Water and Pollution Control Department

April 1999

Purpose

Background

The current surcharge system has been in the city sewer use ordinances since 1985.  The new Water Pollution Control facility began operation in 1989 at which time the current surcharge system went into effect, and the City began billing and collecting surcharges for high-strength wastewater.

Normal domestic wastewater is defined in the City Code, Section 28.302.  The parameters and concentrations used for surcharge program implementation are shown on the table below.

CBOD5 and COD are both measures of oxygen demand.  For any sample, the COD includes the CBOD5.  Therefore, the COD value will be greater than or equal to the CBOD5 value.  The COD analysis is simpler and less time-consuming to perform than the CBOD5 analysis and can be completed within an hour.  The CBOD5 analysis requires a five-day incubation period before the results can be determined.  Because of the above, some contributors may prefer to test the COD only.  In those cases, surcharges will be calculated on COD. 

Based on WPC facility data, the COD is generally 2.5 times the CBOD5 in typical domestic wastewaters.  There are often parameters in non-domestic wastewaters that inhibit the microorganisms used in the CBOD5 test causing a low CBOD5 result.  Examples of these parameters include acetone, alcohols, citric acid, etc.  In the WPC facility, however, the parameters are dilute enough to be able to be treated by the facility microorganisms.  In these cases the COD is a better representation of the oxygen demand for the parameters than CBOD5.  Therefore, if the COD result is more than three times the CBOD5 result, COD will be used to calculate the surcharge.

NH3 and TKN are both measures of the nitrogen content of the wastewater.   For any sample, the TKN includes the NH3-N.  Therefore, the TKN concentration will be greater than or equal to the NH3-N concentration. When proteins break down in the treatment process, they are converted to ammonia; however, at the industrial facility’s discharge point to the city sanitary collection system, the breakdown of proteins has often not occurred yet.  Therefore, for waste streams that have a high protein content (i.e. blood, egg wastes, meat offals, etc.), TKN would better represent the nitrogen concentration that would be treated at the WPC facility.  TKN concentration will be used to assess surcharge except in cases where NH3-N is determined to be a more appropriate method.

Current treatment costs at the WPC facility, including the costs for operation, maintenance, and replacement, are included in the table below.  These treatment costs will be used to determine the surcharges to be paid by contributors discharging wastes higher in strength than domestic wastewater.  The costs for wastewater treatment at the WPC facility will be re-evaluated at least every three years so that the surcharge billings accurately reflect the true costs associated with treating the high-strength wastewater.

Domestic Wastewater Strength and Cost of Treatment

Implementation

The sample sets used for surcharge calculations can also be used to help fulfill the monitoring requirements of the industrial pretreatment program.  Sampling duplications will not be required where a sample can be used for more than one purpose.  The City will work with each contributor to minimize monitoring expenses.  However, either the City or the contributor can request additional sampling if either feels that the data currently being used to assess the surcharge are not representative of wastewater discharge quality.  If the contributor modifies its operations, such as through a plant expansion or elimination of a product line, either the City or the contributor can request a new sampling set to determine a new database on which to base future surcharge payments.  The 1997 average cost for performing three days of surcharge sampling was approximately $350.  All costs associated with the surcharge sampling and/or evaluation will be assessed to the contributor.

For the purposes of the following section, a surcharge sample is defined as a composite sample (either time-based or flow-proportional) collected from a facility that is representative of a 24-hour period during a typical work week.  Samples shall be collected during typical work weeks so that the samples will be representative of the average wastewater flow from the facility.  Samples collected during weekends or shutdown periods will not be accepted.  Samples must be collected in accordance with 40 CFR 136 laboratory practices and shall be analyzed for pH and other domestic wastewater parameters.

If an individual sample (pretreatment or surcharge) indicates that the contributed wastewater has a higher than maximum normal “domestic” strength, one of the following processes (A, B, C, or D) will be followed.

Process A:

The surcharge rate calculated using Process A will be assessed monthly to the facility beginning in the billing cycle following the last surcharge sample.  The surcharge will be evaluated at least once in 12 months but no more frequently than once in six months.

Process B:

The surcharge rate determined by Process B will be billed monthly to the facility beginning with the billing cycle following the last surcharge sample.  The surcharge will be used for 12 months or until results from a new three-day surcharge sample event are submitted or until adequate data are available to use Process A.

Process C:

The calculated surcharge will be billed during the billing cycle it is calculated.  The surcharge rate determined by Process C will only be valid for the month it was calculated with and must be evaluated monthly.

Process D:

Monthly surcharge rates calculated using Process D will be billed monthly (based on metered or estimated flow) to the facility beginning the billing cycle following the evaluation.  The surcharge will be evaluated at least once in 12 months but no more frequently than once in six months.