NITROGEN AND PHOSPHORUS TREATMENT

Denitrification – A Refresher

Urban wastewater contains nitrogen—mainly organic nitrogen and ammonia.
Biological nitrogen removal mobilizes a battery of processes that converge to convert a toxic compound (ammonia) into an inert gas (nitrogen dioxide).

The main steps are:

  • Ammonification
  • Assimilation
  • Nitrification
  • Denitrification

These different steps are driven by various species of bacteria that do not share the same types of metabolism, making it critical to ensure these processes are implemented separately in time and/or space.

The conditions required for nitrification (aerobic—presence of oxygen) then denitrification (anoxic—absence of oxygen) therefore have to be integrated into successive steps.

Phosphate Removal – A Refresher

The phosphorus load in wastewater is generally in the form of phosphates (PO4) — mainly orthophosphates and polyphosphates..

In-water nutrient phosphorus load is the primary rate-limiting factor of algal growth, and once in-water phosphorus climbs over a tipping point, it can trigger eutrophication (total asphyxiation of the waterbody).

Phosphorus uptake by plants as a phytoremediation solution would require excessively vast planted land area to meet the discharge quality objectives.
The wastewater engineering needed for microbial assimilation (dubbed ‘biological’ phosphate removal) would prove far too complex to implement, ruling it out as not an appropriate technology for constructed wetland process systems.

Studies on phosphorus treatment techniques adapted to reedbed-system processes can be collapsed into two main techniques:

  • The ‘passive’ technique: using a high-adsorptive/high-precipitative media.
  • The ‘dynamic’ technique: influent-stream treatment using salts of metals.