Dear All,
We are operating a food industry treatment plant based on an aeration tank (V= 1,000 m3) and settlement tank. To acommodate the increasing loads to;
BOD = 720 kg/d
TN = 70 kg/d
TP = 90 kg/d
I would like to know what retention time should we have in the new anoxic tank and what rate should be the recycling activated sludge for this levels of nitrogen?
The flow into the treatment plant is 600 m3/d.
Thank in advance for your attention.  

The target effluent levels of nitrogen are normally used to calculate the recycle rate.  Roughly speaking a recycle rate of 100% will remove 50% of nitrogen.  300% will remove 75%.

Cheers

A settling tank was set up & sludge was reused as to provide a balanced microorganisms population for the treatment of POME & methane gas production. The effect of sludge recycling rate where possible rate was researched by implementing Natural and organic Launching Charges at different debris recycling where possible rates.

Sharin Martos,
Wastewater math:
Q = inflow
R = recycle rate
TN (inf) = influent total nitrogen

Assuming plant has the denitrifying capacity then the removal is given by the following formula:

Final TN = TN (inf) x Q /(Q + R)

Substituting:
Q = 100%
R = 300%

Final TN = TN (inf) x 100 /(100 + 300) = TN (inf) x 1/4 = TN (inf) x 25%

Therefore removal is 75%.

Q.E.D.

It depends on a range of factors and is an ideal figure.

Things that will affect it:
- significant variations in influent TN
- temperature changes
- changes in MLSS
- consistency of load on process
etc.

It generally applies to the operation of the process and relies on a fairly consistent feed quality.

Cheers

The MLSS represents the mass of solids (that house the bacterial mass) that lives in the aeration tank.  The MLSS is varied depending on the temperature and load applied.


The incoming sewage (food) is balanced with the mass of bugs in the aeration tank (MLSS) to ensure proper treatment.

The overall removal efficiencies of organic & nitrogen & features of membrane fouling in the combined system treating the synthetic wastewater such as high attention of organics & nutritional value were examined. As a result, nitrogen removal efficiency was increased to 67% when the inner recycle rate was 300% of influent flow rate.

In a WWTP that has an anoxic zone ahead of the aeration zone the recycle rate largely determines the removal efficiency of nitrogen.

The nitrate at the end of the aerobic zone is theoretically equal to the ammonia level arriving at the head of the aerobic zone.  The concentration of ammonia there is diluted by the recycle rate.  The higher the recycle rate, the lower the ammonia concentration, therefore the lower the nitrate in the effluent.

If you draw a process flow diagram and do a mass flow calculation you'll see the recycle rate directly controls the removal of N.

There are factors that affect the performance.  The anoxic zone has to be correctly sized.  There can be an extra reduction in N due to simultaneous nitrification/denitrification.

I have used the formula and field testing to confirm N removal at several plants.  It works.

Here is a simple example:
Influent NH3 = 50mg/L
Recycle rate = 300%
Ammonia entering anoxic zone = 50mg/L
Ammonia entering aerobic zone = (50 x Q)/(Q+R)
= (50 x 100%) /(100% +300%) = 12.5mg/L

Assume 12.5mg/L ammonia converts to 12.5mg/L nitrate.

Actually there will be a few mg/L of nitrogen converted to new cells and thus the effluent nitrate will be slightly lower.

Hope that helps.

The detention time in an anoxic zone is about 2 to 3 hours (based on Q).

The denitrification reactions occur quite quickly.

The ammonia leaving the anoxic zone (thus entering the aerobic zone) is converted to nitrite, then nitrate, as it moves along the aerobic zone.

That is briefly what the nitrification process is.