Does Waste Water Treatment Remove Pathogens?

Wastewater treatment is a crucial process for cleaning water before it’s released back into the environment. A key part of this treatment aims to remove pathogens – disease-causing microorganisms like bacteria, viruses, and protozoa. But how effective is wastewater treatment at actually eliminating these threats? As a professional water treatment supplier, I will help you find it out!

An Overview of Wastewater Treatment

Let’s start with a quick overview of wastewater treatment works. I’ll focus specifically on the processes involved in pathogen removal.

Wastewater treatment generally involves three main phases:

1. Primary treatment – Screens, tanks, and settling ponds remove large solids and about 50% of suspended solids. Some sedimentation of pathogens occurs.
2. Secondary treatment – Biological processes eliminate 85%+ of organic matter. Pathogens are removed through natural die-off and predation.
3. Tertiary treatment – Filtration, nutrient removal, and disinfection further clean the wastewater. Additional pathogen removal occurs.

After tertiary treatment, the wastewater (now called effluent) is safe for discharge or reuse.

Obviously that’s a broad overview. But it sets the stage for digging into the key question:

How effective are these processes at removing pathogens?

Do Primary Wastewater Treatments Remove Pathogens?

Primary treatments aim to remove coarse solids from raw sewage through screening and sedimentation.

Pathogen removal does occur in primary treatment, but it’s limited:

• Screening removes larger pathogens like protozoan cysts that are attached to debris.
• Grit removal chambers settle out sand and grit. Some encysted pathogens settle out.
• Primary clarifiers enable separation of floatable greases and sedimentation of organic solids. Around 10-20% of viruses and bacteria are removed.

However, most pathogens have low settling velocities so they remain suspended in the wastewater.

Overall, primary treatment only reduces pathogens by around 1-2 log units (that’s 90 to 99% removal). Significant risks remain for downstream processes and the receiving environment.

Secondary Treatment Pathogen Removal Processes

The main aim of secondary treatment is to drastically reduce organic matter and suspended solids using biological treatment processes.

But there are also opportunities for pathogen removal:

• Natural die-off – Unfavorable conditions (like lack of nutrients) causes some pathogens to die off.
• Predation – Other microorganisms feed on pathogens as a food source.
• Adsorption & filtration – Pathogens attach to media surfaces via physicochemical processes.

According to literature, well-optimized biological treatment processes generally achieve 1 to 4 log reductions for key pathogen groups. However, issues like short-circuiting can reduce removal performance.

Overall, secondary treatment alone doesn’t satisfy modern pathogen discharge guidelines. Additional tertiary treatment is needed.

Role of Tertiary Treatment in Removing Pathogens

Tertiary treatment represents final “polishing” of wastewater to make it safe for discharge or reuse. Processes include granular media filtration, nutrient removal, and disinfection.

Tertiary treatment offers major advantages for pathogen removal:

Nutrient Removal

Denitrification filters utilize woodchip or other biodegradable media to remove nitrogen. As an added benefit, they also filter out pathogens through adsorption and predation.

Field studies show tertiary denitrification achieves 0.2 to 4.5 log reductions for bacteria and viruses – better than typical secondary processes.

Filtration Processes

Sand, textile, and membrane filters physically trap pathogens to achieve targeted removal efficiency. Microfiltration reliably removes protozoa and most bacteria at >4 log reduction. And ultrafiltration eliminates viruses with >3 log reduction.

Disinfection Technologies

Chlorine, UV irradiation, and ozonation all inactive pathogens through chemical damage to cell structures. Over 3 to 6 log reductions are readily achievable.

Does Multi-Barrier Treatment Ensure Pathogen Removal?

Based on the fact above, it’s clear that relying on just primary and secondary processing isn’t enough for wastewater reuse applications.

Tertiary polishing steps like filtration and disinfection are essential to deal with residual viral and bacterial pathogens. And they readily achieve the >4 log reduction benchmark set by international guidelines.

The most reliable approach utilizes multi-barrier treatment – combining complementary technologies like biological digestion, filtration, and chemical disinfection.

In this way, any lapses in treatment efficacy by one process are backed up by the others. Redundancy improves robustness.

And the combined effect is greater than any one process alone. With optimal operation, treated effluent can fully satisfy microbiological guidelines for discharge and reuse.

Emerging Pathogen Risks in Wastewater Treatment

As knowledge advances, new concerns always emerge over risks posed by wastewater pathogens. Recently, focus has turned to antibiotic resistance and specific pathogens like SARS-CoV-2.

Antimicrobial Resistance Risks

Wastewater treatment plants receive sewage from homes and hospitals – both sources of disease-causing bacteria. Treated effluent still contains live antibiotic-resistant bacteria and genes.

There are fears this genetic resistance could transfer to natural microbial populations. Or that inadequetly treated wastewater could spark disease outbreaks.

To de-risk wastewater discharge, some installations now utilize advanced oxidation to damage cell structures and inactivate resistant pathogens.

Coronavirus Contamination

SARS-CoV-2 has been frequently detected in untreated sewage which acts as a useful wastewater-based epidemiology tool. However, tertiary disinfection processes like UV irradiation seem to effectively inactive coronavirus pathogens – reducing risks to downstream environments and workers.

The Bottom Line

It’s clear wastewater treatment provides a vital barrier against waterborne pathogens that impact public and ecological health.

While primary and secondary processing alone achieves reasonable pathogen reduction, tertiary polishing is usually essential to satisfy discharge or reuse guidelines – especially filtration and disinfection steps.

A well-optimized, multi-barrier treatment train reliably eliminates bacterial, viral, and protozoan pathogens to help safeguard the water cycle. And continued technology advances help address emerging contaminant risks like antimicrobial resistance.

Through sustainable water reuse backed by effective wastewater treatment, communities around the world can unlock reliable water resources that are safe from pathogenic threats now and into the future.