Aqueous immersion cleaning is an effective method of parts washing and is considered to be the simplest and least aggressive cleaning method available. Also, it may be the most environmentally friendly cleaning method since its constituents are generally less toxic than other cleaning systems and non-ODS. Aqueous immersion is useful when dirt is easily removed and low equipment cost is important; however, it is generally slower than other methods of washing and more particular about the dirt and substrate materials.

Aqueous immersion cleaning can be enhanced in several ways by using several different techniques either exclusively or together. The most common methods are heat, agitation, and electricity.

1. Heat - the effectiveness of aqueous solutions increases with temperature; however, there is an optimum temperature for each situation that is governed by the physical characteristics of the materials involved.
2. Agitation - achieved by constant movement of the cleaning solution through and around the part being cleaned. Agitation can be maintained mechanically through the use of spargers, mixers, and rotating barrels, or it can be maintained through ultrasonic/megasonic/vibratory means. In general, air agitation is not recommended for highly alkaline solutions, mainly because it can lead to carbonate buildup in the tank.
3. Electricity - electrocleaning should not be used as the initial cleaner to remove the bulk of soils, but should instead be used to remove smuts, light flash rust, light oxides and residues from previous cleaning operations.

Ultrasonic cleaning is a process of agitating the part being cleaned through high frequency sound waves. This action literally shakes the dirt off. It provides excellent penetration and cleaning in the smallest crevices and between tightly spaced parts in a cleaning tank.

A problem that is often associated with immersion tanks is that they can become heavily loaded with soils in a short time. If soil loads are high, separation and filtration systems may help, but may also require additional chemical replenishment since surfactants will also be removed. A prewash station will usually increase the time between bath changes. But between the prewash, the rinse and possibly a drying station, an aqueous process will generally require more floor space than an equivalent solvent process.

The aqueous cleaning solution may be either alkaline or acidic, but the bulk of the industrial mainstay is alkaline. Even with that, the chemistry in the bath must be carefully matched to both the type of soil on the part and the substrate material, since the process depends mainly on chemical solvency. When properly matched, it is the least aggressive form of wet cleaning and works best for the removal of soluble fluids and soils.

Immersion cleaning allows a facility to use non-toxic or non-ODS chemicals instead of solvents such as methyl chloroform, methyl ethyl ketone and other ODSs. The decrease in toxic and ODS solvents on site may reduce the on site storage below any of the reporting thresholds of SARA Title III for those chemicals (40 CFR 355, 370, and 372; and EO 12856). In addition, the use of less toxic chemicals may decrease the need for a facility to obtain an air permit (40 CFR 70 and 71). Switching from a halogenated solvent (i.e., methyl chloroform) may also decrease the need for a facility to meet the NESHAPs for halogenated solvent cleaning (40 CFR 63). Using a non-ODS substance will also help facilities meet the requirements under 40 CFR 82, Subpart D and EO 12843 requiring federal agencies to maximize the use of safe alternatives to class I and class II ozone depleting substances, to the maximum extent practicable. Moreover, depending on what is used for the immersion cleaning (i.e., water and detergent) the facility may decrease the amount of hazardous waste generated (i.e., no waste solvent generated). Waste reduction is required under RCRA, 40 CFR 262, Appendix. The reduction of hazardous waste may also help facilities reduce their generator status and lessen the number of regulatory requirements (i.e., recordkeeping, reporting, inspections, transportation, accumulation time, emergency prevention and preparedness, emergency response) with which they must comply with under RCRA, 40 CFR 262.

The compliance benefits listed here are only meant to be used as a general guideline 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.

The handling and use of cleaners follows general, common chemical handling rules. Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.

• Simplicity of process
• Lower capital costs
• Safer chemicals used

• Can be more time consuming
• May require repeated applications to obtain same results
• Likely to require a larger work area
• Process produces large wastestreams

Immersion cleaning equipment varies in cost. Simple tabletop ultrasonic tanks may cost under $500. Large precision cleaning systems consisting of multiple tanks and drying stations with automated parts handling may cost $500,000. Typically immersion cleaning is a less capital intensive method of cleaning, so lower costs can be expected. The capital cost for medium and heavy duty immersion units will vary considerably, depending upon the unit type and its application. Capital costs for these systems can range from $5,000 to $12,000. According to the Naval Air Station North Island, both energy and material costs are highly dependent on the type of chemical being used and the temperature at which it is being applied.

Assumptions:

• Labor rate: $30/hr
• Total labor requirements for solvent cleaning: 20 hr/wk or 1,040 hr/yr
• Total labor requirements for immersion cleaning: 22.5 hr/wk or 1,170 hr/yr

Immersion Cleaning

• Immersion type cleaning equipment cost: $6,000
• The immersion cleaning system includes wash and rinse stations
• System electrical requirements: 700 kW-hr/yr. at a cost of $0.08/kw-hr
• Process wastewater: 16,500 gal/yr. at a disposal cost of $8.24/1000 gal.
• Alkaline cleaning bath: 150 gal at a cost of $1.30/gal.
• Water usage cost: $1.94/1000 gal.

• Solvent usage: 600 gal/yr. at a cost of $11/gal.
• Waste solvent: 100 gal/yr. at a disposal cost of $2/gal.

Annual Operating Cost Comparison for Immersion Cleaning and Solvent Cleaning

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Approval is controlled locally and should be implemented only after engineering approval has been granted. Major claimant approval is not required.