Fine Air Bubble Diffuser

[Amirasofea]

Dear experts

I have a question.

Previously, our WWTP was using surface aerators for our WWTP, and the results were within recommended limits.

Not long ago, we have installed a fine bubble diffuser system which supplies air for bacteria via air blower. We have invested a lot of money for this project. This system immediately replaced the surface aerators. This system was installed mainly for cost savings. However, the results turned haywire, with a lot of parameters went off limits, until we have to run back some of the surface aerators for backup. Some of surface aerators were already removed from the aeration ponds to make way for bubble diffuser.

What could have been the problem that caused the bubble diffuser to fail to supply oxygen to the bacteria? Before the project, we were informed by the project engineer that bubble diffuser works better than surface aerators, and at much low energy cost.

With bubble diffuser, the MLSS increased from 4000 mg/L to 13,000 mg/L, the color increased from 10 to 200, and everything else collapse. After checking, the air from the air blower is extremely hot, which is 85 deg.C, and you can't touch the air pipe because it's too hot.

I suspect the oxygen in the hot air is low which caused less oxygen to be supplied to the bacteria. Any other possible cause for this disaster?

Can install aftercooler solve the problem? It's difficult to revert back to surface aerators after all the dollars we've spent on the new system.

 

[bimr]

You seem to have some misunderstandings.

The bubble diffuser is just changing the method of air & mixing that is introduced into your wastewater treatment plant, not the overall management of the plant as demonstrated in the MLSS. The MLSS should stay the same for both types of aeration.

Having said that, many bubble aeration systems are sold on the basis of claims about energy savings. Aeration needs are set by two factors: 1. Energy to mix. 2. Energy to supply oxygen. You have to use the higher aeration demands of either 1 or 2.

In most cases, the energy to mix is greater than the energy to supply oxygen. The energy to mix is 10-15 cfm per 1000 cubic feet of aeration basin according to M & E. Check to see if you have enough aeration.

The hot blower piping and air will not affect anything. Unless your aeration tank temperature goes over 100 Deg F, you do not have to worry about the blower air.

 

[semo]

I agree with bimr on his statements and if your DO levels are up, the diffusers are doing their job. During transition from surface aeration to diffused, I would expect to see what you are describing.

Most likely your surface aerators were not reaching the bottom of the lagoon and only mixing the upper levels (and maybe a deeper region in their immediate vicinity). When you installed the diffusers around the lagoon, you placed them deeper in unmixed areas where sludge had settled.

Now, turning them on caused agitation and suspension of this sludge which not only increased the color; but, also placed more solids in suspension causing a higher mixed liquor.

Until this sludge is broken down, displaced, activated, etc. you will have a dark liquid higher in solids. I don't know what type of plant you have (ie HRT); but, this could take days to reach equilibrium. Given time, it will equalize and your MLSS will reduce and the color will return to normal.

 

[Watthour]

The basis of the above is mostly correct, However, I would point out that the aeration mixing rates may be incorrect in general applications. 20 CFM/ MCF is the 10-States Standard for municipal waste, and 30 CFM/MCF is required in municipal digesters. It is likely that TCEQ will adopt those levels as well, and many other jurisdictions are expected to follow once that occurs. Commercial and industrial waste processing will often require even higher mixing energy, and that is a function of MLSS/TSS, HRT, and the resultant F/M.

Semo nailed it. Depending upon the basin configuration and how the diffusers were installed, the resident septic sludge in the basin may now be in suspension, and it may actually take weeks, not days, to stabilize the plant. Surface aerators, even with trunks, have limited ability to mix beyond a specific depth and radius. I have replaced surface aerators with submerged mixer-aerators and experienced the same phenomenon, and to an even greater extent than with diffused air systems. Once the basin is fully mixed with all solids back in suspension, and the biology has had an opportunity to process the septic sludge, some stability should return.

In the words of Aaron Rodgers, chillax.

 

[bimr]

Note that the Ten States standard for mixing is most likely incorrect for optimum performance.

"Considerable confusion exists about the ability of diffused aeration systems to mix basins for various treatment processes. In particular, it seems the use of 20-20 cfm/1000 cf of reactor volume is a common criterion without many people realizing how the numbers were derived. For diffused aeration systems, this criterion can create a very unrealistic mixing requirement unless the fundamental criteria are understood and properly applied."