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Biosolids Management Planning – Nutrient Removal and Storage

Kyle Leonard

Modern wastewater treatment plants are technologically advanced facilities that require a great deal of knowledge and skill to operate. While many people only consider the treated effluent discharged from these facilities, treatment plants must actually process, discharge, and dispose of two separate wastewater components: the liquid component that is treated and discharged to a river, stream, etc. and the solids component that is treated and disposed by land application, landfilling, incineration, etc. Treatment and ultimate disposal of both components are equally important.

A very large fraction of a wastewater treatment plant’s capital and operating costs can be attributed to the systems that capture and treat the solids fraction of sewage entering the wastewater treatment plant.

Biosolids are applied to agricultural fields and/or turf grass lands as fertilizer and soil amendment to improve and maintain productive soils and stimulate plant growth and crop production.
Treated sewage sludges/solids are known as biosolids. Biosolids are used as a valuable fertilizer in many locations. Biosolids are applied to agricultural fields and/or turf grass lands as fertilizer and soil amendment to improve and maintain productive soils and stimulate plant growth and crop production. The controlled land application of biosolids completes a natural cycle in the environment.

Evaluation Study

The Sioux Falls, South Dakota Water Reclamation Facility (WRF) began operations in 1978. Sludge from the treatment processes is treated in three primary anaerobic digesters. The digested sludge (i.e., biosolids) is then transferred to one secondary digester. Biosolids are routinely transferred from the secondary digesters to one of two sludge storage lagoons for holding until final disposal. The City land applied the biosolids to approximately 2,000 acres of leased agricultural land in 2011. The process of pumping the biosolids from the lagoon, loading the tanker trucks, and land applying the biosolids requires extensive effort and expense.

In 2013, HR Green was hired to conduct a biosolids management evaluation study to evaluate alternatives and recommend a biosolids plan for the next 20 years for the City of Sioux Falls. The study included a forecast of biosolids production rates due to growth and future nutrient removal in the liquid component treatment process. The study also included an investigation of digested biosolids processing and storage options and made recommendations as a part of the overall biosolids management plan.

A list of the scope elements of this study included:

  1. Review of current biosolids handling, storage and disposal operations.
  2. Evaluate biosolids processing, storage and disposal alternatives.
  3. Evaluate future Water Reclamation Capital Improvements Plans (CIP’s) for cost consolidation options for processing, storage and disposal alternatives.
  4. Present recommendations for improvements.

The following goals were used as a basis for alternatives, evaluations and recommendations:

  • Capital cost – Recommendations considered cost consolidation potentials and benefits with respect to CIP’s with transition to mechanical dewatering.
  • Life-cycle costs – Capital and Operation and Maintenance (O&M) costs for a 20- year planning period will be used for evaluations.
  • Innovation vs. Reliable Technology – NNew technologies were considered but more focus was given to reliable technologies with flexible operations.
  • Best use of staff and existing infrastructure – AAlternatives incorporated assets of existing infrastructure and focus on best use of staff for O&M.
  • Environmentally Sustainable – Continue to beneficially re-use biosolids while protecting the environment and public health.

This study examined the past five years of biosolids land application history to help determine the future design conditions. WRF staff predicted a 2 percent annual increase in biosolids production due to population growth. More significantly, the Sioux Falls WRF will eventually receive a discharge permit with more stringent nutrient removal requirements. Due to the probable permit limits and the liquid treatment process changes needed to achieve those limits, a 10 percent increase in biosolids production was anticipated due to nutrient removal treatment.

The design period looked out 20 years starting with the most recent biosolids production and projected plant operation costs for the existing liquid biosolids storage, handling, and disposal. Six alternatives for biosolids processing, storage, and disposal were presented in the initial workshops for the project, four modified alternatives were presented, and one additional modified alternative was presented at the draft report meeting. Both economic and non-economic elements were evaluated for each alternative considered.

Results of the Study

Results of the evaluation concluded that the WRF should implement an interim biosolids management plan that keeps the existing infrastructure in place for liquid biosolids storage but contracts out the handling and disposal of the biosolids. This interim biosolids management plan will be in place until approximately the year of 2021 or earlier if funding for the long term recommendation is available sooner.

The long term biosolids management plan, which will startup after the interim plan, will consist of dewatering the liquid biosolids into a cake product and further drying the biosolids into a Class A product. Dewatering equipment will use highly efficient screw presses over the more traditional belt filter, centrifuge, and rotary fan presses. The dewatered cake product will be dried to a 92 percent solids Class A product with an indirect heated screw dryer. Using the indirect heated dryer is a technique not typically implemented; however, this process dries the biosolids product to at Class A product that can be sold for public use, applied to City grounds, and many other applications.

Biosolids Management

The WRF has existing unused infrastructure where the highly efficient dewatering and drying equipment specified by this project will be able to completely fit within this existing building.

The dewatering and drying process/equipment utilized by this project allows for the WRF to maintain compliance with permits while producing a quality Class A product that may potentially be used in City public parks as a soil amendment or partially replace chemical fertilizers and/or be available free-of-charge to the public for use. The WRF has existing unused infrastructure where the highly efficient dewatering and drying equipment specified by this project will be able to completely fit within this existing building. Dried product storage will still need to be built; however, the structure’s footprint will be small. The existing liquid biosolids storage lagoons will be repurposed as the WRF’s new wet-weather equalization basins.

If the WRF continued on their current biosolids management plan they would be required to add a second crew with appropriate biosolids land application equipment within a few years to handle the increasing labor needs to handle the increase in liquid biosolids volume. Additionally, the WRF would also need to replace aging existing biosolids land application equipment within a few years. The project alleviated the need to add a second crew, equipment, and replacement of equipment by providing a treatment process that required minimal personnel labor to operate and maintain, significantly reduced the volume of treated biosolids, and eliminated the current land application process that used the aging equipment. By providing an interim biosolids management plan that utilized the existing liquid biosolids storage lagoons and contracted out the biosolids handling and disposal process, the WRF has time to acquire the required funds needed for the capital cost of the long term biosolids management plan.

By using this comprehensive approach, HR Green was able to fulfil all of the major goals the WRF staff had set for the study. As a follow-up to the final report, HR Green was able to setup a pilot study that tested the long term management plan recommended dewatering and drying equipment. The pilot study provided exceptional results that confirmed the equipment and treatment process recommendations.

If you would like to learn more about our ability to provide a comprehensive review of wastewater treatment plants contact Michael Roth at mroth@hrgreen.com

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