Abstract
The characteristics of the 29 vertical flow systems reported on by Rose and Dietz (2002) have been reevaluated based on 2 years additional experience and revised understanding of acidity. Data on several additional systems have been collected. If measured hot acidity is interpreted as net acidity that includes the effects of alkalinity (vs. net acidity = acidity minus alkalinity), many systems remove slightly less acidity and fewer VFS produce net alkaline water. Rates of acidity removal (g/m 2 /d) are somewhat lower, and the typical value is about 34 rather than 40 g/m 2 /d. If non-Mn acidity is taken as the critical parameter, the sizing parameter for non-acid effluent is about 35 g/m 2 /d. Systems with fine limestone in the compost do about twice as well and appear to be a solution for high-Al discharges. Over the past 2 years, at least 5 of the systems have markedly decreased in effectiveness. Problems have been accumulation of excess ferric hydroxide precipitate on top of the compost, pipe plugging, channeling, and Al coating causing decreased reaction rate with limestone.
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