Sewage sludge

Sludge

Desiccation cracks in dried sludge, the hard final remains from a sewage plant.

Sewage sludge refers to the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term septage is also referring to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems such as septic tanks.

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. 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.

Biosolids is a term often used in conjunction with reuse of sewage sludge 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.

Overview

Classes of sewage sludge (US)

Class A sludge is typically dried and pasteurized, and is also known as "exceptional" quality. However, bacteria in Class A sludge products such as "Milorganite" can actually "rebloom" under certain environmental conditions.

Class B includes all sludge not classified as Class A. Class B sludge is typically "undigested" and is volatile. Residents living near Class B sludge processing sites may experience asthma or pulmonary distress due to bioaerosols released from sludge fields.

Both classes of sludge may still contain radioactive or pharmaceutical wastes.

Micro-pollutants

Micro-pollutants can become concentrated in sewage sludge. Following treatment, sewage sludge is either landfilled, incinerated, applied on agricultural land or, in some cases, retailed or given away for free to the general public. Each of these disposal options comes with a myriad of potential — and in some cases proven — human health and environment impacts. By not mixing human excreta with water and the other contaminants that are disposed of in sewers and septic tanks, UDDTs thus avoid many of the problems associated with these practices.

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:

Pollutant	Ceiling concentration (milligrams per kilogram)
Cadmium	85
Copper	4300
Lead	840
Mercury	57
Molybdenum	75
Nickel	420
Selenium	100
Zinc	7500

There are thousands other components of sludge that remain untested/undetected disposed of from modern society that also end up in sludge (pharmaceuticals, nano particles, etc.) which has been proven to be hazardous to both human and ecological health.

Food production classification

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

In 2014 a large grocery chain in the United States called Whole Foods banned produce grown in sewage sludge

Sewage sludge treatment

Main article: Sewage sludge treatment

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.

Disposal, land application or reuse

After treatment, and dependent upon the quality of sludge produced (for example with regards to heavy metal content), it is either disposed of in landfills or applied to land. 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.

Use of sewage sludge as fertilizer has on occasion resulted in selfset tomato plants becoming common field weeds, from seeds eaten by people and passed through the sewage system. Also, utilization of sewage sludge has shown an increase in level of soil available phosphorus and soil salinity.

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, combustion of biofuel produced from drying biosolids 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) nomethane release (an extremely potent greenhouse gas) if the biofuel is combusted, or (b) in the case of anaerobically digested biosolids a far lower, controlled level of methane released to the atmosphere from the pyrolyzed/digested/combusted sludge rather than the uncontrolled release of methane from untreated or land applied sludge. If methane is captured rather than allowed to outgas, it can be used for fuel, closing the carbon cycle.

Thermal processes can eliminate and anaerobic 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 concreate or asphalt.

Examples of other ways to use dried sewage sludge as an energy resource include (1) the Gate 5 Energy System, an innovative process to power a steam turbine using heat from burning milled and dried biosolids, or (2) combining dried biosolids 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.

Controversies in the US

Risks of land application

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 simply unknown and unassessed. Additionally "Regulations that limit contact with biosolids do not prevent environmental processes in the conceptual model such as aerosolization or erosion and the death or multiplication of pathogens."

Lawsuits and court cases in the US

October 25, 2012 - 2 families won a $225,000 tort lawsuit against a sludge company that contaminated their properties.

An epic battle between the home rule of local government and states rights/commerce rights has been waged between the small town of Kern County, CA and Los Angeles, CA. Kern county passed an ordinance "Keep Kern Clean" ballot initiative which banned sludge from being applied in Kern County. Los Angeles sued and the case has yet to be decided.

On February 26, 2013 in Pennsylvania, the case Gilbert v. Synagro, a judge barred a nuisance, negligence and trespass lawsuit under PA's Right to Farm Act.

November 30, 2009, James Rosendall of Grand Rapids, MI was sentenced by United States District Judge Avern Cohn to 11 months in prison followed by three years of supervised release for conspiring to commit bribery Racketeer Influenced and Corrupt Organizations Act. Rosendall was the former president of Synagro of Michigan, a subsidiary of Synagro Technologies. His duties included obtaining the approval of the City of Detroit to process and dispose of the City’s wastewater. United States Attorney Berg said: “If a businessman seeking a contract with the City knowingly allows third parties to bribe public officials to get that contract, this is a crime that deserves prison, which is what Mr. Rosendall got. In this case, Mr. Rosendall’s significant cooperation and assistance in the investigation and prosecution of others resulted in a lower sentence than he would ordinarily would have received for complicity in such a bribery scheme.”

November 9, 2012 - Synagro accused of illegal dumping that allegedly "risks of chemical exposure and explosions at the plant near Philadelphia International Airport."

Environmental justice

Chosen sludge land application sites tend to be locations where poverty is high and economic prosperity and opportunity is low. Sludge tends to be land applied where minorities live. This is the definition of environmental racism. In the United States, the Environmental Protection Agency (EPA) is charged with investigating allegations of environmental racism, or, violations of civil liberties, under Title 6 of the Civil Rights Act of 1964. The caveat is that the complaint must be logged to the EPA within 180 days of a suspected incident of racism.

Local government ordinances and complaints in the US

In January 27, 2011 Synagro's home state of Texas, Travis County Commissioners declared that Synagro's Solid Waste disposal activities would be inappropriate and prohibited land use according to the towns already established ordinances.

Hazardous waste

Sewage treatment plants receive all types of hazardous waste from hospitals, nursing homes, industry and households.

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 contaminated by this hazardous material. An ongoing criminal investigation is underway to determine the perpetrator of this crime. While investigation was underway, 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 admits PCB's have entered their sewage treatment centers as well.

Biosolids controversy in the US

Biosolids is a term widely used 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 to agricultureal fields and applied to the land. Low levels of constituents such as PCBs, dioxin, and brominated flame retardants, may remain in treated sludge.

A recent, thorough review of literature, and 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.

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 to 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 "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 are used because the sludge produces acids when it oxidizes.

Alternatives to land-based management of biosolids that avoid the above-described potential impacts to the environment and public health are any number of thermally-based processes such as incineration, gasification or the Gate 5 Energy System.

Whistleblower

Dr. David Lewis worked for the EPA for 31 years. In a peer-reviewed article in Environmental Science & Technology he showed that pathogens could easily remain undetected in untreated sewage sludge. Since the government is pushing the use of this sludge for agricultural fertilizer, Lewis was harassed and finally fired in May 2003. At a hearing on the role of science in shaping public policy he charged the EPA with "corrupt[ing] the scientific peer review process in order to support certain political agendas and further the agency's self-interest."

Regulations

European Union

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. 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.

The legal term for biosolids by law is sludge. Treatment processes only remove some cancer causing agents; others remain. As detailed in the 1995 Plain English Guide to the Part 503 Risk Assessment, EPA's most comprehensive risk assessment was completed for biosolids.

United States

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. 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-specific 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 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.

History

Development of sewage sludge treatment

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, then 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. Recent oceanic studies indicate that bacteria from human gut is destroying coral in the Caribbean.

Use as fertilizer

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 public outrage. 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.

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.

Journal articles

• Gattie, David K.; Lewis, David L. (2004). "A High-Level Disinfection Standard for Land-Applied Sewage Sludges (Biosolids)". Environmental Health Perspectives. 112 (2): 126–31. doi:10.1289/ehp.6207. JSTOR 3435552. PMC 1241820. PMID 14754565.
• Snyder, C (2005). "The dirty work of promoting "recycling" of America's sewage sludge". International journal of occupational and environmental health. 11 (4): 415–27. doi:10.1179/oeh.2005.11.4.415. PMID 16350476.
• Harrison, Ellen Z.; McBride, Murray B.; Bouldin, David R. (1999). "Land application of sewage sludges: An appraisal of the US regulations". International Journal of Environment and Pollution. 11: 1. doi:10.1504/IJEP.1999.002247.

External links

• "Biosolids Applied to Land: Advancing Standards and Practices", National Research Council, July 2002
• Sludge blog, SewagesludgeactionNetwork.com
• Biosolids FAQs, Biosolids.com