## 3 Results and Discussion
### Analysis of Influencing Factors on Carbon Emission Intensity
#### 3.2.2 Operating Load Rate  
The relationship between operating load and carbon emission intensity is shown in Figure 6. As the operating load rate increases, the carbon emission intensity decreases, with a clear negative correlation between the two. Specifically, for STPs with a load rate below 40%, the carbon emission intensity is as high as 0.883 kgCO\({}_{2}\)-eq/m\({}^{3}\). For STPs with load rates in the range of 40% to 60% and 60% to 80%, the carbon emission intensity decreases to 0.407 kgCO\({}_{2}\)-eq/m\({}^{3}\) and 0.416 kgCO\({}_{2}\)-eq/m\({}^{3}\), respectively. When the load rate is higher than 100%, the carbon emission intensity of the STP reaches the minimum value of 0.326 kgCO\({}_{2}\)-eq/m\({}^{3}\). High-Emission Mechanisms During Low-Load Operation (<40% Design Capacity)  
(1) Reduced Equipment Efficiency and Elevated Energy Allocation  
a. \"Oversized Equipment\" Phenomenon: Aeration blowers, pumps, and other critical units maintain baseline power consumption even at low loads, resulting in significant energy waste. For instance, blowers operating at reduced frequencies exhibit substantially lower energy efficiency compared to rated conditions. b. Increased Proportional Fixed Energy Consumption: Lighting, control systems, and other fixed energy demands remain constant despite reduced influent volumes, thereby increasing energy allocation per unit of treated wastewater and indirectly elevating carbon intensity. (2) Impaired Microbial Activity and Treatment Performance  
a. Endogenous Respiration Phase: Prolonged exposure to low organic loads induces microbial \"starvation\", forcing microorganisms to utilize endogenous reserves. This metabolic shift necessitates higher aeration demands per unit pollutant degraded, prolongs hydraulic retention time (HRT), and exacerbates energy-related carbon emissions. b. Sludge Aging: Extended solids retention time (SRT) under chronic low-load conditions reduces microbial activity, requiring supplementary carbon sources or chemical coagulants to stabilize treatment processes, both of which contribute to additional carbon footprints. (3) Chemical Addition-Induced Carbon Emissions  
Figure 6: The relationship between operating load rate and carbon emission intensity and electricity consumption per ton of water. (**a**) Operating load rate vs. power consumption per ton of water. (**b**) Operating load rate vs. carbon emission intensity. External Carbon Sources: Supplemental carbon (e.g., sodium acetate) for denitrification processes entails production, transportation, and utilization-phase emissions. Foam/Sludge Bulking Control: Low-load operation often necessitates chemical addition to mitigate operational issues (e.g., foam formation, sludge bulking), further increasing process-related carbon emissions. Emission Reduction Mechanisms During Moderate-to-High Load Operation (40-100% Design Capacity)  
1. Optimized Equipment Performance: Key energy-consuming units (e.g., aeration systems, pumping systems) operate at peak efficiency near design capacity, minimizing specific energy consumption (kWh/m\({}^{3}\)). 2. Fixed Energy Cost Amortization: Fixed operational costs (e.g., lighting, automation) are distributed across larger influent volumes, reducing per-unit treatment emissions. 3. Enhanced Biological Treatment Efficiency a Microbial Activity Surge: High-load conditions stimulate exponential microbial growth, accelerating pollutant degradation rates, shortening HRT, and minimizing aeration requirements. b. Mitigated Anaerobic Zone Formation: Sufficient organic loading prevents localized anoxic/anaerobic conditions in aeration tanks, thereby suppressing methane (CH4) and nitrous oxide (N2O) emissions. 4. Reduced Sludge-Associated Emissions Lower Sludge Yield: High substrate availability enhances microbial assimilation efficiency, reducing excess sludge production. Improved Sludge Stability: Enhanced microbial activity minimizes supplementary sludge disposal requirements and associated treatment emissions.