Phosphorus is a critical nutrient that, in excess, can lead to eutrophication in water bodies, causing harmful algal blooms and deteriorating water quality. Biological phosphorus removal (BPR) is an effective and sustainable method used in wastewater treatment plants to mitigate this issue. This process relies on the activity of specific microorganisms known as phosphorus-accumulating organisms (PAOs) that uptake and store phosphorus within their cells. Phosphorus accumulation is common in the bacteria. For BPR wastewater treatment, we want to cultivate organisms that store the highest levels of phosphate. Normal bacteria solids average 1 – 2% Phosphorus in dry weight solids where Bio-P organisms can increase Phosphorus content to 5 – 7%. In this blog post, we will explore how to favor these organisms, the role of organic acids in the anaerobic zone, the necessity of nitrate removal before the anaerobic zone, and the process of phosphorus uptake in the aerobic zone.
Favoring Organisms that Uptake Phosphorus
To enhance the efficiency of biological phosphorus removal, it is essential to create an environment that favors the growth and activity of PAOs. These organisms thrive in a sequence of anaerobic and aerobic conditions. In the anaerobic zone, PAOs release phosphorus and uptake volatile fatty acids (VFAs) as a carbon source. This process is crucial for their subsequent phosphorus uptake in the aerobic zone.
The Role of Organic Acids in the Anaerobic Zone
Organic acids, particularly VFAs, play a vital role in the anaerobic zone of BPR systems. These acids serve as a carbon source for PAOs, enabling them to store energy in the form of polyhydroxyalkanoates (PHAs). This stored energy is later used in the aerobic zone for phosphorus uptake. The presence of sufficient organic acids and time in a true anaerobic zone is essential for the successful proliferation of PAOs and the overall efficiency of the BPR process.
Nitrate Removal Before the Anaerobic Zone
For the anaerobic zone to function correctly, it is crucial to remove nitrates from the wastewater before it enters this zone. Nitrates can act as alternative electron acceptors, inhibiting the anaerobic conditions necessary for PAOs to release phosphorus and uptake organic acids. By ensuring that nitrates are removed, typically through an anoxic zone where denitrification occurs, the anaerobic zone can maintain the conditions required for PAOs have a competitive advantage over other common wastewater organisms.
Phosphorus Uptake in the Aerobic Zone
In the aerobic zone, PAOs utilize the energy stored in PHAs to uptake phosphorus from the wastewater. This process, known as “luxury uptake,” involves the incorporation of phosphorus into polyphosphate granules within the cells of PAOs. The efficiency of phosphorus uptake in the aerobic zone is influenced by factors such as dissolved oxygen levels and the availability of organic carbon. Maintaining optimal conditions in the aerobic zone is essential for maximizing phosphorus removal.
Common Challenges in Phosphorus Removal
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Maintain sufficient anaerobic zone conditions favorable to Bio-P organisms
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Organics acids & low D.O. lead to more filamentous organisms
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High TKN (ammonia) in the influent can make denitrification important to get low redox conditions in the anaerobic zone.
- GAO occupy the same ecological niche but do not store phosphate. Higher temperatures (>30OC) favor GAO.