Traditional Attached Growth Systems
The earliest known attached growth systems were cases of raw sewage cascading over rock beds in river courses, with microbes growing on the stone surfaces and causing a variety of metabolisms, including aerobic in the upper layers, anaerobic in the benthic layers, and facultative in the intermediate sections.
How to develop of Modern Versions of Attached Growth Systems?
The earliest treatment methods are the trickling filters and intermittent sand filters.
The trickling filter media consisted of loosely arranged rock media ranging in size from 100 mm to 150 mm. Design data for these trickling filters were developed by accumulating data on their performance. The stone medium, on the other hand, has mostly been abandoned across the world as time has passed. In India, the largest known installation has been at the Piranha sewage farm.
The major reasons were clogging and choking of flow channels and under drains caused by gradual stone erosion caused by microbial corrosion and attrition, necessitating the actual removal and repacking of whole filters with diameters of up to 50 m in certain cases. The rotary reverse jet distributor had its own set of issues, with grit accumulating in the arm ducts, the turn table going into differential mode right away, and the rotation abruptly stopping owing to crushed ball races and twisted ball retention rings. As a result, light weight synthetic media with a substantially greater surface area per unit volume have appeared in the market.
The variations in physical arrangement are
a) Fixed Film Reactors (FFR): These are attached systems growing on fixed film over stationary medium, with sewage trickling down the exposed surface.
b) Submerged Fixed Bed Reactors (SFBR):These are attached growth systems on fixed film over submerged stationary medium in an otherwise sewage holding reactor, with sewage flowing primarily upward through the reactor.
Submerged Aeration Fixed Film (SAFF), Rotating Biological Contactor (RBC), Fixed Bed Biofilm Activated Sludge Process (FBAS), and other commercially recognized technologies fall under this category, and can be aerobic or anaerobic.
In each of these scenarios, microns form on the surfaces of the media and thicken as new bacteria adhere to the preceding film; once the thickness reaches a certain point, the microbial film sloughs off the media, and new microorganisms emerge. Because this is a cyclic process, the amount of organic materials removed might fluctuate from time to time.
However, the advantage of this technology is that it eliminates the need for large sewage holding reactors such as aeration tanks, and the attached growth reactor can be designed and built simple with no water pressure on the inside, similar to cooling towers, saving money on civil construction and energy for mixing the contents.
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