Membrane technologies for wastewater reuse: The challenges of oily waste fouling
By Peter S. Cartwright, P.E., and George Alther
For the last 30-plus years, protection of the environment has been steadily moving up on the priority list of concerned citizens. Issues such as the hole in the ozone layer, the greenhouse effect, overflowing landfills, acid rain, destruction of the rain forests and overpopulation have created an attitude of conservation and environmental responsibility throughout the world.
Manufacturers are beginning to realize the potential economic benefits of resource recovery, as well as the somewhat intangible rewards accrued by an image of responsible stewardship.
The membrane separation technologies of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) possess characteristics which make them attractive as wastewater reuse processes. These include:
• Continuous process, resulting in automatic and uninterrupted operation;
• Low energy utilization involving neither phase nor temperature changes;
• Modular design, no significant size limitations;
• Minimum of moving parts, with low maintenance requirements;
• No effect on form or chemistry of the contaminant;
• Discrete membrane barrier to ensure physical separation of contaminants;
• No chemical addition requirements to effect separation.
Background
Membrane technologies are based on a process known as "crossflow" or "tangential-flow" filtration, which allows for continuous processing of liquid streams. In this process, the bulk solution flows over and parallel to the membrane surface, and because this system is pressurized, water is forced through the membrane. The turbulent flow of the bulk solution over the surface minimizes the accumulation of particulate matter. Figure 1 illustrates crossflow filtration compared to conventional filtration.
The crossflow membrane separation technologies of Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO) are defined by some membrologists on the basis of pore size. Other experts prefer to use definitions based on the removal function, as follows: Microfiltration is utilized to remove submicron suspended materials on a continuous basis. The size range is from approximately 0.01 to 1 micron (100 to 10,000 angstroms). By definition, microfiltration does not remove dissolved materials. Microfiltration is illustrated in Figure 2.
Ultrafiltration is the membrane process which removes dissolved non-ionic solute, typically organic materials (macromolecules). Ultrafiltration membranes are usually rated by "molecular weight cut-off" (MWCO), the maximum molecular weight of the dissolved organic compound that will pass through the membrane into the permeate stream. Ultrafiltration pore sizes are usually smaller than 0.01 micron (100 angstroms) in size. Ultrafiltration is depicted in Figure 3.
The above processes (MF and UF) separate contaminants on the basis of a "sieving" process; that is, any contaminant too large to pass through the pore is rejected and exits in the concentrate stream.
Nanofiltration can be considered "loose" reverse osmosis. It rejects dissolved ionic contaminants but to a lesser degree than RO. NF membranes reject multivalent salts to a higher degree than monovalent salts (for example, 99% vs. 20%). These membranes have molecular weight cut-offs for non-ionic solids below 1000 Daltons. Nanofiltration is illustrated in Figure 4.
Reverse Osmosis produces the highest quality permeate of any pressure driven membrane technology. Certain polymers will reject over 99% of all ionic solids, and have molecular weight cut-offs in the range of 50 to 100 Daltons. Figure 5 illustrates reverse osmosis.
Both NF and RO membranes reject salts utilizing a mechanism that is not fully understood. Monovalent salts are not as highly rejected from the membrane surface as multivalent salts; however, the high rejection properties of the newer thin film composite RO membranes exhibit very little differences in salt rejection characteristics as a function of ionic valance. As indicated earlier, this difference is significant with NF membranes. In all cases, the greater the degree of contaminant removal, the higher the pressure requirement to effect this separation.
System design
Figure 6 is a schematic of a complete membrane processing system (or a single membrane element).
Note that the feed stream enters the system (or membrane element), and as the stream passes along and parallel to the surface of the membrane under pressure, a percentage of the water is forced through the membrane polymer producing the permeate stream. Contaminants are prevented from passing through the membrane based on the polymer characteristics. This contaminant-laden stream exits the membrane system (or element) as the "concentrate" stream, also known as "brine" or "reject."
The permeate rate of a given membrane element cannot be changed without varying the applied pressure or temperature. Recovery, however, can be easily changed by varying the feed flow rate to the element, and this is one of the variables that is controlled by the system designer.
It is virtually impossible to accurately design an industrial wastewater treatment system utilizing membrane technologies without a complete and thoroughly comprehensive testing program. This is required to identify the best membrane polymer and element configuration, and to optimize the system design and operating conditions.
In most cases, membrane technologies have likely never been run on this stream before, so the results are impossible to predict with any degree of accuracy. The goal in most non-water purification applications is to make the concentrate stream as small as possible to achieve a high system recovery (that percentage of the feed stream that becomes permeate). These high recoveries exact a price in that the resulting increased osmotic pressures require higher pressure pumps, and the higher concentrate concentrations can result in precipitation of slightly soluble materials.
No reverse osmosis membrane is perfect in that it rejects 100% of the solute on the feed side; this solute leakage is known as "passage." Expressed as "percent passage," the actual quantity of solute which passes through the membrane is a function of the concentration of solute on the feed side. Under high recovery conditions, the concentration of solute on the feed side is increased and therefore the actual quantity of solute passing through the membrane also increases. Because most effluent applications demand that, in addition to a minimum concentrate volume, the permeate quality be high enough to allow reuse or meet discharge regulations, the "catch-22' predicament of permeate quality decreasing as recovery is increased can impose design limitations.
The effect of increasing recovery upon the concentration of contaminants in the concentrate stream is illustrated in Table 1.
For wastewater treatment and water reuse applications, the minimum recovery is usually no less than 90%.
Testing
In general, every stream must be tested to obtain the following design factors:
• Specific membrane polymer
• Optimum membrane element configuration
• Total membrane area
• Temperature
• Optimum pressure
• Maximum system recovery
• Flow conditions
• Membrane element array
• Pretreatment requirements
Specific properties of streams which influence these design factors include:
• Stream chemistry
-- Total solids content
-- Suspended (TSS)
-- Dissolved organic (TOC, MBAS, COD, BOD)
-- Dissolved inorganic (TDS)
• Chemicals of concern
-- Oxidizing chemicals
-- Organic solvents
-- Saturated solutes
• pH
• Operating temperature
• Osmotic pressure as a function of system recovery
• Variation in chemistry as a function of time
Membrane fouling from free oils
The "Achilles heel" of consistent membrane performance is fouling, and whereas fouling from suspended solids and sparingly soluble mineral salts is straight forward and rather easily corrected, oily contaminants present a much greater challenge. Free oils, in particular, are difficult to completely remove, and once they coat the membrane surface, can wreck havoc.
Organoclay treatment
Organoclays are bentonite clays which have been modified with quaternary amine compounds rendering them hydrophobic and organophylic, and have been used as thickeners and antisettling agents in grease, lubricants, putties, and paints since the 1950's. Organoclays have the ability to remove 50% of their weight in oil, which represents a seven-fold increase over activated carbon.
Because they are available in granular form, they can be placed in vessels appropriate for other filter media and operated in a similar fashion.
Less expensive than activated carbon, and more effective at removing oily waste, organoclays make optimum pretreatment technologies for all oily wastewater contamination issues.
A membrane element should be cleaned when at least one of the following conditions occur:
• The permeate rate, normalized for temperature and pressure effects, drops by 10%
• Salts passage into the permeate increases by 10%
• The pressure drop (feed pressure minus concentrate pressure) across the system, increases 15% over that measured during the initial 24 -- 48 hours of operation.
An environment that requires frequent membrane cleaning (more than once/month) will significantly shorten membrane life.
A typical cleaning regimen involves a cleaning system (CIP), designed to mix, heat and deliver the cleaning solution selected for dissolution of the fouling material. For oily waste fouling, the chemistry usually consists of surfactants and perhaps enzymes in a high pH solution.
The membrane system is taken off-line, the pump from the CIP system is connected to the inlet of the membrane system, and the permeate and concentrate streams are directed back to the CIP tank for recirculation. After the membrane elements are filled with the heated cleaning solution, they are usually allowed to soak from one to 24 hours, depending on the degree of fouling. Following the soak period, the cleaning solution is pumped throughout the system, usually at a high flow rate, pumped out, the membrane elements flushed with purified water, system drained, and then put back into service.
This process is labor intensive and relatively expensive.
Economics of cleaning
Although the costs for cleaning membranes are extremely variable (system size, labor requirement, kind of cleaning chemicals, extent of fouling, etc.), a good approximation is $309 per 4'8" x 40" element.
Organoclay costs approximately $100/ft3.
Assume that a water source containing 3 ppm of oil is processed with 8 - 4" x 40" spiral RO elements, producing 10gpm (24 hr/day basis) of permeate. To ensure virtually complete removal of the oil, place a filter housing containing 1ft3 of organoclay upstream of the mixed bed resin. The adsorptive capacity of the organoclay will require that it be changed no more frequently than every 700 days. A conservative design would dictate organoclay replacement annually, at a cost of $100/yr.
Assuming that the membranes (without the organoclay pretreatment) would require cleaning after contact with only 0.25lb oil, the cleaning frequency would be every week at a total cost of $240. This results in an annual cost of approximately $12,500 vs. $100 for organoclay pretreatment. These calculations do not include more frequent membrane replacement resulting from the effect of the oil fouling.
Conclusion
The potential for membrane separations technologies in wastewater and processing applications are very bright. To realize this potential it is imperative that any candidate stream be thoroughly tested. This requires knowledgeable, experienced personnel to run and interpret testing on well designed testing equipment.
About the Authors: A registered professional engineer and member of Industrial WaterWorld's Editorial Advisory Committee, Peter Cartwright is president of Cartwright Consulting Co., of Minneapolis, MN. Contact: 952-854-4911, [email protected] or www.cartwright-consulting.com. George Alther is president of Biomin Inc., of Ferndale, MI. Contact: 248-544-2552, [email protected] or www.biomininc.com
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