Grantee Research Project Results
Final Report: Soil Amendments for Enhanced Phosphorus Retention: Implications forGreen Infrastructure Design
EPA Grant Number: SU839456Title: Soil Amendments for Enhanced Phosphorus Retention: Implications forGreen Infrastructure Design
Investigators: Small, Gaston E , Wihlm, Spencer E , Wallace, Hannah R , Abrahamson, Jenna N , Deile, Madison P , Mahre, Erin K , Fischer, John PH , Jimenez, Ivan J , Shrestha, Paliza , Salzl, Michael T
Institution: University of St Thomas
EPA Project Officer: Page, Angela
Phase: I
Project Period: December 1, 2018 through November 30, 2019
Project Amount: $14,997
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2018) RFA Text | Recipients Lists
Research Category: P3 Challenge Area - Safe and Sustainable Water Resources , P3 Awards
Objective:
Green stormwater infrastructure (GSI) has become a national stormwater focus and is commonly being implemented in cities as part of their stormwater management strategy. Bioretention, also known as raingardens, is one of the popular forms of GSI that offers promising solutions to control pollutants (of nutrients such as phosphorus - P, and nitrogen - N) from urban storm runoff to combat eutrophication and harmful algal blooms (e.g., toxic Cyanobacteria and Microcystis) in the receiving waters. Bioretention comprises of porous soil media and vegetation, where runoff is filtered, stored, and treated by taking advantage of the different chemical sorption and biological uptake/transformation mechanisms in the soil media. The paradigm has been that these bioretention systems remove P and N from stormwater runoff and act as nutrient "sinks" on the landscape, but existing studies have shown that a significant amount of P and N is in fact being leached and exported to the effluent from the soil media used in these systems. While implementation of bioretention is happening at a rapid pace, not enough attention is paid to the different design attributes of a bioretention, especially, the soil media composition which plays a critical role in the removal capacity of pollutants.
Summary/Accomplishments (Outputs/Outcomes):
In this study, we conducted field and laboratory trials to investigate the effects of different substrates (e.g., Ca-based water treatment residuals [WTR] and/or coir) derived from waste materials as bioretention soil media (BSM) amendments to remove dissolved P and/or N from leachate and subsequently improve effluent water quality. Our field mesocosms and laboratory columns were set up to simulate bioretention filter media. Quantitative studies of BSM often use actual or synthetic stormwater to generate a suite of stormwater pollutants. Since our primary pollutants of concern were nutrients, we utilized compost in the overlying BSM as a nutrient source to the effluent following natural and simulated (in field and lab respectively) rain events. Compost is also often utilized in the bioretention media, and many state bioretention guidelines nationwide continue to recommend compost as an organic matter source despite its potential negative effects from nutrient leaching. Both WTR and coir are waste substrates-otherwise landfilled and incinerated respectively- may have the potential to be re-used on land in a sustainable manner. WTR is a waste by-product of municipal drinking water treatment plants, and coir is a by-product of the coconut industry and derived from the coconut shell.
We evaluated the efficacy of Ca-based WTR (also known as spent lime) to reduce dissolved nutrients (of PO43-, NH4+ and NO3-) in the effluent from 32 field-simulated BSM mesocosms receiving different levels of compost applications (e.g., 20%, 40%, 80% and 100% of BSM volume) in the upper layer. Half of the plots received WTR treatment in the bottom 13 cm layer, while the other half received sand as control. Where WTR layer was present, a set WTR: sand mix at 60:40 volumetric ratio was utilized. In the second part of the study, we compared nutrient reduction potential of WTR and coir from laboratory-simulated bioretention columns receiving different rates of compost application (e.g., 20% and 40% of BSM volume). The columns were layered from top to bottom with compost, sand, and WTR or coir respectively in different volumetric ratios (e.g., 5%, 10% and 30% of BSM volume). We hypothesize that WTR will reduce P concentrations due to its high demonstrated sorption potential, but its removal efficiency in terms of the fraction of dissolved P removed might decrease with higher rates of compost application. Effects of Ca-WTR on dissolved N retention is unknown; however, the cation exchange properties of WTR may allow removal of certain dissolved N species such as NH4+. Additionally, since published studies show WTR to be composed of a heterogeneous mixture of inorganic elements including heavy metals, we also measured heavy metal concentrations in the effluent to evaluate trade-offs. This research aims to identify opportunities for future improvement of GSI by providing better understanding of what kind of soil media additives in these systems might improve nutrient retention performance.
Results show WTR to be an effective substrate amendment to BSM for removing dissolved P from leachate, thereby increasing the effluent quality for this pollutant. For the most part, while effluent P concentration did not vary significantly with increasing compost levels with WTR present, it increased significantly with increasing compost levels without WTR (Figure 1). This indicates the ability of WTR to maintain a relatively high performance for P removal even at higher amounts of loading. WTR also reduced effluent volume in all treatments except for the 20% compost treatment. Reduction in effluent volume has been observed in studies which had utilized Al-based WTR as a result of increased soil pore size distribution and soil water holding capacity. Contrary to our hypothesis, P removal efficiency of WTR did not decrease with increasing compost application. Treatments with WTR also led to reduction in effluent NH4+ concentrations, though the reduction was only significant in the treatment with 20% compost (Figure 1). NH4+ mass retention of up to 90% was observed due to WTR in the 80% compost treatment. On the other hand, WTR increased NO3- concentrations in the effluent at all compost levels by up to 5 times (Figure 1). This is not surprising as NO3- is not known to undergo sorption reactions, but instead other mechanism like microbial denitrification plays a critical role in its removal. WTR was not a source of heavy metal pollution to the effluent. Dissolved heavy metal concentrations of Cu, Cd, Pb, Al, Fe, Ni and Zn in the effluent were below the threshold of EPA drinking water standards.
Unlike WTR, coir was not an effective adsorbent for PO43-. When same amounts of coir and WTR was used, the treatment with WTR always appeared to have lower NH4+ but higher NO3- than coir, though never significantly for either nutrient. Therefore, the ability of coir to affect NH4+ and NO3- concentrations any differently than WTR cannot be concluded, nor can the lab study specifically attest to the benefits of either soil amendments for NH4+ and NO3- retention, as they did not significantly vary from control (sand).
Figure 1. Mean leachate nutrient concentrations measured for plots with (+) and without (-) WTR at different compost application levels in field-based mesocosm study. Terms with asterisks denote significant differences between plots with and without WTR at each volume of compost application. A single asterisks (*) is significant at the p < 0.05 level, double asterisks (**) at p < 0.001. Varying letters indicate significant differences among compost volumes within each WTR+ and WTR- plots (p < 0.05). Errors bars indicate ±1 standard error.
Conclusions:
In conclusion, field-applied WTR showed good retention capacity for NH4+, and heavy metal leaching was not a concern, which further validates its application in bioretention systems, although there is limitation in generalizing results due to variation among different sources of WTR. Compared to proprietary engineered media substrates which can be expensive to acquire, WTR is cost-effective and can be easily acquired from local drinking water treatment plants at no cost. Use of WTR for environmental pollution mitigation is beneficial from a waste recycling perspective and reduces burdens on landfills. As traditionally applied sand (or any coarse-textured soil) by itself has poor sorption capacity, a need for modified bioretention media containing substrates with high adsorption properties must be studied to identify opportunities for future improvement of these infrastructures.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 7 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Shrestha P, Salzl MT, Jimenez IJ, Pradhan N, Hay M, Wallace HR, Abrahamson JN, Small GE. Efficacy of spent lime as a soil amendment for nutrient retention in bioretention green stormwater infrastructure. Water 2019;11(8):1575. |
SU839456 (Final) |
Exit Exit |
Supplemental Keywords:
Bioretention, Water treatment residuals, Spent lime, Coir, Compost, Phosphorus, Nitrogen, Water qualityRelevant Websites:
St. Thomas Biology Receives Environmental Protection Agency Grant Exit
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.