Designing explosion-proof equipment for wastewater treatment.
by Jonathan Herlong and Steve Mosley
November 21, 2018

Federal standards protect groundwater by requiring some landfills to use plastic liners to collect and treat leachate. Leachate, which is the resulting liquid that drains or “leaches” through a landfill after it rains, varies widely in composition, depending on the age of the landfill and the type of waste there. When a new wastewater treatment facility was being built next to an existing landfill, the specifying engineers had to consider how leachate entering their pumping processes might affect the safety of plant personnel and pumping equipment.

While the specifying engineers at the consulting design firm were reviewing the requirements of one 15-foot deep sump inside the wastewater treatment plant, they determined that the process required:

leachate landfillImage 1. Leachate from a landfill near a new wastewater facility may have affected the safety of the personnel and pumping equipment. (Images courtesy of BJM Pumps)
  1. Pumps with explosion-proof motors—This indoor sump was classified as a potentially hazardous area because of the gases that could result from the combination of fluids entering it. Explosion-proof zones are defined in areas where flammable gas has the potential to be ignited by the arc of an electrical device.
  2. Pumps that can handle acidic fluid—The sump would be collecting unscreened building wash-down water, membrane bioreactor (MBR) permeate, cooling tower blowdown and dewatering press filtrate. In addition to creating a potentially explosive atmosphere, this combination of wastewater could create an acidic solution with low pH levels, which can corrode standard cast iron submersible pumps.
  3. Pumps capable of passing rigid solids about a half-inch in size and spherical in shape—The leachate entering the wastewater sump was estimated to contain up to 10,000 milligrams per liter (mg/L) of solid materials.
  4. Pumps sized to produce 600 gallons per minute (gpm) at 78 feet of total dynamic head (TDH)—After the specs were fully defined, the consulting design firm chose to work with a company that supplies a range of rotating process equipment to industrial and municipal operations throughout the southeastern United States. The engineers reviewed the pump specs and recommended an explosion-proof pump.
liquid drainsImage 2. Leachate is liquid that drains from a landfill after it rains.

The explosion-proof submersible shredder pumps were recommended because they are engineered with necessary features, such as:

  • The pump has an explosion-proof motor approved for use in Class I, Division 1, Groups C & D hazardous locations. This approval certifies that the submersible pump motor is spark-free, has no source of sparking inside the motor that can cause ignition and, in the case where ignitable gases might enter the inside of the motor housing, the design of the pump motor has the capacity to extinguish the flame from that ignition before the flame can exit the vessel. The vessel can contain the pressure generated by an internal explosion, thereby keeping plant personnel safe.
  • The pump is manufactured from corrosion-resistant stainless steel. The pump end components are made of 316 stainless steel, including all wear and wet parts: impeller, wear plate, seal chamber and pump volute. The elastomers, such as O-rings, lip seals and gaskets, are made of a chemically resistant flouroelastomer (FKM). The motor is treated with an epoxy coating that holds up in aggressive chemical environments. With the right metallurgy and durable construction, this submersible shredder pump would be ideal for pumping this particular process waste.
  • The pump is engineered to shred solids and has been proven to cut and pass solids up to 3.5 inches in size. Specifically designed for industrial shredding applications, the pump uses an impeller to cut against a spiral-shaped diffuserplate and continuously rip apart solids with 360-degree shredding action.
  • The pump has a three-seal motor protection. The motor is protected with an oil-lubricated double seal design where the double mechanical seals are comprised of a lower seal made of silicon carbide/silicon carbide and upper seal faces made of carbon/ceramic. An additional lip seal is installed above the impeller to help prevent abrasives from entering into the seal chamber. Seal leakage detectors are installed in the seal chamber and offers early warning should water enter the chamber. The pump has an air-filled motor with winding protection and National Electrical Manufacturers Association (NEMA) Class F motor insulation, allowing the motor temperature to rise to 221 F before an automatic switch turns the pump motor off if the temperature and/or amp draw raises too high. When the motor cools, the switch is designed to automatically reset and the pump will once again begin operating. Since the fluid temperatures were estimated to range between 50 F and 90 F, this would be a nonissue for the pumps.

With the assurances provided by the consulting firm, the owners of the new wastewater treatment facility approved the purchase of the submersible shredder pumps.

In April 2017, two explosion-proof submersible shredder pumps were installed in the indoor sump. The pump company provided the guide rail system that enabled the installation team to easily install both pumps, which are run off level control.

The submersible shredder pumps are set up to run in an alternating lead-lag configuration, to prolong pump life and to accommodate possible higher inflows into the sump.

Managing safety is a necessity for many industries. The implementation of explosion-proof zones on the job site has created a need for explosion-proof pumping equipment and consulting engineers who must know what proven technology is available to the market.