Case Study: Lake Apopka

Phosphorus Removal in Lake Apopka Using TPX™
NClear, through a grant from the Florida Department of Environmental Protection (FDEP), has demonstrated a novel technological approach to remove phosphorus (P) from Lake Apopka. At 30,000 acres, Lake Apopka is the fourth largest lake in the state of Florida. Located just 15 miles northwest of Orlando, Lake Apopka was one of the most sought-after bass fishing destinations in the US in the middle of the last century. Beginning in the 1940’s, however, significant nutrient loading into the lake gradually reduced water quality, resulting in a highly eutrophic lake. Restoration of Lake Apopka has been a major priority of FDEP and the St. Johns River Water Management District (SJRWMD) since the Lake Apopka Restoration Act of 1996. Restorative measures implemented by SJRWMD have been extremely effective at eliminating a large fraction of external loading inputs to Lake Apopka. However, water column concentrations of P remain well above the TP target restoration goal of 0.055 mg/L established by FDEP as part of the Total Maximum Daily Load (TMDL) process. This reflects the continued effects of internal loading from the sediments which has become increasingly more important in governing water column P concentrations as external inputs have declined. NClear’s solution specifically targets this problem of internal loading, i.e. large releases of dissolved P into the water column occurring in the lake through both episodic sediment resuspension events and continual, relatively passive diffusive fluxes across the sediment-water interface. NClear’s approach uses NClear’s patented calcium-silicate nanocrystal technology (TPX™) to efficiently and permanently sequester P both in the water column and in the sediment.

In Phase I of the FDEP grant, NClear conducted a pilot study in a one-acre sequestered section of Lake Apopka. The results indicated ortho P removal of more than 90%, along with improved water clarity and a significant decrease in the sediment layer. FDEP subsequently authorized NClear, in conjunction with the University of Florida (UF) Soil Microbial Ecology laboratory as a subcontractor, to begin a series of supplemental bench scale studies with sediments from Lake Apopka to, inter alia, identify biogeochemical changes in surficial sediments expected to occur following treatment with TPX™. This involved a series of laboratory microcosm experiments designed to quantify the effects of TPX™ on sequestering P from sediment and water column samples taken from three locations as shown in Figure 1.

Figure 1. Lake Apopka sampling locations from which sediment cores and bulk unconsolidated sediments were collected by University of Florida.
Results of the TPX™ Study
Results from an adsorption isotherm experiment to help define an appropriate TPX™ dosing level for the subsequent mixing and release experiments are presented in Figure 2. The figure plots final observed concentrations in filtered surface water from Lake Apopka that has been spiked with differing amounts of KH2PO4 as a function of expected P concentrations based on the spiking level, the ambient soluble reactive phosphorus (SRP) present in the lake water initially prior to spiking (0.045 mg/L), and assuming no P is removed subsequent to the P additions. Results are shown for spiked lake water both without and with TPX™ (100 mg/L).
Figure 2. Adsorption isotherm showing the response between expected P concentrations in the experimental microcosm based on P spiking additions to filtered surface water from Lake Apopka (and assuming no P removal; see blue line) and the observed response in the lake water both without and with TPX™ added. TPX™ dose was 100 mg/L.

The results shown in Figure 2 illustrate that 0.1 g/L TPX™ should be sufficient to adsorb the SRP in solution following the period of stabilization. For example, equilibrium SRP concentrations were all below 0.017 mg/L for expected concentrations <= 0.108 mg/L. At higher spike levels, equilibrium SRP began to increase with the maximum equilibrium concentration (0.078 mg/L which equates to 87% removal) occurring in response to the highest spike.

Results from a TPX™ mixing experiment are shown in Figure 3. Overlying water concentrations of SRP in the microcosms after the initial stabilization period (216 hours) ranged from 0.009 to 0.021 mg/L. Resuspension of the sediments for an hour following the stabilization period resulted in an immediate increase in SRP for Sites 2 and 3 (0.042 and 0.030 mg/L, respectively measured after allowing the water to clarify and a stable interface to form). Addition of TPX™ (0.1 g/L) resulted in reducing the water column SRP concentrations to all less than 0.009 mg/L. Further resuspension did not result in any measurable release of SRP following the addition of TPX™; restated, the addition of TPX™ to the microcosms not only effectively mitigated the initial resuspension release of SRP but also was sufficient to stabilize the sediments against further net release of SRP associated with additional mixing.

Figure 3. TPX™ mixing study showing effects of mixing (sediment resuspension) and subsequent addition of TPX™ followed by a second phase of mixing on water column SRP concentrations.

The TPX™ mixing experiment was then continued to evaluate the effects of the TPX™ treatment on passive releases of P from the sediments into the overlying water column over an extended period.  After conducting the second resuspension event, the microcosms were separated into two groups (oxygenated and anoxic) and allowed to incubate under quiescent conditions for 28 days.  Results for SRP are shown in Figure 4 as a function of redox regime for each of the three lake sites, and indicate that SRP concentration for all three sites under aerobic conditions remain uniformly low, averaging 0.007 mg/L, after 28 days.  By comparison, SRP concentrations under anaerobic conditions (which are not sustained in Lake Apopka due to its shallowness and wind-wave dynamics) were substantially elevated, averaging 0.070 mg/L. 

Figure 4. Long-term incubation results from the TPX™ mixing study showing the effects of redox status on SRP release to the overlying water in the Lake Apopka sediment-water microcosms used to conduct the initial mixing experiments shown in Figure 3. The box plot shows the variation in triplicate measurements of SRP concentrations measured after an incubation period of 28 days under separate aerobic and anaerobic (N2) atmospheres.

The experimental data from the Phase 1 in situ study and the Phase 2 UF lab study presented here – which are consistent and supported by supplemental thermodynamic modeling done in parallel to this project – indicate that the application of TPX™ can effectively and permanently sequester P in both the water column and the sediment, thereby mitigating the internal loading of P in Lake Apopka. The key to using this technology towards accelerating the recovery of Lake Apopka will be predicated on properly dosing the lake such that the mitigation is not only effective in the short term but also provides sufficient sustained benefits for the sediments to further stabilize and allow for rooted macrophyte recolonization to become more fully re-established.

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