What does a small waste water treatment plant consist of? I got interested in these things too in the course of my reading up on treating ponds. In Japan, they sell small systems for homeowners that you can bury in the ground.
There's definitely a lot of similarities in the biological treatment of waste water and treating pond water, at least the when it comes to aerobic bacteria. Some waste water system do use anaerobic bacteria to treat waste water as well though, in fact the well known "septic" tank system that many people have in their back yards is based on bacteria that thrives in an oxygen free environment.
The waste water treatment systems they use around camp here are all aerobic systems, and they use powerful aerators to oxygenate the crap out of the waste water, literally. LOL The system I'm most familiar with has about 7 large tanks that have these aerators in them, except for the first one which is a settlement tank. After the aeration tanks there is a membrane chamber which acts as a mechanical filter, but also has biological activity going on in there. It is from this tank that the solid waste build up, that doesn't go through the membrane, get recirculated back to the first settling tank. This stuff that gets recirculated is "activated", which means it's rich in active bacteria and helps inoculate the raw waste coming into the system. What does filter through the mechanical membrane next goes through powerful UV sterilizers and is discharged on to the ground in drain fields. Apparently the effluent that goes through the membrane filter and UV system is safe for human consumption, but so far I haven't seen anyone drink it.
It still has a light brown tinge to it.
The thing that I was most curios about was that they are utilizing aerobic bacteria, same as we do in ponds, but the bacteria live in free floating water, they don't have any media in their tanks, or at least not this one I'm most familiar with. This isn't to say that no wastewater treatment facilities use media for the bio-film to grow on, in fact many of the large ones use gigantic
Trickle Filters, and
Rotating Contact Filter which both utilize surface media, exactly the same way our pond bio-filters do.
BTW, the treatment system that I have become familiar with is sized for a 90 man camp. The camp is expanding to 400 man and they have another larger package treatment plant sitting out on the lease in sections, but they haven't set it up yet.
The 90 man one looks like the one in the bottom picture from the outside.