7 answers about this wastewater treatment technology.
by Kevin Bates
September 18, 2018

Two-shafted grinders have been a fixture within sludge treatment systems of wastewater treatment plants since their invention in 1973. The industry has undergone significant changes since then, but the need for grinders has persisted and evolved with the requirements of plant operators and engineers.

Here are common questions about the use of grinder technology in sludge systems and some important things to consider when selecting sludge grinders.

1. Why were grinders first used within a treatment plant’s sludge system?

The number one 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, several kinds of pumping technologies are typically employed to move the thickened materials. Some of these, like progressive cavity pumps and rotary lobe pumps, rely on elements with close tolerances. These pumps can be damaged by harder debris and fouled by rags or other stringy materials in the sludge. The grinders are used to shred the materials small enough so the pumps can do their job efficiently.

2. What makes grinders a good option for protecting sludge systems?

The cutting shafts on most two-shafted grinders operate at a low rotational speed but with a high torque for tearing through tough solids. This is important because one thing that is certain in wastewater systems is that the operator will encounter the unexpected. While most sludge is consistent and benign, it only takes one rag ball or piece of clothing to shut down sludge handling equipment.

two-shafted grinderImage 1. Two-shafted grinder for sludge system (Images courtesy of JWC)

3. What has changed in the applications for grinders in sludge systems?

Protecting pumps remains the top role for grinders in sludge systems, but the sophistication of biosolids processing in treatment plants has seen significant advancements during the last 45 years.

These changes have exposed new areas where grinders are needed to help overall plant efficiency. Grinders are now routinely installed to protect centrifuges and heat exchangers from debris that can put those systems out of commission. The cost of a grinder is a small insurance policy compared to a significant repair on a centrifuge or other expensive piece of capital equipment.

4. With the improvements in primary treatment, especially headworks fine screening, is there less debris in sludge today?

The short answer is “yes” for plants with perforated plate 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, smaller debris like hair, bits of rag materials, and other inorganics still make their way into the plant and the digestion systems, driving the need for grinders. Another common entry point for unwanted trash is through secondary sources like trucked-in septage or sludge from remote facilities that is not passed through the headworks fine screens. This small debris has an amazing ability to come together in the digester and form hair balls, wipes ropes and other tougher solid masses. When sludge is drawn off the bottom of tanks or passes through a recirculation loop, the solids will quickly shut down pumps.

5. What has changed in sludge grinder technology?

It has been an interesting journey with sludge grinder technologies coming and going through the years only for some to return again. Macerator technologies are again gaining traction in 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 are on the rise today. Tougher solids such as rags, hair and other stringy materials need tough equipment to avoid them causing shutdowns.

New technology advancements including serrated cutters are specifically designed to take on disposable wipes and rags. One such cutter first saw overwhelming success in protecting lift station pumps within collection system networks where wipes problems are the worst. More recently, treatment plants have been using the technology for their inline sludge grinders. Users report better performance in cutting through rags and hair within recirculation loops and waste activated sludge (WAS) lines.

sludgeImage 2. Sludge after going through a grinding system

6. What are the maintenance considerations for grinders in sludge systems?

No plant wants another piece of equipment to maintain. Grinders within a sludge treatment system should be able to run for many years with limited need for maintenance. Some manufacturers only recommend an annual inspection through inspection ports. Some two-shafted grinders will require more frequent gearbox oil changes or cleaning of trash traps, which can potentially be a burden for plant staff.

Once the cutters reach the end of their useful life, most manufacturers provide either replacement components or a fully assembled cutter cartridge. The benefit for plants of fully assembled cartridges is multifold.

Cartridges come complete with new seals, new gearing and new shafts, plus factory assembly. Cartridges also lend themselves to a quick change out and less time that the grinder is offline. If a plant is only ordering parts, they may struggle with pulling the equipment apart and develop a plan so reassembly goes as planned.

7. What are other important considerations for two-shafted grinders?

Rugged construction and reliable performance are paramount in selecting a two-shafted grinder. Like everything else in a treatment plant, grinders are working in a harsh and often hazardous environment. If a grinder needs frequent attention, it not only burdens a plant’s limited maintenance staff, it can also expose the staff unnecessarily to biowaste and other hazards as they perform the repairs.

There are various design concepts of two-shafted grinders on the market today. One cost-saving concept is to only support the twin shafts at the top end. This is done by eliminating the bottom shaft bearings and seals and allowing the shaft ends to float freely in the pipeline. With support at only one end, the two cantilevered shafts are allowed to flex as solids pass through them.

A different approach uses top and bottom tungsten carbide seals, which rigidly contain the shafts to deal with loading tough solids and reversals while being able to handle direct exposure to the sludge.

Finding a manufacturer that has the ability to provide service, technical support and, of course, be willing to fulfill its warranty obligations if ever required is important. These traits speak to the long-term commitment of the company to its customers and the products it provides.