Based on what little information is available it is pretty difficult to give you definitive advice. However here is a couple of things that might be worth investigating and may be able to be done easily.

1) Feeding raw sewage to your first anoxic zone thereby increasing the amount of available substrate for denitrification

2) Increasing your recycle rate (you mention you have already done this.) If you are dragging in a lot of high DO ML reducing the recycle rate may also improve things particularly if done in conjunction with other measures.

3) Increasing your SRT by reducing your wasting and increasing your MLSS.This potentially increases the amount of denitrifiers available

4) Reducing the DO slightly in your aerobic zones , so that  mixed liquor entering the  anoxic zones becomes anoxic earlier in the sequenc hence giving more effective anoxic volume.

Most of these changes will take at least a few days to take effect, so i would suggest you change one thing and leave it a week or so to see how the trends look.

It might also be worth looking at the level of nitrates leaving each of the aerobic and anoxic zones to get an idea of where nitrification and denitrification is happening and how much.

Regards

TerryF

First you'll want to assure that you have the mixed liquor environment working for you- assuming you have two anoxic zones- one up front and one later in the process, check that DO levels are low enough to assure anoxic activity (less than 0.5 mg/l) in the anoxic zones and that DO levels in the pass you're drawing recycle from are not too high (say in 2.0 to 3.0 mg/l level).  As Terry points out recycling too much mixed liquor with high DO concentrations to the anoxic zone can reduce it's efficiency and you'll want to optimize recycle rate with that in mind (in theory more is better but not if it retards anoxic activity).  Getting DO levels just right is the first step in optimizing BNR and you can't spend too much time getting it right (be aware optimizing on back shifts when loads are less also needs to be factored in).  Similarly, DO control can be more difficult in the winter than in the summer due to higher DO solubility and transfer rates and lower nitrification, denitrification and respiration rates. 

If you have DO environment problems just geting those right may solve your problem for you.  Response times to DO adjustments are on the order of hours not days so you should see results quickly.  First look at reducing air as needed to get your DO's just right, then balance recycle rates to get as much recyle as possible without carring excess DO into the anoxic zone.  This is done by watching anoxic zone DO with different recycle rates balancing to good effect.  Also looking to see what nitrate levels are through the anoxic zone (are you taking up all the recycle nitrate you recycle there, and if so more recycle won't help). 

Once you've addressed DO (and you want to be very disciplined and thorough in doing that) then you can turn to looking at either lowering MLSS by operating at shorter SRT's and/or reducing tankage on line.  I've had sucess in stimulating respiration rates in the anoxic zones by lowering SRT and MLSS.  This will push the nitrification reaction further into the aeration tank and possibly carry a bit more demand into the second anoxic zone for you.  If the system is underloaded and respiration rates are very low going into the second anoxic zone then not much will happen there.  Response times to mixed liquor adjustments tend to be slower, especially in cold weather, on the order of days and weeks, so it's best to look to environment first, then sludge wasting and mixed liquor inventories.

Food for thought for your initial efforts.  Hope it is helpful.

Wiff Peterson

Wiff Peterson

When you say you don't think the 2nd anoxic is working well , what do you mean and what tests are you doing to show this?

Regards

TerryF

Unless there is something i don't understand about your particular plant , theoretically the nitrate return concentration (which you say is about 3mg/l) should be pretty close to the same as that entering the 2nd anoxic. After all it is the same mixed liquor , or it should be.

The second anoxic zone by its very nature and position in the system achieves perhaps no more than 50% nitrate reduction , depending upon HDT, MLSS , SRT temperature and NO3 levels. Therefore i would suggest that you check what is going in to the 2nd anoxic and what is coming out. Likewise what is your influent TKN and COD and what is the TN level you are trying to achieve.

Regards

TerryF

Agree that it is very helpful to run profile of nitrate concentrations in the each anoxic zone, and of ammonia concentrations in each aerobic zone and use this to get a sense of how each is operating and the degree of stress in each.  Of course results will vary with mixed liquor temperature so it is good to run these profiles monthly, or when you're in an intense tuning phase weekly.  Good to check respiration rates at head of each stage from time to time as this gives good feeling for mixed liquor activity and loadings.

Probably the problem with the second anoxic zone is low oxygen demand.  If you check respiration rate at the head of the anoxic zone (you can grab a sample and aerate it to check RR) you'll probably find that the high SRT and low loadings on the system are such that there's not much BOD left in this zone to drive denitrification so you'll experience very low RR (not much higher than return sludge respiration rate).  Reducing SRT will detune the system with respect to ammonia and BOD a bit and increase the opportunity to drive a bit more BOD further into the process (i.e. a bit more BOD driven to the second anoxic zone) and hopefully increase RR in both anoxic zones to drive more denitrification.  However, the nitrification reaction will also push further into the process.  When ammonia concentrations start to bump up higher than you're comfortable with in the last aerobic stage then you've reached the limit in how much you can lower SRT to help with denitrification.  Keep in mind that if you get to an uncomfortable point on ammonia when this occurs in spring you can anticipate that help will be on the way in the form of higher nitrification rates as temperatures continue to climb during spring months.

Wiff Peterson

The nitrates are lower because of the dilution of the internal recycle rate.  Prior to that, they are more or less equal.  Sorry for the confusion.

Rob