Biosolids, also referred to as treated human sewage, is a term used by the waste water industry to denote the byproduct of domestic and commercial sewage and wastewater treatment. These residuals are further treated to reduce but not eliminate pathogens and vector attraction by any of a number of approved methods and then trucked and land appied to a farm field. Low levels of constituents such as PCBs, dioxin, and brominated flame retardants, may remain in treated sludge.
Recent conclusion of thorough review of literature and 20-year field study of air, land, and water in Arizona concluded that biosolids use is sustainable and improves the soil and crops. Other studies conclude that plants uptake large quantities of heavy metals and toxic pollutents that are retained by produce, which is then consumed by humans.
One of the main concerns in the treated sludge is the concentrated metals content; certain metals are regulated while others are not. Leaching methods can be used to reduce the metal content and meet the regulatory limit. The U.S. divides biosolids into two grades: Class B sewage sludge, and Class A treated sewage sludge. Class A sludge has been treated to reduce bacteria prior to application to land; Class B sludge has not.
Depending on their level of treatment and resultant pollutant content, biosolids can be used in regulated applications for non-food agriculture, food agriculture, or distribution for unlimited use. Treated biosolids can be produced in cake, granular, pellet or liquid form and are spread over land before being incorporated into the soil or injected directly into the soil by specialist contractors. It used to be common practice to dump sewage sludge into the ocean, however, this practice has stopped in many nations due environmental concerns as well to domestic and international laws and treaties. In particular, after the 1991 Congressional ban on ocean dumping, the U.S. Environmental Protection Agency (EPA) instituted a policy of digested sludge reuse on agricultural land. The EPA promoted this policy by presenting it as recycling and rechristening sewage sludge as "biosolids", as they are solids produced by biological activities.
A 2004 survey of 48 individuals near affected sites found that most reported irritation symptoms, about half reported an infection within a month of the application, and about a fourth were affected by Staphylococcus aureus, including two deaths. The number of reported S. aureus infections was 25 times as high as in hospitalized patients, a high-risk group. The authors point out that regulations call for protective gear when handling Class B biosolids and that similar protections could be considered for residents in nearby areas given the wind conditions.
Khuder, Milz, Bisesi, Vincent, McNulty, and Czajkowski (as cited by Harrison and McBride of the Cornell Waste Management Institute in Case for Caution Revisited: Health and Environmental Impacts of Application of Sewage Sludges to Agricultural Land) conducted a health survey of persons living in close proximity to Class B sludged land. A sample of 437 people exposed to Class B sludge (living within 1-mile (1.6 km) of sludged land) - and using a control group of 176 people not exposed to sludge (not living within 1-mile (1.6 km) of sludged land) reported the following:
"Results revealed that some reported health-related symptoms were statistically significantly elevated among the exposed residents, including excessive secretion of tears, abdominal bloating, jaundice, skin ulcer, dehydration, weight loss, and general weakness. The frequency of reported occurrence of bronchitis, upper respiratory infection, and giardiasis were also statistically significantly elevated. The findings suggest an increased risk for certain respiratory, gastrointestinal, and other diseases among residents living near farm fields on which the use of biosolids was permitted."
—Khuder, et al., Health Survey of Residents Living near Farm Fields Permitted to Receive Biosolids
Although correlation does not imply causation, such extensive correlations may lead reasonable people to conclude that precaution is necessary in dealing with sludge and sludged farmlands.
Harrison and Oakes suggest that, in particular, "until investigations are carried out that answer these questions (...about the safety of Class B sludge...), land application of Class B sludges should be viewed as a practice that subjects neighbors and workers to substantial risk of disease." They further suggest that even Class A treated sludge may have chemical contaminants (including heavy metals, such as lead) or endotoxins present, and a precautionary approach may be justified on this basis, though the vast majority of incidents reported by Lewis, et al. have been correlated with exposure to Class B untreated sludge and not Class A treated sludge.
A 2005 report by the state of North Carolina concluded that "that a surveillance program of humans living near application sites should be developed to determine if there are adverse health effects in humans and animals as a result of biosolids application."
In 2009 the EPA released the Targeted National Sewage Sludge Study, which reports on the level of metals, chemicals, hormones, and other materials present in a statistical sample of sewage sludges. Some highlights include:
- Silver is present to the degree of 20 mg/kg of sludge, on average, a near economically recoverable level, while some sludges of exceptionally high quality have up to 200 milligrams of silver per kilogram of sludge; one outlier demonstrated a silver lode of 800–900 mg per kg of sludge.
- Barium is present at the rate of 500 mg/kg, while manganese is present at the rate of 1 g/kg sludge.
- High levels of sterols and other hormones have been detected, with averages in the range of up to 1,000,000 µg/kg sludge.
- Lead, arsenic, chromium, and cadmium are estimated by the EPA to be present in detectable quantities in 100% of national sewage sludges in the US, while thallium is only estimated to be present in 94.1% of sludges.
Recent studies (2010) have indicated that pharmaceuticals and personal care products, which often adsorb to sludge during wastewater treatment, can persist in agricultural soils following biosolid application. Some of these chemicals, including potential endocrine disruptor Triclosan, can also travel through the soil column and leach into agricultural tile drainage at detectable levels. Other studies, however, have shown that these chemicals remain adsorbed to surface soil particles, making them more susceptible to surface erosion than infiltration. These studies are also mixed in their findings regarding the persistence of chemicals such as triclosan, triclocarban, and other pharmaceuticals. The impact of this persistence in soils is unknown, but the link to human and land animal health is likely tied to the capacity for plants to absorb and accumulate these chemicals in their consumed tissues. Studies of this kind are in early stages, but evidence of root uptake and translocation to leaves did occur for both triclosan and triclocarban in soybeans. This effect was not present in corn when tested in a different study.
A PhD thesis studying the addition of sludge to neutralize soil acidity concluded that the practice was not recommended if large amounts (broad - define 'large") are used because the sludge produces acids when it oxidizes.
Read more about this topic: Sludge