How to Remove Nitrogen and Phosphorus in Effluents?
Excessive levels of nitrogen and phosphorus in discharged effluents can lead to eutrophication and degradation of receiving water bodies. These nutrient overloads deplete oxygen and promote algal blooms that disrupt aquatic ecosystems. As discharge regulations tighten, wastewater treatment plants must implement more robust nutrient removal processes to reduce their environmental impact. This article examines common techniques for transforming and eliminating nitrogen and phosphorus species in effluents.
Nitrogen Removal
Nitrogen in wastewater predominantly exists as ammonia, organic nitrogen, nitrate and nitrite. Biological nitrification and denitrification processes are leveraged to remove these nitrogen forms.
Nitrification
Under aerobic conditions, specialised bacteria like Nitrosomonas oxidise ammonia to nitrite, while Nitrobacter further oxidises nitrite to nitrate. This autotrophic nitrification requires sufficient alkalinity, oxygen supply, and residence time. Temperature, pH and other environmental factors impact the optimal growth rates of nitrifying bacteria.
Denitrification
Anoxic heterotrophic denitrifying bacteria convert the nitrate produced in nitrification back to nitrogen gas that is released into the atmosphere. This requires readily biodegradable organic carbon and anoxic conditions achieved through controlled aeration. Electron acceptors for denitrification include nitrates, nitrites, and oxidised nitrogen/sulfur compounds.
Biological Nutrient Removal (BNR)
Activated sludge-based BNR processes combine nitrification and denitrification into cycled aerobic/anoxic sequences within the same treatment basin. Configuration examples include:
1- A/O (anaerobic/oxic): Integrates anaerobic zones for biological P removal
2- A2/O (anaerobic/anoxic/oxic): Improved process stability and nitrogen removal
3- Modified Ludzack-Ettinger: Anoxic zone before aeration for denitrification
4- Bardenpho: Four-stage process with additional aerobic/anoxic cycle
5- Oxidation ditches: Plug flow with aerobic and anoxic zones
Specialised BNR processes offer streamlined operation and can approach total nitrogen levels below 3 mg/L. However, the microorganisms require careful environmental control for optimal performance.
Separate Stage Nitrogen Removal
In applications with variable influent loads, a two-sludge system keeps nitrification separate from denitrification. Nitrified effluent passes to an anoxic denitrification reactor where a heterotrophic culture reduces the nitrates using a supplemental carbon source. While operationally simpler, two reactors require additional space and recycle streams.
Phosphorus Removal
Unlike nitrogen gas stripping, phosphorus must precipitate or be taken up biologically for removal from waste streams.
Chemical Precipitation
Salts of aluminium or iron can chemically react with soluble phosphates to produce stable sediments that settle out as sludge. Chemicals include alum, ferric chloride, and lime. Dose and pH control ensures optimum precipitation. Recovered solids may be recycled or landfilled depending on composition. Chemical costs are high for stringent effluent limits.
Enhanced Biological Phosphorus Removal (EBPR)
Polyphosphate-accumulating organisms (PAOs) can be enriched under anaerobic/aerobic cycling to take up excessive phosphorus beyond typical biological demands. In the initial anaerobic zone, PAOs like Accumulibacter use stored poly-hydroxyalkanoates (PHAs) to take up soluble phosphates and store them as polyphosphate chains. Introducing aerobic conditions causes these bacteria to replenish their PHA reserves then, allowing the luxury phosphorus uptake cycle to repeat. The biomass concentrates the phosphorus, which is wasted in the process.
EBPR challenges include controlling glycogen-accumulating organisms (GAOs) that compete with PAOs, along with managing adequate fermentation and anaerobic/aerobic cycling.
Tertiary Phosphorus Removal
For stringent effluent phosphorus limits, tertiary removal provides a final polishing step after biological treatment. Coagulant addition and clarification, multi-media filtration, and advanced ion exchange are common tertiary phosphorus removal methods. These final barriers protect receiving waters from any remaining soluble phosphates and colloidal particles passed through upstream treatment.
Innovative Processes
Beyond conventional technologies, emerging nutrient removal alternatives include:
1- Membrane aerated biofilm reactors (MABR) using attached growth media for BNR
2- Anaerobic ammonium oxidation (anammox) using chemolithotrophic bacteria
3- Adsorption, ion exchange, or algal/plant uptake systems
4- Thermal oxidation, gasification and nutrient recovery from dewatered biosolids
Nutrient Balancing and Recovery
Minimising supplemental nutrient additions improves treatment costs. Mass balances track influent and effluent nitrogen and phosphorus to optimisethe recycling of these nutrients within the treatment process. Novel nutrient recovery approaches are also being developed, enabling treatment facilities to become resource recovery centres providing fertilisers and soil amendments.
Holistic Nutrient Management
Ultimately, effective nutrient removal requires a holistic approach incorporating:
1- Pretreatment for loadings control
2- Optimized BNR and chemical treatment processes
3- Side-stream nutrient recycles from dewatering and digesters
4- Tertiary polishing if required to meet strict effluent limits
5- Proactive management of metals and other contaminant interferences
6- Nutrient monitoring and process control automation
7- Integration of nutrient recovery resource opportunities
Regulations will continue driving advances in both nutrient elimination and transformation into value-added products.
Conclusion
Meeting strict effluent nutrient criteria requires robust treatment for nitrogen and phosphorus removal. Combined nitrification/denitrification processes convert bioavailable nitrogen to inert nitrogen gas. Chemical, biological and tertiary methods eliminate phosphorus as solid precipitates or enriched biomass. As technologies improve and circular economy principles take hold, the nutrient transformation will shift from mere removal to recovery and regeneration of these valuable resources.
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