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.
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.
WTR is a by-product generated from the addition of alum or ferric salts used in the coagulation–flocculation process during drinking water treatment. Different sources of WTR will have different properties, including adsorption properties, depending on the amount of iron and aluminum coagulants in the source material. Major components of WTR are soil separates, organic materials, and Al and Fe hydrous metal (hydr)oxides, depending on the metal salt used for coagulation. Alum [Al2(SO4)3 × 14H2O] is the most commonly used coagulant in the United States and Canada (Elliott et al., 1990); the iron salts FeCl3 and Fe2(SO4)3 are also used (Ipolito et al., 2011).
In addition to the properties of the source material, WTRs carry the signature of the water being treatment. Ipolito et al. (2011) reported that trace metal concentrations appear to have decreased over time, speculating this is due to reduced concentrations in source waters resulting from increased regulatory controls.
This section provides information on the physical and chemical properties of spent lime.
Geotechnical behavior of WTR material is a function of the physical and chemical composition of its solid contents and the type, amount, and chemical nature of the pore fluid. As the structure and the solids, content of the WTP residual change, interactions between the solid and the liquid phases such as cementation takes place. This affects geotechnical properties such as compaction, shear strength and permeability. Change in the water content alters the floc structure and the particle sizes of the solids but will also change the ion concentration and complex formation within the residuals.
Ipolito et al. (2011) describe Al- and Fe-WTRs to be amorphous in nature, of various shapes and sizes, and highly porous. The following tables provide a summary of physical characteristics of WTRs. Some notes from the information.
Particle size distribution of aluminum and iron water treatment residuals.
Link to this table
|Study||Site/condition||% fines||D10 (mm)||D50 (mm)|
|Basim||Site 1 - dry||48||0.006||0.024|
|Basim||Site 2 - wet||100||0.008||0.0034|
|Basim||Site 2 - dry||30||0.014||0.08|
|Basim||Site 3 - wet||65||0.0065||0.023|
|Basim||Site 3 - dry||27||0.0027||0.16|
|Xu et al.||Al-WTRs||23.7|
|Xu et al.||Fe-WTRs||15.5|
Physical properties of aluminum and iron water treatment residuals.
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|Study||Site/condition||Specific surface (cm2/g)||Liquid limit||Plastic limit (%)||Specific gravity (g/cm3)||Total volatile solids (%)||Bulk density (g/cm3)||Dry density (g/cm3)||Effective cohesion (kPa)||Effective angle of shearing resistance (0)||Compression index||Ksat (cm/s)|
|Basim||Site 1 - wet||2820||371||228||1.66||2.2-4.5||25-32|
|Basim||Site 1 - dry||282||107||Not plastic||2.5||2.2-4.5||25-32|
|Basim||Site 2 - wet||23800||329||200||2.11||38-43||28-35|
|Basim||Site 2 - dry||510||37||Not plastic||2.75||38-43||28-35|
|Basim||Site 3 - wet||2544||690||20||1.93||7.0-7.5||27-33|
|Basim||Site 3 - dry||318||151||Not plastic||2.52||7.0-7.5||27-33|
|Various studies||1.87-2.3||0.94-0.97||10-6 – 10-7|
The following table summarizes chemical data for aluminum and iron water treatment residuals (WTRs). Soil reference values (SRVs) and soil leaching values (SLVs) are included and discussed in the next section. The values shown in the adjacent table represent medians of values from the literature. The dataset used for this analysis is here - File:WTR chemical data jan 13 21.xlsx.
Chemical properties of water treatment residuals. Values are medians for all data collected from the literature.
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|Chemical||Type of water treatment residual||Tier 2 Soil Reference Value (mg/kg)||Soil Leaching Value (mg/kg)|
|Aluminum and iron||Calcium|
nd=not detected. Reporting limits were below SRVs and SLVs.
SLV assumes 3 foot separation from groundwater and a media depth of 3 feet.
Numbers in bold exceed one or more of the risk criteria.
References: Turner et al. 2019; Elliott et al 2002; Lang, 2009; Shrestha et al, 2019; Elliott et al., 1990; 2009; EPA, 2011; Barr Engineering, 2014; Wang et al., 2014
Concentrations of arsenic, manganese, and vanadium in WTRs exceed Tier 2 Soil Leaching Values (SLVs). The Tier 2 Soil Reference Value (SRV) was exceeded for arsenic. When mixed with a typical bioretention media consisting primarily of a sand-compost mixture, it is unlikely SLVs or SRVs would be exceeded. However, the chemical characteristics of any given WTR likely reflect the chemistry of the source water being treated. Thus, waters with elevated concentrations of specific chemicals, particularly manganese or arsenic, could lead to elevated concentrations of these chemicals in the WTRs.
A concern with the application of WTRs is the potential hazard of leached Al/Fe (Basta et al., 2000). This may damage the aquatic environment when leachate enters surface water bodies (Codling, 2008). It becomes more critical at low soil pH values (pH < 5.2), whereby Al becomes more soluble (Foy, 1996). The average pH of WTR is 6.5, while typical stormwater runoff is at neutral pH (6.5–7.5, Lee et al., 2015). In this case, the potential of Al leaching would not be prominent according to the distribution of alum species under neutral pH range. Similar research supported that organic-complexed Al forms were the dominant form detected in the supernatant of WTR samples under the neutral pH condition, rather than Al3+ (Agyin-Birikorang and O'Connor, 2009). Another study conducted on grass cropping grown with WTR soil showed no signs of Al toxicity in the plants studied (Oladeji et al., 2009). However, there are still limited studies on the impact of leaching phenomenon especially on WTRs with metal-modified surface.
Due to their porosity and amorphous nature and the presence of Al and Fe (hydr)oxides, WTRs can adsorb large quantities of anions. Anion sorption is a function of the WTR particle size, charge, and surface area (Ipolito et al., 2011). The incorporation of WTRs may also affect the physical properties of soil or engineered media. The effects of WTRs on chemical and physical characteristics of soil or media are presented below.
Of primary concern for stormwater practitioners is the ability of WTRs to retain phosphorus, particularly dissolved forms of phosphorus. 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 WTRs with different physical and chemical compositions and P sorption capability.
Though not a fully comprehensive review, the following studies provide an indication of the range of phosphorus adsorption by Al- and Fe-WTRs.
Initial P sorption to the external WTR surfaces is fast. Wagner et al. (2008) shook 1.0 gram of Al-WTR with 25 milliliters of 10 mg-P/L for up to 24 h. The authors observed a 50 percent reduction in total dissolved P within 2 min, 90 percent removal in 15 min, and nearly 100 percent removal after 24 h. Other researchers have shown similar rapid sorption (Leader et al., 2008, Makris et al., 2005).
P sorption by Al-WTRs is strong and practically irreversible. Makris et al. (2005) suggested that P sorption with time was kinetically biphasic (i.e., a quick sorption phase followed by longer-term sorption), with the slower sorption kinetic phase being associated with P diffusion into micropores. 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 (Leader et al., 2008; Agyin-Birikorang and O’Connor, 2007; Ippolito et al., 2003; Makris et al., 2005; Makris et al., 2004).
Studies have shown that the phosphorus removal performance (adsorption) is strongly dependent on stormwater runoff pH (Lee et al., 2015). pH dependence may be due to different solubility of Al and Fe and changes to the surface charge of Al-hydroxide at different solution pHs (Kim et al., 2003). Under neutral pH conditions commonly observed in stormwater runoff (6.5–7.5 (Lee et al., 2015)), minimal pH effect on P adsorption capacity was observed and minor fluctuations in pH would not affect P adsorption performance significantly (O'Neill and Davis, 2012).
Other factors that may affect sorption, such as flow concentrations, rate, and volume, have not been extensively studied.
Overall, median removal efficiencies from 86 percent to 99 percent for WTRs containing at least 10 percent WTR by volume can be expected for Al- and Fe-WTRs .
There are limited studies on the effects of Al- and Fe-WTRs on soil physical and hydraulic properties. There are several studies on the effects of WTRs on plant response. Plant response may be used as a surrogate for identifying adverse impacts to soil properties, but most of these studies focused on potential chemical effects, such as aluminum toxicity, phosphorus defieciency, and release of heavy metals.
Ulen et al. (2012) observed no change in soil aggregate stability when applied to clay loam, silty loam and loam sand soils in greenhouse pot experiments. Herrera Environmental Consultants, Inc. (2016) summarized several studies that employed WTRs as an additive to a sand-compost mixture (typical addition of WTR was 10 percent). Generally hydraulic conductivity (Ksat) rates for were lower than the other treatments due to the addition of fine–textured water treatment residuals. Typical reductions in conductivity were about 40 percent from treatments with just sand and compost. WTRs generally had higher water holding capacity. Mahmoud et al. (2015) studied the effects of adding compost and WTRs on aggregate stability, mean weight diameter, pore size distribution and dry bulk density of a salt-affected soil. The addition of compost and water treatment residuals had significant positive effects on the studied soil physical properties, and improved the grain yield of wheat. Bugbee and Frink (1985) observed improved soil aeration with the addition of alum. Kim et al. (2002) observed that addition of alum to soil increased the buffer capacity to acidity, hydraulic conductivity, waterholding capacity, and phosphate adsorption of the soil and decreased the bulk density and mobility of small particles. Howells et al. (2018) studied effects of WTRs on earthworm populations and observed no adverse effects except at low pH (<5). Dayton and Basta (2001) examined the beneficial use of drinking water treatment residuals (WTRs) as a potential source of topsoil and observed no significant different between WTR treatments and topsoil.
The effects of WTRs on soil physical and hydraulic properties appears to be primarily related to the finer particle size of the WTRs. In a standard bioengineered media mix primarily consisting of sand and compost or other organic amendment, addition of WTRs will decrease hydraulic conductivity and increase porosity.
The primary concerns of adding Al- or Fe-TRs to soil are the potential for increased aluminum toxicity (for Al-WTRs), reducing available phosphorus, and potential toxicity from chemicals in the WTRs, such as arsenic and selenium. These concerns can be addressed by ensuring a pH of 6.0 or greater in the engineered media and testing the source material for heavy metals if the source water for treatment contains chemicals of potential concern. Phosphorus deficiency is not a concern if WTRs are added in appropriate quantities, since the purpose for adding WTRs is to remove phosphorus from influent stormwater runoff.
For studies that address the above concerns for pH and source material composition, addition of WTRs has no effect or can improve plant response. A few studies are summarized below, though there are numerous additional studies in the literature..
No standards or classification were found in the literature.
Presently, only dewatered WTRs are utilized in stormwater and other land applied applications. These WTRs are considered to be aged. Birikorang and O'Connor (2009) studied the aluminum reactivity of freshly-generated Al-WTRs and observed that reactivity decreased with time. They estimated that reactivity would continue to decrease over a 6 month period for field-applied WTRs. They recommended that application of freshly-generated Al-WTRs with similar physicochemical characteristics as the one utilized for their study should be avoided.
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).
There is limited information on the safety and handling procedures for Al- and Fe-WTRs. These WTRs are generally not considered hazardous. Materials handling operations may generate fugitive emissions, and these emissions can be managed by installing a proper ventilation system or dust suppression system. Dechlorination may be necessary if the water treatment utilized chlorine as a disinfectant and the WTRs are placed in an environment where they will contribute directly to receiving waters (EPA, 2011).
Because WTRs are a byproduct or residual material from treatment of drinking water, they are considered to be a sustainable source for use in stormwater applications.