Leachate Treatment FAQ

Activated Carbon

Activated carbon, also called activated charcoal, is a form of carbon processed to have small, low volume pores that increase the surface area available for adsorption or chemical reactions. Activated is sometimes substituted with active.

Activated Sludge

The activated sludge process is a type of wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa.

Aerated Lagoon An aerated lagoon is a simple wastewater treatment system consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters.


Leachate treatment FAQ illustration is often done by aeration as shown at this WwTW aeration lane..
CC BY-NC-ND by diamond geezer

Aeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.

Aerobic An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. In contrast, an anaerobic organism (anaerobe) is any organism that does not require oxygen for growth. Some anaerobes react negatively or even die if oxygen is present.

Aerobic Granulation

The biological treatment of wastewater in the sewage treatment plant is often accomplished using conventional activated sludge systems. These systems generally require large surface areas for treatment and biomass separation units due to the generally poor settling properties of the sludge. Aerobic granules are a type of sludge that can self-immobilize flocs and microorganisms into spherical and strong compact structures. The advantages of aerobic granular sludge are excellent settle-ability, high biomass retention, simultaneous nutrient removal and tolerance to toxicity. Recent studies show that aerobic granular sludge treatment could be a potentially good method to treat high strength wastewaters with nutrients, toxic substances.


In chemistry, alcohols are organic compounds that carry at least one hydroxyl functional group (C?OH) bound to their aliphatic substructure. The term alcohol originally referred to the primary alcohol ethanol (ethyl alcohol), which is used as a drug and is the main alcohol present in alcoholic beverages. An important class of alcohols, of which methanol and ethanol are the simplest members, includes all compounds for which the general formula is CnH2+1OH. Simple mono alcohols that are the subject of this article include primary (RCH2OH), secondary (R2CHOH), and tertiary (R3COH) alcohols.


An aldehyde is a compound containing a functional group with the structure CHO, consisting of a carbonyl centre with the carbon atom also bonded to hydrogen and to an R group, which is any generic alkyl or side chain. Aldehydes are common in organic chemistry, and many fragrances are aldehydes.


Ammonia is a compound of nitrogen and hydrogen with the formula NH3. A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a characteristic pungent smell. It is a common nitrogenous waste, particularly among aquatic organisms, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or indirectly, is also a building block for the synthesis of many pharmaceutical products and is used in many commercial cleaning products.

Ammoniacal Nitrogen

Ammoniacal nitrogen (NH3N) is a measure for the amount of ammonia, a toxic pollutant often found in landfill leachate and in waste products, such as sewage, liquid manure and other liquid organic waste products. It can also be used as a measure of the health of water in natural bodies such as rivers or lakes, or in man-made water reservoirs. The term is used widely in waste treatment and water purification systems.


The ammonium cation is a positively charged polyatomic ion with the chemical formula NH4. It is formed by the protonation of ammonia (NH3). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations (NR 4), where one or more hydrogen atoms are replaced by organic groups (indicated by R).

Anaerobic Digestion

Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.


Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface.


Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically, a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on earth and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the earth’s crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory.

Biochemical Oxygen Demand

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C and is often used as a surrogate of the degree of organic pollution of water.

Bio Clogging

Bio clogging or biological clogging is clogging of pore space in soil by microbial biomass; their body and their by-products such as extracellular polymeric substance (EPS). The microbial biomass blocks the pathway of water in the pore space, forming a certain thickness of impermeable layer in soil, and it reduces the rate of infiltration of water remarkably.

Carbon Dioxide

Carbon dioxide is a colourless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth’s atmosphere as a trace gas. The current concentration is about 0.04% (410ppm) by volume, having risen from pre-industrial levels of 280ppm. Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater. It is present in deposits of petroleum and natural gas. Carbon dioxide is odourless at normally encountered concentrations, but at high concentrations, it has a sharp and acidic odour.


Carbon is a chemical element with the symbol C and atomic number 6. It is non-metallic and tetravalent, making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.

Chemical Oxygen Demand

In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution which in SI units is milligrams per litre (mg/L). A COD test can be used to easily quantify the amount of organics in water. The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water or wastewater. COD is useful in terms of water quality by providing a metric to determine the effect an effluent will have on the receiving body, much like biochemical oxygen demand (BOD).


An air diffuser or membrane diffuser is an aeration device typically in the shape of a disc, tube or plate, which is used to transfer air and with that oxygen into the leachate, sewage or industrial wastewater. Oxygen is required by microorganisms/bacteria residents in the water to break down the pollutants. Diffusers use either rubber membranes or ceramic elements typically and produce either fine or coarse bubbles.”


Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated organic compounds that are significant environmental pollutants.

Dissolved Oxygen

Oxygen saturation is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit of oxygen saturation is percent (%).


Ecology is a branch of biology that studies the interactions among organisms and their biophysical environment, which includes both biotic and abiotic components. Topics of interest include the biodiversity, distribution, biomass, and populations of organisms, as well as cooperation and competition within and between species. Ecosystems are dynamically interacting systems of organisms, the communities they make up, and the non-living components of their environment. Ecosystem processes, such as primary production, pedogenesis, nutrient cycling, and niche construction, regulate the flux of energy and matter through an environment. These processes are sustained by organisms with specific life history traits.

Extended Aeration

Extended aeration is a method of sewage treatment using modified activated sludge procedures. It is preferred for relatively small waste loads, where lower operating efficiency is offset by mechanical simplicity.


This Leachate Treatment FAQ illustration shows a large Wastewater Facility.
CC BY by eutrophication&hypoxia

A floc is a type of microbial aggregate that may be contrasted with biofilms and granules, or else considered a specialized type of biofilm. Flocs appear as cloudy suspensions of cells floating in water, rather than attached to and growing on a surface like most biofilms. The floc typically is held together by a matrix of extracellular polymeric substance (EPS), which may contain variable amounts of polysaccharide, protein, and other biopolymers. The formation and the properties of flocs may affect the performance of industrial water treatment bioreactors such as activated sludge systems.


Flocculation, in the field of chemistry, is a process in which colloids come out of suspension in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.


A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and moulds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, fungi, which is separate from the other eukaryotic life kingdoms of plants and animals.


The basic philosophy behind geocomposite materials is to combine the best features of different materials in such a way that specific applications are addressed in the optimal manner and at minimum cost. Thus, the benefit/cost ratio is maximized. Such geocomposites will generally be geosynthetic materials, but not always. In some cases, it may be more advantageous to use a non-synthetic material with a geosynthetic one for optimum performance and/or least cost.


A geogrid is geosynthetic material used to reinforce soils and similar materials. Geogrids are commonly used to reinforce retaining walls, as well as subbases or subsoils below roads or structures. Soils pull apart under tension. Compared to soil, geogrids are strong in tension. This fact allows them to transfer forces to a larger area of soil than would otherwise be the case.


A Leachate Treatment FAQ illustration showing a geomembrane under construction on a landfill site.A geomembrane is very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid migration in a human made project, structure, or system. Geomembranes are made from relatively thin continuous polymeric sheets, but they can also be made from the impregnation of geotextiles with asphalt, elastomer or polymer sprays, or as multi-layered bitumen geocomposites. Continuous polymer sheet geomembranes are, by far, the most common.


Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Typically made from polypropylene or polyester, geotextile fabrics come in three basic forms: woven, needle punched, or heat bonded.


Groundwater is the water present beneath Earth’s surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.


Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO42H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It also forms as an evaporite mineral and as a hydration product of anhydrite.


The halogens are a group in the periodic table consisting of five chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The artificially created element 117 may also be a halogen. In the modern IUPAC nomenclature, this group is known as group 17.

Hydraulic Head

Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum.

Hydraulic Retention Time

The residence time of a particle is the total time that the particle has spent inside a control volume. The residence time of a set of particles is quantified in terms of the frequency distribution of the residence time in the set, which is known as residence time distribution, or in terms of its average, known as mean residence time.

Hydrogen Sulfide

Hydrogen sulfide is the chemical compound with the formula H2S. It is a colourless chalcogen hydride gas with the characteristic foul odour of rotten eggs. It is very poisonous, corrosive, and flammable.

Industrial Waste

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, industries, 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, liquid or gaseous. It may be hazardous or non-hazardous waste. Hazardous waste may be toxic, ignitable, corrosive, reactive, or radioactive. Industrial waste may pollute the air, the soil, or nearby water sources, eventually ending up in the sea. 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 every year. Most countries have enacted legislation to deal with the problem of industrial waste, but strictness and compliance regimes vary.

Industrial Wastewater Treatment

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.


The watt is a unit of power. In the International System of Units (SI) it is defined as a derived unit of 1 joule per second and is used to quantify the rate of energy transfer.


A landfill site is a site for the disposal of waste materials by burial. Landfill is the oldest form of waste treatment, although the burial of the waste is modern; historically, refuse was simply left in piles or thrown into pits. Historically, landfills have been the most common method of organized waste disposal and remain so in many places around the world.

Landfill Gas

Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill. Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Trace amounts of other volatile organic compounds (VOCs) comprise the remainder (<1%). These trace gases include a large array of species, mainly simple hydrocarbons.

Membrane Bioreactor

Membrane bioreactor (MBR) is the combination of a membrane process like microfiltration or ultrafiltration with a biological wastewater treatment process, the activated sludge process. It is now widely used for municipal and industrial wastewater treatment.


A thiol is any organosulfur compound of the form R?SH, where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl group.


Methane is a chemical compound with the chemical formula CH4 (one atom of carbon and four atoms of hydrogen). It is a group 14 hydride and the simplest alkane and is the main constituent of natural gas. The relative abundance of methane on Earth makes it an attractive fuel, although capturing and storing it poses challenges due to its gaseous state under normal conditions for temperature and pressure.


A microorganism, or microbe, is a microscopic organism, which may exist in its single celled form or in a colony of cells.

Mixed Liquor Volatile Suspended Solids MLSS

Mixed liquor suspended solids (MLSS) is the concentration of suspended solids, in an aeration tank during the activated sludge process, which occurs during the treatment of wastewater. The units MLSS is primarily measured in are milligram per litre (mg/L), but for activated sludge its mostly measured in gram per litre [g/L] which is equal to kilogram per cubic metre [kg/m3]. Mixed liquor is a combination of raw or unsettled wastewater or pre-settled wastewater and activated sludge within an aeration tank. MLSS consists mostly of microorganisms and nonbiodegradable suspended matter. MLSS is an important part of the activated sludge process to ensure that there is a sufficient quantity of active biomass available to consume the applied quantity of organic pollutant at any time. This is known as the food to microorganism ratio, more commonly notated as the F/M ratio. By maintaining this ratio at the appropriate level, the biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes the lower the biochemical oxygen demand (BOD) will be in the discharge. It is important that MLSS removes COD and BOD in order to purify water for clean surface waters, and subsequently clean drinking water and hygiene. Raw sewage enters in the water treatment process with a concentration of sometimes several hundred mg/L of BOD. Upon being treated with MLSS and other methods of treatment, the concentration of BOD in water is lowered to less than 2 mg/L, which is considered to be clean, safe to discharge to surface waters or to reuse water.

Municipal Solid Waste

Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. “Garbage” can also refer specifically to food waste, as in a garbage disposal; the two are sometimes collected separately. In the European Union, the semantic definition is ‘mixed municipal waste,’ given waste code 20 03 01 in the European Waste Catalog. Although the waste may originate from a number of sources that has nothing to do with a municipality, the traditional role of municipalities in collecting and managing these kinds of waste have produced the particular etymology ‘municipal.’


Nitrogen is the chemical element with the symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is generally accorded the credit because his work was published first.


A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excreted by cells to create noncellular structures, such as hair, scales, feathers, or exoskeletons. Some nutrients can be metabolically converted to smaller molecules in the process of releasing energy, such as for carbohydrates, lipids, proteins, and fermentation products, leading to end products of water and carbon dioxide. All organisms require water. Essential nutrients for animals are the energy sources, some of the amino acids that are combined to create proteins, a subset of fatty acids, vitamins and certain minerals. Plants require more diverse minerals absorbed through roots, plus carbon dioxide and oxygen absorbed through leaves. Fungi live on dead or living organic matter and meet nutrient needs from their host.


Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial and aquatic environments. It is matter composed of organic compounds that have come from the remains of organisms such as plants and animals and their waste products in the environment. Organic molecules can also be made by chemical reactions that don’t involve life. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and carbohydrates. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet.

Organic Acid

An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group nCOOH. Sulfonic acids, containing the group SO2OH, are relatively stronger acids. Alcohols, with OH, can act as acids but they are usually very weak. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: the thiol group, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally referred to as organic acids.


Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive non-metal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. By mass, oxygen is the third most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colourless and odourless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth’s atmosphere. As compounds including oxides, the element makes up almost half of the Earth’s crust.


In biology, a pathogen in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.


A polychlorinated biphenyl (PCB) is an organic chlorine compound with the formula C12H10xClx. Polychlorinated biphenyls were once widely deployed as dielectric and coolant fluids in electrical apparatus, carbonless copy paper and in heat transfer fluids.


In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7.


A Phosphate is a chemical derivative of phosphoric acid. The phosphate ion (PO4O3) is an inorganic chemical, the conjugate base that can form many different salts. In organic chemistry, a phosphate, or organo-phosphate, is an ester of phosphoric acid. Of the various phosphoric acids and phosphates, organic phosphates are important in biochemistry and bio-geochemistry, and inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry. At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates.


Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth. It has a concentration in the Earth’s crust of about one gram per kilogram. In minerals, phosphorus generally occurs as phosphate.

Polychlorinated Dibenzodioxins (Dioxins)

Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated organic compounds that are significant environmental pollutants.


In meteorology, precipitation is any product of the condensation of atmospheric water vapour that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and “precipitates”. Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate.


Protozoa is an informal term for single celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. Historically, the protozoa were regarded as “one celled animals”, because they often possess animal like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single celled organisms that can move independently and feed by heterotrophy.

Public Health

Public health has been defined as “the science and art of preventing disease, prolonging life and promoting human health through organized efforts and informed choices of society, organizations, public and private, communities and individuals”. Analysing the health of a population and the threats it faces is the basis for public health. The public can be as small as a handful of people or as large as a village or an entire city; in the case of a pandemic it may encompass several continents. The concept of health takes into account physical, psychological and social wellbeing. As such, according to the World Health Organization, it is not merely the absence of disease or infirmity.

Rotating Biological Contactor

A rotating biological contactor or RBC is a biological treatment process used in the treatment of wastewater following primary treatment. The primary treatment process means protection by removal of grit and sand and coarse material through a screening process, followed by a removal process of sediment by settling. The RBC process involves allowing the wastewater to come in contact with a biological medium in order to remove pollutants in the wastewater before discharge of the treated wastewater to the environment, usually a body of water. A rotating biological contactor is a type of secondary (Biological) treatment process. It consists of a series of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of the wastewater. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place.


The rotifers, commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera) of microscopic and near microscopic pseudocoelomate animals.

Sanitary Sewer

A sanitary sewer or foul sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings to treatment facilities or disposal. Sanitary sewers are part of an overall system called a sewage system or sewerage.

Sequencing Batch Reactors

Sequencing batch reactors (SBR) or sequential batch reactors are a type of activated sludge process for the treatment of wastewater. SBR reactors treat wastewater such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the mixture of wastewater and activated sludge to reduce the organic matter. The treated effluent may be suitable for discharge to surface waters or possibly for use on land.

Sewage Fungus

Sphaerotilus natans is an aquatic periphyton organism associated with polluted water. It forms colonies commonly known as “sewage fungus”, but later identified as tightly sheathed filamentous bacteria.

Sewage Treatment

Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater. Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater that is safe enough for release into the environment. A by-product of sewage treatment is a semisolid waste or slurry, called sewage sludge. The sludge has to undergo further treatment before being suitable for disposal or application to land.


A sanitary sewer or foul sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings to treatment facilities or disposal. Sanitary sewers are part of an overall system called a sewage system or sewerage.


Sludge is a semisolid slurry that can be produced from a range of industrial processes, from water treatment, wastewater treatment or onsite sanitation systems. For example, it can be produced as a settled suspension obtained from conventional drinking water treatment, as sewage sludge from wastewater treatment processes 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. Sludge can consist of a variety of particles, such as animal manure.

Sludge Bulking

In treatment of sewage one process used is the activated sludge process in which air is passed through a mixture of sewage and old sludge to allow the microorganisms to break down the organic components of the sewage. Sludge is continually drawn off as new sewage enters the tank and this sludge must then be settled so that the supernatant can be separated to pass on to further stages of treatment.

Sludge Treatment

Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.


Solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.

Sphaerotilus Natans

Sphaerotilus natans is an aquatic periphyton organism associated with polluted water. It forms colonies commonly known as “sewage fungus”, but later identified as tightly sheathed filamentous bacteria.

Submersible Mixers

A submersible mixer is a mechanical device that is used to mix sludge tanks and other liquid volumes. Submersible mixers are often used in sewage treatment plants to keep solids in suspension in the various process tanks and/or sludge holding tanks.

Sulfuric Acid

Sulfuric acid (alternative spelling sulphuric acid), also known as vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with molecular formula H2SO4. It is a colourless, odourless, and viscous liquid that is soluble in water and is synthesized in reactions that are highly exothermic.


Precipitation is the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the ‘precipitate’. The chemical that causes the solid to form is called the ‘precipitant’. Without sufficient force of gravity (settling) to bring the solid particles together, the precipitate remains in suspension. After sedimentation, especially when using a centrifuge to press it into a compact mass, the precipitate may be referred to as a ‘pellet’. Precipitation can be used as a medium. The precipitate-free liquid remaining above the solid is called the ‘supernate’ or ‘supernatant’. Powders derived from precipitation have also historically been known as ‘flowers’. When the solid appears in the form of cellulose fibres which have been through chemical processing, the process is often referred to as regeneration.


In chemistry, a suspension is a heterogeneous mixture that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometre, and will eventually settle, although the mixture is only classified as a suspension when and while the particles have not settled out. A suspension is a heterogeneous mixture in which the solute particles do not dissolve, but get suspended throughout the bulk of the solvent, left floating around freely in the medium. The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the suspended particles are smaller and do not settle. Colloids and suspensions are different from solution, in which the dissolved substance (solute) does not exist as a solid, and solvent and solute are homogeneously mixed.


Waste are unwanted or unusable materials. Waste is any substance which is discarded after primary use, or is worthless, defective and of no use. A by-product by contrast is a joint product of relatively minor economic value. A waste product may become a by-product, joint product or resource through an invention that raises a waste product’s value above zero.


Wastewater is any water that has been affected by human use. Wastewater is “used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff or stormwater, and any sewer inflow or sewer infiltration”. Therefore, wastewater is a by-product of domestic, industrial, commercial or agricultural activities. The characteristics of wastewater vary depending on the source. Types of wastewater include domestic wastewater from households, municipal wastewater from communities and industrial wastewater. Wastewater can contain physical, chemical and biological pollutants.

Wastewater Treatment

Wastewater treatment is a process used to remove contaminants from wastewater or sewage and convert it into an effluent that can be returned to the water cycle with minimum impact on the environment, or directly reused. The latter is called water reclamation because treated wastewater can be used for other purposes. The treatment process takes place in a wastewater treatment plant (WWTP), often referred to as a Water Resource Recovery Facility (WRRF) or a Sewage Treatment Plant (STP). Pollutants in municipal wastewater are removed or broken down.

World Health Organization

The World Health Organization (WHO) is a specialized agency of the United Nations that is concerned with international public health. It was established on 7 April 1948, and is headquartered in Geneva, Switzerland. The WHO is a member of the United Nations Development Group. Its predecessor, the Health Organization, was an agency of the League of Nations.