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Advanced wastewater treatment (AWT)

Treating Reclaimed Water
Flowchart showing all of the potential applications of reclaimed water

Flowchart of reclaimed water applications (Credit: EPA)

In the words of EPA Administrator Andrew Wheeler, “Water reuse represents a major opportunity to support our nation’s communities and economy by bolstering safe and reliable water supplies for human consumption, agriculture, business, industry, recreation and healthy ecosystems.” Therefore, we should make the effort to understand, sustainably acquire, and use for this important resource. Understanding reclaimed water starts with understanding where it comes from and how it is treated. Some of you may have heard the term “AWT” before when referring to reclaimed water. Some of you may have absolutely no clue what that is! For those curious about how your reclaimed water gets treated, read on to discover all there is to know about advanced wastewater treatment, or AWT.

Reclaimed water is already treated to a high standard, but what does it mean to treat reclaimed water? Well, all reclaimed water that gets distributed to homes and other buildings must have undergone two or more treatments already. These treatments make reclaimed water safe to use for irrigation. We use reclaimed water where we can to irrigate parks, gardens, and other large stretches of land. Not to mention, reclaimed water has many other applications, which you can read about here. Recycling water is not only important for reducing strain on our valuable, freshwater resource, but it also enables wastewater to be naturally filtered before reaching our local creeks and bays.

Standard Treatment

Sarasota County collects our wastewater so that it can be treated. The County also monitors the flow while simultaneously removing large solids and grit through a screening process, known as the “headworks.” Wastewater then undergoes biological treatment processes where bacteria or other organisms break down the leftover solids. Biological treatment can be aerobic or anaerobic, depending on whether oxygen is present in the controlled environment. Once the solids have been broken down, the leftover liquid is then given water clarifiers to make it clear.

Three vials of treated wastewater

Wastewater to reclaimed water (Credit: USGS)

The next treatment process removes any suspended solids and dissolved organic matter through filtration. To do this, the treated wastewater is aerated to increase particle flow and then the organic sludge is filtered out through a sand filter. After the water has been filtered and clarified, it enters a disinfection chamber. In this chamber, the water is generally treated with chlorine, which kills any remaining microorganisms. The treatment targets pathogens like the bacteria Salmonella, viruses such as norovirus and adenovirus, and protozoa like Giardia.

Although the water is not safe to drink at this stage, as it contains salts and other nutrients, it is ready to be stored and distributed. Some treatment facilities may differ in their use of technology when treating wastewater, but the end result is usually the same or very similar.

Grizzle-Figg

In Sarasota County, water that gets released into state-regulated waterbodies must follow the Grizzle-Figg Statute. The statute sets strict nutrient standards for wastewater, requiring that is clean enough to be safely discharged into the environment, while also limiting the amount of discharge and where it gets released. Striking a balance between reclaimed water needs, storage, and highly variable rainfall has proven to be a challenge.  Thus, without intention, reclaimed water that does not meet these strict standards has been discharged into state-regulated waterbodies within the county. Today, the county is making every effort to remove nutrients from its reclaimed water in order to keep the environment safe and clean. One way of ensuring that the county’s reclaimed water is up to standard is through switching to advanced wastewater treatment, or AWT.

Advanced Wastewater Treatment

So, after reclaimed water has met the standard treatment requirement, it may be further treated to remove nutrients. The nutrients and other organic compounds leftover in reclaimed water are not necessarily harmful. In fact, they can be a great supplement to fertilizers in your home garden. The primary nutrients found in reclaimed water are nitrogen (N) and phosphorus (P), two of the main components in common fertilizers.

Even though reclaimed water is already a beneficial resource, its distribution is limited. According to David Sell, Facilities and Lift Stations Operations Manager, certain watersheds cannot accept reclaimed water unless it has been given advanced treatment. By treating the water with this advanced process, it can also be safer to discharge into fresh and saline water bodies in case of excess production. Water reclamation facilities can treat their water to federal and state standards in a variety of ways.

Membrane Bioreactor

Certain methods in AWT can allow treated wastewater to match the physical, chemical and biological properties of whatever receiving stream the treated water will be discharged into. One such process is known as membrane bioreactor filtration (MBR). Membrane bioreactors can actually filter raw sludge, often making first and secondary treatments obsolete. Although MBRs have an efficient, integrated design and reduce the effort needed to treat wastewater, their maintenance and potential complications mean that they are not often the first choice of a water reclamation facility. One town that is making use of this technology is Davie, Florida. The Davie Water Reclamation Facility is able to process 3.5 million gallons of wastewater per day, using membrane bioreactors after the initial grit and solids are removed. Charlotte County and its Rotonda Water Reclamation Facility are also making use of MBRs alongside biological nutrient removal in their reclaimed water process.

Modified Ludzack-Ettinger
Modified Ludzack-Ettinger

Modified Ludzack-Ettinger (Credit: Xueming Chen)

Another method, which does not meet the standards of a full advanced wastewater treatment, is the Modified Ludzack-Ettinger (MLE). Sarasota’s Bee Ridge Water Reclamation Facility has piloted this technique in its reclaimed water treatments as it gets ready to switch to Advanced Wastewater Treatment by 2025. Manatee County is also looking into introducing MLE to its water reclamation facilities.

MLE uses a denitrification process where an anoxic zone is created or added upstream of the aerobic zone. In MLE, an anoxic (or oxygen-free) basin is added to complete the process of the nitrogen cycle. In the anoxic basin, microorganisms use the oxygen from the nitrate (NO3-), reducing it to nitrogen dioxide (NO2-), and further to N2 and N2O gas. The gas goes into the air completely, removing nitrogen from the water. This process makes the water cleaner and safer to discharge.

AWT in The City of Tampa
Dissolved air reactors

Dissolved air reactors (Credit: City of Tampa)

One successful model for advanced wastewater treatment can be found in Tampa, Florida. The Howard F. Curren Advanced Wastewater Treatment Plant (HFCAWTP) is part of the City of Tampa’s effort to promote the use of reclaimed water while keeping its quality up to state and federal standards. The plant’s AWT system is comprised of seven steps. It begins with the standard primary and secondary treatments. After, the plant uses dissolved air reactors to convert the sludge from ammonia to nitrogen. The effluent and sludge are combined to form “mixed liquor.” In the plant’s sedimentation tanks, the sludge is removed through a process known as gravity settling. The solids are then either discarded or reintroduced into the mixture for further processing.

Denitrification facility in Tampa

Denitrification facility (Credit: City of Tampa)

After converting the ammonia to nitrogen, the reclaimed water is put through a denitrification process in an anaerobic environment, much like in the MLE. This means that the nitrates are converted into nitrogen gas, which is released into the air. This stage removes about 90% of the nitrogen from the mixture. The reclaimed water is then aerated to contain at least 5 milligrams of dissolved oxygen per liter. The water is then disinfected further with chlorine. If the effluent is being released into Hillsborough Bay, the chlorine is removed through the introduction of sulfur dioxide. The plant processes an average flow of 55 million gallons per day.

The Plan for AWT in Sarasota County
An aerial view of the Bee Ridge Water Reclamation Facility

Aerial view of the Bee Ridge Water Reclamation Facility

We know that reclaimed water is useful, and we now know that it can be used in a variety of ways. Reclaimed water that has undergone AWT can be distributed to more places, meaning that more wastewater can be reused. The Bee Ridge Water Reclamation Facility is making strides to complete its switch to Advanced Wastewater Treatment by 2025. In the meantime, the plant is removing as many nutrients as possible to allow for safe potential discharge into local waterbodies during rainy season.

An Equal Opportunity Institution. UF/IFAS Extension, University of Florida, Institute of Food and Agricultural Sciences, Nick T. Place, dean for UF/IFAS Extension. Single copies of UF/IFAS Extension publications (excluding 4-H and youth publications) are available free to Florida residents from county UF/IFAS Extension offices. Sarasota County prohibits discrimination in all services, programs or activities. View the complete policy at www.scgov.net/ADA.

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