BLOOMINGTON RIALTO WWTP SOLVES DIGESTER MIXING PROBLEM

by Bill Haberstroh,
Plant Maintenance Manager, OMI

A Vaughan H8M10 Horizontal End Suction Chopper Pump completely mixed the contents of the 80' diameter digester at Bloomington Rialto WWTP

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. Add to that the additional burden of not having cleaned a Primary Digester in over 15 years and you can then appreciate the dilemma facing our OMI staff at the Bloomington Rialto Wastewater Treatment Plant in the spring of 1993. The solution, we would find, was a single piece of equipment designed to break down and transfer solids - the Vaughan Chopper Pump.

Located in Southern California, the 8 MGD 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. Would changing to some type of gas mixing be an answer? Probably not - the gas options examined were extremely expensive to retrofit and had shown only limited success in other plants. Perhaps another mechanical type of system using rotary sludge mixers or mixing propellers? This too carried a price tag which was not in the budget. Besides, this type of mixing was prone to high maintenance caused by rags and debris hanging up on mixing paddles, propellers and other internal equipment.

A system involving mechanical mixing with a non clog or screw impeller type pump would continue to be one of the most cost effective and simplest solutions. This type of system offered all kinds of advantages for operations and maintenance; it was totally accessible for adjustments and/or periodic maintenance and was an excellent means of recirculating solids around the digester.

However, there were two big disadvantages in using either a non clog or screw impeller type pump:

    1) The daily removal of debris, which regularly clogged the existing pump, was expending excessive maintenance time and money. Continued efforts to keep the pump from clogging were futile.

    2) 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 would continue to grow as they recirculated through the digester, clogging the pump and piping and decreasing the capacity of the digester.

I  had successfully used Vaughan Chopper Pumps in several operations around the plant including primary scum and sludge transfer, but had never considered them for the greater flows required for mixing an 80 foot, 950,000 gallon digester. After discussing this problem with Rockwell Engineering & Equipment, the Southern California based Vaughan Representative, I agreed to install a new higher flow Chopper Pump which Vaughan had developed specifically for this application.

A bit skeptical at first as to whether it would work any better than a non-clog or screw impeller type pump, I was finally convinced by the Vaughan Performance Guarantee, which went beyond the normal workmanship & materials warranty to offer a guarantee that the pump would not clog. Vaughan also explained the additional benefits attainable using a Chopper Pump in this application:

    - Enhanced digestion process; the chopping of solids within the digester exposes more surface area, promoting the breakdown of organics.

    - Increased Methane Production; the continuous breakdown of organic solids results in an increase in gas, potentially usable as a fuel for cogeneration.

    - No Roping and Weaving of rags, hair and grease normally caused by standard non-clogs as well as vortex type pumps. The chopping performed by the pump doesn't allow longer, stringy materials to combine.

    - Minimal Buildup of Flotables; by continuously chopping the contents of the digester, organic breakdown would be maximized and would allow little accumulation of floating debris.

    - Longer Periods Between Scheduled Digester Cleanouts, since the breakdown of solids would eliminate accumulations which might affect capacity or performance of the digester.

    - Ease of Maintenance: the location of the horizontal Chopper Pump outside the digester allows easy access for adjustments and routine maintenance.

A Vaughan Horizontal Chopper Pump was installed in April of 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. At that point Vaughan Company made some minor modifications which eliminated any blockage at the entrance to the pump.

After Vaughan made the 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.

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

    - Better sludge quality as transferred from the digester to the belt filter press.

    - Higher internal temperatures generated by the digester thanks to a better breakdown of the solids.

    - Lower solids content inside the digester.

    - Increased gas production by the digester.

    - Minimal abrasive wear. While the digester was taken down for cleaning during a plant expansion, the Chopper Pump was disassembled and it's overall condition evaluated. AFTER THREE YEARS OF SERVICE, NO NEW PARTS WERE REQUIRED TO RETURN THE PUMP BACK INTO SERVICE.

    - Although standard non-clog pumps in this application are required to run 24 hours a day, the Vaughan Chopper Pump required only 8-12 hours of operation per day after the initial breakdown of material over the first few months. This shortened operating time resulted in reduced operating costs and energy savings.

One of the greatest benefits which the Chopper Pump provided would be evidenced in 1995 when the Rialto Plant, as part of an upgrade, had it's digester cleaned. Not realizing the impact of the Chopper Pump, the schedule 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 Contractor began cleaning operations in Digester #1 on June 10, 1996, and completed the scope of work 2 days later on June 12. The time spent inside this digester was reduced significantly because all the sludge had been conditioned and completely emulsified, allowing them to simply pump out the contents without encountering any problems. There were no weaved masses of grease hair and rags to deal with, no clogged pumps during the operation and none of the headaches normally encountered when dealing with these types of concentrated solids.

I attribute the overall outstanding performance of the Vaughan Chopper Pump to it's ability to break down solids which enhances the digestion process. The simplicity of using a Vaughan Chopper Pump alone, as compared to more complicated mechanical or gas mixing systems, made their choice the most practical, efficient and cost effective way to deal with digester mixing.