Important Considerations For Enhanced Biological Phosphorus Removal (EBPR)

“The more information received, the less assumptions to be made for design”. [3]

What are the fundamentals of Enhanced Biological Phosphorus Removal (EBPR)?

In a previous post, the following slide was created from [1] which sums up the EBPR process:

Can the ‘Enhanced’ or ‘Excess’ bit of BPR be explained with a simplistic example?

In the Biological Phosphorus (P) Removal system, i.e. the anaerobic zone, both the Ordinary Heterotrophic Organisms (OHO, which do not remove P in excess) and the PAOs coexist; the larger the proportion of PAOs that can be encouraged to grow in the system (ie. no oxygen and no nitrate environment), the greater the percentage P content of the activated sludge and, the larger the amount that can be removed during sludge wasting from the aerobic zone [2].

The relative proportion of the two organism groups (i.e OHOs and PAOs) within the anaerobic zone depend on the fraction of the influent wastewater readily biodegradable Chemical Oxygen Demand (rbCOD) that each organism group obtains. rbCOD is an indicator of Voltaile Fatty Acids (VFA) or soluble COD that can ferment to VFA [3].

To summarise, the non-negotiable for accomplishing Enhanced BPR are:

1. Truly anaerobic conditions which means no oxygen, no nitrates, and no sulphates (any input of oxygen will lead to rbCOD consumption by OHOs instead of PAOs). Higher nitrates will necessitate a larger anaerobic zone fraction.

2. Alternating anaerobic and anoxic / aerobic conditions.

3. Sufficient carbon type (PAOs specifically want rbCOD. Within rbCOD, this refers to VFA. More specifically, this means acetate).

4. Sufficient Mixed Liquor Suspended Solids (MLSS) mass to maintain adequate PAO population.

5. Mixing intensity should be such that MLSS don’t settle within the anaerobic zone and air is not entrained that can lead to a thriving OHO population [2].

What’s the role of readily biodegradable COD within EBPRs?

A simple overall total COD fraction has been shown below with the key sub fractions that impact BPR:

There are two types of readily biodegradable COD (fbs):

1. Complex (fermentable)

2. Short chain VFAs

VFAs are essential for EBPR to occur. Typically, they can be found in WRRFs:

(1)  Within the influent. Acetate which are a form of VFA are typically present in the range of 5 – 15 ppm [5]. VFAs can be fermented within the collection system that bring the wastewater to the Water Resource Recovery Facility (WRRF). But the fermentation can be impacted by the residence time within the collection system, gradient, cold temperatures, wet weather flows etc.  

(2)  Generated within the anaerobic zone through fermentation of influent rbCOD

But in general, the above two sources may not be enough to drive EBPR. Therefore, it will need to be supplemented with:

(3)  Hydrolysis of slowly biodegradable particulate COD (fbp)

(4)  Decay of active biomass (supplied via the mixed liquor recycle) which is a slow process and therefore the VFA availability is slow for utilisation by PAO [4].

To make use of (3) and (4), providing sufficient mixed liquor concentration (to increase hydrolysis rates) and sufficient time in the anaerobic zone are key design considerations.

NOTE: The readily biodegradable portion is measured by flocculated and filtered COD (ffCOD).

The filtered COD is measured after the sample has been filtered through a 0.45 µm filter and represents both very fine colloidal and soluble material. The colloidal material can be removed by flocculating the sample before filtering it. ffCOD represents only the soluble readily biodegradable and truly soluble unbiodegradable COD [6].

What role does process modelling play in design of EBPR configurations?

After the wastewater fractionation has been carried out satisfactorily, the next step is to carry out process modelling as part of the design.

Simulators (eg: Biowin, Sumo, GPS-X etc) are software that execute the process models containing biological, physicochemical, separation processes and process control schemes. Over the past many years, they have become a standard within the water industry as they have accumulated various process mechanisms in one location (BOD oxidation, nitrification and denitrification, shortcut nitrogen removal, chemical and biological P removal etc).

This helps us to understand how these different processes are interacting amongst each other. These models are accurate about sludge production, oxygen demand, mass balance etc. They also help with evaluating the impacts of the diurnal flow pattern that every WRRF is subjected to. At existing full scale WRRFs, an orthophosphate profiling can be done along the basin length or in multiple basins. The data from this exercise can be plugged into the simulators to evaluate PAO population establishment in the anaerobic zone, P release or uptake etc [2].

For EBPRs, simulators can be used to check for the following:

• Adequacy of anaerobic zone sizing

• Impacts of recycle streams and rates on the zone performance

• Evaluate effects of placing the anaerobic zone in the mainstream or sidestream as part of the design

The most important data for any process model is the detailed understanding of the influent wastewater concentrations and their fractionations. The following parameters are useful while using process models for EBPR [3]:

Despite the salient features and the help we can get from process models, it is also worthwhile remembering that process models wouldn’t provide the answers to the following:

• Is the mixing within anaerobic zone sufficient?

• Is the basin geometry ideal?

• Will there be any back mixing (from anoxic or aerobic zone) to the anaerobic zone?

• Are the WRRF operators sufficiently trained to handle the process?

• Will there be growth of filamentous organisms and odour generation? [3]

What should not and should be the basis of sizing anaerobic zones within the bioreactor?

When the design Solids Retention Time (SRT) is high, it usually translates to an increase in bioreactor volume and CAPEX. To combat this, the tendency is to reduce the unaerated mass fraction, but this affects the Biological Nutrient Removal (BNR) system performance.

Sizing the zone directly based on Hydraulic Retention Time (HRT) of 90 – 120 minutes is also not the right approach. That’s because, if the anaerobic zone is too small, then the fermentation of readily biodegradable COD (rbCOD), a key design parameter, may not be complete.

Of the total mass fraction in a bioreactor (anaerobic + anoxic + aerobic), it is recommended that a minimum 0.12 – 0.15 is provided for the mainstream anaerobic zone. This is because if the influent has limited rbCOD, a larger zone will help to promote hydrolysis of slowly biodegradable COD. The VFA production from this reaction will help promote PAO growth.

Based on [7], the following anaerobic mass fraction is recommended:

 What are some of the numbers and ratios to keep in mind while designing EBPR facilities?

A high-quality substrate means better BPR. The following table is a good starting point for design:

Presence of anaerobic conditions can be confirmed via Oxidation Reduction Potential (ORP) measurement. In general:

• For mainstream BPR, Oxidation Reduction Potential (ORP) value: <-100mV

• For sidestream BPR, Oxidation Reduction Potential (ORP) value: <-300mV [2]

As per [8]:

• 1 ppm of nitrate (NO3-N) will take up rbCOD required for the removal of 0.7 ppm of P by supporting denitrification

• 1 ppm of Dissolved Oxygen (DO) will use up the rbCOD for the removal of 0.3 ppm of P by supporting carbon oxidation by OHO

If the adopted EBPR configuration has been designed and operated in the right manner, it is possible to achieve TP levels of 0.3 – 0.5ppm without chemical dosing. For TP < 1ppm and below, for EBPR facilities, it is recommended to have chemical dosing backup (if the license is based on daily limits, then chemical dosing is required but for annual average limit, dosing backup may not be required) [3].

Though a thumb rule of 5 – 10 W / m3 of energy input is commonly cited for mixing, the actual power requirement is dependent on the size and shape of the anaerobic zone. Mixer manufacturers must be consulted for the sizing [8].

What are some of the challenges and benefits of EBPR?

As compared to chemical removal, EBPR is more complex. The process performance can be affected due to wet weather flows, sudden temperature changes, salinity intrusion etc. If the WRRF has a combination of EBPR and anaerobic digesters (for biosolids), there is potential for increased struvite formation and challenges with biosolids dewatering performance.

If there is good Total Nitrogen (TN) removal within the WRRF, it leads to better biological TP removal too. With EBPR, Sludge Volume Index (SVI) reduces which translates to better sludge settleability and lower N2O emissions [2].

The following table is a good reference [3] while evaluating chemical (C) vs biological (B) treatment to remove the TP:

Do PAO face competition (in addition to OHO) within the anaerobic zone?

Glycogen Accumulating Organisms (GAO) compete with PAO for VFA uptake in an anaerobic environment using energy stored in glycogen [6]. They do not make any contribution to BPR and compete for the available carbon source under the following conditions:

• Warmer temperatures (>28°C)

• High coagulant dosage

• Influent wastewater with low N content and pH < 7

• High glucose content in feed [2,6]

What are the references for the information provided in this article?