Elliot Lewis

Introduction

Enhanced Biological Phosphorus removal (EBPR) is the process whereby the biomass in an activated sludge plant is cultivated to absorb the most phosphorus possible. This transfers the phosphorus from the bulk liquid into the wasted sludge, thus removing it from the secondary effluent.

Mechanisms at Work

By selection of heterotrophic bacteria, polyphosphate-accumulating organisms (PAO) are proliferated within the biomass. This is a biological process where the biomass is trained to consume phosphorous. In the anaerobic zones, state of famine, all non-PAOs would be outcompeted by PAOs by having phosphorous energy stores (releasing the phosphorus back into the liquid). Then the PAOs are able to take back up the phosphorus in the feast environment, to prepare for anaerobic zones again. This is known as Luxury Phosphorus Uptake.

In aerobic zones, the PAOs will uptake phosphorus to form polyphosphate crystals within their cell mass. Typical PAOs have 5% to 7% phosphorus by mass within the cell mass and common garden variety biomass bacteria species have 0.5% to 1.0% (to form the DNA and cell membrane phospholipids), therefore the combined mix of well performing and well proliferated EBPR plants can absorb 3.5% to 4.5% within the total organic mass. The balance of wasted sludge to influent phosphorus then becomes the limiting factor for absorption.

By controlling the sludge age to as low as possible, the specific sludge yield can be increased, maximising the mass in which phosphorus can be absorbed within. Working this through, the following is derived,
$$ M_{\text{WAS}} = \frac{M_{\text{P}}}{f_{\text{P}_\text{WAS}}} = \frac{Q \cdot X_{\text{P}_0}}{0.05} $$
\( M_{\text{WAS}} \) = Mass of waste activated sludge (\( \text{t} \cdot \text{d}^{-1} \))
\( f_{\text{P}_\text{WAS}} \) = Fraction of P in WAS (typically 0.05)
\( Q \) = Flowrate of influent (\( \text{ML} \cdot \text{d}^{-1} \))
\( X_{\text{P}_0} \) = Concentration of phosphorus in the influent (\( \text{t} \cdot \text{ML}^{-1} \))

By linking this to the specific sludge yield \( Y \), which is typically \( 0.4 \text{ kgTSS/kgCOD} \).
$$ Y = \frac{M_{\text{WAS}}}{Q \cdot X_{\text{COD}_0}} $$
\( X_{\text{COD}_0} \) = Concentration of COD in the influent (\( \text{t} \cdot \text{ML}^{-1} \))

This gives,
$$ \frac{X_{\text{COD}_0}}{X_{\text{P}_0}} = \frac{1}{Y \cdot f_{\text{P}_\text{WAS}}} $$

This typically solves as the influent COD to Phosphorus ratio requiring a minimum of 50:1 (as \(Y = 0.4 \) and the concentration of P in WAS being 5%). Therefore, from this it is plain to see that when Y increases (for younger sludge ages) or when \( f_{\text{P}_\text{WAS}} \) increases, the amount of carbon required decreases and the hurdle is lowered for EBPR. Sensitivity for the mass of P in biomass from plant performance and sludge yield is very important when assessing the EBPR potential. As well, this can be re-produced for BOD (instead of COD) and for volatile solids (instead of total solids).

Phosphorus Release

Phosphorus Release is the process where in anaerobic conditions, the PAOs use their internal polyphosphate energy stores to survive the famine conditions of the anaerobic zone. This is the primary selection process to ensure that bacteria which absorb phosphorus have a competitive advantage over the rest, allowing for proliferation of PAOs within the heterotrophic bacteria.

Phosphorus release occurs within the anaerobic zone, this requires a complete absense of oxygen and nitrate, therefore, any compromised unaerated zones will prevent EBPR. Similarly, any anaerobic conditions before the secondary effluent will cause phosphorous release, undoing all of the benefit of EBPR.

Typically, an oxidation reduction potential (ORP) measurement is used to quantify the anaerobic conditions. First, complete denitrification is required between 0 to -50 mV, followed by anaerobic phosphorus release between -50 to -250 mV. Extremely deep anaerobic conditions (less than -250 mV) can cause acid formation (fermentation) and methane production, which is undesirable for EBPR.

Glycogen vs Phosphorus

Phosphorous Accumulating Organisms (PAOs) and Glycogen Accumulating Organisms (GAOs) are in direct competition for carbon in anaerobic-aerobic cycling. Both thrive in the same alternating environment and compete for the same food source: volatile fatty acids (VFAs). GAOs do not store phosphorus (as polyphosphate) and so do not contribute to luxury phosphorus uptake in the aeration zone. So, when GAOs out compete PAOs, the process failure is hard to correct as the re-proliferation of PAOs will take some time.

Slight process tweaks can provide environments which are marginally advantageous for PAOs.

• PAOs prefer lower temperatures
• PAOs prefer slightly higher pH in anaerobic zones (above 7.1 to 7.2) as this increases the energy cost of VFA transport for GAOs. Alkalinity dosing (up to 7.8) may aid in GAO supression.
• PAOs prefer acetate and propionate as VFA food sources. GAOs specifically struggle with propionate. Changing carbon sources can aid GAO supression, including primary sludge fermentation.
• GAOs are prone to inhibition in high nitrate conditions. Nitrate cycling (intermittent aeration) in a swing zone for prolonged nitrate exposure can aid in PAO competition.
• GAOs are slower growing and therefore benefit from longer sludge ages, so PAOs can outcompete in shorter SRT (towards 8 days).
• Sidestream EBPR (such as sidestream anaerobic fermentation tanks) can create deep anaerobic conditions that force absolute VFA consumption and starvation which PAOs tolerate better than GAOs.

Plant Configurations

There are many process configurations for EBPR. The primary driver for plant designs is the nitrogen. The carbon nitrogen ratio will determine how much carbon is available for phosphorus release and complete denitrification. If low effluent total nitrogen is required or there is high carbon feed, EBPR will be simple. If there is low carbon, EBPR will require more complicated and delicate processes.

Troubleshooting

If an EBPR plant isn’t perfoming as it should, follow these steps:

• Anaerobic P Release
  Measure the phosphorus at the end of the anaerobic zones. This should be 3 to 4 times higher than the influent. If there isn’t P Release, check the ORP of the anoxic and anaerobic zones.
• Mass Balance
  Is there enough WAS to get the P out? Check the COD:P (or BOD:P) ratio and wasting rates. If the carbon has dropped (or P has increased) then there may not be enough wasted sludge. Make sure the sludge age is nice and low.
• Denitrification
  If there’s too much nitrate going around, the ORP in the anaerobic zone might not be low enough. This can be too little carbon for denitrification or an inhibition event.
• RAS Denitrification
  The RAS in the bottom of the clarifiers could be anaerobic, releasing the bound phosphorus back into the liquid. Check the P in the pre-clarifier flow.
• P Stain Testing
  Neisser Staining (for Polyphosphate Globules) can examine the biomass to see if the PAOs are accumulating phosphorus. This can also check to see if there is the right proportion of bacteria within the biomass. General microscopy is also recommended to ensure that PAOs are outcompeting GAOs. Strains such as Candidatus Accumulibacter phosphatis and Candidatus Phosphoribacter (Tetrasphaera) are signs of healthy PAO populations.