When we flush the toilet, take a shower, wash dishes, or even brush our teeth, that water heads down the drain and is whisked away, out of sight and out of mind. But where exactly does that wastewater go? What happens to it after it disappears down the pipes in our homes and businesses?
In this article, we’ll walk through the journey that wastewater takes after leaving our homes, traveling through sewers and wastewater treatment plants to be cleaned before being released back into the environment. Gaining an understanding of this complex treatment process helps highlight the importance of properly handling our wastewater and protecting our precious water resources.
An Overview of Sewers and Wastewater Treatment Systems
Across cities and towns, a vast underground network of sewer pipes and tunnels collects the wastewater from homes, businesses, schools and other sites. This sewer system channels the wastewater to the municipal wastewater treatment plant for processing before it can be safely released into the environment.
But this complex sewer infrastructure doesn’t function alone. It’s actively managed and maintained by skilled wastewater professionals across public works departments and wastewater utilities. From specialized operators, engineers and technicians to administrative support staff, keeping sewers and treatment plants running requires a dedicated workforce with technical skills and expertise treating high volumes of wastewater 24 hours a day, seven days a week.
Where Our Water Goes When It Heads Down the Drain
When we go about our daily routines inside our homes and businesses, we use a considerable amount of water in the process. Activities like flushing the toilet, taking showers, doing laundry, and washing hands and dishes all contribute to water going down our drains. We call this wastewater.
Household drains collect all of this wastewater and transport it into underground sewer pipes, which are normally located beneath streets. These pipes combine with other building’s wastewater as it travels through neighborhood sewer lines into increasingly larger pipes. This forms an interconnected underground piping system known as a sewer collection system.
Gravity causes the wastewater to flow downhill through the sewer system until it reaches even larger sewer mains. These large pipes transport exceptionally high volumes of wastewater from across the sewershed to the city’s or region’s wastewater treatment plant. This critical facility treats the massive influx of wastewater before releasing the clean water back to replenish waterways or for other approved uses.
The Scale of Sewer Systems and Wastewater Treatment
To appreciate the immensity of many sewer systems and wastewater plants, it helps to glance at a few statistics:
- The New York City Department of Environmental Protection manages over 6,000 miles of sewers and 14 wastewater treatment plants.
- The City of Los Angeles maintains over 6,500 miles of public sewers, 48 pump stations and 4 wastewater treatment plants.
- The Milwaukee Metropolitan Sewerage District handles wastewater for 1.1 million customers across 28 communities. Its two treatment plants can handle 600 million gallons of wastewater per day combined.
And the list goes on for sewer systems and wastewater treatment plants large and small across the country. Dealing with such high volumes of wastewater on a continuous basis is a formidable task, but vitally important for clean and healthy communities.
Primary Treatment – Letting Solids Settle Out
Once wastewater reaches the treatment plant through the sewer network, the first treatment phase begins – primary treatment. This involves flowing the wastewater into large settling tanks and allowing the solid material to settle to the bottom over several hours. The settling process extracts between 50 to 70% of solid pollutants.
These solids that settle out are referred to as sludge, comprised of small particles of human waste, food scraps, oils and grease, plastics and other inorganic material. It also contains beneficial microorganisms that help treat wastewater. Sludge is pumped out from the bottom of settling tanks and piped to digesters or dewatering facilities for further processing before disposal or beneficial use as biosolids.
Following sludge removal, the clarified wastewater in the tanks flows out and continues to the next treatment phases to further clean and polish it.
Secondary Treatment – Using Microbes to Remove Contaminants
After settling processes extract solids and pollutants in primary treatment tanks, the partially cleaned wastewater flows to secondary treatment processes utilizing complex biological and chemical processes.
This phase relies on microorganisms and bacteria that occur naturally in wastewater to further break down organic matter and residual contaminants. Carefully controlled and maintained bacteria cultures are essential to secondary treatment.
There are a variety of secondary treatment methods, but most utilize some form of activated sludge process. This involves adding nutrients and air to foster ideal conditions for microbial growth and activity that “eat” away at waste material. The microbes convert ammonia, nitrogen and other substances into safer compounds like nitrates. Some systems also incorporate filtration processes in conjunction with microbial treatments.
Following rigorous biological and chemical processing, secondary treatment removes 85 to 95% of pollutants from wastewater. The cleaned effluent then continues to an additional treatment phase depending on the system.
Advanced Treatment – Polishing for Safe Water Reuse
After completing primary and secondary treatment phases, plants may utilize advanced treatment methods as an extra purification step or to meet higher water quality standards for effluent disposal or reuse. These tertiary processes further “polish” wastewater by incorporating more advanced technology like filtration, nutrient removal, membrane treatments and disinfection processes.
Two common advanced systems include:
Filtration: Effluent passes through filters of varying porosity that trap particles and impurities, including some microbes. This polished water undergoes final disinfection treatment before reuse or release back into surface waters.
Membrane Bioreactor (MBR): A membrane process combined with activated sludge biological treatment. Allows simultaneous pollutant removal along with complete solid separation. Produces exceptional quality effluent for recycling applications.
Reclaimed wastewater completing advanced treatment can be safely reused for irrigation, industrial processes, toilet flushing, replenishing wetlands and more. These applications conserve valuable drinking water supplies.
Disinfection – Killing Harmful Microbes Before Water Reuse or Discharge
The final step at wastewater plants uses disinfection to kill off any remaining harmful microbes before treated effluent is approved for reuse or released back into receiving waters. This protects public health and the downstream environment.
Common disinfection methods utilized at plants include:
- Chlorine: Treated effluent flows into contact tanks where chlorine gas or sodium hypochlorite is added to kill pathogens.
- Ultraviolet Radiation: Powerful UV lamps emit light that penetrates and destroys microorganisms as water flows past lamps.
- Ozone: Ozone gas bubbled through effluent oxidizes and damages pathogens.
By effectively eliminating residual bacteria and viruses, final disinfection treatment allows treated effluent to meet regulatory standards and water quality goals prior to reuse or returning to the natural environment.
Bringing Clean Water Back to the Environment
Traveling through primary, secondary and advanced treatment phases transforms raw sewage and wastewater entering plants into sparkling clean water. Testing confirms trace pollutants and pathogens are eliminated to meet stringent regulations before release.
The treated effluent’s journey ends back in the natural environment, flowing into streams, rivers or other surface waters to replenish this precious resource. Or it may divert to reuse applications. This cycle continually repeats, with wastewater plants processing massive volumes day in and day out – out of sight but vitally important to communities and ecosystems.
Our Role in Keeping Sewers and Waterways Pollution-Free
We all play a part in keeping our wastewater treatment systems running efficiently to protect community and environmental health. Our everyday habits and what we send down drains directly impacts sewers and plants.
Follow these best practices for stress-free sewers and sustainable water:
- Only flush toilet paper and human waste down toilets
- Never pour fats, oils or grease down drains
- Limit use of garbage disposals
- Take household chemicals to disposal sites
- Use mesh screens to catch hair in showers/sinks
- Avoid flushing wet wipes or other non-flushables
- Fix any water leaks promptly
- Conserve water with efficient fixtures/processes
Working together, we can maintain a smoothly running wastewater operation that keeps our rivers clean and our water supplies viable for generations to come through responsible use, conservation and pollution prevention.