Dear Nimantha Wickramage,

Increasing the amount of oxygen from 2mg/l to 5mg/l does not have any negative effect. In the contrary, the microscopic investigation showed a tendency towards poorer sludge properties at a DO concentration of 2 mg/l compared to at 4 mg/l.

The only negative thing is the increase of the cost when employing high quantities.

For more information, please use the following link:

http://publications.lib.chalmers.se/records/fulltext/192796/local_192796.pdf

Variation in dissolved oxygen concentration and its effect on the

activated sludge properties studied at a full scale wastewater

treatment plant

B-M Wilén*, A Markiewicz* and Å Nilsson**

* Water Environment Technology, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

(E-mail: [email protected]; [email protected])

** Gryaab AB, Norra fågelrovägen 3, SE-418 34 Göteborg, Sweden

(E-mail: [email protected])

Abstract

Aeration of activated sludge is very costly and therefore it is desirable to operate the treatment plant at as low DO concentration as possible without risking poor effluent quality. This paper presents a study of the impact of DO concentration on sludge properties at a full scale wastewater treatment plant which utilizes pre-denitrification followed by aeration in activated sludge and with post-nitrification in nitrifying trickling filters. The DO concentration has in the past generally been kept high at the plant (> 5 mg/l) as it has been believed to give better effluent quality. During later

years attempts have been made to reduce the DO concentration to 2-3 mg/l. In this study the DO set-point was changed every 3 weeks between 2 and 4 mg/l for a few months. Also short term effects of change in DO concentration was assessed by changing the DO concentration in the three parallel activated sludge tanks to 1, 3 and 5 mg/l for a few hours. The results show that the impact of DO concentration is hard to assess as the process is subjected to constant changes in the

environmental conditions. The settling properties were not influenced significantly whereas slight negative effects of reduced DO concentration on effluent quality could be seen. The microscopic investigation showed a tendency towards poorer sludge properties at a DO concentration of 2 mg/l compared to at 4 mg/l.

Hoping this will be helpful,

Rafik

The main problem is that system is wasting money in form of electricity. If the removal efficiency and the characteristics of the sludge were ok at 2 mg/L, this extra oxygen (5 mg/L) seems not necessary. The use of less blowers or the use of a variable frecuency converter in the blower can help save a lot of energy and money

Apart from increased power costs, carry-over dissolved oxygen may impact on de-nitrification in the anoxic zones of a BNR plant depending on plant configuration eg Oxidation Ditch or 4 stage Bardenpho.

Nimantha,

Mr. Nimantha 

What Michael Lever said is right however what you can do now is turn off some of the blowers you use in your WWT facilities and then monitor; if you get again 2-3 mg/L dissolved oxigen and save the money related to energy costs; there you go. You get an optimal oxigen concentration without spending more money.

The only negatives besides cost can be increased foam if the stream has a large surfactant component and the production of smaller, sheared, and less treatable*, floc if you are using mechanical aeration. These are rare and I have never seen the latter but have had a facility written up for it. It was listed on a Virginia inspection checklist, no matter the solids situation. Haven't seen it since.

*started to use the word "coagulate-able", but is that a word?

I agree with the answers carrying out the concept that increasing DO is negative, think about the impact of wasting electricity on the emission of CO2, 

There is also a possible link between increased oxygenation and increased N20 emission.

I remember also that filamentous bacteria presence can be enhanced by increased oxygenation

If you increase the DO in aeration tank, the capability of sludge's sedimentation will be decreased in the final sedimentation tank considering our results. These results are coming from our six-month experiments in one of the Iran's wastewater treatment plants.

And we could see a nearly reduction in the amoumt of MLSS in activated sludge reactor by increasing DO from 2 to 5 mg/L. Then we added plastic carriers to it to make a MBBR reactor. After that, we could see a good settleabilty of the sludge and amount of MLSS. 

Mehran, at the same time the DO was increased was a system of sludge wasting based on sludge age also implemented? That should take care of the problem in most cases.

minimum required oxygen should be 2mg/L to4.6 mg/L in aeration tank  however  30-55cubemeter of oxygen is required for 1kg of BOD.Increasing oxygen may  lead  to more cost

Dear Henry, We could not see a reduction in sludge age because the MLSS and MLVSS concentration decreased

From a Chemical Engineers point of view, transfer of Oxygen from the Gas phase ( the air you blow in) to the bulk liquid in the aeration tank will see a drastic reduction for the simple reason that the driving force will be less. Assuming Oxygen saturation value of around 7.5 PPM in wastewater, maintaining 4 PPM D.O results in a diving force of 3.5 PPM only. If we maintain 1.5 PPM in the tank, driving force almost doubles to 6 PPM, hence inherently more energy efficient by 70 % or more.

How about High DO in a MBBR tank? Does it cause any effects there? The MBBR we have, do not have anoxic zones for denitrification, unless you consider a DAF, which is the last unit in train before effluent to city WWTP. This is a industrial pretreatment MBBR system.

The relationship between the dissolved oxygen concentration (DO) and the equilibrium partial pressure at the contacting gas phase is given by Henry's law. DO is often measured with a Clark polarographic oxygen electrode, which calibration and conversion from percent saturation to molarity is addressed at § 2.2.1 (pp. 11-12) at the signalled reference.

The KLa mass transfer coefficient also determines the dissolved oxygen concentration (DO) of the aerobic fermenter. The air to fermenter's media mass-transfer rate of oxygen (Na) should be dependent on the local oxygen deficit: Na = KLa·(Cs-C). Here, Cs is the DO that would apply for saturation of the fermenter's broth and C is the measured DO. A mean concentration difference can be defined (ΔCm) after accepting some convenient mixing model, so that: KLa = OTR / ΔCm, where OTR stands for oxygen transfer rate. Agitation should contribute for KLa. Such a modelling-based approach implicitly takes into account other variables that may influence KLa; including temperature (T) or pH. Further details on this kind of approach, were given at the following reference (MSc Thesis):

Thesis Controlo do Oxigénio Dissolvido em Fermentadores para Minimi...

DO should be maintained between 2 to 4 ppm. I have read some literatures that say that with higher DO, denitrification process is hindered and pH starts reducing, which is undesirable. So take care of pH

If we maintain 1.5 PPM in the tank, driving force almost doubles to 6 PPM, hence inherently more energy efficient by 70 % or more. However, the oxygen saturation concentration also decreases, so that the driving force is not increased so much. The OTR would really be given by: OTR = Klaf (C*inff - C) - R where R is the respiration rate of the biological microbes. See attached paper.

how DO analyzer level can be calculated according to the blower capacity?