Environmental engineering
Environmental engineering is the branch of engineering concerned with the application of scientific and engineering principles to improve and maintain the environment for protecting human health, preserving the quality of air, water, and land resources, and enabling sustainable use of natural resources.[1] Practitioners design and implement systems to manage water supply and treatment, wastewater disposal, air pollution control, solid and hazardous waste handling, and site remediation.[2] The field integrates disciplines such as chemistry, biology, physics, and hydrology to address pollution sources and mitigate environmental degradation caused by human activity.[3] Emerging in the mid-19th century amid urbanization and industrialization, environmental engineering traces its modern origins to efforts in sanitation and public health, including the development of municipal water filtration systems in the 1820s and centralized sewage treatment in the late 1800s, which significantly reduced waterborne diseases like cholera in cities.[4] Key milestones include the engineering of wastewater treatment plants that process billions of gallons daily, preventing untreated discharge into waterways, and air quality controls such as catalytic converters and scrubbers that have lowered urban pollutant levels by orders of magnitude since the 1970s.[5] These interventions have empirically extended life expectancies and enhanced ecosystem services, though large-scale projects like dams have occasionally disrupted aquatic habitats and sediment flows, highlighting trade-offs in causal environmental impacts.[6] Today, environmental engineers confront grand challenges including resilient infrastructure against climate variability, remediation of emerging contaminants like per- and polyfluoroalkyl substances (PFAS), and resource-efficient waste-to-energy conversion, often balancing empirical efficacy against regulatory and economic constraints.[7] Despite institutional tendencies toward precautionary approaches that may overlook cost-benefit analyses, the discipline's reliance on measurable outcomes—such as reduced effluent toxicity and restored groundwater quality—drives verifiable progress in sustaining habitable conditions for expanding populations.[8]Fundamentals
Definition and Scope
Environmental engineering is the branch of engineering that integrates scientific and engineering principles to develop solutions for environmental problems, emphasizing the protection of human health, preservation of ecosystems, and sustainable management of natural resources. This discipline addresses challenges such as water and air pollution control, waste treatment, and resource recovery through the design, construction, and operation of infrastructure systems.[3][2][9] The scope of environmental engineering encompasses several core areas, including water supply and treatment systems to ensure safe drinking water, wastewater management to prevent contamination of water bodies, and solid and hazardous waste disposal methods that minimize environmental impact. Practitioners also focus on air quality engineering to mitigate emissions from industrial and vehicular sources, soil and groundwater remediation to restore contaminated sites, and the integration of sustainability principles in urban planning and infrastructure development. These applications often involve interdisciplinary collaboration with fields like chemistry, biology, and hydrology to model pollutant transport, assess risks, and evaluate long-term ecological effects.[3][10][11] In practice, environmental engineers conduct environmental impact assessments, design pollution prevention technologies, and ensure compliance with regulatory standards such as those set by the U.S. Environmental Protection Agency for effluent limitations and emissions controls. The field extends to emerging challenges like climate change adaptation, including resilient infrastructure against flooding and strategies for carbon sequestration in engineered systems. As of 2023, the U.S. Bureau of Labor Statistics reports that environmental engineers held approximately 47,000 jobs, with median annual wages around $100,090, reflecting demand driven by regulatory needs and industrial growth.[3][1]Core Principles
Environmental engineering operates on foundational principles that integrate scientific rigor, engineering design, and ethical imperatives to address human impacts on natural systems while prioritizing human health and ecological integrity. Central to the discipline is the application of physical, chemical, and biological sciences to quantify and model environmental processes, such as pollutant transport and fate, enabling predictive analysis for system optimization.[12] This empirical foundation ensures interventions are evidence-based rather than speculative, with mass balance equations and reaction kinetics serving as core tools for assessing treatment efficacy in water and air systems.[13] A key principle is pollution prevention over remediation, favoring designs that inherently minimize waste generation—such as through process integration and material selection—rather than relying on downstream controls like filtration or incineration. This approach, rooted in thermodynamic efficiency and resource conservation, reduces long-term costs and environmental burdens; for instance, the U.S. Environmental Protection Agency's green engineering framework emphasizes maximizing energy and material efficiency while minimizing hazard potential in product lifecycles.[14] [15] Sustainable development further underpins practice, requiring engineers to balance immediate societal needs with long-term ecological capacity, as codified in professional standards that mandate compliance with principles avoiding resource depletion or irreversible degradation.[16] Risk assessment constitutes another cornerstone, involving probabilistic evaluation of exposure pathways, dose-response relationships, and uncertainty quantification to inform regulatory compliance and project feasibility. Techniques such as Monte Carlo simulations and fault tree analysis enable quantification of failure probabilities in infrastructure like dams or wastewater plants, ensuring decisions reflect causal chains from emissions to health outcomes rather than unverified assumptions.[17] Systems-level thinking integrates these elements holistically, recognizing feedback loops in ecosystems—e.g., nutrient cycling in watersheds—and demanding interdisciplinary collaboration to avoid siloed solutions that exacerbate unintended consequences, such as groundwater contamination from surface runoff mismanagement.[18] Professional ethics reinforce these principles, obligating engineers to uphold public welfare paramount and disclose limitations in data or models that could mislead stakeholders.[19]Interdisciplinary Foundations
Environmental engineering integrates foundational principles from civil engineering, which provides expertise in infrastructure design for water conveyance, wastewater collection, and solid waste management systems, enabling the construction of resilient environmental control facilities.[20] Chemical engineering contributes process design and optimization techniques, such as unit operations for pollutant separation, reaction kinetics in treatment processes, and mass transfer modeling for contaminant removal in air and water streams.[21] These engineering disciplines form the applied backbone, allowing for scalable, cost-effective interventions grounded in physical laws of fluid dynamics and thermodynamics. Biological and chemical sciences underpin the understanding of natural degradation pathways and engineered bioremediation, where microbiology informs activated sludge processes in wastewater treatment—relying on bacterial consortia to break down organic matter—and phytoremediation strategies using plant-microbe interactions for soil decontamination.[22] Chemistry provides the analytical framework for identifying pollutants, predicting speciation in aqueous environments, and designing coagulants or adsorbents based on equilibrium constants and reaction rates, as seen in advanced oxidation processes for refractory organics.[23] Physics contributes through hydraulics and transport phenomena, essential for modeling groundwater flow and atmospheric dispersion of emissions using equations like Darcy's law for porous media permeability. Earth sciences, including hydrology and geology, supply site-specific data on aquifer recharge rates—typically ranging from 10% to 30% of annual precipitation in temperate zones—and geotechnical stability for landfill liners, integrating subsurface characterization to prevent leachate migration.[24] Mathematics enables quantitative risk assessment via stochastic models and optimization algorithms, such as linear programming for resource allocation in multi-objective environmental systems. This synthesis ensures solutions account for causal interactions, like nutrient loading causing eutrophication with phosphorus thresholds around 0.02 mg/L triggering algal blooms in lakes.[25]Historical Development
Ancient and Pre-Industrial Contributions
The Indus Valley Civilization (circa 2600–1900 BCE) constructed one of the earliest known urban drainage systems, featuring brick-lined underground sewers that connected individual homes to main street channels, with widths up to 91 cm and depths of 1.5 m in sites like Lothal.[26] These systems incorporated covered drains, soak pits for wastewater filtration, and evidence of planned waste disposal, enabling sustained population densities in cities such as Mohenjo-Daro without widespread contamination.[27] Such infrastructure predated similar developments elsewhere and reflected empirical adaptations to flood-prone riverine environments for hygiene and flood control. In ancient Egypt (circa 3000–30 BCE), basin irrigation harnessed the Nile River's annual floods through constructed dikes, canals, and basins to retain silt-rich water for agriculture, with local-scale operations documented from the Old Kingdom onward.[28] Nilometers measured flood heights to predict inundation levels, while early hydraulic structures like the Sadd el-Kafara dam (circa 2686–2181 BCE) attempted sediment trapping and flow regulation, though it failed due to flash floods.[29] These practices prioritized causal flood dynamics over large-scale reservoirs, supporting agricultural surplus without mechanical pumping. Minoan Crete (circa 3000–1450 BCE) pioneered plumbing with terracotta pipes for water distribution and sewage evacuation, as seen in the Palace of Knossos, where conduits channeled wastewater from latrines to external drains.[30] Systems included sloped channels for gravity flow and early flushing mechanisms, marking initial steps in enclosed sanitation to mitigate odors and disease in multi-story complexes.[31] Mesopotamian settlements around 3500 BCE initiated wastewater diversion via simple pits and channels, evolving into brick sewers in Babylonian cities by the 18th century BCE, though less integrated than Indus designs.[32] In the Roman era (from circa 600 BCE), the Cloaca Maxima sewer drained marshes and urban waste into the Tiber River, complemented by aqueducts delivering over 1 million cubic meters of water daily to Rome by the 1st century CE via gravity-fed channels spanning hundreds of kilometers.[33] These feats combined empirical surveying with stone arch construction, reducing urban flooding and enabling public baths for hygiene, though lead pipes posed unrecognized toxicity risks.[34] Pre-industrial Europe and Asia saw incremental refinements, such as Persian qanats (underground galleries from circa 1000 BCE) for arid-zone groundwater access and medieval European cesspits, but lacked the systemic integration of earlier empires until 19th-century mechanization.[29] These ancient efforts laid causal foundations for environmental control by addressing water sourcing, conveyance, and disposal through observation of natural flows and material durability, without formal engineering theory.19th-Century Origins and Urban Sanitation
The rapid urbanization accompanying the Industrial Revolution in Europe during the early 19th century exacerbated public health crises, with overcrowded cities lacking adequate waste disposal and clean water supplies, leading to recurrent cholera epidemics that killed tens of thousands.[35] In Britain, cholera outbreaks in 1831–1832 and 1848–1849 highlighted the causal link between contaminated water sources and disease transmission, though prevailing miasma theory attributed illnesses to foul air rather than pathogens.[36] Edwin Chadwick's 1842 Report on the Sanitary Condition of the Labouring Population of Great Britain provided empirical documentation of urban filth, linking poor sanitation to high mortality rates among the working class, with infant death rates exceeding 50% in some industrial towns due to inadequate sewage and water systems.[37] Chadwick advocated centralized public interventions, including piped water supplies and sewer networks to remove waste, influencing the Public Health Act of 1848, which established local boards of health to enforce sanitation reforms despite initial resistance from property owners fearing costs.[38] These measures prioritized engineering solutions over individual hygiene, marking the shift toward systematic urban infrastructure as a public good. John Snow's investigation of the 1854 Soho cholera outbreak in London further evidenced waterborne causation, as he mapped 616 deaths clustered around the Broad Street pump, contaminated by a nearby cesspit leaking fecal matter; removing the pump handle on September 8, 1854, halted new cases, demonstrating filtration and source isolation as preventive engineering tactics.[36] Snow's dot map and statistical analysis challenged miasma proponents, though acceptance was gradual, spurring investments in water quality monitoring and early filtration experiments.[39] The "Great Stink" of 1858, when sewage overflow polluted the Thames amid hot weather, compelled parliamentary action, leading engineer Joseph Bazalgette to design London's Main Drainage system starting in 1859, featuring 82 miles of brick sewers, 1,100 miles of local pipes, and pumping stations to intercept waste from 3.5 million residents and divert it downstream.[40] Completed by 1865 at a cost of £4.6 million, this network reduced cholera incidence by over 90% in subsequent decades through hydraulic separation of stormwater and sewage, establishing scalable models for urban sanitation engineering.[41] Parallel developments included slow sand filtration, pioneered in Scotland in 1804 and adopted in London by 1829, where gravel and sand beds removed sediments and organics from river water, achieving up to 99% bacterial reduction in urban supplies serving populations exceeding 1 million.[42] These innovations, driven by empirical outbreak data rather than theoretical purity, laid the foundational practices of sanitary engineering—later formalized as environmental engineering—emphasizing causal interventions like source protection and waste conveyance to mitigate urban disease vectors.[35]20th-Century Formalization and Expansion
The early 20th century saw incremental advancements in sanitary engineering practices that addressed urban health risks, including the first systematic air pollution studies around 1900 and the inaugural U.S. municipal water chlorination in Jersey City, New Jersey, on February 26, 1908, which reduced typhoid fever incidence by over 90% within months.[43] These developments, rooted in civil engineering, focused on water quality standards established in 1902 and early mathematical modeling of water flow in 1925, but lacked a unified discipline amid rising industrial effluents.[43] Incidents like the 1948 Donora smog event, which killed 20 and sickened thousands due to sulfur dioxide emissions, underscored causal links between emissions and health, prompting localized controls but no broad formalization.[4] Mid-century industrialization amplified pollution crises, leading to environmental engineering's emergence as a distinct field by the 1950s–1960s, distinct from sanitary engineering's narrower sanitation focus. Rensselaer Polytechnic Institute introduced one of the earliest dedicated environmental engineering degree programs in the mid-1950s, emphasizing pollution prevention over mere treatment.[44] The American Academy of Environmental Engineers (now AAEES) was founded in 1955 to certify specialists in water, wastewater, and air quality, marking professional codification amid post-World War II economic booms that tripled U.S. industrial output and emissions by 1960.[45] This shift reflected first-principles recognition that unchecked waste discharges caused irreversible ecosystem degradation, as evidenced by lake eutrophication from phosphorus detergents. The 1960s–1970s expansion was propelled by regulatory mandates responding to empirical evidence of transboundary pollution, including Rachel Carson's 1962 documentation of pesticide bioaccumulation in Silent Spring, which correlated DDT with avian population declines of up to 50% in affected areas. The National Environmental Policy Act (NEPA), effective January 1, 1970, required environmental impact assessments for federal projects, formalizing engineering roles in predictive modeling.[46] Subsequent laws—the Clean Air Act (1970), mandating emissions standards that reduced U.S. sulfur dioxide by 92% over decades, and the Clean Water Act (1972), targeting point-source discharges—spurred demand for engineers in compliance design, with the U.S. Environmental Protection Agency's creation in December 1970 institutionalizing oversight.[46] University programs proliferated, with over 100 U.S. institutions offering degrees by 1980, professionalizing the field through peer-reviewed standards from bodies like the American Society of Civil Engineers' Environmental and Water Resources Institute.[6] This era's causal focus on source control over end-of-pipe fixes expanded applications to hazardous waste remediation under the 1980 Resource Conservation and Recovery Act, addressing sites like Love Canal where chemical leaching affected 21,000 residents.[47]Education and Professional Practice
Academic Curriculum and Training
Undergraduate programs in environmental engineering typically confer a Bachelor of Science degree and emphasize foundational engineering principles applied to environmental challenges. In the United States, these programs are often accredited by ABET, which mandates at least 30 semester credit hours (or equivalent) in mathematics and basic sciences, alongside at least 45 semester credit hours in engineering topics, including design and professional practice integrated throughout the curriculum.[48] Core coursework generally includes fluid mechanics, hydrology, environmental chemistry, microbiology, water and wastewater treatment processes, air pollution control, and solid waste management, building on prerequisites in physics, calculus, and general chemistry.[49][50] Programs often incorporate laboratory work, field studies, and capstone design projects addressing real-world issues like pollution mitigation or sustainable infrastructure.[51] Graduate-level training advances specialization through Master of Science degrees, typically requiring 28 to 42 credit hours depending on the institution and whether the program emphasizes coursework or thesis research.[52][53] These programs assume undergraduate proficiency in core engineering and delve into advanced topics such as groundwater remediation, atmospheric modeling, hazardous waste engineering, and sustainable systems analysis, often with electives in areas like climate adaptation or renewable energy integration.[54] Research-oriented tracks may span 18 to 24 months and culminate in a thesis, while professional variants focus on practical applications without a research component.[55] Doctoral programs, leading to a PhD, extend this with original research, typically requiring 3 to 5 years beyond the bachelor's, preparing graduates for academia, policy, or high-level R&D roles.[56] Professional training complements formal education through internships, co-op experiences, and licensure pathways. Entry-level engineers often pursue the Fundamentals of Engineering (FE) exam post-bachelor's, followed by 4 years of supervised practice to qualify for the Principles and Practice of Engineering (PE) exam in environmental engineering, administered by state boards under the National Council of Examiners for Engineering and Surveying. Advanced certifications, such as the Board Certified Environmental Engineer (BCEE) from the Academy of Board Certified Environmental Professionals, require PE licensure plus at least 8 years of relevant experience in environmental engineering practice or teaching.[57] These credentials emphasize competency in areas like risk assessment and regulatory compliance, with continuing education units often mandated for renewal to address evolving standards in pollution control and sustainability.[58]Licensure, Ethics, and Career Paths
Environmental engineers seeking to practice professionally in the United States must obtain licensure as a Professional Engineer (P.E.), regulated by state licensing boards through the National Council of Examiners for Engineering and Surveying (NCEES). Core requirements include earning a bachelor's degree from an ABET-accredited environmental or related engineering program, passing the Fundamentals of Engineering (FE) exam shortly after graduation, accumulating at least four years of qualifying work experience under the supervision of a licensed P.E., and passing the Principles and Practice of Engineering (PE) exam, which may focus on civil engineering with environmental applications or a dedicated environmental discipline where offered by states.[59][60] Licensure ensures competence in protecting public health and the environment, and while not mandatory for all roles such as research or internal corporate positions, it is required for independent practice, signing engineering documents for public projects, or providing services directly to the public. Advanced certifications, like Board Certified Environmental Engineer (BCEE) from the American Academy of Environmental Engineers and Scientists, build on P.E. licensure and require additional demonstrated expertise in environmental practice areas.[61] Professional ethics in environmental engineering emphasize prioritizing public safety, environmental integrity, and truthful representation, as codified by organizations like the American Society of Civil Engineers (ASCE) and the National Society of Professional Engineers (NSPE). The ASCE Code of Ethics, originally adopted in 1914 and last updated in October 2020, mandates that engineers hold paramount the safety, health, and welfare of the public; undertake assignments only within their competence; and avoid deceptive acts or conflicts of interest, with specific relevance to environmental work in avoiding harm to ecosystems or human populations through flawed designs or assessments.[19] The NSPE Code reinforces these with fundamental canons requiring adherence to laws, client honesty, and sustainable practices that do not impair future generations' welfare.[62] For environmental specialists, the National Association of Environmental Professionals (NAEP) Code of Ethics, effective since August 2018, extends these principles to mandate objective analysis, full disclosure of limitations in data or methods, and rejection of projects involving known environmental deception.[63] Violations can lead to disciplinary actions, including license revocation, underscoring ethics' role in countering pressures from economic or regulatory incentives that might compromise causal assessments of environmental impacts. Career paths for environmental engineers span government, private sector, and consulting, often starting in entry-level roles focused on compliance, design, or monitoring before advancing to project management or policy advisory positions. In federal agencies like the U.S. Environmental Protection Agency (EPA), engineers develop regulations and oversee remediation; in industry, they manage waste systems or pollution controls for utilities and manufacturing firms; consulting firms employ many for site assessments and sustainable infrastructure projects.[3] The U.S. Bureau of Labor Statistics reports a median annual wage of $100,090 for environmental engineers as of May 2023, with the top 10 percent earning over $125,520, varying by sector—highest in federal government at approximately $115,000—and location, such as California or federal hubs.[64] Employment is projected to grow 7 percent from 2023 to 2033, adding about 3,000 jobs, driven by demand for clean energy transitions, water infrastructure upgrades, and regulatory enforcement amid aging systems and climate adaptation needs, though competition exists in saturated urban markets.[65] Advanced degrees or certifications enhance prospects for senior roles, with many engineers transitioning to interdisciplinary fields like sustainability consulting or executive environmental management after 10–15 years of experience.[66]Primary Applications
Water Supply and Treatment
Water supply systems in environmental engineering encompass the sourcing, conveyance, treatment, and distribution of potable water to meet urban and rural demands while minimizing environmental impacts such as over-extraction from aquifers and ecosystem disruption. Primary sources include surface water from rivers and lakes, which accounts for approximately 70-80% of municipal supplies in many developed regions, and groundwater, which provides a more stable but vulnerable alternative prone to depletion and contamination.[67] Engineers design intake structures, reservoirs, and pipelines to optimize yield, with conveyance often involving large-scale infrastructure like aqueducts to transport water over distances, as seen in systems serving arid cities where inter-basin transfers mitigate local shortages.[68] Conventional treatment processes form the core of potable water preparation, beginning with coagulation, where alum or ferric salts are added to destabilize colloidal particles and pathogens, followed by flocculation to aggregate them into larger flocs via gentle mixing. Sedimentation then allows these flocs to settle under gravity in clarifiers, reducing turbidity by up to 90%, while subsequent filtration through sand or multimedia beds removes remaining particulates to achieve clarity standards typically below 0.3 NTU. Disinfection, often via chlorination at doses of 1-4 mg/L, inactivates microorganisms like E. coli and viruses, ensuring compliance with health guidelines that limit coliforms to zero detectable per 100 mL.[69][70] Advanced technologies address limitations of conventional methods, particularly in regions facing salinity or emerging contaminants. Membrane filtration, including ultrafiltration and nanofiltration, provides finer particle and dissolved solids removal with rejection rates exceeding 99% for salts in reverse osmosis configurations, enabling desalination plants like those in the Middle East to produce over 100 million cubic meters daily from seawater. Recent innovations, such as high-permeability anti-fouling membranes, reduce energy demands from 3-5 kWh/m³ to below 2 kWh/m³, enhancing feasibility for inland brackish water treatment.[71][72] Distribution networks, comprising pressurized pipes and pumping stations, deliver treated water but incur losses of 10-40% globally due to leaks and inefficiencies, prompting engineers to integrate smart monitoring with sensors and AI for real-time leak detection and pressure management. Challenges like water scarcity, projected to affect 1.7-2.4 billion urban residents by mid-century amid climate variability, drive solutions such as aquifer recharge and hybrid systems combining desalination with wastewater reuse to bolster resilience without over-relying on finite freshwater. Contamination risks from agricultural runoff or industrial pollutants necessitate source protection zones and advanced oxidation processes using UV or ozone to degrade persistent organics like PFAS, with treatment efficacy verified through pilot-scale testing.[73][74][75]Wastewater and Stormwater Management
Wastewater management in environmental engineering involves the engineering of systems for collecting, treating, and disposing of domestic sewage, industrial effluents, and other liquid wastes to mitigate water pollution and protect public health.[76] Treatment processes are staged: preliminary treatment employs screening and grit removal to eliminate large debris and settleable inorganic particles; primary treatment uses sedimentation to remove 50-70% of suspended solids and 25-40% of biochemical oxygen demand (BOD); secondary treatment applies biological methods, such as activated sludge processes where microorganisms aerobically degrade organic matter, achieving 85-95% BOD reduction; and tertiary treatment incorporates advanced filtration, nutrient removal via chemical precipitation or enhanced biological processes, and disinfection typically with chlorine, ultraviolet light, or ozone to inactivate pathogens.[76] Sludge generated across stages is thickened, digested anaerobically for stabilization and biogas production, dewatered, and either landfilled, incinerated, or applied as biosolids fertilizer under strict regulations.[77] Globally, only 56% of domestic wastewater flows—equivalent to 332 billion cubic meters annually—were safely treated as of 2024, with disparities stark in developing regions where untreated discharge exacerbates waterborne diseases and ecosystem degradation.[78] Stormwater management addresses precipitation-induced surface runoff in urban and developed areas, where impervious surfaces amplify volume, velocity, and pollutant loads including sediments, heavy metals, nutrients, and hydrocarbons, leading to flooding, erosion, and receiving water impairment.[79] Engineering techniques prioritize infiltration, detention, and treatment: permeable pavements and vegetated swales promote groundwater recharge and initial pollutant filtration; detention basins and retention ponds temporarily store runoff to attenuate peak flows before controlled release; and low-impact development practices like rain gardens and bioswales leverage soil and vegetation for natural filtration, reducing total suspended solids by 60-90% and nutrients by 40-70% in field applications.[79] [80] Computational models such as the EPA's Storm Water Management Model (SWMM) simulate hydrology, hydraulics, and water quality to optimize designs, supporting objectives like reducing runoff by 20-50% through integrated green-gray infrastructure.[81] In cities with legacy combined sewer systems, stormwater surges overwhelm capacity, causing combined sewer overflows (CSOs) that discharge billions of gallons of untreated wastewater annually into waterways, as seen in U.S. systems where CSOs contribute 850 billion gallons yearly despite regulatory efforts.[82] Engineering mitigations include full or partial sewer separation to isolate stormwater, inline storage tunnels or tanks to capture excess during peaks (e.g., projects holding 20-100 million gallons), high-rate treatment via vortex separators or ballasted flocculation for overflows, and hybrid approaches integrating real-time sensors for adaptive gate operations.[82] [83] Green infrastructure complements these by capturing rooftop and street runoff, with studies quantifying 42% CSO volume reductions in implemented catchments, though scalability depends on space constraints and maintenance demands that can limit efficacy in dense urban cores.[84] Emerging challenges include emerging contaminants like pharmaceuticals and microplastics, which conventional processes remove inefficiently (e.g., <50% for many trace organics), prompting advanced integrations such as membrane bioreactors or adsorption for higher efficiencies up to 99%.[85]