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ATTACHMENT 4 To 05-06-04 Draft MWMC Minutes removed; were overly optimistic. This has resulted in insufficient capacity to manage peak flows at the treatment plant and has increased the risk of sanitary sewer overflows (SSOs) at a nUmber of locations in the collection system. Example problems include basement and street flooding and discharges to stormwater facilities and receiving waters. Although the magnitude' of wet weather flows differs greatly, they are significantly diluted because the source of the majority of the flow is rainwater, not sanitary sewage. Treatment plant flow · data indicates that wet weather flow is diluted such that the concentration of typical Pollutants in wet weather flow is 50 percent to 60 percent of that in dry weather flow (Executive Summary, page 1465). The Master Plan also talks of the lack of peak flow capacity. Peak flows have approached or exceeded the'hydraulic design capacity of the plan in seven instances, but no NPDES permit violations have occurred - mass limits have been suspended in those instances. Flows greater than the peak design capacity have been pumped by relying on redundant, spare pumps. The frequency · of peak flow exceedances will increase as the base, average wastewater flow increases. This could potentially lead to NPDES permit violations caused by sanitary sewer overflows or exceedance of effluent quality permit limits. All of the documents agree that the peak flow design capacity is 175 mgd. Everyone also agrees that under our permit we are required to have enough capacity during the wet weather months to treat the wastewater flow that would occur during a storm event described as "the one-in-five-year, 24-hour duration storm" and that we do not have sufficient capacity to do so. Effluent Biosolid Loads.: BOD (now often seen as CBOD) and TSS can be measured in either lbs/day or dry tons. In the reports, you will see both used. Sometimes you may wish to compare data from different reports that are expressed in different measurements (lbs/day or tons/year). To convert lbs / day to dry tons ! year, multiply the lbs / day by 0.1825. To convert dry tons per year to lbs / day, multiply by 5.4795. Under our permit, we are required to remove at least 85% of the monthly average for BOD and TSS. The existing capacity for BOD is 66,000 lbs per day and for TSS is 71,600 lbs per day (Master Plan, page 455; System Development Charge Methodology, page C-l). The Master Plan identifies the aVerage dry weather BOD load as 28,682 lbs/day and that of TSS as 31,056 lbs/day (Table 3-3, page 455).[Yhese were average dry season values, not the 30-day maximum that the plant experiences in the dry season and which permit compliance is based on] It cst~atcs that there is a remaining life BOD average month capacity in thc facility of 40 years and maximum month remaining capacity of 33 years. TSS has a remaining average month capacity of 35 years and a maximum month capacity of 29 years (Table 3-5, page 457).[These values are based on prior NPDES permits. For purposes of the Attachment 4 Page 12 of 17 4-15