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

Overview and description

Water treatment residuals are the by-products from the coagulation and flocculation phase of drinking water treatment. These 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). Research has shown that Aluminum (Al), Iron (Fe) and Calcium (Ca) based water treatment residuals have the potential to improve water quality treatment in bioretention systems. Due to the large number of water treatment facilities that use metal salt during the the water treatment process, water treatment residuals are widely available. Water treatment residuals are also generally available at a low cost.

Water treatment residual benefits may include but are not limited to the following.

• WTRs reduce soluble phosphorus (P) in soils, runoff, and land-applied organic wastes
• WTR use in bioretention media is a sustainable alternative to disposal of WTRs at a landfill
• WTR use in bioretention media is a cost-effective media material
• WTR improves water holding capacity of soil

Applications for water treatment residuals in stormwater management

Water treatment residuals have potential applications for stormwater management. Below is a brief summary.

• WTRs can be very effective in reducing soluable phosphorus
• WTRs increases water holding capacity of soil
• WTRs may reduce the bulk density and improve saturated hydraulic conductivity in compacted soil

Properties of water treatment residuals

This section includes a discussion of chemical and physical properties of water treatment residuals, and potential contaminants in water treatment residuals.

Chemical-physical properties of water treatment residuals

Water treatment residuals have a highly variable physical and chemical structure.

Noguera et al. (2003) varied particle size of coir dust, studying the properties of coir passing through sieves 0.125, 0.25, 0.5, 1.0, and 2.0 mm in diameter. They observed the following.

• As particle diameter increases, air content increased and water holding capacity decreased
• Electrical conductivity and micro-element concentrations were greatest in the smallest diameter coir
• Bulk density decreased from 0.122 to 0.041 g/cm³ as particle size increased from <0.125 to >2 mm
• Pore space increased from 92.3% to 97.3% as particle size increased from <0.125 to >2 mm
• Water holding capacity (ml/l) decreased from 855 to 165, with the greatest change occurring with 0.5-1 mm particles
• Shrinkage (volume loss on drying) decreased as particle size increased (38% to 15% as particle size increased from <0.125 to >2 mm)
• Nutrient availability decreased with increasing particle size, but there were no significant differences between 0.125 and 2 mm. There was a large increase for the smallest particle size.

Based on generally recommended plant specifications, the researchers concluded the 0.25-0.5 mm size appears most suited for plant growth, with some addition of larger particles recommended. Abad et al. (2005) similarly concluded that a mix of particle sizes is likely to be optimum for use of coir as a plant medium.

Another factor affecting chemical properties of coir are the conditions under which it is prepared. In particular, if soaking in a saline solution is used in the preparation of coir, concentrations of potassium, sodium, chloride can be very high and may interfere with plant growth.

The following table summarizes data from the literature on physical and chemical properties of coir. Some general conclusions include the following.

• Coir is slightly acidic but not as acidic as peat
• Available nitrogen, phosphorus, calcium, magnesium, iron, copper, and zinc are low, while sodium, chloride, and potassium are high, particularly if the coir was prepared in a saline solution
• Coir has a very high water holding capacity
• Coir has a high germination index compared to compost (Lodolini et al., 2017)
• Coir dust does not collapse when wet or shrink excessively as it dries (Cresswell)

Chemical and physical properties of coir.
Link to this table

Property	Range found in literature1	Median value from literature
Total phosphorus (% dry wt)	0.036 - 0.41	0.036
Total nitrogen (% dry wt)	0.24 - 0.5	0.45
Total potassium (% dry wt)	0.4 - 2.39	0.819
Total carbon (%)	42 - 49	47.1
Total hydrogen (%)		4.4
pH	4.9 - 6.9	5.9
Cation exchange capacity (cmol/kg)	31.7 - 130	50
Electrical conductivity (ds/m)	39 - 2900	582
Total calcium (%)	0.18-0.47	0.40
Total magnesium (%)	0.11-0.47	0.36
Total copper (mg/kg)	3.1-10.3	4.2
Total zinc (mg/kg)	4.0-9.8	7.5
Total manganese (mg/kg)	12.5-92	17
Bulk density (g/cm3)	0.025 - 0.132	0.06
Water holding capacity (% by wt)	137 - 1100	566
Total pore space (%)	85.5 - 98.3	95.2

Primary references for this data:

• Cresswell
• Abad et al., 2002
• Abad et al., 2005
• Asiah et al., 2004
• Kumar et al., 2010
• Lodolini et al., 2017
• Shrestha et al., 2019

Potential contaminants in water treatment residuals

The largest source of potential contamination from Al-based or Fe-based water treatment residuals is the presence of the aluminum or iron itself which is essential for the phosphorus removal benefit. These metals have the potential to cause hazards from leaching. Excess aluminum in the environment is known to have negative impacts on aquatic environments. There are also risks to plants and the soil biota from aluminum leaching. The risk of aluminum leaching becomes a bigger concern at low pH levels (pH<5) due to the increase solubility of aluminum at these levels. Several studies have stated that since stormwater is typically at a neutral pH level, there is minimal concern with Al leaching.

Effects of coir on physical and chemical properties of soil and bioretention media

In this section we provide information on effects of coir on pollutant attenuation and on physical properties of soil and engineered media.

Effects of coir on retention and fate of phosphorus

There are limited studies on coir retention of phosphorus at concentrations typically found in stormwater runoff (less than 0.5 mg/L). Adsorption studies show that phosphorus adsorption at higher concentrations (greater than 1 mg/L) occurs through ion exchange and chemisorption being mechanisms for adsorption, with sulfate competing with phosphate for adsorption sites)(Takaijudin and Ghani, 2014; Kumar et al., 2010; Namasivayam and Sangeetha, 2004).

Shrestha et al. (2019) studied phosphorus leaching from columns containing mixtures of soil, compost, spent lime, and coir. Using tap water with no detectable phosphorus, they observed that adding coir (10% by weight) to a 70-20 soil-compost mix did not decrease phosphorus leaching compared to an 80-20 soil-compost mix. Similar results were observed for media with 40% compost. Hongpakdee and Ruamrungsri (2015) observed reduced phosphorus leaching at the flowering stage, possibly due to increased plant vigor and uptake in treatments containing coir. Herrera Environmental Consultants (2015) conducting flushing and leaching experiments for a variety of media mixtures, including mixtures containing coir. Mixtures of coir and granular activated carbon (GAC) or ash showed orthophosphorus concentrations of 0.021 and 0.052 mg/L, respectively, when flushed with solutions containing less than 0.004 mg/L. For leaching experiments, influent orthophosphate concentrations were 0.323 mg/L and effluent concentrations for coir-GAC and coir-ash mixtures were 0.025 and 0.164 mg/L, respectively. However, the researchers attributed retention of phosphorus to the GAC and ash rather than coir. The researchers also observed decreasing orthophosphorus leachate concentrations with time.

Additional research is needed to understand the phosphorus retention or leaching from media containing coir. Research to date suggests coir will not retain phosphorus in stormwater runoff but will not significantly contribute to leaching from engineered media.

Effects of coir on retention and fate of other pollutants

There is limited research on retention and leaching of pollutants from coir. Shrestha et al. (2019) observed that media containing coir performed similar to spent lime for ammonium and nitrate retention and leached significantly less of these chemicals than treatments containing compost. Herrera Environmental Consultants (2015) observed similar results and also observed that mixtures of coir and either granular activated carbon or ash reduced copper and zinc leaching compared to media mixtures consisting of just soil and compost. Because concentrations of potential pollutants are low in coir, leaching at concentrations of concern appears unlikely. An exception is coir that was soaked in salt water, which may contribute to high sodium, potassium, and chloride concentrations.

Effects of coir on soil physical and hydraulic properties

Coir has several properties that may improve soil physical and hydraulic properties (Cresswell; Noguera et al., 2003; Abad et al., 2005; Small et al., 2018; Lodolini et al., 2018; Arachchi and Somasiri, 1997).

• Coir dust remains relatively hydrophylic (water attracting) even when it is air dry
• Coir dust does not collapse when wet or shrink excessively as it dries
• Increases water holding capacity
• Increases soil porosity
• Decreases soil bulk density

Effects of coir on soil fertility, plant growth, and microbial function

Advantages of coir over peat (Source:Ministry of MSME, Government of India. 2016)

Requires lesser amount of lime due to high pH

Quick and easy rewetting after drying, while peat becomes hydrophobic on drying

Requires short time for irrigation to replace loss of water and drainage from pot, saving fertilizer due to non leaching of nutrients

Higher capillary wetting property

Able to provide aeration in base of mix

Very resilient and exceptional physical stability when wet or dried

Pure coir is not suitable for plant growth. It has a high C:N ratio (>100) and a high lignin content, resulting in slow decomposition and immobilization of plant nutrients. In addition, polyphenols and phenolics acids in the coir can be phytotoxic and inhibit plant growth (Ministry of MSME, Government of India, 2016).

When composted and added as an amendment to a growing media, coir improves plant growth, with coir outperforming peat in several studies. In the absence of composting, nitrogen and phosphorus additions will likely be necessary, depending on plant requirements. Calcium and magnesium additions may also be needed. Concentrations of other nutrients and micronutrients are generally acceptable for most plant species (Cresswell; Asiah et al., 2004; Noguera et al., 2003; Abad et al., 2002; Meerow, 1997; Lodolini et al., 2017; Hongpakdee and Ruamrungsri, 2015; Small et al., 2015; Scagel, 2003; Arachchi and Somasiri, 1997). Noguera et al. (2003) showed that, based on generally recommended plant specifications, 0.25-0.5 mm diameter coir particles appear most suited for plant growth, with some addition of larger particles recommended. Abad et al. (2005) similarly recommended a mix of particle sizes.

Standards, classification, testing, and distributors

Coir standards and specifications

Recommended values for coir used in a growth media (Source: see reference list in this section)
pH	5.2 - 6.8
Electrical conductivity (ms/cm)	0.50 – 1.20 (lower part of range typically preferred)
Cation exchange capacity (meq/100g)	20 - 40
Nitrogen (%)	0.10
Phosphorus (%)	0.01
Potassium (%)	0.50
Copper (% minimum)	1.5
C:N ratio (minimum)	110
Lignin (%)	30 - 35
Total organic matter (% minimum)	75
Moisture (%)	15 - 20
Ash content (%)	1.0 - 1.5
Impurities	<3%
Fiber content	<2%
Expansion	> 12 l/kg
Water holding capacity	3-4 l/kg

Recommended specifications for coir when used in a growing media are shown in the adjacent table and include the following.

• Moisture content less than 20%
• Compression ratio 5:1
• pH 5.4-6.0
• Electrical conductivity less than 0.65 millimhos/cm (this ensures K, Na, Cl, Ca, and Mg contents are within acceptable limits)
• Not be more than two years old and should not be decomposed
• Golden brown in color
• Free from other contamination, sand and other foreign materials
• Free from weeds and seeds

Coir should be composted or incorporated into media containing a nutrient (N and P) source, such as compost. Alternatively, liming or addition of microorganisms may enhance decomposition of coir, which subsequently aids in release of nutrients from the coir. The Ministry of MSME, Government of India (2016) provide a discussion of different composting materials and methods, including specifications.

References containing specifications are provided below. Note that most of these references include information on the packaged material (e.g. bags, blocks, briquettes), such as weight and size.

• Coir Exports
• Coir pith
• Rudra
• Reiziger
• Williams Enterprises
• Ministry of MSME, Government of India

Distributors

Distributors of coir for use in bioretention media (e.g. horticultural use) can readily be found on the internet and we do not make specific recommendations. When purchasing coir, the following questions should be asked.

• Were the husks loosened using fresh water or salt water?
• If salt water was used, has the coir been desalinized (e.g. residual salt washed out)?
• How was the coir dried (air or mechanical drying)?
• If the material was compacted (e.g. bricks), does it meet specifications (see above)?
• How long has the coir been left to mature (>6 months preferred)?
• Does the coir meet specifications described above?
• Has the coir been treated to prevent infestation?
• Has the material been sieved to achieve desired particle size distribution?

Test methods

Packaged coir is typically tested and meets specifications as described above. Standardized testing does not appear to exist for coir, but several methods for testing different characteristics appear to be appropriate.

The following references provide information on testing of coir.

• The Ministry of MSME, Government of India (2016) provide a discussion of test methods for pH, moisture content, ash content, organic matter and organic carbon content, electrical conductivity, total nitrogen, phosphorus content, C:N ratio, and potassium content.
• Williams Enterprises provides test methods for electrical conductivity, pH, moisture content, fiber content, impurities (sand), expansion or breakout volume, and water retention
• Evergreen Coirs provides test methods for electrical conductivity, pH, impurities (sand), expansion volume, moisture, and weed content

Effects of aging

Several studies have looked into the life cycle of water treatment residuals. Due to the high phosphorus adsorption capabilities WTRs in bioretention media will reach saturation capacity. Bayley et al. (2008) observed that during a land application study WTR was able to maintain its capacity to immobilize phosphorus and remained stable after 13 years. Zhao et al. (2009) estimated that Al-based WTR in constructed wetlands had a life cycle of 9-40 years based on a study designed to determine phosphorus removal from agricultural wastewater. Life cycles of WTR for phosphorus adsorption is largely dependent on many factors such as the influent phosphorus concentration, particle size, particle surface area, particle surface charge and pH. More research is needed on the life cycle of water treatment residuals used for stormwater phosphorus adsorption.

Storage, handling, and field application

• Store in a cool dry place
• Keep away from weedkillers and other garden chemicals
• If material is containerized, reseal after use
• Recommended application rates are 10-15 tons per hectare.

There are few handling concerns. Dust may be an eye irritant. Examples of material and safety data sheets can be found at the following links.

• Organic garden coir
• Coconut fiber
• Fiber dust
• Coco coir
• Coco peat

Sustainability

Water treatment residuals are a sustainable option as a bioretention media. Using metal salts as a coagulant is a common practice at water treatment plants all over the world making WTRs widely available. Landfilling is the most commonly used disposal method of these residuals in many countries. However, due to high disposal costs and limitations on available space, disposal of WTRs at landfills is not a efficient or sustainable option. In recent years a lot of research has been done to look into more sustainable disposal methods. Land application of WTRs is currently becoming the most common reuse method of disposal.

References

• Agyin-Birikorang, S., O'Connor, G. A., & Obreza, T. A. (2009). Drinking Water Treatment Residuals to Control Phosphorous in Soils Soil and Water Science (Vol. SL300, pp. 7). University of Florida: University of Florida.
• Bayley, R., Ippolito, J.,Stromberger, M., Barbarick, K., Paschke, M. (2008). Water Treatment Residuals and Biosolids Co‐applications Affect Phosphatases in a Semi‐arid Rangeland Soil. Communications in Soil Science and Plant Analysis. 39. 10.1080/00103620802432733.
• Ippolito, J., Barbarick, K., Elliott, H. (2011). Drinking Water Treatment Residuals: A Review of Recent Uses. Journal of environmental quality. 40. 1-12. 10.2134/jeq2010.0242.
• Keeley, J., Jarvis, P., Judd, S. J., (2014) Coagulant Recovery from Water Treatment Residuals: A Review of Applicable Technologies, Critical Reviews in Environmental Science and Technology, 44:24, 2675-2719, DOI: 10.1080/10643389.2013.829766
• Turner, T., Wheeler, R., Stone, A. et al. Potential Alternative Reuse Pathways for Water Treatment Residuals: Remaining Barriers and Questions—a Review. Water Air Soil Pollution. 230, 230. 10.1007/s11270-019-4272-0.
• Xu D, Lee LY, Lim FY, et al. Water treatment residual: A critical review of its applications on pollutant removal from stormwater runoff and future perspectives. Journal of Environmental Management. 2020 Apr;259:109649. DOI: 10.1016/j.jenvman.2019.109649.
• Zhao, Y., Liu, R., AWE, O., Yang, Y., Shen, C. (2018). Acceptability of land application of alum-based water treatment residuals – An explicit and comprehensive review. Chemical Engineering Journal. 353. 10.1016/j.cej.2018.07.143.
• Zhao, Y., Zhao, X., & Babatunde, A. (2009). Use of dewatered alum sludge as main substrate in treatment reed bed receiving agricultural wastewater: long-term trial. Bioresource Technology, 100, 644–648.