By David Peterson and Stephen Geribo
Not long ago, the city of Cape Coral, FL, was one of the nation's fastest growing cities. Anticipating future growth, the city embarked on a program to expand water and sewer utilities, as well as build a new $32 million regional biosolids treatment facility, which required investment in new sludge drying and pelletizing equipment. Unfortunately, the downturn in the economy caused Cape Coral to go from one of the nation's fastest growing cities to one of the nation's leaders in home foreclosures.
In response, the city stepped back to reassess its infrastructure needs. It put the construction of the biosolids treatment facility on hold, though they had already invested $15 million on centrifuges, odor control, drying and pelletizing equipment for the facility. An evaluation of the current treatment systems found that the rising cost of lime severely impacted the cost-effectiveness of staying with the existing lime stabilization system.
In search of a feasible biosolids management strategy, the city used an open market request for information (RFI) approach to identify a more cost-effective solution that would meet the appropriate capacity requirements.
Lime Limits
Cape Coral's wastewater treatment is split between the Southwest Water Reclamation Facility (SW WRF) and the Everest Water Reclamation Facility (EWRF). All biosolids from these two plants are treated at the SW WRF. Biosolids are pumped from the EWRF and enter the SW WRF along with raw influent.
The SW WRF processes biosolids through a multi-step process that includes producing waste activated sludge (WAS) as a part of the biological treatment process, which is then stored in two 1.16 million gallon aerated WAS storage tanks. WAS is pumped through sludge thickening tanks and then to parallel belt filter presses (BFPs). After dewatering, the sludge cake is moved by a belt conveyor to a lime stabilization process. Dewatered and stabilized biosolids cake is then stored in an open air windrow building which provides three to five days of storage time. Ultimately, a private contractor hauls the processed biosolids cake off site for disposal.
The existing biosolids equipment is approaching 20 years old, and the city is concerned about the reliability of the lime process feed auger and the underperforming belt filter presses. As well, a previous study found that monthly operating costs increased by approximately 70 percent between 2006 and 2008, in part due to the cost of lime. The monthly operating cost of the system was approaching $100,000 per month.
In addition to those expenses was the investment in new sludge drying and pelletizing equipment that was to be used for the new biosolids treatment process.
In an effort to redefine its biosolids investment to match current growth patterns, the city issued an RFI in the winter of 2011 to the open market. As written in the RFI summary, the intent of the RFI was to solicit "the broadest set of creative responses that will optimize …full life cycle costs over a minimum of 20 years, or more, to process and dispose of biosolids."
By February 2011, the city had received 10 technical responses offering a range of solutions, including the haul and disposal of Class B biosolids, as well as production of Class A biosolids through chemical stabilization, drying and pelletizing, and drying with pellet combustion.
Cape Coral retained the services of outside consultants to evaluate the RFIs, evaluate other options and alternatives not included in RFIs, and formalize a short-term and long-term implementation plan.
Engineered Approach
In general, the responses received represented a broad cross-section of the industry, ranging from local contractors and haulers to multi-national firms with extensive experience in the biosolids industry. However, the range of technical solutions received was somewhat limited.
The aggregate of all the solutions received through the RFI process resulted in three fundamental biosolids handling approaches and seven specific technical concepts. Two of the approaches would create Class A biosolids, while the third approach would create Class B biosolids. The majority of respondents included the installation of the thermal drying equipment as one of the proposed options. There would be some merit to installing and using the drying equipment that was purchased by the city: the equipment was on site and already paid for, so installing the equipment would utilize the city's prior investment. The centrifuges could be used to replace the underperforming belt filter presses in conjunction with any stabilization technology proposed, and therefore, did not figure into the comparison.
After a comprehensive study of available options, engineers found that staying with the city's current lime stabilization process was the most costly option. Installing and operating the dryers was the second most costly option.
The most cost-effective, lowest risk option was simply hauling unstabilized, dewatered sludge (Class B biosolids) to a permitted regional landfill. However, additional research found all sludge disposal facilities in the are to be focused on agricultural land amendment or too small for consideration.
The best short-term solution available was composting Class B biosolids at the Lee County or Charlotte County facilities. Both facilities are located in close proximity to the SW WRF. This approach is highly cost-competitive with any of the other technologies or biosolids management approaches offered. Furthermore, both facilities have the advantage of being permitted already and having limited odor concerns.
Now and Later
In the near term (next five years or less), Kleinfelder/ARI recommended that the city contract with a private hauler to take Class B sludge from the SW WRF. Hauling alleviates the costly lime process and provides time for the city to explore other biosolids management options.
In addition, Kleinfelder/ARI recommended that the city sell or lease those pieces from the thermal drying equipment that they would not require. They recommended that the city keep the dewatering centrifuges for future use to increase the percent solids of the dewatered sludge, thereby reducing trucking requirements and hauling costs.
For the long term, the engineering consultant recommended that the city fully explore the complete market for cost-effective biosolids management through a RFQ/P process to identify suitable technologies and regional solutions that can be compared and ranked on a cost basis, risk allocation basis and technological basis.
About the Authors: David Peterson is a Project Professional with Kleinfelder. He can be reached at [email protected]. Stephen Geribo is the Vice President and Water Infrastructure Segment Manager with Kleinfelder. He can be reached at [email protected].
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