George R. Alther – The Analyst, Winter 2000
Cationic and anionic resins are both capable of remaining in use for long periods of time (up to 20 years and four years, respectively).
However, they frequently become fouled by any of several compounds, thus requiring increased use of regenerant chemicals and rinsewater, waste management expense, and higher labor costs.
This problem can be successfully averted by prepolishing waste streams with organically modified clay.
Resin Fouling
Cationic resins of both a weak- and strong-acidic type should last up to 20 years; but they frequently become fouled by oils, grease, fats, proteins and nitrogenous matter, resulting in a lowering of both exchange rate and exchange capacity.
Gelatinous resins are oxidized by certain oils, resulting in an immediate decrease in exchange capacity of up to 20%. Other forms of resin fouling are caused by iron, aluminum and suspended solids.
Anionic exchange resins, which should last four years in normal service, can be fouled by the same compounds as above, but humic acids are their main foulants.
In 1959, A. L. Wilson showed that humic acids present in the feedwater supply of a power station were removed by the anion exchange resin, resulting in silica leakage. Regeneration did not completely remove the humic acids, thus reducing the removal capacity of the resin by 15%.
The fouling mechanism is primarily ion exchange, according to M. McCoy. The chloride ions or hydroxyl anions within the humic acid structure (or within other high molecular weight organic molecules) will exchange with the chloride, sulfate or hydroxyl anion on the quaternary amine that is grafted to the backbone of the anion exchange resin molecule.
Because organic matter renders the resin hydrophobic, such fouling also reduces the moisture content of the resin, which is detrimental to removal capacity because it decreases pore space. Furthermore, the foulants exert a physical strain on the resin beads, which results in cracking and eventual breaking. These problems result in leakage of silica or other materials that should be removed by the resin.
The end user is faced with increased use of regenerant chemicals and rinsewater, waste management expense, and higher labor costs. Humic acids are amphoteric (i.e., they are both anionic and cationic), which means they can foul both anionic and cationic resins. Particularly susceptible to this phenomenon are power plants which derive their feed water from surface waters that contain humic acids.
Oil-Contamination Example
The following case-study example on handling oil-contaminated wastewater is based on the variables:
- Water flow – 100 gpm, 24 hr per day, 7 days per week;
- Oil content from leaking pumps – 3 ppm;
- One vessel with 32 ft3 or 1,280 lb of gelatinous cationic resin.
Resin manufacturers point out that it takes about 0.5 lb of oil to completely coat 100 Ib of resin. If this system operates for 45 days without regeneration, 164 Ib of oil would deposit on the resin, or more than 1/10 of its weight, rendering it useless.
A normal conservative regeneration cycle in this system would be every 48,000 gal of water, or three times a day. Assuming a regeneration cost of $1 per ft3 of resin, this amounts to $96 per day, plus downtime of 30 min per cycle, or 1.5 hr per day.
If the oil content is 3 ppm, regeneration would be required every 2 hr, or 12 times a day for a cost of $384 per day plus 6-hr downtime. Because the oil oxidizes the resin and migrates into the matrix, the resin would quickly lose 20% of its removal capacity.
The monthly costs amount to the following:
- Standard cationic resin regeneration – $2,688 per month, downtime not included;
- Oily water, cationic resin without prepolish – $7,952 per month, downtime not included – a 200% increase,
- If the resin is anionic, standard regeneration – $26,880 per month, downtime not included;
- Oil water, anionic resin with prepolish – $3,840 per day, or $107,520 per month – a 300% increase;
- Replacement costs of the resin, at $4 per lb., are $5,120.
The regeneration water has to be hauled away by truck, usually at a cost of at least 10 cents per gallon. For a cationic resin, 120,950 gal per month would be generated at a hauling expense of $12,950 per month ($155,400 per year). 483,840 gal per month of oily waste water would be generated for a hauling cost of $48,384 per month, or $580,608 per year, a 275% increase (or $425,208) unless the water is prepolished.
The total cost of increased regeneration plus wastewater disposal amounts to $116,928 per month. Downtime and disruption have not been included.
Activated carbon or membranes are sometimes recommended for prepolishing, but both are quickly blinded by oil. The problem would be transferred from the resin to the carbon or membrane.
If sacrificial resin were used to trap the oil, it would have to be the anionic type, which costs about $4 per lb, or $170 per ft3.
Clays to the Rescue
The prepolishing problem can be solved cost effectively by the use of organically modified clay (organoclay). This material has found acceptance in the groundwater remediation industry for its ability to remove 50% of its weight in oil, seven times the capacity of activated carbon. The technology for prepolishing water to protect the integrity of ion exchange resin and membranes is the same as for groundwater treatment.
Depending on the solubility of the oil, organoclay may be used in the stand-alone mode, or followed by activated carbon, to reduce the oil content to a non-detectable level. The carbon’s job would also be to remove chlorine.
Organoclays are bentonites that have been chemically modified with quaternary amines, which render the bentonite hydrophobic or organophylic. Bentonites consist primarily of montmorillonite-type clay. They are a natural cation exchange resin, the exchangeable ions being primarily sodium, calcium and magnesium. These ions are exchanged with the nitrogen end of the quaternary amine.
Since the 1950s, organoclays have been used as thickeners and antisettling agents in greases, lubricants, putties and paints.
In the granular form, organoclays are placed into the same filter vessels as activated carbon. Once immersed in water, the quaternary amine chains will stand up perpendicular to the clay platelets.
As an oil droplet passes by and encounters the amine, the amine will partition (i.e., dissolve) into that droplet and fixate it by coulombic forces, permanently removing the oil.
In this fashion, and by anion exchange, they also remove humic acids, PCBs and other chlorinated hydrocarbons, and any organic associated with oil. Only anionic and non-ionic surfactants can remove the oil once it is attached to the organoclay.
Prepolishing Costs
The additional costs for clay prepolishing in the above example follow:
| Oil generated per year: | 1,968 lb |
| Organoclay requirement to completely remove oil: | 3,280 Ib |
| Vessel for clay prepolishing: | $5,200 |
| Organoclay changeout cost after one year: | $200 |
| Initial cost of organoclay: | $4,264 |
| Freight costs (initial and changeout): | $1,500 |
| Clay landfill disposal cost: | $600 |
| TOTAL: | $11,764 |
| Annual cost for organoclay, labor for replacement, freight, disposal: | $5,264 |
Cost increase due to oil without prepolishing
| Wastewater hauling costs per year without prepolishing: | $580,608 |
| Standard wastewater hauling costs per year: | $155,400 |
| Savings in wastewater hauling costs per year: | $425,208 |
At a savings of $425,208 per year or $35,434 per month, this system pays for itself, including operation for an entire year, in less than two weeks based on savings in wastewater disposal costs alone.
If an activated carbon tank is required, an additional cost of some $6,000 per year would be added, and the system still pays for itself in less than a month. If regeneration and resin replacement costs are figured in, it pays for itself within one week, not including downtime.
Case Histories
Plating Wastewater Example
A metal plating company uses a rust preventive in a cleaning bath, resulting in a wastewater that contains suspended solids, 10-ppm to 20-ppm oil, heavy metals, and calcium and magnesium. The system operates at 36 gpm.
The treatment train for the water cleanup is as follows: a sand filter, bag filters, an ion exchange resin tank with both cationic and anionic resins and a reverse osmosis (RO) system, which removes salts and soaps.
Each day, the operation spent $60 on bags, and the resin had to be regenerated weekly. Both the resin tanks and RO membranes had to be cleaned every week.
The sand in the sand filter was replaced with organoclay. The oil content of the water is now less than 1 ppm. The operation is saving $50 per day on bags. Cleanup of the RO membranes and ion exchange tanks plus regeneration of resins is now done every two to three weeks.
The plating company reduced the cleanup and regeneration costs by one half to one third. Considering that it costs up to $10 per ft of resin for regeneration, plus all the disruptions associated with it, the use of organoclay for water cleanup is extremely beneficial.
Tumble water from a plating operation contains zinc and oil. To remove the zinc and bring the water into compliance with local permits, a bed of natural zeolite (clinoptilolite) was installed, preceded by a bed of organoclay to prevent the oil from plugging up the pores of the zeolite. The company is in compliance with regulations, changeout is necessary only once every six months, and disruptions are minimal.
Membrane Protection
A manufacturer of air craft components generates a free emulsified oily waste water which results from their machining, milling, cutting and other operations.
Presently, they are using skimmers and dissolved air flotation to remove oil from the water. To remove the heavy metals they follow up with flocculation, precipitation and clarification.
Due to the rapidly increasing cost of water, and the possibility of discharge permit violations, due to potential failures in their present treatment system, the company installed a 25 gallon-per minute (100 liters per minute) reverse osmosis system (RO).
The RO system separates dissolved salts, heavy metals and organics from water by producing two streams; one high in contaminants and dissolved solids (the concentrate) and the other containing relatively pure water (the permeate). The concentrate is returned to the clarifier for additional treatment, and the permeate is recycled back into the manufacturing process. Most of the highly soluble salts from the clarifier, such as sodium, are removed in the liquid portion of the sludge. However, the operator is prepared to “purge” the system of high TDS (total dissolved solids) in case-the concentration gets too high.
Reverse osmosis membranes are very susceptible to fouling by free and mechanically emulsified oils. The oil creates a continuous film on the surface of the membrane which plugs the pores, rendering it inoperable. Under certain flow rates, less than 5 ppm of oil can totally blind the membranes. This means the membrane can no longer remove the salts and metals from the waste water. If the membrane is not overly-fouled, it can be cleaned and most of its performance characteristics restored. This results in costly down time and added expense for cleaners.
The company installed an adsorber vessel filled with organoclay in front of the RO unit. The organoclay removes the oil to the non-detect level (the vessel and quantity or organoclay was sized that way), allowing the RO unit to function unencumbered.
The original RO system required cleaning every two days. Pre-treatment of the water with organoclay has reduced cleaning frequency to once every 30 days.
The details: Cleaning requires 4 hours of labor, cleaners and extra energy, amounting to $200 for each cleaning, or a total annual cost of $36,400. Furthermore, frequent cleaning slowly degrades the elements of the membrane, reducing the membrane’s life from its usual 4 years to 2 years. Replacement cost of the membranes, including installation labor, is $8,500/year. This amounts to a total annual maintenance cost of $44,900.
After implementing organoclay pre-treatment, their cleaning costs were reduced to $2,400/year. In addition, the reduced stress on the membrane elements doubled their life to 4 years, slashing replacement costs to $4,250/year.
Conclusion
Annual operations cost savings by prepolishing with organoclay are $38,200.
These examples show that prepolishing water with organoclay and possibly activated carbon is highly beneficial. The organoclay is blended with anthracite to prevent early plugging of the pore spaces between the granules.
At a pH of about 7.8, soluble iron converts to a ferric iron and plates out on the anthracity, resulting in a drastic reduction of iron and coprecipitation of other metals, which is a further benefit of prepolishing with organoclay.
The grain size of the organoclay/anthracity is -8+30 mesh, causing the material to act like a sand filter, reducing the amount of suspended solids that enter the resin bed.
Table 1. Regeneration costs of cation- and anion exchange resins.
| Activity | Cation resin | Anion Resin |
| Backwash | 15 min 45 gal per ft3 | 15 min 20 gal per ft3 |
| Chemical regenerant | 10% HCI, 15 gal per ft3 30 min 60 min Dosage: 4 lb to 15 lb acid per ft3 | 2% to 6% NaOH, 30 gal per ft3 Dosage: 4 lb to 10 lb caustic per lb resin |
| Slow rinse (displaced rinse) | 0.5 gpm for 20 min 10 gal per ft3 |
0.5 gpm for 20 min 10 gal per ft3 |
| Fast rinse | 2 gpm for 20 min 40 gal per ft3 |
2 gpm for 20 min 40 gal per ft3 |
| Total | 110 gal per ft3 | 100 gal per ft3 |
| Chemical cost | Bulk HCI: 12 cents per lb In totes: 24 cents per lb |
Bulk NaOH: 18 cents per lb In totes: 36 cents per lb (solution) |
| Total regeneration cost | $1 per ft3 resin | $6 to $10 per ft3 resin |
Source: Resin-Tech, Inc.
References
1. Wilson, A. L., “Organic fouling of strongly basic anion-exchange resins,” J. Applied Chemistry July 1959, pp. 352-359.
2. McCoy, M., “Ion Exchange,” Ultrapure Water, Tall Oaks , Publishing, Jan/Feb 1996, pp. 20-31.
About the Author
George Alther has an MS in Geology for the University of Toledo in Ohio. He is the owner of Biomin, Inc., a producer of organoclay, located in Ferndale, MI. He has been manufacturing organoclays since 1987. He has been in the environmental business for 25 years. He is the author of over eighty technical and scientific articles, and holds two patents. For more information please call Biomin, Inc. at (248) 544-2552.
