HOW IS WASTEWATER TREATED?

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How are contaminants removed from wastewater?

Wastewater Treatment Facilities, Water Reclamation Facilities, and Water Resource Recovery Facilities


Water used in our homes and businesses flows through the wastewater collection system to a wastewater treatment facility (WWTF), also called Publicly Owned Treatment Works (POTW) if  owned by a state or local government, which many are.

Water Reclamation Facilities (WRFs) are those that provide advanced treatment, cleaning the water to a higher level to produce reclaimed water. Reclaimed water can be reused for irrigation, cooling towers, wetlands enhancement or other uses. 

Image by Michal Jarmoluk from Pixabay

Water Resource Recovery Facilities (WRRFs) take things a step further by extracting valuable materials from wastewater, such as creating biogas from excess biolsolids that can be used for energy. WRRFs may also recover valuable metals or nutrients like nitrogen and phosphorus to be used as fertilizer.

For simplification, I'll just refer to them all as WRFs unless discussing a specific treatment plant.

Each WRF is unique, and designed to treat the amount flow, level of treatment, and wastewater characteristics for the area served. The type of use or available receiving water are other important factors. However, most facilities use similar treatment steps.

Check out this interesting video about the City of Orlando's Iron Bridge Water Reclamation Facility.



You might think a lot of chemicals would be used at a WRF, but the major treatment method is biological. That's right--microorganisms like bacteria, ciliates, flagellates and others consume the waste out of the polluted water. 

Typical treatment steps and levels 

Note that all WRFs in Florida must have a minimum of Secondary Treatment. 

PRELIMINARY TREATMENT - HEADWORKS

Preliminary treatment usually occurs right where the incoming wastewater (called "influent") enters the WRF. This is called the Headworks, and may include several processes such as pumping, odor control, screening, shredding, grit removal, flow equalization and managing flow direction and rates.
  • Influent Pump Station

WRFs typically have an influent pump station ("master pump station") pumping the wastewater into the treatment plant.  Flow meters record the volume of flow into the WRF.

  • Screening

Screening removes large debris, like sticks, rags, wipes, plastic, rocks and other debris from the wastewater.  Some WRFs also use a mechanical shredder called a comminutor to chop debris into smaller pieces. The purpose is to remove this debris from the waste stream as well as prevent downstream clogging  or damage to pumps, pipes and valves. Screenings are typically trucked to a landfill.

  • Grit Removal

Inorganic materials like sand, grit, and small rocks are removed from the waste stream. This can be done by slowing the flow in a chamber where the heavier grit falls to the bottom and is removed - or by using centrifugal force to separate the grit. The removed grit is typically trucked to a landfill along with screenings. Removing grit helps to prevent abrasion and damage to downstream equipment.

  • Odor Removal
Odor removal can be accomplished with certain chemicals, or through a biofilter that uses microbes to remove the odors.

PRIMARY TREATMENT
  • Sedimentation

Wastewater flows to primary clarifiers, which are sedimentation (settling) tanks. This allows suspended solids to settle to the bottom of the tank, while oil, grease, and other floatable material rises to the top. The solids (called primary sludge) are pumped from the bottom of the tank and sent for further treatment. The floating material is skimmed from the top, then typically trucked to a landfill with grit and screenings. The cleaner water flows over a weir at the top of the tank to the Secondary Treatment step.

SECONDARY TREATMENT
  • Aeration
This is where the biological treatment takes place. The wastewater is sent to aeration tanks, mixing with activated sludge - a combination of beneficial microorganisms,  to form a "mixed liquor." Air is pumped into the mixture through diffusers to make tiny bubbles. (The beneficial microorganisms are aerobic, meaning they need oxygen to live). 

These microorganisms remove harmful contaminants such as dissolved organic matter. They form larger particles called floc. This heavier floc can then settle in the next step which is secondary clarification.

Aeration levels can also be modified at various stages to remove nutrients like nitrogen and phosphorus. (Nutrients can also be removed by adding chemicals).
  • Secondary Clarification
The activated sludge mixture is sent to secondary clarifiers which are sedimentation tanks. In the secondary clarifiers the activated sludge settles to the bottom of the tanks. It is then pumped back to the aeration tanks to be used again. 

The clean water goes over the weirs at the top of the tank.The outgoing flow (effluent) from the secondary clarifiers now flows either to disinfection or for additional treatment.
Secondary Clarifier at a WRF
Photo by Donna Kaluzniak©

Secondary treatment removes suspended solids and carbonaceous biochemical oxygen demand (CBOD). Too much CBOD removes oxygen from the water, which makes it harmful to fish and other aquatic organisms. 

ADVANCED TREATMENT

Advanced wastewater treatment (AWT) uses additional treatment steps, usually so the effluent  can be reused (reclaimed water). The specific treatment technologies will depend on how and where the reclaimed water will be used. Technologies may include high-level disinfection, coagulation, and filtration, among other steps. 

DISINFECTION

After the clean water flows out of the clarifiers, disinfection removes any possible remaining pathogens (disease causing microorganisms). The most common methods of disinfection are:
  • Chlorination
Chlorine is injected to the effluent, which then passes through a contact chamber allowing time and mixing for the chlorine to complete the disinfection process. Treatment plants may use chlorine gas or sodium hypochlorite (strong bleach) for disinfection.
  • Ultraviolet Light
Treated effluent flows through ultraviolet (UV) modules. UV lamps emit electromagnetic energy that destroys the DNA of any remaining microorganisms so they cannot reproduce. UV has proven to be extremely effective at destroying pathogens without chemicals. 
  • Ozone
Ozone (O3) is a powerful oxidizing agent that can destroy pathogens, including bacteria and viruses. It must be generated onsite, as it is unstable, so it can't be stored and transported like chlorine. It works by breaking down the cell walls and disturbing the metabolism of the pathogens. Ozone also works well for odor control. The Water and Wastewater website has a good, detailed description of ozone systems and how they work.

BIOSOLIDS TREATMENT

As the activated sludge (beneficial microorganisms that consume the waste out of the water) continues its work in the aeration tanks, the microorganisms multiply. To keep the population at the right level, some of the sludge must be removed (wasted) to obtain further treatment. This excess sludge is called biosolids. 

  • Biosolids Stabilization
The biosolids are pumped to tanks called digesters. There, a biological process is used to break down organic matter and create a stable product. Some WRFs use aerobic digesters, where air is entrained in the sludge while others use anaerobic digesters, which are covered and have no air added. 

After the biosolids are stabilized, they can be applied to land in liquid form as a fertilizer or dewatered.

Note that biosolids must go through specific treatment processes to ensure the removal of pathogens before being applied to land!

  • Dewatering
After stabilization, the liquid is removed from the biosolids with a filter press, centrifuge or drying beds. The liquid is sent back to the treatment plant headworks. The dewatered biosolids can either be landfilled or used as a fertilizer or soil amendment after going through pathogen removal processes. The Florida Department of Environmental Protection (FDEP) has additional information about biosolids. 

Here's a good video on the City of Palm Coast's process at their water reclamation facility:

https://www.youtube.com/watch?v=_RYfDkER1AM

Geek Out

Here's more information about wastewater treatment:


Florida Dept. of Environmental Protection - Biosolids Rule

Here's a 2-hour symposium on Biosolids that was held in Martin County in 2019:






Location: Florida, USA

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