Catch and Release Strategies: Using Available in-Plant Storage for Wastewater Peak Flow Attenuation

摘要

The City of Los Angeles’ Bureau of Sanitation provides wastewater collection, treatment, and disposal services for approximately 4 million residents within a 600-square-mile service area, including 29 contract agencies outside the City. The City’s more than 6,700 miles of public sewers that can convey about 500 million gallons per day (MGD) of flow to the City’s four wastewater treatment and water reclamation plants. The wastewater collection system is made of two separated service areas. The larger Hyperion System serves approximately 95 percent (95%) of the population. In the Hyperion System, trunk sewers convey wastewater to two upstream skimming treatment plants and subsequently to the larger Hyperion Wastewater Treatment Plant (HTP). One of such upstream treatment plants is the Donald C. Tillman Water Reclamation Plant (DCTWRP). DCTWRP began continuous operation in 1985. DCTWRP is a skimming plant. Its facilities were designed to treat 80 million gallons (MG) dry weather flow (ADWF) and serve a 225 square mile area. The treatment process is made of two trains designed to handle 40 MGD each, Phase Ⅰ and Ⅱ. The DCTWRP is the leading producer of reclaimed water in the San Fernando Valley. The plant also provides critical hydraulic relief to the City’s major sewers downstream, which badly need the additional capacity to serve other portions of the city downstream in the Hyperion System. The returned flow from DCTWRP gets conveyed to a 96-inch pipe. This 96-inch diameter pipe is forced into a 78-inch diameter pipe. Approximately 1.5 miles further downstream the trunk line is further reduced to a 42-inch diameter pipe. Flows in excess of 42 MGD are transferred at the diversion structure into an overflow trunk line. This trunk line has numerous capacity constraints as well as lateral and local sewer connections. This relief trunk line is prone to surcharges during wet weather and peak wet weather wastewater flow conditions. In years past, DCTWRP was able to handle 160 MGD peak wet weather flow (PWWF), for BOD removal. The increased treatment capacity provided a critical hydraulic relief to the City’s major sewers downstream by skimming additional 62 MGD to 80 MGD. In 2007, DCTWRP was converted to achieve full biological nitrogen removal (BNR) with 80 MGD ADWF, for biological oxygen demand (BOD) and full BNR. The additional treatment capacity was not longer available. Using a sophisticated hydrodynamic model of the City’s wastewater collection system, City staff figured out that storm events with frequency return of 10 year design storm will produce wet weather pass-through flows that will exceed 108 cfs (70 mgd). At this threshold rate, pressurization will occur along all secondary sewer lines and private laterals connected to the wastewater collection system relief trunk line. Modelling also indicated that the excess volume associated with a 24-hour 10-year return event was 22.5 million gallons. This excess volume was likely to exit the wastewater collection system in a form of a sanitary sewer overflow (SSO). Subsequently, further modelling showed that running only Phase Ⅰ and utilizing Phase Ⅱ as an in-plant storage (estimated capacity of 20 mg) provided the peak flow attenuation required to prevent SSOs. Staff recommendation was to shutdown Phase Ⅱ of DCTWRP and utilizes its structures for an in-plant storage during the wetweather season. There was going to be no treatment taking place in Phase Ⅱ during this time period. As a result, DCTWRP operates only Phase Ⅰ to produce only 40 MGD Title 22 recycled water for beneficial use. On March 20, 2011, the City experienced a severe and intense rainstorm. The peak influent into the Hyperion Treatment Plant (HTP) during the storm was 584 MGD, compared to typical peak dry weather flow of 290 MGD. The in-plant storage basins in Phase Ⅱ were activated. Flow rates upstream DCTWRP reached 70 MGD, which is the trigger flow at which the wet wea