Wastewater Treatment in Plant: How Sewage Treatment Plants Fit Into Water Infrastructures and Planning

“Wastewater treatment in plant” sounds like a purely technical topic—pipes, tanks, and discharge permits. But the monitoring frameworks behind the United Nations Sustainable Development Goal (SDG) Target 6.3 frame it as something bigger: a cornerstone of public health protection, environmental quality, and climate resilience, tightly linked to how countries build water infrastructures and manage water planning over time.¹

That broader lens matters because wastewater is not only what happens inside the fence line of a facility. Monitoring frameworks track flows from generation to collection, delivery to treatment, treatment level, and final discharge—because failures at any step can leave wastewater “not safely treated,” even if treatment plants exist.²

What “Wastewater” Means in Practice (And Why Definitions Matter)

The SDG monitoring framework defines wastewater broadly as water that is of no further immediate value to the purpose for which it was used, excluding cooling water.³ From there, it distinguishes key categories that show up in national statistics and plant responsibilities:

That last distinction is critical: industrial wastewater treatment may happen in dedicated industrial facilities or inside industrial sites before discharge to a municipal sewer—so the boundary between “industrial” and “urban” treatment can be operationally blurred, especially in reporting.³

Global Context: The Treatment Gap

The 2024 SDG progress report provides a sobering global snapshot. Of approximately 268 billion m³ of household wastewater generated worldwide in 2022, only 155 billion m³ (58%) was estimated to have been collected, delivered to treatment, safely treated, and discharged.⁴

This gap reveals a critical planning insight: many flows that are not safely treated are never collected in the first place. Collection systems and “delivery to treatment” belong in water infrastructure planning, not just treatment capacity expansion.⁵

Treatment Levels: How Regulators Define Performance

Engineers often think of treatment levels as technology packages. But regulatory monitoring defines them by performance outcomes and quantifiable removal thresholds:

Primary Treatment

Physical and/or chemical treatment involving settlement of suspended solids, in which BOD₅ is reduced by at least 20% and total suspended solids are reduced by at least 50% before discharge.⁶ These percentage thresholds come from the WHO/UNICEF Joint Monitoring Programme definitions used to track SDG indicator 6.3.1—a global monitoring convention, not a binding national or EU regulatory limit.

Secondary Treatment

Post-primary biological treatment with secondary settlement, resulting in BOD removal ≥ 70% and COD removal ≥ 75%. Under the EU Urban Wastewater Treatment Directive, compliance for these parameters can be demonstrated either by these percentage reductions or by meeting absolute discharge concentrations—typically BOD₅ ≤ 25 mg/L, COD ≤ 125 mg/L, and total suspended solids ≤ 35 mg/L—whichever the permit specifies. Natural biological treatment processes can also qualify if effluent constituents match conventional secondary treatment.⁶

Tertiary Treatment

Additional treatment beyond secondary, targeting nitrogen and/or phosphorus and/or other pollutants affecting water quality. Important caveat: different removal efficiencies (e.g., organic pollution removal, nitrogen removal, phosphorus removal) are exclusive and cannot simply be added together.⁶

Sewage Treatment Plants: Why They’re Central to National Performance

Urban wastewater treatment plants sit at the center of national wastewater statistics and reporting—not because they’re the only source of treatment, but because they’re often where data exist and where flows converge.⁷

This matters strategically: what decision-makers can measure, they tend to fund and regulate. If municipal plant data dominate national statistics, industrial treatment and smaller facilities often remain invisible in planning processes—creating blind spots in water infrastructure strategy.⁷

The EU Case: The Urban Wastewater Treatment Directive

The European Union’s Urban Wastewater Treatment Directive (UWWTD), first adopted in 1991, illustrates how a long-term regulatory framework structures wastewater treatment as part of water planning. In its 1991 form it requires:

• Collection and treatment systems for agglomerations ≥ 2,000 population equivalents
• Discharge concentration standards for organic pollution, suspended solids, phosphorus, and nitrogen (based on agglomeration size and sensitivity of receiving waters)
• Ongoing monitoring of compliance by Member States

As of 2018, overall EU compliance with the directive was 82%—a strong signal of long-term regulatory commitment, but also a reminder that full compliance is elusive even in advanced economies.⁸

What “Performance” Means: The Pollutants Plants Are Built to Reduce

Treatment performance is tied directly to the pollutants targeted by regulation and monitoring. The major quantified outcomes center on:

• Organic matter (measured as BOD₅ and COD)
• Nutrients (nitrogen and phosphorus)
• Microbiological indicators (coliforms and faecal indicators)

The Bathing Water Test

A key outcome measure in European monitoring is improvement in bathing water standards. The UWWTD allowed clear improvements in meeting excellent and good standards for faecal coliforms in inland and coastal waters—showing that regulatory mandates can produce measurable environmental improvement when implemented consistently.⁹

Industrial Wastewater Treatment: Where the Hardest Gaps Show Up

Industrial wastewater is a core piece of wastewater treatment monitoring, but global assessments are candid about how difficult it is to measure at scale. Barriers include:

• Confidentiality constraints (companies treat disclosure as competitively sensitive)
• Widespread self-supplied water sources (rivers and groundwater not captured in public statistics)
• Fragmented institutional responsibility across multiple agencies
• Potential double-counting when industrial flows are treated at source and then routed to municipal systems

This matters for water planning because many “industrial” discharges never appear in the same data systems used for municipal water plans. A water plan that only optimizes sewage treatment plants may miss a large fraction of industrial flows entirely.¹⁰

Why “In Plant” Performance Is Not Enough: Collection, Delivery, and Overflows

Wastewater treatment plants cannot treat what never reaches them. The SDG framework explicitly tracks the entire chain:

Generation → Collection → Delivery → Treatment → Discharge

Failures anywhere in this chain can prevent safe treatment. Sewer wastewater that is not delivered to an urban plant may be discharged through direct end-pipes, combined sewer overflows, or leaking pipes. Septic tank wastewater may not reach treatment if systems are not properly contained or if emptied sludge is disposed of unsafely.¹¹

Combined Sewer Overflows: A Persistent Challenge

Even under full compliance with the UWWTD, remaining loads from combined sewer overflows and urban runoff can be comparable to other sources of pollution. During storm events, part of collected wastewater may be released untreated through CSOs—sources the 1991 directive did not directly address, and a gap the 2024 recast explicitly sets out to close.¹²

For practitioners, the planning implication is clear: as dry-weather wastewater becomes better treated through upgraded plants, storm-driven discharges become proportionally more important in overall environmental loading.¹³

Chemicals and Micropollutants: Where Secondary Treatment May Fall Short

One of the strongest cautions in emerging wastewater science is about chemical micropollutants—substances including pharmaceuticals, personal care products, industrial chemicals, and pesticides. These include a very high number of substances and were not an explicit target of conventional urban wastewater treatment, though the 2024 UWWTD recast now mandates dedicated quaternary treatment for micropollutants at large plants.¹⁴

Critically, removal efficiency for many micropollutants is highly variable and depends on technology and operating conditions, making it difficult to predict from treatment level alone. A useful predictor is the octanol-water partition coefficient (log K_ow): hydrophobic compounds with a high log K_ow (roughly above 4) tend to adsorb onto sludge solids, while hydrophilic compounds with a low log K_ow stay in the aqueous phase and pass largely untreated. Some substances are virtually unaffected by conventional treatment; others can shift pollution pathways (e.g., from water to sludge) rather than being removed.¹⁴

A Practical Planning Checklist

If you are building water plans that include “wastewater treatment in plant” upgrades, these frameworks suggest a planning logic broader than equipment selection:

1. Define “success” as safely treated outcomes, not just “installed capacity.” In these frameworks, “safely treated” means compliance with standards or treatment commensurate with secondary or higher processes.¹⁵
2. Plan across the entire chain: collection → delivery → treatment → discharge. The SDG framework explicitly tracks failures such as CSOs, leaking sewers, and septic containment issues that can prevent safe treatment even where plants exist.¹¹
3. Address storm-driven pollution as a structural part of the system. CSO and urban runoff loads can be comparable to remaining loads even under strong compliance scenarios.¹²
4. Separate (and reconcile) industrial and urban accounting. Fragmented responsibility, self-supplied water, confidentiality barriers, and potential double counting make industrial flows invisible in municipal statistics.¹⁰
5. Be cautious about micropollutants. Many chemicals are not removed reliably by conventional treatment, and removal can be highly variable; substances such as PFAS are virtually unaffected. Where the 2024 UWWTD recast applies, factor in the coming quaternary treatment obligations.¹⁴

Frequently Asked Questions

Conclusion: “In Plant” Matters—But Only as Part of the Whole System

Wastewater treatment plants are the visible anchor point of water infrastructure strategy. But their performance can only be understood as part of a larger system: collection networks, delivery reliability, storm management, industrial integration, and emerging contaminant control.

That is exactly where water infrastructure strategy meets water planning reality: not only deciding what treatment level exists on paper, but ensuring wastewater is collected, delivered, treated to at least secondary (or better) outcomes, and discharged in ways that genuinely protect receiving waters and public health.¹⁵¹¹