Energy Efficiency Optimization, Indirect Carbon Emission Reduction and Benchmarking Management

Abstract

Mismatches between design parameters (inflow volume, water quality, temperature) and actual operating conditions commonly lead to excessive carbon emissions in existing wastewater treatment plants (WWTPs). Based on energy consumption composition data of WWTPs meeting Class A effluent standards, this paper analyzes the main influencing factors of indirect carbon emissions, and proposes systematic low-carbon operation strategies covering lifting system flow and head management, aeration flow and intensity control, and mixing intensity optimization. Full-scale application across 8 WWTPs demonstrates that these strategies deliver significant cost reduction and efficiency improvement, with remarkable indirect carbon emission abatement. Total electricity consumption decreases from 3.8377×10⁷ kWh/a to 3.3485×10⁷ kWh/a, reducing corresponding CO₂ emissions from 3424.0 t/a to 2987.5 t/a, a 12.7% reduction. Phosphorus removal agent consumption drops from 4491.1 t/a to 3751.9 t/a, and carbon source consumption drops from 5145.6 t/a to 2013.4 t/a, cutting chemical-related indirect carbon emissions from 5084.9 t/a to 2488.3 t/a, a 51.1% reduction. WWTPs with low-strength influent can be upgraded to Level 3 low-carbon operation rating, while plants with high influent total nitrogen, high-energy-consuming equipment or high industrial wastewater proportion require further technologies such as carbon capture, anaerobic digestion combined heat and power, autotrophic nitrogen removal, denitrifying phosphorus removal and sewage latent heat recovery to achieve deeper carbon reduction.

1. Introduction

Municipal wastewater treatment is an essential public infrastructure, but also an energy- and material-intensive industry. Statistics show that by the end of 2022, China had 4695 urban and county WWTPs, treating 72.7 billion cubic meters of wastewater annually. As one of the largest carbon emission sources among public infrastructures, the wastewater treatment sector is estimated to contribute 2.95% of total greenhouse gas emissions by 2030, driven by increasingly strict discharge standards and the traditional “water quality at the cost of energy” operation mode.

China’s carbon peaking action plan for urban construction clearly requires promoting low-carbon governance, intelligent operation and renewable energy application in wastewater treatment facilities. Existing studies have mostly focused on low-carbon process selection for new plants, while systematic, operable optimization strategies for in-service WWTPs without major process modification remain insufficient. This paper summarizes practical low-carbon operation approaches based on years of water utility management experience, and validates their effectiveness through multi-plant case studies, providing actionable guidance for low-carbon retrofitting of existing WWTPs.

2. Carbon Emission and Energy Consumption Profile of WWTPs

2.1 Carbon Emission Composition

Carbon emissions from wastewater treatment are divided into direct emissions and indirect emissions:

• Direct emissions: CH₄ and N₂O released during wastewater and sludge treatment processes, generated by microbial metabolic activities.
• Indirect emissions: Carbon emissions embedded in purchased electricity and chemical production consumed during plant operation.

For conventional WWTPs adopting “pretreatment + biochemical treatment + advanced treatment” processes with Class A effluent standards, indirect emissions from energy and chemical consumption account for the largest share of total carbon footprint. For in-service plants without core process reconstruction, direct emissions remain relatively stable, so optimizing indirect emissions through refined operation management is the most feasible and cost-effective path to achieve low-carbon operation in the short term.

2.2 Energy Consumption Distribution

For typical AAO + secondary sedimentation + high-efficiency sedimentation + filtration processes, four core units account for 75%–85% of total plant energy consumption: raw water lifting, biochemical aeration, biochemical mixing, and internal/external recirculation. The specific proportion varies with influent concentration:

• For low-strength influent (COD < 200 mg/L, ammonia nitrogen < 30 mg/L): lifting accounts for 28.6%, aeration 34.9%, mixing 10.6%, recirculation 6.6%
• For medium-strength influent (COD < 500 mg/L, ammonia nitrogen < 60 mg/L): lifting accounts for 8.36%, aeration 46.44%, mixing 16.36%, recirculation 7.60%

These high-consumption units are the core targets of energy-saving and carbon-reduction optimization.

3. Core Low-Carbon Operation Strategies

3.1 Lifting System Optimization: Flow and Head Management

Lifting systems include coarse grid pump stations, intermediate pump stations, sludge return pumps, nitrification liquid return pumps and process wastewater lifting pumps. Pump power is determined by flow rate, head and pump efficiency, so optimization is carried out from three dimensions.

3.1.1 Invalid Flow Reduction

Eliminate repeated lifting of internal recirculating wastewater to reduce unnecessary flow:

• Optimize pretreatment flushing, filter backwashing and sludge dewatering wastewater volumes to reduce internal circulating water volume
• Implement precise control of nitrification liquid internal return flow based on actual total nitrogen removal demand, instead of operating at design maximum flow
• Adjust external sludge return ratio to 40%–80% under the premise of meeting effluent SS requirements, reducing lifting flow while improving biological phosphorus removal efficiency

3.1.2 Head Loss Minimization

Reduce unnecessary head consumption to lower pump energy consumption:

• Optimize the routing of internal process wastewater to lift it to the nearest treatment unit instead of returning to the front-end collection tank, reducing lifting height
• Adopt “one pump, one pipeline” layout and eliminate redundant outlet valve groups, reducing pipeline head loss by 2–3 m. For a 10,000 m³/d WWTP, this modification can reduce annual power consumption by about 29,200 kWh
• Operate the wet well at high liquid level to reduce net pump lift, and match pump operating points with actual demand through variable frequency control

3.1.3 Pump Efficiency Improvement

Select high-efficiency energy-saving pumps, conduct regular efficiency evaluation, and timely resolve blockage, impeller wear and other issues that reduce operating efficiency.

3.2 Biochemical Process Energy Optimization

Biochemical treatment is the largest energy consumption unit, including aeration, mixing and recirculation systems.

3.2.1 Precision Aeration Control

Aeration usually accounts for 35%–50% of total plant energy consumption. Optimization measures include:

• Replace mechanical aeration with submerged disc microporous aerators, and replace multi-stage centrifugal/Roots blowers with air suspension or magnetic suspension blowers
• Implement precise aeration control based on actual oxygen demand. Identify the position where ammonia nitrogen drops to 1 mg/L along the biochemical tank, and adopt intermittent alternating aeration or staged blower configuration to avoid over-aeration. Field practices show that this strategy can reduce aeration energy consumption by 30%–50% and save carbon source dosage by 50%–90%.

3.2.2 Mixing Energy Reduction

For anaerobic and anoxic tank mixers with design power of 2–8 W/m³:

• Install frequency converters to reduce stirrer speed for small-blade high-speed submersible mixers
• Replace small-blade high-speed mixers with large-diameter vertical curved-blade mixers for rectangular tanks
• Adopt intermittent alternating stirring operation to reduce running time while maintaining uniform sludge suspension

3.2.3 Internal/External Recirculation Regulation

Optimization of recirculation pumps follows the same flow and head management principles as lifting systems described in Section 3.1.

3.3 Chemical Consumption Reduction

Chemical consumption not only directly increases operating costs, but also contributes a large proportion of indirect carbon emissions. Core optimization measures:

• Phosphorus removal agent reduction: Optimize return sludge ratio, control dissolved oxygen in anaerobic tank influent below 0.5 mg/L, and return coagulation sedimentation sludge to enhance biological phosphorus removal efficiency and reduce chemical dosage
• Carbon source reduction: Control effluent dissolved oxygen from the aerobic zone below 1 mg/L, eliminate aerator fouling and damage, and reduce nitrate carryback to the anoxic zone to improve denitrification efficiency and carbon source utilization
• Dewatering agent reduction: Increase dewatering feed sludge concentration to above 20 g/L through intermittent sludge discharge and supernatant control, and optimize flocculant type and dosage

3.4 Intelligent Operation and Management System

For municipal-dominated WWTPs with load rates above 80%, influent quantity and quality show regular, repeatable patterns. An intelligent management system is established based on:

• Smart sensing: Install independent smart meters, remote level gauges, flow meters, and online water quality analyzers (MLSS, DO, ORP, nitrate, phosphate) at key nodes
• Strategy library: Develop targeted operation strategies for dry/wet seasons, peak/valley flow periods, and winter/summer temperature conditions
• Closed-loop optimization: The system automatically issues alarms and optimization suggestions based on real-time data comparison with energy efficiency benchmarks

3.5 Energy Efficiency Benchmarking and Dynamic Management

Establish a standardized energy efficiency benchmark system covering core indicators such as lifting energy consumption, aeration energy consumption, unit treatment power consumption, carbon source consumption ratio, invalid water ratio and dissolved oxygen control targets. Conduct annual benchmarking evaluation following the “data collection → gap analysis → optimization implementation → re-benchmarking” closed-loop process to achieve continuous improvement of energy efficiency level.

4. Full-Scale Application Case Study

4.1 Project Overview

Eight WWTPs operated by a water utility group in Shaanxi Province were selected for low-carbon optimization, with treatment scales ranging from 18,000 to 70,000 m³/d. The main processes include AAO, CASS and hydrolytic acidification + AO, all with Class A effluent standards. Two of the plants have industrial wastewater accounting for more than 30% of influent.

4.2 Optimization Measures Implementation

Based on 2019–2021 operational data and energy efficiency benchmarking gap analysis, targeted optimization measures were implemented, including blower replacement/configuration adjustment, intermittent aeration for low-load plants, pump replacement for efficiency improvement, mixer frequency conversion and timing operation, and central control linkage of recirculation flow with water quality parameters. All optimization works were completed by the end of 2021.

4.3 Performance Results

4.3.1 Energy and Chemical Consumption Reduction

After one year of stable operation, comprehensive performance comparison shows:

• Total annual electricity consumption decreases by 12.7%, from 38.377 million kWh to 33.485 million kWh
• Total phosphorus removal agent consumption decreases by 16.5%, from 4491.1 t/a to 3751.9 t/a
• Total carbon source consumption decreases by 60.9%, from 5145.6 t/a to 2013.4 t/a
• Comprehensive annual cost saving reaches 9.492 million yuan, including 3.278 million yuan in electricity costs, 0.577 million yuan in phosphorus removal agents and 5.638 million yuan in carbon source costs

4.3.2 Indirect Carbon Emission Mitigation

Calculated according to Technical Specification for Low-Carbon Operation Evaluation of Wastewater Treatment Plants:

• Electricity-related indirect CO₂ emissions decrease from 3424.0 t/a to 2987.5 t/a, a reduction of 12.7%
• Chemical-related indirect CO₂ emissions decrease from 5084.9 t/a to 2488.3 t/a, a reduction of 51.1%

Carbon source optimization contributes the largest share of emission reduction, as carbon source production has a high carbon emission factor and its consumption can be significantly reduced through operational optimization.

4.3.3 Low-Carbon Operation Rating Improvement

After optimization, 4 plants with low-strength municipal influent were upgraded to Level 3 low-carbon operation rating (60 ≤ score < 75). The remaining 4 plants also achieved score increases of 19.0%–34.5%, but failed to reach the minimum Level 3 rating due to the following constraints:

• High total nitrogen removal rate leading to high N₂O direct emissions
• High proportion of industrial wastewater leading to high energy consumption intensity
• Multiple internal lifting points or electric low-temperature sludge drying leading to high indirect power emissions

5. Engineering Application Insights

5.1 Applicable Scenarios

The proposed low-carbon operation strategies are most suitable for the following types of WWTPs:

• Municipal-dominated WWTPs with influent concentration lower than design value
• In-service plants with over-designed equipment and obvious over-aeration or over-lifting phenomena
• Plants aiming to reduce operating costs and carbon emissions without major process reconstruction
• Regional water utility groups carrying out batch energy efficiency improvement and benchmarking management

5.2 Key Design Considerations for Retrofit Projects

• Minimal structural modification: Most optimization measures can be implemented through electrical control transformation and operation adjustment, without large-scale civil works
• Instrumentation foundation: Reliable online monitoring instruments for DO, nitrate, flow and liquid level are the prerequisite for precise control optimization
• Gradual implementation: Adopt phased optimization from easy to difficult, starting from operation parameter adjustment, then equipment frequency conversion transformation, and finally intelligent system upgrading
• Effluent quality priority: All optimization measures must be premised on stable effluent compliance, and safety margins should be reserved for load fluctuation

5.3 Operation and Maintenance Best Practices

• Establish a dedicated energy management team to conduct monthly energy consumption statistics and quarterly efficiency analysis
• Regularly calibrate online monitoring instruments to ensure the accuracy of sensing data
• Formulate differentiated operation strategies for different seasons and working conditions, and conduct regular drill verification
• Include energy efficiency indicators in the performance assessment of operation teams to establish a long-term incentive mechanism for low-carbon operation

5.4 Advanced Carbon Neutrality Technology Roadmap

For plants with high carbon emission intensity that cannot meet low-carbon requirements through operation optimization alone, the following deep carbon reduction technologies are recommended:

• Source-side carbon reduction: Adopt autotrophic nitrogen removal (anammox), denitrifying phosphorus removal and other new processes to fundamentally reduce aeration energy and carbon source demand
• Energy recovery: Implement anaerobic digestion of sludge with combined heat and power generation, and sewage source heat pump technology to recover latent heat from effluent, realizing energy self-sufficiency or even net energy output
• Emission control: Adopt N₂O and CH₄ emission reduction and recovery technologies to reduce direct greenhouse gas emissions
• Carbon offset: Develop carbon trading benefits from renewable energy utilization and emission reduction projects

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we believe that refined intelligent operation is the most cost-effective path to achieve low-carbon operation for existing WWTPs, delivering both environmental benefits and economic returns for plant owners.

Our integrated intelligent low-carbon operation platform combines real-time multi-parameter sensing, machine learning-based load prediction, and automatic optimized control of aeration, lifting and dosing systems, which can further reduce comprehensive energy consumption by 8–15% on the basis of manual optimization. We also provide professional energy efficiency benchmarking and diagnostic services for regional water utility groups, helping clients identify energy-saving potential and formulate phased low-carbon retrofit roadmaps. For plants pursuing deeper carbon neutrality goals, our modular energy recovery solutions including sludge anaerobic digestion and sewage source heat pump systems can help achieve energy self-sufficiency and even net zero carbon operation, supporting the transformation of wastewater treatment plants from pure consumption facilities to resource recovery centers.

7. Conclusion

Systematic low-carbon operation optimization is a feasible and cost-effective approach for in-service municipal WWTPs to reduce emissions and increase efficiency, with the following core conclusions:

1. Lifting, aeration, mixing and recirculation systems are the core energy consumption units of WWTPs. Optimized management of flow, head and operating intensity can achieve significant energy saving effects without major process modification.
2. Full-scale application across 8 plants verifies that the proposed strategies reduce total electricity consumption by 12.7% and chemical-related indirect carbon emissions by 51.1%, with annual comprehensive cost savings of nearly 9.5 million yuan, achieving both environmental and economic benefits.
3. Low-strength municipal WWTPs can achieve significant low-carbon rating improvement through operation optimization, while plants with high total nitrogen load, high industrial wastewater proportion or high-energy-consuming equipment require further process upgrading and energy recovery technologies to achieve deeper carbon reduction.
4. Intelligent sensing combined with dynamic benchmarking management can ensure continuous low-carbon operation under fluctuating influent conditions, and is the foundation for digital and low-carbon transformation of wastewater treatment facilities.

FAQ

Q1: Will low-carbon operation optimization affect the effluent quality of WWTPs?
All optimization strategies are implemented on the premise of stable effluent compliance. By precisely matching process input with actual pollutant load, the system avoids both insufficient treatment and excessive waste. The case study shows that 100% effluent compliance rate is maintained after optimization, and some indicators even become more stable.

Q2: What is the typical investment payback period for low-carbon retrofit of existing WWTPs?
For most municipal WWTPs, simple optimization through parameter adjustment and operation management optimization requires almost no additional investment and takes effect immediately. For retrofits including frequency conversion, instrument upgrade and intelligent control system, the typical static payback period is 2–4 years, which is highly economical.

Q3: Why does chemical consumption contribute more to carbon emission reduction than electricity consumption in the case?
Carbon source and coagulant have higher embedded carbon emission factors per unit mass than electricity. In particular, carbon source dosage can be reduced by more than 60% through optimizing dissolved oxygen control and denitrification efficiency, so its contribution to indirect carbon emission reduction is more significant than electricity saving.

Q4: Can all WWTPs achieve Level 3 or higher low-carbon operation rating only through operation optimization?
No. Operation optimization mainly reduces indirect carbon emissions. For plants with high total nitrogen removal rate leading to high N₂O direct emissions, or plants with high industrial proportion and high treatment difficulty, core process upgrading and source reduction measures are required to further reduce total carbon emission intensity and achieve higher low-carbon ratings.