What Does Wastewater Contain

Wastewater is a fundamental byproduct of human activities, ranging from residential and industrial uses to agricultural applications. The composition of wastewater is a complex mixture of organic and inorganic matter, pathogens, nutrients, and various pollutants. Understanding what wastewater contains is crucial for developing effective treatment processes and strategies to prevent environmental contamination, protect public health, and conserve water resources. As the foundational knowledge base within the broader discipline of the Wastewater Treatment Process, wastewater composition defines what treatment systems must remove, the regulatory standards they must meet, and the technologies most appropriate for achieving the required effluent quality — making wastewater characterization the essential first step in every treatment plant design, process selection, and operational troubleshooting effort.

Sources of Wastewater

Wastewater originates from various sources, categorized into three main types: domestic, industrial, and agricultural — each with distinct composition characteristics that drive different treatment approaches.

Domestic Wastewater (Sewage): Generated from household activities including bathing, cooking, cleaning, and sanitation. Typical domestic wastewater contains BOD of 150–300 mg/L, TSS of 100–300 mg/L, total nitrogen of 20–70 mg/L (predominantly as ammonia), total phosphorus of 4–15 mg/L, and fecal coliform concentrations of 10⁶–10⁹ CFU/100 mL. Per capita generation in the US averages approximately 80–100 gallons per person per day, with significant diurnal variation (peak hour flows typically 2–3× average daily flow).

Industrial Wastewater: Derived from manufacturing processes, industrial wastewater composition varies dramatically by industry sector — from relatively dilute cooling water to extremely concentrated process effluent with BOD above 10,000 mg/L, heavy metals at mg/L concentrations, and toxic organic compounds requiring specialized pretreatment before discharge to municipal sewers (under industrial pretreatment programs) or direct treatment before environmental discharge.

Agricultural Wastewater: Runoff from fields laden with fertilizers, pesticides, herbicides, and organic compounds from animal waste. Agricultural wastewater typically contains elevated nitrogen (predominantly as nitrate from fertilizer application) and phosphorus, as well as pesticide and herbicide residuals that are not effectively removed by standard secondary biological treatment — requiring specialized treatment for potable water applications downstream.

Components of Wastewater: Detailed Characterization

Organic Matter

Organic matter in wastewater is measured primarily through two aggregate parameters: biochemical oxygen demand (BOD) — the oxygen consumed by microbial decomposition of biodegradable organic matter over 5 days at 20°C — and chemical oxygen demand (COD) — the oxygen equivalent of all chemically oxidizable compounds. The BOD/COD ratio provides information about the biodegradability of the wastewater: a ratio above 0.5 indicates readily biodegradable wastewater suitable for conventional biological treatment; a ratio below 0.3 indicates significant refractory (non-biodegradable) organic content requiring physical-chemical treatment.

Biodegradable Organics: Proteins, carbohydrates, and fats from human waste, food residues, and certain industrial processes. Their decomposition by heterotrophic bacteria consumes dissolved oxygen, creating the oxygen-depleting BOD load that secondary biological treatment is designed to remove. In municipal wastewater, the COD/BOD ratio is typically 1.5–2.5, with the fraction above BOD representing slowly biodegradable and refractory compounds.

Volatile Organic Compounds (VOCs): Common in industrial wastewater, VOCs including benzene, toluene, ethylbenzene, and xylenes (BTEX compounds) can be toxic to biological treatment microorganisms at elevated concentrations and may volatilize in aeration systems, creating air quality concerns and regulatory air permit requirements.

Emerging Organic Contaminants: Pharmaceuticals, personal care products, microplastics, PFAS (per- and polyfluoroalkyl substances), and endocrine-disrupting compounds (EDCs) are detected in municipal wastewater at nanogram-to-microgram per liter concentrations. These compounds are incompletely removed by standard secondary treatment — typically achieving 20–80% removal depending on the specific compound and treatment configuration — and the fraction discharged to receiving waters or remaining in biosolids is subject to increasing regulatory scrutiny.

Inorganic Substances

Nutrients — Nitrogen: Municipal wastewater contains total nitrogen of 20–70 mg/L, predominantly as ammonia (NH₄⁺) from urine decomposition, with smaller fractions as organic nitrogen, nitrite, and nitrate. Ammonia exerts an additional oxygen demand on receiving waters (nitrogenous BOD — approximately 4.57 mg O₂ per mg NH₄⁺-N) and is directly toxic to aquatic organisms at elevated concentrations. Biological nitrification-denitrification or physical-chemical processes (air stripping, ion exchange) are required to reduce nitrogen to levels meeting discharge limits in nutrient-sensitive watersheds (typically TN below 3–10 mg/L).

Nutrients — Phosphorus: Municipal wastewater contains total phosphorus of 4–15 mg/L, predominantly as orthophosphate (PO₄³⁻) from detergents and human waste, with smaller fractions as polyphosphate and organic phosphorus. Even small concentrations of phosphorus (above 0.01–0.1 mg/L TP) can trigger eutrophication in phosphorus-limited water bodies — driving the increasingly stringent effluent TP limits (below 0.1 mg/L in many TMDL-impaired watersheds) that require enhanced biological phosphorus removal (EBPR) combined with chemical polishing.

Heavy Metals: Arsenic, lead, mercury, cadmium, chromium, nickel, and zinc are typically present in municipal wastewater from industrial contributions, stormwater infiltration, and household product use. EPA’s industrial pretreatment program limits heavy metal concentrations in industrial discharges to POTWs to protect treatment processes and biosolids quality for land application — exceedances can compromise the Class B or Class A biosolids classification that enables beneficial reuse.

Total Dissolved Solids (TDS): Municipal wastewater TDS of 500–1,500 mg/L represents the cumulative dissolved mineral content from water supply minerals, household chemical contributions, and infiltration from groundwater. TDS increases progressively as water passes through the collection system due to evaporative concentration and groundwater infiltration; standard secondary treatment does not reduce TDS, which accumulates in wastewater recycling programs and requires RO treatment for potable reuse applications.

Pathogens

Pathogens in wastewater include bacteria (fecal coliforms 10⁶–10⁹ CFU/100 mL, Salmonella, Vibrio cholerae), viruses (enteric viruses 10³–10⁵ PFU/L including norovirus, rotavirus, adenovirus, hepatitis A), protozoa (Cryptosporidium 10⁻²–10³ oocysts/L, Giardia 10⁻¹–10³ cysts/L), and helminths (roundworm, tapeworm eggs). These pathogens pose significant health risks; untreated or poorly treated wastewater transmits cholera, typhoid, hepatitis A, giardiasis, and cryptosporidiosis through multiple exposure pathways including water ingestion, recreational contact, and food consumption.

Secondary treatment with disinfection achieves 4–7 log pathogen reduction from typical municipal influent concentrations, reducing effluent pathogen levels to those meeting recreational contact or restricted irrigation reuse standards. Advanced treatment including MF/UF membranes achieves additional 3–4 log removal credit for Giardia and Cryptosporidium that are resistant to chlorine disinfection.

Suspended Solids

Total suspended solids (TSS) in municipal wastewater of 100–300 mg/L include particles of organic and inorganic materials that contribute to effluent turbidity, harm aquatic life by reducing sunlight penetration, smother aquatic habitats through deposition, and carry associated pathogens and contaminants. Primary clarification removes 55–65% of influent TSS as primary sludge; secondary biological treatment with clarification removes an additional 85–95% of the remaining TSS to achieve effluent TSS typically below 30 mg/L meeting secondary treatment standards.

Chemicals and Pharmaceuticals

Pharmaceuticals including antibiotics, hormones, analgesics, antidepressants, and antiepileptics enter wastewater through human excretion and direct disposal. Antibiotic resistance gene (ARG) proliferation in wastewater treatment environments — where antibiotics selectively pressure microbial communities — is an emerging public health concern, as treated effluent discharged to receiving waters can spread ARG-carrying bacteria to environmental reservoirs. Pesticides including organochlorines (historically prevalent, now largely banned but persistent in legacy contamination) and currently-used pesticides (glyphosate, chlorpyrifos, imidacloprid) enter wastewater through stormwater runoff and direct household use.

Subtopic Overview: Wastewater Composition Knowledge Topics

Understanding wastewater composition requires not just characterizing what the influent contains, but also understanding what treatment produces as byproducts, what constitutes wastewater versus other water types, and what treatment cannot remove — three questions that define the boundaries of wastewater treatment performance and drive ongoing regulatory and research attention. The subtopics below address these three primary wastewater composition knowledge areas covered in depth on this site.

What Are the By-Products of Wastewater Treatment

What are the by products of wastewater treatment — the residuals generated as treatment processes convert influent wastewater into treated effluent — include biosolids (the stabilized solid residual from primary and secondary sludge treatment), biogas (methane-rich gas from anaerobic digestion of sludge), grit and screenings (the coarse inorganic and gross solid materials removed at headworks), and increasingly, recovered resources including struvite phosphorus, cellulose, and biogas-derived electricity that transform the byproduct profile from liabilities into revenue streams. Biosolids represent the largest by-product stream by mass — a 10 MGD municipal plant with secondary treatment typically produces 5,000–15,000 dry tonnes of biosolids per year, which must be managed through land application (Class B or Class A biosolids applied to agricultural land as a soil amendment), composting, thermal drying, incineration, or landfill disposal. Biogas produced by anaerobic digestion of primary and secondary sludge contains 60–70% methane with a heating value of approximately 600 BTU/SCF — enough energy, when converted through a cogeneration engine, to supply 30–100% of a large plant’s electrical demand from the chemical energy inherent in the wastewater influent organic matter. Grit and screenings removed at the headworks — inorganic mineral particles (sand, gravel) and gross solids (rags, plastics, food waste) — are typically landfilled after dewatering and washing, representing a disposal cost rather than a revenue opportunity at most facilities, though research into recovered cellulose from screenings and fine grit beneficiation for construction sand applications is emerging.

Which of the Following Is Not Considered Wastewater

Understanding which of the following is not considered wastewater — distinguishing regulated wastewater from stormwater, groundwater, uncontaminated process water, and other water streams that require different regulatory treatment — has direct regulatory and operational implications for how water-generating facilities classify, permit, and manage their water discharges. The EPA’s Clean Water Act regulatory framework distinguishes point source discharges of “pollutants” from a “point source” (requiring an NPDES permit) from non-point source pollution (addressed through best management practices and watershed programs) — a distinction that determines whether a facility must obtain a discharge permit and comply with numeric effluent limits or can manage its runoff through non-regulatory means. Stormwater from industrial sites is regulated as wastewater under the NPDES Multi-Sector General Permit (MSGP) and requires best management practices and in some sectors benchmark monitoring — distinguishing it from uncontaminated stormwater that can be discharged without a permit. Cooling water from heat exchange systems that has not contacted process materials or waste streams is generally not considered wastewater in the regulatory sense — though thermal discharge to receiving waters is regulated under NPDES permits when the temperature increase could harm aquatic life. Understanding these classification boundaries enables facilities to correctly categorize their water streams for regulatory compliance, avoid over-reporting obligations for unregulated streams, and correctly identify which streams require treatment before discharge.

Which Substance Is Not Removed by Wastewater Treatment Facilities

Which substance is not removed by wastewater treatment — identifying the classes of contaminants that pass through standard secondary treatment essentially unchanged — defines the treatment performance gap that drives advanced treatment requirements for reuse applications and motivates ongoing regulatory development for emerging contaminants. Standard secondary treatment (activated sludge + clarification + disinfection) effectively removes BOD (85–95%), TSS (85–95%), pathogens (4–7 log), and a portion of nutrients — but does not remove dissolved salts (TDS passes through essentially unchanged), most pharmaceuticals and personal care products (typically 20–80% removal depending on compound), PFAS compounds (essentially zero removal by biological treatment; RO achieves 95–99% rejection), dissolved inorganic ions (nitrate, chloride, sulfate), and synthetic microplastics (partial removal of larger particles in clarification, but nano-sized particles pass through). PFAS compounds — including PFOA and PFOS, now regulated in drinking water at 4 ng/L each under EPA’s 2024 PFAS National Primary Drinking Water Regulation — are not removed by standard biological treatment, accumulate in biosolids at concentrations of hundreds to tens of thousands of ng/g dry weight, and are detectable in treated effluent at concentrations that may impair groundwater recharge and surface water downstream uses. This treatment gap for PFAS has driven intense regulatory attention, research into ion exchange and activated carbon treatment for PFAS removal from influent and centrate streams, and debate over biosolids land application as a PFAS source to agricultural soils and groundwater.

Environmental and Health Impacts of Wastewater Components

Eutrophication

Excessive nutrients accelerate plant growth, particularly algae, depleting oxygen levels and harming aquatic life. The “dead zone” in the Gulf of Mexico — driven by Mississippi River nitrogen and phosphorus loads from municipal and agricultural sources — covers approximately 6,000 square miles during summer stratification, with bottom dissolved oxygen below 2 mg/L killing or displacing essentially all aerobic aquatic organisms. Advanced nutrient removal at municipal plants discharging to impaired watersheds demonstrably improves receiving water quality when implemented at sufficient scale.

Toxicity to Aquatic Life

Heavy metals and organic pollutants are toxic to fish and other aquatic organisms at concentrations well below regulatory thresholds for human health protection. Endocrine-disrupting compounds including synthetic estrogens from oral contraceptives (ethinyl estradiol — active at 1 ng/L in fish) cause feminization of male fish and reproductive disruption in aquatic populations at concentrations present in treated municipal effluent.

Human Health Risks

Pathogens in untreated or inadequately treated wastewater cause waterborne illnesses responsible for approximately 2 million deaths per year globally (primarily in developing countries without adequate sanitation). Pharmaceuticals and industrial chemicals can disrupt endocrine function, increase cancer risks, and contribute to antibiotic resistance proliferation in environmental microbial communities that interact with human populations.

Soil Degradation

When inadequately treated wastewater is applied to agricultural land — a common practice in water-scarce developing countries — it can cause soil salinization, heavy metal accumulation above phytotoxic thresholds, and pathogen contamination of edible crops, reducing agricultural productivity and creating food safety risks that proper wastewater characterization and treatment would prevent.

Comparison of Wastewater Constituent Classes by Treatment Removability

Emerging Technologies and Innovations

Membrane Bioreactors (MBRs): Combine biological treatment and membrane filtration, providing effective solid-liquid separation and pathogen removal that standard secondary clarification cannot achieve — producing effluent turbidity below 0.2 NTU and TSS below 5 mg/L from a single combined biological-filtration process.

Anammox Process: Anaerobic ammonia oxidation using Candidatus Anammoxoglobus propionicus and related organisms achieves nitrogen removal at 60% lower energy than conventional nitrification-denitrification and without the external carbon source requirement — particularly valuable for treating high-ammonia streams (digester reject water, industrial effluent) where its economy advantage is most significant.

Phytoremediation: Using plants including constructed wetland macrophytes to absorb and remove contaminants from wastewater provides an eco-friendly alternative to energy-intensive chemical treatments for final polishing of nutrients and trace metals in appropriate climate and land availability contexts.

Nanotechnology: Nanoscale materials including iron oxide nanoparticles, graphene oxide composites, and carbon nanotubes provide adsorptive capacity for heavy metals and PFAS at laboratory scale — with scale-up challenges including nanoparticle recovery, potential ecotoxicity, and regulatory uncertainty for treated water applications under active research.

Field Notes: Practical Guidance for Wastewater Characterization

Comprehensive Influent Characterization as the Design Foundation

Every treatment system design and operational optimization effort begins with wastewater characterization — the systematic measurement of the influent’s physical, chemical, and biological parameters under representative flow and loading conditions. A complete characterization for design purposes requires 24-hour flow-weighted composite samples across multiple days representing seasonal variation, weekday/weekend differences, and wet/dry weather conditions; grab sample data misses diurnal and event-driven peaks that govern design. For context on how these influent characteristics drive treatment plant type selection, the Types Of Wastewater Treatment Plants resource covers how different plant configurations are matched to different wastewater compositions and flow profiles. For the first treatment stage that addresses the gross solids and settleable fraction of the composition described in this article, the What Is The First Stage Of Waste Water Treatment resource covers preliminary and primary treatment — the headworks processes sized on the basis of influent TSS, grit content, and hydraulic peak flow that wastewater characterization establishes. For how the composition of wastewater connects to the benefits that its treatment delivers, the What Are The Advantages Of Wastewater Treatment resource addresses the environmental, public health, and economic benefits that systematic removal of each constituent class achieves.

Common Characterization Mistakes

The most frequent wastewater characterization error for design purposes is relying on literature “typical” values for municipal wastewater composition rather than measuring the actual influent at the specific site — particularly for industrial-heavy collection systems, tourist-dominated communities with highly seasonal population, or facilities with significant groundwater infiltration that dilutes dry-weather concentrations. A second common mistake is failing to characterize the inorganic fraction of suspended solids separately from the volatile fraction — a plant receiving significant grit carryover from a poorly functioning headworks or industrial discharge of inorganic solids will show high TSS at the secondary clarifier influent that does not correspond to equivalent BOD load, and secondary treatment system design based on TSS/BOD ratio assumptions from typical municipal data will be mis-sized for the actual loading.

Conclusion