Simultaneous nitrification and denitrification using stored substrate (PHB) as the electron donor in an SBR

Understanding how microbial communities manage nitrogen in wastewater is a key challenge in environmental engineering. Traditional methods rely on separate nitrification and denitrification processes, which require distinct oxygen conditions. However, recent studies have explored the possibility of combining these steps in a single reactor. While acetate is commonly used as a carbon source, it is rapidly consumed, making it unsuitable for simultaneous processes. This gap motivated researchers to investigate alternative substrates that could allow both nitrification and denitrification to occur under the same conditions. Prior research has shown that poly-beta-hydroxybutyrate (PHB) can be stored in microbial cells and slowly released. That uncertainty drove the need to determine if PHB could serve as a stable electron donor for both processes. No prior work had resolved whether PHB degradation could match the timing of ammonium oxidation. This study aimed to clarify the role of PHB in enabling simultaneous nitrification and denitrification (SND). The researchers also wanted to understand how dissolved oxygen (DO) levels influence SND efficiency and rate. Their findings could help optimize wastewater treatment systems by reducing energy and chemical inputs.

The goal of this study was to assess whether poly-beta-hydroxybutyrate (PHB) could act as an effective electron donor for simultaneous nitrification and denitrification (SND). The researchers focused on a sequencing batch reactor (SBR) system using a mixed microbial culture. They wanted to determine if PHB's slower oxidation rate compared to acetate could allow both nitrification and denitrification to occur in the same reactor. A key question was whether the timing of PHB degradation could align with ammonium oxidation to support SND. The study also aimed to explore how dissolved oxygen (DO) levels affect SND efficiency and rate. The researchers were motivated by the need to reduce the energy and chemical demands of wastewater treatment processes. By using a stored substrate like PHB, they hoped to create a more sustainable and efficient treatment system. The investigation was designed to provide insights into microbial metabolism under controlled oxygen conditions.

The study used a 2-liter sequencing batch reactor (SBR) with a mixed microbial culture. Acetate was provided as the organic substrate during the feast period. The reactor was operated in cycles with alternating feast and famine phases. During the feast phase, acetate was available, and heterotrophic respiration was high. In the famine phase, acetate was depleted, and the focus shifted to PHB degradation. Dissolved oxygen (DO) levels were manipulated to observe their effects on SND. The researchers measured the oxidation rates of PHB and acetate to compare their electron donor potential. They also tracked the specific oxygen uptake rate (SOUR) and the accumulation of nitrate over time. The study monitored the percentage of nitrogen removed via SND at different DO concentrations. Electron flux analysis was used to determine the timing and intensity of SND activity within the reactor.

PHB degradation was significantly slower than acetate oxidation, with a rate up to six times lower. This slower rate allowed nitrification to occur once acetate was depleted. The oxidation rate of PHB matched that of ammonium, making it suitable for SND. At a DO concentration of 0.5 mg/L, up to 78% of nitrogen was removed via SND. However, this high efficiency came at the cost of a two-fold slower nitrogen removal rate. A moderate DO level of 1 mg/L achieved 61% SND efficiency with a rate of 4.4 mmol N per Cmol per hour. The highest SND activity occurred during the first hour of the aerobic famine period. This period coincided with the highest specific oxygen uptake rate (SOUR). The researchers observed that higher SOUR reduced oxygen penetration into microbial flocs, promoting anoxic zones for denitrification. PHB degradation followed first-order kinetics, with a rate constant of -0.19 per unit time. These findings suggest that higher PHB concentrations could enhance SND performance.

The study found that PHB can serve as an effective electron donor for simultaneous nitrification and denitrification (SND). The slow degradation rate of PHB allowed nitrification to proceed without interference from heterotrophic bacteria. The timing of PHB oxidation matched that of ammonium, supporting SND throughout the aerobic period. At low DO concentrations (0.5 mg/L), SND efficiency reached 78%, but the removal rate was slower. A moderate DO level of 1 mg/L provided a balance between efficiency (61%) and rate (4.4 mmol N per Cmol per hour). The highest SND activity occurred during the first hour of the famine period when the specific oxygen uptake rate (SOUR) was highest. This increased SOUR reduced oxygen penetration into microbial flocs, creating anoxic zones for denitrification. PHB degradation followed first-order kinetics, suggesting that higher concentrations could improve SND performance. The researchers propose that process control techniques to increase internal PHB concentrations could enhance SND efficiency. These findings support the use of stored substrates like PHB in wastewater treatment systems.