What's Best For Wastewater Solids Reduction? Sewage Grinder or Macerator?

Author: Sara Peters | July 25, 2017 | Category: Troubleshooting, Wastewater Treatment

It's inevitable. Wet wipes, sticks, croquet balls (yup, we've seen it) and other random objects somehow make their way to wastewater treatment plants. That's what screens are for right? Despite step, perforated or drum screening systems at wastewater treatment plants, debris finds a way through. Where screens fail, solids reducing equipment steps in. But which is right for your process?

230_x_300_Plover_Rags.jpgFlushable wipes, string, rags, hair, plastic fragments, applicators, plastic bags, can all pass by screens, causing clogging, blockages, and damage to downstream pumps, valves, and processing equipment. We don't have to tell you how this affects your overall maintenance time and contributes to parts and labor costs.

If your plant has considered implementing a solids reduction system to curb the rising costs attributed to debris, read on. We'll break down the differences between popular solids reduction equipment, and where they're best applied.

Wastewater Twin-Shaft Grinders vs. Macerators

Outside of screening systems, solids reduction equipment falls into two categories: macerators and twin-shaft grinders.


Maceration technology provides the best results when installed after primary screening or headworks systems. These units are specifically designed to sheer cut any material that flows through. Some designs provide heavy debris removal traps, as macerators are not designed to cut hard solids like rocks and metal.

How a wastewater macerator works

Twin-shaft grinders

Twin-shaft grinders operate using two counter operating shafts with cutting blades attached. As the cutting blades turn, they pull moving solids around the radius of the housing into the center between the cutting blades. This is a straight through process, moving product through the grinder without a debris trap.

twin shaft grinder cross section.png

Which type of Solids Reduction Equipment should I use?


Macerators are designed for in-line pipe installations. Macerators need to have liquid flowing through the pipe, as they cannot run dry. These units tend to do better with small solids like hair, wipes, rags, and plastics as compared to large heavy solids. But don’t be fooled, wadded up rags and wipes are just as troublesome to pumps as wood or metal. In this case, macerators can provide ideal protection. Remember, macerators are best suited for refined cutting after the headworks of a wastewater treatment plant.

  • Defined Scissor-Like Cut
  • Liquid stream moves solids through cutting element
  • Solids must be suspended in liquid stream. Does not run dry
  • Heavy solids separation (built in “rock trap”)
  • Medium Coarse to Fine Cutting
  • Pressure to 87 psi
  • Lower Torque Required

vogelsang-xripper.jpgTwin Shaft Grinder

Twin shaft grinders are designed for both in-line pipe installations and channels. These units can run dry continuously and can handle both medium-to-large solids.

  • Rips and tears solids apart
  • Rotation of the cutter blades pull solids into cutting elements
  • Wet or dry solids, runs dry
  • Typically not outfitted with a solids separator or “rock trap”
  • Coarse to medium-coarse grinding
  • Pressure to 232 psi
  • Higher torque required
Solids Reduction Examples

Here are two examples of cutting results between the two solids reduction categories.

Solid: 10mm Nylon Rope: The twin-shaft grinder shredded the rope. The macerator exposed and cut the inner filament.

nylon rope.png

Solid: Q-Tips: The twin-shaft grinder cut most of the shafts and shredded the cotton. The macerator produced fine consistent pieces.


Because a twin-shaft grinder is often placed in front of the headworks or in lift stations far from a treatment plant, it could encounter much larger debris. That might not always include a boot or a pair of jeans, but know that a well-designed unit should be able to handle such material as shown here in a testing run done by Vogelsang USA using an XRipper Twin Shaft Grinder.

boot.jpgchopped boot.jpg

jeans.jpgchopped jeans.jpg

Where is Solids Reduction Equipment Placed in the Wastewater Process?

In wastewater treatment plant applications, twin-shaft grinders and in-line macerators can be added or retrofitted into several applications. Some of these applications include:

  • Headworks
  • Alternative to two-stage screening
  • Pump stations
  • Septage Receiving
  • Grease Receiving
  • FOG Receiving
  • Primary Scum
  • Primary & Secondary Sludge
  • RAS, WAS, TWAS Sludge
  • Dewatering Equipment Protection
  • Heat Exchanger Protection
  • Pump Protection

This diagram will provide you with a general sense of where the two units are most applicable.


Every situation is different, and while a twin-shaft grinder is suited for most any application where solids reductions is needed, you’ll find that a macerator often provides much better performance as you move further into the wastewater process.

Still not sure which is right for your process? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.

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Sara Peters

Sara Peters

Sara leads Crane Engineering's blogging team, coming up with fresh stories and insights for our readers to apply to their every day work.

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