# High Rate Stormwater Treatment for Water Reuse and Conservation--Review
## 3 Removal of Nutrients and Heavy Metals Removal Using Different Filter Materials  
Introducing suitable filters or adsorption media is an effective approach to enhancing organic matter, nutrients, and heavy metals removal. Nutrient Removal: Studies have investigated several filtering materials--namely, calcite, zeolite, sand, and iron filings--to evaluate their effectiveness in removing nitrate and phosphate from simulated stormwater over a 24 h period with varying initial concentrations [33]. Nitrate removal rates ranged from 39-65% for calcite, 42-77% (zeolite), 40-70% (sand), and 74-100% (iron filings). For phosphate, removal efficiencies were 35-41% for calcite, 59-100% (zeolite), 49-100% (sand), and 73-100% (iron filings). Nitrate reduction primarily occurs through electrostatic adsorption, while iron filings also promote removal through electrochemical reduction, ligand complexation, and precipitation. Phosphate removal relies largely on electrostatic adsorption for all materials, though precipitation may become dominant at higher concentrations, except with calcite. * Iron filings proved to be the most effective medium, achieving maximum removal rates for both nitrate (73-100%) and phosphate. * A maximum nitrate adsorption capacity of 16 ug/g of dry material was reached by calcite, zeolite, and sand. Iron filings showed a higher capacity, surpassing 30 ug/g for nutrient adsorption. Figure 1: Schematic of biofilter system. * Phosphate adsorption was typical for calcite, while zeolite, sand, and iron filings had higher adsorption up to concentrations of 0.5-1 mg/L. Beyond this limit, precipitation played a larger role. Heavy Metals: Batch tests using synthetic stormwater containing varying concentrations of individual metal pollutants were conducted to evaluate the removal effectiveness of four potential absorption inorganic filter materials (calcite, zeolite, sand, and iron filings) on six common toxic metals (cadmium (Cd), copper (Cu), lead (Pb), nickel (Ni), chromium (Cr), and zinc (Zn)) [34]. The efficiency of metal removal was influenced by the metal type, its concentration, and the choice of filter material. The highest removal efficiencies recorded were 95-100% with calcite, zeolite, and iron filings for Cd, Cu, Pb, and Zn; 90% (zeolite) for Ni; and 100% (iron filings) for Cr. As no single material achieved maximum removal for all metals, enhancing the simultaneous removal of various heavy metals may be achieved by combining multiple filter materials. Stormwater runoff significantly contributes to heavy metal pollution, transporting contaminants such as Cd, Cr, and Ni, which can adversely affect ecosystem health and human wellbeing. A variety of filter materials were tested for their effectiveness in adsorbing heavy metals from stormwater, typically utilizing methods that are chemically intensive or energy consuming. Byproducts from drinking water purification processes referred to as water treatment residuals (WTR) which are aluminum based are non-toxic and present a cost-effective solution for removing heavy metals. However, WTR's low permeability necessitates blending it with materials like sand and carbon to enhance water flow, which in turn diminishes its capacity to adsorb. In response, a granulated WTR sorbent was created through a low-energy, organic method, demonstrating significant potential as a sustainable filter medium for heavy metal removal from stormwater [35]. Five inexpensive and easily accessible sorbents were evaluated for their ability to remove polycyclic aromatic hydrocarbons (PAHs) like pyrene, phenanthrene, acenaphthylene, and naphthalene from synthetic stormwater. The materials tested included three unprocessed waste products--waste tire crumb rubber (WTCR), coconut coir fiber (CCF), and blast furnace slag (BFS)--as well as two modified substances: biochar (BC) and iron-coated biochar (FeBC) [36]. The findings showed that carbon-based sorbents had high PAH removal efficiency, while BFS was comparatively less effective.