Sorption

Any XOC that is sorbed onto the biomass and other solids in a bioreactor will be removed from the system with the waste biomass. Since the sorbed XOC will not be chemically altered, the potential exists for it to desorb from the waste biomass during its handling, processing, and disposal, potentially leading to release of the XOC to the environment. Because of the potential of such releases, it is important to quantify them so that they may be controlled or eliminated if necessary.

Mechanisms and Models. Sorption of an XOC onto biomass is a complex process, involving both adsorption to the surface of the solids and absorption into cellular components, particularly the lipids. Because the exact mechanism is seldom known, the term "sorption" is typically used to describe the phenomenon and the determination of sorption coefficients is accomplished empirically. Unlike sorption to soil and sediments, which typically display two distinctly different rates with each contributing significantly to the removal,14 sorption onto biomass is very rapid, with the vast majority of the sorption occurring in a matter of minutes, followed by slow sorption of an additional small amount over a period of hours.145" For example, Wang"" found that the liquid phase concentration of di-n-butyl phthalate was essentially the same after two minutes of contact with biomass as it was after 72 hours of contact, whereas others have found that equilibrium was approached within an

hour.

Desorption is the release of a sorbed chemical from the sorbent, and as such, is the opposite of sorption. In some cases, sorption is fully reversible, with the desorption relationship being the same as the sorption relationship. For example, the sorption of di-n-butyl phthalate,"" lindane/5 diazinon,4' and 2-chlorobiphenyl4" were all found to be fully reversible. In other cases, sorption may be irreversible, suggesting that chemical reactions are involved/4 or the reversibility may change over time,59 suggesting a shift in the relative importance of adsorption and absorption. All of this suggests that the reversibility of sorption will depend on the nature of the chemical and biosolids, and must be determined on a case by case basis. In spite of this, reversibility appears to be common and is often assumed.

A number of models are available to express the equilibrium relationship between the concentration of an XOC in the liquid phase and the quantity of the XOC on the solid phase.1S4K All are referred to as isotherms because the equilibrium re lationship is influenced by temperature, requiring it to be quantified for a fixed temperature. For most purposes, the Freundlich isotherm is adequate:

where Cs X(K is the concentration of the XOC on the solid phase, ks xo< is the sorption coefficient, and n is an empirical coefficient. The units of k^xnc depend on the value of n and the units of Cs.XOc and Sxor. Typically, Cs xot has units of mg/g and SXO( has units of mg/L. Thus, when n has a value of 1.0, k^o, has units of L/g. At the low concentrations at which XOCs are typically present in wastewaters, isotherms are often linear, allowing n to be taken as l.O.4* Such a value should not be assumed for all cases, however; particularly for higher XOC concentrations. The best policy is to determine the values of 1c,XIk and n experimentally.

The major loss of an XOC due to sorption onto biomass in the activated sludge process comes from biomass wastage. Consequently, the rate of loss by sorption is given by:

where and XM„ are the flow rate and concentration of the wasted mixed liquor from the bioreactor.

Estimation of Coefficients. Although it is preferable to evaluate the coefficients kvX1K and n experimentally for a given biomass, the procedure is tedious and expensive. Thus, it would be desirable to have a way to estimate them from the literature, particularly for preliminary engineering studies in which only an estimate of the relative importance of sorption as a removal mechanism is needed. Two methods exist. One allows transfer of information on a given XOC from one biomass to another, whereas the other allows extrapolation of information on one XOC to another XOC for the same biomass.

The vast majority of sorption to the biomass in an activated sludge system is to the organic fraction. Since the materials constituting the sorptive organic fractions of various sludges are similar, if a sorption coefficient is expressed per unit of organic carbon it can be used for any type of wastewater solids, even those from different plants.'^ Furthermore, since the mixed liquor volatile suspended solids (MLVSS) concentration is proportional to the organic carbon content of an activated sludge, the same should be true when the sorption coefficient is expressed per unit of MLVSS. Thus, a literature value for the sorption coefficient for a given XOC on activated sludge can be used to approximate the sorption coefficient on biomass from another plant, provided that the coefficients are expressed per unit of organic carbon or MLVSS.|s

The octanol:water partition coefficient, k<)W, is a commonly reported characteristic of organic chemicals and is representative of their hydrophobicity, with larger k,,w values indicating more hydrophobic compounds.4" Because sorption is related to the tendency of an XOC to leave the water phase, it is related to its hydrophobicity. Consequently, the sorption coefficient, k„.X(K-, is related to the octano!:water partition coefficient. A number of researchers have developed correlations of the type:

where t,X(H is expressed on a per unit carbon basis. Schwarzenbach et al.4*' summarized values for the coefficients a and b for a number of types of XOCs likely to

be found in wastewater. The values of a were around 1.0 (0.81, 0.88, 1.01, and 1.12), suggesting that a value of 1.0 can be assumed with which to estimate the sorption coefficient for one XOC from a measured sorption coefficient for another on the same biomass. If a can be set equal to 1.0, then it follows from Eq. 22.15 that:

Thus, the sorption coefficient for XOC #2 can be estimated from a measured sorption coefficient for XOC #1 using handbook values of the octanol.water partition coefficients for the two XOCs.

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