1.2 Understand and manage nutrient loads to areas irrigated with non-advanced wastewater

Key Message: Reclaimed water used for irrigation contains significant amounts of nitrogen, especially if it originates from wastewater treatment facilities without advanced treatment. If the nitrogen content of reclaimed irrigation water is ignored when calculating synthetic fertilizer application, overfertilization can occur. Greater understanding of nutrient fate in reclaimed water, coupled with targeted education for homeowners, lawn care professionals, and golf course managers, is needed to align irrigation and fertilizer practices and reduce nutrient pollution.

Importance

Homeowners and landscape managers may inadvertently apply excessive nutrients if they do not adjust synthetic fertilizer rates to account for nutrients already present in reclaimed irrigation water. A 2021 UF/IFAS Sarasota County Extension survey found that about 66% of golf course managers (n=39) did not account for reclaimed water nutrients before fertilizing. This is a significant source of overfertilization, especially during the summer fertilizer blackout period.

Reclaimed water nutrient concentrations vary based on treatment level. Secondary treatment facilities produce reclaimed water with higher nitrogen and phosphorus levels than advanced wastewater treatment (AWT) facilities (see Chapter 1.1). Sarasota County’s three main wastewater facilities that provide reclaimed water are working towards AWT upgrades with the Bee Ridge facility upgrade completed by late 2025. In the meantime, knowing the quality and quantity of nutrients in reclaimed water would allow fertilizer applicators to reduce synthetic fertilizer use, achieving the same landscape outcomes with lower costs and reduced nutrient runoff (see Chapter 4).

Overview

Irrigating with reclaimed water can potentially add to nutrient loading to waterways and waterbodies. The 2018 annual nutrient load of reclaimed irrigation water from Sarasota County’s six largest wastewater treatment facilities (WWTFs) totaled 581,951 pounds TN and 118,082 pounds TP (Table 1.2.1).

In addition, fertilizer ordinances adopted by Sarasota County and municipalities allow application of nitrogen and phosphorus up to 4.0 and 0.5 pounds per 1000 square feet per year, respectively (see Chapter 4.3). Although golf courses are exempt from these ordinances, they must follow FDEP’s manual for “Best Management Practices for the Enhancement of the Environmental Quality on Florida Golf Courses.” (see Chapter 4.5). This manual, the adopted fertilizer ordinances, the Southwest Florida Water Management District’s AGMOD irrigation calculator, and interviews with long-term and active golf course operations that do not use reclaimed water for irrigation were consulted to estimate water and fertilizer application rates (Table 1.2.2). Assuming typical fertilizer application rates, reclaimed water may significantly contribute to nutrient loading. Reducing nutrient concentrations through AWT reduces combined fertilizer and irrigation nutrient impacts.

Nutrient Loading Calculation

To demonstrate the approach, the nutrient loading from reclaimed water and fertilizer application was evaluated for the Jacaranda West Golf Course (Figure 1.2.1), irrigated with reclaimed water from the Venice Gardens WWTF.

The evaluation included the following steps (Table 1.2.3):

1. Estimate the irrigated acreage for the Jacaranda West Golf Course (JWGC).
2. Compute the average annual water use for JWGC using SWFMWD AGMOD.
3. Estimate the annual nitrogen fertilizer load for JWGC.
4. Estimate the annual phosphorus fertilizer load for JWGC.
5. Obtain 2017 and 2018 monthly reclaimed water irrigation (RWW) use for JWGC from the Venice Gardens WWTF.
6. Obtain TN and TP concentrations for the Venice Gardens WWTF from monthly discharge monitoring reports available from FDEP and compute monthly average concentrations for 2017 and 2018 (see Chapter 1.1).
7. Compute monthly and annual TN and TP loads for 2017 and 2018 for JWGC as the product of (5) and (6).
8. Compare result of (7) with (3) and (4).

Based upon this preliminary demonstration of approach, reclaimed water from the Venice Gardens WWTF may be providing 106% (in 2017) and 104% (in 2018) of the recommended annual nitrogen fertilizer application for the JWGC. This estimate assumes that all reclaimed water delivered to JWGC originated from the Venice Gardens WWTF, even though that facility is part of Sarasota County’s interconnected south master reclaimed water system, which includes the City of Venice AWT-WWTF. We also assumed that JWGC used all reclaimed water delivered. While these assumptions introduce some uncertainty around the estimate, we believe them to be reasonable.

This evaluation indicates that the potential supplemental nutrient loads delivered by reclaimed water can be significant and that reclaimed water alone could satisfy turf nutrient needs. Based on the two years examined, loads may be relatively consistent from year to year. However, the potential supplemental nutrient loads delivered by any WWTF may vary and should be evaluated on a case-by-case basis.

In 2021, UF/IFAS Sarasota County developed a Reclaimed Water Tool to help homeowners, community associations, and turfgrass professionals adjust their fertilizer use based on the nutrients in their reclaimed water assuming an irrigation rate of 0.75 inches per week (Figure 1.2.4).