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OZONE-ELECTRON BEAM TREATMENT FOR GROUNDWATER REMEDIATION.

P. GEHRINGER. H. ESCHWEILER and H. FIEDLER

Auxlnan Research Ccntrc Seibcrsdorl. .4-Z&4 Seibersdorf. Austria.

ABSTRACT

Dose rate effect and low penetration XL‘ malor d1sadv;u~~agcs of electron beam irradiation. Addition of ozone before or during irradiation may elin;inate [he dose rate ct’fec~ as shown for the remediation of a groundwater contaminated with trace amou111s (II’ chlorinated solvents such as trichloroethylene (TCE) and perchloroethylene (PCE). Beside the elimination 01’ Ihe dose ra[e effect the necessary radiation dose for pollutant decomposition is considerably Iowcrcd hy lhe presence of ozone during irradiation. As a consequence the economy of the combined o/oilc/irnvli3rion procchs becomes better than that of irradiation alone. Irradiation of turbulent water ilows enables clean-up of water layers thicker than the maximum penetration r)f the electrons used. This el’f‘ecl is improved by the presence of ozone during irradiation, too.

KEL’WORDS

Electron beam irradiallon. ozone, atlva~wed osltla~ic!n: groundwater. remcdiation, chlorinated solvents, trichlomethylene, perchtorc~elhyleil~,

IURODI j(‘TlOh

Radiation processing 01’ groundh;IIcr I’or reniot al 01 low level contaminants or for disinfection suffers from some major limitations which prevenr the dirccr use of such a treated water as drinking water. Although the radiation doses usually applied are camparatively small formation of nitrite and hydrogen peroxide above the limit values for drinking water are mostly unavoidable (Gehringer et al., 1992a).

These problems may he overcomc by addition oI‘ tmnc before irradiation (Gehringer et nl., 1992b; 1993a). By the presence of o,vme both the above by-products are completely removed. Moreover, the “androgynous” irraclia~ion process with its almost equal amounts of oxidiz,ing and reducing intermediates is converted into an oxidation proccbs a so-called Advanced Oxidation Process (AOP).

Remediation of groundlvater for subaequenr II\C a\ drrnking water should be done with electron beam accelerators as radiation source instead of Cobalt-60 because of public acceptance. However, as compared to y-sources electron beam acceleratorx usually show two major disadvantages: a so-called dose rate effect and in the range of interest (0.5 10 3 Me\‘) a rather low penetration of the electrons.

Both effects are very important issues in the design and, of course. in the economy of electron beam processing of water. The present paper examines these problems for a combined ozone/electron beam process applied for remediation of a groundwater contaminated with trace amounts of chlorinat -d solvents such as trichloroethylene (TCE) and percliloroethylcne (PCE), respectively with special emphasis to economical aspects.

1076

EXPERIMENTAL

For the experiments Vienna City drinking water (195 ppm bicarbonate; 6.5 ppm nitrate and 0.6 ppm DOC) was spiked with TCE and PCE. respectively. All the experiments described were run both in a prototype and a bench scale installation which have been already described elsewhere (Gehringer et al., 1992b; 1993b). In some experiments gaseous ozone contained in an oxygen gas flow (about 100 mg 03 per litre 02) was introduced directly into the irradiation chamber by means of large metal frits placed at the bottom of the chamber.

RESULTS

Fig. 1. TCE decompc)sition in gh)utidwater by various irradiation treatments.

Fig. 1 shows both the dose rate effect of the TCE decomposition in groundwater and its elimination by ozone addition before irradiation. Moreover, the signilicant reduction in the radiation dose requirement of the combined ozone/irradiation treatments is also clearly demonstrated. This effect is especially pronounced with the electron beam irradiations: To reduce, for instance, 120 ppb TCE to a residual concentration of 5 ppb the electron beam irradiation requires a dose of 370 Gy. When 3 ppm 03 is added to the groundwater before irracliation the same TCE reduction is already achieved with a dose of 45 Gy. Using the known equation for capacity calculations

kW kg H,O /h = 3600 .- .r)

l&y

these dose values (0.37 kGy and O.tO45 kGy) C;III be converted now for a given electron beam accelerator (25 kW) and beam utilization factor (TJ = 0.6) into throughput capacities (kg H20/h). The corresponding values are: 146 m3/h for 370 Gy (electron beam only) and 1200 m3/h (combined 3 ppm 03/electton beam). In this way the dose rate effect is expressed in terms of production capacities and becomes easier to handle for economic evaluations. Based on these conversions an economic comparison between these two processes has been made (Table 1).

Table 1. Economic comparison for the reduction of 120 ppb TCE to 5 ppb in groundwater hy electron beam irradiation with and without addition of ozone.

Cost

Capital requirement (in IO00 US % units)

. 25 kW electron beam accelerator (incl. auxiliary equipment. transport installation, building and vault)

l Ozone generator l Water handling equipmerit

Total

Capital cost (in US $/hour) (9.5 % over 10 pear\; 8OOC~ hcmr\iycar)

Electron beam irratl. 370 Gy = 146 m?/h

1100

c

7h

? 7 4.5

1.5 32.5 9

1.5 3

Ozone-Electron beam irrad. 3 ppm 01 + 45 Gy) = 1200 m3/h

1200

270 250

1720

34

Operating cost (in US $/hour-) l Electric power (Y, O.l4/k\%‘h)

Accelerator Odmc gcncrator Pumpa

l Oxygen ($ 0.23/m3) + storage l Maintainance

TOTA I. I

vl $/I1 90 s/h (Capital + Operating) 0.25 S/m3 0.075 S/m3

Ahhough h)th capital co\t and operating co\t ar-c h~~hcr tor the o7onc/electron beam treatment the combined process is more co+ef!‘cct~c.e (by a lact[~r of :1bout 3) hecauhc of the higher throughput rate.

Next to the tloac rate ~‘l‘i’cc~ 111~ short pcnctratioll OI tllc cllcr-getic electrons represents a serious problem in the design of’ an electron treatment process. HOWCV~:I-, cxpcrunellts under turbulent flow conditions have demonstrated that water layers of 3 mm thicknchs car1 bc treated successl’ully with 500 keV-electrons although their maximum penetration is 1.4 mm only. It was found that the presence of ozone improved the turbulence effect (Gchringer (jr 01.. 1989). Rcmediation of water layers thicker than the maximum penetration of the electrons applied is equivalenl lo a dose reduction and of great economic consequence, therefore. The amount of dose reduction with incrc;rsing water layer thickness together with its consequericc to pollutant tlccompositic)n is SIWWII ill Fig. 2 for 5(K) kc\/-electrons combined with 2 ppm initial 03 concentration. In all these experiments a constant Ilow rate of IO5 m/min was maintained, the corresponding throughput r-,ltcx wcrc: 030 L/h (2 IIIIII j; 945 1Jh ( 3 mm) and 14 17 L/h (4.5 mm).

Optimum turbulence durm, u ~rradia~ioll C;UI bc acluc\ CCI when O/OIK I\ lntroducctl gaseously into the irradiation chamber. Such ;I process has. morcovcr. two other inlpc)rtant aspects. (1) The density of the gas/water mixture is less than waler the penetration of‘ the electron becomes higher, therefore. (2) The ozone of the water phase is consumed very fast durmg irradiation. Accordingly ozone may cross the gas/water boundary much faster as it does without chemical rcuctions; its dissolution is considerably driven by that. As a consequence much lower residual pollutant concentrations may be attained as compared with aqueous ozone addition (Fig. 3). Both clfccts are important options on the radiation processing of water.

107X

50

TCE (wb)

r

100 200 3iNl 4.i mm

Ihe (Gy)

Fig. 2 TCE decomposition in grountlwter 13~ combined c1~one/500 kcV

clectmn beam irradiation 3s ;I fulxtioli (11

the water layer thickness (rc;ilctl.

I nnu

500

PCE

(ppb) 100

91

I I I I

500 keV- electrons / 3 mm layer - water flow 50 L/h

0 0zOne water 5 ppm 0,

l ozone water 8 ppm 0,

A 0, IO, - gas flow 6 L/h

0 0, / 0, - gas flow 16 L/h -

I I inn 200 300 400

Dose (Cy)

Fig 3 PCE decomposition in groundwater by combined ozone/electron beam irradiation treatment.

REFEREYCES

Gehringcr, P.. E. Proksch, H. Eschwcilcr and M’. S/inovatz ( 19X9). The Elimination of Chlorinated Ethylenes from Drinking Water. f?cic. 19th .Iu~u/I Co/$ Rrrdicrr. Rudioisotopes. Tokyo, 393-413.

Gehringer, P., E. Proksch, H. Eschweiler and W. Szinovatz (1992a). Clean-up of polluted groundwater by radiation-induced oxidation. Pmt. Spp. Applic~ttiot~ r~flsntopes and Rrrdictriotz in Conservation ufE~zvi~~~~zm~v~t. Karlsruhe, X3-205. IAEA, Vic~lna.

Gehringer. P., E. Proksch. H. Eschwciler and \i’. S/illovat7 (1992b). Remediation of Groundwater Polluted with Chlorinaletl E~hylcncs by O/one-clcclron Ream Irradiation Treatmcnl. Apple. Rudiat. Isot. 41% 1 lU7- 11 IS.

Gehringer. P.. H. Esch\vcilcr. W. Szinoval/. H. Ficdlcr. R. S~cincr and G. Sonncck (1993a). Rrtdiat. Phys. Chcm. 42, 7 1 l-7 14.

Gehringer. P., H. Eschweilcr, W. Szinovatz. H. Ficdlcr, G. Sonncck and R. Sleiner (lYJ3b). Groundwaler Treatment for Chlorinated Erhylc~~~ using Ozone and Elcclron Beam Irradialion. Proc. 11th Ozot1e War/d Cottgress. San Francisco. S- 13-26 to S- 13-32.