Biological Nutrient Removal Basics II

 

The goal of this article is:

1.       To discuss the purpose of recycle in configurations other than SBRs.

2.       To review inhibiting BNR processes.

Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it! (Extraordinary Leadership: Thinking Systems, Making a Difference, 2009, Reberta M. Gilbert, M.D, pg 5)

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The last article discussed some basic BNR concepts as it relates to sequencing batch reactors. This article will cover how these concepts can be applied to configurations other than SBR’s. Figure 1 below illustrates the configuration of a 5-Stage Bardenpho Process.

FIGURE 1 The 5-Stage Bardenpho Process

Recycle

The primary difference between an SBR and more conventional configurations is the SBR is divided TIME while other treatment configurations are divided by SPACE. In other words, in facilities other than SBR’s, the anaerobic, aerobic and anoxic periods are processed in separate tanks. These tanks are fixed in size, which means the amount of time the wastewater spends in each tank is determined by the flow entering that tank (hydraulic detention time). The only ways the hydraulic detention time can be altered is by changes in the influent flow, or the addition of internal recycle. In the illustration above, there are two internal recycles: one to return nitrate-rich wastewater from the effluent of the aerated tank to the influent of the first anoxic tank. The second one returns the MLSS from the final clarifier to the head of the anaerobic tank. One recycle that is not shown that has become more common is a recycle line to return phosphate-rich wastewater from the aerobic tank back to the anaerobic tank for additional processing.

The recycle rates in conventional activated plants (plants not required to nitrify or remove any other nutrients) are typically in the range of 50% - 150% of the average daily flow. In BNR systems, the recycle rates can be as high as 600% (a six to one ratio) of the average daily flow. The primary purpose of the internal recycle is to return partially processed wastewater back to the previous tank for additional processing. The way recycle does this is by increasing the hydraulic retention time in that specific portion of the process. To express it in another way, using the illustration in figure 1 above as the example, when flow is returned from the effluent end of the oxic tank to the anoxic tank, the hydraulic retention time is increased in those two tanks, and in those two tanks only.

Typical Factors Inhibiting Biological Nutrient Removal

Besides ensuring there is enough time for the separate BPR processes to be completed, there are other factors that can inhibit BPR. The most common factors are expounded on below.

Factors Inhibiting Nitrification

 


Nitrification consumes approximately 7.1 parts of alkalinity for every part of ammonia converted to nitrate. It also consumes around 4.6 parts of oxygen for every part of nitrate produced. Concerning oxygen, oxidation ditches have been known to fully nitrify at DO levels of 0.7 mg/L, so it is not necessary to jack up the oxygen levels above 1.0 mg/L when having ammonia issues. The optimum pH range is 7.5 to 9.0 units, but they do not like sudden changes in pH. The effluent alkalinity should be maintained at 70 mg/L but not allowed to drop below 40 mg/L for long periods. The BOD:TKN ration should be above 4:1 and the SRT should not be allowed to drop below five (5) days.

If accepting leachate, leachate tends to produce a tan color in the effluent. This color can interfere with the colorimetric test method for ammonia (anything using a spectrophotometer), so the electrometric test method (probe) is recommended instead.

Factors Inhibiting Denitrification

 


The definition of anoxic for wastewater treatment is MLSS with ‘No Free Oxygen and Some Nitrates’. For optimum denitrification, a DO level well below 0.4 mg/L is required. Because denitrification always follows nitrification, the endogenous BOD may be depleted prior to the nitrated reaching the anoxic period of treatment. There needs to be at least 2.9 parts of COD (BOD takes too long to analyze) for every part of nitrate and the proper range for the other environmental factors (pH, temp, SRT). For every 1.0 part of NOx converted to Nitrogen gas, 3.6 parts of alkalinity will be recovered, reducing the need for alkalinity addition.  

One condition that can occur is oxygen creep from the aerobic phase or tank into the anoxic phase or tank, especially during winter when the oxygen saturation rate of the water is very high, microbial activity is very low and/or the DO levels in the aerobic tank are very high. Oxygen can carry over into the anoxic tank converting it into an aerobic tank.

Concerning DO Levels, practically speaking for both optimum treatment performance and power consumption, the DO should be kept as low as possible in the aerobic portion of treatment. Generally speaking, unless there is something else going on in the aeration basin, the DO does not have to exceed 1.0 mg/L in non-SBR systems and in SBR systems the maximum optimum DO may fall between 1.5 – 2.5 mg/L . This will almost ensure that oxygen creep into the anoxic period of treatment will not occur.

Factors Inhibiting Luxury Phosphorous Uptake

 


The definition of anaerobic for wastewater treatment is MLSS with ‘No Free Oxygen and No Nitrates’. Facultative organisms will utilize oxygen from various molecules in the following order: O2, NO3, SO4 and CO2. There is normally very little SO4 and CO2 in the waste stream, therefore tracking the NO3 levels entering the “anaerobic” period of treatment is sufficient to ensure the system is truly in an anaerobic condition.

Concerning NOx in the influent, the NOx concentrations are typically well below 1.0 mg/L. However, there is a popular chemical often added to the collection system for odor control. This chemical is Calcium Nitrate. It does a good job of reducing or eliminating sulfide production in the collection system but will ‘poison’ a biological phosphorous removal (BPR) process. Additionally, Nitrate/Nitrite or total nitrogen limits are being added to NPDES permits and adding calcium nitrate may have an adverse effect on the effluent stream as well.

Side Stream Management. Plants just beginning biological phosphorous removal have never had to be concerned with high phosphorous levels in the digester decant. The phosphorous that has been absorbed by the biosolids will be released again when the air is turned off for decant. If left off for more than 24 hours, the phosphorous in the decant can exceed 700 mg/L. This will be returned to the head of the plant and will have an impact on the effluent phosphorous levels.

Plants that are monitoring phosphorous may want to consider operating the digester in a manner that reduces the amount of phosphorous discharged back to the head of the plant. The main objective would be to decant a little every day. One method would be to only aerate the digester for three hours immediately after decanting, then turning the air off, letting it settle overnight, then decant first thing in the morning. This would have to be done every day.

Conclusion

These are just some of the more typical issues that can affect nutrient removal in the typical WWTP. Paying attention to these factors can serve as signposts for process control and can assist in the ability to troubleshoot the plant. There are may other factors that may occur as well.

The next article will focus on what I call “The Fifteen Minute BOD” and the University of Tennessee says is the “most valuable yet underutilized test available for operators of the activated sludge process”.

 The Georgia Rural Water Association is always available to assist any municipality that serves a population of <10,000 with process control, troubleshooting, training, lift station maintenance, smoke testing, leak detection, standard operating procedure development, asset management development, or any other compliance and operational issues that may crop up. To request their assistance, please contact them at Contact Us - Georgia Rural Water Association (grwa.org).

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Feel free to contact me should you serve a population >10,000 or have any questions or issues that the GRWA is unable to assist you with. Also, if you would like to contribute an article feel free to email me at the address below. I am always looking for contributors that have an interesting perspective, topic or has an interesting case that they would like to share – especially if the solution is a direct result of applying the principles from this forum.

Dennis Brown, Wastewater Specialist and Trainer, Retired

dbrown.grwa@gmail.com

678.750.3996



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