The Biochemistry of Enhanced Phosphorus Removal by Activated Sludge
Authors: Heymann J. B. 
Published: May 2018

The design and operation of an activated sludge plant which aims to achieve maximum phosphorus removal should be based on sound biochemical principles. This poster presents the biochemical reactions believed to be responsible for observed variations in the concentration of different phosphorus species in such plants.

Excess phosphorus removed in the aerated stage of plant configurations such as the Phoredox process (i.e. more than is necessary for sludge metabolism) is accumulated in the sludge in the form of polyphosphate. The most likely major function of this compound is to supply energy for active membrane transport of specific carbon sources into the cells in the anaerobic stage, acetate being one such compound. The energy for this process is derived from the hydrolysis of polyphosphate to orthophosphate which is then released into the medium around the cells. Five possible mechanisms are proposed to describe this process.

From the microbial viewpoint the main advantage of active membrane transport is the high rate at which compounds transported becomes available inside the cell. In the absence of a terminal electron acceptor such as oxygen or nitrate, carbon compounds absorbed cannot be oxidized. Accumulation as such would inhibit further absorption and may lead to disturbance of intracellular conditions. Absorbed material is therefore converted to a reserve to be stored for later use. Indications are that sewage feed composition favours the formation of one reserve compound: poly-β-hydroxybutyrate (PHB).

In the presence of oxygen, the cell is able to oxidise PHB at a high rate relative to other reserves such as glycogen and triglycerides. The energy produced in this way is far in excess of what the cell needs for normal metabolism and results in an ‘over-production' of adenosine triphosphate (ATP) far in excess of that needed for biosynthesis. In its role as energy carrier, ATP formed due to the oxidation of PHB cannot be accumulated. Instead, to control the ATP:ADP ratio (ADP = adenosine diphosphate) in the cell, the energy is transferred from ATP to polyphosphate by polyphosphate kinase. When the sludge is recirculated to the anaerobic stage, some of this polyphosphate is used for active membrane transport.

The success of excess phosphorus removal by activated sludge revolves firstly around the degree of PHB formation in the anaerobic stage. Any chemical or physical agent inhibiting this process would reduce the extent of phosphorus removal. The second important factor is the extent of ATP ‘over-production' which is mainly determined by biosynthetic activities. An increase in biosynthesis of structural cell components would lead to a higher rate of ATP utilization for energy and thus decrease polyphosphate formation. These are the central concepts on which enhanced phosphorus removal in activated sludge plants should be based.

It seems that phosphorus removal in the Phoredox process represents an interplay between structural cell biosynthesis and reserve material synthesis. The organic compounds in the sewage feed play a decisive role in determining the set of processes which is going to occur. It is hypothesized that compounds needed in the biosynthesis of cell material would result in the so-called ‘normal' phosphorus removal, while substances with high energy content and lower biosynthetic value would favour ‘excess' phosphorus removal. The building blocks of cell material usually contain high amounts of nitrogen (such as proteins and nucleic acids) while the compounds usually associated with high energy content contain very little nitrogen (such as carbohydrates and lipids). This concept is reflected in the importance and value attached to the COD:TKN ratio, which roughly approximates the ratio of non-nitrogen containing compounds to nitrogen-rich compounds. In view of the above-mentioned ideas, this parameter can be refined to the following proposed ratio:

In the formulation of these concepts, conventional biochemical principles were applied to old and new observations in a relatively new field. However, very little quantitative information regarding the validity of these hypotheses are available and care should be taken not to see these as the final answer.