Liquid Waste Management

Fluid wastes consisting of sewage and residential wastewater, or treated water or other liquids, generated by industrial operations, particularly in the pulp and paper industry, the food processing industry, and the chemical manufacturing industry.

  • As such, liquid waste can be defined. Liquids such as wastewater, fats, oils, or grease (FOG), used oil, liquids, sediments, gases, or sludges; hazardous liquids from the home.
  • These hazardous or potentially hazardous liquids pose a threat to human health or the environment.
  • They can also be the byproducts of manufacturing processes or abandoned commercial products designated as “Liquid Industrial Waste,” such as cleaning fluids or pesticides.
  • There are broad waste restrictions, and special regulations apply to the generation, storage, transportation, treatment, and disposal of hazardous and liquid wastes.

What is Liquid Waste Management?

  • Liquid waste management is a method for preventing the discharge of pollutants into waterways by collecting and disposing of hazardous liquid items.
  • Trash management entails overseeing all phases of waste production and disposal. It consists of trash collection, transportation, treatment, and disposal.
  • Waste Management, in its simplest form, is the process of managing and differentiating waste based on its relevance to the environment, in which useful wastes are reduced, reused, and recycled, while non-usable wastes are disposed of properly without or with minimal impact on the environment.

Characteristics of Sewage/Liquid Waste

  • Sewage has extraordinarily high Biochemical Oxygen Demand (BOD) and Oxygen Consumption (OC) values.
  • Due to the lack of oxygen, sewage organic matter undergoes anaerobic or partial decomposition, resulting in the generation of toxic gases such as CH3, CO, and H2S. In addition to being hazardous, these gases react with water to form acids.
  • Large quantities of acid production increase the acidity of sewage, rendering it unsuited for supporting life.
  • Sewage typically contains abnormally high concentrations of heavy metals.

In sewage, the death of oxygen-dependent organisms such as aerobic microbes, plants, and animals is caused by the sewage’s anoxic condition, high acidity, high concentration of heavy metals, and reduced photosynthetic rate due to inadequate illumination. This is why organisms capable of thriving in anaerobic environments predominate in sewage.


Compositions of liquid wastes/wastewater

According to their physical, chemical, and biological features, liquid wastes can be defined.

Physical characteristics of liquid wastes


  • Solid particles may be transported together with the flow of wastewater. These solids may be settleable or suspended.
  • When the flow rate is reduced, settleable solids sink to the bottom (settle out), such as when wastewater is kept in a tank.
  • Suspended solids are microscopic particles that remain suspended in water; they do not dissolve in wastewater but are transported by it.
  • The concentration of solids can be determined by filtering and weighing the solids in a given amount of water.
  • The laboratory process involves weighing a filter paper, pouring a measured volume of water through the paper, drying the paper, and weighing it once more.
  • The mass difference equals the mass of solids, which can be expressed as milligrammes of solid matter per litre of water, using the unit symbol mg l-1.


  • Generally, wastewaters are warmer than the ambient temperature. This is due to the fact that warm or hot water may be present in the waste stream from home activities such as showering or industrial processes.
  • The temperature is expressed in Celsius (oC).


  • Typically, the odour of wastewater is caused by the wastewater’s biodegradation, which results in the release of gases.
  • Biodegradation is the destruction (breakdown) of organic compounds by microorganisms and bacteria.
  • Organic matter is any substance derived by living organisms, including human and animal waste, food waste, paper, and agricultural waste.
  • Odor detection is typically a subjective process, however it is possible to quantify it using smell units.

Chemical characteristics of liquid wastes

Organic matter

  • Wastewaters from numerous sources contain organic materials, which is a common cause of surface water contamination. If organic matter is dumped into a river or lake, bacteria and other naturally occurring microorganisms will breakdown the trash while consuming dissolved oxygen from the water.
  • If there is an abundance of organic matter, most or all of the dissolved oxygen may be consumed, depriving other aquatic organisms of this vital ingredient. The oxygen required for the degradation of organic matter is known as its oxygen requirement.
  • This is determined using a metric known as the biochemical oxygen demand (BOD). BOD tests are conducted in a laboratory and entail monitoring the quantity of oxygen consumed, often over five days, as organic matter in wastewater decomposes.
  • In milligrammes per litre, the result represents the quantity of oxygen consumed in decomposing the organic stuff in the effluent (mg l-1).
  • The chemical oxygen demand (COD) test is an additional chemical method for assessing the amount of organic materials. This test is far faster than the BOD test, taking only two hours to complete. Rather than biological disintegration, it relies on the chemical oxidation of organic materials.
  • To oxidise the organic materials, a sample of wastewater is boiled with a mixture of strong acids and a determined amount of oxidising agent. At the conclusion of the test, the remaining amount of oxidising agent is measured.
  • The amount consumed corresponds to the amount of organic stuff present in the sample. Again, the result is reported in mg l-1. Because the chemical process can oxidise more material than the biological process, COD outcomes tend to be greater.

Inorganic material

  • Additionally, wastewater contains inorganic compounds. This comprises a vast variety of chemicals as well as inert materials such as sand and silt, as well as all substances that are not derived from animals or plants.
  • Numerous inorganic compounds are dissolved in water, and while some are safe, others are harmful pollutants that can harm aquatic life, such as fish and other aquatic species.
  • Ammonia (NH3), which is present in human and animal excreta, is one example. Similar to organic stuff, ammonia is decomposed by natural processes in the environment. If ammonia is spilled into a river, microbes convert it into nitrate (NO3), which is less hazardous. This spontaneous conversion of ammonia to nitrate requires oxygen and is restricted when ammonia levels are excessive.
  • Chloride (from salt), phosphates (from chemical fertilisers and human and animal wastes), and metal compounds are further examples of inorganic chemicals found in wastewaters (from mining operations or metal-plating plants).

Biochemical oxygen demand (BOD)

The biochemical oxygen demand (BOD) is a measurement of the quantity of oxygen utilised in the respiration of bacteria to oxidise organic materials in sewage and for the subsequent metabolism (oxidation) of cellular components generated from wastes. One of the fundamental reasons for treating wastewater before returning it to a body of water (e.g., a stream or lake) is to lessen the demand on the dissolved oxygen supply of the receiving body of water. The BOD is proportional to the amount of oxidizable organic material in the wastewater; the more oxidizable organic material, the greater the BOD. BOD level is used to quantify the “strength” of wastewater.


Microorganisms content

Coliforms Bacteria

  • Coliforms, including Escherichia coli, belong to the Enterobacteriaceae family.
  • These bacteria are typically found in the intestines of humans and other animals, and they are widely employed as indicator organisms.
  • They lose viability in freshwater more slowly than the majority of important gut pathogens. When such “foreign” enteric indicator bacteria are not identifiable in a particular volume of water (often 100 millilitres), the water is deemed drinkable (Latin potabilis, fit to drink).
  • Included in the Order Enterobacteriales are pathogens and mutualists. However, coliforms contain a variety of bacteria whose primary source may not be the gastrointestinal tract. To address this, assays for the presence of faecal coliforms have been developed.
  • These are coliforms from the intestines of warm-blooded animals that can only develop at 44.5 °C.
  • Other indicator bacteria include enterococci isolated from faeces. These microorganisms serve as indicators of faeces contamination in brackish and marine water.
  • These bacteria die more slowly than faecal coliforms in saltwater, making them a more trustworthy indicator of recent pollution.

Fecal Streptococcus

  • Fecal streptococci are enteric bacteria present in the intestines of humans and other warm-blooded animals.
  • This group is represented by Streptococcus foecalis; additional species include S. foecium, S. bovis, and S. equinus. Since faecal streptococci, specifically S. foecalis, are prevalent in the large intestines of people, their presence in water indicates faecal contamination.

Slime-Forming Bacteria

  • Numerous bacteria are capable of producing gelatinous or mucilaginous substances, either as capsular structures or extracellular excretions.
  • The organic and inorganic components of the water, which provide nutrition for the bacteria, influence the production of slime and the organisms responsible for its production.

Iron Bacteria

  • Iron bacteria are among the most prevalent nuisance species in water.
  • They convert soluble iron compounds to insoluble iron compounds (ferric hydroxide), which may be deposited surrounding the organism (SphaerotiIus) or secreted to form stalks or ribbons connected to the cell (Goflionella).
  • This deposition and accumulation of insoluble material in the piping system could potentially have a substantial impact on the water flow rate.
  • Additionally, iron bacteria can produce slime, discolour water, and cause unpleasant aromas and flavours.

Sulfur Bacteria

  • Some sulphur bacteria can produce and tolerate extremely acidic conditions.
  • Organisms of the genus Thiobocillus oxidise elemental sulphur to sulfuric acid and can produce an acidity within the range p1-I 1; therefore, they may be responsible for pipe corrosion.
  • Desulfovibrio desulfuricans transforms sulphates and other sulfur-containing molecules into hydrogen sulphide.


  • When water is exposed to sunlight, algal growth frequently occurs; the occurrence of algae in water is comparable to weed development in a garden.
  • Algae are present in all natural aquatic settings. Their nuisance qualities include producing turbidity, discoloration, odour, and flavour in water.
  • Frequently, algae are the leading cause of filter blockage during water purification.
  • In this regard, diatoms are the most significant, although green and yellow algae are also involved.
  • In addition to these undesirable qualities, certain algae are capable of creating chemicals that are harmful to humans and animals.


  • Numerous viruses are known to be expelled by people via the gastrointestinal tract, and they may enter drinking water sources via sewage.
  • The most prevalent viruses in sewage are the enteroviruses, which include the polio, coxsackie, and echo viruses.
  • The infectious hepatitis virus has been isolated from dirty water and seafood; these sources have been linked to the spread of this disease. Rotaviruses are also extremely significant.
  • The likelihood that virus infections, particularly enteric virus diseases, may be waterborne suggests that procedures for evaluating the virological safety of a water supply should be devised.
  • Significant research is currently being conducted to create a standard test for the detection of viruses in water and wastewater. Simultaneously, increased emphasis is being placed on evaluating the efficacy of water treatment procedures for the removal and/or inactivation of viruses.

Sources of liquid waste

Liquid wastes from residential areas

  • In metropolitan settings, residential liquid wastes are commonly referred to as domestic wastewaters. These wastewaters are the result of our daily activities, including meal preparation, washing, bathing, and toilet use.

Liquid wastes from commercial areas

  • The wastewaters from commercial areas, including businesses, stores, open markets, restaurants, and cafes, will resemble those from residential areas the most.
  • This is due to the fact that only human-related activities are conducted in such regions, as opposed to industrial production.
  • The presence of excessive levels of cooking oil in the effluent of restaurants and cafes can be mitigated by installing a grease trap in their outflow pipes.
  • A grease trap is a tiny tank or chamber that reduces the flow rate of wastewater. In the grease trap, fats, oils, and grease float to the surface of the wastewater and form an internal scum layer. This can then be disposed of as solid garbage. The grease trap expels relatively clean water for disposal.

Liquid wastes from industrial areas

  • In industrial regions, liquid wastes are produced by processing or manufacturing companies as well as service sectors, such as auto repair shops. The type of industry determines the waste’s composition.
  • Other industries’ wastewaters may contain a variety of chemical compounds, some of which may be detrimental to human health (and therefore potentially harmful).
  • Before being discharged into the environment, industrial wastewaters containing hazardous compounds must be treated and the pollutants eliminated.
  • The presence of hazardous substances is one characteristic that distinguishes industrial wastewater from home wastewater.
  • Moreover, the flow rate might change significantly in certain industries, such as those whose output rates fluctuate with the seasons, such as the processing of certain food crops.


  • Although stormwater is not a form of liquid waste in the same sense as wastes from residential, commercial, or industrial locations, it is a form of wastewater.
  • Stormwater can be contaminated by a variety of pollutants, including faeces, silt, rubber from vehicle tyre wear, litter, and motor oil.
  • During the rainy season, enormous volumes of stormwater affect several regions of Ethiopia.
  • In areas with a sewerage network (a system of sewers), rainfall may flow into the sewers or into open ditches.

Liquid Waste Disposal Methods


  • In this process, water is extracted from liquid waste until only solid trash remains. The solid waste is subsequently discarded, but the water can be reused or recycled.
  • This disposal method is only appropriate for nonhazardous liquid wastes.
  • Using the idea of density difference, centrifugation can be used to separate solid contaminants from liquid waste that contains physically distinct components.
  • Sludges and slurry wastes can be filtered using belt filter presses that use gravity to separate sludge from water and collect the leftover sludge in a trough. This sludge is then compressed between two filters using the pressure of two rollers to extract any remaining water.
  • Dewatering is often utilised in the construction industry for liquid waste disposal.


  • This method is also limited to nonhazardous liquid wastes. Frequently, it is the first step in municipal sewage water treatment and trash disposal.
  • Sedimentation is a straightforward process that does not require sophisticated machinery. It utilises gravity to separate solid trash from liquid waste.
  • The liquid is left undisturbed in a large tank known as a sedimentation basin, where the solid waste settles down due to the difference in density while contaminants with lower density, such as oil and grease, rise to the top.
  • Once this procedure is complete, the wastes are removed carefully and the water is recycled for further use, while the wastes are disposed of safely.


  • Incineration can be used to dispose of hazardous material such as chemicals, scrap metals, acids and bases in liquids and water.
  • In this process, liquid waste is burnt at extremely high temperatures in combustion chambers, producing hot gases and ashes. The ash residue can be discarded, whilst the treated gases can be released into the environment. Heat energy can also be collected from the gases for residential or industrial applications, such as cooking or powering turbines. The remaining water is free of pollutants and acceptable for consumption.
  • There are two types of furnaces utilised in this process:
    • Fluidized-bed heater: Fluidized-bed furnaces are industrial furnaces that use pressure to transform a bed of solid particles or solid-fluid mixtures into a fluid-like state. These incinerators include one boiling, heated bed of sand, ash, or limestone, with oxygen fed in to enhance heat combustion. Their size enables for complete and efficient combustion.
    • Multiple-hearth furnace: A multiple-hearth furnace utilises numerous chambers piled on top of one another to incinerate massive volumes of trash at various stages, all at continuous, constant rates. Because the chambers are layered, they are compact, easy to fit into tight spaces, and relatively inexpensive to construct and install.
  • Incineration is not always the best approach to dispose of liquid waste. In contrast to the procedures described above, incineration is detrimental to the environment since it emits hazardous pollutants and greenhouse gases. It can diminish air quality, worsen asthma and other respiratory disorders, and contribute to climate change.
  • Also expensive to install, maintain, and operate are incinerators. In certain instances, however, establishments opt for incineration since it is efficient and leaves behind little waste that requires further disposal.

Root Zone

  • Root zone waste treatment is generally utilised for household wastes such as water from toilets, sinks, and bathtubs.
  • This method of waste disposal uses both biological and physical mechanisms to remediate water. It is somewhat sophisticated and costly, but it recycles water entirely, rendering it safe for reuse in the environment.
  • The water is treated in many stages: preliminary treatment in a settler, initial treatment in an anaerobic baffled reactor, secondary treatment in an anaerobic filter, and final treatment in a planted gravel filter.
  • Root zone approach requires less external energy since it relies primarily on gravity. It also doesn’t require frequent maintenance.
  • Root-zone treatment may involve a series of filtration techniques such as the following:
  • Pretreatment sedimentation: The water is initially allowed to settle in a sedimentation basin so that some of its solid particles can precipitate for simple removal.
    • Anaerobic reactor: In the subsequent phase, the liquid waste may be sent through an anaerobic reactor. Typically, the reactor’s baffled construction provides numerous interior compartments for the water to flow through. As the water flows through, the microorganisms that have accumulated on the compartment surfaces consume an increasing amount of suspended materials.
    • Anaerobic filter: An anaerobic filter has a filter medium on which microorganism colonies can develop. These microbes consume more suspended particles, hence purifying the liquid waste.
    • Plant-filled gravel filter: After the water has been subjected to the initial treatments, it passes through a gravel bed containing living plant roots. The plants are often robust reeds that provide resistance to the flow of water. As the plants respire, they contribute oxygen to the effluent and aid in the elimination of any leftover pollutants.
  • Root-zone therapy has numerous advantages. The water normally flows downhill from one stage to the next, reducing the need for pumps and valves.
  • Root-zone technology is also extremely environmentally friendly, consuming only 20% of the energy of a normal sewage treatment facility. In addition, a well-established plant bed often requires less upkeep.
  • However, because it comprises so many components, root-zone therapy can be costly and may not be offered in many places due to its intricate installation.


  • Composting involves the use of microbes for the decomposition of liquid organic wastes by placing them in a pit for an extended period of time.
  • The nutrient content of organic wastes, such as nitrogen, salt, and potassium, is particularly useful for the land and the ecosystem.
  • Once the water has been extracted, the solid waste can be converted into compost, while the water can be recycled and returned to the environment.
  • The bacteria in the composting pit can metabolise solid organic waste and reduce its volume by up to 50 percent.
  • This is an exceptionally eco-friendly technique of garbage disposal.


  • With this method, liquid waste disposal firms may easily transform liquid waste into solid garbage without adding bulking agents such as fly ash, sawdust, or lime dust to increase its volume.
  • Depending on the type of bulking agent employed, solidification may increase greenhouse gas emissions, despite being an environmentally safe alternative to the hazardous subterranean injection procedure.
  • Transporting a substantial amount of solid garbage is also expensive.

Considerations When Choosing Your Liquid Waste Disposal Method

There is no single garbage disposal method that is optimal in every circumstance. When deciding on a liquid waste disposal method, you must analyse the advantages and downsides, evaluate your waste creation patterns and disposal demands, and make a decision that meets your needs the most effectively. As you deliberate, please bear the following factors in mind:

  • Soil formation and stability: The disposal site you’re considering must have stable soil capable of containing waste. Soils that are softer and looser may permit shifting and leakage. If this is the case in your location, you may need to choose an alternative to land disposal, such as incineration.
  • Land space: The availability of suitable land for liquid waste disposal will also influence your decision. If space is at a premium, your disposal options may be limited, and you may be required to avoid solidification and other processes that generate enormous amounts of trash.
  • Waste quantity: Similarly, if your facility generates large quantities of liquid waste, you will need to select an appropriate disposal strategy. Even while composting is beneficial for the environment, if you have an excessive amount of garbage, you may not be able to provide the necessary resources for it.
  • Necessary treatment: Certain liquid wastes contain little contaminants and require just minor treatment. Others are highly contaminated and will have rigorous treatment before to disposal. For sewage with a high proportion of biosolids, for example, root-zone treatment would be insufficient. Ensure that the proposed disposal procedure is thorough enough to keep you in compliance with legislation.
  • Well water sources: Investigate whether or not residents in your neighbourhood use well water. If so, determine the origin of the water supply. You must ensure that your waste disposal site is far from any water source.
  • Surface water sources: Similarly, if the suggested disposal site for your liquid waste is in close proximity to surface water sources, you will also need to avoid these. A leak at the waste disposal site could allow contaminated runoff to enter the surface water sources, endangering the health and safety of neighbours.
  • Water table level: The level of the water table for groundwater is also an important factor to consider. If the water table is high, dumping sites must stay shallow to prevent water contamination.
  • Cost: In addition to environmental considerations, the cost of liquid waste disposal is a major factor. Determine which of the disposal technologies you are considering fits your facility’s budget the best by comparing their relative costs.

Disposal of inadequately treated wastewater leads to

  • Greater likelihood of harmful germs spreading.
  • Increased risk associated with consuming natural bodies of water.
  • Pollution has contaminated oysters and other shellfish, rendering them unsuitable for human consumption.
  • Pollution of the winter feeding sites of ducks has caused significant population declines.
  • Swimming in the water becomes more hazardous, and the water loses its utility for other recreational activities.
  • Unstable organic debris in sewage depletes the oxygen supply of the water, destroying aquatic life.
  • The development of a variety of unpleasant conditions, such as foul scents and debris collection, which lower property values.
  • Accumulation and diffusion of harmful substances that damage ecosystems and public health.


  • Beiras, R. (2018). Liquid Wastes. Marine Pollution, 53–67. doi:10.1016/b978-0-12-813736-9.00005-2 
  • Hocking, M. B. (2005). Raw Water Processing and Wastewater Treatment. Handbook of Chemical Technology and Pollution Control, 139–174. doi:10.1016/b978-012088796-5/50008-9 
  • Prabhu, S.Venkatesa & T.G, Nithya & Masi, Chandran & Chinnasamy, Gomadurai & Abda, Ebrahim M.. (2021). Recent advances and prospects for industrial waste management and product recovery for environmental appliances. Physical Sciences Reviews. 10.1515/psr-2021-0063. 
  • Syed, Sabir. (2006). Solid and Liquid Waste Management. Emirates Journal for Engineering Research. 11. 19-36.

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