Rates and extents of phenanthrene desorption were studied for more than 250 days as functions of sorbent type, initial loading level, and aging. Apparent first-order desorption rate constants for the slowly desorbing fraction were found to (i) range from 0.00086 to 0.148 days-1 for geosorbents that contain geologically mature kerogen and less rigid humic-type soil organic matter, respectively, (ii) decrease by as much as an order of magnitude with decreasing initial sorbed solid-phase phenanthrene concentration, (iii) decrease by a factor of 2 with increasing aging time for a humic topsoil but remain unaffected by aging time beyond 3 months for a shale, and (iv) be 1-2 orders of magnitude lower than rate constants for the rapidly desorbing phenanthrene fractions for any given contaminated sample. Six models were used to fit the desorption rate data. Biphasic diffusion and biphasic first-order models with three fitting parameters possess broad utility and are potentially useful in a variety of environmental applications. Disadvantages of a five-parameter triphasic first-order desorption model, a two-parameter gamma-function model, and a one- or two-parameter pore diffusion model are also discussed.

The rate of sorption of copper(II) and lead(II) onto activated carbons Nuchar SA and Filtrasorb 400 was observed to occur rapidly at the outset, followed by a slow and prolonged sorption. This indicates that previous sorption studies conducted for short equilibration periods likely underestimated the actual capacity of activated carbon for heavy metals. Although copper and lead ions desorbed from the activated carbon surface rapidly, the rate of desorption of lead was slower, prevailing over several days. Sorption of copper and lead ions on Nuchar SA was found to be fully reversible.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Water Environ. Res.,zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 65, 238 (1993).

No abstract

An earlier paper by the authors (1988) introduced a correlation between the Vesilind settling parameters, Vo and K, and SVI performed in a stirred, 1‐L graduated cylinder even though SVIs also were determined in more commonly used containers such as an unstirred, 1‐L graduated cylinder and an unstirred, 2‐L Mallory settlometer. Only the stirred graduated cylinder relationship was given because stirring increases the precision and decreases the variability of the SVI test, and Standard Methods (1975) has included stirring in the SVI test since 1975. Nonetheless, because the SVI test continues to be performed in unstirred containers, the correlations for the other SVI test methods measured during the study are presented herein so that they can be used by the wastewater treatment plant operations community.

The success of gravity separation of activated sludge from a treated effluent depends on the flocculent nature of the mixed liquor entering the secondary clarifier. Despite its importance to the overall effectiveness of the activated sludge process, flocculation phenomena are not routinely considered in the design and operation of the process. Further optimization of the activated sludge process to meet higher performance demands requires that the competing reactions of floc aggregation and breakup be maximized and minimized, respectively. Accordingly, the goal of this study was to develop an improved understanding of activated sludge flocculation.
A theoretically based and easily performed batch flocculation procedure was developed. The procedure enabled the quantification of the flocculation characteristics of activated sludges. The procedure was field applied, testing 30 activated sludges obtained at 21 full‐scale facilities. Results obtained during the field study indicated that the equilibrium concentration of supernatant suspended solids following batch flocculation and settling is comparable for a wide variety of activated sludges regardless of the initial aggregative state of the mixed liquors or the aeration device employed. The results indicated that flocculation of activated sludge cannot be used to reduce supernatant suspended solids below a certain limit. Moreover, the results indicated that attainment of equilibrium is rapid; the activated sludge flocculation reaction in batch reactors was 99% complete within 10 minutes for all but six of the activated sludges studied. Field‐determined estimates of activated sludge flocculation characteristics can be used to predict the performance of flocculators placed either upstream of or in secondary clarifiers. These estimates also can be used to determine the impact of altering process variables on flocculation, thereby affording a procedure for optimizing an activated sludge's flocculation potential.