3.1 Quantify nutrient loads from biosolids disposition

Key Message: Biosolids are a valuable byproduct of wastewater treatment that can be landfilled, spread on ranches and fields, or used as a fertilizer. However, the high concentration of nutrients in biosolids may contribute to water quality problems. Quantifying nutrient loads from biosolid disposal methods is an important first step in assessing their role in an overall nutrient management plan.

Importance

Biosolids are a nutrient-rich byproduct of wastewater treatment. As these nutrients are considered a valuable commodity for crop growth and production, biosolids have been applied to landscapes as a well-intentioned disposal solution. However, depending on the treatment level of the biosolids, these nutrients can find their way into soils and waters. Quantifying nutrients from biosolids and their disposition is an important consideration in nutrient budgeting and management.

Overview

In Florida, permitted wastewater treatment facilities (WWTFs) are required to report the quantity and disposition of biosolids to the Florida Department of Environmental Protection (FDEP). This information is part of monthly Discharge Monitoring Reports (DMRs) mandated by their permit (see Chapter 1.1). Review of submitted DMRs and discussion with representatives from the six major WWTFs in Sarasota County confirmed that all transport the biosolids to the Charlotte County Bio-Recycling Facility. This facility is operated under contract by Synagro and co-located with the Charlotte County solid waste landfill. The facility meets FDEP and EPA rules governing production of Class AA biosolids. There is no known land spreading of biosolids from WWTFs in Sarasota County other than those purchased from Synagro or a similar Class AA composting operation. Land spreading of biosolids is allowed by Sarasota County Ordinance (Sec. 54-188) under strict permitted conditions consistent with Florida law (F.A.C. Ch. 62-620) (see Chapter 3.3).

Approach

To demonstrate the approach, monthly DMRs from 2018 were reviewed for each of the six major WWTFs in Sarasota County. These DMRs allow for the quantification of the nutrient loads and disposition of biosolids. The DMR data is provided by the WWTF and maintained in a publicly accessible database by FDEP. The distribution of dry tons transferred to the Charlotte County Bio-Recycling Facility as well as the corresponding pounds of total nitrogen (TN) and total phosphorus (TP) were calculated. Nutrient loads were calculated as the product of the reported dry tons transferred and the average percentage of nitrogen and phosphorus by dry weights of 2.21 and 1.24, respectively, as reported by Synagro. In 2018 the six major WWTFs in Sarasota County generated 5,498 tons of biosolids, resulting in 243,012 pounds of TN and 139,649 pounds of TP (Figures 3.1.1-3).