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Nutrient Removal - Denitrification (nitrate/nitrite removal)

2/16/2015

Often I have seen people confuse nitrification and denitrification during conversation. In the last post, I covered nitrification which is the conversion of ammonium (ammonia nitrogen) into nitrate via a two-step biological pathway. The organisms in the nitrification process are most commonly Nitrosomonas sp. and Nitrobacter sp. (There are other microbes, but they occupy the same ecological niche and have similar growth characteristics). Key points are the nitrification microbes are:

Need for high levels of dissolved oxgyen
Consumption of alkalinity
Relatively slow growth
Susceptibility to numerous toxic chemicals

Once ammonia is converted to nitrate, it is good to remove the nitrate from the water. In receiving streams, nitrate can promote algae blooms. Nitrate is removed under anoxic conditions (dissolved oxygen needs to be at effectively 0). Many waste water bacteria when faced without free oxygen in the water can use nitrate/nitrite as an alternative electron acceptor. This just means that the bacteria use the oxygen from the nitrate/nitrite.

When it comes to nitrate/nitrite removal the following conditions are needed:

Effective 0 dissolved oxygen - anoxic conditions
A source of soluble "food" ~ often called BOD. Note the solubility factor, as using nitrate as electron acceptor does not allow for rapid metabolism on complex or insoluble compounds such as fats, oils, or long chain hydrocarbons.

In wastewater treatment we usually find denitrification setup involving an initial anoxic step at the front of the system where return sludge rich in nitrate mixes with influent high BOD waste. The bacteria use the oxygen in the nitrate, producing nitrogen gas that escapes to the atmosphere.

Denitrification can also be a problem when there is excessive nitrate in the secondary clarifier. If the solids are held in the clarifier too long (usually >3 hours) - the bacteria can use the nitrate to degrade adsorbed organics. The sludge then floats to the top of the clarifier and can carry over the weir. In this case the most often used solution is to increase recycle rates which lowers residence time in the clarifier.

Biological Nutrient Removal - Nitrification

2/8/2015

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

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.

Biological Nutrient Removal - Part 1

2/3/2015

I am going to start a series on biological nutrient removal – in this case we are referring to ammonia, nitrate/nitrite, and phosphorous. Each component of nutrient removal requires differing microbes and environmental conditions. To allow for each to occur in a wastewater system requires a combination of engineering and operational techniques. In the following series of posts, I will go through the basic biological concepts and how we can influence microbial activity & treatment efficiency.

Biological Nutrient Removal (BNR) reduces the amount of nitrogen and phosphorous discharged from water treatment plants. BNR is usually subdivided into:

·         Nitrification – biological conversion of ammonia nitrogen into less harmful nitrate nitrogen.

·         Denitrification – conversion of nitrate/nitrite nitrogen into nitrogen gas ~ this actually removes the nitrogen from the water.

·         Biological Phosphorus Removal – certain microbial cells uptake phosphate in excess. These cells with high phosphate stored are wasted into biosolids, thereby removing phosphate from the water discharge.

Nutrient Removal - Denitrification (nitrate/nitrite removal)

Biological Nutrient Removal - Nitrification

Biological Nutrient Removal - Part 1

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