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Sludge is 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). Commonly 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.

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.


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 Commissionmarker 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.

Treatment process

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 stabilisation. The more treated the wastewater the more toxic the sludge. See also List of waste water treatment technologies.

Benefits of treatment

The treatment process reduces the water content of the sludge. The basic principal is the cleaner the water is after the sludge is removed, the more toxic the sludge is going to be. The toxicity of the sludge will vary dependant on the source of the waste water. Varying combinations of domestic and industrial customers will effect the composition of the sludge collected. This has been proven when random samplings of treated sludge are found to be filled with heavy metals, as well as chemical residues that are not removed by the treatment process. The treatment process does not remove 100% of the pathogens, and in many cases pathogen regrowth after spreading is significant.

Final product

Treated biosolids can be produced in cake, granular, pellet[25386] or liquid form and are spread over land before being incorporated into the soil or injected directly into the soil by specialist contractors.

Digested sewage sludge can be used as a soil conditioner, but may contain pathogenic or toxic materials. It used to be common practice to dump sewage sludge into the ocean, however, this practice has stopped in many nations due 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. 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.

According to Harrison and Oakes, the EPA has no system to track and respond to health complaints related to exposure to sludge, and further, agencies receiving reports of health complaints have not generally been attentive to the complainants. They further report that there are at least three hundred and fifty people who have reported health issues correlated with or suspected to be caused by sewage sludge exposure, including at least one death.

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 of sludged land) - and using a control group of 176 people not exposed to sludge (not living within 1 mile 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.

Lewis, Gattie, Novak, Sanchez, and Pumphrey report that sludge-correlated symptoms in humans have included asthma, weight loss, fatigue, eye irritations, flu-like symptoms, gastrointestinal complications, headaches, immunodeficiency problems, lesions, nausea, nosebleeds, rashes, respiratory complications, abscesses, reproductive complications, cysts, and tumors. Lewis gives two accounts of human mortality correlated with sludge application: Lewis goes on to state that adverse effects reported by humans in the area of biosolid applications decreases linearly with distance from the field upon which biosolids were applied.

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.

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.

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.

See also


  1. Drinking water treatment sludge production and dewaterability
  2. Basque Research. (2009). Adding high doses of sludge to neutralise soil acidity not advisable. The University of the Basque Country.

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