§ 18-108  EXCESSIVE STRENGTH CHARGES.
(A)   For any user, when the BOD5, suspended solids or other pollutant concentration exceeds the range of concentrations of these pollutants in normal domestic wastewater, a surcharge shall be added to the basic charge for, volume as determined under this article.  This excessive strength charge shall be calculated in accordance with the following formula which is based on the Federal Water Pollution Control Act of 1972, as amended, 40 CFR Part 35.935-13 and 40 CFR Subpart E, Appendix B, Model #2:
Cs = [Bc(B)) = Sc(S) + Pc(P] Vu
(B)   (1)   Cs equals a surcharge for wastewaters of excessive strength.
(2)   Bc equals O&M costs for treatment of a unit of BOD5.
(3)   B equals concentration of BOD5 from a user in excess of normal domestic wastewater concentration.
(4)   Sc equals O&M cost for treatment of a unit of TSS.
(5)   S equals concentration of any TSS from a user in excess of normal wastewater concentration.
(6)   Pc equals O&M cost for treatment of a unit of any pollutant.
(7)   P equals concentration of any pollutant from a user in excess of the base level of that pollutant in normal domestic wastewater.
(8)   Vu equals volume contribution from a user per unit of time.
(C)   Example:  It is determined that a system user has excessive strength wastewater, as defined in this article, for BOD5 and suspended solids.  Laboratory test results indicate wastewater concentrations of 600 mg/l for BOD5 and 800 mg/l for suspended solids.  The user's flow volume is 2,000 gallons per day or a total of 40,000 gallons during the subject month is (.04 mg).
(D)   (1)   Based on city records of current wastewater treatment costs \$140,000/year it is determined that 30% of the cost can be allocated to the cost of treating BOD5 and 40% can be allocated to the cost of treating suspended solids.
(2)   Total annual loading for the system is determined to be 583,804 pounds each for BOD5 and suspended solids.  Thus, the cost to treat a pound of BOD5 is determined to be \$.07, while the cost to treat a pound of suspended solids is \$.10.
(E)   Therefore, in this example, and using the excessive strength formula presented above, the system user would be required to pay a surcharge for 350 mg/l excess BOD5 (600 mg/l - 250 mg/l) and for 550 mg/l excess TSS (800 mg/l - 250 mg/l).  Prior to determining the surcharge these concentrations must be converted to pounds using the factor 8.34.  Thus 350 mg/l of BOD5 equals 2,929 pounds (350 mg/l x 8.34 lbs./million gallons) and 550 mg/l of TSS equals 4,857 pounds (550 mg/l x 8.34 lbs/million gallons).  Note: 1 mg/l - 1 ppm.  The amount of the surcharge can now be determined for the month in question by using the formula presented in this section.
Cs = [Bc(B) + Sc(S) + Pc(P)] Vu
= (0.07 (2,919) + 0.10(4,587)) .04
= (204.33 + 458.7) .04
= (663.03) .04
= \$26.52
(`90 Code, § 18-50)