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Sewage sludge is the residual, quasi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term "septage" also refers to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems, such as .
After treatment, and dependent upon the quality of sludge produced (for example with regards to heavy metal content), sewage sludge is most commonly either disposed of in , dumped in the ocean or applied to land for its fertilizing properties, as pioneered by the product Milorganite.
The term "Biosolids" is often used as an alternative to the term sewage sludge in the United States, particularly in conjunction with reuse of sewage sludge as fertilizer after sewage sludge treatment. Biosolids can be defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting.
Sewage sludge is usually treated by one or several of the following treatment steps: lime stabilization, thickening, dewatering, drying, anaerobic digestion or composting. Some treatment processes, such as composting and alkaline stabilization, that involve significant amendments may affect contaminant strength and concentration: depending on the process and the contaminant in question, treatment may decrease or in some cases increase the bioavailability and/or solubility of contaminants. Regarding sludge stabilization processes, anaerobic and aerobic digestion seem to be the most common used methods in EU-27.Kelessidis and Stasinakis, 2012. COMPARATIVE STUDY OF THE METHODS USED FOR TREATMENT AND FINAL DISPOSAL OF SEWAGE SLUDGE IN EUROPEAN COUNTRIES, Waste Management, vol. 32, June 2012, p. 1186-1195. Kelessidis and Stasinakis, 2012
When fresh sewage or wastewater enters a primary settling Storage tank, approximately 50% of the suspended solid matter will settle out in an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. The sludge will become putrefaction in a short time once anaerobic bacteria take over, and must be removed from the sedimentation tank before this happens.
This is accomplished in one of two ways. Most commonly, the fresh sludge is continuously extracted from the bottom of a hopper-shaped tank by mechanical scrapers and passed to separate sludge-digestion tanks. In some treatment plants an Imhoff tank is used: sludge settles through a slot into the lower story or digestion chamber, where it is decomposition by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. secondary treatment process also generates a sludge largely composed of bacteria and protozoa with entrained fine solids, and this is removed by settlement in secondary settlement tanks. Both sludge streams are typically combined and are processed by anaerobic or aerobic treatment process at either elevated or ambient temperatures. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then .
Following treatment, sewage sludge is either landfilled, dumped in the ocean, incinerated, applied on agricultural land or, in some cases, retailed or given away for free to the general public. According to a review article published in 2012, sludge reuse (including direct agricultural application and composting) was the predominant choice for sludge management in EU-15 (53% of produced sludge), following by incineration (21% of produced sludge). On the other hand, the most common disposal method in EU-12 countries was landfilling.
United States municipal wastewater treatment plants in 1997 produced about 7.7 million dry tons of sewage sludge, and about 6.8 million dry tons in 1998 according to EPA estimates. As of 2004, about 60% of all sewage sludge was applied to land as a soil amendment and fertilizer for growing crops. In a review article published in 2012, it was reported that a total amount of 10.1 million tn DS/year were produced in EU-27 countries.Kelessidis and Stasinakis, 2012. COMPARATIVE STUDY OF THE METHODS USED FOR TREATMENT AND FINAL DISPOSAL OF SEWAGE SLUDGE IN EUROPEAN COUNTRIES. Waste Management, vol.32, June 2012, p. 1186-1195. Kelessidis and Stasinakis, 2012 As of 2023, the EU produced 2 to 3 million tons of sludge each year. Worldwide it is estimated that as much as 75 Million Mg of dry sewage sludge per year.
Production of sewage sludge can be reduced by conversion from to dry toilets such as urine-diverting dry toilets and composting toilets.
As of 2023, 11% of sludge produced in the EU was disposed of in landfills. The EU is attempting to phase out the disposal of sludge in landfills.
Thermal processes can greatly reduce the volume of the sludge, as well as achieve remediation of all or some of the biological concerns. Direct waste-to-energy incineration and complete combustion systems (such as the Gate 5 Energy System) will require multi-step cleaning of the exhaust gas, to ensure no hazardous substances are released. In addition, the ash produced by incineration or incomplete combustion processes (such as fluidized-bed dryers) may be difficult to use without subsequent treatment due to high heavy metal content; solutions to this include leaching of the ashes to remove heavy metals or in the case of ash produced in a complete-combustion process, or with biochar produced from a pyrolytic process, the heavy metals may be fixed in place and the ash material readily usable as a LEEDs preferred additive to concrete or asphalt. Examples of other ways to use dried sewage sludge as an energy resource include the Gate 5 Energy System, an innovative process to power a steam turbine using heat from burning milled and dried sewage sludge, or combining dried sewage sludge with coal in coal-fired power stations. In both cases this allows for production of electricity with less carbon-dioxide emissions than conventional coal-fired power stations.
As of 2023, 27% of sludge produced in the EU was incinerated.
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. Such use was pioneered by the production of Milorganite in 1926.
Use of sewage sludge has shown an increase in level of soil available phosphorus and soil salinity.
The findings of a 20-year field study of air, land, and water in Arizona, concluded that use of biosolids is sustainable and improves the soil and crops. Other studies report that plants uptake large quantities of heavy metals and toxic pollutants that are retained by produce, which is then consumed by humans.
A PhD thesis studying the addition of sludge to neutralize soil acidity concluded that the practice was not recommended if large amounts are used because the sludge produces acids when it oxidizes.
Studies 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 Glycine max. This effect was not present in zea mays when tested in a different study.
A cautionary approach to land application of biosolids has been advocated by some for regions where soils have lower capacities for toxics absorption or due to the presence of unknowns in sewage biosolids. In 2007 the Northeast Regional Multi-State Research Committee (NEC 1001) issued conservative guidelines tailored to the soils and conditions typical of the northeastern US.
Use of sewage sludge is prohibited for produce to be labeled USDA-certified organic. In 2014 the United States grocery chain Whole Foods banned produce grown in sewage sludge.
Treated sewage sludge has been used in the UK, Europe and China agriculturally for more than 80 years, though there is increasing pressure in some countries to stop the practice of land application due to farm land contamination and negative public opinion. In the 1990s, there was pressure in some European countries to ban the use of sewage sludge as a fertilizer. Switzerland, Sweden, Austria, and others introduced a ban. Still, the dominant method for disposal of sewage sludge in the EU is via application to agricultural lands. As of 2023, 40% of sludge produced in the EU was used on agricultural land. Since the 1960s there has been cooperative activity with industry to reduce the inputs of persistent substances from factories. This has been very successful and, for example, the content of cadmium in sewage sludge in major European cities is now only 1% of what it was in 1970.
Recycling of phosphate is regarded as especially important because the phosphate industry predicts that at the current rate of extraction the economic reserves will be exhausted in 100 or at most 250 years.
Phosphate can be recovered with minimal capital expenditure as technology currently exists, but municipalities have little political will to attempt nutrient extraction, instead opting for a "take all the other stuff" mentality.One potential drawback of extracting products from sludge — as opposed to land application — is that only some of the sludge is used and the rest still needs disposal. It can also be very expensive to develop and use appropriate technologies for extracting resources.
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:
In 2013, in South Carolina PCBs were discovered in very high levels in wastewater sludge. The problem was not discovered until thousands of acres of farm land in South Carolina were discovered to be contaminated by this hazardous material. SCDHEC issued emergency regulatory order banning all PCB laden sewage sludge from being land applied on farm fields or deposited into landfills in South Carolina.
Also in 2013, after DHEC request, the city of Charlotte decided to stop land applying sewage sludge in South Carolina while authorities investigated the source of PCB contamination. In February 2014, the city of Charlotte admitted PCBs have entered their sewage treatment centers as well.
Contaminants of concern in sewage sludge are plasticizers, PDBEs, PFASs ("forever chemicals"), and others generated by human activities, including personal care products and medicines. Synthetic fibers from fabrics persist in treated sewage sludge as well as in biosolids-treated soils and may thus serve as an indicator of past biosolids application.
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The NRC published "Biosolids Applied to Land: Advancing Standards and Practices" in July 2002. The NRC concluded that while there is no documented scientific evidence that sewage sludge regulations have failed to protect public health, there is persistent uncertainty on possible adverse health effects.
The NRC noted that further research is needed and made about 60 recommendations for addressing public health concerns, scientific uncertainties, and data gaps in the science underlying the sewage sludge standards. The EPA responded with a commitment to conduct research addressing the NRC recommendations.Residents living near Class B sludge processing sites may experience asthma or pulmonary distress due to released from sludge fields.
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.
In 2007, a health survey of persons living in close proximity to Class B sludged land was conducted. A sample of 437 people exposed to Class B sludge (living within of sludged land) - and using a control group of 176 people not exposed to sludge (not living within of sludged land) reported the following: 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 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 "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."
Studies of the potential uses of sewage sludge around homes, such as covering lead-contaminated soil in Baltimore, have created debates over whether participants should have been informed about potential risks, when there remains uncertainty about those risks.
The chain of sewage sledge to biosolids to fertilizers has resulted in PFASs ("forever chemicals") contamination of farm produce in Maine in 2021 and beef raised in Michigan in 2022. The EPA PFAS Strategic Roadmap initiative, running from 2021 to 2024, will consider the full lifecycle of PFAS including health risks of PFAS in wastewater sludge.
European countries that joined the EU after 2004 favor landfills as a means of disposal for sewage sludge. In 2006, the predicted sewage sludge growth rate was 10 million tons of sewage sludge per year. This increase in the amount of sewage sludge accumulation in the EU can be due to the increase in the number of households that are connected to the sewage system. The EU has directives in place to encourage the use of sewage sludge in agriculture, in a way that the soil, humans, and the environment are not harmed. A guideline the EU has put into place it that sewage sludge should not be added to fruit and vegetable crops that are in season. In Austria, in order to dispose of the sewage sludge in a landfill, it must first be treated in a way that reduces its biological reactivity." Disposal and Recycling Routes for Sewage Sludge"
/ref> Sweden no longer allows sewage sludge to be disposed in the land fills. In the EU, regulations regarding sewage sludge disposal differ because legislation regarding landfill disposal in not in the national regulations for the EU.
EU member states are tasked with implementing and enforcing the Directive within their borders, as well as monitoring and reporting on sludge production, treatment, characteristics, and use. Member states are allowed to set more stringent limits for heavy metals than set out in the Sewage Sludge Directive, and can set limits for other pollutants. As of 2021, more than half of the EU member states had stricter limits for mercury and cadmium than required under the Directive.
Member states are also allowed to limit or promote the use of sewage sludge for agriculture as they choose, meaning that some countries prohibit the use of sludge in agriculture, while some use up to 50% of the sludge they generate in agriculture. Spain, France, Italy, and the United Kingdom (while it was still part of the EU) have particularly promoted the use of sludge in agriculture. Each of Austria's federal states has its own regulations for the use of sewage sludge in agriculture, including different limits for heavy metals. For example, Tyrol has banned the use of sludge on agricultural lands, while in Salzburg it is only allowed under certain conditions.
Since the Directive's passage, there has been the substantial decrease in heavy metal residues in agricultural soils over time (well below the limits set), though it is not possible to determine what proportion of the decrease is due to the Directive itself, as opposed to other national and EU legislation.
The Sewage Sludge Directive has been evaluated several times under EU proposals to build a circular economy through the reduction and reuse of wastes. In 2014, a European Commission evaluation of the Sewage Sludge Directive suggested it was appropriate for its goals, and did not need revision. In 2023, as part of the European Green Deal and Circular Economy Action Plan, the EU re-evaluated the Sewage Sludge Directive, and found that it should be maintained – as the use of sewage sludge as fertilizer aligns with circular economy goals and potentially reduces the EU carbon emissions – but that the potential pollutants and contaminants regulated under the Directive should be reviewed and potentially revised. This evaluation noted that, as of 2023, the original Directive had not been seriously updated since its original passage in 1986, even though in the intervening decades there had been many developments in both environmental policy, expectations, and research, as well as member states' national policies around sewage sludge. The evaluation particularly emphasized concerns about methane emissions, microplastic contamination, and antibiotic resistances.
The Sewage Sludge Directive has not yet set limits for other contaminants, such as organic pollutants, pathogens, microplastics, pharmaceutical residues, and personal care product residues. With the identification of these new contaminants in sludge since the Sewage Sludge Directive originally passed, several researchers have suggested that the EU should consider revising the Directive to address their potential risks to health and environment.
According to the EPA, biosolids that meet treatment and pollutant content criteria of Part 503.13 "can be safely recycled and applied as fertilizer to sustainably improve and maintain productive soils and stimulate plant growth." However, they can not be disposed of in a sludge only landfill under Part 503.23 because of high chromium levels and boundary restrictions.
Under the Obama Administration, the Biosolids Center of Excellence (headquartered in EPA Region 7) was created to monitor and enforce compliance with biosolids regulation.U.S. Environmental Protection Agency Office of Inspector General. (2018-11-15). EPA Unable to Assess the Impact of Hundreds of Unregulated Pollutants in Land-Applied Biosolids on Human Health and the Environment. Report No. 19-P-0002. The Center receives and reviews annual reports from the major producers of biosolids.
Eight U.S. states oversee their own biosolids programs: Arizona, Michigan, Ohio, Oklahoma, South Dakota, Texas, Utah, and Wisconsin; other states' programs are overseen by the EPA.
Class A sludge must be treated so that specific pathogens (like Salmonella) are no longer detected. This class of sludge can be used for all land applications, including where the public may come into contact with it (i.e. agricultural land, home use, for public sale). Biosolids that meet Class A pathogen reduction requirements or equivalent treatment by a "Process to Further Reduce Pathogens" (PFRP) have the least restrictions on use. PFRPs include pasteurization, heat drying, thermophilic composting (aerobic digestion, most common method), and beta ray or gamma ray irradiation.
Class B sludge also requires treatment to reduce pathogens, but pathogens are still detectable in the sludge (such as some parasitic worm eggs). This class of sludge has much stricter restrictions on its use. Biosolids that meet the Class B pathogen treatment and pollutant criteria, in accordance with the EPA "Standards for the use or disposal of sewage sludge" (40 CFR Part 503), can be land applied with formal site restrictions and strict record keeping.
As detailed in the 1995 Plain English Guide to the Part 503 Risk Assessment, the EPA's most comprehensive risk assessment was completed for biosolids.
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