The opinion of the court was delivered by: Justice Rathje
The petitioner, the Village of Fox River Grove (Village), appeals from a decision of the Illinois Pollution Control Board (IPCB) refusing to eliminate certain restrictions attached to the National Pollution Discharge Elimination System (NPDES) permit issued to the Village by the Illinois Environmental Protection Agency (IEPA) for the wastewater treatment facility (the facility) owned and operated by the Village. We affirm the order of the IPCB.
On appeal, the Village has identified seven separate issues for our consideration. For clarity purposes, we restate the issues raised as follows: (1) whether the standard of review applicable in this case is de novo; (2) whether the IPCB erred in utilizing the facility's hydraulic flow rating to arrive at population equivalents for purposes of determining whether stricter effluent limits under IPCB regulations applied; (3) whether the IPCB erred in refusing to consider the prior interpretations of the applicable regulations by the IEPA; and (4) whether the imposition of the stricter effluent limits will inhibit the ability of the facility to treat sewage within the facility's designated service area.
In order to understand fully the issues raised on this appeal, it is necessary to review the NPDES permit history of the Village's treatment facility, as well as the history of the facility itself.
The original treatment facility was built in 1926; it was last upgraded in 1978. The facility is located in a residential area. Construction of a new facility on the site is not possible since under current regulations the entire site is in a flood plain. The sewer system, which is a tributary to the facility and which was constructed in 1926-27, allows for more than the normal amount of infiltration of storm water and groundwater flow. When the facility was redesigned in 1977, it was determined that it would be easier to treat the infiltration rather than to try to remove it from the facility. The wastewater flow rates for the facility were based on a combination of sewage and infiltration. Thus, while the facility was designed for and is rated as having a hydraulic or design flow capacity of 1.25 million gallons per day, the facility was designed to treat only 1 million gallons of sewage per day.
Before turning to the history of facility's NPDES permits, it is necessary to understand the term "population equivalent" as defined in the regulations of the IPCB. "Population equivalent" (P.E.) is defined as follows:
"a term used to evaluate the impact of industrial or other waste on a treatment works or stream. One population equivalent is 100 gallons (380 liters) of sewage per day, containing 0.17 pounds (77 grams) of BOD (5) (five day biochemical oxygen demand) and 0.20 pounds (91 grams) of suspended solids. The impact on a treatment works is evaluated as the equivalent of the highest of the three parameters. Impact on a stream is the higher of the BOD (5) and suspended solids parameters." 35 Ill. Adm. Code §301.345 (eff. date _________). Based upon the 1976 planning report done for the redesign of the facility, the population equivalent was shown to be a residential population at 8,500 equivalents and Good Shepard Hospital at 1,500 equivalents for a total of 10,000 P.E.
An NPDES permit is a permit that allows water from the facility to be discharged into the Fox River. The NPDES permit contains limits on the amount of deoxygenating or organic waste that can be contained in the water that flows from the facility into the Fox River. These limits are expressed in terms of suspended solids and either five-day carbonaceous biochemical oxygen demand (CBOD5) or five-day biochemical oxygen demand (BOD5). For example, 30 milligrams per liter (mg/l) of BOD5 is the equivalent of 25 mg/l of CBOD5.
The first NPDES permit for the facility was issued in 1977 by the United States Environmental Protection Agency (USEPA) and provided for effluent limitations of 30 mg/l for BOD5 and 30 mg/l for suspended solids. In 1986, the Village received a proposed NPDES permit for the facility that lowered the effluent concentrations from 30 mg/l of BOD5 to 20 mg/l and from 30 mg/l of suspended solids to 25 mg/l. In a letter dated November 3, 1986, to the IEPA, which administered the issuance of the permits for the USEPA, the Village stated its position as follows:
"We believe your changing of our monthly average effluent limits for BOD and SS (suspended solids) from 30/30 mg/l to 20/25 mg/l is an incorrect interpretation of the Illinois Pollution Control Board Rules and Regulations Subtitle C, Chapter I, Section 304.120b. This section requires that no effluent from any source whose untreated waste load is 10,000 population equivalents or more (emphasis in original), shall exceed 20 mg/l of BOD5 and 25 mg/l of suspended solids. Our plant design capacity is for untreated wastes [sic] loads up to 10,000 population equivalents. Our present untreated waste load is less than 10,000 population equivalent. Therefore, we believe we correctly come under Section 304.120a which requires an effluent limit of 30/30 mg/l of BOD/SS."
In a letter dated December 5, 1986, the IEPA responded as follows:
"The basic premise of Section 304.120 of the Illinois Environmental Protection Act, Subtitle C, Chapter I is to establish effluent Standards based on the design average flow of the facility. ***
A literal interpretation of 304.120(b) would establish effluent standards that could vary from month to month, week to week, or day to day depending on the influent characteristics of the waste at any one point in time. The Agency does not agree with this interpretation."
The Village appealed the IEPA's decision to the IPCB but withdrew its appeal when a settlement was reached with the IEPA. Under the settlement, the facility was rerated from a 10,000 P.E. to a design organic equivalent of 9,900 P.E. An NPDES permit for the facility was issued providing effluent limitations of 30 mg/l for BOD5 and 30 mg/l for suspended solids. In 1992, the facility's NPDES permit was renewed and provided effluent limits of 25 mg/l CBOD5 and 30 mg/l for suspended solids. At the time of the 1992 renewal, the USEPA reviewed the facility's NPDES permit and indicated no problems with the above-stated limits.
The Village sought renewal of the facility's NPDES permit in 1996. In a November 8, 1996, letter to the Village, the IEPA issued a draft permit providing for effluent limits of 20 mg/l of CBOD5 and 25 mg/l of suspended solids. After the Village questioned the imposition of the more stringent ...