The nitrogen cycle from organic nitrogen to ammonium to nitrite and then nitrate is important for many systems on earth. Humans participate in the nitrogen cycle in many ways, although fertilizer production is no doubt one of the more influential ways. In addition to manufactured sources of nitrogen used in fertilizer human waste also has a high degree of organic nitrogen. Removing this nitrogen from human waste is an important part of wastewater treatment systems. A particular kind of waste treatment system called a Living Machine takes advantage of the nitrogen cycle when processing waste.
Influent enters the systems anaerobic settling tanks and here human waste with organic nitrogen is converted into ammonium, through ammonification. Water then flows into the closed aerobic tanks where nitrification takes place. Microbes facilitate both the anaerobic and aerobic steps in converting the nitrogen. Nitrate now moves through the system into the artificial wetland where nitrate is removed through denitrification, producing nitrogen gas. Artificial wetlands need to be anaerobic with high enough levels of carbon for this process to occur.
If not enough nitrogen is being removed through the artificial wetland, it has been suggested that the effluent could be moved through the settling tanks again, causing denitrification due to the anaerobic environment. However, since denitrification produces nitrogen gas the settling aspect of the first tank (which decreases smell and helps with the ammonification process) could be messed up. Adding nitrate to the beginning of the system actually causes there to be more downstream nitrate. The first tank moves water to the second tank through a pipe at the top of the water level. Because of the nitrogen gas lifting up more nitrogen rich material in the first tank and the design of the system, more organic nitrogen is shifted downstream.
Thus the living machine way of processing wastewater ingeniously takes advantage of the nitrogen cycle, but also is an example of how the process can be used poorly. Dynamic systems modeled off of natural processes can more naturally clean wastewater, but a thorough understanding of the various feedback loops is required before effectively using these processes.