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Secondary treatment of wastewater is a crucial process that ensures the removal of biodegradable organic pollutants, including sugars, fats, and organic short-chain carbon molecules. This process primarily targets the reduction of organics and
| Characteristics | Values |
|---|---|
| Purpose | Removal of biodegradable organic matter from sewage or similar kinds of wastewater |
| Target pollutants | Biodegradable organic pollutants, dissolved and suspended organic materials, nitrogen, phosphorus |
| Mechanism | Biological processes, using microorganisms in a managed aerobic or anaerobic process |
| Minimum level of pollutant removal | 90% (for surface-aerated basins with retention times of 1-10 days), 98% (for treated water discharged into Oconomowoc River) |
| Parameters | Biochemical oxygen demand (BOD5), total suspended solids (TSS) removal, pH |
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What You'll Learn
Biochemical oxygen demand (BOD)
BOD is typically measured over a period of five days at 20°C, and the results are expressed as the amount of oxygen required for aquatic microorganisms to stabilize decomposable organic matter under aerobic conditions. This process is known as the BOD5 test and has been endorsed by the United States Environmental Protection Agency (EPA). The BOD5 test is considered a standard for assessing the efficiency of secondary treatment processes in wastewater treatment plants.
During secondary treatment, biological processes are employed to remove dissolved and suspended organic matter, which is measured as BOD. These biological processes are carried out by microorganisms in a managed aerobic or anaerobic environment, depending on the treatment technology. The microorganisms consume biodegradable soluble organic contaminants, such as sugars, fats, and organic short-chain carbon molecules, reducing the BOD of the wastewater.
The efficiency of BOD removal in secondary treatment processes can vary depending on the technology employed. For example, aerated lagoons, a type of suspended-growth method, can achieve 80 to 90% removal of BOD with retention times ranging from one to ten days. On the other hand, more advanced activated sludge systems can achieve even higher levels of BOD removal, with some treatments claiming pollutant removal of 98% or greater.
It is important to monitor and control BOD levels to prevent adverse effects on aquatic ecosystems. High BOD concentrations can lead to a rapid depletion of oxygen in the water, causing stress, suffocation, and even death among aquatic organisms. Additionally, sudden increases in BOD can disrupt the balance of ecosystem populations, favouring species with lower oxygen requirements and potentially impacting the overall health of the ecosystem.
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Total suspended solids (TSS) removal
Total suspended solids (TSS) refer to the solid particles suspended in water, which can cause turbidity and inhibit plant growth by reducing photosynthesis when present in elevated concentrations. TSS must be removed before water demineralization to prevent clogging and fouling of membranes.
TSS removal is a crucial aspect of secondary treatment in wastewater management. Secondary treatment involves biological processes to remove dissolved and suspended organic matter, which is measured as biochemical oxygen demand (BOD). This process is carried out by microorganisms in managed aerobic or anaerobic environments, depending on the technology employed.
The US Environmental Protection Agency (EPA) establishes secondary treatment standards for publicly owned treatment works (POTWs), which include minimum requirements for TSS removal in municipal wastewater treatment plants. These standards are designed to ensure that treated wastewater meets specific quality criteria before being discharged into the environment, typically rivers, lakes, or oceans.
Several methods are employed to achieve TSS removal during secondary treatment. One common approach is the activated sludge process, where aeration tanks provide an environment for microorganisms to break down organic matter, including suspended solids. The resulting biomass settles in secondary clarifiers, separating the treated water from the biology.
Another technique is the use of trickling filters, where wastewater is distributed over a bed of media. Biofilms of microorganisms develop on this media, degrading the organic matter. The water is then clarified to remove any remaining solids. Rotating Biological Contactors (RBCs) are also used, where rotating discs support biofilm growth, alternately exposing the film to wastewater and air to facilitate organic matter degradation.
For more stringent TSS removal requirements, advanced treatment methods are employed. Membrane filtration, for example, uses microfiltration (MF) and ultrafiltration (UF) membranes to physically separate suspended solids from water, achieving high-efficiency TSS removal. Chemical coagulation is another advanced method that can enhance overall treatment performance.
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pH
The minimum level of pollutant removal accomplished by secondary treatment targets the removal of biodegradable organic pollutants and the reduction of biological oxygen demand (BOD). This process aims to prevent oxygen depletion in receiving water bodies, which occurs when microorganisms break down organic matter and consume the oxygen required by aquatic life.
Regarding pH, secondary treatment standards reflect specific requirements. The pH of wastewater is a critical parameter that can impact the effectiveness of treatment processes and the quality of the treated water. The pH level can affect the solubility and toxicity of pollutants, as well as the activity of microorganisms involved in the treatment process.
For context, pH is a measure of how acidic or alkaline a substance is and ranges from 0 to 14, with 7 being neutral. Wastewater streams can have varying pH levels depending on their sources and the nature of the pollutants they contain.
In the case of Iowa, the effluent values for pH must be maintained within the limits of 6.0 to 9.0 for publicly owned treatment works. This range ensures that the treatment process can effectively remove pollutants and that the treated water is safe for discharge or reuse. Maintaining the correct pH is essential for the success of the treatment process. For example, in the case of nitrogen removal, the pH is raised to about 10 to facilitate the conversion and removal of ammonia.
Additionally, variations in pH, along with temperature and pollutant concentrations, can disrupt the microbial population in treatment systems, requiring careful monitoring and adjustments to restore balance.
Overall, while specific pH requirements may vary based on local regulations and the characteristics of the wastewater, maintaining the appropriate pH range is a critical aspect of secondary treatment to ensure the effective removal of pollutants and the protection of aquatic ecosystems.
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Anaerobic treatment
Secondary treatment of wastewater involves the biological removal of biodegradable organic matter from sewage or similar types of wastewater. The primary treatment step involves the physical removal of settleable solids, followed by a secondary treatment where biological processes are used to remove dissolved and suspended organic matter. This is achieved through biological degradation by microorganisms in an aerobic or anaerobic process.
Anaerobic lagoons are a common type of anaerobic treatment system, consisting of large man-made ponds where wastewater is piped to the bottom, forming a liquid layer that prevents oxygen from reaching the semi-solid sludge layer. This creates an environment for anaerobic digestion to break down organic materials, which can take a few weeks to several months. Anaerobic treatment offers advantages such as low sludge production, lower nutrient requirements, and net energy production in the form of methane. However, it may have longer start-up and retention times due to low microbial growth rates and potential issues with odor and corrosion.
The minimum level of pollutant removal achieved by secondary treatment can vary depending on the specific technologies and processes employed. Aerated lagoons, for example, can achieve 80-90% removal of biochemical oxygen demand (BOD) with retention times ranging from 1 to 10 days. In general, secondary treatment aims to produce an effluent quality suitable for the intended disposal or reuse option, meeting the standards set by regulatory bodies such as the US EPA for publicly-owned treatment works (POTWs).
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Activated sludge treatment
Secondary treatment of wastewater involves the removal of biodegradable organic matter and suspended solids. This is mostly achieved through biological means, with microorganisms consuming pollutants and transforming them into cell tissue, water, and nitrogen.
The activated sludge process typically begins with an aeration tank, where air or oxygen is injected into the wastewater, mixing with the activated sludge. This is followed by a settling tank, where the biological flocs (the sludge blanket) are allowed to settle, separating the biological sludge from the clear treated water. This settling tank is usually referred to as a "final clarifier" or "secondary settling tank."
The activated sludge process can also be used to treat nitrogenous or phosphorous matter through the addition of an anoxic compartment inside the aeration tank. This allows for the more efficient performance of the nitrification-denitrification process. First, ammonia is oxidized to nitrite, which is then converted into nitrate in aerobic conditions. Facultative bacteria then reduce the nitrate to nitrogen gas in anoxic conditions.
Overall, activated sludge treatment is an effective method for treating wastewater, particularly in centralized treatment facilities, and is widely relied upon by municipalities.
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Frequently asked questions
Secondary treatment is a process that targets the removal of biodegradable organic pollutants from sewage or wastewater.
The goals of secondary treatment are to achieve a certain degree of effluent quality, reduce organic pollutants, and protect aquatic habitats and the environment.
Common methods used in secondary treatment include activated sludge treatment, rotating biological contactors (RBCs), and aerated lagoons.
Activated sludge treatment uses an aeration tank with agitators that mix oxygen into the wastewater. The tank contains microorganisms that break down organic material, forming sludge which then settles at the bottom of a settling tank.
Secondary treatment can achieve high levels of pollutant removal, typically exceeding 80-90%. In some cases, such as in the City of Oconomowoc, WI, pollutant removal is maintained at 98% or greater.
