4.2 Phosphorus removal processes

Phosphorus removal in wastewater treatment is crucial for preventing eutrophication in water bodies. Two main methods are used: biological and chemical removal. Biological removal uses special bacteria to store phosphorus, while chemical removal adds metal salts to form insoluble precipitates.

The biological method, called Enhanced Biological Phosphorus Removal (EBPR), relies on phosphorus accumulating organisms (PAOs). These bacteria store phosphorus under specific conditions. Chemical removal involves adding metal salts like ferric chloride or aluminum sulfate to create phosphate precipitates that can be filtered out.

Biological and Chemical Phosphorus Removal

Mechanisms of phosphorus removal

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• Biological phosphorus removal
  • Enhanced biological phosphorus removal (EBPR) process alternates between anaerobic and aerobic conditions to promote the growth of phosphorus accumulating organisms (PAOs)
    • During the anaerobic phase, PAOs uptake volatile fatty acids (VFAs) and store them as poly-β-hydroxyalkanoates (PHAs) while releasing phosphorus
    • In the aerobic phase, PAOs use the stored PHAs as an energy source to uptake and store large amounts of phosphorus as polyphosphate granules
  • Phosphorus is ultimately removed from the system through the waste activated sludge (WAS) process, which discards excess bacterial biomass containing the stored polyphosphate
• Chemical phosphorus removal
  • Involves the addition of metal salts, such as ferric chloride ($FeCl_3$) or aluminum sulfate (alum, $Al_2(SO_4)_3$), to the wastewater
    • Metal ions react with soluble phosphorus species to form insoluble metal phosphate precipitates (e.g., $FePO_4$, $AlPO_4$)
    • Precipitates are removed from the wastewater through sedimentation in clarifiers and subsequent filtration processes
  • Chemical phosphorus removal can be implemented as a standalone treatment or in conjunction with biological processes to enhance overall phosphorus removal efficiency

Microorganisms in EBPR

• Phosphorus accumulating organisms (PAOs)
  • Key microorganisms responsible for the enhanced biological phosphorus removal process
  • Possess the unique ability to store large amounts of polyphosphate within their cells, typically in the form of intracellular granules
  • During the anaerobic phase, PAOs uptake volatile fatty acids (VFAs) and store them as poly-β-hydroxyalkanoates (PHAs) while simultaneously releasing phosphorus
  • In the subsequent aerobic phase, PAOs utilize the stored PHAs as an energy source to uptake and store phosphorus, replenishing their polyphosphate reserves
• Glycogen accumulating organisms (GAOs)
  • Compete with PAOs for volatile fatty acids (VFAs) during the anaerobic phase of the EBPR process
  • Unlike PAOs, GAOs do not contribute to phosphorus removal as they lack the ability to store polyphosphate
  • The presence of GAOs in significant numbers can reduce the overall efficiency of the EBPR process by consuming VFAs that would otherwise be available for PAOs

Factors affecting EBPR performance

• Carbon source availability
  • Sufficient volatile fatty acids (VFAs) must be present during the anaerobic phase for PAOs to uptake and store them as poly-β-hydroxyalkanoates (PHAs)
  • A lack of readily biodegradable carbon sources (e.g., acetate, propionate) can limit the performance of the EBPR process by restricting PHA storage in PAOs
• Anaerobic and aerobic conditions
  • Strict anaerobic conditions must be maintained during the anaerobic phase to promote VFA uptake and PHA storage by PAOs
    • The presence of nitrate or oxygen can disrupt the anaerobic environment and hinder the selective advantage of PAOs
  • Adequate aerobic conditions are necessary during the aerobic phase for PAOs to efficiently uptake phosphorus and replenish their polyphosphate reserves
    • Insufficient aerobic retention time can limit the amount of phosphorus removed by PAOs
• Competing microbial populations
  • The presence of glycogen accumulating organisms (GAOs) can reduce the amount of VFAs available for PAOs, thereby limiting EBPR performance
  • Nitrate or oxygen intrusion during the anaerobic phase can favor the growth of denitrifying bacteria, which compete with PAOs for carbon sources and can diminish EBPR efficiency

Chemical vs biological phosphorus removal

• Metal salt addition
  • Common metal salts used for chemical phosphorus removal include ferric chloride ($FeCl_3$), aluminum sulfate (alum, $Al_2(SO_4)_3$), and calcium hydroxide (lime, $Ca(OH)_2$)
  • Metal salts can be added at various points in the wastewater treatment process, such as the primary clarifier, secondary clarifier, or as part of a tertiary treatment step
  • Factors affecting the performance of chemical phosphorus removal include pH, alkalinity, and the presence of competing ions (e.g., carbonate, sulfate)
• Integration with biological processes
  • Chemical precipitation can be employed as a standalone treatment or in combination with biological phosphorus removal processes like EBPR
  • Chemical addition to the secondary clarifier can provide additional phosphorus removal capacity and serve as a "backup" for EBPR during periods of reduced biological performance
  • Simultaneous precipitation involves adding metal salts directly to the aeration basin, allowing for concurrent biological and chemical phosphorus removal within the same reactor