What happens to your water and solids after they go down your sink, shower, or toilet?
Below is an outline of a conventional wastewater treatment plant, which is the most common treatment process for many mid-size to large communities.
THE JOURNEY OF WATER THROUGH WASTEWATER TREATMENT
Step 1 (Water is Used): First, the water that will eventually become wastewater is used. Whether it is used for showering, cooking, cleaning, or consuming, ultimately, all of this water ends up leaving our homes and makes its way to the treatment plant.
Step 2 (Water Travels to Plant): Once the water exits our homes through drains and pipes, it will travel through the wastewater distribution network to the wastewater treatment plant. In larger cities, the water will pass through pump stations, which pump water from lower locations to higher locations. Depending on how far away you live from the treatment plant, this journey could be short to miles long.
Step 3 (Screen Filters): Once the water makes it to the treatment plant, it will interact with what is typically referred to as the "headworks." At the headworks, the water will pass through screen filters to remove large solids and debris. This filtering increases the efficiency of downstream processes and protects equipment like pumps. The solids that are filtered out at this stage are usually trash and often get disposed of in a landfill. Most wastewater treatment operators have interesting stories about things that have shown up in this filtering process!
Step 4 (Primary Clarification): Once large debris is removed, then the water is slowed down in a step called primary clarification. Up until this point, the water is typically moving quite quickly. Depending on the capacity of the plant it could be multiple million gallons a day. Primary clarifiers are typically large, long rectangular tanks. Since the water is slowed down in this stage, more solids are able to settle out. These solids are collected in hoppers at the bottom of the tank, and transported to the solids treatment process.
Step 5 (Aeration): After initial solids removal, the water travels to an aeration basin. In this step, air is pumped into the water to enable the removal of dissolved gases and to support a robust microbial community that continues to break down any remaining solids. The microbes in this process are aerobic, which means that they need oxygen to survive. There are other processes in wastewater treatment, specifically in solids treatment, that utilize anaerobic microbes, which means they cannot survive in the presence of oxygen.
Step 6 (Secondary Clarification): Secondary clarification is very similar to primary clarification. The water is again slowed down and any remaining solids are settled out of the water. Often these solids include microbes from the previous step, aeration. In many wastewater treatment plants, some of these solids are pumped back into the aeration basin to replenish the microbial communities. This is a really neat closed-loop process!
Step 7 (Disinfection): At the end of the conventional treatment train, many wastewater treatment plants disinfect the cleaned water to ensure there are no lingering microbes. Many treatment plants use chlorine for this disinfection process.
Step 8 (Discharge into the Environment): Finally, water is released back into the environment, typically into a nearby river. Often, wastewater treatment plants discharge into rivers that are also used for drinking water. This illuminates how interconnected the water cycle already is. Potable reuse provides an opportunity to skip this environmental buffer step through advanced treatment, saving water and chemicals. If you want to learn more about potable reuse, check out our potable reuse page!
THE JOURNEY OF SOLIDS THROUGH WASTEWATER TREATMENT
The solids treatment process isn't as sequential as the water treatment process, thus each step is labeled with a letter, but many of these steps work in conjunction with each other.
Step A (Screen Filtering): As noted above in the water treatment section, large pieces of debris are screened out at the screen filtering (think leaves, sticks, trash). This is primarily to protect downstream processes and pumps. The solids that are filtered out at this stage are typically sent straight to a landfill.
Step B (Primary Clarification Solids to Digester): During primary clarification, the water slows down and solids are able to settle out. These solids settle to the bottom of the clarifiers and then are moved into large hoppers by slow-moving bars. From the primary clarifiers, the solids are then pumped into a primary anaerobic digester. Digesters are essentially large tanks that provide a home for microbes to assist in the decomposition of the solids. Anaerobic means that the microbes thrive in environments without oxygen. As the microbes break down and consume the organic matter in the solids, they produce gas. This gas is often referred to as biogas. In many treatment facilities, this gas is used to power other parts of the treatment process. In Portland, they complete additional treatment on this gas, such that it adheres to natural gas standards. Then, the treatment plant pumps this gas directly into commercial natural gas lines. This is an example of a "renewable" source of natural gas (RNG). So cool!
Digesters can also be used for more than just wastewater solids. Digesters can also break down food waste, thus allowing for the capture of the gas that is a byproduct of the decomposition of food, for beneficial use.
Step C (Primary Digestion to Secondary Digestion): After primary digestion, the solids are then pumped into secondary digestion. Essentially this step is the same as primary digestion, except the secondary digesters might be operating at slightly different parameters to effectively treat the solids that come from secondary clarification and aeration.
Step D (Return Activated Solids): Post aeration, the water is again slowed down and the solids are settled out. Some of these solids are pumped back into the aeration basin to replenish the microbial communities. This type of sludge is referred to as return-activated sludge (RAS).
Step E (Secondary Solids to Digesters): The portion of solids that are not RAS are then pumped to digestion for further treatment. Depending on plant layout, these secondary solids may be pumped to primary digestion or directly to secondary digestion.
Step F (Solids Dewatering): After digestion, the goal is to remove as much water as possible from the solids. During dewatering, machines like belt presses, filter presses, or centrifuges help separate the remaining water from the solids.
Step G (Dewatered Solids to Application): Depending on regulations and the treatment process that is applied by the plant, these solids are often loaded onto trucks and then sent for land application for growing crops. Depending on the quality of the solids, these may be used for livestock crops or for human consumption crops.