3.2 Quantify the cost and effectiveness of biosolids disposition

Key Message: Because most biosolids are generated by municipal wastewater treatment facilities, emerging markets for biosolids products may offer economic and environmental benefits to communities. Understanding the costs and benefits of various technologies for processing biosolids can inform their sustainable use.

Importance

Biosolids represent a significant source of nutrients and are primarily generated by government wastewater treatment facilities (WWTFs). Their disposition presents both challenges and opportunities for nutrient budgeting and management. Subject to regulation, biosolids can be landfilled, incinerated, spread on permitted agricultural lands, golf courses, parks, and forests, or processed into fertilizer (see Chapter 3.1). In Sarasota County, biosolids from the six major WWTFs are transferred for a fee to the Charlotte County Bio-Recycling Facility (CCBF) for processing. Operated by Synagro, this facility processes up to 10,000 tons per year into Class AA compost for sale. Quantifying the economic and environmental cost-benefit of biosolids disposition is critical for holistic nutrient management. Emerging technologies and markets offer communities sustainable options with economic and environmental co-benefits.

Overview

Alternatives for Biosolids Disposition

Several alternatives to landfilling biosolids exist, including composting, thermal drying (TD), and thermal hydrolysis processes (THP). In Florida, compost and TD biosolids have been widely marketed, though no THP facilities currently operate in the state.

Composting blends dewatered biosolids with a carbon source, such as wood chips or sawdust, creating a suitable environment for microbial activity. Composting can transform Class B biosolids into Class AA products. Florida’s citrus industry, landscaping, and nursery production have extensively used biosolids compost (MDWSD 2016).

Thermal drying produces granulated fertilizer products by drying biosolids. Though energy-intensive, operational costs are often offset by reduced handling costs and fertilizer sales. TD biosolids can also be landfilled, land-applied, or burned for energy at facilities like cement kilns. However, this market has not yet been developed because, to date, fertilizer has been a more marketable commodity.

In the THP process with soil blending, dewatered Class AA biosolids can be used in the production of blended landscaping soils. This is an emerging management technique that has been gaining momentum nationally in horticultural markets (MDWSD, 2016). Tacoma Grow (TAGRO) in Washington is a well-known soil blending program.

THP blends dewatered Class AA biosolids into landscaping soils or processes them under pressure before anaerobic digestion. The THP process can significantly reduce biosolids volumes and generate methane gas as a renewable energy source. Examples include:

• Raleigh’s Neuse River Resource Recovery Facility, which reduces biosolids volume by 50% and captures enough methane to fuel 50 city buses daily (Stradling 2019). By capturing and burning the methane rather than allowing it to escape from landfills during decomposition, the facility reduces greenhouse gas emissions equivalent to removing 4,000 cars from the road each day.
• DC Water’s Blue Plains AWT plant, achieving a 50% biosolids reduction, generating 30% of its energy needs, and cutting greenhouse gas emissions by 41%.

Wholesale market values vary substantially (Table 3.2.1). High-quality granules fetch significantly higher prices than compost, though future markets may shift depending on regulatory changes (see Chapter 3.3).

Challenges of Biosolids Disposition

Reusing nutrients captured in biosolids presents a sustainable disposal opportunity, but existing and emerging challenges impact the cost, effectiveness, and safety of disposal. Challenges include contaminants of emerging concern (CEC), which may be present in domestic and/or industrial wastewater biosolids applied to soils. These CECs include (EPA, 2009):

• Pharmaceuticals and Personal Care Products (PPCPs)
• Steroids and Hormones (S/H)
• Microplastics
• Pesticides
• Alkylphenols and Alkylphenol Ethoxylates (APEs) used in some detergents and cleaning products
• Bisphenol A (BPA) used primarily to make polycarbonate plastic and epoxy resins
• Polybrominated Diphenyl Ethers (PBDEs) used as flame retardants and in plastics and electronics
• Dioxins and Polychlorinated Biphenyls (PCBs) used in industrial activities and products like coolants, lubricants, plasticizers, and flame retardants
• Per- and Polyfluoroalkyl Substances (PFAS) used widely in consumer products. Recently regulated for drinking water, EPA’s proposes to develop studies and limits on PFAS in industrial wastewater discharges as well.

Additional challenges impacting the use of biosolids in Florida include (MDWSD, 2016):

• The Food Safety Modernization Act (2015) imposes strict “days to harvest” restrictions for crops grown with biosolids that have not been treated for pathogens. Although the regulation does not prohibit the use of biosolids, it allows growers cooperatives and purchasers to implement restrictions.
• Declines in Florida citrus production of 40 to 50 % due to Citrus Greening disease, reducing biosolids markets.
• Nutrient limits imposed under the Lake Okeechobee and Estuary Recovery Plan, requiring fertilizer standards for urban and turf use.
• FDEP’s 2021 rule revisions for land application of biosolids may reduce available application areas by 75% due to stricter site criteria as well as surface and groundwater monitoring requirements (FDEP 2019b). More biosolids are expected to be landfilled or transferred to Class AA biosolids treatment facilities or transferred out-of-state for management or disposal (see Chapter 3.3). Small rural facilities lacking economies of scale may face disproportionate burdens.

Approach

Evaluation of sustainable biosolids disposition should consider cost, market return, and environmental and social impacts. Based on a Miami-Dade Water and Sewer Department study (MDWSD 2018), annualized costs are approximately:

• Base Case: $500–$700/ton
• Class B Composting: $700–$800/ton
• Class AA Thermal Drying (TD): $550–$700/ton
• Class AA Thermal Hydrolysis Process (THP): $550–$650/ton

These estimates reflect economies of scale in large urban areas and would likely be higher in smaller communities. The “Triple Bottom Line” costs should be evaluated, including:

• Economic – capital and operating costs
• Environmental – carbon footprint, impacts to soil, water, and air quality, and regulatory compliance risks
• Social – traffic, odor, public safety, beneficial reuse, aesthetics, and noise