Planning for an Uncertain Future - Sometimes Simple is Best

Authors:

  • Matt Van Horne, Deborah Mahoney - Hazen and Sawyer
  • Hans Tuneblum - Veolia
  • Roger Brooks - City of Leominster

This figure shows the sensitivity of the analysis for the thickening and dewatering alternatives across a range of final hauling/disposal costs.

Across the United States, the biosolids management environment is evolving. Land application regulations, air pollutant mitigation requirements, landfill bans on organic waste and increasing public pressure are forcing municipalities to reassess their biosolids management approaches to best accommodate all of these externalities while providing a technically appropriate and economically viable solution. For the City of Leominster (Massachusetts), a critical crossroads in their biosolids management needs arose when the incinerator facility at a neighboring municipality opted to close down rather than upgrade to meet the new air emission regulations. This incinerator, and its associated dewatering infrastructure, had been the destination for the liquid sludge from the City of Leominster Water Pollution Control Facility (LWPCF) for several decades. This event cause the LWPCF to use an emergency gravity belt thickener to reduce the trucked volume prior to hauling to another incinerator facility significantly further away, all the while dealing with operational, logistical and economic impacts from this practice.

This closure of the local incinerator forced the City of Leominster to undergo a process to evaluate and select the best processing and disposal option for the City both on a short term basis and long term basis, realizing the changes occurring in the biosolids management environment. This presentation will discuss the evaluation performed by the City which would inevitably change the course of biosolids processing for the City long into the future. This fresh look at the available options for residuals handling provided the City with the opportunity to develop a project with an eye on short term opportunities to provide the needed flexibility and reliability for day-to-day residuals management as well as looking towards future opportunities for the City.

The LWPCF is a 9.3 million gallon per day water resource recovery facility whose solids consist of a co-settled mix of primary solids, waste activated solids and seasonal chemical solids (from ballasted flocculation phosphorus removal facilities). The original facility design included vacuum filter dewatering units to produce a cake material, however these facilities were never utilized on a consistent basis due to the attractive economics of trucking liquid sludge material a short distance for dewatering and incineration by a neighboring municipality. The recent cessation of incineration at this facility resulted in an increase in hauling distance which necessitated the use of a gravity belt thickener unit, originally intended for emergency use only, and sludge storage tanks to allow for adequate management of the residual material. This was not a sustainable solution for the LWPCF and a study was commissioned to identify the appropriate pathway forward.

The analysis looked at the 20-year potential biosolids generation from the facility with an eye towards maximizing the utilization of existing facilities to provide the best technical and economic solution. The options considered included a no-action alternative as well as thickening, dewatering, digestion and thermal drying. One of the main considerations in the evaluation was the total disposal cost, including hauling as well as final disposal charges. A total disposal cost sensitivity analysis was performed to identify the point at which a different recommended solution would have been reached.
One option that was thought to be initially attractive was to repurpose the existing sludge storage tanks back to their original intent as anaerobic digesters. This would have allowed the City to possibly benefit from the implementation of the ban on landfilling of certain organic wastes and possibly generated an additional revenue stream. However, the costs required to rehabilitate the tanks and provide the necessary ancillary equipment to support anaerobic digestion proved to be significant and the capacity limitations due to the size of the tanks would not allow a sufficient quantity of imported material to be received to aid in the overall economics of this approach.

The economic analysis showed that capital costs for the various alternatives were the main driver in the 20-year life cycle evaluation, largely driven by the final disposal opportunities for the various biosolids products. In the final evaluation, the option to provide a permanent thickening facility was chosen as it provided a definitive near-term solution as well as the possibilities to move to advanced levels of processing in the future as economic, technical and regulatory drivers changed. The thickening option allows for disposal at a relatively close-by incinerator facility that can only receive material below approximately 7% total solids. Improving the final product to a cake material (approximately 20% total solids) would have required significant increases in hauling distances and significant increases in project disposal fees.

The City is currently in the startup phase of the implementation process with rotary drum thickening equipment. The overall implementation includes pre-purchasing of the thickening equipment, on-site validation testing of the selected manufacturer’s equipment and a detailed sequence of construction to ensure that the facility can continue operations through the construction period. The overall project included significant demolition work to repurpose the vacuum filter dewatering room, rehabilitation of the major sludge pumping facilities, addition of an emulsion polymer storage and feed system, rehabilitation of the sludge storage tanks and improvements to the control system. All of these improvements were designed to provide flexibility to route the residual material to various points at the LWPCF and increase the options available to operating staff to best manage day-to-day variations in handling needs.

The key elements of this presentation include: – How small plants can address holistic solids handling improvements in a cost effective manner, – Why, despite the perceived economic benefits, anaerobic digestion was not the recommended solution, – How long term assessment of disposal alternatives can drive the recommended solution, and – How maximum flexibility can be included to provide handling options to address variations in biosolids management needs

This presentation will describe in detail the various elements of the evaluation as well as the major drivers that informed the ultimate decision. The various considerations identified and addressed in the process will serve as a template for other utilities looking to rehabilitate or change their residuals handling process for both short-term and long-term solutions.

For more information, please contact the author at mvanhorne@hazenandsawyer.com.

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