Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 399

TABLE 8.3
Properties of GAC Used at SLAC
Parameter AquaCarb ® 1230C

Carbon type Coconut shell

Mesh size, U.S. Sieve 12 × 30

Effective size (mm) 0.6-0.85

Uniformity coefi cient 2.0

Iodine number (mg I 2 /g) 1100

Hardness no. (wt%) 95

Abrasion no. (wt%) 85

Apparent density (g/cm 3 ) 0.46-0.52

Water soluble ash (weight percent) 2

Contact pH 9-10

Source: Siemens, 2008, Westates® coconut shell based granular activated
carbon. www.siemens.com/water (accessed September 15, 2008).

a low ash content designed for use in potable water systems and in high-purity water systems for the
microelectronics and other industries. This grade of carbon passes the ANSI/NSF Standard 61 for
use in potable water applications; its specii cations are summarized in Table 8.3 (Siemens, 2008).
A third plausible theory for consideration is a combination of chemical, physical, and biological
mechanisms that may have developed within the GAC system operating conditions at SLAC. In this
theory, part of the 1,4-dioxane in the inl uent is chemically oxidized by the addition of chlorine, and
the remaining 1,4-dioxane is biodegraded by the appropriate microbial consortia in the GAC.
Microbial consortia may thrive in the GAC if organic compounds and/or toxic inhibitory degrada-
tion by-products that would otherwise minimize or negate microbial degradation are removed
through physical adsorption. In such a case, the enhanced environmental conditions possibly created
within the GAC vessel might support growth of a microbial population of 1,4-dioxane-degrading
microorganisms. For example, cometabolites such as toluene may facilitate biodegradation by sus-
taining the growth of monooxygenase-expressing strains of bacteria. Research conducted at
University of California, Berkeley has shown that monooxygenase-expressing bacteria are able to
degrade 1,4-dioxane, including those induced by toluene (Mahendra and Alvarez-Cohen, 2006).
Toluene is consistently present in the GAC inl uent at concentrations exceeding 1 mg/L (SLAC,
2008). Similar physical-biological interactions have been observed in i xed-i lm GAC biological
reactors as well as in wastewater treatment processes employing powdered activated carbon. At
SLAC, the potential occurrence of physical-chemical-biological removal mechanisms is facilitated
by the unusually long system retention times (i.e., > 2 days) and the in-series operation of the GAC

vessels (SLAC, personal communication, 2008). *

8.6 1,4-DIOXANE AND ORANGE COUNTY WATER DISTRICT OCWD
GROUNDWATER REPLENISHMENT SYSTEM
The unexpected discovery of 1,4-dioxane in advance-treated wastewater used in OCWD's
Groundwater Replenishment System underscores the challenge of using recycled water for ground-
water basin management. Fortunately, most recycled water is not tainted by the large-volume industrial
discharge of 1,4-dioxane that caused problems in Orange County; however, as this study shows,
1,4-dioxane is nevertheless ubiquitous at low concentrations in treated wastewater because of the

* Personal communication with SLAC staff on possible mechanisms for 1,4-dioxane removal by GAC, 2008; SLAC
National Accelerator Laboratory Environmental Health and Safety Division, Menlo Park, CA.