Technology Tips – Grinders for Sewage Sludge Systems

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Technology Tips – Grinders for Sewage Sludge Systems

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Ever since the development of activated sludge processes in the early 20th century, wastewater engineers have had to contend with debris getting into them. Unwanted materials cause pump clogging, fouling of thickening technologies like centrifuges, and potential damage to a variety of critical treatment equipment. To combat this, engineers began to deploy reduction and removal equipment into sludge systems, including additional screening and single-shaft comminutors. 

In the early 1970s, the two-shafted grinder was introduced to the market and has been a fixture in wastewater treatment plant sludge systems ever since. The slow speed but extremely high torque operation of a two-shafted grinder proved itself as a reliable solution to shred the toughest solids and make the life of a wastewater operator much easier. The industry has seen significant changes since the 1970s but the need for grinders has persisted and evolved. JWC Environmental started offering two-shafted grinding more than 45 years ago with its Muffin Monster grinders. Kevin Bates, director of product management and marketing, recently answered several questions about the use of grinder technology in the wastewater industry.

Q) Why were grinders first used with a treatment plant’s sludge system?

Bates: The No. 1 reason grinders have become a staple in treatment plants around the world is their ability to protect sludge pumping equipment. Due to the viscous nature of sludge there are a variety of pumping technologies that are typically employed to move the thickened materials. Some of these like PC pumps and rotary lobe pumps rely on elements with very close tolerances. These pumps can be easily damaged by harder debris and be fouled by rags or other stringy materials in the sludge. The grinders are used to simply shred the materials small enough so that the pumps can do their job efficiently.

Q) What has changed with how grinders are used in sludge systems?

Bates: While protecting pumps still remains the top application, the sophistication of biosolids processing in treatment plants has evolved significantly over the last 45 years. This has exposed new areas where grinders are needed to help overall plant efficiency. Muffin Monster grinders are routinely used to protect centrifuges and heat exchangers from debris that can put those systems out of commission. The cost of a grinder is a very small insurance policy compared to a significant repair on a centrifuge.

Q) With the increase in primary treatment, especially headworks fine screening, isn’t there less debris in sludge today?

Bates: The short answer is yes, for plants with fine screening systems. Some of the largest items that would have historically been found in a plant’s treatment systems are now pulled out at the headworks. That said, we see that smaller debris will still make its way into a plant and into digestion systems. This can include hair, smaller wipes materials, and other inorganics. Additionally, there are secondary sources like trucked-in septage or sludge from other facilities that can introduce additional trash. Small debris has an amazing ability to come together in digesters and form hairballs, wipes ropes and other tougher solid masses. When sludge is drawn off the bottom of digesters or through a recirculation loop the solids will quickly shut down pumps.

Q) What has changed in sludge grinder technology?

Bates: It has been an interesting journey with sludge grinder technologies coming and going through the years, only for some to return again. We have recently seen some companies try to reintroduce macerator technologies to the North American market for sludge pump protection.

Macerators, which use a high-speed blade rotating against an orifice plate, were tried in the 1980s and faced the same problems they are seeing today. The high-speed cutting cannot handle the tougher solids, while the rags, hair and other stringy materials accumulate in the equipment and finally shut them down.

The only consistently accepted technology has been two-shafted grinders like JWC’s Muffin Monsters. Two-shafted grinders use low-speed and extremely high torque to cut through tough solids. The two shafts of cutters with minimum clearance between them are able to slice like scissors through stringy materials while crushing the harder solids. 

One new advancement from JWC Environmental has been the Wipes Ready technology for the Muffin Monster two-shafted grinders. This technology includes new serrated cutters that are specifically designed to take on disposable wipes. The Wipes Ready cutter first saw overwhelming success in protecting lift station pumps within collections system networks where the wipes problems are the worst. More recently treatment plants have been using the technology for their inline Muffin Monster sludge grinders. Users are reporting even better performance in cutting through rags and hair within recirculation loops and WAS pipelines.

Q) For two-shafted grinders what are important considerations? 

Bates: Rugged construction and a history of reliable performance are paramount in selecting a two-shafted grinder. Like everything else in a treatment plant, they are working in a harsh and often hazardous environment. If a grinder needs frequent attention they are not only burdening a plant’s limited maintenance staff, they are also exposing the staff unnecessarily to bio-waste and other hazards as they perform the repairs.

There are various concepts of two-shafted grinders on the market today that have real trade-off to be considered. One is the idea to only support the twin shafts at the top end of the grinder. This is done by eliminating the bottom bearings and seals and allowing the shaft ends to float in the pipeline. This approach eliminates seals having to survive in the sludge, as well as lowers the cost. Unfortunately, with support at only one end, it also allows the two shafts to flex as solids pass through them, which can lead to eventual failure of the top seals as well as the shafts. JWC’s Muffin Monsters always provide top and bottom tungsten carbide seals that rigidly control the shafts to deal with the loading of tough solids and reversals.

Finally, it's important to find a manufacturer that will stand behind its products and promises. This means having the ability to provide service and technical support, and of course to be willing to fulfill its warranty obligations if ever required. These traits tend to speak to the long-term commitment of the company to its customers and the products it provides.

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