Design Accuracy and Performance Outcome - Case Studies of two Large Wastewater UV Systems Started in 2013

In the Pacific Northwest, Carollo Engineers has recently been working with several utilities to replace their existing (and outdated) UV reactors with new equipment. This technical paper focuses on the work at Pierce County WA and Clean Water Services (OR). At Pierce County, they are in the process of upgrading various components of the Chambers Creek Wastewater Treatment Plant (WWTP). Replacement of the UV disinfection system was a priority project that addressed capacity and operational issues associated with the existing UV system, which had a capacity of 54 mgd. The new system has a firm capacity of 57 mgd with provisions for future expansion to 80 mgd. The County replaced the existing UV system with a new low-pressure, high-output (LPHO) system, installed within a portion of an out of service chlorine contact basin (CCB), which was used for emergency storage of secondary effluent during power outages. The construction of the new UV system was completed in October 2013. In coming months, the existing UV disinfection system will be removed and the remaining basins will be re-configured for emergency storage.The other treatment plant that is discussed in this paper is the Clean Water Services Hillsboro Wastewater Treatment Facility (WWTF). The Hillsboro WWTF provides service to the western region of the City of Hillsboro, the community of North Plains, and the south eastern area of the City of Cornelius, the community of Banks, and the northwestern portion of the City of Forest Grove. Previously, the facility has two operational modes. During the dry weather season (typically May through October), screened and degritted influent is pumped to the Rock Creek Advanced Treatment Facility (AWWTF) for treatment. During the wet weather season (typically November through May), approximately 14 to 17.5 mgd of flow receives secondary treatment at the WWTF. This flow is disinfect ed using ultraviolet light (UV) prior to discharge to the Tualatin River.The planning studies(Hillsboro and Forest Grove Flow and Load Projections, Draft April 2012 and West Basin Scenario Evaluation, Draft November 2012) identified future capacity and operating plans for the Hillsboro WWTF. Planning considerations included converting the Hillsboro WWTF to year-round treatment, discharge of disinfected effluent to a natural treatment system (NTS), and increasing UV treatment capacity to approximately 20 mgd.The previous UV disinfection system at the Hillsboro WWTF was installed in 1997. The system was a medium pressure unit in a single channel rated at 14 mgd. The unit is considered to have reached the end of its useful life. The UV upgrade project replaced the existing system with a newer UV technology, increased UV process capacity, and added a second parallel system for redundancy.This paper reviews the detailed scientific-based design approach used during design of the se two UV systems and details the successful performance outcome of the UV systems.