Incineration Goes Green for the Water & Sewer Authority of Cabarrus County, NC

As in many urbanizing areas, management of biosolids from wastewater treatment facilities in the fast-growing piedmont region of North Carolina continues to increase in cost and complexity. Utilities continue to seek more cost-effective biosolids management options in addition to traditional landfill disposal and agricultural land application. Factors contributing to increasing biosolids management costs include rising energy and transportation costs, longer hauling distances to suitable agricultural land, more complex land application requirements, increasing landfill tipping fees, and increasing volumes of biosolids production.The Water & Sewer Authority of Cabarrus County, NC (WSACC) owns and operates the 24 mgd Rocky River Regional Wastewater Treatment Plant (RRRWWTP), which is within 100 miles transport distance of at least 17 major wastewater treatment facilities treating at least one million gallons per day (MGD) in North and South Carolina. The solids treatment train at the RRRWWTP consists of gravity thickeners for primary sludge and waste activated sludge, centrifuge dewatering of co-thickened solids, and incineration of dewatered cake in a multiple-hearth furnace (MHF). Dewatered solids are undigested and high in volatile solids content, enabling autogenous combustion (without use of auxiliary fuel) in the MHF.The permitted mass-flow based capacity of the MHF is 20,000 wet lbs/hour, but RRRWWTP solids production rates are considerably less and likely to remain below permitted capacity through at least 2024. Currently, the RRRWWTP produces only enough solids to allow the MHF to operate for 4 to 5 days per week. For the remainder of each week, WSACC is forced to maintain MHF temperatures by combusting fuel oil at substantial additional expense.With uncertainty regarding future fuel costs and the desire to implement a more cost-effective and sustainable solution to its own solids management issues, WSACC retained CH2M HILL to examine the potential for green power generation using heat recovered from the MHF. It became apparent that the potential to import biosolids from other wastewater treatment facilities in the area could justify the capital for power generation and imported solids handling facilities. The evaluation included the following three basic elements:Quantity and quality of available biosolids from surrounding utilitiesPower generation potential within the permitted limits of the MHFEstimated capital and operation and maintenance (O&M) costs for such a system, along with associated payback period analysis.Given the relatively high solids concentrations and volatile content of dewatered solids produced at the RRRWWTP, it was determined that sufficient quantities of biosolids from other utilities in the region are available for supplementing solids produced at the RRRWWTP, and that the blended solids would be sufficiently high in total volatiles to maintain autogenous combustion in the MHF. The potential ranges of RRRWWTP solids and imported solids mixes were evaluated and compared with the permitted mass-flow capacity of the MHF, along with operational constraints, to establish the power generation potential of heat recovery facilities at 1.2 to 1.5 megawatts (MW).Estimates for capital and O&M costs were developed for imported solids handling facilities and heat recovery/ power generation facilities. Then payback periods were calculated to predict whether potential economic returns were acceptable for the project to move forward. The original estimates of capital, O&M, potential savings/revenue, and simple payback periods for different cases of onsite biosolids storage capacity are provided in the body of the manuscript. Cost estimates provided indicate simple payback periods ranging from approximately 7 to 10 years.Based on progress to date, WSACC has chosen to move forward with design of imported solids receiving and heat recovery/ power generation facilities, while concurrently updating cost estimates to confirm the project's financial viability. The recommended facilities and operations are expected to establish a sustainable, regional solids processing and energy-recovery facility, allowing WSACC to meet its biosolids management needs, produce green energy, and provide an alternative for neighboring wastewater treatment facilities, while reducing its power costs and earning additional revenue from imported biosolids.