How Can STP Plants Minimise Greenhouse Gas Emissions?
As global climate change increases due to increasing greenhouse gas (GHG) levels, industries across all sectors are facing heightened pressure to curb their emissions footprint. Municipal sewage treatment facilities find themselves squarely in this spotlight since their very operations generate significant volumes of harmful gases like methane, carbon dioxide and nitrous oxide.
Sewage contains biodegradable organic matter from residential and industrial waste streams. As this sludge decomposes through treatment processes, it releases methane, a potent GHG with over 25 times greater global warming potential than carbon dioxide. Nitrous oxide emitted from denitrification also carries major climate impacts. Wastewater handling accounts for an estimated 3% of total global GHG emissions.
With more strict emission regulations, sewage treatment plants need comprehensive strategies to minimise their carbon footprint. Increasingly, facilities that proactively cut emissions will access financial incentives, public funding, and rate benefits that reward sustainability. We will discusses the primary sources of sewage treatment GHGs and outlines innovative approaches plants can implement to rein in harmful emissions.
Sewage GHG Emission Sources
There are several key process areas within a typical sewage treatment facility that contribute to greenhouse gas emissions:
Sludge Digestion and Handling
Anaerobic digesters stabilise sewage sludge by breaking down organic matter in the absence of oxygen. While generating useful biogas, this process is also the single largest source of methane from treatment plants.
Lagoons and Sludge Storage
When sludge and wastewater are stored in open lagoons and holding tanks, the liquid surfaces provide a significant opportunity for methane and ammonia to volatilise into the atmosphere.
Aeration Processes
Diffused aeration systems release carbon dioxide as a byproduct of transferring oxygen into wastewater during activated sludge treatment. Smaller amounts of nitrous oxide emerge during nitrification.
Combined Sewer Overflows (CSOs)
During significant rainfall events, combined municipal sewers can overflow and discharge untreated Sewage along with methane and nutrients that stimulate algal growth and oxygen depletion.
Onsite Combustion
Burning fossil fuels like natural gas or digester biogas to heat digesters and sludge handling areas produces greenhouse gas emissions. Electricity consumption for treatment processes also carries emissions.
Effluent Discharge
Municipal treatment effluents still contain residual nitrous oxide along with nitrogen and phosphorus nutrients that generate emissions downstream.
Sludge Digestion Optimisation
Given the predominant methane volumes generated by sludge digesters, optimising this treatment step provides a high-impact opportunity for cutting GHG emissions:
Anaerobic Codigestion
Supplementing sludge feedstocks with organic wastes like food waste, fats/oils/grease (FOG) and other biodegradable carbon sources boosts biogas/methane yields while diverting those organics from landfills.
Advanced Anaerobic Digestion
Emerging anaerobic digestion processes like temperature-phased, hyper-thermophilic and micro-aerobic improve methane capture and biogas quality while stabilising sludge more rapidly.
Biogas Capture/Utilisation
Better containment and conveyance of biogas eliminates fugitive methane releases. Collected biogas can then be utilised to generate renewable electricity, heat, or transportation fuel.
Extensions like thermal hydrolysis pretreatment, enzyme augmentation and microbial optimisation of digestion boost gas generation further, displacing more fossil fuel use.
Lagoon and Sludge Emissions
Liquid surfaces at sewage plants provide major opportunities for greenhouse gas mitigation, including:
Aerobic Digestion
Converting anaerobic lagoons and tanks to aerobic sludge digestion eliminates the anaerobic methane generation conditions while stabilising biosolids through a smaller overall biomass.
Advanced Liquid Treatment
Adding sludge thickening, dewatering and volume reduction steps prior to liquid handling minimises methane emissions from open surfaces. Sludge can be composted or dried for reduced emissions.
Combustion Destruction
Flaring, combusting or diverting captured methane to boilers or reciprocating engines mitigates its greenhouse impact by converting it to less potent carbon dioxide.
Combining these mitigation measures essentially dries and shrinks the sludge volumes exposed for methane release while capturing any fugitive gas for destruction or utilisation.
Aeration Emissions
During aerobic wastewater treatment, mechanical aerators and diffused aeration systems contribute significant carbon dioxide emissions through their operation:
Advanced Aeration
Novel aeration designs like fine-bubble diffusers, automated controls, and highly efficient turbo blower systems drastically reduce the energy intensity required for oxygen transfer.
Alternative Biological Treatment
Innovations like membrane-aerated biofilm reactors (MABR), which utilise passive gas transfer membranes, granular activated sludge, and attached growth processes, reduce aeration demands by enhancing treatment kinetics.
Primary Treatment and High-Rate Clarification
Upstream physical and chemical treatment processes maximise solids separation and reduce aeration basins' oxygen demand, cutting emissions.
Optimised aeration not only reduces direct CO2 emissions but also compounds benefits that accrue by shrinking electricity/fuel demands, chemical consumption, and the overall treatment plant footprint.
Nutrient Removal
Excess nitrogen discharge helps produce nitrous oxide, a potent greenhouse gas around 300 times more impactful than carbon dioxide. Enhancing nutrient removal cantherefore slash emissions:
Advanced Biological Nutrient Removal
Optimising bioreactor conditions for hydrolysis, nitrification and denitrification ensures complete nitrogen removal. Technologies like anaerobic ammonium oxidation (Anammox) offer more efficient nitrogen gas release.
Effluent Reuse
By treating effluent to standards for non-potable reuse, facilities recycle water back into their headworks instead of emitting nutrients to the environment.
Nutrient Recovery
Emerging methods stabilise and concentrate nutrients into value-added fertiliser products, eliminating nutrient emission pathways.
Proper nutrient removal and recovery decreases downstream impacts while potentially creating new revenue sources for treatment facilities.
Operational Strategies
Proven emission mitigation is not solely based on capital upgrades.Optimising day-to-day sewage plant operations makes an immediate difference:
Energy Management
Monitoring and reducing energy usage through equipment scheduling, load shedding and conservation measures minimise indirect emissions from grid/generator electricity.
Process Control Automation
Advanced process monitoring and control automation tools allow fine-tuning treatment processes for optimal emissions footprints.
Flow Optimisation
Mitigating infiltration and implementing green stormwater infrastructure controls sanitary sewer wet weather flows that contribute to emissions during overflows.
Inter-Process Integration
Integrating different treatment stages, material streams, and considering circular resource recovery opportunities drives synergistic emissions reductions across facility operations.
Collecting robust data to conduct greenhouse gas audits quantifies current baselines so operators can implement strategic mitigation plans.
Conclusion
As public awareness of climate change grows, regulations targeting industrial GHG emissions will only tighten further for treatment plants. This not only creates increasing sustainability pressures but also opportunities for facilities to pursue comprehensive emissions mitigation programs. Through optimising sludge treatment, liquid handling, aeration, nutrient removal and overall operational processes, sewage treatment professionals can significantly reduce their facilities' greenhouse gas footprint through available and emerging technologies.
By taking strategic steps now to inventory emissions sources, model reduction pathways, and implement mitigation measures, municipal treatment plants can get ahead of this challenge.
Sewage treatment plantsare responsible for contributing to climate change. Implementing thoughtful greenhouse gas strategies across all plant processes is rapidly becoming a mandate for responsible, sustainable public service.
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