According to the Deputy Force Environmental Advisor, N451 at U.S. Naval Forces Japan, the oil/water separator should be protected from precipitation with a cover as part of the equipment, or provided with a shelter when installed at the facility (Navy activity action). The oil/water separator should also be provided with an expanded metal catwalk with railings for operator safety during operation and maintenance of the unit. This protects the operator from slipping or falling.

Oil/water mixtures are separated in the unit by conventional gravity separation and by enhanced coalescence in the coalescing pack. These two steps may take place in separate compartments or in a single chamber. The selection of the appropriate type of system depends on the amount of wastewater to be treated, its physical and chemical characteristics, and the treatment objectives. Some units incorporate a hopper-type bottom for accumulation and ultimate discharge of settled material.

Free oil is removed from the water by gravity separation; a process that takes advantage of the density difference between the oil and water. In general, the greater the difference in densities, the more effective will be the oil removal. The effectiveness of the process also depends on the rate that oil droplets rise through the water column, which in turn depends on the size of the oil droplets. Small oil droplets may not rise fast enough to be captured for removal unless the unit is very large (unsuitable for pierside operation) or a coalescing pack is used.

The coalescing pack may consist of a series of parallel plates or a volume of packing. The purpose of this material is to provide surface area to contact and intercept small oil droplets. Oil droplets become removed by adhering to the packing. On the packing, the small oil droplets will agglomerate, forming larger oil droplets. As these oil droplets grow in size, their buoyancy and rise rate increases and they will rise to the surface of the unit where they will be removed.

The treated effluent from these units should not contain free oil. Free oil is retained in the units by a baffle located upstream of a fixed level control device, usually a weir. Free oil is removed from the unit by skimming the surface with a variety of devices, which are specifically designed for this purpose. Selection of a skimming device depends on the amount and necessary quality (e.g., for recycling) of the oil to be removed. Both manual and automatic skimming devices are available. Sludge blowdown from the bottom hoppers can also be automated. Recovered oil and sludge is generally transferred to holding tanks for storage prior to disposal. Additional processing of these waste materials prior to disposal may be warranted for larger systems.

Oil/water separators can be located pierside to treat oily wastewater off-loaded from ships. Depending on the type of wastewater treated in the unit and the local water quality standards, the unit may or may not provide sufficient treatment for discharge to a sanitary sewer without further pretreatment.

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.

• Minimizes the need for disposal of some oily wastewaters as hazardous wastes.
• Effluent product may be discharged directly into industrial wastewater sewers or sanitary sewers.
• No filters or filter media to purchase, replace, or dispose.
• Can handle most mixtures of free oil and water in a continuous operation.
• Operation and maintenance is not complex; operations can be automated.
• Can be installed by in-house military personnel.
• May meet environmental regulations regarding discharge into sewers.

• Cannot effectively treat emulsified oils.
• Can not remove dissolved materials such as fuels (gasoline), solvents, or heavy metals to most discharge limits.
• May require storage tanks for collected oils and sludges.
• Space requirements to place the separating unit on the pier.
• Requires electrical service and plumbing connections.
• Treatment effectiveness depends strongly on wastewater characteristics. Performance may not be consistent.

From this analysis, it was determined that an activity must handle at least 104,000 gallons per year of oily wastewater to justify installation of a pierside oil/water separator if a one year payback period is required. This analysis assumed that the wastewater would receive additional treatment at the activity and if the wastewater characteristics were such that this was not necessary, the payback period would be shorter or a smaller volume could be treated economically. However, this analysis is most sensitive to the assumptions pertaining to hazardous waste disposal costs, and care should be taken in applying these results to a specific activity that may have substantially different hazardous waste disposal costs.

Assumptions:

• Oily wastewater will be unloaded pierside into a storage tank from which it can be fed into the oil/water separator.
• Treated water from the oil/water separator will be discharged to the activity’s industrial wastewater sewer and will be treated in the activity’s industrial wastewater treatment plant. The cost of this supplemental treatment will be $0.10/gallon.
• Oily wastewater can be disposed offsite without treatment for $0.75/gallon.
• Oil/water separator unit: $30,000 for 10 gpm unit (includes installation and one time manufacturer training).
• Electrical power required for operating the oil/water separator is 7.5 kW for operation of necessary pumps and controls.
• Electrical power required for transferring wastewater from ships to pier are equal for both alternatives and therefore not included.
• Electricity rate: $0.08/kw-hr.
• Labor associated with operating unit: 2 personnel while unit is in operation plus an additional 20 percent for mobilization, demobilization, and hazardous waste handling.
• Sludge production in the oil/water separator is 5 percent of the process flow.
• Sludge will be disposed in drums at $200/55-gallon drum as hazardous waste.
• Oil production in the oil/water separator is 10 percent of the process flow.
• Oil will be recycled offsite at a cost of $0.75/gallon.
• Burdened Labor rate: $60/hr.

Table 1. Annual Operating Cost Comparison for Pierside Oil/Water Separator vs. Hazardous Waste Disposal
166,000 gallons per year production basis

Economic Analysis Summary:

• Annual Savings for Pierside Oil/Water Separator: $28,100
  
• Capital Cost for Diversion Equipment/Process: $30,000
  
• Payback Period for Investment in Equipment/Process: < 2 years

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None.