Aluminum and iron water treatment residuals and application in stormwater Revision as of 20:49, 14 January 2021 by Mtrojan (talk | contribs) (→‎Effect of aluminum and iron water treatment residuals on retention and fate of phosphorus)(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff) 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.
• 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.

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

• Hydraulic conductivity (Ksat) rates for the 60sd/30comp/10wtr treatment were generally lower than the other treatments due to the addition of fine–textured, water treatment residuals (WTR). WTR may also have a high water holding capacity

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

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.

Standards, classification, testing, and distributors

Distributors

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

• One of the most cost-effective and sustainable additives is WTR, which is locally sourced and is the only additive tested that is a low-energy byproduct of another industrial process. However, based on the results from the WSU Mesocosm Study, this additive actually decreased system performance. This result was inconsistent with at least one other study that indicate WTR can improve total phosphorus removal (Lucas and Greenway 2011).

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).