• +1 800 982 4489
  • sales@watertechnologiescanada.com

Three-stage modified Bardenpho process for BNR

Three-stage modified Bardenpho process for BNR

The overall objective of wastewater treatment is to treat waterborne waste to a degree that it may be discharged at an environmentally acceptable level, prevent the pollution of surface waters and protect downstream users.

This is done by reducing the concentration of nutrients and trace chemicals, and by eliminating heavy metals that may be present in the waste stream. Disinfection of the clarified effluent (by UV for example) reduces the chances of disease by eradicating pathogens and viruses.

This three stage activated sludge process – in sequential anaerobic, anoxic, and aerobic zones – removes both nitrogen and phosphorus from the wastewater and achieves a high carbonaceous BOD removal.

The modification of having the ability to add a small portion of Primary Effluent (PE) to the anaerobic zone and the rest split between the anoxic zones is known as the Westbank Process.

Primary clarifier effluent enters the bioreactor and is diverted into the anaerobic and anoxic zones.

Primary effluent that enters the anaerobic zone mixes with the RAS from the secondary clarifier before flowing into the anaerobic zone. Primary effluent has a high BOD (biochemical oxygen demand) content and will readily take up oxygen in almost any form. This eliminates any nitrates that may be present in the RAS before entering the anaerobic zone – which must be free of oxygen.

The primary reason for returning activated sludge (RAS) to the anaerobic zone is to reintroduce microorganisms into the process. The RAS maintains the population in the bioreactor by returning mixed liquor (sludge containing microorganisms) that has settled to the bottom of the secondary clarifier.

The remaining flow enters the anoxic zones and is used as a carbon source for the microorganisms.

Suitable conditions such as food, dissolved oxygen and temperature allow the bacterium (mixed liquor) to reproduce and increase in concentration. To maintain a balanced population, some of the organisms are taken out of the process as waste activated sludge (WAS). The WAS pumps withdraw mixed liquor from the last aerobic zone and pump it to the DAF tank for thickening.

Anaerobic Zone

The anaerobic zone is the first zone in the bioreactor and ideally is devoid of oxygen and nitrates. The wastewater entering the anaerobic zone is:

  • High in BOD.
  • High in ammonia, with little to no nitrates present.
  • Nutrient rich with phosphorus.

The purpose of the anaerobic zone is to make the microorganisms release phosphorus stored in their cells. In this environment, the microorganisms are deprived of oxygen and nitrate and prompted to transport orthophosphorus across their cells while taking up substrate (VFA) from fermentation process.

The phosphorus release into the wastewater is crucial for the uptake of phosphorus in the aerobic zone. The phosphorus that is released by the microorganisms is 4 – 6 times the amount of phosphorus that is contained in the raw wastewater.

Anoxic Zone

As mentioned previously, a portion of the primary effluent flow is directed into the anoxic zones to provide an additional organic carbon source (food), since most of the carbon was taken up in the anaerobic zone.

The anoxic zone receives nitrates through the internal recycle of mixed liquor from the aerobic zone. This wastewater is high in ammonia, nitrates and phosphorus, while being low in dissolved oxygen (<0.10 mg/L DO). Mixers keep the mixed liquor in suspension.

Denitrification takes place in the anoxic zones. Denitrification is the reduction of nitrate to nitrogen gas. Pseudomonas and Bacillus are primarily responsible for the denitrification process.

The BOD is also reduced in the anoxic zones as a result of the oxidation of organic matter through anaerobic respiration.

The degree of nitrogen removal is controlled by the rate of the internal recycle flow.

Aerobic Zone

The flow from the anoxic zones flows hydraulically into the aeration zones. The air is dispersed to the aerobic zones through a grid of fine bubble diffusers. The movement of the bubbles from the diffusers also acts to keep the solids fully mixed.

The wastewater is high in phosphorus and ammonia and low in nitrates. Dissolved oxygen levels are kept greater than 1.0 mg/L DO, and vary according to plant operations (typically ranges from 1 to 2 mg/L DO).

The aerobic zones are responsible for two types of biological processes – luxury uptake of phosphorus and nitrification.

Luxury Phosphorus Uptake is the excessive uptake of phosphorus by Acinetobacter that takes in more phosphorus (PO) then is required for the metabolic activity of the cell.

This reaction takes up the surplus of phosphorus that was expelled in the anaerobic zone. The phosphorus is synthesized (combined) and stored as a poly-phosphate. The result is a phosphate rich sludge of which a portion is removed from the end of the aerobic zone.

Nitrification Processnitrifying bacteria are able to oxidize ammonia to nitrate (and other oxidized nitrogen compounds). This process is known as nitrification and takes place in an aerobic environment.

There are two subgroups of nitrifiers – the ammonia (NH3) oxidizers and the nitrite (N02-) oxidizers. Ammonia oxidizing bacteria (AOB), most commonly Nitrosomonas, convert ammonia to nitrite. This is a rate limiting step that relies on temperatures above 8°C to proceed.

The nitrite oxidizer that converts the nitrite to nitrate is Nitrobacter.

The rate at which nitrifiers grow is slow because of the way they obtain their energy, which is relatively inefficient. These chemolithotrophs must oxidize simple inorganic compounds, such as ammonia and nitrite, and used dissolved carbon dioxide to make sugars for cell material.

Water Technologies Canada Inc.

Leave a Reply