Biological Nutrient Removal - Nitrification

Nitrification is a very delicate process. Several parameters must be certain in order to oxidize NH3.  They include the following:

• Typically in the field 70-80% organic reduction should have occurred prior to trying nitrification.  Usually COD levels at effluent should be 100-150 mg/L while BOD5 < 40 mg/L.

• The majority of organics must be degraded since the biomass contains 93-97% heterotrophs and 3-7% autotrophs.  Thus, if high carbon content is present, the heterotrophic organisms will out-compete the slower growing autotrophic nitrifying bacteria for essential nutrients.

• Rule of Thumb:  As the BOD:TKN ratio decreases, nitrification kinetics increase.

• Dissolved Oxygen (DO) is vitally important.  Although critical DO (in vitro) is 0.2 mg/L, field DO should never drop below 0.5 mg/L.  Optimal range is 2.0 mg/L for no inhibition whatsoever. 

• The oxygen required to oxidize 1 gram of NH3-N to NO2-N is approximately 3.5 grams O2.  From NO2-N to NO3-N, it is 1 gram of O2 per gram of NO2-N.

Step 1

2NH4+ + 3O2    -->      2NO2 + 4H+ + 2H2O + energy                  
                        (Nitrosomonas sp)

Step 2

2NO2 + O2    -->    2NO3 + energy                           
                       (Nitrobacter spp)

• Temperature ranges for nitrification vary from ideal temperature of 30-36OC to a total range of 10-38OC.  No nitrification will occur below 5OC or above 45OC.  Severe inhibition will occur below 10OC and above 38OC.

• pH is also important to the oxidation of nitrogen.  Optimal pH is about 7.5 – 7.7 with an operating range of 6-9. The rate of nitrification becomes inhibited at pH values lower than 6-6.7 and at higher pH's of 8.5 and above.

• Alkalinity or the ability to buffer a system is extremely important as the oxidation of ammonia utilizes 7.1 mg of alkalinity measured at CaCO3 /mg NH3-N oxidized.  Thus highly buffered systems are necessary for efficient nitrification.

• Several compounds/substances directly inhibit nitrification by being toxic to nitrifiers.  The following interfere with nitrification:

                    Heavy metals (Cu, Co, Pb, etc.)
                    Cyanides/Cyanates
                    Phenols
                    Mercaptans
                    Thiourea
                    Aniline
                    Certain Halogenated Compounds

• UV radiation (sun light) has also been found to inhibit nitrification.  However, wastewater turbidity usually eliminates this threat.

Troubleshooting Checklist

When trouble arises in nitrification, examining each requirement will help to systematically eliminate parameters until the problem is identified.  The following lists optimal conditions for nitrification in three wastewater treatment systems.

Troubleshooting checklist - system requirements for nitrification in industrial WWT.

Activated Sludge

pH                                                     6.5-8.0, 7.0-7.5, optimum for MLSS

Temperature                                      10 - 38OC, 30OC optimum

Effluent BOD5                                    < 30 mg/L

Effluent COD                                     <100-150 mg/L

Effluent TOC                                      <45 mg/L

MLSS                                                  2,500 mg/L

Sludge Age (MCRT)                             5 - 15 days

DO                                                      2.0 mg/L, 2.0 - 4.0 in systems subject to shock         

Aerated Lagoons

pH                                                       6.5-8.0, 7.0.-7.5 optimum for MLSS

Temperature                                       10 - 38OC, 30 OC optimum

Effluent BOD5                                    < 30 mg/L

Effluent COD                                     <100-150 mg/L

Effluent TOC                                      <45 mg/L

MLSS                                                  100 mg/L

MCRT = HRT                                      15 days, optimum 40-60 days

DO                                                      2.0 mg/L, 2.0-4.0 in systems subject to shock

Rotating Biological Contractors

pH                                                     6.5-8.0, 7.0.- 7.5 optimum for MLSS

Temperature                                      10 - 38 OC, 30 OC optimum

Effluent BOD5                                    < 30 mg/L

Effluent COD                                     <100-150 mg/L

Effluent TOC                                      <45 mg/L

DO                                                      2.0 mg/L, 2.0-4.0 in systems subject to shock


Nitrification

Summary

Critical Requirements

Step 1

2NH4+ + 3O2   -->     2NO2 + 4H+ + 2H2O + energy                  
                        (Nitrosomonas sp)

Step 2

2NO2 + O2   -->   2NO3 + energy                           
                        (Nitrobacter spp)

Step 1 is the rate limiting step. Once achieving nitrite (NO2), nitrate formation is very rapid.

Nitrosomonas and Nitrobacter are both autotrophic aerobes with relatively slow generation time.

Autotrophs require only CO2 and simple forms of inorganic nitrogen for cell synthesis as opposed to heterotrophs which require complex forms of carbon and nitrogen for cell synthesis.

Dissolved Oxygen Requirements

• The oxygen required to oxidize 1 part of NH4-N to NO2-N is 3.0-3.5 parts of O2.
• The oxygen required to oxidize 1 part of NO2-N to NO3-N is 1.0 to 1.3 part of O2.
• DO levels should be 2.0 mg/L.
• DO levels below 1.5 mg/L will have a negative influence on nitrification and some inhibition will occur.
  
  

Temperature

• Optimum range 28 - 360C
• Nitrification may not occur below 50C or above 450C
• Deleterious effects are observed above 370C
  
  

pH

• The rate of nitrification begins to be severely inhibited at a pH range of 6.3 to 6.7
• The rate of nitrification is assumed to be constant in the range of 7.2-8.0
  
  

Alkalinity

• The oxidation of ammonia destroys alkalinity by virtue of the release of hydrogen ions.
• Approximately 7.1 mg of alkalinity as CaCO3 is destroyed per mg NH3-N oxidized.
• A highly buffed system is necessary to ensure process control.
  
  

Substrate Loading

• A theoretical threshold of 0.45 to 0.55 mg of ammonia applied per kg of MLVSS exists for nitrification to occur.
  
  

Inhibition

• Nitrifiers are extremely sensitive organisms that can be inhibited by very small concentrations of toxic substances.

• Free Ammonia inhibits Nitrosomonas at a concentration of 10 ppm to 150  ppm and Nitrobacter at a concentration of 0.1 ppm to 1.0 ppm. (This is why pH is important!)
  

• Free Nitrous Acid inhibits nitrification at concentrations of 0.2 ppm to 2.8 ppm.