5 Colo. Code Regs. § 1002-62.10
This Statement of Basis, Specific Statutory Authority and Purpose is adopted in compliance with Section 24-4-103: 25-8-202(1)(c) and (2); and 25-8-205, C.R.S.
The amendments allow substitution of the CBOD5 test for the BOD5 test contained in Section 10.1.3 of the regulation. The Federal requirements for secondary treatment have been revised to permit the substitution. The BOD5 pollutant parameter and its associated testing procedure have been widely used to determine the operating efficiency and effluent quality of wastewater treatment facilities. The oxygen demand for the BOD5 parameter is primarily exerted through the bacterial oxidation of carbonaceous organic matter which results in a carbonaceous biochemical oxygen demand or CBOD. However, an additional nitrogenous oxygen demand (NOD) may also be exerted through the action of nitrifying bacteria that oxidize ammonia nitrogen to nitrate in the two stage process known as nitrification. Secondary treatment requirements are based on controlling the oxygen demand due to the carbonaceous component of the organic material in the effluent because secondary treatment facilities can effectively remove carbonaceous organic material but may not consistently remove nitrogenous materials.
Problems have arisen with the use of the BOD5 test in many municipal secondary facilities. When sufficient numbers of nitrifying bacteria are present in the test sample they can exert a significant NOD in the BOD5 test that would not be exerted in the absence of nitrifying bacteria. Since many of the factors conductive to improving secondary bacteria, the BOD5 test can erroneously indicate poorer effluent quality when, in fact, effluent quality and plan performance have actually improved. As a result of this, some facilities must be operated in a mode that inhibits the growth of nitrifying bacteria to improve BOD5 test results and show compliance with secondary treatment requirements. These procedures usually result in poorer effluent quality, although BOD5 test results indicate the opposite and may result in greater sludge production and higher operation and maintenance costs.
A nitrification inhibited BOD5 test or CBOD5 test allows the oxygen demand due to decomposition of carbonaceous organic matter to expressed but eliminates any extraneous measurement of NOD.
A statistical analysis of effluent samples from well operated treatment plants indicated a CBOD5 of 25 mg/l which corresponds to a BOD5 of 30 mg/l. Therefore, the use of the CBOD5 test and the corresponding 25 mg/l limit based on a 30-day BOD5 average is not expected to allow an additional oxygen demand on receiving waters and no negative water quality impacts are anticipated.
FISCAL IMPACT STATEMENT
Since the substitution of CBOD5 for BOD5 will give a more accurate indication of true effluent quality in many instances, a positive indication of true effluent quality in many instances, a positive economic impact may be gained by municipal dischargers. As BOD5 may falsely indicate poorer-than-actual quality, thereby forcing more intensive treatment and/or the potential for fines as a result of permit violations, this substitute methodology may result in lessened treatment expenses and/or decreased risk of permit violation.
As these regulations result in no potential impact upon beneficial uses and as costs may be saved, the Water Quality Control Commission finds this final agency action to be economically reasonable.
5 CCR 1002-62.10