Many drinking water treatment facilities mix non-toxic aluminum-based or iron-based chemicals into the water as part of the water cleaning process.  After these coagulants have bound with a variety of trace contaminants (bacteria, salts, particles, etc.) in the water, they are removed by settling, taking the contaminants with them.

Water treatment residuals are sometimes dewatered by natural freeze-thaw processes.  Photo courtesy Northern Tilth.

These water treatment residuals (WTR), sometimes called hydrosolids, "alum sludge," or "ferric sludge," are managed in a variety of ways, including via dischage to a wastewater treatment facility, landfill disposal, or land application.  They are mostly water and aluminum or iron; they also contain a variety of trace amounts of metals, suspended solids, organic chemicals, and biological particles.

Land Application of Water Treatment Residuals

Water treatment residuals stockpiled for use in soils blends to reduce phosphorus (P) environmental availability.

Research has demonstrated that these residuals - especially alum-rich water treatment residuals - bind with phosphorus (P) in biosolids, manures, and soils, thus reducing the solubility of phosphorus (e.g. Elliott et al., 2001).  Because of this, alum WTR are recognized as a tool useful in controlling phosphorus run-off that can be damaging to surface waters, especially in areas where excess P is already impairing the health of lakes and streams.  Thus, wastewater treatment plants that process alum WTR find that their biosolids have lower soluble P, a useful characteristic when the biosolids are land applied.  Likewise,  alum WTR that are directly land applied result in lower soluble P in the soil.  (See more about biosolids and phosphorus, P.)

Concern About Potential Aluminum Toxicity

Questions are sometimes raised about the potential impacts of the high levels of aluminum in WTR and whether, when WTR are land applied, the aluminum can have an impact on soils, crops, and ecosystems.   This concern is based on the fact that it has been long known that natural aluminum in soils can cause phytotoxicity (harm to plants) when the pH of the soil is low (pH <5). 

Soils are already high in aluminum (Al) naturally.  Al, along with silica, are primary ingredients of soils and bedrock (Al is about 7% of the earth's crust).  Therefore, the addition of alum WTR - which may have Al concentrations averaging about 140,000 mg/kg - that are tilled into a much larger volume of soil, creates only a small increase in aluminum concentration in the soil.  For example, some WTR have been applied to soils at rates as high as 60 dry tons / acre.  Yet, even at that rate, there is no noticeable difference in comparison to control soils.  The fact is, if the soil is very acidic, there is chance of aluminum phytotoxicity whether or not alum-rich WTR has been used.  Of course, pH management is a primary concern for any agricultural operation, and most soils, biosolids, and composts are close to neutral (pH of 7), which keeps aluminum in an insoluble form. When alum WTR are directly land applied, the aluminum in them does not likely pose any significant risk, as long as the WTR application rate is moderate and controlled and the soil pH is maintained at an appropriate agronomic level. 

The Al concentration and potential for any risk is much lower when WTR are processed through a wastewater treatment plant and mixed with biosolids and/or composted with other ingredients.  Testing of some Massachusetts composts that include WTR do not show unusually high levels of aluminum: 3,000 - 6,500 mg/kg (ppm).  Compare that range to natural soils that can easily have 1% or more Al (1% = 10,000 mg/kg) and Florida's residential soil clean-up target level of 80,000 mg/kg. 

It is important to note, however, that in some rare instances, there can be elevated concentrations of other contaminants that the WTR removed from drinking water, such as arsenic (As).  Therefore, WTR that are to be land applied should be tested for trace metals and/or other potential contaminants.  This is true - and what is routinely done โ€“ for any residuals destined for land application.