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 nutrient-rich byproducts intended for beneficial reuse, especially for crop growth. However, depending on treatment level and land application methods, nutrients can leach into soils and waterways. Different wastewater treatment processes remove more or less nitrogen from effluent, while phosphorus largely remains in the biosolids. Proper land application techniques, such as mixing biosolids into soil rather than surface spreading, are necessary to reduce runoff. Quantifying biosolids nutrients and their disposition is critical to nutrient budgeting and water quality management. This requires multiple years of data to establish trends.

Overview

In Florida, permitted WWTFs must report biosolids quantity and disposition to the Florida Department of Environmental Protection (FDEP) through monthly Discharge Monitoring Reports (DMRs) (see Chapter 1.1). These data are publicly available and submitted by WWTF operators.

Review of DMRs and discussions with operators of the six major WWTFs in Sarasota County confirm that all biosolids are transported to the Charlotte County Bio-Recycling Facility. This facility, operated by Synagro and co-located with the Charlotte County landfill, meets FDEP and EPA regulations for producing Class AA biosolids. There is no known direct land spreading of WWTF-generated biosolids in Sarasota County, aside from commercial Class AA compost products like those from Synagro. Land spreading remains allowed under strict permitting consistent with state law (F.A.C. Ch. 62-620) and local ordinance (Sec. 54-188) (see Chapter 3.3).

Approach

To demonstrate the approach to quantifying biosolids nutrient contributions, monthly DMRs from 2018 were reviewed for Sarasota County’s six major WWTFs. These reports provided data on dry tons transferred to the Charlotte County Bio-Recycling Facility and were used to calculate associated nutrient loads. Total nitrogen (TN) and total phosphorus (TP) were calculated as the product of the reported dry tons transferred and average dry weight nutrient concentrations 2.21% nitrogen and 1.24% phosphorus provided 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 (Figure 3.1.1 and Figure 3.1.2).