Shredder Pump Sizing

[TurtleboyTom]

I am looking to change the pump and piping for the wet well lift station that feeds a small packaged extended aeration activated sludge wastewater treatment plant. The current pump is a submersible solids handling pump and it is oversized, which is causing intermittent, infrequent and high flows. In order to go to a smaller pump I would need to size the discharge pipes to 2" and equip the pump with a VFD.

I'm seeking some advice and trying to see if my concerns are valid.
1. Going to a small diameter pipe might cause plugging issues, but will the shredder take care of that?
2. Should a shredder pump be operated on a VFD at low speeds?

 

[LittleInch]

You might want to try the water treatment and distribution forum who have more experience of waste systems.

I can only guess that the vendor would help you decide if the smaller pump can cope with your unknown solids, but why the VFD?

I would guess that the VFD isn't a great plan for a shredder pump.

Any links to a particular pump you have in mind?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

look for Muffin Muncher shredder, Google is your friend

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TurtleboyTom]

https://www.bjmpumps.com/product/sk08cf-460t-4/

Here's the pump I'm looking at, simply because it's the lowest flow I can find. The run time on our 200 gpm pump is often less than 1 hour a day.

 

[LittleInch]

What is your head requirement?

Note those pumps are limited to 10 starts per hour and don't seem to mention VFD.

The head range is about 14 to 38 ft, but ideally about 30 ft. which is about 50gpm.

So would run for 4 hours/day?

This seems like a very small overall flow going on here. Can't you use a bucket?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TurtleboyTom]

Haha. It is a very small flow. If I could size the pumps at 25 gpm I would.
Head is 16 feet.

 

[LittleInch]

Well at 16 feet head you'll be doing about 85 gpm and operating far right of the curve.

My guess is that your bigger pump was operating off the end of the curve if this was the same head requirement.

Ask the vendor if it can work effectively at a lower speed or a smaller impellor.

I was only half joking about the bucket - flows this low will be difficult to move effectively.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IHQ22]

What is the size of the wet well and how is the pump controlled? floats or a level sensor?

The number of start/stop is normally determined by the wet well size. If at all possible, you can lower the amount of daily start/stop by raising the pump start float.

On the pipe size, I would not go smaller than 2".

 

[ashtree]

There are shredder pumps out ther that are a lot smaller than 85gpm but you are going to be in a pump designed for single dwelling systems. Longevity might not be good if you are doing a lot of hours.
A shredder will not be very good at low speed, as they rely on flywheel effect to a degree to "power through" the solids which typically come intermittently.

Two other options:

Recirculate some of your flow back to the pumpwell. This is inefficient but it will help with your problem.

Put a mixed equalisation tank in the front of your plant and feed the plant with a VFD controlled positive displacement pump giving you the flow that the plant needs. The pump in the well just feeds the equalisation tank. Sizing the equalisation tank may be problematic and this will require footprint and capital to implement.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"