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Using Vibrating Membranes to Treat Oily Wastewater
Guest article by Greg Johnson of New Logic Research

Overview

A unique membrane filtration system has been installed at several major waste oil hauling operations and manufacturing plants that handle or produce oily wastewater. The system is also being used to process used crankcase waste oil and produce filtrate that can be sold as a higher value bunker oil.

This system called VSEP, (Vibratory Shear Enhanced Process), uses a membrane module with special construction for service with high temperature solvents and waste oils and is able to recover up to 90% of the oily wastewater as clean water. The use of high temperature polymeric membranes has many significant advantages over the conventional methods of oily water treatment.

There are dozens of methods used for oil water separation. Each technique has advantages. No one technique is suitable for all situations. Membranes have the advantage of being simple efficient separating devices to hold back oil, grease, metals, BOD, and COD. They can provide clear permeate which can be sewered or re-used.

Membranes

Membrane separation technology has been around for many years. Initially, the use of membranes was isolated to a laboratory scale. However, improvements over the past twenty years have made it possible to use membranes on an industrial level. A membrane is simply a synthetic barrier, which prevents the transport of certain components based on various characteristics. Membranes are very diverse in their nature with the one unifying theme to separate. Membranes can be liquid or solid, homogeneous or heterogeneous and can range in thickness. They can be manufactured to be electrically neutral, positive, negative or bipolar. These different characteristics enable membranes to perform many different separations from reverse osmosis to micro-filtration. There are four main categories of membrane filtration. These are determined by the Pore size or Molecular Weight Cut off

Reverse Osmosis Membranes

The first category of membranes is for reverse osmosis. These are the tightest membranes for separating materials. They are generally rated on the % of salts that they can remove from a feed stream. However, they can also be specified by molecular weight cutoff. Usually, the rejection of NaCl will be greater than 95% in order to be classed as an RO membrane. The molecular weight cutoff is shown in the table above.

An example of their use would be for filtering seawater in order to remove the salt. They are also used to remove color, fragrance and flavor from water streams. Reverse osmosis membranes don't have structural pores. Filtration occurs as ionic species are able to diffuse their way through the membrane itself.

Nanofiltration Membranes

A great deal of recent research has led to the improvement of membranes in the range of nanofiltration. As the name suggests, these membranes are used to separate materials on the order of nanometers. These membranes are not usually rated based on their pore size because the pores are very small and difficult to measure accurately. Instead they are rated based on the approximate molecular weight of the components that they reject or the % of salts that they can remove from a stream. These membranes are used predominately for wastewater treatment but they are also used to concentrate material that has a wide range of particle sizes.

Ultrafiltration Membranes

Conventional ultrafiltration membranes are composed of some type of polymer material with pores ranging from a little less than 0.01 µm to 0.1 µm. These membranes are used for many different separations including oily wastewater treatment, protein concentration, colloidal silica concentration and for the treatment of various wastewaters in the Pulp & Paper Industry.

Microfiltration Membranes

These membranes tend to be porous, with pores greater than 0.1µm. These types of membranes are used to separate larger particulate matter from a liquid phase.  Some examples would be coarse minerals or paint particles, which need to be concentrated from an aqueous solution. Oily wastewater oil/water separation covers a broad spectrum of industrial process operations. There are many techniques employed depending on the situation.

This case study illustrates those separations, which are suited to the VSEP membrane technology. The oily wastewater application can be broken down into categories determined by the type of user and the oil/ water separation desired.

Types of Oily Wastewater customers:

• Barge/Bilge Water from marine operations
• Manufacturing where oily water is a waste product
• Waste Haulers & recyclers

There is a saying: "Oil and Water don't mix". This is true, but they can exist as an emulsion. Oil is not soluble in water but it can exist evenly dispersed as globules in water. The concentration of these globules is a function of mixing or stirring. If allowed to stand the emulsion will separate because oil is lighter than water, although, some amount of oil globules will remain in the water.

Another interesting fact is that this emulsion can exist two ways. If the concentration of oil is less than 50%, the water will be the suspension fluid and the oil will be the globule. A phase transition occurs if the oil content is more than 50%. When this happens, the oil is the suspension fluid and the water forms globules. For this reason, hydrophilic membrane separations will be possible only when the oil content is less than 50%.

Commercial Uses

Sometimes mixing of oil and water is intentional and some times it is an unavoidable necessity. The following are instances of oil water mixtures:

• Produced Water: Water is injected into drilling shafts to displace oil.
• Barge/Bilge Water: Wash down cleaning operations contaminated by oil.
• Machining Coolant: Oil mixed with water acts as a lubricant to reduce tool wear.
• Washwater with Degreaser: Fluid used for cleaning oily or greasy parts.
• Lubricant Manufacturing Wastewater

Methods used for Oil Water Separation

• Centrifuge
• Rotary Drum Vacuum Filter
• Dissolved Air Flotation (DAF)
• Slope Plate Clarifiers
• Biological Treatment
• Evaporators
• Gravity Separating Devices
• VSEP

Comparisons of Oil Water Separation Technologies

Centrifuge: Uses large horsepower motors and because of the number of moving parts is subject to high maintenance. While centrifuges are effective at removing suspended solids, they do not account for dissolved solids and heavy metals in solution. The effluent from a centrifuge would need further treatment prior to disposal.

Rotary Drum Vacuum Filter: Quite effective at rejecting large solids. Sometimes filtrate must be sent back around to get all of the smaller particles. Usually employs coarse filtration. Vacuum filters require large floor areas and have high capital costs

Dissolved Air Flotation (DAF): Large tanks where air is bubbled into the bottom and with the use of flocculants, solids are floated to the top and skimmed off. A very large tank is required due to the residence time required. Also chemical addition is a daily if not hourly process and is a significant operating cost.

Slope Plate Clarifiers: Cheap and easy to use. The process relies on gravity to drop out heavy solids. Here again colloidal materials with small mass and dissolved constituents do not settle. Sometimes it is used in conjunction with flocculation chemicals. These chemicals have limited effect in dropping out heavy metals, BOD, and COD.

Biological Treatment: This process relies on biological activity to digest the solids in the wastewater. The problem is that the system is extremely temperature and pH sensitive. Also loading must be done at a set rate. The operation of this kind of system usually requires a very skilled operator. It also can take up a lot of floor space due to the amount of residence time required for the bugs to digest the materials.

Evaporators: Can reduce wastewater to dry solids that can be landfilled. Of course water re-use is not possible. Evaporators have very high capital costs and consume huge amounts of energy even for the most efficient models.

VSEP:  Able to produce drinking water quality filtrate from any wastewater. Extremely energy efficient and the vertical design allows for a very small footprint. Does not require pre-treatment or post-treatment for that matter. Wide range of membranes available allows for precise separations based on the process objectives. There is no chemical addition required except for periodical membrane cleaning.

Suitability of VSEP for Oily Wastewater

As with other waste streams, volume reduction is the goal. Hauling and disposal costs are king. Wastewaters normally have very strict sewering rules and surcharges are attached to anything that is sewered. Since oil is normally limited to 100 ppm, oily wastewaters cannot be sewered and must be taken care of in other ways. Oil can also not be landfilled as long as it is a liquid. Therefore, disposal of oily wastewater is an expensive operation.

Table #1 shows a typical discharge requirement for metals and oil. Volume reduction of the oily wastewater will reduce the treatment costs to dispose of the material. There are also many types of membrane solutions for oil water separation. A common membrane device used is a tubular membrane system. One common problem with Tubular Membrane Systems is the permeate quality. VSEP can offer competitive installed costs along with RO quality permeate requiring less post treatment.

Concentration polarization is the main limiting factor to membrane filtration with oily wastewaters. Therefore the existence of a boundary layer of highly concentrated oil and solids next to the membrane surface must be eliminated.

Spiral membranes employ crossflow and fluid velocity to accomplish this. Tubular membranes use the same technique with greater efficiency. None of these has the degree of efficiency of the vibrating membrane surface of VSEP which can use both high crossflow velocities as well as high vibrational energy at the membrane surface which is oscillating back and forth 55 times per second. Performance comparisons based on GFD of permeate flow is difficult because there are so many variables to consider.

Permeate flow rates will vary depending on the initial concentration of oil and other materials in the feed material as well as the % recovery which is being achieved. Table #2 shows some typical production capable with VSEP.

Effects of Temperature

Temperature needs to be considered with regard to design. Temperature can be used to increase filtration performance. A stream that appears to be too expensive to filter at 25°C may be well within the budget for a project at 40 or 50°C even though you have a cost associated with heating the feed.

The reason is because increased temperature decreases the viscosity of the liquid and enables the material to flow through the membrane faster. It also makes it possible to reach a higher endpoint solids because generally the material remains more fluid at a higher temperature. As many streams are water-based, the following table provides the viscosity correction factors for water at various temperatures. The following empirical relationships between viscosity and temperature are based on measurements taken with viscometers calibrated with water at 20°C.

0°C to 20°C
log10hT = ((1301)/(998.333+8.1855(T-20)+0.00585(T-20)2)-1.30233

20°C to 100°C
log10(hT/h20) = ((1.3272(20-T)-0.001053(T-20)2)/(T+105))

For example, if a wastewater stream had a flux of 110 GFD at 25°C and you wanted to know what the flux would be at 50°C then you could set up the following ratio to give you an estimate based on the change in viscosity.

(h @25°C)(Flux @25°C) = (h@50°C)(Flux@50°C)
(0.8904)(110) = x(0.5468)
x = 179 GFD (@50°C)

These types of calculations can also be completed for other materials given the viscosity versus temperature relationships for the feed liquid. As seen in the above example, doubling the temperature nearly doubles the flow rate. The result of this is that it requires about half as much equipment to do a filtration separation at 50ºC as it would at 25ºC. This means lower capital cost as well as lower operating costs. The VSEP has been designed to withstand temperatures of up to 150ºC

Volume Reduction

With oily wastewater, hauling for disposal is the conventional method of remediation. Since hauling costs can be very expensive, reducing the volume that needs to be hauled can have a significant effect on operating costs. VSEP is capable of volume reducing wastewaters by up to 98% leaving a small amount to be hauled and clean water that can be sewered or reused in the process.

The recovery ratio is the amount of liquid, which is recovered as clean, permeate from the feed flow. In other words, it is the ratio of liquid that passes through the membrane versus what is fed to the membrane. This is usually a critical factor for membrane filtration because for a product dewatering or wastewater application, a person is generally looking to remove as much of the water as possible.

Process Conditions

A process schematic for treatment of a typical oily wastewater process using a VSEP system is presented in Figure 1. When the residual oily wastewater has been settled so that oil and water can separate naturally, the result is a process effluent, at 1.5 to 2% by weight total solids (TS). This process effluent is normally sent to a multi-train chemical treatment step by a filter press or a dryer or an evaporator in order to concentrate the solids to 60 to 65% by weight. As you can see in the diagram, the addition of VSEP to concentrate the process effluent improves the process efficiency. The permeate can be reused in the process or discharged.

The oily wastewater is fed to the VSEP treatment system at a rate of 44 gpm and a pressure of 250 psig. One industrial scale VSEP unit, with a nano-filtration membrane is used to treat the process effluent. The produced concentrated stream at a flow rate of 10 gpm and a solids concentration of 10% TS is sent to a coalescer and stored for hauling.

The VSEP generates a permeate stream of about 34 gpm which is recycled to the process or discharged to the sewer. The permeate concentration is reduced to ~ 1 mg/L of total suspended solids (TSS), and a low level of total dissolved solids (TDS), all well below the design requirements for process recycling or discharge. Membrane selection is based on material compatibility, flux rates (capacity) and concentration requirements. In this example, the TSS reduction is over 99% while the oily waste is concentrated from a starting feed of 1.5-2% to a final concentrate of 10% by weight. The permeate quality from the VSEP can be controlled though laboratory selection of membrane materials available to fit the application parameters.

Summary

Successful pilot tests have been conducted for many kinds of oily wastewater treatment. Depending on process temperatures, membrane selection and the requirement for solids concentration or BOD/COD removal for effluent streams, the permeate flux rate in the VSEP can range from 15 to over 150 gallons per day per square foot.

New Logic Research has supplied VSEP separation technology successfully into many industrial processes. Manufacturing plants' as well as the oil waste hauling industries' efforts to meet environmental regulations will be enhanced with the utilization of membrane filtration combined with "Vibratory Shear Enhanced Processing". The availability of new membrane materials and VSEP technology make it possible to treat the more difficult streams with very successful, economic results.

For more information contact our author:  

Mr. Greg Johnson
New Logic Research Inc.
1295 Sixty Seventh Street
Emeryville, CA 94608
Phone: 510-655-7305
Fax: 510-655-7307
E-mail: gjohnson@vsep.com
Web site:  http://www.vsep.com/

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