Why and how to treat municipal sewage?

3.1. Biological transformation of pollution

Encyclopedia of the Environment - Sewage - Organic pollution transformation process

Figure 4. Conversion process of organic contamination in the presence of dissolved oxygen or nitrate ions in the bioreactor. [©Joseph De Laat]

Biodegradable organic matter (consisting of proteins, fats, and carbohydrates) is used as food by microorganisms called heterotrophic bacteria because they use organic carbon as a source of carbon for their development and reproduction (anabolic) and for their energy requirements (catabolism) (Figure 4). The production of new cells also requires the participation of ammonia nitrogen and phosphate, and their concentrations in untreated sewage can greatly meet the demand. In the presence of dissolved oxygen or nitrates, organic matter in sewage is converted to carbon dioxide and biological matter in roughly equal proportions.

Encyclopedia of the Environment - Sewage - Nitrogen pollution transformation process

FIG. 5. Transformation pathways of nitrogen pollution in wastewater treatment plants. [©Joseph De Laat]

The main conversion reactions of nitrogen pollution in wastewater treatment plants are ammoniation, assimilation, nitrification and denitrification

Ammoniation (reaction 1) converts organic nitrogen (mainly contained in sewage urea) to ammonia nitrogen. This reaction is quick; Many types of microbes can perform this reaction:

Urea [CO(NH2)2]→ Ammonia [NH3]+ Carbon dioxide [CO2]

Assimilation (reaction 2) is when ammonia nitrogen is assimilated by bacteria to form new organic nitrogen biomolecules for the synthesis of new bacteria.

Biological nitrification (reactions 4a and 4b) converts ammonia nitrogen (ammonium, NH4+) to nitrite nitrogen (nitrite, NO2 -) by aminooxidating bacteria, which then converts it to nitrate nitrogen (nitrate, NO3 -) by nitrifying bacteria:

Ammonium [NH4+]→ Nitrite [NO2 -]→ nitrate [NO3 -]

These reactions occur only under aerobic conditions and are carried out by microorganisms called autotrophic bacteria because they use inorganic carbon (CO2 or HCO3 -) as a carbon source to synthesize new bacteria.

Biological denitrification (reaction 5) reduces the nitrate ion (NO3 -) to nitrogen (N2). In sewage treatment plants, denitrification can only occur in the absence of oxygen. Denitrification is done by heterotrophic bacteria and requires the consumption of organic matter. Take methanol (a small organic molecule that is easily biodegradable) as an organic matter in the denitrification process, which is accompanied by the elimination of organic pollution (being oxidized to CO2) :

Nitrate [NO3 -]+ methanol [CH3OH]→ Nitrogen [N2]+ Water [H2O]+ carbon dioxide [CO2]

As with the removal of ammonia nitrogen, the growth of biomass during nitrification is accompanied by the partial removal of phosphorus through assimilation (synthesis of phosphorus into new biomolecules).

In order to further biological phosphorus removal, it is necessary to alternate the biological material through anaerobic and aerobic stages, resulting in the formation of phosphorus-removing bacteria called phosphorus-accumulating bacteria, which have the property of excessive accumulation of phosphorus in their cells. Phosphorus can account for 10%-12% of the dry weight of phosphorus removing bacteria, while the proportion of phosphorus in the dry weight of non-phosphorus removing bacteria is 1%-2%.

In wastewater treatment plants, biological phosphorus removal only removes about 40-60% of phosphorus. In order to meet the discharge standards (see Table 1), it is also necessary to supplement physical and chemical phosphorus removal, that is, to add iron salt (usually ferric chloride, FeCl3) by forming iron phosphate precipitation removal.