Sewage sludge

For other meanings see, Sludge (disambiguation).

Sludge

Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes. The term is also sometimes used as a generic term for solids separated from suspension in a liquid; this 'soupy' material usually contains significant quantities of 'interstitial' water (between the solid particles). Sludge is also a derrogatory term for 'dump', when people talk sludge i.e. talk aimlessly without any purpose or meaning. Pear and Turd G are examples of sludge talkers. Wall to wall sludge.

When fresh sewage or wastewater is added to a 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. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. After digesting for an extended period, the result is called "digested" sludge and may be disposed of by drying and then landfilling. More commonly with domestic sewage, the fresh sludge is continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.

Excess solids from biological processes such as activated sludge may still be referred to as sludge, but the term biosolids, is more commonly used to refer to the material, particularly after further processing such as aerobic composting. Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.

Background and history

Biosolids, the treated form of sewage sludge, have been in use in UK and European agriculture for more than 80 years, though there is increasing pressure to stop the practice of land application. 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. 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.

European legislation on dangerous substances has eliminated the production and marketing of some substances that have been of historic concern such as persistent organic micropollutants. The European Commission has said repeatedly that the "Directive on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture" (86/278/EEC) has been very successful in that there have been no cases of adverse effect where it has been applied. The EC encourages the use of sewage sludge in agriculture because it conserves organic matter and completes nutrient cycles. 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.

Of general interest on this topic is the Swanson et al. (2004) brief history of sewage management in New York City [Swanson, R.L., M.L. Bortman, T.P. O’Connor, and H M. Stanford. 2004. Science, policy and the management of sewage materials; The New York City experience. Mar. Poll. Bull. 49: 679-687]. Since 1884 when sewage was first treated the amount of sludge has increased along with population and treatment technology. At first the sludge was discharged directly along the banks of rivers surrounding the city, a bit later piped further into the rivers, and then further still out into the harbor. In 1924, to relieve a dismal condition in New York Harbor which actually putrefied in places as a result of decay of sewage, New York City began dumping sludge at sea at a location in the New York Bight called the 12-Mile Site. This was deemed a successful public health measure and not until the late 1960s was there any examination of its consequences to marine life or to humans. There was accumulation of sludge particles on the seafloor and consequent changes in the numbers and types of benthic organisms. In 1970 a large area around the site was closed to shellfishing From then until to 1986, the practice of dumping at the 12-Mile Site came under increasing pressure stemming from a series of untoward environmental crises in the New York Bight that were attributed partly to sludge dumping. In 1986, sludge dumping was moved still further seaward to a site over the deep ocean called the 106-Mile Site. Then, again in response to political pressure arising from events unrelated to ocean dumping, the practice ended entirely in 1992. Since 1992, New York City sludge has been applied to land (outside of New York state). That practice, now employed by two/thirds of the sewage treatment plants in the US, has been under continued scrutiny. The wider question is whether or not changes on the sea floor caused by the portion of sludge that settles are severe enough to justify the added operational cost and human health concerns of applying sludge to land. The cost versus benefit question is moot in the U.S. because ocean dumping of sludge is banned, but international treaty (London Dumping Convention) allows the practice so, on a global basis, as more and more sewage is treated, every sludge management option deserves practical consideration.

The term biosolids was formally created in 1991 by the Name Change Task Force of the Water Environment Federation (WEF), formerly known as the "Federation of Sewage Works Associations" to differentiate raw, untreated sewage sludge from treated and tested sewage sludge that can legally be utilized as soil amendment and fertilizer. The Federation newsletter published a request for alternative names. Members sent in over 250 suggestions, including "all growth," "purenutri," "biolife," "bioslurp," "black gold," "geoslime," "sca-doo," "the end product," "humanure," "hu-doo," "organic residuals," "bioresidue," "urban biomass," "powergro," "organite," "recyclite," "nutri-cake" and "ROSE," short for "recycling of solids environmentally." In June 1991, the Name Change Task Force finally settled on "biosolids," which it defined as the "nutrient-rich, organic byproduct of the nation's wastewater treatment process."

The legal term for biosolids by law is sludge. Treatment processes do not remove cancer causing agents. As detailed in the 1995 Plain English Guide to the Part 503 Risk Assessment , EPA's most comprehensive risk assessment was completed for biosolids.

Treatment process

Main article: Sewage Treatment

Sewage sludge is produced from the treatment of wastewater and consists of two basic forms — raw primary sludge (basically faecal material) and secondary sludge (a living ‘culture’ of organisms that help remove contaminants from wastewater before it is returned to rivers or the sea). The sludge is transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime/alkaline stabilisation. Some treatment processes such as composting and alkaline stabilization involve significant amendments may dilute contaminant concentrations; depending on the process and the contaminant in question, treatment may decrease or in some cases increase the bioavailability and/or solubility of contaminants. In general, the more effectively a wastewater stream is treated, the greater the resulting concentration of contaminants into the product sludge. See also List of waste water treatment technologies.

Biosolids

Biosolids, also referred to as treated sludge, 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 pathogens and vector attraction by any of a number of approved methods. 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.

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 [5] 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 sludged land. A sample of 437 people exposed to 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."

— Health Survey of Residents Living near Farm Fields Permitted to Receive Biosolids, in x, x, Khuder, et al.

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.

The EPA has recently (as of 2009) 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. It is unknown whether mineral speculators have yet invested in the sludge stocks of the United States.
• 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.

For produce to be USDA-certified organic, sludge (biosolids) cannot be used.

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.

United States

According to the United States Environmental Protection Agency (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. After the 1991 Congressional ban on ocean dumping, the US EPA promulgated regulations - 40 CFR Part 503 - that continued to allow the use of biosolids on land as fertilizers and soil amendments which had been previously allowed under Part 257. The EPA promoted biosolids recycling throughout the 1990s. The EPA's Part 503 regulations were developed with input from university, EPA, and USDA researchers from around the country and involved an extensive review of the scientific literature and the largest risk assessment the agency had conducted to that time. However, there was no risk assess for pathogens or chemicals and heavy metals were not considered to be cancer causing agents. The Part 503 regulations became effective in 1993.

United States municipal wastewater treatment plants in 1997 produced about 7.7 million dry tons of biosolids, and about 6.8 million dry tons in 1998 according to sources relying on EPA estimates. As of 2002, about 60% of all biosolids were applied to land as a soil amendment and fertilizer for growing crops. 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. 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 or gamma ray irradiation. Processes to reduce pathogens have no effect on heavy metals and may or may not have effects on the levels of other trace pollutants in biosolids. Treatment processes that involve significant amendments such as composting and alkaline stabilization may dilute total trace metals concentrations, but, depending on the process and the element in question, may decrease or in some cases increase the bioavailability and/or solubility of trace elements. "Composting is not a sterilization process and a properly composted product maintains an active population of beneficial microorganisms that compete against the pathogenic members. Under some conditions,explosive regrowth of pathogenic microorganisms is possible."

Often thought to consist of only "human waste", treated sewage sludge or "biosolids" contain any contaminants from sewage that are not broken down in the treatment process, or which do not remain with the water effluent leaving the treatment plant. The most commonly detected trace contaminants of concern are heavy metals (arsenic, cadmium, copper, etc., some of which are also critical plant micronutrients), and toxic chemicals (e.g. plasticizers, PDBEs, and others generated by human activities, including personal care products and medicines). Synthetic fibers from fabrics persist in biosolids as well as in biosolids-treated soils and may thus serve as an indicator of past biosolids application. Pathogens are not a significant health issue if biosolids are properly treated and site-sepcific management practices are followed; there is generally a greater concern for products that have been fertilized with un-treated animal wastes and which may be eaten raw.

The National Research Council 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. EPA responded with a commitment to conduct research addressing the NRC recommendations.

The EPA Office of the Inspector General (OIG) completed two assessments in 2000 and 2002 of the EPA sewage sludge program. The follow-up report in 2002 documented that "the EPA cannot assure the public that current land application practices are protective of human health and the environment." The report also documented that there had been an almost 100% reduction in EPA enforcement resources since the earlier assessment. This is probably the greatest issue with the practice: under both the federal program operated by the EPA and those of the several states, there is limited inspection and oversight by agencies charged with regulating these practices. To some degree, this lack of oversight is a function of the perceived (by the regulatory agencies) benign nature of the practice. However, a greater underlying issue is funding. Few states and the US EPA have the discretionary funds necessary to establish and implement a full enforcement program for biosolids. To do so would require substantial spending that most legislatures are unwilling to support. Some states and companies involved in biosolids management have willingly agreed to use a "fee" per unit of biosolids managed to help fund such programs, and generally, where such programs are in place, biosolids land application proceeds without incident, however these fees are seldom sufficient to fully fund a rigorous inspection program.

A cautionary approach to land application of biosolids has been advocated by some for regions where soils have lower capacities for toxics sorption 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.

Alternative pathways for sludge reuse

Feridun of the United Sludge Free Alliance suggests that sludge can be recycled in a variety of ways that are both environmentally beneficial and sustainable, and which do not involve application of biologically active materials to croplands that humans live close to. These include using anaerobic digestion to produce biogas, pyrolysis of the sludge to create syngas and potentially biochar, or incineration in a waste-to-energy facility for direct production of electricity and steam for district heating or industrial uses. Synergies from these processes include a far lower, controlled level of methane release (an extremely potent greenhouse gas) to the atmosphere from the pyrolyzed/digested/combusted sludge rather than the uncontrolled release of methane from untreated sludge. If methane is captured rather than allowed to outgas, it can be used for fuel, closing the carbon cycle. Thermal or anaerobic processes greatly reduce the volume of the sludge, as well as achieve remediation the biological concerns. Direct waste-to-energy incineration systems require multi-step cleaning of the exhaust gas, to ensure no hazardous substances are released. In addition, the ash produced by incineration is difficult to use without subsequent treatment due to its high heavy metal content; solutions to this include leaching of the ashes to remove heavy metals followed by reuse of the ash as aggregate for concrete, or if biochar is used, the heavy metals may be fixed in place by the char structure. An other way to use dried sewage sludge as a energy resource is to burn it together with coal in coal-fired power stations. This is considered as biomass co-firing, which allows to produce the same amount of electricity with less carbon-dioxide emissions.

See also

• By-product
• Ecological sanitation
• Oil sludge
• Sanitation
• Sewage treatment
• Suspended solids
• Wastewater treatment facility

Further reading

Books

• John Stauber; Sheldon Rampton (1995). "Chapter 8: The Sludge Hits the Fan". Toxic Sludge is Good For You (PDF). Common Courage Press. ISBN 1-56751-060-4.

Government/NGO reports

• "Biosolids Applied to Land: Advancing Standards and Practices", National Research Council, July 2002
• Guidance For Controlling Potential Risks To Workers Exposed to Class B Biosolids, Center for Disease Control (CDC), July, 2002
• Why USDA Should Just Say No: Executive Summary, Environmental Working Group, April 1998
• Why USDA Should Just Say No: Policy Memo, Environmental Working Group, April 1998
• Human Health Risk Evaluation of Land Application of Sewage Sludge/Biosolids, North Carolina Health Department: Occupational and Environmental Epidemiology Branch, November 2005.

Journal articles

• "A High-Level Disinfection Standard for Land Applying Sewage Sludges (Biosolids)" (PDF). Environmental Health Perspectives. February 2004.
• Caroline Snyder (2005). "The Dirty Work of Promoting "Recycling" of America's Sewage Sludge" (PDF). International Journal of Occupational and Environmental Health. 11: 415–427.
• "Land application of sewage sludges: an appraisal of the US regulations". International Journal of Environment and Pollution. 11 (1): 1–36. 1999.

Web

• Sewage sludge, SourceWatch
• Sewage Sludge (aka Sewage Biosolids), Cornell Waste Management Institute
• Sewage Sludge Basic Information from United Sludge-Free Alliance