Biological processes in wastewater treatment can be classified into two main categories: suspended growth and attached growth/ biofilm processes.
In suspended growth processes the microorganisms responsible for wastewater treatment are maintained in liquid suspension by appropriate mixing and aeration. The most common suspended growth process is activated sludge.
Instead in the attached growth process, the microorganisms are attached to an inert packing material such as rock, gravel, redwood and especially a wide range of plastic fill media packings.
Plastic fill media blocks consist of corrugated sheets usually made of PVC or Polypropylene material.
Either by vacuum or thermo forming the corrugation is pressed into a flat plastic sheet. In detail you can see a corrugation substructure within the main corrugation. On the one hand this provides more surface area for attached biofilm growth but on the other hand the corrugation substructure functions like stiffeners giving the flat sheet more resistance to compression-buckling. Details in the corrugation shape differ from manufacturer to manufacturer and they are often a patented secret.
When the corrugation is formed into flat sheets, their material thickness is changed as well. However, the material thickness change is not random, instead known weak points are reinforced with more material whereas other areas are thinned out to save material costs. Also, the overall sheet thickness of plastic fill media packings can be varied by using different flat sheets to optimize stability and material usage depending on the application. A good example for different fill media thicknesses are the layers within a trickling filter – please click on the above link for more information.
The corrugated sheets are either welded or glued together at dedicated connection points to build plastic fill media blocks. Polypropylene is very difficult to glue and is therefore always thermally welded whereas PVC can be glued together easily.
Depending on the application and degree of wastewater pollution several different plastic fill media designs are available. In general, they are divided into two categories: cross-flow and vertical flow. Within the two categories plastic fill media blocks differ by their channel sizing. Smaller channels mean more surface area which equals more attached microorganisms and more treatment capacity. But at the same time the risk of fill media channel clogging increases as smaller the channels are. Typically, the surface area of plastic fill media blocks is between 30 and 70ft2/ft3 (100 to 240m2/m3).
Crossflow fill media blocks are used where good air and water mixing is required. A typical application is a trickling filter: The sprayed wastewater from a rotary distributor must moisten the plastic fill media surface area evenly throughout the whole filter. At the same time natural draft from the trickling filter bottom must reach all plastic fill media blocks.
Finally, vertical-flow fill media blocks are used where a particular flow direction must be maintained. A typical application is a submerged fixed bed reactor: Produced bubbles from aerators underneath the fixed bed reactor raise vertically upwards. To minimize power costs and optimize contact time with the attached biofilm, the bubbles must stay inside the fixed bed reactor until they reach the water surface.