Sewage system designs vary across the country and can differ county-to-county in some places. In simpler models, a reasonably cheap and efficient option is the use of waste stabilization lagoons, where water is allowed to pass through a series of ponds across several days. Microbial action and natural forces allow solids to settle in the form of sludge, remove toxins, and digest chemical and organic waste. However, these lagoons require a lot of land, and are nowhere near as time efficient as industrial, aerated lagoons that process out chemicals via activated sludge and disinfectants. In some cases, semi-treated or untreated water is re-used in the form of irrigation, applied to fishponds, or allowed to return to the groundwater/evaporated into the atmosphere.
Globally, around 80% of wastewater is released without any treatment at all. Sudden increases in phosphorus, ammonia, or other chemicals like these found in waste can over-fertilize delicate ecosystems in a process called eutrophication. This is where massive amounts of oxygen are suddenly depleted from the environment. Eutrophication is toxic to aquatic organisms, and can cause sudden death in fish, bacteria and shellfish alike. They also promote excessive flora, which reduces available oxygen, which is a major concern for ecosystems where both microorganisms and bigger organisms are heavily reliant on dissolved oxygen to survive. Decaying organic matter, and fish and human waste, can all carry pathogens. These include disease-carrying bacteria and viruses like giardia, cholera, E.coli, and pneumonia. If these are allowed to travel further into the water system, they can end up being in our tap water, or in the water used to grow the food we eat.
In more advanced designs, sewage is pumped through a series of treatment steps before being released back into surface water or carried further into the treatment process in order to be used by humans. In order to extend HRT times, ensure sediment does not enter input valves or pumps, and to separate input and output flows, baffles are very common within sewage treatment designs.
If you have a septic tank in your backyard, all your wastewater is delivered to the tank by a main sewage pipe. The septic tank is a waterproof container made of concrete, fiberglass, polyethylene, or a combination thereof. Wastewater is held within the tank long enough to allow solids to settle to the bottom of the container in the form of sludge. Oil and grease float to the top of the water as scum. Liquid wastewater is released into a drain field, while solids are kept from leaving the tank through valves and screens. The drain field is where water is allowed to flow through porous, unsaturated soil. This drainage process purifies the water as it passes through the soil, capturing and filtering out excess nutrients, chemicals, and other pollutants found in the waste. From here, the water can eventually percolate into the groundwater.
In larger sewage treatment plants, the water is first passed through screens where debris and solids can be caught and trapped in order to remove large sediments like grit or rock. Oil, grease, and fats are caught by skimming them off the top of the water. Then, the primary processes for sewage include sedimentation tanks, where smaller solids are given time to settle. HRT (hydraulic retention time) for these primary tanks is typically between 1 and 2 hours, but this number varies widely. In order to maintain proper HRT in these tanks, baffles are used to divert water flow and extend the path that water must follow. Sludge gathering at the bottom of the tank and scum gathering on top of the water is removed, and water is moved into the secondary part of treatment.
The secondary stage of treatment for sewage water is mostly microbial, where bacteria is allowed to absorb suspended particles of organic waste. Aeration improves conditions for microorganisms encouraging microbial growth. Activated sludge refers to a process where pre-grown bacteria and microbes are injected into earlier stages in the treatment series. This encourages sediment and chemical breakdown in the primary tanks and helps remove solids and sludge from the secondary reservoirs.
Water that needs further treatment goes through a tertiary stage. This is sometimes referred to as effluent polishing, and involves a series of disinfectants like ozone, chlorine, or UV light; sand and carbon filter; or spending longer times within aerated lagoons with more complex ecosystems. Sometimes water is passed through fine gravel, sand or other sediments in order to filter out smaller dissolved solids. These processes remove toxins, pathogens, nutrients, and any remaining suspended matter within the wastewater.
Baffles are located at both the entrance and exit of most any tank found in a sewage transportation or treatment system. The ‘inlet baffle’ is found at the inlet pipe, where water is removed from the previous source. The ‘outlet baffle’ is understandably located at the outlet pipe. This is a critical baffle because it keeps solids from exiting the tank and making their way further into the system, where they can cause blocks or damage.
