Project Background

Since 1961, the City of Williston has treated Missouri River water at a facility located approximately five miles southwest of the City adjacent to the river.  With a design capacity of 6 million gallons per day (MGD), the Williston Water Treatment Plant (WTP) treats water through the processes of presedimentation, lime softening, recarbonation, gravity filtration, corrosion control, fluoridation, and chlorine disinfection.  The City of Williston has experienced intermittent population growth since the facility was constructed.  In addition to producing potable water for the City of Williston, the Williston WTP serves as the regional WTP for two rural water systems.  The Williston WTP is currently faced with several challenges including aging infrastructure, insufficient capacity, and the lack of redundant raw water transmission, treatment processes, chemical feed systems, and finished water transmission.  Additionally, extensive regulatory requirements for drinking water, stemming from the Safe Drinking Water Act (SDWA), pose significant challenges for the treatment systems.

Project Description 

Advanced Engineering and Environmental Services, Inc. (AE2S) was retained by the City of Williston to complete a Master Plan of the water treatment system in 1997 and prepared a prioritized plan of capital improvements through a 23-year (2020) planning period.  In order to establish the prioritization, the plan took into consideration the strength of the justification for each project, hydraulic and process optimization opportunities, anticipated deadlines to meet capacity and regulatory milestones, and the effects of individual improvements on other improvements.

Under Phase 1 of the project, major improvements consisted of filter rehabilitation, transfer pump improvements, instrumentation and control upgrades, electrical upgrades, and intake improvements.  The filter rehabilitation consisted of separating the three two-bay filters into six individual filters and included replacement of the filter underdrains, filter media, backwash troughs, piping, valves, filter controls, vertical diffusion vane pumps, compressed air system, and related appurtenances.  The transfer pump motors and reservoir fill pump were replaced and new surge relief valve, isolation valves, magnetic flowmeters, and pump control valves were installed.  The existing control system was replaced by a new programmable logic controller (PLC) based control system and a computer based supervisory control and data acquisition system (SCADA), all programmed by AE2S control system integration engineers.  A new electrical room was constructed and new switchgear, distribution panels, motor control centers, and electrical power and control wiring were installed.  At the intake the traveling water screen and three intake pumps, valves, piping, and appurtenances were replaced.

Under Phase 2, the treatment train will be changed to include pretreatment, lime softening, recarbonation, and gravity filtration as well as corrosion control, fluoridation, a combination of free chlorine and ultraviolet (UV) light primary disinfection, and chloramine secondary disinfection.  Powdered activated carbon will be added to the raw water line in the intake building to address taste and odor problems.  The improvements will also include the decommissioning of the existing grit basin and presedimentation basin and installation of two 10 MGD Actiflo® ballasted flocculation treatment units for pretreatment.  Actiflo is a microsand enhanced rapid mix, flocculation, and sedimentation process that has been shown to work effectively at high-rise rates, resulting in smaller footprint areas.  The process combines flash mixing, coagulation, maturation, and settling in a single process unit.  Each Actiflo® process unit is configured as a separate treatment train containing four primary basins referred to as the coagulation tank, injection tank, maturation tank, and lamella tube settling tank.  Water flows successively from one tank to the next within each unit.  A metal salt coagulant (alum) and a cationic polymer will be added during the process to increase treatment performance.

After pretreatment, the water will flow to one of two lime softening basins.  Phase 2 will include the removal of an existing Infilco Degremont Inc. (IDI) “Hybrid Type NS” Accelator and the replacement with new softening equipment as well as construction of a second softening basin in the new addition.  The two softening basins will permit the City of Williston to use two flow trains and provide more operational flexibility.  Both softening basins treat 4,200 gallons per minute (gpm) and measure 53 feet square in plan by approximately 21.5 feet deep.  Each softening basin will include stainless steel components, a sludge scraper, automatic sludge blowdown, and a complete peripheral tube settler package to increase the net effective surface area and minimize floc carryover.  The chemicals added to the lime softening process include slaked pebble lime, sodium aluminate, and a polymer.  The lime system improvements required under Phase 2 include two new 1,000 pound per hour U.S. Filter lime slakers with gravimetric feeders, grit removal, and replacement of the existing screw conveyance and bucket elevators.  Lime slaker slurries will be transferred to the softening basins via stainless steel troughs and discharged into the primary mixing zones of each basin.  Concentrated sludge from the softeners will be blown off to a viewing pit and pumped to on-site sludge lagoons for dewatering and drying.  The grit from the lime slakers will be removed by grit conveyors and discharged into refuse containers for disposal.  Recarbonation will be implemented after lime softening to convert the carbonate alkalinity to bicarbonate alkalinity and stabilize the water prior to filtration.  The new recarbonation equipment will consist of a pressurized solution feed system manufactured by TOMCO2 Equipment Company.  The recarbonation basins will be compartmentalized with overflow and underflow baffles, and new carbon dioxide diffusers will be installed in each recarbonation basin.  In addition to carbon dioxide, chlorine and polyphosphate will be added to the recarbonation basins.

A final component of the project includes construction of a redundant 30 inch ductile iron transmission line from the WTP to the existing 4 million gallon ground storage reservoir.  The length of the transmission line is approximately 5 miles and includes isolation, blow-off, and air release valves, cathodic protection at gas main crossings, and highway crossings.