The Bismarck WWTF Facility Planning had most recently been completed in 2002. AE2S was hired to provide an update to the planning effort prior to implementation of the trickling filter rehabilitation design to confirm that the plan was still “on the right track.”
The Basis of Design Update included updated population projections, updated flows and loads, updated regulatory review, and future regulatory requirement projections. The approach for the future regulatory projections included using the USEPA’s 2013 ammonia criteria guidance that had not yet been incorporated into the State water quality standards, as well as projecting future nutrient (nitrogen and phosphorus) limits based on recently adopted standards from neighboring states. The future regulatory projections proved to be very valuable in confirming that the City could move forward with the improvements project, while also leaving the flexibility to meet regulatory compliance for the future. A thorough review of the existing WWTF performance in conjunction with future projections clearly indicated that Bismarck was fast approaching the capacity of the trickling filters and would need to add additional capacity, which the previous facility planning efforts had not addressed. Another major revelation was that they were “recycle” limited in the trickling filters and could not operate at wetting rates that would provide optimized treatment potential in the trickling filter system.
The design of the treatment improvements was phased. The first project consisted of odor control system (biofilter) rehabilitation for the WWTF headworks due to a failure in the media underdrain system. The second phase included construction of a new trickling pump station to provide better control of the influent and optimize the wetting rate of the trickling filter system. The second phase includes a new 21 MGD Screw Pump (54-inch open screws) lift station for primary effluent; a 40 MGD Vertical Turbine solids handling pump station for trickling filter effluent and trickling filter recycle; a new third 136-foot diameter trickling filter; and other miscellaneous improvements. The recycle and effluent pumps were configured into a split self-cleaning wet well to minimize maintenance requirements associated with solids deposition in the wet well and offer operational flexibility. All vertical turbine pumps were provided with VFDs to allow for greater energy efficiency and better flow control to the final clarifiers. The third trickling filter is now operational, and the last phase is in construction to rehabilitate the two existing trickling filters and complete the associated yard piping modifications. To ensure that all three trickling filters have the same equipment (rotary distributors and media), a preprocurement with direct purchase by the City was used for the purchase of equipment for all three trickling filters with a staggered schedule of delivery.