Introduction

Whole Systems Approach to Ecological Design

Appropriate Technology: Water

Organic Agriculture and Local Food

Appropriate Technology: Energy

Green Building & Retrofitting
Appropriate Technology: Water
Whole Systems Approach to Water Constructed Wetlands Treatment Rainwater Harvesting Lagoon and Tank Based Treatment

Nitrification

Nitrification is the process by which nitrifying bacteria convert ammonia to nitrate.  The step happens in two distinct steps.  Nitrosomonas sp.oxidizes the ammonia to the intermediate product, nitrite.  Nitrobacter sp. converts the nitrite to nitrate.  Nitrifying bacteria are considered autotrophic bacteria, or bacteria that utilize CO2 as their carbon source for growth.

The nitrifying bacteria are environmentally sensitive organisms.  A variety of environmental factors can inhibit their growth.  These inhibiting factors include high ammonia concentrations, low temperatures, pH outside of the 6.5 to 8.6 range and low dissolved oxygen (<1 mg/l).  The pH of the system can change dramatically due to the nitrification process.  The alkalinity in the wastewater is consumed at a rate of 7.14 mg of alkalinity, as CaCO3, for every milligram of ammonia oxidized.  If there is not a sufficient alkalinity concentration in the wastewater, the pH will be depressed as the ammonia is oxidized.  Alkalinity addition is not typically necessary for domestic sewage treatment and is not expected to pose a problem for a typical ecovillage treatment system.

The rate of nitrification is also significantly affected by the fraction of nitrifiers present in the wastewater.  When the concentration of biodegradable organics, measured as BOD5, is high, the heterotrophic bacteria, or bacteria that use organic carbon as a carbon source, dominate the bacterial population.  Typically when the BOD5 is reduced below 80 mg/l the population of nitrifying bacteria is large enough to begin the nitrification process.  For this reason, nitrification happens in the last stages of treatment.  For systems requiring enhanced ammonia treatment, consideration of recycling internally to a gravel mound is excellent for nitrification, being primarily a simple aerobic trickling filter reactor towards the end of the treatment train.  Some nitrification will also take place in the constructed wetland.