Oily Waste Treatment by UF

By Gil K. Dhawan | July 1978

Abstract

Emulsified oily water presents a difficult treatment problem in a large number of metal fabricating and machining industries.  The conventional methods of disposal include chemical treatment, incineration and haulage by the waste disposal companies.  This paper describes a new approach for the treatment of the oily waste water.

A typical system would include pre-treatment (such as settling or screening), ultrafiltration and some post-treatment.  If the oil can be recycled, the post treatment is generally settling to increase the concentration of the oil.  If the oil cannot be recycled, the post-treatment is usually incineration. The concentration of the recovered oil can be as high as 90% oil by volume.

The ultrafiltration system dewaters the oily water solution up to 90% as it passes over the membrane system developed by Electrohome.  Each module contains tubes that act as the pressure vessels supporting the cylindrical membranes.  With each pass through the tubes water is removed and the oily water gets more concentrated.

The present systems are being used on a variety of oily water situations in a number of metal industries.  This paper discusses the data collected from these operating systems.  Examples of some of the plants in operation are given.  In most cases the savings in the haulage costs and reduction in the incineration costs can pay for the system in less than two years.  Where recovery is possible the payback is even more attractive.  Other advantages include the low energy consumption, minimal use of chemicals, and the simplicity of the systems.

Introduction

Ultrafiltration is filtration on a molecular level.  Its basis is a membrane of controlled pore size which discriminates between large and small molecules.  The molecules retained by the membrane may be dissolved in solution or they may be visible aggregates.  The principle of ultrafiltration is shown schematically in Figure1 and Figure 2.

The liquid to be treated is applied against a membrane surface under controlled conditions of pressure and flow.  The membrane allows water and smaller molecular components in water to go through the membrane.  This stream is called the permeate.   The larger molecular components do not go through the membrane and get concentrated in the water.  This stream is called the concentrate.

The performance of an ultrafiltration process is defined by two parameters.  These are: 

• Permeate Rate:   This is the rate at which the permeate is produced and is usually measured as gallons per day per square foot of the membrane surface.
  
  
• Volume Reduction:  This is defined as:    [Volume of Permeate Removed/Original Volume of the liquid] x 100.

These factors will be discussed in more detail in the following sections.

Figure 1:   Process of Ultrafiltration (UF)

Figure 2:  Schematic of Ultrafiltration UF System

Factors Affecting the Performance of Ultrafiltration

There are several factors that can affect the performance of the ultrafiltration system.  These are listed below:

•  Flow Across the Membrane Surface
  The permeate rate is directly proportional to the velocity of the liquid across the membrane.  Increasing the velocity of the liquid also reduces the fouling of the membrane by suspended solids.  Generally an optimum velocity of the liquid is arrived at by a compromise between the pump horse power and the increase in permeate rate.
  
  
• Operating Pressure
  The permeate rate is also directly proportional to the applied pressure across the membrane.  Excessing pressure can, however, lead to an irreversible compaction of the membrane.  The compaction will cause a permanent drop in the permeate rate.   
  
  Generally the maximum recommended pressure for ultrafiltration membranes is about 100 pounds per square inch.
  
  
• Operating Temperature
  The permeate rate is directly proportional to the liquid temperature.  Normally the operating temperature is the highest temperature the membrane and the system can withstand.  Ultrafiltration membranes are now available in materials that can be steam cleaned

Application of Ultrafiltration to Emulsified Oily Waste

Emulsified oily wastes are produced in metal working plants.  These include machine shops, automotive plants, rolling mills, and others.  Emulsified oily waste is also generated in teh textile industry.  Typical characteristics of this waste are as follows:

• Contaminants
  The emulsified waste invariably contains other contaminants.  These include unemulsified hydraulic and machine oils, metal fines, dirt, lint, etc.
  
  
• Oil Content
  The oil content of hte emulsified oil waste is generally between 1 and 10% oil by volume.
  
  
• pH
  Most emulsified wastes are alkaline.  The pH is usually between 6 and 12.
  
  
• Temperature
  The temperature of the waste varies between 60° and 140°F.  

The emulsified oily waste produces a disposal problem and is not acceptable to the municipal treatment systems.  The conventional solutions to this problem are:

• Chemical Method
  This process uses mixing tanks where an acid and emulsion breaker are added.  This is followed by settling of the liquid to remove the free oil which can be incinerated.  The oil free liquid is then neutralized before it is discharged to the sewer.  
  
  This method is not economical for small sizes due to the relatively high cost for tanks, chemical injection and control systems.  In addition, it requires large space and high operating costs.
  
  
• Incineration
  The other method commonly used by most plants is to transport the waste to the nearest disposal company that will incinerate the waste.  The disposal cost by this method varies between 25 cents and 45 cents per gallon.  This is a rather expensive way to treat the emulsified oil problem.
  
  

• Ultrafiltration
  Ultrafiltration can be used to dewater the emulsified oil waste.  Up to 95% of teh water can be removed by ultrafiltration.  This water can either be reused for making fresh emulsions or sewered.  The concentrate, which is as little as 5% of the original waste, can either be reused or incinerated.  The disposal costs by incernation are now only 5% of the original disposal costs.   
  
  In addition. the ultrafiltration process is simple, continuous, and uses  relatively very small amounts of chemicals.  When the oil can be reused, the ultrafiltration process provides a closed loop operation

Operation of Ultrafiltration Systems for Oil Waste

The effect of certain operating parameterws on the performance of ultrafiltration systems was outlined in an earlier section of this paper.  Generally, for this application the following operations are used:

The  high flow across the membrane surface reduces the rate of fouling of the membrane.  Fouling may be defined as the deposition of suspended solids on the membrane.  The rate of fouling is dependent on the concentration of free  oil and contaminants, type of oil, and type and concentration of other additives in the oil (such as emulsifiers, rust inhibitors, etc.).

Membranes are therefore periodically washed using a detergent solution.  The frequency of cleaning is determined in each case by monitoring the performance of the system.  Usually a weekly cleaning is sufficcient to maintain a satisfactory level of performance.

Performance of Ultrafiltration Systems

Performance data for ultrafiltration systems have been obtained for several emulsified oil situations.  These include cutting oils from machining operations, wash waters from automotive plants, and rolling mill coolants.  The information was obtained from laboratory tests, pilot plant and operating systems. Although there are variations that can be made for most emulsified oil treatments.  These are listed below:

• Ultrafiltration can be used ot remove between 90-95% of hte water in emulsion (for typical test results see Table 1).
  
  
• The permeate rate is independent of the oil concentration up to a certain oil concentration  This concentration is different for each emulsion, and generally indicates the point at which the emulsion breaks down (See Figure 3 and Figure 4).
  
  During the concentration of emulsified oily water a certain amount of the emulsifiers are removed in the permeate.  This causes the emulsion to break down and release some free oil.  The free oil forms an oily layer on the membrane.  It is this oily layer on the membrane that produces a sharp drop in the permeate rate (see Figure 3 and Figure 4).
  
  
• In some cases the permeate can be reused.  Where even higher quality of permeate is required, it can be further treated by reverse osmosis or carbon absorption system.
  
  
• The final concentrate of the emulsified oil can be either incinerated or recycled.  Normally reuse of the concentrate is possible when there is only one type of oil-emulsifier package in the combined effluent.  One example of this recovered oil is given in Table 2 where 200 gallons of emulsified oil is recycled every day, resulting in savings of about $500/day.
  
  
• The permeate rate decreases with time but can be recovered to its original value by a regular washing of the membrane (Figure 5).  The wash solution removes any oily deposit on the membrane surface, and generally takes about two hours of re-circulation in the system.  The frequency of the wash cycle will vary with each application.

Table 1 - Typical Test Data for an Emulsified Oil Application

*Note:  Percent Volume Reduction equals  [Volume of Water Removed/Original Volume of Liquid] x 100

Table 2 - Quality of Concentrate vs. Original Oil

 Figure 3 - Membrane Flux vs. Total Solids

32°; 30 GPM; 50 psig; 4-5% Oil Initial Feed

Figure 4 - Performance of Ultrafiltration Systems Using Oily Water

Test 5; 50 psig; 30 GPM; pH 6-7

 Figure 5 - Performance of UF System at Budd Automotive, Kitchener

Economics of the Ultrafiltration System

The following example will illustrate the attractive payback of using an ultrafiltration system:

Design Conditions

Capital Costs

The capital cost for an ultrafiltration system to treat 2,000 gpd of emulsified oil waste water is $40,680.

Operating Costs

Payback Calculations

A survey of the automotive plants in Ontario indicated that the disposal of oily wastewater costs an average of about $0.25 per gallon.  A summary of payback calculations for the ultrafiltration system is given below:

Conclusions

Ultrafiltration is an efficient and economically attractive solution to the emulsified oil waste problem.  The process is simple and requires no chemicals.  Where the oil can be reused the ultrafiltration makes a closed loop system possible.  The payback for the system is very attractive, even when there is no recovery of the oil.