thumb|[[Desiccation cracks in dried sludge, the hard final remains from a sewage plant]]
Sewage sludge is the residual, semi-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 septic tanks.
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 landfills, 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. Opponents of sewage sludge reuse reject this term as a public relations term.
Treatment process
Sewage sludge treatment is the process of removing contaminants from wastewater. Sewage sludge is produced from the treatment of wastewater in sewage treatment plants and consists of two basic forms — raw primary sludge and secondary sludge, also known as activated sludge in the case of the activated sludge process.
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.
When fresh sewage or wastewater enters a primary settling 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 putrescent 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 decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. thumb|Sewage sludge in a beaker from a treatment plantThe 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 landfilling.
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. 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. 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 flush toilets to dry toilets such as urine-diverting dry toilets and composting toilets.
Disposal
Landfill
Sewage sludge deposition in landfills can circulate human-virulent species of Cryptosporidium and Giardia pathogens. Sonication and quicklime stabilization are most effective in inactivation of these pathogens; microwave energy disintegration and top-soil stabilization were less effective.
A Texas county has launched a first-of-its-kind criminal investigation into waste management giant Synagro over PFAS-contaminated sewage sludge it is selling to Texas farmers as a cheap alternative to fertilizer.
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.
Ocean dumping
It used to be common practice to dump sewage sludge into the ocean. However, this practice has stopped in many nations due to environmental concerns as well to domestic and international laws and treaties. Former USA president Ronald Reagan signed the law that prohibited ocean dumping as a means of disposal of sewage sludge in the US in 1988.
Incineration
Sludge can also be incinerated in sludge incineration plants, but incineration comes with its own set of environmental concerns (air pollution, disposal of the ash). Pyrolysis of the sludge to create syngas and potentially biochar is possible, as is combustion of biofuel produced from drying sewage sludge or incineration in a waste-to-energy facility for direct production of electricity and steam for district heating or industrial uses.
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 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) might 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 may be 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 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.
Use
Land application
Biosolids is a term widely used to denote the byproduct of domestic and commercial sewage and wastewater treatment that is to be used in agriculture. National regulations that dictate the practice of land application of treated sewage sludge differ widely and e.g. in the US there are widespread disputes about this practice.
alt=A yellow shovel excavator with a man visible at the wheel in the cab, is pushing a huge pile of sewage sludge which looks like very dark colored dirt, which is filling most of the bottom half of the image. This is taking place on a bare field, with light brown soil visible, whose color contrasts strongly with the dark sewage sludge. In the background, there is a green field, and a yellow field, and trees. |thumb|A shovel excavator loading solid sewage sludge for land application.
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 soybeans. This effect was not present in corn when tested in a different study. 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.
Transformation into products
Sewage sludge is an agglomeration of concentrated wastes, and therefore it contains many potentially extractable and useable components. These can include using sludge to produce energy, create carbon-based components, extract phosphorus and nitrogen, or make bricks or other construction materials. 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.
Pathogens
Bacteria in treated sludge products can actually regrow under certain environmental conditions. Pathogens could easily remain undetected in untreated sewage sludge. Pathogens are not a significant health issue if sewage sludge is properly treated and site-specific management practices are followed.
Heavy metals
One of the main concerns in the treated sludge is the concentrated metals content (lead, arsenic, cadmium, thallium, etc.); certain metals are regulated while others are not. Leaching methods can be used to reduce the metal content and meet the regulatory limit.
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:
- 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.
- Silver is present to the degree of 20 mg/kg of sludge, on average, while some sludges 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.
alt=Grey outline map of Europe with yellow circles for countries, sized to show the amount of microplastic sewage sludge spread on fields per year in tonnes in 2016. The amounts are: France 11653, United Kingdom 11455, Germany 9696, Spain 8394, Italy 5528, Poland 2253, Portugal 1579, Finland 1234, Austria 890, Sweden 655, Romania 244, Estonia 197.|thumb|Microplastic contamination from use of sewage sludge on agricultural land in Europe, 2016.
Micro-pollutants
Micro-pollutants are compounds which are normally found at concentrations up to microgram per liter and milligram per kilogram in the aquatic and terrestrial environment, respectively, and they are considered to be potential threats to environmental ecosystems. They can become concentrated in sewage sludge. Each of these disposal options comes with myriad potential—and in some cases proven—human health and environment impacts.
Sterols and other hormones have also been detected. There are potentially thousands of other components of sludge that remain untested/undetected disposed of from modern society that also end up in sludge (pharmaceuticals, nano particles, etc.) which have been proven to be hazardous to both human and ecological health.
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.
Pollutant ceiling concentration
The term "pollutant" is defined as part of the EPA 503 rule. The components of sludge have pollutant limits defined by the EPA. "A Pollutant is an organic substance, an inorganic substance, a combination of organic and inorganic substances, or a pathogenic organism that, after discharge and upon exposure, ingestion, inhalation, or assimilation into an organism either directly from the environment or indirectly by ingestion through the food chain, could, on the basis of information available to the Administrator of EPA, cause death, disease, behavioral abnormalities, cancer, genetic mutations, physiological malfunctions (including malfunction in reproduction), or physical deformations in either organisms or offspring of the organisms."
The maximum component pollutant limits by the US EPA are:
{| class="wikitable"
|-
! Pollutant!! Ceiling concentration (mg per kg)
|-
|Cadmium||85
|-
|Copper||4300
|-
|Lead||840
|-
|Mercury||57
|-
|Molybdenum||75
|-
|Nickel||420
|-
|Selenium||100
|-
|Zinc||7500
|}
Health risks
In 2011, the EPA commissioned a study at the United States National Research Council (NRC) to determine the health risks of sludge. In this document the NRC pointed out that many of the dangers of sludge are unknown and unassessed.
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 bioaerosols 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: