WTP solids tested as FGD reagent

Issue 5 and Volume 100.

WTP solids tested as FGD reagent


Recently, I had the opportunity to assist with two projects in which water treatment plant (WTP) clarifier solids were tested on a full-scale basis as a supplemental reagent in wet-limestone flue-gas desulfurization (FGD) systems. Although testing is still being conducted, preliminary results are favorable. Efficient scrubber operation depends on a number of key factors including limestone grind size and calcium carbonate (CaCO3) content.

At one of the demonstration sites, we performed reactivity tests on the water treatment plant solids and the utility`s limestone, which typically has a 95-percent or greater calcium carbonate content. The active components of the WTP solids (75 percent CaCO3 plus an estimated 7 percent magnesium hydroxide) reacted completely before the test was half finished, whereas the limestone had not completely dissolved even at the conclusion of the reactivity test period. Because the limestone had always worked well in the scrubber, this laboratory demonstration was an initial indicator that the WTP solids would react suitably during the full-scale demonstration.

Full-scale observations

Fundamentally, the two projects were different in that previously ponded but dried WTP residual was used in the first demonstration process, whereas at the second location, which is a combined electric and water utility, the WTP clarifier discharge was pumped directly to one of the FGD system`s wet-grinding mills. Because the material for the first project was in dry form, utility personnel loaded it into the ball mill to be wetted and transformed into slurry. Another significant difference between the two projects is that at the first site, the scrubber removes fly ash, thus providing an additional source of alkalinity. At the second site, an upstream electrostatic precipitator removes virtually all of the fly ash before the gas reaches the scrubber.

The tests presented some revealing information. The WTP solids reacted as well or better than limestone. Reagent utilization, which has always been very good, stayed at the same level. Because the WTP solids for the first demonstration project came from sludge lagoons, the material contained rocks and other hard debris that caused problems with the ball mill and reagent preparation equipment.

Utility personnel had to mechanically remove the rocks prior to introduction of the WTP solids into the system. Reagent feed at the second site was much simpler. Some initial problems occurred with the piping system used to transfer the materials from the water treatment plant to the scrubber, but once utility personnel solved these, flow to the ball mill was steady.

Because the WTP solids at the second demonstration site contained approximately 15 percent inert material, a thin but visible layer of inerts began to form on top of the by-product slurry. At the conclusion of the three-day test, however, no inert buildups had occurred in the dried by-product. The utility plans a month-long test to determine if using WTP solids has an effect on the vacuum-drying system and the by-product quality.

For electric utilities, the use of this material may represent a method to reduce limestone purchase and hauling costs, and perhaps introduce a better-reacting reagent to the system.

Despite some drawbacks, the process shows a great deal of promise. In this era of increased recycling, the use of WTP solids in FGD systems has merit both from an ideological and an economic standpoint. More details will be forthcoming in later reports. z