In wastewater treatment, size does matter, at least the surface area of the material to be treated matters. The more surface area, the quicker the bacteria can do their job. To accomplish this, the industry uses comminutation or grinding.
At some facilities, comminutors are used to grind coarse solids without removing them from the flow stream. They are found mainly in smaller communities and should follow the grit removal process to prolong the life of the equipment and reduce wear on the cutting surfaces.
If the comminutor precedes grit removal, the cutting teeth will require frequent sharpening or replacement. Units usually consist of a vertical stationary section and an oscillating or rotating section equipped with cutting teeth. The close tolerance between the shear bars and cutting teeth allow solids to be cut into smaller, more uniform sizes, which are taken out by subsequent treatment stages. If the cutting adjustment is improper, the solids will not be cut cleanly or shredded as desired.
For larger facilities, there are robust grinder units that reduce the solid material into very small pieces to help make it flow through the plant more easily and cut the floatables into small enough pieces that they tie up with the sludge as it settles and is wasted out of the system. Comminutation also helps in the removal of BOD and TSS in that it breaks the organic solids into smaller pieces, allowing for more surface area for the bacteria to attack and degrade the BOD more rapidly.
To ensure continuous operation in a flow process, it is desirable to have the screens or comminutors installed in parallel in the event of a breakdown or to provide for overhaul of a unit. With this arrangement, flow is primarily through the comminutor and diverted to the course (bar) screens only when necessary to shut down the comminutor.
Another type of solids reduction unit in a chopper pump is a centrifugal pump, which is equipped with a cutting system to cause chopping of the solids that are present in the pumped liquid. The main advantage of this type of pump is that it prevents clogging of the pump itself and of the adjacent piping, as all the solids and stringy materials are reduced by the chopping system. This type of pump may be a submersible pump in a lift station or a wet well dry well pump with a chopper or grinder attached to the impeller. It is also used in pressure sewer systems.
A comminutor or grinder reduces the size of the floatable material entering the treatment plant, allowing it to settle out with the sludge or be removed by the scum removal system. It also increases the surface area of organic material to the bacteria for biological degradation.