What Substances Remain in Wastewater After Effective Secondary Treatment

Wastewater treatment is a critical process for maintaining environmental and public health by removing contaminants from sewage and industrial effluents before they are released into natural water bodies. These processes ensure that the wastewater does not pose a risk to ecosystems or human health. Treatment typically occurs in several stages, including primary, secondary, and sometimes tertiary processes. Each step is designed to target specific types of contaminants. Secondary treatment is particularly important as it employs biological processes to significantly reduce the concentration of biodegradable organic matters and suspended solids. However, even after effective secondary treatment, some substances persist in wastewater. These residues raise concerns about environmental and health impacts, stimulating discussions on the need for advanced treatment processes.

Overview of Wastewater Treatment Processes

1. Primary Treatment

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Primary treatment is the initial stage aimed at removing large solids and suspended particulate matter through physical processes such as screening and sedimentation. The primary goal is to decrease the wastewater’s solids content to make it amenable to subsequent biological treatment. Typically, primary treatment removes about 60% of suspended solids and 35% of Biological Oxygen Demand (BOD), a measure of organic matter.

2. Secondary Treatment

Secondary treatment focuses on the biological degradation of dissolved organic material that remains after primary treatment. This phase often employs microorganisms, predominantly bacteria, which consume organic molecules as food. Biological processes, such as activated sludge systems, trickling filters, or bio-towers, are commonly used in this phase, reducing BOD by approximately 85-90% and further removing suspended solids.

3. Tertiary (or Advanced) Treatment

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Although not always implemented, tertiary treatment addresses specific contaminants not removed during secondary treatment. This phase could involve chemical, physical, or biological processes to remove nutrients (like nitrogen and phosphorus), heavy metals, residuals of pharmaceuticals, and any remaining pathogens. The use and necessity of tertiary treatment depend on local regulations and the intended use of the treated wastewater.

Even with these established processes, a range of substances can persist post-secondary treatment. Understanding these substances and their potential impacts is integral to advancing wastewater management and environmental protection.

Substances Remaining After Secondary Treatment

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1. Nutrients: Nitrogen and Phosphorus

One significant concern with secondary-treated wastewater is the presence of nutrients, primarily nitrogen and phosphorus. These nutrients, although essential for plant growth, can cause eutrophication when introduced in excessive amounts into aquatic environments. Eutrophication leads to algal blooms, which deplete dissolved oxygen in water bodies, thus harming fish and other aquatic organisms.

• • Nitrogen: Nitrogen typically remains in forms like ammonia, nitrate, and nitrite. Biological processes in secondary treatment convert a portion of ammonia to nitrate through nitrification. Still, complete nutrient removal often requires additional treatment steps. 

• • Phosphorus: Phosphorus may persist in organic and inorganic forms. Its removal in secondary treatment is generally limited; therefore, further chemical treatment (such as precipitation with iron or aluminum salts) in tertiary processes is often necessary to achieve regulatory standards.

2. Pathogens and Microorganisms

Secondary treatment using biological processes vastly reduces pathogenic microorganisms, yet it seldom results in their complete elimination. Remaining pathogens include bacteria, viruses, protozoa, and helminths. While secondary treatment can reduce pathogen levels by 90-99%, the risk persists for waterborne diseases if effluents are improperly managed or discharged into sensitive areas.

Disinfection during tertiary treatment (using chlorination, ozonation, or UV irradiation) is crucial for pathogen control, especially for waters intended for human contact or reclaimed water uses.

3. Pharmaceuticals and Personal Care Products (PPCPs)

PPCPs are emerging contaminants of interest due to their increasing presence in the environment and potential effects on wildlife and human health. These substances originate from a variety of sources, including household waste, hospitals, and pharmaceuticals industries. Typical examples include:

• • Antibiotics

• • Hormones and contraceptives

• • Painkillers and anti-inflammatory drugs

• • Preservatives and disinfectants in personal care products

Secondary treatment removes PPCPs only partially, as these compounds are designed to be biologically active and stable in various environments. Advanced oxidation processes (AOPs) in tertiary treatment are often required for effective PPCP degradation.

4. Heavy Metals

Metals like copper, lead, mercury, zinc, and cadmium may persist after secondary treatment due to their non-biodegradable nature. Such metals originate from industrial discharges, urban runoff, or household plumbing. Their presence in the environment can be toxic to aquatic life and humans, interfering with cellular processes and bioaccumulating through food webs.

The removal of heavy metals often necessitates specific chemical treatments, such as precipitation, ion exchange, or membrane technology, typically integrated into tertiary treatment stages.

5. Endocrine Disrupting Compounds (EDCs)

EDCs comprise various chemicals capable of interfering with hormone systems, causing developmental, reproductive, neural, and immune effects in both humans and wildlife. Similar to PPCPs, these compounds enter water systems through domestic sewage and industrial waste. Common EDCs include:

• • Bisphenol A (BPA)

• • Phthalates

• • Pesticides and herbicides

Secondary treatment generally reduces EDC levels through microbial degradation, but complete removal often requires enhanced biochemical or physicochemical treatments. EDCs remain an area of active research, aiming to understand their impact and develop effective removal technologies.

6. Dissolved Solids and Salts

While secondary treatment effectively reduces organic matter and some suspended solids, dissolved solids, including salts and minerals, usually remain at high levels. These may include sodium, potassium, calcium, and magnesium ions, often originating from household waste and commercial effluents.

High levels of dissolved solids can affect the use of treated wastewater for irrigation or discharge into sensitive aquatic environments. Reverse osmosis or electrodialysis in tertiary treatment are potential solutions for reducing these dissolved constituents.

7. Microplastics

Microplastics, defined as plastic particles less than 5 millimeters in size, are an increasing concern due to their persistence and potential impacts on aquatic life and food safety. These originate from consumer products, synthetic clothing, and deteriorating plastic waste. Secondary treatment typically captures larger particles, but smaller microplastics can escape biological treatment processes.

Research continues on effective methods for microplastic removal, highlighting the importance of mechanical filtration and advanced tertiary treatments to address this emerging pollutant class.

Environmental and Health Implications

The presence of these residual substances in secondary-treated wastewater poses various environmental and health challenges:

• • Ecosystem Impact: Eutrophication, caused by excess nutrients, leads to hypoxic conditions detrimental to aquatic life. Heavy metals and microplastics can accumulate in organisms, disrupting food chains and leading to toxicity. 

• • Human Health Risks: Pathogens and PPCPs in water sources can result in waterborne diseases and antibiotic resistance. EDCs pose threats to human development and reproductive health, demanding caution in areas reliant on recycled water. 

• • Regulatory Considerations: National and international regulations, like the European Union’s Urban Wastewater Treatment Directive and the U.S. Clean Water Act, increasingly focus on reducing pollutant discharge, prompting stricter effluent quality standards.

Advances and Innovations in Wastewater Treatment

As understanding of residual substances in treated wastewater expands, treatment technologies continue to evolve. Notable advancements include:

• • Membrane Filtration: Technologies like ultrafiltration and nanofiltration can enhance the removal of micropollutants and pathogens in tertiary treatment stages. 

• • Biochar and Constructed Wetlands: These eco-friendly approaches offer additional removal capacity for nutrients, heavy metals, and organic contaminants through adsorption and phytoremediation. 

• • Advanced Oxidation Processes (AOPs): Techniques using ozone, hydrogen peroxide, or UV light effectively degrade persistent organic pollutants, including PPCPs and EDCs. 

• • Integrative Management: Encouraging source control measures, such as minimizing industrial discharge and promoting the use of biodegradable products, complement treatment technologies in reducing pollutant loads.

Conclusion

Even after effective secondary wastewater treatment, a range of substances persists, presenting ongoing environmental and health challenges. Addressing these concerns requires a multifaceted approach, including advanced treatment methods, regulatory frameworks, and public awareness initiatives. Continued research and innovation in wastewater management are essential to mitigate these impacts and safeguard our water resources. By refining treatment processes and integrating sustainable practices, we can achieve cleaner effluents and better protect natural ecosystems and human populations.