The recycling system consists of collection, settling, filtration, reconditioning and reuse of the hydroblasting water. The water is continuously recycled during the hydroblasting operations until the operation has been completed. Initially, potable water is mixed with sodium nitrite before being pumped under pressure to high pressure nozzles. The water is pressurized and sprayed on to the steam side surfaces of the boiler tubes to remove scale build-up. The wastewater from the blasting operation is collected and pumped out of the boiler.

Utilizing the recycling unit, wastewater is sent to two 650-gallon settling tanks. Once enough water has been collected, the fresh water is shut off and the wastewater is recycled through the unit. The wastewater is pumped to the first collection tank, where it is gravity fed to the second tank, allowing larger, suspended particles to settle out. It is then pumped from the second settling tank through a parallel train of six cartridge filters each side rated at 50, 25, and 5 microns, respectively. The recycled water passes through a heat exchanger before being returned to the hydroblasting operation for reuse. The filters remove small suspended material that cannot be removed through gravity settling. The filters do not remove sodium nitrite, which remains above 1,200 ppm, the level needed to prevent flash rusting. As a result, no additional sodium nitrite needs to be added to recycled water.

The recycling unit has resulted in no known mechanical failures during the hydroblasting operations, nor does it cause a decrease in the quality of the cleaning operation. The sodium nitrite stays above the 1,200 ppm threshold, preventing rusting and surface damage. After inspection, no residuals were observed on the tube surfaces of the boilers cleaned with the hydroblasting recycling unit and the cleaning operation was completed satisfactorily.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.

• With the use of the recycling unit, the amount of wastewater generated by hydroblasting operations can be reduced by 90%.
• Since less potable water is used in the operation, the amount of sodium nitrite used can be reduced dramatically.
• No additional personnel are needed to operate the recycling unit.

• This process generates spent filters that must be disposed as hazardous waste.

The cost of the recycling unit is around $25,000, which includes the equipment, spare filters for a year, training, and support. The savings generated will vary from activity to activity depending on the number of boiler cleanings performed.

Assumptions:

• 10 boiler cleanings are conducted a year.
• 10,000 gallons of wastewater are generated per boiler cleaning.
• Cost to dispose of wastewater is $2.50/gallon (including transport).
• 2 hours of labor is needed to set up hydroblast recycling equipment.
• No extra labor is needed to operate the recycling system.
• 80 labor hours are required per boiler cleaning.
• The labor rate is $45/hr.
• 1,000 gallons of wastewater needs to be disposed of after hydroblasting the boiler using the recycling system.
• 10 pounds of sodium nitrite is used with the recycling system and 100 pounds of sodium nitrite is used without the recycling system.
• Sodium nitrite costs $2.35/lb.
• Water costs $4/1,000 gallons.
• Disposal of used filters costs is $50/cleaning.
• The yearly maintenance cost for recycling system is $200.

Table 1. Annual Cost Comparison for Recycling Versus No Recycling of Hydroblast Wastewater

Economic Analysis Summary:

• Annual Savings for Secondary Use: $225,875
  
• Capital Cost for Diversion Equipment/Process: $25,000
  
• Payback Period for Investment in Equipment/Process: 2 cleanings

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