**** Ulrich Bretscher's Wastewater Page ****



Cleaning Clogged Aerators in Place
No diffuser clogging anymore!

Necessity is the mother of invention

Like so many inventions, this one was inspired by an emergency. It occurred back in 1982 while pilot plant tests were being run to evaluate the most effective means of upgrading the sewage treatment plant of the city of Gossau SG in Switzerland. Six months after beginning our pilot plant tests and with the end of the tests in sight, the fine bubble diffusers clogged so much that the successful completion of the tests were endangered. That would have required emptying the aeration tanks, replacing the clogged aerators, restarting the plant and repeating the last test. Thus incurring a loss of several weeks of labor plus a lot of money.

I didn't know then what caused the clogging of the diffusers. Was it scaling by calcium carbonate? Was it scaling of the pores by bacteria? From my experience as a chemical engineer I knew, formic acid was an excellent solvent of carbonates as well as of microbial films since formic acid was the strongest available organic acid. So what to do? Since I had nothing to lose, I went to a hardware shop, bought a pump head designed to attach to a portable drilling machine (commonly used for lawn irrigation). Then I went to a local drug store and bought a liter of formic acid.

Formic acid has a boiling point little higher than water. Right behind the blower we had an air temperature of about 80 °C. My plan was to fix a nozzle near the blower exit and hope the acid would evaporate quickly enough inside the main pipe so that the distribution of the gaseous formic acid to and in the manifolds at the bottom of that tank would be uniform enough to dissolve the obstructions inside the diffuser plates.

Surprise, surprise!! About 20 minutes after adding the formic acid the pressure loss was nearly normal and my pilot test was saved. I didn't notice a decline of the plants biologic performance nor a deviation of the pH of the activated sludge.

Fig. 1: One of the activated sludge tanks in the pilot plant in Gossau, seen from the upper level of the isolated cabin, we had built around the plant. Each aeration tank had a volume of 11 m3 and a depth of 2.2 meters.

Fig. 2: The diffuser manifold, consisting of 15 diffuser plates attached to plastic (PVC) pipes. The diffusers used were Nokia HKL-215. The diffuser material was polyethylene foam. Dia. 186 mm.

The above photos were taken with a Polaroid camera in 1982.

What did the obstruction consist of?

 To answer this question I waited another month, until the diffusers of  the second tank were clogged, too. When new, the blower delivered 42 m3/h air to this tank. But at the end of the pilot tests, the blower blew a mere 8 m3/h. The rest blew off through a relieve valve.

Fig. 3: Flow sheet of the examined aeration tank.
Volume = 11 m3, Depth = 2.2 meters
Next I emptied the tank and removed diffuser plates #1, 2 and 3 (fig. 3) and replaced them with solid plates. Then I refilled this tank with the original activated sludge that I had put aside and started the blower again.

In consideration of the high carbonate hardness of this wastewater (5 mmol/l) I injected 0.5 liters of 85% formic acid. This guaranteed that the pH of the activated sludge wouldn't become acidic.

After six hours of reaction time I emptied the tank again and took diffusers #4, 5 and 6 as samples to investigate how much of the cloggings had dissolved. Here is what I found:

Fig. 4: Diagram of the composition of the cloggings inside of the diffuser plates. before and after fumigating with formic acid.

This graph demonstrates that about 36% of the cloggings was of organic nature, 64% was inorganic, - predominantly calcium carbonate and iron phosphate. Using formic acid, 70% of the obstructions were dissolved; however the iron phosphate buildup remained relatively unchanged.
After this treatment, the blower conveyed air again as it did at the beginning of the pilot tests.


Corrosion of the piping by formic acid

In 1982 the piping of waste water treatment plants in Switzerland usually consisted of pig iron (found in very old plants) or zinced steel. We checked what harm would be done to the piping by regularly injecting formic acid. Unfortunately the results of our corrosion tests conducted 25 years ago are lost. But we know by now from our experience that an optimum concentration of formic acid lies between 85 to 98 %.

 To update the information for this paper, I repeated the corrosion tests and analyzed the data on the computer; a convenience not available in 1982. To repeat these tests I used samples from sheet steel-37 and sheet zinc, each sample with a total surface of 12 cm2. I exposed these to a series of different formic acid dilutions ranging from 70% to 98% and determined their weight-loss over time.

 As is demonstrated by the following two graphs, an over all optimum formic acid concentration is observed between 85 and 98%. This is because zinc corrodes significantly when exposed to a formic acid dilution of less than 85%, while steel-37 is relatively resistant to any concentration of formic acid.  

Fig. 5: Corrosion of steel-37 in different dilutions of formic acid. Fig. 6: Corrosion of zinc. Note the vastly increased solubility in formic acid of less than 85 %

Hence we usually apply inexpensive formic acid, technical grade 85%. It costs about Swiss Fr. 2.- per kilogram botteled in 60 kg containers and its effect as a solvent of the cloggings is satisfactory.

The causes of clogging

A diffuser which is clog-proof hasn't been invented so far, although many of them have come onto the market claiming this attribute. From our experience we know that at least halve of all sewage treatment plants will experience diffuser clogging sooner or later.

 At first, the cause appeared to be a property of the sewage. But we haven't found out yet exactly what causes it, though we keep working hard on it. Clogging is rarely found in sewer districts with soft water. But on the other hand, there are also plants in areas with hard water that are free from clogging.

 We found just one case where an extreme loss of oil from the blower fans was the cause of the clogging. In two cases we found the fault to be with the air mufflers. There the mufflers were coated inside with glass wool which tore loose and accumulated inside the diffusors. Of course, in this case formic acid couldn't help and that's why we discovered the damage.

 There are the diffusers which use a perforated polyurethane membrane. It generally is assumed that any obstruction within the perforation can be dislodged by a short blast of air. The pores in the membrane are supposed to expand and eject the obstruction, but this proves to be wishful thinking. Plus there it is another disadvantage: Polyurethane is not resistant to formic acid. It balloons up irreversibly like bubble gum. Some plants have replaced formic acid with acetic acid. Acetic acid acts somewhat less aggressively on polyurethane, but nevertheless, after multiple applications, the membrane looks blistered and floppy.

Fig. 7: A clogged diffuser plate from polyethylene foam. After nine months all diffuser plates in this plant were entirely clogged and showed massive calcinations of peculiar patterns like this example demonstrates.

The next picture shows a microscopic section of a polyurethane membrane with pores that were pierced by needles the way sewing machine needles would. Friction probably caused the needles to become hot, melting some of the material. On their withdrawal this caused a funnel-like structure inside the pierced holes which acted as check valves. Not knowing about this defect, the manufacturer had allways marked the membrane which side has to be fixed upwards.

Fig. 8:
A microscopic section of a polyurethane membrane. Note the obstructed pore with an inadverted "check valve" inside, prone to clogging.

(This membrane was charged with air from the underside.)


All types of diffusers and membranes can be cleaned with formic acid, particularly those made from rubber, polystyrene and polypropylene or ceramics, but not those of polyurethane.

Always use formic acid 85% or stronger. Inject it at a place where the pressurized air is as hot as possible. This place is usually right behind the blower where commonly temperatures of 70 to 80° C prevail. You won't need to spray it into the piping with a spray nozzle. Just pumping it slowly into the pipe is O.K.

 New fine pore diffusers characteristically cause a pressure loss of ca. 0.3 meter. That means the air pressure in the main may be calculated as diffuser depth (in meters) plus 0.3 meter.

It's usually acceptable to allow clogging up to a pressure loss of 1 meter. Then you probably need to take action.

 There are two strategies to treat clogging:  

  1. Wait until the diffusers are clogged up to a pressure increase of 1 meter, recorded by a manometer in the mains.
    Then inject ca. 80 ml formic acid/m2 aerator-surface. Wait one hour and inject the same amount of acid again if you haven't gotten the desired effect. And before doing this, remove as much water from the manifolds beneath the diffusers as possible
  2. If your plant is constantly prone to severe clogging, then install a batcher and dose once a week with 35 ml formic acid/m2. This will keep your aeration juvenile and give you the option to adjust the weekly amount of acid according to your experience. 


Fig. 9:
An early dosing station for formic acid.
Note the very old-fasioned blowers in the rear where formic acid was injected for 25 years.
Before, the diffusers of this plant had to be exchanged and cleaned at least once a year (Look Fig. 7). After this treatment none have had to be replaced.


This paper was first published by U.Bretscher and W.H. Hager "Die Reinigung von Abwasser-Belüftern", gwf 1983

The process was patented in all European countries plus the USA and Canada in 1983. This patent has expired now. So this process may be applyed by anyone for free. But I'm still interested hearing about the experiences of my readers.

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