Preserving the Clean and Life-Giving Flow
Humanity’s Early Struggles With Waste
Most of us enjoy clean drinking water wherever we go. We turn on the faucet in our kitchens to wash our dishes. We use the clean water in our bathrooms to take care of our business and clean ourselves. We also take some cool water from our refrigerators after working in the yard or playing outside on a hot day.
The Early Years
However, this was not always the case. When humans were primarily hunter-gatherers around 200,000 years ago, they would form tribes and communities as a means of survival. Due to the impermanent structure of these early tribes and communities and lack of resources to clean water, if the conditions caused by human disposal and waste became unlivable, these communities would leave, relocate, and let the waste decompose naturally. However, humans were not to be traveling hunter-gatherers forever.
Evolving Methods for Human Disposal
This changed as permanent establishments were being created approximately 10,000 years ago, where new ways of containing human waste were being discovered and utilized. This included digging holes in the ground (Deuteronomy 23:14), advanced sewer systems developed by the ancient Minoan civilization in what is known today as Crete, and sewage and drainage systems throughout the Indus Valley in modern-day Pakistan that date back to 2600 B.C.. Various uses of aquaculture also utilized human disposal and was practiced in 1100 B.C. throughout China during the Yin dynasty. Thankfully, these wastewater treatment practices only advanced along with societies.
The bathroom-toilet structure of a house in Lothal (Courtesy of The Archaeologist)
Societies from then on only kept evolving as they moved into the Bronze Age, especially in ancient Greece and the Roman Empire. Archaeologists have discovered an area of Greece southeast of the Acropolis that consisted of sewerage and drainage containing a central sewer.
The Romans adopted their own sewage system with systems such as the Cloaca Maxima, but it did not handle solid waste like the sewage system in the Minoan civilization did.
The Cloaca Maxima (Courtesy of Encyclopaedia Britannica, Inc.)
This led to Roman residents dumping their waste into the streets and forced officials to implement extensive street cleaning programs. These programs and advancements demonstrated that even during ancient times, humanity demonstrated incredible resolve to improve the quality of life for everyone.
The Unfortunate Regression
Even though great progress was made in sewage and wastewater treatment during the age of the ancient Greeks and Romans, this progress didn’t carry on into the Middle Ages as this period saw comparatively little to no progress. The main focus was on wars with neighboring countries rather than building society, and many people suffered because of it.
Many towns and villages reverted and adopted primitive ways of handling human waste. As a result, many of these towns and villages were vulnerable to disease outbreaks. During this time, it is estimated that approximately 25% of Europeans died due to cholera outbreaks and other water born diseases. Even though society reverted in this area and health declined, there was hope around the corner, especially with diseases such as cholera.
Reclaiming Healthy Living and Modernizing to the Present
Cholera has an important place in water treatment, thanks to a man named John Snow. In 1854, he was able to demonstrate that a contaminated well in London was the main culprit behind cholera outbreaks in an area. This led to greater awareness of public health and safety and helped spur advancements in wastewater treatments in the 1900’s and even today. So, thanks to people like John Snow, many of us are able to enjoy safe, clean drinking water in the cities and towns we live in today.
Photographic portrait of John Snow (Courtesy of Encyclopaedia Brittanica, Inc.)
It’s really in the 1900’s when we begin to see the emergence of wastewater treatment plants (WWTPs). They began to be used worldwide primarily by major urban centers as they were able to handle large volumes of wastewater without occupying so much space. Because of the discharge from these early WWTPs to rivers and the ocean, demand for nutrients and organic matter began to decrease. More recently, due to global population and economic growth demands, water reuse and reclamation has become of greater focus and is an area wastewater treatment plants today are honing in on.
So, now that we know how the modern-day WWTP came to be, how do they work? How are they able to separate solid waste from the liquid wastewater itself? What kinds of methods do they use to kill bacteria and turn that wastewater into water that’s safe for us to drink and for the environment? Let’s take a dive into the different components of the modern-day wastewater treatment plant and the engineering flowing through them.
From Waste to Essential-The Journey of Water Treatment
Diagram showing the route of wastewater through a wastewater treatment plant
A Long and Windy Road
Let’s begin from when you flush the toilet. Unless you live in an area where you rely on a septic system, the waste that you flushed down the toilet moves through a labyrinth of pipes until it reaches the wastewater treatment plant serving your residence or workplace. This is where the wastewater begins the first stage of the waste removal process, which is more formally known as pretreatment.
How does the fluid move through the pipes? This is due to gravity influencing pipeflow and can be explained using an equation known as Bernoulli’s Equation. Here’s what it looks like:
p + (½)⍴V2 + ɣz = PT
So, what does this equation even mean? This equation measures the constant total pressure of the fluid, or in this case, wastewater in the system (PT) by using the wastewater’s static pressure, or initial pressure, plus the density of the wastewater (⍴) and the velocity it’s moving at (V) plus the specific weight (ɣ) and the elevation of the wastewater (z). For those who aren’t familiar with specific weight, it’s a measure of mass per unit volume, or density (⍴) times gravity (g). Here’s what specific weight looks like:
ɣ = ⍴g
I’d like to point out that this is for ideal cases. The wastewater coming from your home or workspace most likely is compressed to some degree and changes pressure from your home to the wastewater plant, doesn’t maintain a constant flow along its path, nor isn’t inviscid, which means the fluid’s viscosity, or its resistance to flow, is negligible. Let’s explain why we need more than Bernoulli’s equation.
Wastewater in pipes is pressurized due to methane and other gases within the wastewater and also because it’s traveling in an enclosed pipe. Depending on the design of the sewage system, your wastewater may experience both laminar and turbulent flow, which isn’t constant flow. Furthermore, the wastewater will likely have differing levels of viscosity depending on the wastewater’s profile, or the different compounds and materials within it.
How do we know if a fluid flow isn’t constant? That’s when we use what is known as Reynolds Number, which tells us if the flow is laminar or turbulent. This is what Reynold Number looks like and what it means:
Re = (⍴DV)/μ
Laminar: Re < 2,000
Transitional: 2,000 <= Re <= 4,000
Turbulent: Re > 4,000
Let’s break down what this means. First, the Reynolds Number is calculated by the density (⍴) times the pipe diameter (D) times the velocity of the wastewater (V) all divided by the wastewater’s viscosity (μ). The resulting value will fall into one of three categories: laminar, transitional, or turbulent flow. Laminar flow is the calm flow we’re all used to seeing with a gentle stream; turbulent flow is violent that we’re used to seeing with water flowing through a powerful river with waves and crests; transitional flow is, as the name implies, a state where the flow of the wastewater is in between laminar and turbulent flow.
So, how does this relate to wastewater treatment and why should we care? Well, how the wastewater moves in the sewage pipe will determine how the wastewater will interact with the pipe walls. So, even though Bernoulli’s equation can tell us how your wastewater should flow based on initial static pressure, the density of the wastewater, its velocity, and how the elevation will change along the way (which is where the variable z comes in to play), the Reynolds Number gives us more insight as to how the fluid will interact with the sewage pipe itself.
Another thing to keep in mind is how fluids lose pressure in enclosed pipes. As I discussed in the Actual Vapor Compression Refrigeration Cycle in “Pushing Out Heat to Keep Refrigerators Cold”, friction forces as a result of shearing stresses cause this phenomenon. Since the goal is to try to get the wastewater from your toilet to the wastewater treatment plant as safely and as quickly as possible, this is the reason why durable pipes such as polyvinyl chloride (PVC) pipes with smooth interior surfaces are widely used in these applications.
There’s still one more thing to take into consideration before the wastewater reaches the wastewater treatment plant, and that’s the minor losses the wastewater will experience along the way. We can all understand that wastewater most likely won’t travel along a straight path throughout its journey. There will be turns, twists and gravity forces along the way that will result in flow loss. These losses are called minor losses.
So we can better understand what minor losses are, let’s take a look at some more formulas:
KL = (hL/(V2/2g)) = Δp/((½)⍴V2
Δp = KL(½)⍴V2
hL = KL((V2)/2g)
Using the loss coefficient (KL) as a starting point is a common way to determine the minor losses in a fluid system like wastewater. With enough information, it can help determine the velocity of the fluid (V), head loss (hL), change in pressure (Δp), and density (⍴) of the wastewater. If you haven’t heard the term head loss before, it is the loss the wastewater will experience traveling through a pipe based on friction, pipe length, pipe diameter, and velocity of the fluid.
As you can see, even before the wastewater reaches the plant, there are plenty of factors to take into consideration. Understanding how the wastewater’s initial static pressure, its projected velocity, and the elevation and geography the wastewater is traveling through (ɣz components of Bernoulli’s Equation) are influential with the high-level design. How the type of flow interacts with the pipe and the wear and tear it will cause will help engineers determine which type of material the pipe will be made from.
Last, but not least, understanding how all the minor losses in the system will affect wastewater transport is a major component in determining how the wastewater will flow and its final design. Once the wastewater reaches the treatment plant, the magic turning this unsanitary water into water that can be used by the environment and all of us begins.
First Things First
When the wastewater first reaches the plant—often transported via a lift station, which is a pumping system that’s used in circumstances where gravity isn't enough—it begins the first phase of treatment, known as Preliminary Treatment. This is the phase where solids are removed from the wastewater itself, including trash, toilet paper, rags, and large items that will prevent the water from being treated.
This is done through bar screens, comminuting devices, grit chambers such as horizontal-flow and aerated, proportional weirs, and Parshall flumes. We’ll only be discussing how these techniques apply to wastewater treatment and why they’re important, but if you’d like a deep dive into these processes, Continuing Education and Development, Inc. has a wonderful resource which can explain this in more detail.
The first part of the preliminary treatment process is the bar screens. Bar screens consist of parallel bars where trash, rags, and toilet paper are separated from the wastewater. They can be designed with different spacing usually ranging from ½” to 1 ½” between bars, allowing for efficient transport of wastewater into the plant. Some operations have up to 4” spacing while others have ¼” spacing or smaller. All of this is dependent upon the kinds of solids found in the wastewater along with the areas and industries the wastewater treatment plant serves.
Once the wastewater gets through the bar screens, it then goes to the comminuting devices in the plant. This is the part of the preliminary treatment phase where remaining solids are cut without removing them from the sewage flow.
The comminuting devices’ role is to protect the plant from potential damage further on into the plant from debris while also reducing odors, flies and sparing plant operators from the rather more unpleasant sights of solid waste. It should be noted, however, that solids in this phase produce more scum at the digesters and it is important to handle these solids appropriately at this stage rather than deal with the consequences later.
The wastewater will then flow from the comminuting devices to the grit chamber. This is where grit, which consists of sand, gravel, cinder or other materials that are heavier than the organic biodegradable solids, are removed from the wastewater. There are two different types of grit chambers that wastewater treatment plants use: horizontal-flow and aerated.
In a horizontal-flow grit chamber, the wastewater moves at a rate of around 0.3-1 ft/s. This allows grit to be removed from the wastewater and settle at the bottom. The velocity is controlled by proportional weirs and Parshall flumes. Proportional weirs are weirs that are built based off of the water depth and fluid flow proportions to alter the flow of water by raising the water level upstream. Parshall flumes are fixed, hydraulic structures that measure the fluid flow of the wastewater.
The other type of grit chamber, known as an aerated grit chamber, uses air diffusion, or the process of integrating air into the wastewater, to move grit. This is accomplished via air moving through the wastewater in a vortex-like pattern through the wastewater inside the aerated grit chamber. The vortex movement of the air helps grit move to the outer edges of the wastewater and eventually settle to the bottom of the tank floor. Lighter organic particles will be suspended and eventually be carried out of the tank as a result of this process.
A Primary Cleaning Force
Once the wastewater flows from the Preliminary Treatment phase to the Primary Treatment phase, the water will then enter structures known as clarifiers. The wastewater will enter and stay in the clarifier for 1 to 2 hours. While the wastewater is in the clarifier, it’s important that the clarifier deliver on the objectives it was designed to accomplish before the wastewater enters secondary treatment:
Flocculation: a process where smaller, dispersed hydrophobic, or water-repelling, particles aggregate into larger particles known as “floc”. This affects clarification efficiency, or how well a clarifier removes suspended solids, based upon how well flocculation works in this stage. The floc is then scraped using a skimmer at the top of the clarifier, which is basically a large arm that scoops the floc from the top of the wastewater and stores it in a designated location for further processing.
Clarification: this is where the solids in the influent stream, or wastewater coming into the clarifier, are separated from the wastewater until the effluent stream, or the wastewater leaving the clarifier, doesn’t contain either the floc or the heavy sediments that rest at the bottom
Thickening: this is where hydrophilic, or water-absorbing, sludge at the bottom of the clarifier forms. It’s important to ensure this is transported to a proper unloading site, such as a landfill, a digester, or other designated place where this material can degrade without causing harm to the environment.
Storage: no matter the weather, clarifiers are expected to hold solid storage adequately. This might result in certain clarifier designs only working in certain environments, but not necessarily a one-size-fits-all solution.
While it’s necessary to include these things in the design and engineering of a clarifier, another thing to consider as the water is stored here—very much like in the grit chamber—is the concept of hydrostatic forces exerted onto the walls and floor of these storage units. Here is the formula that encapsulates this concept well:
FR = ɣHL
Here, the specific weight (ɣ, just like in Bernoulli’s Equation) times the depth of the storage container (H) times the length or width of the storage container (L) will give you the resultant force of the wastewater on the clarifier or in the grit chamber. Depending on where you start (i.e. if you use the floor as the base or one of the side walls as the base) can help you determine the magnitude and direction of the hydrostatic forces the wastewater is exerting on different components of the structure; thus, this formula is important for engineers to use so that they can design and engineer an effective and durable clarifier or grit chamber that can withstand the load and volume of the wastewater the system requires.
Once the wastewater completes the primary treatment, 90%-95% of settleable solids, 50%-75% of total suspended solids (TSS), and 25%-40% of biochemical oxygen demand (BOD) will have been removed from the wastewater. To make primary treatment even more effective, enhancements have been made in primary treatment of wastewater. Examples of this include RapiSand Ballasted Flocculation from WesTech and also Dissolved Air Flotation technologies like the ones made by Evoqua Water Technologies.
While the primary treatment process is mainly driven by gravity and fluid mechanics, centrifugal pumps, positive displacement pumps and progressive cavity pumps at the bottom of the primary clarifier help move sludge for further processing.
Secondary Cleaning Power
Now, the wastewater moves from primary treatment into secondary treatment, which is more driven by microbiological organisms. The secondary treatment process starts by the primary effluent being mixed in with the return activated sludge (RAS) containing the necessary microorganisms to break down the remaining BOD in the wastewater. This mixture goes through an aeration tank, where the bacteria continue to break down BOD in the wastewater. After leaving the aeration tank, the wastewater moving to the secondary clarifier from the aeration tank is called mixed liquor suspended solids (MLSS).
Most of the sludge is reused for the next cycle (RAS) while the portion of the sludge to be wasted (no longer needed) is called waste activated sludge (WAS) and is sent to a designated location for further processing. According to the EPA, the main two pumps used in secondary sludge (RAS and WAS) are centrifugal pumps and progressive cavity pumps.
Now that the wastewater is now MLSS as it leaves the aeration tank, it moves into the secondary clarifier and begins to interact with four different settling regimes rather than one like in the primary clarifier. This is due to the fact that there are biological solids and activated sludge present in the secondary clarifier. Let’s explain them in detail.
The first one—Discreet Settling or Type I—is the first kind of settling regime and was the only kind of settling regime found in the primary clarifier. This is where independent units have little interaction with their neighbors and where TSS concentration is less than 600 mg/L
The wastewater then moves to the Flocculent Settling (Type II) regime, where the TSS concentration increases to between 600 mg/L to 1200 mg/L. This is caused by particles starting to interact with each other through collision and differential settling, or different settling rates due to gravity, causing larger particles to form.
The wastewater then moves to Hindered or the Zone Settling (Type III) regime, where so much interacting has occurred between the particles that a matrix of particles begins to settle together. In Zone Settling, the TSS concentration is between 1200 mg/L to 5000 mg/L.
The fourth settling regime, the Compression Settling (Type IV) regime, is the last phase the wastewater will undergo in the secondary clarifier. This is where the solid matrix formed by the particles begins to compress and release more water particles into the wastewater. In this regime, the TSS concentration is over 5000 mg/L.
A skimmer is present at the top of the secondary clarifier just like it was in the primary clarifier to scrape off the solid matrix formed at the top of the wastewater. Also similar to the primary clarifier is the sludge present at the bottom of the secondary clarifier. However, the sludge in the secondary clarifier needs to be regulated and watched more closely since the pumps that are used to remove the sludge are in the RAS/WAS cycle.
Two more things that engineers need to consider when designing wastewater treatment plants are Surface Overflow Rate (SOR) and Underflow Rate (UFR). The SOR measures the maximum flow rate the secondary clarifier can handle before overflowing, while the UFR explains the lowest flow rate that the RAS can return to the system. Here’s how you can calculate both:
SOR = Q/A
UFR = QRAS/A
For SOR, it’s measured by flow rate of wastewater (Q) divided by surface area of the clarifier (A). For UFR, it’s measured by flow rate of RAS (QRAS) over the surface area of the clarifier.
Now, once the wastewater passes through the secondary treatment phase, where will it go? According to the 2025 Report Card for America’s Infrastructure conducted by the American Society of Civil Engineers (ASCE), only 38% of all WWTPs provide advanced, or tertiary, treatment of wastewater. Even though so many processes are completed by the time wastewater completes secondary treatment, it is still not suitable for human consumption and can only be released into the environment.
However, for the 38% of WWTPs that do have tertiary treatment, the wastewater can be cleaned even further for not just the environment, but for irrigation purposes and human consumption. How does tertiary treatment make that possible? Here’s a look into the processes that govern the tertiary treatment process.
Completing the Perfect Trinity
Tertiary treatment begins with surface filtration to remove those last few remaining solids that might have made it through secondary treatment. By this point in the treatment process, there are only 5-10 mg/L of TSS in the wastewater, so the spacing in the filters needs to be small enough to capture these solids. That’s not the only kind of filtration in tertiary treatment, though.
Another type of filtration found in tertiary treatment is depth or volume filtration. This is where the wastewater passes through granular material, like sand filters, or compressible material, like free fiber cloth filters. Since this type of filtration requires multiple development processes, they offer greater filtration efficiency compared to other methods found in similar kinds of machinery.
One more type of filtration is collectively called membrane filtration. As the name implies, this type of filtration utilizes membranes that consist of pores that are 1 μm or smaller. This process starts by having the wastewater undergo microfiltration (MF) where the wastewater travels through membranes that are 1 μm, or 1 x 10-6 m in diameter. This is then followed by the wastewater undergoing ultrafiltration (UF) where the pores in the membrane the wastewater is passing through is even smaller and can block larger dissolved particles.
The process continues through nanofiltration (NF), where the water passes through membrane filters containing pores that are 1 nm, or 1 x 10-9 m, in diameter and the membrane can reject even smaller dissolved particles. The last stage in membrane filtration is reverse osmosis (RO). The membrane in this stage doesn’t have pores and is reliant upon pressure from osmotic and hydraulic forces across its polymer membrane.
Altogether, membrane filtration has proved to be an effective method, with studies suggesting many toxic compounds were rejected 99% of the time or more. It is important to note that membrane filtration does not include any chemical treatment. These results in studies like this could have huge implications on how chemicals will be used in tertiary treatment in the future.
Another tertiary treatment method is known as Biological Nutrient Removal (BNR) and is used to remove the nutrients from wastewater, primarily total nitrogen (TN) and total phosphorous (TP) compounds. As the name implies, BNR uses live microorganisms to remove these compounds from the wastewater.
Depending on the environment and budget constraints of the project, some WWTPs might incorporate only TN removal, TP removal, or both. Either way, using microorganisms to remove these compounds rather than chemicals could be a more environmentally friendly alternative to wastewater treatment.
Whether or not the wastewater went through BNR, it will then be treated with chlorine in a process known as chlorination. Unlike BNR, this does not remove nutrients like TN or TP from the wastewater itself. Instead, the goal of chlorination is to react with organic compounds and microorganisms to denature the organic compounds or kill the microorganisms. It’s very effective in this role by killing vast quantities of E. coli (showing 2.5 log unit to 2.6 log unit reduction) and also some viruses (showing 0.7 log unit reduction) and is very cost-effective (treatment cost of 0.0003 to 0.006 EUR/m3).
Many of us, including regulatory bodies such as the Environmental Protection Agency (EPA), are aware of the danger of chlorine in our water. Thankfully, there are methods to remove the residual chlorine found in water through a process called dechlorination. Compounds such as sulfur dioxide have been used in the past with promising results. Even though chlorination and dechlorination of wastewater makes using chlorine a viable treatment method, it still might not be able to get all the microbes and pathogens remaining in the water.
Safe and Clean for All
This, ladies and gentlemen, is where ultraviolet (UV) radiation comes in. UV radiation works by damaging DNA and RNA in microbes, which effectively kills them. Studies have suggested that UV radiation reduces antibiotic resistance gene (ARG) detection by 2.6 log units. It is more expensive than chlorination with treatment costs estimated to be approximately 0.296 USD/m3.
n order to make sure that the wastewater will be sufficient for human consumption, it will need to have its pH, or acid-base balance, re-evaluated and returned to a pH value at or very close to 7. Here is more information from the EPA showcasing critical pH values for various animals and how to gauge different pH values.
To ensure the water isn’t corrosive to pipes or is harmful to human health after all the different treatments the wastewater has undergone, the water will then go through a final process called stabilization that helps restore the chemical and pH balance of the water. The EPA outlined these guidelines in its 2017 Potable Use Compendium (page 6-16) to ensure Direct Potable Reuse (DPR), or wastewater that can be used for human consumption, is safe for human consumption and the environment. If the wastewater successfully completes all these requirements, the water will most likely be deemed safe not just for the environment but also for human consumption.
Confronting the Forces of Nature
Unfortunately, the weather is not always kind. Storms come through with torrential rains and are sometimes responsible for large volumes of water in relatively short periods of time. This can create undue stress on the area’s infrastructure, especially on WWTPs. So, how do WWTPs manage to get through these storms?
WWTPs manage to navigate through these storms through the use of overflow systems. These structures allow excess water that would overwhelm the WWTP to safely divert excess flow away from the plant so the plant can resume normal operations. With the exception of major storms such as hurricanes, typhoons or other tropical storms, overflow systems are designed to withstand significant amounts of rainfall. Without them, their operations would be severely compromised.
Some cities have adopted alternate methods to handle the issue of excess rainfall, industrial waste and domestic sewage. One such method uses a single pipe to handle the entire flow of all that water. These structures, known as Combined Sewer Overflows (CSOs), are used by countless cities across the United States and have historically been proven effective in their mission. Here’s more information from the EPA describing how they work and how they are used in U.S. cities.
Other cities have adopted bypass systems to divert the excess rainwater during periods of extreme inflow away from the wastewater treatment plant. These bypass systems must empty into surface waters maintained by a state such as Minnesota. They are also subject to other state and federal regulations and require special permits to be constructed. Here is a document from the Minnesota Pollution Control Agency (MPCA) explaining the process in more detail.
Built from Materials to Last a Lifetime
With all of the inner workings of modern-day WWTPs, the materials within them need to be made of the highest quality. High-quality materials allow these various components and structures to perform their intended functions well. The clarifiers and grit chambers must withstand the hydrostatic forces the wastewater within them exerts on the walls. The pipes within the wastewater treatment plant must be chemically resilient and be able to allow wastewater to flow with minimal resistance and turbulence.
The different fluid dynamic components—the weirs, the Parshall flumes, etc.—must be structurally sound to withstand the forces of water and also control its flow. Let’s explore the materials most often used in wastewater treatment plants that allow these magnificent engineering masterpieces the ability to effectively carry out their mission.
Concrete is used to handle the hydrostatic pressure in clarifiers and grit chambers since they are well known to handle compression. Polyvinyl Chloride (PVC) is durable, chemically resistant with a smooth internal surface, making it a perfect material to handle wastewater flow throughout the plant without degrading or corroding over time. Some pipes are made from 304L and 316L steel because they are tolerant to corrosive materials as well.
Some materials in the plant, like the membranes used in tertiary treatment, are hydrophilic and absorb water to help them carry out their intended function. Other materials, like the metals used to manufacture the various pumps used for sludge management, have hydrophobic coatings that are resistant to water and prevent the pumps from corroding.
Grit chambers have special lining to prevent damage from grit, just like this grit classifier from KWS Manufacturing Company, LLC is lined with ceramics. HDPE and epoxy liners found in wastewater treatment plant storage tanks prevent leakage of wastewater into the concrete, thus preventing water erosion.
Even with the right materials, wastewater treatment is still an energy-intensive process. The pumps moving the sludge, the aeration process with RAS/WAS systems, the lifting station, and the other electrical equipment to monitor and ensure plant operations function properly need to be fully operational 24/7. So, how much energy does the WWTP need? Where does it get its energy from?
Strength Through Power
According to a December 2017 report from the Department of Energy, municipal wastewater treatment plants across the U.S. used approximately 30 terawatt hours per year in electricity. Even though this is a large amount of energy, a report from Continuing Education and Development, Inc. authored by Ely Greenberg, P.E., CFM states that WWTPs constitute 4% of total energy usage in the U.S. The report also explains how most of the energy consumption in WWTPs is used in the secondary and tertiary treatment phases—which makes sense when you begin to understand the aeration process utilizing RAS/WAS systems in secondary treatment and the UV and RO treatments used in the tertiary treatment process.
However, not all WWTPs consume energy the same way. A January 2015 report by Energy Star states that energy use intensity (EUI) among water plants ranges between 5,000 to 50,000 British thermal units per gallon per day (kBtu/gal/day). Despite the varied range, the report also suggests that the median energy consumption among WWTPs in the U.S. was a little higher than 10 kBtu/gal/day and the WWTPs consuming near 50 kBtu/gal/day were statistical outliers and far less common.
Well, where do WWTPs get their energy from? Do these facilities place significant strain on the power grid, or can they generate some power on their own from the byproducts produced within the treatment plant itself? Let’s examine how WWTPs get the energy they need to purify wastewater—24 hours a day, 7 days a week.
Resilience and Strength
Many WWTPs rely on their local power grids for energy, but companies like Lakeside Equipment Corporation are already partnering with facilities to implement alternative energy sources to reduce operating costs and improve energy efficiency. These primarily include using solar energy and wind power to offset grid dependence and improve the environmental sustainability and resiliency of the WWTP. As Enchanted Rock states in their article, improving resilience helps WWTPs weather storms like Winter Storm Uri that came through Texas in February 2021, a storm that caused nearly 15 million Texans to lose access to clean water and be put on boiler notices.
A WWTP operating in freezing conditions similar to those observed during Winter Storm Uri (Courtesy of TPO-Treatment Plant Operator)
Solar and wind aren’t the only power mitigation methods WWTPs have at their disposal, however. One of the byproducts from the primary and secondary treatment processes—sludge—can be used for this process. If you can recall earlier in the article, sludge from the primary and secondary treatment processes are moved from the primary clarifier, aeration tank, and secondary clarifier for further processing. The sludge is moved to the WWTP’s digesters where the sludge undergoes anaerobic digestion (AD), which is a process where bacteria digest the sludge in the absence of oxygen.
The main products from AD are biogas and carbon dioxide (CO2). Biogas produced from AD can be used to power the WWTP’s various processes and can also be used in other biofuel applications. The CO2 from AD can also be used in emerging carbon capture technologies, which can be used in carbonating drinks or be used to create chemicals.
When biogas is used to produce both electricity and heat for the WWTP, it is referred to as a Combined Heat and Power (CHP) system. A CHP system is used to provide thermal energy demands of the WWTP while also providing it the electrical power it needs to keep operations fully functional. In addition to biogas, other CHP sources a WWTP may use include natural gas, oil, and landfill gas.
The Energy of the Future
However, there may be even more energy potential within wastewater than we realize. It not only could power the WWTP itself, but it could also provide power in other areas of life as well. According to a report from Fluence Corporation Limited, there could be enough energy in wastewater to power 158 million homes, which is the number of homes in the United States and Mexico combined. Treated wastewater may also provide power to automobiles, to 7.5 million American homes, and create thousands of new jobs. If this were to come true, the cars you drive and the homes you live in could very well be powered by the energy created from WWTPs.
A Cost Worth More Than Its Weight in Gold
As with all things in life, everything comes with a price, and the cost to build a wastewater treatment plant that can handle these processes is not cheap. According to AUC Group, the capital expenditure (CAPEX), or the total cost, to build the WWTP depend upon capacity, technologies incorporated into the WWTP, raw water parameters, effluent wastewater quality targets, construction costs, site conditions, consultant fees, among many other regulatory requirements and unexpected variable costs.
In terms of small-scale municipal plants that are designed to handle a few million gallons a day, the cost to build a WWTP can range from $1 million to $5 million. There are WWTPs that command much higher pricetags, such as the WWTP in Maryville, MO which cost $50 million to build and also the Northwest Water Treatment Facility in Wichita, KS, which commanded a respectable $494 million price tag.
Also worth considering when discussing costs are operations and maintenance (O&M) expenses. Depending on the chemicals, the wastewater flow rate through the plant, the effluent quality expected after the primary and secondary treatment phases, these O&M costs can range from hundreds of thousands of dollars per year to millions of dollars per year. Here are some examples from the California State Water Resources Control Board and from Pennsylvania’s Department of Environmental Protection that provide more detailed analyses of O&M costs.
The Reservoir Combining It All Together
Even though many of us are not aware of WWTPs since they work quietly behind the scenes of everyday life, they are no less crucial to human survival than homes or medical clinics are. As costly as they can be, they are able to safely clean wastewater into water that can be safely expelled into the environment for further natural purification. They can also be a source of safe direct potable reuse (DPR) water that can help mitigate the effects of drought in many places around the world.
Not only can the wastewater create byproducts that can be used to help energize the WWTP, but they can also potentially provide energy in other areas of life and create thousands of jobs. No matter how one looks at these magnificent feats of human ingenuity and engineering, there’s no denying that WWTPs not only serve us now, but will continue to serve society in increasing ways in the future.
How can we improve the efficacy and efficiency of WWTPs? Will WWTPs be able to be fully self-reliant and more dependable, even in the midst of inclement weather? How can WWTPs make more of an impact on society and less on the environment? Let us know in the comments below!
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