All AC applications end up with exhausted (spent) AC. This article will run in two parts and will touch on some considerations of generating spent AC and its regeneration:
- 1. Spent AC regeneration
- 2. How to determine the remaining adsorption capacity in your AC system
- 3. How to predict AC feasibility for adsorbing specific aqueous pollutants.
This article does not consider legal and safety problems; it is intended to get the AC user to think about some existing AC opportunities instead of a black box treatment.
AC is the most economical treatment option to remove trace pollutants from drinking water. AC can remove the last traces of organics better than any known technology. The key attribute of AC is its ability to remove a wide variety of toxic organic and inorganic compounds to non-detectable levels in potable water applications.
Spent Activated Carbon (AC) Regeneration
During the last few years Professional Analytical and Consulting Services (PACS), of Corapolis, Pennsylvania, has been developing a new liquid phase technology to regenerate spent activated carbons. Table 2 contains a list of spent AC regenerated with this new liquid phase regeneration process. Liquid phase offers several advantages over the classical technologies to regenerate (regen) exhausted AC. This new liquid phase process is based on three well-established phenomenon:- competitive adsorption,
- pH effects on the adsorption/ desorption of acidic and basic organic compound adsorption, and
- solubility in bulk solvent.
Competitive adsorption is due to AC preference for specific molecules and their variable attractive forces. The higher the adsorbate capacity (grams adsorbed per 100 grams of AC) the stronger the adsorptive forces. AC capacity is typically expressed in grams of adsorbate per 100 grams of AC at equilibrium. Adsorption of molecules which can exit in ionic and non-ionic forms, depending on the pH, is useful in the new liquid phase concept.
In AC columns ionic forms pass through the AC column, but the non-ionic form has a high capacity, loading can be to about 51 grams per 100 grams of AC. AC can have a 100-fold or more difference in capacity for an ionic/non-ionic form of a compound.
Adsorption/desorption can be reversed simply by changing the pH. The pH only needs to change two pH units above or below the pKa or pKb of the acidic or basic competitive adsorber. This new liquid phase regeneration process consists of two steps: 1. Pushing the original adsorbates off of the AC with a pH sensitive competitive adsorbate (a good solvent to accept the adsorbatesis important), and 2. Changing the pH to convert the new adsorbate to its ionic form which will desorb and wash out of the column.
As you can see from the data in Table 2, the iodine numbers are improved by use of liquid-phase regeneration. These improvements are competitive with other regeneration technologies. Iodine number is an American Society for Testing Materials (ASTM) test method to determine AC adsorption activity. This test method is the most commonly used to evaluate AC for water applications. It is the number of milligrams of iodine adsorbed per gram of AC, in the defined ASTM test method. Many AC users buy AC based on the iodine number and other ASTM physical and chemical test method results. PACS provides ASTM test methods for the AC industry. In the drinking water AC application, bituminous coal is the common raw product to make granular AC. AC can be manufactured from a wide variety of natural and synthetic materials which have a high carbon content. In drinking water applications a 1000 iodine number for coal based AC is desirable because it maintains the hardness of the AC. As the iodine number goes higher, the hardness is expected to decrease. AC hardness is important in regeneration because of the AC handling. Soft AC literally breaks apart to cause problems (water flow through the bed decreases) and AC dust is created. Small dust particles are not desirable.
AC operators often back wash (flow is reversed at a higher flux, gallons/square foot/minute, which can be 10 times the operating flow). AC beds do remove suspended solids which buildup on the bed. Also, water flow pressure will compact the bed to a smaller volume and possibly restrict the flow. Back washing removes the accumulated grit between the AC particles and increases the bed height. Small AC particles which are pushed down in the bed are redistributed. Back washing results in an increase in bed height and better flow. Back washing is an important form of regeneration. Regenerators often request the AC user to back wash and drain the excess water before shipment of the spent AC for regeneration at their facility.
Sending the spent AC to a regeneration facility spares the AC user the inconvenience of handling spent AC. The AC user can have fresh (regenerated or virgin) AC brought to their facility and use the same truck to take the spent AC away for regeneration. When it is shipped off-site, spent AC is usually considered as non-hazardous due to their reclaimed status. The cost of regenerated AC is usually lower than virgin AC.
The EPA has a test procedure called the Toxicity Characteristic Leaching Procedure (TCLP) to evaluate materials potential to release toxic organics and metals, which could threaten groundwater quality. The TCLP method mimics a landfill; an acetate solution is used to leach the material to obtain extracts which are analyzed by gas chromatography- mass spectrometry (GC-MS) and inductively coupled plasma spectroscopy (ICP) instruments. These instruments can accurately measure low nanograms. Results from the TCLP are used to determine if the spent AC is hazardous or non-hazardous. Non-hazardous AC has a lower cost to regenerate. Almost always in drinking water application cases, the spent AC will pass TCLP. Spent AC is typically classified as hazardous only if it was used in a waste water application listed in the CFR as hazardous waste activities.
Classical regeneration of spent AC is performed off-site in a furnace, rotary kiln or multiple hearth furnaces. The thermal regen process is similar to that used to manufacture the original AC. The AC user should expect to lose about 5-10% of the AC during regeneration. Some AC particles will break during handling and the regeneration process burns off some of the AC to produce slightly smaller particle on average. This loss will be made up with other AC. Thermal regeneration can be provided as a segregated or mixed AC regen process. Multiple hearth furnaces regenerate large amounts of AC using a continuous process. The regenerator has a hard time separating the spent AC from different clients. Often, the AC user may not receive the same AC back from the regenerator because of co-mingling spent AC from several AC users to feed the furnace. Rotary kilns are smaller units and the operator can feed small batches of spent AC and provide a segregated AC; the client receives back the same AC sent for regeneration, plus any needed make up AC. Regeneration can save money for the AC user and manage the AC adsorbates, literally from their cradle to their grave, RCRA law intent.
Thermal regeneration plants can cost 1 to 3 million dollars and need environmental permits (which may take a year to obtain) and have many other business problems. The not in my back yard (NIMBY) is always a problem if you want to start a regeneration plant. Science education in the United States is low and citizens can be easily persuaded to resist new chemical plants in their area. Most AC users will not erect their own regeneration facility. However, if you generate enough spent AC or have a group of spent AC generators to form a consortium, it may become economically feasible to establish your own regeneration facility. The liquid phase regeneration process discussed above may not have these problems faced by the thermal regen technology; it is expected to be a lower cost process and avoid AC attrition due to handling.
If you have spent AC, it is possible to use laboratory scale regeneration studies to evaluate its recovery back to useful AC. PACS provides laboratory regenerability evaluation of spent AC with both liquid and thermal regeneration. This lab scale evaluation lets the AC user know what to expect with their spent AC when they work with a commercial regenerator.
During the last few years AC users can purchase good quality AC from many suppliers. Historically there were only a few suppliers. With the increased number of players in the AC industry it is not uncommon for an AC user to buy fresh AC from one vendor and send spent AC to another vendor. Many of the new players do not provide the technical services provided by the established firms. á
