Consideration and Approval of Contract:  Wildcat Hill WWTP Temporary Digested Solids Dewatering System Project

Award of the contract will authorize the construction of the Temporary Digested Solids Dewatering System project at the Wildcat Hill Wastewater Treatment Plant in accordance with the approved Public Improvement Plans prepared by Carollo Engineers. This is a regulatory project required to comply with the Consent Order issued by the Arizona Department of Environmental Quality signed May 2013.

Funding is available in the Fiscal Year 2013-2014 Utilities Division Capital Budget.
This project is budgeted in Account # 201-5709-772-4306 (FY2014) in the amount of $1,200,000 dollars. To date $150,076 has been spent on engineering fees, leaving $1,049,923 remaining in the project fund balance for the completion of this project without change orders.

  1. Repair Replace maintain infrastructure (streets & utilities)

Staff approached City Council on February 26, 2013 who approved $1.2M in upgrades to Wildcat Hill WWTP solids handling operation. The City hired Carollo Engineers who completed the design in July 2013 on a temporary solution to better handle the solids at Wildcat Hill WWTP. This new design was recently approved by ADEQ in October 2013. 

• Approve the award as recommended
• Reject bids and re-advertise the project

Significant concerns with the existing solids dewatering system at the Wildcat Hill Wastewater Treatment Plant (WWTP) have caused the city to implement a temporary solids (digested sludge) dewatering project. Currently, the two Solids Settling basins (SSBs) at the Wildcat Hill facility are completely full of solids. This results in an overflow condition where the supernatant being returned to the plant process contains high levels of solids, nitrogen and ammonia. The supernatant being returned to the WWTP is thought to be causing issues with the treatment process. This new project is generally described as temporarily installing a supplemental geotextile fabric tube (Geotube®) dewatering system that will work in conjunction with the existing solids settling basins (SSBs) and Dedicated Land Disposal (DLD) site, to maintain compliance with the plant’s existing permits.
     
The Wildcat Hill WWTP was originally designed by Brown & Caldwell in 1979 to handle 1083 tons per year of digested solids. Since that time the plant has continually added solids from other sources such as Rio De Flag WRF, other Coconino County subdivisions, and septic haulers while never increasing the solids handling capacity of the plant. Currently the plant is handling 1480 tons per year of solids loading which is 400 tons per year above the original design of 1083 tons per year. This is a 37% 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 digested sludge. Sludge is pumped from the anaerobic digesters to the solids settling basins 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 DLD area. The sludge is disked, or knifed, into the earth on the DLD when frost or saturation conditions allow. When the DLD is shut down due to seasonality/weather, the digested solids are stored in the solids settling basins until the warmer dry weather returns.

The existing two SSBs have been experiencing operational issues since 2009. The SSBs are not functioning to drop the solids out of the dewatering system. Some of the digested sludge solids and high nitrogen loads are being returned to the WWTP in the supernatant. The supernatant flow comes back into the WWTP downstream of the primary clarifiers. The digested sludge solids that are returned to the WWTP do not settle out, they float, so they are not removed in the secondary clarifiers, causing bulking and solids overflow to the tertiary filters, which causes additional problems with filtration and disinfection. The solids and nitrogen loads that are being returned are thought to be causing issues with the Integrated Fixed-Film Activated sludge (IFAS) process. 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 plant; reduce the amount of ammonia being returned to the plant; process the solids on-site versus hauling costs and disposal fee to take to a landfill; and install a temporary system that will function during cold weather months.

The current project will develop a temporary solids dewatering system solution to allow the city to return dewatering supernatant to the WWTP with a lower solids content and ammonia load than the current system does. It is estimated that the temporary dewatering system will be in place at for approximately 2-3 years.

The design of the temporary solids dewatering system includes the following:
 

• Installation of a temporary digested sludge solids dewatering and polymer injection system to dewater an average daily sludge flow of approximately 70,000 gallons per day at 1.5 – 2.5 percent solids.
• Constructing a paved lay down area for the placement of the geotextile fabric dewatering bags (Geotubes®). The lay down area will generally consist of grading a pad, installing a geogrid or geotechnical fabric, placing and compacting an aggregate subbase, and installation of an asphalt cement running surface for trucks or earthmoving machinery. The dewatering concept will utilize a new header piping system with detachable hose to fill the Geotubes® with digested sludge. The supernatant (or filtrate) from the Geotubes® will be returned to the WWTP utilizing the existing 12-inch Supernatant line. The filtrate will return to the plant in its current location.
• The plant will inject polymer into the digested sludge line prior to filling the Geotube® bags.
• The Geotube® bags will be designed to allow for filling from more than one location.
• The plant will have the option of either disposing of the dewatered solids at the existing DLD, or having them taken to a landfill for disposal.

Staff issued an Invitation for Bid for construction services on September 18, 2013. The solicitation ran for three consecutive weeks with two advertisements in the Az Daily Sun. Staff received eight responses, Felix Construction Company was the lowest responsive bid.     

This project is required in order to maintain our existing permit and regulatory compliance with the Arizona Department of Environmental Quality(state)  and the EPA (federal).

This temporary solids dewatering system project was approved and permitted by ADEQ in October 2013.
 

This is a temporary solution  estimated to be in place 2-3 years until a permanent Solids Handling process can be designed and implemented at the Wildcat Hill Wastewater Treatment Plant.

Using geotextile fabric dewatering bags to dewatering sludge is a very cost-effective temporary method versus mechanical dewatering of digested sludge which requires high energy demand. This process uses gravity rather than electrical energy to dewater the sludge.

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