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

Key Message: Reclaimed water used for irrigation contains a significant amount of nitrogen, especially if it comes from a wastewater treatment facility that does not use advanced treatment protocols. If the nitrogen content of reuse irrigation water is ignored when calculating the recommended amount of synthetic fertilizer to apply to landscapes and golf courses, over fertilization can occur. Greater understanding of the nutrients in reclaimed water is needed, coupled with targeted education for homeowners, lawn care professionals and golf course managers to align irrigation and fertilizer management to reduce nutrients.

Update (November 2021): UF/IFAS Sarasota County Extension developed a Reclaimed Water Tool to help homeowners, community associations, and turfgrass professionals adjust their fertilizer use based on the amount of nutrients in reclaimed water used for irrigation. See Progress section for specific details.

Importance

Homeowners and landscape managers can inadvertently apply excessive nutrients to landscapes and golf courses if they don’t offset the amount of synthetic fertilizer they apply by the amount of nutrients already in reclaimed irrigation water. [Update: A 2021 survey conducted by UF/IFAS Sarasota County Extension for golf course managers indicated that about 66% (n=39) of respondents do not account for nutrients in their reclaimed water before fertilizing.]

Reclaimed water is a byproduct of wastewater treatment, and its nutrient content varies significantly depending upon level of treatment. Reclaimed water produced in secondary treatment wastewater facilities contains higher concentrations of nitrogen and phosphorus than water produced in advanced wastewater facilities. There are three main reclaimed water providers in Sarasota County, none of them use advanced treatment. [Update: Sarasota County Government approved a strategy to convert all three facilities to advanced treatment (see Chapter 1.1).]

If the quality and quantity of nutrients in reclaimed water can be reliably known, fertilizer applicators can reduce the amount of synthetic fertilizer they apply and achieve the same result with reduced costs and risks of over fertilizing (see Chapter 4). Therefore, it is important to understand the quantity and fate of nutrients from reclaimed irrigation water versus fertilizer application as part of an overall nutrient management plan.

Overview

Even if communities adopt and abide by fertilizer ordinances, irrigating with reclaimed water can potentially add to the nutrient load. 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 (Figure 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 1.2.1).

Assuming fertilizer use follows these application rates, reclaimed water may contribute significant nutrients to surface and groundwaters, potentially polluting receiving waters. Reducing nutrient concentrations in effluent through Advanced Wastewater Treatment reduces the potential combined impact of nutrient loading from fertilizer and irrigation with reclaimed water.

Approach

To demonstrate the approach, the nutrient loading from reclaimed water and fertilizer application was evaluated for the Jacaranda West Golf Course (Figure 1.2.2), irrigated with reclaimed water from the Venice Gardens WWTF.

The evaluation included the following steps (Table 1.2.2):

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).

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. 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. The fate of potential excess nutrients also needs to be better understood. Depending on the hydraulic conductivity and gradient, surficial geology, distance to receiving water body, and other factors, natural denitrification may occur in the groundwater leachate, similar to that suspected in some instances for onsite wastewater treatment and disposal system leachate (see Chapter 2.1).

Understanding the nitrogen contribution of reclaimed water to water bodies requires a tracer unique to wastewater effluent that diminishes in the environment under similar conditions and rates as nitrate. Tracers can be used to calculate the nitrogen concentration in reclaimed water versus other sources such as septic systems and fertilizers (Oppenheimer et al., 2018).

The Pinellas County Water and Sewer Utilities website includes an interactive map of areas where reclaimed water from WWTFs is applied (Figure 1.2.3). This map provides guidance for adjusting fertilizer use based on irrigation with one inch of reclaimed water twice a week. This is a promising educational tool for creating awareness that reclaimed water is effectively “fertilizer water” and a means for customers to manage/reduce nitrogen fertilizer applications. It is an initiative worth replicating.