WATER RECYCLING COST COMPARISON USING ION EXCHANGE AND REVERSE OSMOSIS

There are four basic ways to recycle/reclaim water and metal in a plating type operation. To date, we have not seen a good cost comparison between the four types of units so we thought we would write one. The four basic types of systems used in plating waste treatment are: 1. Reverse Osmosis with prefiltration and carbon after a precipitation system. 2. Deionization type ion exchange for recirculation using deionized water feeds. 3. Metal recovery ion exchange without recirculation. 4. Metal recovery ion exchange with reverse osmosis recirculation. These four approaches all provide some advantages and all also have some disadvantages. There is a capital cost comparison but do not take the numbers as absolute because the equipment cost can vary up to 300% between various vendors selling the same basic system. Type 1 - Reverse Osmosis with prefiltration. RO recovery systems are subject to fouling without good prefiltration and pH adjustment. Following a precipitation system, with good filtration, an reverse osmosis system makes sense when properly sized. With an automated cleaning procedure, up to 80% of the water can be recycled. When used in this manner the thing to watch for is precipitation in the membrane when the concentration is taken too high and the flow is too low. As any metal is sludged prior to the reverse osmosis unit, there should be little to precipitate in the RO. The disadvantage is that there is no reduction of sludge volume using this method. Type 2 - Deionization type ion exchange. When a system is primed with deionized water, only the chemistry you add to the system is removed. Incoming water is eliminated as a source of ions. A deionization type system removes all ions from solution and recycles deionized water. The metal, Sodium, Potassium, and other cations are removed on one resin, and the anions such as sulfate, fluoride, chloride, sulfamate, EDTA, etc. are removed on another resin. The quality of the water recycled is very high. When regenerated you get all of these ions back in the regenerant which then has to be dealt with. Metals can be plated out of the cation regenerant. The limiting factor in these systems is the high regenerant use required. If you have a total ion concentration of 200 PPM in the effluent with a metal (Copper) concentration of 20 PPM, you are recovering 9 pounds of "stuff" to one pound of Copper to get deionized quality water. Regeneration will result in water usage of 8-10% of the total deionized water flow or, for every 10,000 gallons recirculated, you will have to treat and dispose of 800-1000 gallons of solution. Dionizations's main advantage is on very low TDS streams with a high percentage of target ions. When target ion concentrations are a small percentage or TDS is very high, deionization becomes very expensive. Type 3 - Metal recovery ion exchange. MRIX systems uses chelated resins to remove only the target ions from the effluent stream. The most efficient uses are for removing Lead, Copper and Nickel. They exchange Sodium for the target metal and pass the hardness ions, Calcium and Magnesium. They are effective on industrial water rinse without any preconditioning and are very effective in the direct replacement of sludge generating systems. Regeneration is very efficient and the target metal is usually recovered in relatively small volumes of regenerant. The rinse waters from regeneration are recycled through the system to remove any contaminants. The effluent water from these systems are high in TDS and pH and may be used for non-critical rinses after acid baths where neutralization is necessary. The main disadvantage of higher resin cost is offset by much lower operating costs and low maintenance. Type 4 - Metal recovery ion exchange with reverse osmosis. Metal recovery ion exchange with reverse osmosis systems combine Type 3 and Type 1 systems into a single recycling unit. The advantages of Type 3 systems (efficient metal recovery) and Type 1 systems (low operating cost and efficient TDS rejection) work together to provide a low cost, low maintenance water recirculation system. The capital cost is higher than any of the other systems but the efficiency of compliance and recirculation is enhanced. Rinse water is first stripped of metal by exchanging it for Sodium. The high Sodium gray water is then passed through a reverse osmosis system where approximately 80 % of the volume is recycled and the TDS is passed to the 20% going to the drain. The main disadvantage of higher capital equipment expenditures is offset by much lower operating costs and disposal costs.


................Type 1.............Type 2...............Type 3..............Type 4 

Capital Cost......Mod .............. Mod................ Mod................ High
Operating Cost....Low .............. High............... Low................ Mod 
Maintenance.......Mod .............. Low ...............V.Low............... Mod 
Efficiency 
Metal Recovery....Low............... High............. V.High .............. V.High
Chemical usage....Low ............. V.High ............. V.Low ............. V.Low
Electrical Use....Mod ............... Low ................ Low ............... Mod
Can be recycled?..Yes ............... YES ................ Maybe ............ Yes
Compliance........High .............. High ............... High ............. High
Disposal Costs....High .............. Low ................ V.Low ............ V.Low 

$/1000 gallons/hr 
Capital Equipment$71K+ ..............$60K ...............$80K ..............$105K

REGENERATION COSTS

A quick comparison of operating cost between a deionizing ion exchanger and a metal recovery ion exchange system. Parameters: Deionization system is recirculated, 200 ppm total TDS in return effluent, 20 PPM Copper. Metal Recovery Ion Exchange system is not recirculated, uses 600 PPM TDS industrial water feed and 20 PPM Copper.


                        		  Deionization	     Metal Recovery IX
Gallons processed /cu.ft. 
  Before regeneration   			800           		11,350 

Regenerant use/cu.ft. 
   Sulfuric Acid        			6.5 lb.        		14 lb. 
   Caustic Soda         			6.0 lb.         	6 lb. 

Ratio Deionization/Metal Recovery Ion Exchange
   Acid                   		 	2.92/1
   Caustic              			6.3/1
Gallons per 300 day year of caustic and 
acid at 20,000 gal/day water usage. 
   Acid (93%)                              1550 gals/yr.            532 gals/yr.
   Caustic (50%)                           3333 gals/yr.            532 gals/yr.

WATER USE PER YEAR

Total water to dispose of per year, 20,000 gpd system. This covers treatable water that must be handled outside of the system and cannot be run through the system for treatment. Metal Recovery Ion Exchange systems recover a concentrated regenerant. Water used for rinsing columns is passed to the buffer tank and processed through the columns to drain automatically. No other treatment of rinse water is required. Deionization systems generate both regenerant and rinse waters that must be treated and disposed of. The rinse water is high TDS and would severely reduce the capacity of the columns if it were bled back into the system.


                        	Deionization	  Metal Recovery Ion Exchange
Gal regenerant to treat
   Caustic(5%)          	42,662 gals    	  0 gals (10,640 processed)
   Acid(5%DI,10%MRIX)   	48,060 gals    	  8405 gals(no recycling)
Cu recovered/cu.ft./cycle 	137 grams      	  862 grams
Gals. of Rinse/yr. to treat 
   Acid Cycle           	183,315 gals   	  0 (15,960 processed during regens)
   Caustic Cycle        	216,645 gals   	  0 (18,620 processed during regens)