Aluminum and iron water treatment residuals and application in stormwater

This page provides information on aluminum and iron in water treatment residuals. While providing extensive information on water treatment residuals, there is a section focused specifically on stormwater applications for aluminum and iron in water treatment residuals.

Overview and description

Water treatment residuals are the by-products of water treatment for drinking water. Drinking water treatment residuals are primarily sediment, metal (aluminum, iron or calcium) oxide/hydroxides, activated carbon, and lime removed from raw water during the water purification process (Agyin-Birikorang et al., 2009). Aluminum sulphate (commonly known as alum), ferric chloride and lime are added as flocculants in the water treatment process. This process results in the generation of vast quantities (generally between 10 and 30 mL of WTRs for every litre of water clarified) of a sludge-like waste (or by-product) known as water treatment residuals (WTRs), which require an outlet for their disposal or end use (Dassanayake et al. 2015). The most common method of disposing these WTRs is by sending them to landfills. However, WTRs have many physical and chemical properties that lend them to potential positive reuse routes.

Sources of material, including variants

WTR is a by-product generated from the addition of alum or ferric salts used in the coagulation–flocculation process during drinking water treatment, and contains minimal amount of toxic substance (Ippolito et al., 2011) Water treatment residuals are widely available and generally low cost. The use of WTR in bioretention media will prevent this material of ending up in a landfill. WTR was shown to efficiently remove more than 90% of phosphorus from wastewater (Lee et al., 2015) Different sources of WTR will have different adsorption levels due to the amount of Iron and Aluminum coagulants using. The adsorption of phosphorus in stormwater is correlated to the amorphous hydrous metal oxide content in the WTR due to their strong affinity for oxyanions like phosphate.

Phosphorus saturation index (PSI) of two WTRs were relatively low (Fe-WTR: 0.01 and Al-WTR: 0.02). When PSI was less than 1, there would be excess Fe and Al for binding of P or the presence of low P availability, which suggested that WTR has high P adsorption capacity (Elliott et al., 2002).

97% removal with WTR, 7-21% without WTR used in Babatunde et al. (2009)'s study had excellent P removal capacity (23.1 mg P/g)

Effects of aluminum and iron water treatment residuals on physical and chemical properties of soil and bioretention media

Effect of aluminum and iron water treatment residuals on retention and fate of phosphorus

Shrestha et al. (2019) Efficacy of Spent Lime as a Soil Amendment for Nutrient Retention in Bioretention Green Stormwater Infrastructure

• The relative effectiveness of WTRs in reducing soluble P depends on several factors, including source water characteristics, water treatment methods, and length of residual storage time prior to land application. Each water treatment facility uses unique source water and different treatment chemicals and processes, producing WTR with different physical and chemical compositions and P sorption capability.
• P sorption by Al-WTRs is practically irreversible. Once P reaches the WTR microsites, the adsorption is very strong, and little or no desorption is likely. Thus, once immobilized by the WTR particles, P is likely irreversibly bound, barring destruction of the WTR particles associated with extremely low soil pH values.
• Nair and Harris (2004) developed a technique [soil phosphorus storage capacity (SPSC)] to predict the amount of P a soil can sorb before exceeding a threshold soil equilibrium concentration. The SPSC values are calculated from oxalateextractable P, Fe, and Al concentrations of a soil as: SPSC (mgP/kg)=(0.15 - PSR)* (Alox + Feox)*31

PSR=Phosphorus sorption ratio = (Pox)/(Alox+Feox) SPSC values can indicate the risk arising from P loading as well as the inherent P sorption capacity of the soil. The SPSC values range from negative values (for highly P-impacted soils with no remaining P retention capacity) to positive values (for less P-impacted soils, excess P retention capacity)

• Freshly produced WTRs are in a liquid state with high water content (2–4% solids) making them expensive to transport and challenging to handle, particularly in the volumes generated by large-scale water treatment plants (i.e. > 1000 t year−1 of liquid WTRs produced); hence, dewatering or thickening processes are commonly employed (Dassanayake et al. 2015). Generally, after the full mechanical dewatering process (e.g. Fig. 2), the solids content of these WTRs increases to between 17 and 35% solids (Dassanayake et al. 2015).
• The sorption capacity of WTRs is a function of particle size, surface area and surface charge (i.e. WTRs with a smaller mean particle size can sorb greater quantities of P (Yang et al. 2006a))
• 97% adsorption of Phosphorus
• Negative effects on fish, invertibrates.
• Al toxicity generally a concern when soil pH below 5 because of increased solubility.

Effect of aluminum and iron water treatment residuals on retention and fate of other pollutants

• Nitrogen.
• Metals.
• Organics.
• Bacteria and viruses. For recycled materials such as WTR and biochar, relatively good reductions of pathogen have been observed, but were still unable to meet the stormwater criteria of >1.5 log. Findings from existing literature have shown robust technical feasibility and promising evidence that metal surface modification on WTR could enhance its bacterial removal capacity when applied to stormwater runoff treatment.
• Dissolved organic carbon.
• Greenhouse gas emissions.

Standards, classification, testing, and distributors

Distributors

Caution: The Minnesota Pollution Control Agency does not endorse specific distributors or products

Effects of aging

High phosphorus adsorption capability of WTR would lead to an increase of P concentrations in soil and would gradually reach the media saturation after long-term application. Bayley et al. (2008) studied the long-term effects of WTR. It was demonstrated that the WTR maintained its capacity to immobilize the significant inorganic P and remained stable after 13 years application. Similarly, the study reported that WTR reduced total labile P in runoff by 60% compared with control after 7.5 years (Agyin-Birikorang and O'Connor, 2009).