Part 7: Impact of Dissolved Oxygen Reduction on Nitrogen Removal Efficiency and Microbial Community in a Large-Scale Activated Sludge Process** ...
Published on by Hossein Ataei Far, Deputy Manager of the Research, Technology Development, and Industry Relations Center at NWWEC
### **DO Control and Aeration Strategies Implemented (2011-2016)** 🌬️
**Overview:**
This section summarizes the changes in dissolved oxygen (DO) control and aeration strategies at the Changi Water Reclamation Plant between January 2011 and July 2016. The study is divided into two phases:
- **Phase I:** January 2011 - March 2014
- **Phase II:** April 2014 - July 2016
**Phase I: Higher DO Concentration**
- **DO Levels:** 🌟 Consistently maintained at 1.7 (±0.5) mg O₂/L.
- **Daily Air Supply:** 660,814 m³/day.
- **Air/Primary Effluent Ratio:** 3.4 (±0.5) m³/m³.
- **Performance:** ⚙️ The system operated stably with no significant changes in performance.
**Phase II: Reduced Aeration Air Supply**
- **DO Levels:** 💡 Reduced to 1.0 (±0.4) mg O₂/L.
- **Daily Air Supply:** Reduced to 605,130 m³/day.
- **Impacts:**
- **COD Mass Loading:** 📉 Decreased by 6%.
- **Total Nitrogen (TN) and Ammonium (NH₄-N) Loading:** 📈 Increased by 9%.
- **Conditions:** 🌡️ Temperature, pH, alkalinity, and sludge retention time (SRT) remained stable.
**Primary Cause of Changes:**
- **DO Reduction:** 🧪 The main driver of changes in process performance and microbial dynamics.
**Impact on Microbial Community and Nitrogen Removal Efficiency:**
- **Microbial Shifts:**
- **AOB (Ammonia-Oxidizing Bacteria):** 🦠 Decreased significantly.
- **NOB (Nitrite-Oxidizing Bacteria):** 🔬 Increased in activity.
- **Performance Impact:**
- **AOB/NOB Activity Ratio:** 📉 Decreased.
- **Nitrite Accumulation:** 📉 Reduced.
- **Anammox Activity:** 📉 Declined.
- **Nitrogen Removal Type:** 🔄 Shifted from autotrophic to heterotrophic.
**Overview:**
This case study examines the impact of dissolved oxygen (DO) reduction on the partial nitritation/anammox (PN/A) process and excess biological phosphorus removal at the Changi Water Reclamation Plant in Singapore. It focuses on the 200,000 m³/d step-feed activated sludge process (Train 2) from 2011 to 2016, evaluating process performance and microbial community dynamics.
#### **Key Findings:**
- **Dissolved Oxygen (DO) Reduction:**
- **DO Levels:** 📉 Dropped from 1.7 to 1.0 mg O₂/L.
- **Effluent Concentrations:**
- **Ammonium and Nitrate:** 🌟 Increased.
- **Nitrite:** ❌ Decreased.
- **Total Inorganic Nitrogen (TIN):** 📈 Increased by 2.4 mg N/L.
- **Nitrogen Removal Shifts:**
- **Autotrophic Removal:** 🌿 Higher under high DO conditions.
- **Heterotrophic Removal:** 🔄 Increased under low DO conditions.
- **Microbial Community Changes:**
- **Ammonia-Oxidizing Bacteria (AOB):** 🦠 Reduced by 30 times.
- **Nitrite-Oxidizing Bacteria (NOB):** 🔬 Increased; shift from Nitrobacter to Nitrospira.
- **AOB/NOB Activity Ratio:** 📉 Dropped from 12.8 to 1.6.
- **Nitrite Accumulation Ratio:** 📉 Reduced from 76% to 29%.
- **Anammox Bacteria Activity:** 📉 Decreased.
**Conclusion:**
The reduction in DO levels from higher to lower values resulted in significant shifts in both microbial communities and process performance. Specifically, lower DO levels decreased ammonia-oxidizing bacteria (AOB) while increasing nitrite-oxidizing bacteria (NOB), impacting nitrogen removal efficiency and microbial activity. Effective management of DO levels is essential for maintaining optimal performance and balance in activated sludge processes. 🌟
DO reduction significantly impacts microbial communities and PN/A process performance, affecting nitrogen conversion and highlighting implications for biofilm and hybrid PN/A processes.
**References:**
1. Cao, Y., et al. (2018). The influence of dissolved oxygen on partial nitritation/anammox performance and microbial community at the Changi Water Reclamation Plant (2011 to 2016).
Figure 1: The volumetric ratio of air supply to primary effluent flow (m3/m3) and the DO (mg O2/L) in the aerobic zones of Train 2.