Sewage treatment

• 1. Introduction

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  Sewage treatment is the work of taking the wastewater that drains from households and businesses and turning it into an effluent clean enough to send back into a river, a bay, or a field. At the global level, an estimated 52% of sewage is treated. That single figure hides a deep divide. High-income countries treat roughly 74% of their sewage, while developing countries treat an average of just 4.2%. Behind those percentages sits a question of survival. When the breakdown of human waste is left entirely to a river or an ocean, the result can be water pollution, eutrophication, and disease. So how does a treatment plant strip contaminants from a stream of filth? Why do some places spend fortunes on energy-hungry tanks while others rely on ponds and sunlight? And what happens when the things we flush, from drugs to detergents, refuse to break down at all?

• 2. Stages Of Removal

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  Preliminary treatment meets the raw sewage first, removing coarse material before it can damage or clog pumps and lines. A bar screen catches cans, rags, sticks, and plastic packets carried in the flow. In large modern plants the screen is a mechanically raked bar screen; in smaller or less modern plants, a manually cleaned screen may be used. Grit removal follows, where the velocity of the incoming sewage is slowed so sand, gravel, and rocks can settle. This protects closely machined surfaces in equipment like comminutors, centrifuges, and high pressure diaphragm pumps from abrasion.

  Primary treatment lets sewage pass slowly through a basin where heavy solids settle to the bottom and oil, grease, and lighter solids float to the surface to be skimmed off. These basins are called primary sedimentation tanks or primary clarifiers, and they typically hold the flow for 1.5 to 2.5 hours. Mechanically driven scrapers push the collected sludge toward a hopper, from which it is pumped to sludge treatment. Primary sedimentation tanks remove about 50 to 70% of the suspended solids and 25 to 40% of the biological oxygen demand.

  Secondary treatment goes after the dissolved material that settling cannot catch, especially the organic fraction. Microorganisms feed on the organic matter, grow, and multiply, forming flocs or biofilms that clump into a settleable sludge. A percentage of those settled bio-solids are reintroduced at the influent as an innoculant to keep the colony alive. This biological work, done through aerobic or anaerobic processes, reduces the organic matter measured as biological oxygen demand. Tertiary treatment adds a final polishing stage to push effluent quality higher before discharge into a sensitive ecosystem such as an estuary, a low-flow river, or a coral reef.

• 3. Killing The Pathogens

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  Disinfection aims to kill disease-causing microorganisms before treated sewage returns to the environment or is reused. Its effectiveness rises as more of the earlier treatment steps are completed, because water with high turbidity shields organisms from harm. Short contact times, low doses, and high flows all work against a clean kill. The common methods are ozone, chlorine, ultraviolet light, and sodium hypochlorite.

  Chlorination remains the most common form of treated sewage disinfection in many countries, thanks to its low cost and a long history of working. It carries a cost beyond the chemical itself. Chlorinating residual organic material can generate chlorinated-organic compounds that may be carcinogenic, and because residual chlorine is toxic to aquatic species, the effluent must be chemically dechlorinated afterward.

  Ultraviolet light avoids chemicals entirely, damaging the genetic structure of bacteria, viruses, and other pathogens so they cannot reproduce. Its weaknesses are the need for frequent lamp maintenance and the need for a highly treated effluent, since any solids present can hide microorganisms from the radiation. Ozone offers another path. It is generated on-site from oxygen in the ambient air, which makes it safer than chlorine that has to be stored on site and is highly poisonous in an accidental release. Ozone is unstable and reactive, oxidizing most organic material it touches, though the generation equipment is costly and demands special operators. Heat sterilization can go where light cannot, penetrating liquids that are not transparent and even solid materials within wastewater.

• 4. Nitrogen And Phosphorus

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  Sewage may contain high levels of nitrogen and phosphorus, and excessive release feeds eutrophication. That process can trigger algal blooms, and some algal species produce toxins that contaminate drinking water supplies. Free ammonia is toxic to fish, and high nitrite or nitrate in drinking water carries public health significance through a disease called methemoglobinemia.

  Nitrogen is removed by biological oxidation from ammonia to nitrate, called nitrification, followed by denitrification, which reduces nitrate to nitrogen gas that escapes to the atmosphere. Nitrification is a two-step aerobic process. The oxidation of ammonia to nitrite is most often facilitated by bacteria such as Nitrosomonas spp., while nitrite oxidation to nitrate is now known to be carried out predominantly by Nitrospira spp., rather than the Nitrobacter spp. once believed responsible. Denitrification requires anoxic conditions, meaning oxygen is absent but nitrate is present, and it needs an electron donor that can be organic matter from the sewage itself, sulfide, or an added donor like methanol.

  Phosphorus is a limiting nutrient for algae growth in many fresh water systems, so an excess can drive eutrophication and foul downstream equipment like reverse osmosis membranes. Studies of United States sewage in the late 1960s estimated mean per capita contributions of 500 g in urine and feces and 1000 g in synthetic detergents. Reformulated detergents later cut that largest contribution, while the phosphorus in urine and feces stayed unchanged. Phosphorus can be removed biologically through enhanced biological phosphorus removal, where polyphosphate-accumulating organisms store up to 20% of their mass as phosphorus, or by chemical precipitation with iron, aluminum, or lime. Recovered as phosphate-rich sludge, it can return to the soil as fertilizer; recycling residential wastewater could satisfy 22% of the world's phosphorus needs.

• 5. The Fourth Stage

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  Micropollutants such as pharmaceuticals, household chemicals, and pesticides slip through the commonly used primary, secondary, and tertiary stages and reach the water beyond. Their concentrations are quite low, yet there is still a chance of harming aquatic organisms. Among pharmaceuticals, the toxicologically relevant ones include substances with endocrine disrupting effects, genotoxic substances, and substances that enhance bacterial resistances.

  Techniques for removing these pollutants through a fourth treatment stage are in place in Germany, Switzerland, Sweden, and the Netherlands, with tests ongoing elsewhere. In Switzerland the requirement has been enshrined in law since 2016. The European Union followed with a recast of the Urban Waste Water Treatment Directive, rewritten on the 27th of November 2024 as Directive (EU) 2024/3019, published on the 12th of December and entered into force on the 1st of January 2025. Member states have until the 31st of July 2027 to adapt their national legislation.

  The amendment sets a target, similar to Switzerland, that 80% of 6 key substances out of 12 must be removed between discharge into the plant and discharge into the water body. At least 80% of the investment and operating costs for the fourth stage will fall on the pharmaceutical and cosmetics industry under the polluter pays principle. The directive also calls for the municipal wastewater treatment sector to be energy neutral by 2045. Plants with over 150,000 PE have priority and should be adapted immediately, with the wider rollout staggered so 100% of plants comply by the 31st of December 2045. The process steps mainly use activated carbon filters that adsorb the micropollutants, and a combination of advanced oxidation with ozone followed by granular activated carbon has been suggested as cost-effective for pharmaceutical residues.

• 6. High Tech Versus Low Tech

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  Waste stabilization ponds sit at one end of the spectrum: a low cost treatment option with practically no energy requirements that nonetheless demand a lot of land. Because of their technical simplicity, most of the savings compared with high tech systems come from operation and maintenance costs. They belong to the family of low-tech, extensive, nature-based processes that often use little or no energy, alongside septic tanks, Imhoff tanks, constructed wetlands, sand filters, and vermifilters. Many rely on natural treatment processes and serve rural areas or small to medium-sized communities.

  The activated sludge process anchors the other end. Broadly speaking, it achieves a high effluent quality but is relatively expensive and energy intensive, because it includes an aeration step. It joins other high-tech, mechanized options such as membrane bioreactors, moving bed biofilm reactors, rotating biological contactors, trickling filters, and ultraviolet disinfection.

  Energy tells much of the story. For activated sludge treatment plants in the United States, around 30% of the annual operating costs usually goes to energy, most of it spent on aeration, pumping, and the dewatering and drying of sludge. Some plants claw energy back by producing biogas from sludge through anaerobic digestion, enough to meet most of the plant's own energy needs. Small rural plants using trickling filters may run with no net energy requirement, the whole process driven by gravitational flow, though this is usually only practical in hilly terrain and where the plant is remote enough that odors do not trouble housing.

• 7. Choosing And Designing

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  Population equivalent is the parameter that sizes a plant around the per person organic matter load. Commonly used definitions worldwide hold that 1 PE equates to 60 grams of BOD per person per day, and also equals 200 liters of sewage per day. The same concept lets engineers express the strength of industrial wastewater against ordinary sewage.

  Choosing a suitable process is complicated and requires expert inputs, often through feasibility studies, because the factors run long: applicable flow, influent characteristics, climatic aspects, process kinetics, sludge processing, energy, personnel, reliability, and area availability among them. A life cycle assessment can weigh the aspects, which makes the final decision subjective to some extent. In industrialized countries the most important parameters are typically efficiency, reliability, and space requirements; in developing countries the focus may shift to construction and operating costs and process simplicity.

  Industrial effluent complicates the choice further. In highly regulated developed countries, industrial wastewater usually receives at least pretreatment at the factories to keep toxic compounds out of the biological stage and out of the sewage sludge, protecting both treatment efficiency and the sludge's reuse value. Environmental impacts weigh on selection too, from odors and vector attraction to soil and groundwater contamination and the devaluation of nearby areas. Odors usually signal an anaerobic or septic condition, with hydrogen sulfide the most common source of complaints, managed in urban plants with carbon reactors, bio-slimes, small doses of chlorine, or chemicals like iron salts and calcium nitrate.

• 8. Effluent Out Into The World

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  Before the 20th century in Europe, sewers usually discharged into a river, lake, or ocean with no treatment, leaving the breakdown of human waste to the ecosystem. That could work where the assimilative capacity was sufficient, which is now often not the case given rising population density. The history of treatment then moved through land application, known as sewage farms, in the 1840s in England, followed by chemical treatment and sedimentation in tanks, then biological treatment in the late 19th century, leading to the activated sludge process starting in 1912.

  Reuse has become its own story. Increasingly, people use treated or even untreated sewage for irrigation, since cities are lucrative markets for fresh produce and farmers often have no alternative to water polluted with sewage. The World Health Organization developed guidelines for safe use of wastewater in 2006, advocating a multiple-barrier approach where farmers stop irrigating a few days before harvest so pathogens die off in sunlight and apply water carefully to avoid contaminating leaves eaten raw.

  The consequences of failure are still measured in public health. In 2024, The Royal Academy of Engineering released a study into the effects of wastewater on public health in the United Kingdom, outlining 15 recommendations to raise water quality in rivers and lakes. Interviewed by The Guardian, Sir Chris Whitty called improving water quality and sewage treatment a public health priority, comparing it to eradicating cholera in the 19th century. That same year the Paris Olympics had to delay swimming-focused events like the triathlon because of high levels of sewage in the River Seine, a reminder that the gap between an estimated 52% global treatment rate and full coverage is still being closed plant by plant, with wastewater production anticipated to rise by 51% by 2050.