4.4 Microbial ecology in nutrient removal systems

Wastewater treatment relies on diverse microorganisms to remove nutrients. Bacteria like nitrifiers, denitrifiers, and phosphate accumulators play key roles in breaking down pollutants. Understanding these microbes is crucial for optimizing treatment processes.

Factors like substrate availability, redox conditions, and operational parameters shape microbial communities in treatment systems. Advanced molecular tools help identify and monitor these microbes. Balancing microbial interactions is essential for efficient nutrient removal.

Microbial Diversity and Roles

Microorganisms in nutrient removal

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• Nitrifying bacteria play a crucial role in converting ammonia to nitrate in a two-step process
  • Ammonia-oxidizing bacteria (AOB) such as Nitrosomonas and Nitrosospira oxidize ammonia ($NH_3$) to nitrite ($NO_2^-$)
  • Nitrite-oxidizing bacteria (NOB) like Nitrobacter and Nitrospira further oxidize nitrite ($NO_2^-$) to nitrate ($NO_3^-$)
• Denitrifying bacteria, primarily heterotrophic bacteria (Pseudomonas, Paracoccus), reduce nitrate ($NO_3^-$) to nitrogen gas ($N_2$) under anoxic conditions, completing the nitrogen removal process
• Phosphate accumulating organisms (PAOs) like Accumulibacter and Tetrasphaera contribute to phosphorus removal by storing phosphorus as polyphosphate under aerobic conditions and releasing it while taking up volatile fatty acids (VFAs) under anaerobic conditions
• Glycogen accumulating organisms (GAOs) such as Competibacter and Defluviicoccus compete with PAOs for VFAs under anaerobic conditions but do not contribute to phosphorus removal, potentially hindering the process efficiency

Factors Influencing Microbial Communities

Factors shaping microbial communities

• Substrate availability significantly influences microbial community structure and function
  • Carbon source type and concentration affect the growth of heterotrophic bacteria and the competition between PAOs and GAOs
  • Nitrogen and phosphorus concentrations impact the growth and activity of nitrifying and denitrifying bacteria
• Redox conditions in different zones of the treatment system selectively favor specific microbial groups
  • Aerobic zones promote the growth of nitrifying bacteria and PAOs
  • Anoxic zones support the activity of denitrifying bacteria
  • Anaerobic zones encourage phosphorus release by PAOs and VFA uptake
• Operational parameters such as solids retention time (SRT), hydraulic retention time (HRT), temperature, and pH shape the microbial community
  • Longer SRTs favor the growth of slow-growing nitrifying bacteria
  • HRT affects the contact time between microorganisms and substrates
  • Temperature influences microbial growth rates and enzyme activities
  • pH impacts the growth and activity of nitrifying bacteria and PAOs

Molecular Tools and Techniques

Molecular tools for microbial analysis

• 16S rRNA gene sequencing identifies and quantifies microbial populations based on their unique 16S rRNA gene sequences, providing insights into the diversity and relative abundance of microorganisms in the system
• Fluorescence in situ hybridization (FISH) uses fluorescently labeled oligonucleotide probes to visualize and quantify specific microbial populations, allowing for the spatial distribution analysis of microorganisms within the system
• Quantitative PCR (qPCR) quantifies the abundance of specific microbial populations based on the amplification of targeted genes, enabling the monitoring of changes in microbial community structure over time
• Metagenomics analyzes the collective genomes of microbial communities, revealing the functional potential and metabolic pathways of microorganisms in the system

Microbial Interactions and Process Performance

Microbial interactions in nutrient removal

• Competition between microbial groups can impact process performance
  1. PAOs and GAOs compete for VFAs under anaerobic conditions; excessive growth of GAOs can reduce phosphorus removal efficiency
  2. Nitrifiers and heterotrophs compete for oxygen and space in the biofilm or flocs; high organic loading can favor heterotroph growth and inhibit nitrification
• Symbiotic relationships among microbial groups contribute to efficient nutrient removal
  1. Nitrifiers produce nitrite and nitrate, which serve as substrates for denitrifiers; denitrifiers consume these compounds, preventing their accumulation and potential inhibition of nitrifiers
  2. Fermenters break down complex organic matter into VFAs utilized by PAOs; PAOs release phosphorus, which can be used by other microorganisms for growth
• Maintaining a balanced microbial community and optimizing operational parameters are essential for process stability and efficiency
  • Ensures efficient and stable nutrient removal while preventing the dominance of undesirable microorganisms (GAOs)
  • Monitoring and controlling operational parameters optimizes the growth and activity of key functional groups and minimizes the impact of disturbances on process performance