Trickling Filters Return to Process Page

What is secondary treatment and why is it done?   The secondary treatment stage follows primary settling.  During secondary treatment, the wastewater is pumped to one of two treatment units.  One of  the units is a large circular distributor that moves and sprays the water over layers of plastic media.  This unit is called a "Rotary trickling filter".  The wastewater that is sprayed on the plastic media then "trickles down" (hence the name) the plastic to an underdrain system at the bottom where the water is collected, then transferred.  While the water is trickling down the plastic media, bacteria that are growing on the media "eat" the leftover dissolved organic material and nutrients that are in the water.  As the bacteria age and die, they no longer hold onto the plastic media and the bacteria are washed from the plastic  by the wastewater in a process called "sloughing".    What essentially happens during this stage, is that bacteria are removing dissolved pollutants that would be very difficult to eliminate into a solid mass that can then be settled with gravity during the next stage of treatment.   This same process is being performed by the other unit which is called a "Fixed Bed trickling filter unit".  Unlike the Rotary trickling filter, the Fixed Bed unit has a series of sprayer nozzles in a fixed position (hence its name) that spray the water out over a large rectangular area.  This unit is from the original Treatment Plant design that was built in 1934 and was considered the state-of-the-art at that time.  Instead of a plastic media, the fixed bed unit has several feet of porous rock that the water is sprayed over.  Beyond that, the unit processes the wastewater in the same manner as the rotary system above.  Bacteria grow on the rock, "eat" the nutrients from the water, and are washed off the rock when they die.   The primary differences between the two units is the volume of wastewater they can treat versus the amount of space they occupy.  The Rotary trickling filter can treat almost twice as much wastewater as the Fixed Bed unit and does it in half the space that the Fixed Bed system uses.  Additionally, since a Fixed Bed system does not move, it has to use many more sprayer nozzles to cover the same amount of area.  This results in a significant amount of additional maintenance as each of these nozzles must be cleaned monthly. It is for this reason that Fixed Bed systems have fallen out of favor compared to the more modern Rotary systems. After the wastewater has been processed through the Secondary Treatment stage, the wastewater (which now contains the remains of the dead bacteria) proceeds to Secondary Settling.