Waste Water Recycling and Pre-treatment Systems:
An Alternative to Oil/Water Separators

Introduction

The Air Force has typically used oil/water separators to remove oil from an influent waste water stream through gravity separation. This occurs when oil, lighter than water, is skimmed off of the surface of the incoming waste water. Once the oil is skimmed off the surface, it is then routed to a holding sump for collection at a later time. The rest of the water simply passes into the sanitary sewer system. A very simple diagram of an oil/water separator is shown in Figure 1. Unfortunately, oil is not the only substance going to an oil/water separator. In many industrial facilities, solvents and surfactants also enter separators. The problem here is that the oil tends to emulsify into the water when such substances are introduced into the waste stream. This in turn tends to have a negative impact on the efficiency of the oil/water separator. Since there are operations that inherently have surfactants in the waste streams, such as aircraft and vehicle wash racks, it is safe to assume that oil/water separators receiving waste water at such facilities are often not very efficient.

In this report, an alternative to oil/water separators is introduced, especially in processes that have waste streams that are incompatible with the principle of operation of the oil/water separator in the first place. After providing background information about oil/water separators, an economic analysis is conducted to compare conventional vehicle washing with a closed loop waste water recycling system. Other techniques of reducing the waste generation by oil/water separators are also discussed. Finally, recommendations are given to implement some of the concepts presented in this paper.

According to Air Combat Command Environmental Program Guidance Document 93-019, Guidance on Management of Oil/Water Separators⁽¹⁾, oil/water separators function as physical pass through treatment devices to remove residual oils, petroleum products, and other floating constituents from waste water. Waste fuels/oils must never be intentionally dumped or drained into oil/water separators. It is important to note that surfactants, or detergents, will lessen the effectiveness of a separator by causing the oil to disperse and become soluble in water.

According to the Air Force Center for Environmental Excellence (AFCEE) Fact Sheet on Oil/Water Separators⁽²⁾, the following factors have a direct impact on the efficiency, use and management of oil/water separators:

• Frequency and intensity of use
• Design capacity
• Periodic maintenance practices
• Presence or absence of emulsifying agents
• Type of oil/water separator
• Contaminants contained in the waste water stream

Since most of these factors are directly dependent on oil/water separator operation, it becomes the user’s responsibility to ensure they are properly maintained. Unfortunately, common problems with oil/water separators are usually related to the presence of emulsifying agents or other contaminants contained in the waste water stream. If the user has problems with their oil/water separator, such as, the waste sludge being characterized as a hazardous waste, or poor separation, it may be time to consider alternatives to using oil/water separators. The following section discusses some alternatives to oil/water separators. Two alternatives to oil/water separators are presented. The first is a zero discharge closed loop recycling system used primarily for aircraft or vehicle wash racks. The other system is a sanitary sewer pre-treatment discharge system that is useful in treating waste water such as shop mop water.

Some of the information presented here, is based upon data obtained from the Tri-Services’ Pollution Prevention Opportunity Software System, Naval Facilities Engineering Services Center.⁽³⁾ The pollution prevention professional should consider the pay back period, advantages, and disadvantages for each technology before making any decisions. Also, the information presented here is not an endorsement for any specific product, as there are other manufacturers and vendors of these types of products.

The closed loop wash rack allows complete recycling of wash water for aircraft, heavy equipment, and vehicle cleaning. A vehicle is placed over a wash pad collection basin. Wash water will then be processed through the recycling unit. As used water enters the recycling unit (see figure 2), it undergoes several different processes before returning to use for washing the vehicle. As water enters the unit it will pass through a settling unit that uses coalescing plates usually made of polypropylene. These plates aid in the efficient separation of oil from the waste water. Alum (Aluminum Sulfate) is also added as a flocculant which will cause the heavier particles such as heavy metals soluble in the waste water to settle out. An oil skimmer is used to remove oil from the surface of the waste stream. This oil is deposited into a waste oil container where water is decanted for enhanced waste minimization. Once the waste water has been treated in the settling unit, it passes through several filters. A special blend of sand, gravel, and anthracite screen out dirt and other solids to about 20-40 microns in size. With an easy to operate top valve, the filter is back washed under pressure for more effective media cleaning. The clean water is then placed under high pressure and reused to wash the vehicle where the process is repeated.

Sample Results: ⁽³⁾ The sample results below (Table 1) are typical constituents found in a water sample taken from the effluent of a pressure cleaning operation.

Table 1: Typical sample results from pressure cleaning operation. ⁽³⁾

• Usually, recycling units are above ground systems which will reduce the risk of undetected leaks and the need for large, costly below-ground pits.
• The recycling unit sits on a single platform making for simple and quick installation
• There are reusable and back-washable filters for lower operational and maintenance costs.
• Most recycling systems have a unique, steep-pitched coalescing pack that optimizes oil/water solids separation.
• Some systems are equipped with chemical monitoring and injection systems for introducing flocculants and other chemicals to further enhance the treatment and recycling process.
• Most of the water recycled through the unit is used over and over again. This advantage greatly reduces the cost of fresh water required which produces less waste water that needs to go down the drain.
• By using the wash water over and over again, there is a savings realized by not having to keep injecting fresh cleaning solution into the process water. The majority of the cleaning solution is recycled with the waste water.

• Unlike typical three stage oil/water separators, these recycling units require routine maintenance to ensure proper operation and efficiency. Maintenance of this system usually requires routine checks of pressures and verifying proper amount of chemicals are added.
• Although recycling units greatly reduce the quantity of waste effluent, it does not eliminate it. The unit will still produce waste sludge that will need proper disposal. The operator of the unit will need to characterize this waste stream to determine if it is a hazardous waste.
• Training and proper use is a definite must because if the unit is used improperly, damage could result to the machine which will lead to expensive repair bills. Therefore, someone must always be fully trained and available for proper maintenance and operation of the unit.
• Security and weather protection may become a factor if the machine is exposed to a harsh environment. Also, the machine should be secured in some manner when not in use to prevent unauthorized access and possible damage to the machine.

Economic Analysis

An economic analysis is conducted to determine what the cost would be to recycle the waste water (option 1) compared to directly discharging the water to the sanitary sewer. The cost values used here are based on national survey data published in previous papers. ⁽⁴⁾

Table 2: Economic Evaluation. ⁽⁴⁾

Based on the above economic analysis, the user of the recycling system could save $1.04 per vehicle washed. This may not seem like a significant savings at first, but if a facility washes a great deal of vehicles, the savings could add up. The following example shows how the pay back period in years may be estimated.

10 vehicles are washed on average at a facility five days per week. The cost for a new recycling unit, including installation and maintenance contract for one year is $15,000.

Number of vehicles washed per year = 10 vehicles/day * 5 work days/week * 52 weeks/year

There are 2600 vehicles washed per year.

Using the following formula for pay back period in years:

Start up costs are $15,000.

Current costs are $6.97 per vehicle * 2600 vehicles/year = 18,122.00

Future costs are $5.93 per vehicle * 2600 vehicles per year = 15,418.00

Pay back = $15,000/ ($18,122.00 - $15,418.00) = 5.5 years

The recycling unit should pay for itself in six years time assuming 10 vehicles are washed per day. If 20 vehicles were washed per day, the pay back period would drop to about 3 years. Likewise, if only 5 vehicles were washed per day, the pay back period would become 11 years. Based upon the above economic analysis, one could draw several conclusions. For one, this investment makes good economic sense depending directly on the number of vehicles washed per day. Additionally, costs for soap and maintenance contracts and initial capital purchases are subject to change. It is therefore important to conduct an economic benefit analysis at the time this investment is being considered. Finally, since the pay back period is a direct function of the number of vehicles washed per year, it is important to be as accurate as possible in calculating the number of vehicles that will be washed using the recycling unit. Of course , this will apply to aircraft as well because the common denominator used in the economic analysis is knowing how many gallons of water are required to wash a vehicle. Simply substitute the number of gallons to wash an aircraft and continue the analysis.

Shop mop water may eventually have a build-up of oil, metal chip, and particle contamination. Excessive quantities of oil and metal contamination should be removed before this waste water may be discharged to the sanitary sewer system. Unfortunately, the oil/water separator was not designed to handle this type of discharge. In such instances, a sanitary sewer pre-treatment discharge system may be an appropriate consideration for a facility or installation. An advanced sanitary sewer discharge pre-treatment system may be used to treat the shop mop water (see figure 4).

The waste stream enters the Coalescing Centrifugal separator where centrifugal motion forces the solids to separate to the sides of the separator where they eventually fall to the bottom. The waste stream then enters the aeration tower where it is mixed with air to release volatile organic compounds. From the aeration tower, the waste stream passes through a number of different solids and oil removers. The first compartment is an inclined plate coalescor, which separates the oils, solids, and water. Solids settle to the bottom of the tank and oils are skimmed from the surface. The second compartment contains a solids filter and oil absorber. The hydrocarbon accumulator absorbs the oils and the water overflows into the third compartment. The third compartment contains a multi-media filtration bed. The first layer is a solids filter. The second layer is the ion exchange layer which removes heavy metals. The third layer is the carbon layer, where oils, odors and organics are adsorbed. Finally, underneath the multi-media filter is the aerator where ozone is bubbled for controlling organic loading. The water then leaves the unit and is discharged.

• Does not require any chemical additives
• Fully automatic, easy to use, versatile
• Provides overflow protection

• Does not have automatic back flushing
• Requires periodic maintenance

Below is a partial listing (Table 3) of some vendors that offer recycling systems, pre treatment devices, installation, and maintenance support. Figure 3 shows what one of these recycling units might look like.

Table 3: Partial Vendor Listing

It makes good economic sense to consider installing a zero discharge recycling unit at wash racks that wash a significant number of vehicles or aircraft per day. Since each facility is different, costs change, the size, shape, and level of cleaning required also change, an economic analysis should be conducted on a case by case basis. One could use the example shown above as a guide for conducting such an analysis and use the provided contractor information as a means of gathering additional information.

A study being conducted by Pacific Environmental Services⁽⁶⁾, Inc. for the Air Force has shown that oil/water separators contribute significantly to the hazardous waste cost of an installation. In some cases, they have shown that oil/water separators represent the majority of hazardous waste being disposed. This is primarily due to oil/water separator sludge containing heavy metals. If the shop can eliminate the discharge of oil based wastes from discharging to an oil/water separator, then there is no longer a use for that separator and it should be removed. By removing an oil/water separator, an entire waste stream is eliminated and there is no longer the risk of having leaks in the separator that may eventually become a clean-up site.

Shop mop water should be discharged directly to the sanitary sewer and not to an oil/water separator. In most cases pre treatment of shop mop water is not necessary if the sewage treatment plant is capable of removing heavy metals from the influent. A study should be conducted on a case by case basis to determine the need to install a flow through pre-treatment system at an installation. The cost of the pre-treatment system and installation should be compare to the saving of not having to dispose of oil/water separator sludge.

Implement dry cleanup procedures and only use floor drains to carry water directly to the sanitary sewer. Plug floor drains to oil/water separators that carry industrial waste water from maintenance facilities. Industrial waste should be disposed of separately. The Bioenvironmental Engineer (BEE) should take an active role to ensure that floor drains in these facilities are plugged. Processes that generate a waste water should be examined to ensure that there is no discharge occurring to an oil/water separator. If a waste water can not go to the sanitary sewer, it should be treated first so that it could go to the sanitary sewer. An oil/water separator is not a treatment device and is not capable of removing anything, other than oil and some sediment, from the waste water.

Eliminate pollutants and prohibit discharge of waste water from operations containing hazardous wastes, including heavy metals. The BEE should look at what kinds of constituents are being released into the environment by sampling effluent from oil/water separators. The analysis results could be used to identify sources from the analysis results. This will help to pinpoint what products are being discharged and where they are coming from within the facility.

The base BEE is the one person that visits every industrial facility on an installation. That person should ensure practices, procedures, and processes do not generate a waste stream that ends up in an oil/water separator. A close examination of sample results from sludge or effluent from an oil/water separator will help reveal the materials used. The BEE could trace those constituents to find what processes they came from and recommend corrective actions like chemical substitution, preventing wastes from going down the drain, and more efficient use of materials.

1. Headquarters, Air Combat Command Environmental Program Guidance Document 93-019, Guidance on Management of Oil/Water Separators, 22 Oct 1993
2. United States Air Force Center for Environmental Excellence (AFCEE) Fact Sheet on Oil/Water Separators, Dec 1996
3. The Tri-Services’ Pollution Prevention Opportunity Software System, Naval Facilities Engineering Services Center, 1996
4. "Waste Wash Water Recycling Makes Dollars and Sense for Military Equipment", Ronald G. Fink, RGF Environmental Group, Joint Service Pollution Prevention Conference and Exhibition, First Annual Proceedings, pages 352-357, August 1996
5. Bolling Air Force Base Pollution Prevention Opportunity Assessment Workshop Handbook, US Air Force Center for Environmental Excellence and Bolling Air Force Base, District of Columbia
6. Pollution Prevention Opportunity Assessments for Air Mobility Command Bases, contract number F41624-95-9017, delivery order number 14, PES project F114