Sludge

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Fecal sludge collected from pit latrines near Durban, South Africa, awaiting further treatment by drying Faecal sludge after some drying (stock piling) - this is the feed material to the process (8152055703).jpg
Fecal sludge collected from pit latrines near Durban, South Africa, awaiting further treatment by drying

Sludge is a semi-solid slurry that can be produced from a range of industrial processes, from water treatment, wastewater treatment or on-site sanitation systems. It can be produced as a settled suspension obtained from conventional drinking water treatment, [1] as sewage sludge from wastewater treatment processes [2] :23–25 or as fecal sludge from pit latrines and septic tanks. 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 can consist of a variety of particles, such as animal manure. [3] [ not specific enough to verify ]

Contents

Industrial wastewater treatment plants produce solids that are also referred to as sludge. This can be generated from biological or physical-chemical processes.

In the activated sludge process for wastewater treatment, the terms "waste activated sludge" and "return activated sludge" are used.

Sludge from the food-processing and beverage-making industries can have a high content of protein and other nutrients. Thus, it can be processed for beneficial uses such as animal feed, rather than being landfilled.

Types of Sludge

There are several types of sludge, often categorized by their origin or processing stages:

Sludge Composition

Sludge composition varies significantly based on its source and the treatment process used. It generally includes:

Treatment and Disposal Methods

Proper sludge treatment and disposal are crucial to minimize environmental and public health impacts.

Primary Settling tank for sludge collection and treatment in Prague Primary settling tank, Prague Central Wastewater Treatment 4587.jpg
Primary Settling tank for sludge collection and treatment in Prague

Common methods include:

  1. Thickening and Dewatering: Thickening and dewatering reduce sludge volume, making it easier to transport and process further. Dewatering techniques include centrifugation and filtration. [8]
  2. Anaerobic Digestion: This process decomposes organic matter in sludge in the absence of oxygen, producing biogas that can be used as an energy source. Anaerobic digestion reduces pathogen levels and stabilizes sludge. [2]
  3. Composting: Composting sludge with other organic materials can create a soil amendment product. This requires proper management to control pathogens and odors. [7]
  4. Incineration : Sludge can be incinerated, converting it into ash and reducing volume significantly. However, this process requires energy input and can emit pollutants if not properly controlled. [6]

Reuse and Environmental Concerns

Some treated sludge, known as biosolids, can be used as fertilizer in agriculture due to its nutrient content.  However, the presence of contaminants like heavy metals and pathogens requires careful regulation and management. In many countries, guidelines limit the application of biosolids to protect soil health and groundwater quality. [9] There is also increasing concern over "forever chemicals" like PFAS (per- and polyfluoroalkyl substances) that can accumulate in sludge and pose long-term environmental risks. [9]

Regulatory Standards and Management

United States Environmental Protection Agency Seal Seal of the United States Environmental Protection Agency.svg
United States Environmental Protection Agency Seal

Many countries have established regulatory frameworks for sludge management. In the United States, for instance, the Environmental Protection Agency (EPA) oversees the safe disposal and reuse of sludge through its "Part 503" regulations. These regulations set limits on pathogens, heavy metals, and other contaminants to ensure biosolids used in agriculture or land application are safe. [9] Similarly, the European Union has strict directives regarding sludge, emphasizing sustainable practices and environmental protection. [10]

40 CFR Part 503

The EPA, under CWA section 405(d), established regulations for the use and disposal of sewage sludge (biosolids) found in 40 CFR Part 503. These standards regulate sludge applied to land, incinerated, or placed in surface disposal sites, addressing pollutant limits, pathogen and vector reduction, management practices, monitoring, recordkeeping, and reporting. They apply to anyone handling, applying, or disposing of sewage sludge, as well as operators of disposal sites. Initially finalized in 1993, 40 CFR Part 503 has been amended several times. The original regulation is in the Federal Register , while the updated version is in the Code of Federal Regulations . [11]

Sewage Sludge Directive 86/278/EEC

The directive aims to promote the safe use of sewage sludge in agriculture while protecting human health, soil, water, and the environment. It prohibits untreated sludge on agricultural land unless properly incorporated into the soil, mandates adherence to plant nutrient requirements, and prevents soil and water contamination. The Directive also supports the EU's waste hierarchy by encouraging safe recycling of nutrients like phosphorus, aligning with circular economy principles and the European Green Deal's zero pollution goals.

Using treated sludge as an alternative to chemical fertilizers reduces dependence on raw material extraction but requires strict control to avoid spreading contaminants. A 2014 evaluation of the SSD highlighted shortcomings due to its outdated framework, including gaps in addressing modern pollutants (e.g., pharmaceuticals, microplastics) and its alignment with the EU's circular economy goals. It also identified a need to regulate other sludge uses and consider interactions with newer policies, such as the Urban Waste Water Treatment Directive (UWWTD). [12]

Since then, scientific advances, policy changes, and new EU strategies (e.g., Circular Economy Action Plan, Farm to Fork Strategy, Biodiversity Strategy 2030) have underscored the need to update the SSD. A comprehensive evaluation is underway to determine whether revisions are necessary to meet contemporary environmental, health, and resource efficiency needs. [13]

Related Research Articles

<span class="mw-page-title-main">Compost</span> Mixture used to improve soil fertility

Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by decomposing plant and food waste, recycling organic materials, and manure. The resulting mixture is rich in plant nutrients and beneficial organisms, such as bacteria, protozoa, nematodes, and fungi. Compost improves soil fertility in gardens, landscaping, horticulture, urban agriculture, and organic farming, reducing dependency on commercial chemical fertilizers. The benefits of compost include providing nutrients to crops as fertilizer, acting as a soil conditioner, increasing the humus or humic acid contents of the soil, and introducing beneficial microbes that help to suppress pathogens in the soil and reduce soil-borne diseases.

<span class="mw-page-title-main">Sewage sludge</span> Semi-solid material that is produced as a by-product during sewage treatment

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.

<span class="mw-page-title-main">Waste management</span> Activities and actions required to manage waste from its source to its final disposal

Waste management or waste disposal includes the processes and actions required to manage waste from its inception to its final disposal. This includes the collection, transport, treatment, and disposal of waste, together with monitoring and regulation of the waste management process and waste-related laws, technologies, and economic mechanisms.

<span class="mw-page-title-main">Industrial waste</span> Waste produced by industrial activity or manufacturing processes

Industrial waste is the waste produced by industrial activity which includes any material that is rendered useless during a manufacturing process such as that of factories, mills, and mining operations. Types of industrial waste include dirt and gravel, masonry and concrete, scrap metal, oil, solvents, chemicals, scrap lumber, even vegetable matter from restaurants. Industrial waste may be solid, semi-solid or liquid in form. It may be hazardous waste or non-hazardous waste. Industrial waste may pollute the nearby soil or adjacent water bodies, and can contaminate groundwater, lakes, streams, rivers or coastal waters. Industrial waste is often mixed into municipal waste, making accurate assessments difficult. An estimate for the US goes as high as 7.6 billion tons of industrial waste produced annually, as of 2017. Most countries have enacted legislation to deal with the problem of industrial waste, but strictness and compliance regimes vary. Enforcement is always an issue.

<span class="mw-page-title-main">Septic tank</span> Method for basic wastewater treatment (on-site)

A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater (sewage) flows for basic sewage treatment. Settling and anaerobic digestion processes reduce solids and organics, but the treatment efficiency is only moderate. Septic tank systems are a type of simple onsite sewage facility. They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed in a septic drain field, which provides further treatment. Nonetheless, groundwater pollution may occur and is a problem.

<span class="mw-page-title-main">Water pollution</span> Contamination of water bodies

Water pollution is the contamination of water bodies, with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources. These are sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater. Water pollution may affect either surface water or groundwater. This form of pollution can lead to many problems. One is the degradation of aquatic ecosystems. Another is spreading water-borne diseases when people use polluted water for drinking or irrigation. Water pollution also reduces the ecosystem services such as drinking water provided by the water resource.

<span class="mw-page-title-main">Wastewater treatment</span> Converting wastewater into an effluent for return to the water cycle

Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on the environment. It is also possible to reuse it. This process is called water reclamation. The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater the treatment plant is called a Sewage Treatment. Municipal wastewater or sewage are other names for domestic wastewater. For industrial wastewater, treatment takes place in a separate Industrial wastewater treatment, or in a sewage treatment plant. In the latter case it usually follows pre-treatment. Further types of wastewater treatment plants include Agricultural wastewater treatment and leachate treatment plants.

<span class="mw-page-title-main">Clean Water Act</span> 1972 U.S. federal law regulating water pollution

The Clean Water Act (CWA) is the primary federal law in the United States governing water pollution. Its objective is to restore and maintain the chemical, physical, and biological integrity of the nation's waters; recognizing the responsibilities of the states in addressing pollution and providing assistance to states to do so, including funding for publicly owned treatment works for the improvement of wastewater treatment; and maintaining the integrity of wetlands.

<span class="mw-page-title-main">Biosolids</span> Decontaminated sewage sludge

Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use sewage sludge from local wastewater treatment plants. Scientific research over many years has confirmed that these biosolids contain similar nutrients to those in animal manures. Biosolids that are used as fertilizer in farming are usually treated to help to prevent disease-causing pathogens from spreading to the public. Some sewage sludge can not qualify as biosolids due to persistent, bioaccumulative and toxic chemicals, radionuclides, and heavy metals at levels sufficient to contaminate soil and water when applied to land.

<span class="mw-page-title-main">Agricultural wastewater treatment</span> Farm management for controlling pollution from confined animal operations and surface runoff

Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation water. Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production. It may be performed in plants using mechanized treatment units similar to those used for industrial wastewater. Where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles. Animal slurries are usually treated by containment in anaerobic lagoons before disposal by spray or trickle application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes.

<span class="mw-page-title-main">Industrial wastewater treatment</span> Processes used for treating wastewater that is produced by industries as an undesirable by-product

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans. This applies to industries that generate wastewater with high concentrations of organic matter, toxic pollutants or nutrients such as ammonia. Some industries install a pre-treatment system to remove some pollutants, and then discharge the partially treated wastewater to the municipal sewer system.

Onsite sewage facilities (OSSF), also called septic systems, are wastewater systems designed to treat and dispose of effluent on the same property that produces the wastewater, in areas not served by public sewage infrastructure.

<span class="mw-page-title-main">Milorganite</span> Brand of biosolids fertilizer produced by treating sewage sludge

Milorganite is a brand of biosolids fertilizer produced by treating sewage sludge by the Milwaukee Metropolitan Sewerage District. The term is a portmanteau of the term Milwaukee Organic Nitrogen. The sewer system of the District collects municipal wastewater from the Milwaukee metropolitan area. After settling, wastewater is treated with microbes to break down organic matter at the Jones Island Water Reclamation Facility in Milwaukee, Wisconsin. The byproduct sewage sludge is produced. This is heat-dried with hot air in the range of 900–1,200 °F (482–649 °C), which heats the sewage sludge to at least 176 °F (80 °C) to kill pathogens. The material is then pelletized and marketed throughout the United States under the name Milorganite. The result is recycling of the nitrogen and phosphorus from the waste-stream as fertilizer. The treated wastewater is discharged to Lake Michigan.

<span class="mw-page-title-main">Wastewater quality indicators</span> Ways to test the suitability of wastewater

Wastewater quality indicators are laboratory test methodologies to assess suitability of wastewater for disposal, treatment or reuse. The main parameters in sewage that are measured to assess the sewage strength or quality as well as treatment options include: solids, indicators of organic matter, nitrogen, phosphorus, indicators of fecal contamination. Tests selected vary with the intended use or discharge location. Tests can measure physical, chemical, and biological characteristics of the wastewater. Physical characteristics include temperature and solids. Chemical characteristics include pH value, dissolved oxygen concentrations, biochemical oxygen demand (BOD) and chemical oxygen demand (COD), nitrogen, phosphorus, chlorine. Biological characteristics are determined with bioassays and aquatic toxicology tests.

<span class="mw-page-title-main">Secondary treatment</span> Biological treatment process for wastewater or sewage

Secondary treatment is the removal of biodegradable organic matter from sewage or similar kinds of wastewater. The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.

<span class="mw-page-title-main">Sewage sludge treatment</span> Processes to manage and dispose of sludge during sewage treatment

Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge treatment is focused on reducing sludge weight and volume to reduce transportation and disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.

<span class="mw-page-title-main">Sewage treatment</span> Process of removing contaminants from municipal wastewater

Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges. Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. There are a high number of sewage treatment processes to choose from. These can range from decentralized systems to large centralized systems involving a network of pipes and pump stations which convey the sewage to a treatment plant. For cities that have a combined sewer, the sewers will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage treatment often involves two main stages, called primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment stage with polishing processes and nutrient removal. Secondary treatment can reduce organic matter from sewage,  using aerobic or anaerobic biological processes. A so-called quarternary treatment step can also be added for the removal of organic micropollutants, such as pharmaceuticals. This has been implemented in full-scale for example in Sweden.

<span class="mw-page-title-main">Sewage</span> Wastewater that is produced by a community of people

Sewage is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality. Sub-types of sewage are greywater and blackwater. Sewage also contains soaps and detergents. Food waste may be present from dishwashing, and food quantities may be increased where garbage disposal units are used. In regions where toilet paper is used rather than bidets, that paper is also added to the sewage. Sewage contains macro-pollutants and micro-pollutants, and may also incorporate some municipal solid waste and pollutants from industrial wastewater.

<span class="mw-page-title-main">Reuse of human excreta</span> Safe, beneficial use of human excreta mainly in agriculture (after treatment)

Reuse of human excreta is the safe, beneficial use of treated human excreta after applying suitable treatment steps and risk management approaches that are customized for the intended reuse application. Beneficial uses of the treated excreta may focus on using the plant-available nutrients that are contained in the treated excreta. They may also make use of the organic matter and energy contained in the excreta. To a lesser extent, reuse of the excreta's water content might also take place, although this is better known as water reclamation from municipal wastewater. The intended reuse applications for the nutrient content may include: soil conditioner or fertilizer in agriculture or horticultural activities. Other reuse applications, which focus more on the organic matter content of the excreta, include use as a fuel source or as an energy source in the form of biogas.

<span class="mw-page-title-main">Jacqui Horswell</span> Environmental microbiologist

Jacqueline Horswell is an English-born New Zealand environmental microbiologist who specialises in research into the waste society produces, its effect on the environment, and how it can be managed. Her work focuses particularly on measuring the effect of microbial and chemical contaminants in sewage sludge and the safe reuse of biosolids as fertilizer by the planting of native trees to filter and inactivate pollutants from the sludge and the use of vermiculture. Horswell is involved in consultation with communities in New Zealand and has contributed to official guidelines for the management of biosolids. Her research has also provided information about soil microbial communities for forensic science using microbial cultures and DNA sequencing. Since 2018, Horswell has been a lecturer at Massey University.

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