Which type of Organic contaminants is used in Onsite STP Plants?
Onsite sewage treatment plants, also known as septic systems, are commonly used in rural areas where connection to a municipal sewer system is not available. These systems collect and treat household wastewater onsite before dispersing it underground. While septic systems provide an effective means of wastewater treatment, concerns have been raised about the potential for organic contaminants to pass through these systems and pollute ground and surface waters. Organic contaminants, such as pharmaceuticals, personal care products, and household chemicals, are increasingly being detected in wastewater and the environment.
Sources of Organic Contaminants in Septic Systems
Organic contaminants originate from a variety of household products and activities. Pharmaceuticals and personal care products, such as prescription and over-the-counter drugs, fragrances, cosmetics, sunscreens, and antimicrobials, are extensively used in homes. Cleaning agents, detergents, disinfectants, pesticides, and plasticizers also contribute organic contaminants to wastewater. Sources specific to the kitchen include food waste, cooking oils, and chemicals leaching from food packaging materials. Outdoor uses, such as recreational pools, can introduce organic contaminants as well. The wide variety of consumer products containing organic chemicals, coupled with large usage volumes, make households a major source of these contaminants entering septic systems.
Transport and Fate of Organic Contaminants
Organic contaminants enter the septic tank through the wastewater influent pipe. Septic tanks provide partial removal of suspended solids and oils, which reduces the organic load. However, many organic contaminants have low affinity for solids and will remain dissolved in the wastewater. These dissolved compounds are transported with the clarified effluent to the subsurface drainfield.
The fate of organic contaminants in drainfields depends on their sorption and biodegradation properties. Nonionic organic compounds, such as pharmaceuticals and personal care products, tend to be more mobile and persistent in the environment. They can migrate through the unsaturated zone under the drainfield and eventually reach groundwater. Anionic and cationic organic compounds interact more strongly with soils and are often attenuated in the drainfield. However, overloading of the drainfield can exceed the sorptivecapacity of the soil and allow contaminants to break through. Biodegradation removes some organic compounds, but many degrade slowly or resist microbial processes. Overall, the natural attenuation processes in drainfields only provide partial and variable removal of organic contaminants.
Effects on Treatment Performance
The presence of organic contaminants in septic system wastewater has several concerning implications for treatment performance. Many organic compounds are not effectively removed by conventional septic tank and drainfield treatment. This allows contaminants to persist through the system and impair effluent quality. High concentrations of organic chemicals can potentially disrupt biological treatment processes. Some compounds are toxic to microorganisms at sufficient doses. Mixtures of organic contaminants could have additive or synergistic inhibitory effects on the microbial ecosystems within septic tanks and drainfields. Disinfection of effluent by soil infiltration may also be hindered if pathogenic microbes develop increased resistance to antimicrobial organic contaminants. Furthermore, surfactants and emulsifiers can facilitate mobilization of other organic and inorganic contaminants. Overall, the complex mixtures of organic contaminants found in septic system wastewater present challenges for consistent and effective treatment.
Several strategies can help mitigate risks associated with organic contaminants in decentralized wastewater systems:
1- Source control- Reducing usage and disposal of hazardous household chemicals to septic systems. Product substitutions, proper dosing, and diversion of high risk wastes could lessen contaminant volumes. Education programs can encourage homeowners to be aware of their chemical use and disposal habits.
2- Advanced treatment- Supplementing existing septic tanks with organic adsorbents (e.g. activated carbon) or advanced oxidation processes can enhance organic contaminant removal prior to drainfield discharge. Package treatment plants using membrane bioreactors are also increasingly being used for decentralized systems.
3- Improved siting and design- Larger drainfield size, proper soil assessment, separation distances to groundwater/surface water, and diversion of excess flows can optimize contaminant attenuation. Advanced drainfield designs, such as pressurized shallow narrow drainfields, can improve treatment.
4- Performance monitoring- Regular inspection and maintenance checks, combined with effluent and groundwater monitoring where feasible, allows early identification of any performance issues.
5- Clustered systems- Decentralized cluster systems serving multiple households can provide economy of scale for more advanced treatments. Mini-utilities operated by local governments are a potential management model.
A combination of these strategies will likely be needed to adequately control organic contaminant impacts from septic systems as their use increases. Further research is still needed to characterize contaminant fate and effects, evaluate best mitigation options, and promote sustainable decentralized wastewater management.
Onsite sewage treatment systems provide a flexible wastewater treatment option for rural and suburban areas, but concerns persist about their ability to remove potentially harmful organic contaminants originating from household chemicals and personal care products. These contaminants can pass through septic tanks and drainfields to impair effluent quality and pollute water resources. High contaminant concentrations may also disrupt treatment processes relying on biological degradation. There are several mitigation techniques available to enhance organic contaminant removal and minimize environmental impacts, but a suite of source control, system design, and performance monitoring measures may be necessary as decentralized systems expand. Further research can help optimize solutions to this wastewater treatment challenge. Addressing the issue of organic contaminants will allow onsite sewage treatment to remain a viablecomponent of sustainable wastewater infrastructure.
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