Submerged media in residential aerobic treatment tanks.

[lippy]

Hello friends,
My question is this:
Does submerged media in aerobic treatment tanks really help
the microorganism colony? I was under the impression that they flocculate freely in the tank. Thanks, Joe

 

[ptmoss]

I see quite a few of these home treatment tanks being developed in the last few years. Many, if not most, have a media for the bugs to attach to and colonize. One benefit I've seen is the start-up time is very quick whereas with straight aerobic tanks it takes a while (don't ask me exactly how long!)for the bugs to develop an adequate colony to achieve the stated treatment levels.

 

[lippy]

Hello ptmoss and thanks for input,
I too have poked around in these tanks.
The film of microorganisms on these medias
is not much more than the thickness of dust
on an old book. It certainly is not like
moss growing on a rock. If the media helps
retain oxygen/bubbles in contact with the
mixed liquor/wastewater than I can see some
good. However, the ones I have seen just let
the bubbles rise to the top with hardly any
delaying action. Again is not the flocculated/
free floating colony of microbes much larger than
what is attached. Say in the order of 9 to 1 or
greater. Thanks, JOe

 

[mmenarry]

I've designed & manufactured systems in Ireland using submerged filter media, and it does have it's advantages. The microbial mass on the surface of the media can be very large, especially with some of the better made media, which tends to come in large blocks rather than discrete pieces. Also, although the depth is not much on the media, it does significantly help against shocks to the system generally, such as the housewife throwing gallons of bleach down the drain at spring cleaning time! The layers are a good protection against such shocks, which can put a sytem back to no microbes at all. Media that we used had area ranges from 95 to 150 square metres per cubic metre.

 

[Muggle]

Submerged aerboic action has been used for a real long time in aquacultrue - fish rearing. The growth rates,etc. are well known and widely published for practically all media types. The bacteria constantly grows and sloughs off- as it replicates. Nitrosomonas and nitrobacter are the main bacaterial types - the slick, sticky biofilm is nitrosomonas and is pretty hardy- nitrobacter is very thin and is what converts nitrite to nitrate. It is easily dislodged if your filter media is disturbed. All work best with alkalinity in the 120-180ppm range (as calcium carbonate). Nitrosomonas needs a substrate to replicate at full efficiency - otherwise suspended bacteria is working at maybe 20% of capacity in removing pollutants.

 

[lippy]

Responding to mmenarry and muggle,
How do attached microbes resist shock better than suspended?
Most small municipal wwtp use suspended growth/activated sludge without media don't they? Where can I find growth rate information for media types? Is not a main function of media to detain oxygen from making the surface?
Also would say 4in.sq. surface area for every 1in.cubed tank volume be considered appropriate surface area density?
Sorry for all the question but this is truely fascinating stuff. Thanks, JOe.

 

[Muggle]

Libby - There are numerous types of submerged media. Most of your aquaculture sites have tons of data on each type. Do a Google search under "biomedia" and you'll get a bunch of hits.

air into the biomedia is more to make sure DO level can support the bacterial needs. Also helps to off-gas CO2, but if the DO level of water is sufficient (4.0ppm or greater) then you do not have to have aeration into the media. By having a growth media - you increase the number of bacterial colonies due to the increased surface area. This helps eliviate "shock". The mor porous the media, the greater the colony counts. All bio medias provide growth/surface area surface areas for comparisons.

I would not use "tube" ceramic media - the anaerobic microbes growing on the inside will eventually go anaerobic as the tube plugs from bacterial growth. They are hard to clean. Look for round to angular media with numerous small holes. The alrger the number of holes, the greater the surface area.

dave/Muggle

 

[mmenarry]

Lippy,
The media we used was like expanded corrugated plastic, made by Marley (UK) Marpak, I believe the name was. It had a surface area to volume ration of 110 square metres per cubic metre. We mainly used this in small-scale (household to 200 cubic metres/day).
Bacteria are a lot stronger if given the physical damage through agitated onditions from hitting off other flocs and side walls, etc. The layers of active bacteria are generally much thicker on media than they are on flocs. This allows the active bacteria to handle occaisional differences in water conditions (such as the householders doing a spring clean and dumping a total of two gallons of chlorine based cleaners down the drain!)
The reason that most municpal plants avoid submerged media is probably cost. We tend to use media to reduce the overall size of our treatment units.
With regard to retaining oxygen from reaching the surface, I've never seen a practical way of doing this, nor do I think that that would have any benefit. Oxygenate and agitate is the best way of keeping the O2 level up in wastewater.
Michael

 

[Muggle]

The greater the porosity of the media - the greater the bacterial growth. Not a big fan of the plastic corrigation type media as it's good for nitrosomonas (bacteria that converts ammonia to nitrite) but bad for nitrobacter( converts nitrite to nitrate). The greater the number of "holes" per inch or cm - the greater the surface area, and thus the greater the colonization of the media.