A Hydro International Technology Efficiently Extracts, Cleans and Dries Grit

The Hydro GritCleanse system achieves low organic content and high total solids in washed grit along with efficient particle retention.

A Hydro International Technology Efficiently Extracts, Cleans and Dries Grit

The system uses a structured laminar flow that helps retain fine and slow-settling grit particles while an interior baffle prevents short-circuiting.

Clean-water plants increasingly look for higher removal efficiency in headworks equipment, including grit systems.

The goal generally is cleaner, drier grit combined with efficient grit capture and retention to protect downstream equipment and processes and extend asset life.

In line with this, Hydro International has introduced the Hydro GritCleanse fluidized bed grit washing and dewatering system. The device receives slurry from a grit capture system, which can include Hydro International’s Grit King HeadCell, and then efficiently removes organic solids and dewaters the material before discharge.

Cleaner and drier grit costs less to landfill and reduces odor issues. Patrick Herrick, regional sales manager, talked about the technology in an interview with Treatment Plant Operator.

TPO: What market trends led to the development of this technology?

Herrick: California and other West Coast states are looking at limits on organic content in material going to municipal landfills. There is also a general desire among plant operators to reduce organics and the associated odors in the headworks.

TPO: What else is driving greater grit processing performance?

Herrick: In the entire market, we’re seeing a trend toward higher performance expectations from headworks processes. For example, years ago 1/2- or 3/8-inch screens were pretty typical. Today, quarter-inch is the coarsest most plants will look at. We’re seeing a similar trend in grit management. The market perceives that as an industry, we need to target finer grit particles to maximize capture efficiency and thereby protect downstream processes.

TPO: How much of an improvement does this equipment achieve in producing cleaner and drier grit?

Herrick: Our legacy grit washing equipment typically has been able to achieve 15 to 20 percent volatile solids or less in the output, while the Hydro GritCleanse system consistently achieves less than 5 percent volatile solids. Whereas the legacy equipment was guaranteed to produce 60 percent or greater total solids, this product achieves about 90 percent.

TPO: How well does this technology perform in grit retention?

Herrick: At flows up to 300 gpm, it retains 95 percent of grit particles greater than or equal to 106 microns. At flows from that point to 400 gpm, we characterize retention as 85 percent.

TPO: In brief, how does this process operate?

Herrick: Flow enters the conical clarifier tangentially. This forces the grit to contact the vessel walls, creating drag and establishing a rotary flow. The grit settles into the low-velocity boundary layer at the inside vessel wall. A structured laminar flow helps retain fine and slow-settling grit particles while an interior baffle prevents short-circuiting. Once in this boundary layer, the grit decelerates further and settles into a gently agitated fluidized bed.

There, physical forces separate the grit and volatile solids by density. The grit falls to the bottom of the bed, and the lighter organics stay in suspension. Organic material attached to the grit particles is scrubbed away by friction between particles, and the grit descends to the bottom. Organics then flow out through a valve that opens on a timed basis. The cleaned grit descends to a dewatering screw at the bottom of the unit that runs intermittently to discharge the grit.

TPO: What exactly makes the technology effective in separating organics?

Herrick: The keys are residence time in the bed, the action of a stirrer that keeps the grit from ratholing and getting stagnant and the four-point injection of the fluidization water from below. This forces the organics toward the top of the bed, where they stay until the organics discharge valve is opened.

TPO: What is the key to efficient grit retention?  

Herrick: It’s the tangential feed, where we put the grit slurry in immediate contact with the clarifier itself. We give the grit very little opportunity to escape with the effluent. That, coupled with a low surface overflow rate on the clarifier, helps provide a residence time and surface loading rate low enough to retain the finer grit while washing the organics off.

TPO: What maintenance does this system require?

Herrick: The shafted dewatering screw has a bearing at the bottom that we consider a low-maintenance item. It’s essentially the same design as on our SpiraSnail grit dewatering unit. We have a five-year history with the SpiraSnail, and that bearing has proven to be a very solid design. Another aspect of the Hydro GritCleanse is that the screw is on a single-point pivot. If the maintenance staff needs to separate the screw from the clarifier, the screw pivots to the side without the use of a lifting device for easy maintenance access. Otherwise operators really don’t have to do anything except a weekly check to make sure everything is working.

TPO: Is the process fully automated?

Herrick: Yes. It comes with a PLC control panel. If it is integrated with our primary grit removal system, we control the entire system, including the pumps that feed the grit slurry to the Hydro GritCleanse unit. Everything is incorporated into a single panel, which typically would be a PLC-driven system with a human-machine interface panel for the operators. If the plant has SCADA or any other distributed control system, we can integrate the control system through that.


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