Dissolved Oxygen

Alkalinity (pH)

Wastewater Temperature

Nitrogenous Food

Detention Time

MCRT, F/M, or Sludge Age

Toxic Materials

Dissolved Oxygen-

Nitrification has a substantial oxygen requirement. 4.5 lb O2 is required per lb NH4+-N to be converted. Aeration Tank D.O. usually maintained 1.0 to 4.0 mg/L .

Nitrification rates are rapidly depressed as the pH is reduced below 7.0. pH levels of 7.5 to 8.5 are considered optimal. Typical refineries run at a pH of 8-9 with no problems. 7.14 lbs of M-alkalinity are destroyed per lb of ammonia-nitrogen oxidized.

Temperature

Nitrification is inhibited at lower wastewater temperatures in wastewater treatment plants. Up to five times as much detention time may be needed in the winter versus the summer months since the activity drops significantly. During winter months, increasing MLVSS, MCRT will help as well as lowering wasting. A desired range is 60° to 95° degrees F. Below 40 nitrification will probably not occur.

BOD5/TKN Ratio -The fraction of nitrifying organisms decreases as this ratio increases. Many times influent TKN is not measured. Refineries and Chemical plants often have amines in the influent and are used to measuring this and accounting this in the total nitrogen balance. Municipals are not used to doing this, but probably should if they have any industrial pretreatment adding to their influent, or if they have a ton of food plants or restaurants adding high loading to their plant. Many times the new cleaning and sanitation compounds have quaternary amines and biocides as a component of the cleaning and sanitizing compounds. These can add a significant loading to the plant. Especially since it takes time to break down the amines first, before the ammonia is released and can be recognized or utilized by the nitrifiers.

The time required for nitrification is directly proportional to the amount of nitrifiers present. Because the rate of oxidation of ammonia is essentially linear, short-circuiting must be prevented. Minimum detention time is approx. 4 hours at 22 ° to 24°C. Many plants run very old sludge ages. Be careful of this. A sludge age that is too old will have problems with sudden changes in the influent and will have a higher problem with toxicity issues than a medium to old sludge. Let as much of the ammonia removal happen by the carbonaceous bacteria first, and use your nitrifiers as a final polishing only.

When reviewing the performance of your activated sludge process for the selection of an optimum F/M ratio, MCRT, oxygen requirements, etc. , the requirements of the nitrification microorganisms must be taken into account in addition to that of the heterotrophic (carbon degrading) bacteria.

Many compounds can be toxic to nitrifiers- Nitrosomonas and Nitrobacter. Cooling tower biocides have many amines also present in them as well as zinc that can be toxic to the nitrifiers. Check your MSDS sheet on the chemicals used onsite.

Soluble Carbonaceous BOD > 25 mg/l in final effluent

High pH (both Nitrosomonas and Nitrobacter are inhibited by un-ionized ammonia (NH3). Since Nitrosomonas is more sensitive than Nitrobacter, the result may be a high level of NO2- in the final effluent.

Heavy Metals 3 mg/L to 23 mg/L

-silver, mercury, nickel, chromium, copper, zinc

Halogenated Solvents 0 mg/L

Phenol and Cresol <20 mg/L

Cyanides <20 mg/L

**Nitrite nitrogen concentrations greater than 20 mg/l can be inhibitory

towards Nitrosomonas.

Salinity does not appear to inhibit the nitrification process in wastewater treatment plants.

Chlorides do not appear to be toxic unless very high. Typical chlorides concentrations were in the range 1000-2000mg/l. Tests with concentrations as high as 4000mg/l did not show a measurable inhibition of the nitrification process.

So what is a Total Nitrogen Balance?
Since nitrification is very "mathematical" compared to carbonaceous bacteria, it is easier to do the math with nitrifiers and measure what is going on in the wastewater treatment system.

Example from a customer site:
The wastewater influent has ammonia levels of 25 ppm and BOD of 300. Theoretically, 15 parts of ammonia will be consumed by the carbonaceous bacteria and 10 ppm will be left for the nitrifiers. Wait, the ammonia levels in the effluent were coming out at 30 ppm. How is that possible? He assumed his nitrifiers were dead and were not working. The customer never checked for amines in the influent or measured nitrates in the final effluent. There were amines present in the wastewater influent after supplementary testing. After measuring TKN on the influent and nitrates and nitrites on the effluent, the missing N was accounted for.

What should be measured?
Influent TKN, nitrates, Nitrites and ammonia at least occasionally to get a good picture if any are present. If there are amines present, TKN and ammonia should be measured daily. Hach makes Test N Tube reagents that can be used with your COD reactor to measure Total Nitrogen. There are other companies out there also that make it now very easy to measure these items in your own lab.

Final clarifier effluent- measure at least occasionally TKN and nitrites, daily measure ammonia and nitrates. The N not used by the carbonaceous bacteria as a nutrient source will only be converted by the nitrifiers and will still be in the system in one form or another as ammonia if not converted, or nitrate or nitrite if utilized by the nitrifiers.

OK, so the math does not add up. One thing people do not pay attention is the supernatant from the sludge digestor or belt press. Usually, when the carbonaceous bacteria go into endogenous respiration, they have started to run out of food. Some of the bacteria die off and re-release some of the nutrients. Two of the largest costs at a wastewater treatment plant are electricity and solids handling. Electricity can be high for mechanical pumps and aerators, especially if nitrifying.

It takes more time and electricity for mechanical energy for sludge digestion and nitrification processes. Balancing the cost of solids handling, vs. how much ammonia is returned from the supernatant can be critical. If the cost of solids handling is cheaper than electricity and nitrification, cut back a bit on your sludge digestion. Measure the ammonia present in the supernatant. If it is very high, you might be shooting yourself in the foot, (quite often seen at municipalities). The amount of savings on solids cost more in the long run to nitrify at a wastewater treatment plant.

Another thing where the math might not add up is if solids are held too long in the clarifier and gassing or denitrification is occurring. When the carbonaceous bacteria run out of oxygen- the will look for another energy source, first nitrates, then sulfates for O2. Gassing and floating sludge are signs in a clarifier that oxygen is short and the solids are being held too long in the clarifier or sludge digestion tank. Adjust the bed level on your clarifier and make sure that if aerobic sludge digestion is occurring, that there is proper mixing and pH adjustment. Septic supernatant can also impact nitrifiers.

If the nitrifiers run out of air in the clarifier, this can also inhibit their growth, since many of the nitrifiers are returned to the front of the system in the RAS. Watching your bed in the clarifier is critical to nitrifiers.