Adsorption 101 |
Part 1 Adsorption is the adhesion of a chemical species onto the surface of a solid, such as phosphate molecules coordinating with the surface of an iron hydroxide particle. This process is different than coagulation or precipitation. Phosphorus has been removed from wastewater through coagulation by the addition and mixing of a coagulant; however, this process is limited by a solubility constant – phosphorus in municipal wastewater cannot be lowered beyond approximately 0.020 mg/L TP by coagulation. Adsorption is not limited in the same way. Through adsorption, phosphorus may be lowered to single-digit ppb levels. Recent research supports the concept of adsorption of phosphorus onto iron oxide particles occurring in municipal wastewater systems. An example of this research may be found here:Smith et al., 2008, Phosphate complexation model and its implications for chemical phosphorus removal. WER, 80(5):428-38. The Blue PRO® process harnesses adsorptive power in a reactive filter system. Using a continuous backwash filter to continuously regenerative adsorptive media and filter water, Blue PRO® not only overcomes equilibrium limitations to phosphorus removal, but also overcomes the diffusion limitations of a coagulant dosed in bulk solution. This is how Blue Water is able to offer a cost-effective solution for total phosphorus removal to extremely low levels.
The Blue PRO® process was developed a decade ago based on the knowledge that adsorption is the key to achieving extremely low levels of phosphorus in wastewater. At Blue Water we know the reactive filtration process is most effective when the adsorption removal mechanism is optimized, including formation of hydrous ferric oxide (HFO) coated sand and adjustments to our Centra-flo™ continuous backwash filters to achieve the most efficient separation of iron-oxide and phosphorus-containing waste particulates. The wastewater industry is now beginning to recognize that adsorption plays a role in chemical phosphorus removal; an article in the April issue of Water Environment & Technology highlights this removal mechanism: “Chain Reaction: How Chemical Phosphorus Removal Really Works” (WE&T April 2009, Vol. 21, No. 4) The WE&T article describes the role of adsorption as is pertains to chemical use in bulk solution. The Blue PRO® process offers additional advantages: the ability to overcome equilibrium and diffusion limitations. Our filters are designed to act more like adsorption columns than bulk solution reactors. As water flows up through our filters, forced into contact with HFO-coated sand grains, the solution phosphorus concentration is progressively decreased. Therefore equilibrium limitations are overcome through decreasing phosphorus, and phosphorus is forced into contact with HFO, overcoming diffusion limitations. Significant reserve adsorption capacity is available in the waste iron particulates from the Blue PRO® process. These particulates are continually removed from the system in the reject line, and through “reject recycle” this material is recycled to the front of the secondary system. This is the most efficient use of chemical within a plant, since now more phosphorus may be adsorbed on the particulates before dropping out in secondary clarification. More information about this process, including a mass balance across a plant with reject recycle installed may be found in these papers: Newcombe et al., Part I and II, 2008. The Blue PRO® reactive filtration process has the low-level phosphorus removal capability of an adsorption column, but with the added advantage of continuous regeneration of the adsorptive media and continuous removal of the waste material from the process stream. The HFO adsorptive media never needs to be replaced, removal efficiency will never decline, and adsorption capacity will never be used up.
In the first two installments of this series we explored the contaminant removal mechanism of adsorption, the key to Blue PRO®'s ability to reach extremely low phosphorus levels, such as 0.010 mg/L P. The Blue PRO Zinc can be removed by the Blue PRO® reactive filtration process in a couple of different ways. In one installation, zinc was lowered over 84% through the reject recycle option. The recycling of Blue PRO® rejects within the plant allowed zinc to be adsorbed onto the iron through the secondary portion of the plant; the zinc was removed from the process stream through secondary clarification. In the pilot project in Colorado described above, the efficiency of zinc removal by the Blue PRO® process was increased from 10% to 70% when pH was raised from 6.5 to 7.5. This minor adjustment in pH was enough to overcome the point of zero net charge associated with the zinc, allowing it to be absorbed by the hydrous ferric oxide (HFO) coated sand within the Blue PRO® system. Other contaminants that may be reduced by Blue PRO® under the right conditions include other metals, selenium, mercury, and radium. Pilot projects may be required to determine the ability of the process to remove other contaminants. A treatability study is available to determine the likelihood of success with the process.
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