Aluminum and iron water treatment residuals and application in stormwater 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. 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.

Properties of aluminum and iron water treatment residuals

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.

Physical properties

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 ranges widely depending on the source of the WTR
• Dry WTRs are currently used in stormwater applications. Physical properties vary considerably between wet and dry WTRs. Wet WTRs are highly plastic and not recommended for stormwater applications.
• The tables provide a summary of a limited dataset. Additional information may be found in various studies for specific applications.

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-wet	86	0.004	0.04
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
Gersten	Al-WTRs	31
Gersten	Fe-WTRs	15

Physical properties of aluminum and iron water treatment residuals.
Link to this table

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)
Turner			100-550	80-250	1.8-2.2	10-60	1.0-1.2	0.12-0.36	0	28-44
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

Chemical properties

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.
Link to this table

Chemical	Type of water treatment residual		Tier 2 Soil Reference Value (mg/kg)	Soil Leaching Value (mg/kg)
	Aluminum and iron	Calcium
Aluminum	51000	2211	100000
Ammonia		nd
Antimony	nd	1	100	10.8
Arsenic	35	4.3	20	11.6
Barium	205	30.3	18000	3370
Beryllium	1.55	nd	230	5.44
Boron		nd	47000	124
Cadmium	0.8	0.2	200	17.6
Calcium	16700	494367
Carbon	172500	114000
Chromium III	52.5	1.6	650	200000
Copper	48	2	9000	1400
Iron	51000	956.5	75000
Kjeldahl nitrogen		nd
Lead	49.5	2.8	700	5400
Magnesium	2300	8530
Manganese	1950	73	8100	260
Mercury	nd	nd	1.4	6.58
Molybdenum	2	0.2		32.2
Nickel	32.5	1.6	2500	352
Nitrogen	8350	330
Phosphorus	2995	74
Potassium	5550	846
Selenium	nd	1	1300	5.28
Silver	nd	1	1300	15.7
Sodium	855	335
Strontium	45	274	100000	5620
Vanadium	68	3	250	8
Zinc	183	5.4	75000	6010

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

Potential contaminants in aluminum and iron water treatment residuals

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.

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

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.

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

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.

• Ippolito et al. (2003) used a particle size range of 0.1 to 0.3 millimeters, showing that Al-WTR retained 12,500 mg-P/kg after a 1 day shaking period.
• Makris et al. (2004) observed P sorption to be 7700 and 2000 mg/kg over 1 day for Al- and Fe-WTRs, respectively (particle size < 2mm). After 80 days of shaking the adsorption was approximately 10,000 and 9100 mg-P/kg for Al- and Fe-WTRs, respectively.
• Dayton and Basta (2005a) observed sorption ranging from 10,400 to 37,000 mg-P/kg for Al-WTRs less than 0.15 mm in size over a 6 day shaking period.
• Yang et al. (2006) used an Al-WTR particle size of 0.063 mm and determined P sorption as a function of pH (4.3, 6.0, 7.0, 8.5, and 9.0). The Al-WTR P sorption capacity was maximized near 3500 mg-P/kg at a pH of 4.3 and 6.0 and decreased with increasing pH (700 mg/kg at pH 9). Babatunde et al. (2008) used a 48 hour shaking period and found that Al-WTR (1.18 millimeter mean particle size) adsorbed 4520 mg-P/kg at a pH of 4.0, whereas P sorption decreased with increasing pH up to 9.0 (1740 mg-P/kg).
• Razali et al. (2007) also performed P batch adsorption research using Al-WTR of less than 2.36 millimeter in size. They observed a sorption capacity of 10,200 mg-P/kg for PO₄ at a solution pH of 4.0. Fu et al. (2008) optimized Al-WTR P removal from solution, observing 99.6 percent removal efficiency for orthophosphate, or 2990 mg-P/kg, by holding the pH at 4.2 and using a WTR particle size of 0.125 millimeter.

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 [1].

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

• Nitrogen. Several studies show Al- and Fe-WTRs to be effective at retaining nitrogen, but most studies have been conducted using wastes or source waters with very high initial nitrogen concentrations. Observed removal rates are approximately 80 percent for ammonia-N and 50 percent for total nitrogen, depending on source concentrations and charatceristics of the WTRs (Cheng et al., 2017; Hu et al., 2012; Ren et al., 2020; Liang, 2011). Liang (2011) reported more than 99 percent reduction of nitrate for wastewater, with the mechanism being denitrification associated with high concentrations of bioavailable organic carbon in the wastewater.
• Metals. Chiang et al. (2012) studied the sorption capacity of WTRs for As(V), Cd²⁺, Pb²⁺ and Zn²⁺ from synthetic solutions. The maximum sorption capacity, estimated by Langmuir equation fitting, ranged between 0.5 and 0.6 mmol/g, except for cadmium, which had a maximum sorption capacity of 0.19 mmol/g. At the highest WTRs dosage (250 mg/g), concentrations of the cationic contaminants decreased by at least 80%, while approximately 40% removal was obtained with 50 mg/g dosage. Wolowiec et al. (2019) provided a literature review of metal sorption by WTRs and cited the following sorption capacities (mg/g): AsIII 9.0-59.7; AsV 9.2-49.9; CrIII 19.2; CrVI 10.9; SeIV 2.1-22.11; SeVI 1.9-11.05; Pb 53.9; Hg 79.0; Cd 5.3; Co 17.3; Ni 11.6. Ghorpade and Ahammed (2018) conducted batch tests with simulated wastewater in single- and multi-metal solutions of Cu(II), Co(II), Cr(VI), Hg(II), Pb(II) and Zn(II). Removal of cationic metals such as Pb(II), Cu(II) and Zn(II) increased with increase in pH while that of anionic Cr(VI) showed a reduction with increased pH values. All metals were effectively removed, with complete removal of copper and 78-92% removal for chromium. Turner et al. (2019) conducted a literature review for metal sorption by WTRs. Their review indicated effective removal of arsenic, cadmium, copper, lead, nickel, and zinc by WTRs.
• Organics. There is limited information on sorption of organic chemicals by Al- and Fe-WTRs. Leiva et al. (2019) observed Langmuir sorption isotherms (L/kg) of 3.36, 88.0. and 3275.4 for Matalaxyl-M, Paclobutazol, and Imidacloprid, respectively. Punimaya et al. (2016) showed that Al-WTR is very effective in immobilizing tetracycline (TTC) and oxytetracycline (OTC). Caporale et al. (2013) observed high organic carbon concentrations on WTRs resulting from sorption of organic material from the source water (243 and 155.4 g kg−1 in the Al- and Fe-WTR samples, respectively).
• 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.

Effect of aluminum and iron water treatment residuals on soil physical and hydraulic properties

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.

Effects of aluminum and iron water treatment residuals on soil fertility, plant growth, and microbial function

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..

• Juan et al. (2017) observed no negative effects of al- and Fe-WTRs on algal survival and growth.
• Juan et al. (2016) observed no negative impacts to the bacteria Aliivibrio fischeri.
• Wang et al. (2014) observed no increase in metal lability under anaerobic conditions, but recommended further study since some metals showed increased lability following anaerobic incubation.
• Xie and Kurosawa (2016) observed that an organic amendment such as compost was necessary for growth of vegetables in soil amended with WTRs. Phosphorus deficiency was a limiting factor in plant growth in treatments employing only WTRs.
• Heil and Barbarick (1989) found that low (5 g kg−1) application rates of Al and Fe WTRs to soils increased yield of sorghum-sudangrass (Sorghum bicolor L. Moench), attributed to their Fe contributions and pH-increasing abilities.
• During trials of co-application of WTRs and vermicomposts, Ibrahim et al. (2015) found that a 2:1 ratio of WTR to vermicompost with a combined application rate of 5 g kg−1 resulted in greater wheat yields than all other treatments tested
• WTRs were found to serve as a better planting medium for peppers (Capsicum annuum ‘Takanotsume’) than granite parent material–based soils when both were amended with 10% additions of compost (Park et al., 2010).
• Tay et al. (2017) reported reductions in Chinese cabbage (Brassica pekinensis (Lour.) Rupr.) biomass with WTR application rates above 2% (w/w) despite fertilizer addition.
• Sotero-Santos et al. (2005) studied Fe- and Al-WTR survival and reproduction toxicity effects on water flea (Daphniasimilis) performed in 25, 50, or 75% (wt/vol) WTR-diluted systems. In general, Fe- and Al-WTR did not cause acute toxicity, but long-term Fe-WTR exposure caused some mortality and decrease in reproduction potential. The Al-WTR caused reductions in reproduction.
• Ippolito et. al (2011) stated "The limited amount of research regarding the effects of WTR on microfauna, insects, and animals are optimistic. Land application of WTR may trigger a perceived P deficiency response in microorganisms, but microbial community structure is not affected. Long-term exposure to Fe-based WTR may cause increased water flea mortality, but a correlation between mortality and WTR characteristics could not be made. It has also been shown that no risk is associated with Al-WTR consumption by grazing herbivores. Although these findings are enlightening, more research is needed to ensure that terrestrial and aquatic ecosystem health is maintained across sites receiving WTR applications."
• Garau et al. (2014) found that Fe WTR amendments (3% w/w addition rate) led to increased culturable heterotrophic bacteria and actinomycetes while having the opposite effect on heterotrophic fungi. Overall, they found the soil microbial biomass remained constant.

Standards, classification, testing, and distributors

Aluminum and iron water treatment residuals standards

No standards or classification were found in the literature.

Effects of aging

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).

Storage, handling, and field application

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).

Sustainability

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.