Chopper Pump Helps Solve Digester Mixing Problems

Mixing the contents of a digester under normal conditions can be a challenging task, given the various types of solids which can end up either floating in a layer along the top or settled out on the bottom. The OMI staff at the Bloomington Rialto Wastewater Treatment Plant also faced the problem of having a primary digester that had not been cleaned in over 15 years.

Mixing the contents of a digester under normal conditions can be a challenging task, given the various types of solids which can end up either floating in a layer along the top or settled out on the bottom. The OMI staff at the Bloomington Rialto Wastewater Treatment Plant also faced the problem of having a primary digester that had not been cleaned in over 15 years.

Located in Southern California, the 8 mgd treatment plant needed an answer to an ongoing problem - how to efficiently mix and maintain a failing digester whose ineffective mechanical mixing system could not keep grease, hair, rags and other floatables from forming a thick layer of scum, decreasing both the capacity and the efficiency of the digester.

The daily removal of debris that regularly clogged the existing pump was expending maintenance time and money. Also, the swirling vortex effect created by the pump tended to weave stringy materials (rags, hair) together with grease and other debris. These masses of roped and weaved material grew as they were recirculated through the digester, clogging the pump and piping and decreasing the capacity of the digester.

After discussing this problem with Rockwell Engineering & Equipment, a Southern California based company representing the Vaughan Co., the staff decided to install Vaughns Horizontal Chopper Pump, which was developed specifically for this type of application.

The pump was installed in April 1993 using an existing motor supplied by the Rialto WWTP. Its operation was controlled by a Variable Frequency Drive, running the unit at approximately 700 RPM. The initial chopping action and high flow of the pump churned up the contents of the digester to the extent that solids, broken loose from the digester, clogged the inlet piping leading to the pump.

Vaughan representatives made some modifications which eliminated the blockage problem at the entrance to the pump. After that initial adjustments, the pump ran flawlessly over the next 36 months pumping approximately 3300 GPM at 7 to 8 feet of total head. It remained in operation 24 hours a day only for the first 4 months, after which it was placed on a timed operation of 8-12 hours a day.

Benefits noted by the OMI operations and maintenance staff at Rialto included:

  • Enhanced digestion process: the chopping of solids within the digester exposed more surface area, promoting the breakdown of organics.
  • Increased methane production: the continuous breakdown of organic solids resulted in an increase in gas, potentially usable as a fuel for cogeneration.
  • No roping and weaving of rags, hair and grease. The chopping performed by the pump doesnt allow longer, stringy materials to combine.
  • Minimal buildup of floatables: by continuously chopping the contents of the digester, organic breakdown was maximized and allows for little accumulation of floating debris.

The staff also saw an improvement in the sludge being transferred from the digester to the belt filter press. Higher internal temperatures were generated by the digester thanks to a better breakdown of the solids, which contributed to a decrease in solids content.

While the digester was taken down for cleaning during a plant expansion, the Chopper Pump was disassembled and its overall condition evaluated. After three years of service, no new parts were required to return the pump to service.

While the digester was taken down for cleaning during a plant expansion, the Chopper Pump was disassembled and its overall condition evaluated. After three years of service, no new parts were required to return the pump to service. In 1995, as part of an upgrade at the Rialto Plant, the digester was cleaned. The schedule initially called for the contractor to spend approximately four weeks inside the tank, pumping out sludge, scum and large masses of hair, rags and grease normally found matted & weaved together. The cleaning operation took only two days. Time spent inside the digester was reduced significantly because all the sludge had been conditioned and emulsified, allowing the contractor to pump out the contents without encountering any problems.

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