0.34 MGD , which occurred on March 11.
Annual Average Flow
The annual average flow is the arithmetic average of all daily flow determinations taken during the previous 12-month period. It is calculated by adding the individual daily flow measurements together and dividing by the number of measurements taken during the previous 365 days. For example, if the total flow recorded during a 12-month period is 600 MG and during that period of time flow measurements were obtained once per day, the annual average would be calculated: (600 MG) / (365 days) = 1.64 MGD annual average flow