Conference Agenda

The Sand Island WWTP is the largest treatment plant in Hawaii, serving Honolulu and surrounding areas and treating an average flow of 65 MGD. The facility currently provides preliminary treatment, chemically enhanced primary treatment, and UV disinfection prior to discharge through a 2.4-mile outfall. The WWTP is located on a highly constrained site adjacent to public recreation facilities, industrial facilities, and the primary access road from the island of Oahu to Sand Island located in Honolulu Harbor.

The City and County of Honolulu entered into a Consent Decree to upgrade the Sand Island facility to secondary treatment standards, and recently began construction of a 20 million gallon per day (MGD) membrane bioreactor (MBR) facility as Phase 1 of the secondary expansion. Phase 2 of expansion will provide the additional 90 MGD of secondary treatment capacity required to meet full secondary standards and support future growth. Phase 2 will also add peak flow equalization, upgrade preliminary and primary treatment, and expand solids treatment processes to treat the additional waste activated solids generated by the new secondary process.

Previous planning evaluated a wide range of secondary treatment technologies for the Phase 2 expansion, however since the site was not intended to support full secondary treatment, only intensified processes including MBR, biological aerated filters (BAF), and aerobic granular sludge (AGS) are able to provide the full secondary treatment capacity required within the footprint available. The 90-MGD Phase 2 expansion would represent one of the largest facilities in North America using any of these technologies, therefore the City conducted extensive investigation including detailed process evaluation and site visits to nine large facilities in the United States and Europe. A large group of City stakeholders evaluated the secondary process alternatives based on qualitative and quantitative criteria, relying on technical information from the design team and operational experience and performance shared by utility peers during site visits.

This presentation will describe the evaluation conducted to assess the large-scale intensified treatment processes considered for the Phase 2 expansion, review the City’s selected process, and describe how the technology is being implemented.

A major barrier to quantifying the true cost of grit is these costs are typically accepted as routine maintenance assigned to different parts of the plant so the root cause may not be identified as the common denominator. If grit is not managed by removal in the headworks it will continue onto downstream processes and must be managed throughout the plant. Grit deposits in downstream process tanks, taking up space, reducing retention time and increasing velocity which impacts treatment efficiency. Grit deposition in the aeration basin can increase energy requirements needed to achieve proper treatment. Accumulation in digesters can create unstable operating conditions by reducing volatile solids destruction, impairing mixing, and reducing gas production. To remove deposited grit basins must cleaned. Digester cleaning is expensive, dangerous and time-consuming, requiring taking the basin offline, exposure to toxic gasses, confined space entry, repairs and recommissioning, not to mention removing, handling and disposing of the deposited material. Grit is also responsible for abrasive wear to virtually all mechanical equipment.

All plants are interested in improving efficiency, reducing O&M costs, improving plant performance and reducing energy requirements or moving towards energy neutrality. If plant staff could monetize the cost of bypassing grit process improvements could be justified.

The purpose of this paper is to provide tools to understand effective design points for grit removal systems and assess the true cost of grit while shedding light on why the grit removal process is important.

Water and Wastewater Membranes need to be protected against fibrous & sharp debris. This not only protects all downstream systems & monitoring equipment, including water & wastewater membranes from premature surface abrasion & clogging, but site studies have shown that 1.0 mm & 0.5 mm aperture membrane pre-screening significantly reduces the annual number of membrane desludging/cleaning cycles by up to 3-fold. It also reduces the labor intensity of each cleaning cycle while eliminating clogging of aeration manifolds that scour membrane surfaces. Fine Pre-screening has become extremely important in protecting both membrane warrantees and lifespans. Laboratory & field experiments prove the vast majority of debris is caused by chopping & maceration equipment installed at pump stations or inside plants to protect downstream equipment from disposable wipes & plastics flushed into our sewer systems. The presentation will emphasize the importance of fine screening with data showing how short, fine cotton-wool-polyester fibers, human hair, as well as filamentous algae will pass through 6.0 mm, 3.0 mm and 2.0 mm aperture screens and “Recombine” downstream into larger rag type debris that increases the operating maintenance & cleaning cycles of all downstream systems and monitoring equipment. The Presentation will also include supporting pilot plant & field data that focuses on why velocity & headloss are the two most important operational characteristics in understanding how screens are properly sized, maintained & operated. Computational Fluid Dynamics Analysis (CFD) with (SCR) Screening Capture Ratio data will reveal the direct relationship velocity & headloss have on a screen’s capture efficiency & performance. It will also emphasize the aperture requirement for removing 2 dimensional versus 3 dimensional solids. A screen’s effluent quality affects the lifespan, operation & maintenance of all downstream systems & monitoring equipment. Headworks and membrane protection screens share the same velocity and headloss limitations that affect their performance and debris capture. Both lab and site test data will demonstrate and support the shared conclusions of this presentation. Whether the drivers are regulatory, reuse or to improve downstream process efficiencies, the trend has been to remove non-biodegradable debris, algae & aquatic remains from influent flow.