Sewer
A sewer is an underground infrastructure network of pipes, pumps, and other appurtenances designed to collect and transport wastewater—primarily from domestic, commercial, and industrial sources—along with limited stormwater or infiltrated groundwater, to publicly owned treatment works (POTWs) or discharge points for processing before release into the environment.[1] These systems operate largely by gravity, with wastewater flowing downhill through progressively larger mains from individual laterals connected to buildings, though lift stations are employed in low-lying areas to pump flows upward.[2] By removing sewage from populated areas, sewers play a critical role in maintaining public health, preventing the spread of waterborne diseases such as cholera and typhoid, and protecting water quality in rivers, lakes, and coastal areas.[2] Sewer systems are classified into three main types based on their purpose and contents: sanitary sewers, which exclusively convey wastewater without stormwater to avoid overwhelming treatment capacities; storm sewers, dedicated to channeling rainwater and surface runoff directly to natural water bodies to mitigate flooding; and combined sewers, which handle both wastewater and stormwater in a single set of pipes, a design common in older urban areas but prone to overflows during heavy rains.[3] Sanitary systems predominate in modern infrastructure, comprising over 80% of U.S. municipal collections, while combined systems serve about 700 communities and affect roughly 40 million people, often requiring upgrades to separate flows and reduce pollution.[4] Maintenance challenges, including pipe corrosion, root intrusion, and blockages, necessitate regular inspections and rehabilitation to prevent sanitary sewer overflows (SSOs), which can release untreated sewage into waterways.[1] The development of sewer systems traces back to ancient civilizations, with the Minoans on Crete constructing covered stone drains around 2000 BCE to manage wastewater in palace complexes like Knossos,[5] and the Indus Valley Civilization featuring sophisticated brick-lined sewers in cities such as Mohenjo-Daro by 2500 BCE.[6] The Romans advanced this technology significantly from the 6th century BCE, engineering extensive networks like the Cloaca Maxima in Rome—a 1,300-meter-long arched tunnel that drained marshes and carried sewage to the Tiber River—integrated with aqueducts to supply fresh water and flush waste.[7] In the United States, early colonial settlements relied on rudimentary privies and ditches, but urban growth in the 19th century spurred widespread adoption; by the 1850s, cities like Chicago and New York installed combined systems to combat epidemics, with sewer construction accelerating post-Civil War to serve expanding populations. Today, global sewer infrastructure spans tens of millions of kilometers, supported by engineering standards from organizations like the American Society of Civil Engineers, emphasizing sustainable materials and smart monitoring to address aging networks and climate impacts.Etymology and Terminology
Definition and Scope
A sewer is an underground conduit or pipe system designed to convey wastewater, sewage, or stormwater away from populated areas to treatment facilities or discharge points.[2] These systems primarily serve to remove human waste, industrial effluents, and surface runoff, thereby preventing the spread of disease, mitigating flooding risks, and reducing environmental contamination of water bodies.[8] By channeling potentially hazardous materials through controlled pathways, sewers play a critical role in public health protection and ecosystem preservation, particularly in densely populated regions where untreated discharge could lead to widespread sanitary issues.[9] Key components of sewer systems include pipes for conveyance, manholes for access and inspection, pumping stations to handle elevation changes or low-flow areas, and outfalls for final discharge into treatment plants or receiving waters.[10] The scale of these systems varies significantly: urban networks often comprise extensive grids spanning hundreds of miles to serve millions of residents, while rural implementations may involve simpler, decentralized setups integrated with septic tanks or smaller collection lines.[2] This infrastructure ensures efficient transport under gravity or pressure, adapting to local topography and population density. Sewers differ from drainage systems in purpose and content: sanitary sewers handle contaminated wastewater requiring processing to avoid health and ecological harm, while storm sewers (also called storm drains) manage stormwater runoff, often without treatment, to prevent localized flooding by directing it to natural water bodies. In contrast to building-internal plumbing, which consists of private pipes and fixtures distributing water and collecting initial waste within structures, sewers form the public, external infrastructure connecting multiple properties to centralized treatment.[11][12]Historical and Regional Terms
The word "sewer" entered the English language around 1400 AD, derived from Middle English "sewer" or "seuer," which originated in Anglo-Norman "sewere" meaning a watercourse or ditch.[13] This term traces back to Old French "sewiere," denoting an overflow channel for a fishpond or marshy land, ultimately linking to Vulgar Latin "*exaquaria" (a channel to drain water out), formed from Latin "ex-" (out) and "aquaria" (water pond or container).[13][14] Note that "sewer" is a homonym; it also refers to a person who sews, derived separately from Old English "siwian" (to sew).[13] Regional terminology for sewer-like structures varies significantly, reflecting local engineering and environmental needs. In the United States, "culvert" commonly refers to a conduit or drain—often a pipe or tunnel—that channels water under roads or embankments, sometimes used interchangeably with small-scale sewers for stormwater management.[15] In France, "fosse septique" describes a septic tank or on-site wastewater treatment system, distinct from municipal sewers but integral to rural sanitation.[16] Terminology evolved with urbanization, particularly in the 19th century, as societies transitioned from isolated waste pits to networked systems. Before this shift, "cesspool"—a simple underground holding tank for human waste—was the dominant term in English-speaking regions, often unlined and prone to overflow in growing cities.[17] The rise of public health reforms and infrastructure prompted the broader adoption of "sewer" for piped drainage networks, supplanting cesspools in urban contexts. Obsolete or dialectal terms persist in specific locales, such as "rhyne" in southwestern UK dialects (e.g., Somerset), referring to artificial drainage ditches that reclaim wetland for agriculture.[18] Cultural influences on sewer terminology often highlight societal priorities like cleanliness and renewal. In ancient Roman engineering, "cloaca"—meaning a drain or sewer—embodied purification, as seen in the Cloaca Maxima, a monumental channel that removed waste from the city, symbolizing the expulsion of impurities to maintain urban health and order.[19]History of Sewers
Ancient and Classical Periods
The earliest evidence of organized drainage systems appears in Neolithic settlements, such as Skara Brae in Scotland, dating to approximately 3100–2500 BCE. This prehistoric village featured stone-built houses with underfloor drainage channels and simple latrines that directed wastewater to exterior gullies, demonstrating an early response to sanitation needs in communal living spaces.[20][21] In ancient Mesopotamia, urban centers like Uruk and Babylon developed rudimentary sewer networks by the third millennium BCE, consisting of covered brick drains along streets that channeled household waste and stormwater to nearby rivers. These systems, documented in cuneiform texts and archaeological remains, supported growing populations in densely packed cities by mitigating flooding and waste accumulation.[22][23] The Indus Valley Civilization advanced these concepts significantly around 2600–1900 BCE, particularly in Mohenjo-Daro, where a sophisticated municipal drainage infrastructure included brick-lined sewers embedded in street margins, connected to household bathrooms via chutes and soak pits for wastewater absorption. This grid-planned system, constructed with standardized baked bricks and gypsum mortar, covered the entire urban area and emphasized public health in one of the world's first planned cities.[24][25][26] In ancient Greece, particularly Athens during the Archaic and Classical periods (c. 600–300 BCE), drainage systems utilized terracotta pipes and stone-lined channels to manage runoff and sewage, with networks beneath the Agora directing waste to the Eridanos stream. These installations, often integrated with public fountains and aqueducts, reflected engineering adaptations to hilly terrain and seasonal rains.[5][27] The Romans elevated sewer engineering to monumental scale with the Cloaca Maxima, constructed around 600 BCE under Etruscan influence during the reign of Tarquinius Priscus. This vaulted stone-arch channel, originally a canal for marsh drainage that later served as a sewer, extended approximately 1,600 meters as the primary system to the Tiber River, with branches forming a broader network to handle urban effluent.[28][29] These developments were driven by rapid urbanization, which necessitated waste management to prevent disease in expanding cities, as seen in Mesopotamian and Indus settlements where population densities exceeded 20,000 inhabitants. In Rome, sanitation was codified in the Twelve Tables (c. 450 BCE), which prohibited burials within city limits to avoid contamination and emphasized cleanliness as a civic duty tied to religious purity and public order. Greek city-states similarly linked hygiene to philosophical ideals of moderation and community welfare, influencing laws on water use and waste disposal.[30][31]Medieval to Industrial Era
Following the fall of the Roman Empire in the 5th century, sewer systems across Europe largely deteriorated, with urban centers reverting to rudimentary waste management practices that persisted through the medieval period until the 15th century. Large-scale engineered networks like those in ancient Rome gave way to simple cesspits—brick-lined pits for collecting human waste and household refuse, often allowing liquids to seep into the ground—and open ditches that channeled sewage into nearby rivers or streets.[17] These methods were inadequate for growing populations, leading to frequent contamination of water sources and outbreaks of diseases such as dysentery. In contrast, some medieval monasteries maintained more organized sanitation, incorporating basic drainage channels and latrines that directed waste away from living areas, as evidenced by the extensive stone-lined drain system discovered beneath Paisley Abbey in Scotland, dating to the 12th century and spanning over 100 meters.[32] During the Renaissance, modest advancements emerged in select European cities, marking a shift toward structured underground infrastructure. In Paris, Provost Hugues Aubriot oversaw the construction of the city's first vaulted stone sewer along Rue Montmartre around 1370, a covered channel designed to carry wastewater more efficiently beneath streets and reduce open flooding.[33] Similarly, in London during the Tudor era under Henry VIII, early regulatory efforts laid groundwork for sewer improvements; the Statute of Sewers enacted in 1532 established commissions to oversee drainage and repair of watercourses, addressing the growing nuisance of stagnant ditches in an expanding urban area, though implementation remained piecemeal and focused on flood control rather than comprehensive waste removal.[34] The Industrial Revolution in 19th-century Britain accelerated urbanization, overwhelming these primitive systems and catalyzing widespread public health crises. Rapid migration to cities like London swelled populations, with over a million residents by 1800 crammed into slums lacking proper sanitation, resulting in sewage overflowing into streets and the River Thames; this directly contributed to the cholera epidemic of 1832, which killed around 6,000 in London alone by spreading through contaminated water.[35] In response, Edwin Chadwick's seminal 1842 report, The Sanitary Conditions of the Labouring Population in Great Britain, documented how poor drainage shortened life expectancy in industrial towns to as low as 16 years for laborers and advocated for centralized sewage removal to prevent disease, influencing the Public Health Act of 1848.[35][36] A pivotal breakthrough came with engineer Joseph Bazalgette's design for London's metropolitan sewer network, authorized in 1858 amid the "Great Stink" crisis when Thames pollution forced Parliament to act. Constructed from 1858 to 1875, the system comprised 130 kilometers of main brick-lined intercepting sewers and over 2,000 kilometers of local tunnels, diverting waste eastward to treatment sites and preventing backflow into the river.[37] This engineering feat, built with egg-shaped brick arches for efficient flow, virtually eradicated cholera outbreaks in London by the 1870s, validating germ theory and serving as a model for urban sanitation worldwide.[37]20th Century Developments
In the early 20th century, sewer systems saw widespread adoption across major U.S. cities as urbanization accelerated and public health concerns over waterborne diseases prompted infrastructure investments.[38] Cities like Chicago extended their foundational sewer networks from the late 19th century, with the Metropolitan Water Reclamation District constructing large intercepting sewers to redirect sewage away from local waterways and toward treatment facilities.[39] These developments built on precursors to modern environmental legislation, such as the 1948 Federal Water Pollution Control Act, which provided initial federal funding for pollution abatement and influenced subsequent sewer expansions.[40] Following World War II, rapid suburban growth in North America and Europe drove a boom in sewer infrastructure, emphasizing separated systems to handle sanitary and stormwater flows independently and reduce overflows.[38] In the U.S., federal aid facilitated extensions of sewer lines into new suburban developments, accommodating the housing surge for returning veterans and supporting urban sprawl.[41] Similarly, European reconstruction efforts integrated modern separated sewers into post-war urban planning, prioritizing efficiency in expanding residential areas.[42] This era also marked the advancement of reinforced concrete pipes, with key theoretical developments in the 1920s and 1930s by engineers like W.J. Schlick and M.G. Spangler, enabling durable, large-diameter conduits suitable for high-volume urban flows. The global spread of sewer systems in the 20th century reflected colonial legacies and post-independence challenges, particularly in Asia and Africa. In Mumbai, British-era sewers established in the 1880s were upgraded during the 1950s as administrative boundaries expanded to include suburbs, incorporating new treatment infrastructure to address population growth.[43] However, developing nations faced persistent issues, such as widespread open sewers in Latin America during the 1970s, where inadequate covered systems contributed to river contamination and health risks amid rapid urbanization.[44] Key milestones included the 1972 U.S. Clean Water Act, which mandated nationwide upgrades to wastewater treatment and sewer systems through federal grants, leading to over $1 trillion in investments and significant reductions in point-source pollution.[40][45] In Scandinavia, vacuum sewer technology emerged in the 1960s as an innovative alternative for challenging terrains, using differential air pressure for efficient transport and gaining popularity for its shallow installation and reduced excavation needs.[46]Types of Sewer Systems
Sanitary Sewers
Sanitary sewers are underground pipeline systems specifically engineered to collect and convey human and domestic wastewater from households, commercial buildings, and industries to centralized treatment plants, deliberately excluding stormwater to avoid dilution and ensure efficient sewage processing.[47] This separation allows for targeted treatment of concentrated wastewater, minimizing the volume and variability of flows reaching facilities and reducing the risk of overflows during wet weather.[48] Key components include gravity-fed main pipes, typically with diameters of 200 to 300 mm for collection lines, connected by smaller laterals of 100 to 150 mm diameter that extend from building inlets to the mains.[49][50] Where terrain prevents continuous gravity flow, lift stations equipped with pumps elevate sewage to higher elevations for continued transport.[51] These systems rely on a network of manholes for access and maintenance, constructed from durable materials like PVC or ductile iron to withstand corrosion and pressure.[47] Separate sanitary sewers offer significant advantages, such as lower treatment costs due to consistent, undiluted flows that simplify biological and chemical processes at plants, though they require parallel stormwater infrastructure, increasing initial capital outlay.[48] Challenges include vulnerability to groundwater infiltration through cracks, which can elevate flows by up to several hundred cubic meters per kilometer per day in systems with cracks, particularly older ones, and risks from illegal stormwater connections that undermine separation.[51] Since the post-World War II era, particularly from the 1950s onward, these systems have become standard in new suburban developments across the United States, driven by urban expansion and federal clean water initiatives.[52] In urban areas, sanitary sewer flow rates generally range from 100 to 500 liters per person per day, encompassing domestic use and minor industrial contributions, with averages around 200 liters in residential suburbs.[53][47] This design predominates in U.S. suburban regions, where it supports controlled wastewater management without the mixing seen in older combined systems.[52]Storm Sewers
Storm sewers, also known as stormwater drainage systems, are engineered networks designed to collect and transport rainwater and surface runoff from urban and suburban areas to designated outlets such as rivers, streams, or retention basins, primarily to prevent flooding and manage excess water during storms.[54] Unlike sanitary sewers, these systems handle relatively clean water from precipitation falling on impervious surfaces like streets, roofs, parking lots, and sidewalks, directing it away from developed areas to reduce the risk of water accumulation that could damage infrastructure or disrupt daily life.[55] The core purpose is to mitigate urban flooding by providing a controlled pathway for stormwater, which can otherwise overwhelm natural drainage in densely built environments.[56] Key components of storm sewer systems include larger-diameter pipes typically ranging from 300 mm to 2000 mm (12 to 80 inches) to accommodate high-volume flows, catch basins or inlets that capture surface runoff from gutters and streets, and culverts that channel water under roads or railways.[57] These systems often operate on gravity flow, relying on natural slopes to move water toward outlets, supplemented by detention ponds or basins that temporarily store excess runoff to control release rates and prevent downstream surges. Access structures like manholes allow for inspection and maintenance, ensuring the network's longevity in handling variable storm intensities.[58] Design of storm sewers focuses on calculating peak flows using methods like the Rational Method, where discharge is estimated as Q = C × i × A (with C as runoff coefficient, i as rainfall intensity, and A as drainage area), tailored to handle events with intensities of 50-100 mm per hour for typical 10- to 25-year return periods.[59] Pipe sizing and layout account for hydraulic capacity to avoid surcharging, with slopes optimized for self-cleansing velocities around 0.6-0.9 m/s to prevent sediment buildup. In modern designs, environmental integration is emphasized through features like bioswales—vegetated channels that infiltrate and filter runoff, reducing pollutant entry into pipes and enhancing overall system sustainability.[60] Prominent examples include the extensive stormwater networks in flood-prone cities like Tokyo, where the Metropolitan Area Outer Underground Discharge Channel (G-Cans), completed in 2006, features massive underground reservoirs and tunnels capable of storing and pumping 200 cubic meters of water per second to protect against typhoon-induced flooding.[61] Despite these advancements, untreated discharge from storm sewers remains common, often carrying first-flush pollutants—initial runoff laden with accumulated urban contaminants like oils, metals, and debris—that can degrade receiving water bodies during early storm phases.[62] This highlights the need for complementary treatment practices in stormwater management.[63]Combined and Hybrid Systems
Combined sewer systems utilize a single network of pipes to convey both domestic, commercial, and industrial wastewater along with stormwater runoff, a design prevalent in many older urban areas of Europe, such as London and Paris, where 19th-century infrastructure was built to handle mixed flows efficiently during eras of rapid urbanization.[17][37] These systems direct all flows to wastewater treatment facilities during dry weather conditions, but during heavy rainfall, the combined volume often exceeds pipe capacity, leading to overflows of untreated or partially treated mixtures directly into nearby water bodies.[3] Historically, combined systems emerged in the mid-19th century as cities like New York adopted them to manage growing populations and prevent street flooding, with New York's network, dating back to the 1850s and spanning over 7,500 miles as of 2024.[64][65] This approach was a significant advancement over open ditches but proved problematic as urban density increased, causing frequent combined sewer overflows (CSOs) that release pollutants into waterways.[66] To mitigate CSOs, modern management strategies incorporate structural controls such as deep storage tunnels, retention basins, and weirs or regulators that divert excess flows away from sensitive outfalls, allowing gradual treatment during off-peak periods.[67] According to the U.S. Environmental Protection Agency (EPA), approximately 700 communities across the United States operate combined systems serving about 40 million people (as of 2023), with CSOs estimated to have discharged around 850 billion gallons of untreated wastewater and stormwater annually as of 2004, though volumes have decreased due to ongoing mitigation efforts.[4][68][69] In 2024, the EPA issued proposed guidance providing more flexibility for CSO control after long-term control plan implementation, aiming to accelerate reductions in overflows.[70] Hybrid systems represent an evolutionary approach, often involving partial separation where sanitary wastewater follows dedicated pipes during dry weather, while controlled storm inlets allow limited runoff entry only during wet conditions to prevent overloads.[67] These retrofits, common in upgrading legacy combined networks, have demonstrated substantial pollution reductions; for instance, implementations in communities like Bremerton, Washington, have cut annual CSO volumes by more than 80 percent through integrated storage and diversion techniques.[71] Such hybrids balance capacity constraints with environmental protection, transitioning older infrastructures toward more sustainable operations without full separation.[72]Design Principles
Hydraulic and Flow Considerations
Hydraulic design in sewer systems relies on principles of fluid dynamics to ensure efficient transport of wastewater while preventing sedimentation and blockages. The primary equation used for calculating flow velocity in gravity sewers is Manning's equation, which models uniform open-channel flow:V = \frac{1}{n} R^{2/3} S^{1/2}
where V is the average velocity (m/s), n is the Manning's roughness coefficient, R is the hydraulic radius (m), and S is the slope of the energy grade line (dimensionless). [73] For concrete sewer pipes, a typical roughness coefficient n value is 0.013, accounting for the smooth interior surfaces often coated with sewage slime. [74] Sewer flow is predominantly gravity-driven in most systems, where wastewater moves downhill through sloped pipes to maintain a self-cleansing velocity greater than 0.6 m/s (2 ft/s) when flowing full or half full, sufficient to prevent solids deposition. In areas with uphill terrain or flat topography, pressurized systems employ pumps—such as grinder pumps or septic tank effluent pumps—to propel wastewater through small-diameter pipes, enabling transport over longer distances or elevations without relying on gravity. [75] Flow rates vary significantly, with peak flows in sanitary sewers typically 2 to 4 times the average dry weather flow due to diurnal patterns and wet weather influences. [76] Sizing sewer pipes involves estimating future demands based on population equivalents, which represent the wastewater contribution per person or unit (often 100-200 gallons per capita per day), adjusted for commercial and industrial inputs. Infiltration and inflow (I/I) must also be factored in, as groundwater seepage and surface runoff through cracks or illegal connections can increase flows by up to 35% in older systems during high groundwater periods. [77] These considerations ensure pipes are sized to handle peak wet weather conditions without surcharging, typically using a safety factor to accommodate growth and uncertainties. Advanced hydraulic modeling is essential for simulating flow dynamics, routing, and capacity under various scenarios. Widely adopted tools include the Storm Water Management Model (SWMM), a dynamic hydrology-hydraulics simulator from the U.S. Environmental Protection Agency that handles both stormwater and sanitary sewer flows, including infiltration and surcharging effects. [78] The Hydrologic Engineering Center's River Analysis System (HEC-RAS), developed by the U.S. Army Corps of Engineers, supports one- and two-dimensional unsteady flow computations suitable for complex sewer networks integrated with natural channels. [79] These software packages enable engineers to optimize designs by predicting velocities, depths, and potential overflows.