344 Organic waste recycling: technology and management
the oxygen used in nitrification must be added to that used to metabolize
soluble/colloidal BOD. The rate of nitrification also depends on water
temperature, and is very slow at temperatures below 10 °C.
Denitrification is an anoxic metabolic pathway used by specific bacterial
genera in aquatic environments characterized as having little or no dissolved
oxygen, an adequate supply of carbon for cell synthesis and neutral pH. These
criteria are met readily in the bottom sediments and detrital layer of aquatic
systems thus leading to rapid denitrification. The rate of denitrification depends
on:
- the metabolic activity of the bacteria, i.e., environmental factors
such as organic carbon availability, pH, and wastewater
temperature; - the effective surface area of the bottom sediments, and
- the potential for the produced N 2 to escape to the atmosphere rather
than be fixed in the overlying water of vegetation.
Nitrogen uptake by plants and its subsequent removal by harvesting plants is
another mechanism of nitrogen removal. The N-uptake is quite high in plants
grown in primary sewage effluent, but is very low for plants cultured in
secondary sewage effluent. This is due to the following reasons:
- Low concentration of the inorganic N in the influent.
- Plant-available N in the secondary sewage effluent is present in
NO 3 - (nitrate) form compared with NH 4 + in the primary sewage
effluent. A significant portion of NO 3 - in the secondary sewage
effluent is lost through denitrification (Reddy and Sutton 1984).
Thus, harvesting plants, especially those grown in secondary sewage
effluents, remove little nitrogen directly. However, the indirect effects of plant
harvesting on oxygen transfer rates and the quantity of bacterial support
structure in the aerobic zone may affect the rates of bacterial nitrification and
denitrification significantly.
Phosphorus removal
Phosphorus removal mechanisms in aquatic systems are plant uptake and
several chemical adsorption and precipitation reactions (occurring primarily at
the sediment/water column interface).