RESPONSABILIDAD DE LA DIRECCIÓN

For biological phosphorus removal, it is essential to have anaerobic and aerobic zones in the treatment line. The most convenient arrangements of both zones are discussed in Chapter 36.

The early explanations for the mechanism of biological phosphorus removal referred to the anaerobic zone as causing a condition of bacterial stress that would result in phosphorus being released in this zone. After that, high assimilation of the phosphorus available in the liquid medium would occur in the aerobic zone at a higher level than the normal metabolic requirements of the bacteria. When removing the excess biological sludge, bacteria with high phosphorus levels are also removed.

As more information has become available through intense research in this area in the past years, a mechanistic model has been developed, which includes fundamental biochemical aspects. It should be noted that, in spite of the great progresses made in this area, some knowledge gaps still need to be filled in. Several of the organisms involved are taxonomically unknown. The current mathematical models for biological phosphorus removal, such as the IWA models (IAWQ, 1995, and subsequent versions), are extremely complex and are still being tested in full-scale activated sludge plants. However, the merit exists as the increasing knowledge in the area has allowed better designs and operational control strategies to be developed.

This book deals with biological phosphorus removal in a simplified way. More recent and deeper information should be obtained from specific publications, since the theme has developed significantly.

Biological phosphorus removal is based on the following fundamental points (Sedlak, 1991, IAWQ, 1995):

• Certain bacteria are capable of accumulating excess amounts of phos-phorus in the form of polyphosphates. These microorganisms are named phosphorus accumulating organisms (PAOs). The bacteria most frequently mentioned as an important PAO is Acinetobacter.

• These bacteria are capable of removing simple fermentation substrates produced in the anaerobic zone and then assimilate them as products stored inside their cells.

• In the aerobic zone, energy is produced by the oxidation of these stored products. The storage of polyphosphates in the cell increases.

The anaerobic zone is considered a biological selector for the phosphorus ac-cumulating microorganisms. This zone has an advantage in competition terms for the phosphorus accumulating organisms, since they can assimilate the substrate in this zone before other microorganisms, which are not phosphorus accumulating organisms. Thus, this anaerobic zone allows the development or selection of a large

Figure 35.2. Variation of the soluble BOD and orthophosphate concentrations in the anaerobic and aerobic zones in an activated sludge system designed for biological phosphorus removal (adapted from EPA, 1987)

population of phosphorus accumulating organisms (PAOs) in the system, which absorb substantial amounts of phosphorus in the liquid medium. Phosphorus is then removed from the system with the excess sludge (Sedlak, 1991).

Figure 35.2 presents typical profiles of soluble BOD and orthophosphates in anaerobic and aerobic zones in an activated sludge system designed for phospho-rus removal. The concentration of soluble BOD decreases in the anaerobic zone, even if there are no aerobic or anoxic electron acceptors. In the anaerobic zone, while the soluble BOD concentration decreases, the soluble phosphorus concen-tration increases. Subsequently, in the aerobic zone, the phosphorus concenconcen-tration decreases, while the soluble BOD concentration continues in its decline.

The biological phosphorus removal mechanism is summarised in Figure 35.3 and is described in the following paragraphs (EPA, 1987b; Sedlak, 1991; Henze, 1996).

Alternation between anaerobic and aerobic conditions

• Alternation of conditions. The PAO require the alternation between anaero-bic and aeroanaero-bic conditions, to build their internal energy, organic molecules and polyphosphate storage components.

Anaerobic conditions

• Production of volatile fatty acids by facultative bacteria. Part of the biodegradable organic matter (soluble BOD) is converted, through fermen-tation processes in the raw sewage or in the anaerobic zone, into simple organic molecules of low molecular weight, such as volatile fatty acids.

This conversion is usually made by facultative organisms that normally oc-cur in the sewage and in the anaerobic zone. The volatile fatty acids become available in the liquid medium. There is not enough time for hydrolysis and the conversion of the particulate influent organic matter.

• Accumulation of the volatile fatty acids by the PAOs. The phosphate accu-mulating organisms give preference to these volatile fatty acids, which are

988 Activated sludge

Figure 35.3. Schematic mechanism of biological phosphorus removal

quickly assimilated and accumulated inside the cells. PAOs assimilate these fermentation products better than the other organisms usually occurring in the activated sludge process. As a consequence, there is a selection of the population of these phosphorus accumulating organisms in the anaerobic zone.

• Phosphate release. The release of phosphate that was previously accu-mulated by the organisms (in the aerobic stage) supplies energy for the transport of the substrate and for the formation and storage of organic metabolic products, such as PHB (polyhydroxybutyrate).

Aerobic conditions

• Consumption of the stored substrate and assimilation of phosphate. PHB is oxidised into carbon dioxide and water. The soluble phosphate is removed from the solution by the PAOs and is stored in their cells for generation of energy in the anaerobic phase.

• Production of new cells. Due to the use of substrate, the PAO population increases.

Phosphorus removal

• Phosphorus removal by the excess sludge. The phosphorus is incorporated in large amounts into the PAOs cells and is removed from the system through the removal of the biological excess sludge, which discards a fraction of the mixed liquor containing all the organisms in the activated sludge, including PAOs.