Significant concerns with the existing solids handling system at the City of Flagstaff Wildcat Hill Wastewater Treatment Plant (WWTP) have prompted Utilities staff to develop alternative handling options.  The alternatives address the need to dewater and remove the waste solids from the Plant.  All alternatives will require a FY14 capital project to implement a temporary (2-3 yr) solution. 

The Wildcat Hill WWTP was originally designed in the 1970s to handle 700 tons per year of digested solids. Since that time the plant has continually added solids from other sources such as the Rio De Flag Water Reclamation Facility and septic haulers from Coconino County subdivisions, while never increasing the solids handling capacity of the Plant. Currently the plant is handling 1100 tons per year of solids loading which is 400 tons per year above the original design of approximately 700 tons per year.  This is a 57% increase in solids handling due to growth in our community and the surrounding area. The upgrade to Wildcat Hill WWTP completed in 2010 did not include an upgrade to its solids handling and treatment process.

The Wildcat Hill WWTP has no mechanical dewatering of its digested sludge. Sludge is pumped from the anaerobic digesters to solids settling basins (SSBs or lagoons) that are located southeast of the WWTP for storage. At appropriate times, (seasonal, or when ponds are full) the solids from SSBs are disposed of using on-site land application in an approximately 40-acre “Dedicated Land Disposal” (DLD) area. The sludge is disked or injected into the soil within the DLD when frost or saturation conditions allow. When the DLD is shut down due to cold or wet weather, the SSBs are subject to overloading due to the inability to handle solids.  During this time, the digested solids pumped to the SSBs are returned again to the treatment plant..

The Utilities Division contracted with Carollo Engineers to analyze, prepare preliminary cost estimates and provide recommendations for designing and installing a temporary solids dewatering system on-site. Currently the WWTP sends approximately 70,000 gallons per day (gpd) of digested sludge to the SSBs, which are experiencing operational issues.  The SSBs and DLD (during rainfall/snowmelt events) are not functioning properly by adequately dropping the solids out of the dewatering system. Some of the digested sludge solids (with high nitrogen concentrations) are being returned back to the treatment plant in a centrate or liquid sidestream. This sidestream liquid flow comes back into the treatment process downstream of the primary clarifiers, which challenges the Plant’s ability to adequately remove nitrogen.



Currently, the centrate or liquid sidestream that is being returned to the treatment plant is thought to be one of the major problems with the treatment plants ability to remove nitrogen.  The elevated nitrogen levels impact the City’s ability to make Class A+ reclaimed water. Additionally, the digested sludge solids that are returned to the treatment plant do not settle out which causes bulking and solids overflow in other parts of the treatment process (i.e., tertiary filters)..

Some of the issues the City would like to address with the temporary dewatering system are: dewater the solids; reduce the amount of solids returning to the treatment plant; reduce the amount of nitrogen (i.e., ammonia) being returned to the treatment plant; and install a system that will function during cold weather months allowing solids handling year round.

The initial capital project is generally described as installing a temporary dewatering system,, although it could be part of a permanent solution.   This system will work in conjunction with the existing SSBs and DLD site, to maintain compliance with the WWTP’s permits. Carollo Engineers compared the following traditional solids dewatering options; cleaning existing basins, Geofabric containers, Belt filter press, and Centrifuge. Attached are the engineers initial recommendations and preliminary cost estimates for each option.

The purpose of this review is to describe the alternatives being considered as well as the recommendations from the City's engineering consultant (Carollo Engineers).


FISCAL IMPACTS

Continuing to utilize the existing SSBs and DLD method has historically been the least expensive method of dealing with sludge.  This method has saved the City millions of dollars in operational and capital costs throughout the years compared to alternative solids handling methods such as belt filter presses and centrifuges. However, this method is antiquated and no longer a recommended industry practice. Thus continuing with this method in the long term is no longer a viable option. 

The proposed capital project was originally budgeted for $800,000 in FY14, but only included dredging the existing solids basins. The total project cost for the recommended Geofabric option is estimated to be $1.2 M dollars. The increased cost is proposed to be funded from sewer capacity fees since the elevated solids volume coming into the Wildcat Hill WWTP is due to growth.  This will have no impact to the Utility Fund balance in outgoing years and does not affect the current 5 year plan.

This project is budgeted for FY14; however staff proposes to use FY13 contingency funds to provide funding in order to accelerate the project schedule and begin construction in May 2013. Staff proposes to proceed immediately in the current fiscal year, using $800,000 in contingency funds and the remainder from sewer capacity fees.