(418e) Role of Membrane in Iemb Treatment of Groundwater Highly Contaminated with Oxyanions

Perchlorate
contamination of ground water is a worldwide concern. Several sites in Israel's
coastal aquifer are contaminated with perchlorate, with hundreds of ppm found
in one area accompanied with significant concentrations of nitrate and chlorate 
as well.  This has prevented water production from wells in the area. 

The
ion-exchange membrane bioreactor (IEMB)  [1] is a hybrid process for safe treatment of groundwater
highly contaminated with oxyanions (perchlorate, nitrate, and chlorate).  By Donnan
dialysis anionic contaminants migrate across an anion exchange membrane (AEM)
from a feed-water compartment to a bio-compartment. Once in the bio-compartment,
the anions undergo microbial reduction to safer species such as chloride ions
and nitrogen. The AEM acts as a barrier and keeps both compartments completely
separate. Glycerol is used as an exogenous carbon and electron source for the
biodegradation process [2]. This arrangement keeps the carbon source, reaction
byproducts and bacteria confined in the bio-reactor thus preventing secondary contamination of the treated water.  This
contrasts with standard anaerobic bioreactors for treating such contaminations,
for which the Ministry of Health requires additional barriers including MF, UV
and GAC.

The
present study examined the role of membrane type in performance of the IEMB in
removing perchlorate and other anions (nitrate and chlorate) at levels of
hundreds mg L^-1 from ground water with a mixture of high levels of
oxyanions.   This was based on Donnan dialysis studies and was studied initially
for synthetic and actual ground water fed to the water side while feeding 0.1 N
of NaCl to the bio-compartment. To understand the results, conductivity and
selectivity measurements were carried out on the different membranes.  The
anion exchange membranes examined included ACS (Tokuyama Soda),  PCA-100 (PC-Cell
GmBH), and AMI-7001 (Membrane International).

For
ACS membrane and all experiment conditions studied here, perchlorate dominates
the flux across the AEM and fluxes deviated from linear dependence on the
driving force at less than 0.5 mM. Even though perchlorate concentration is
considerably lower than nitrate and chlorate, its flux is greater than the sum
of the other anion fluxes. At an effective perchlorate driving force (EDF)
value above 0.7 [mM] perchlorate had a negative effect on the other anions
resulting in a decrease in the flux for both nitrate and chlorate while its own
flux only increased.  

Figure
1 – Results of
Donnan dialysis on ACS membrane.  Oxyanions specific flux vs. the Effective
Driving Force (EDF) of (A) perchlorate, (B) chlorate, (C) nitrate and (D)
anions total flux vs. the total EDF, for IEMB Donnan dialysis experiments fed
with (○) RHGW and bio media, (▼) synthetic
anions solution and NaCl 0.1N and RHGW and NaCl 0.1N (■) into the water
compartment and bio-compartment, respectively.  

Figure 2: Specific flux in Donnan dialysis on
synthetic mixture of oxyanions using PCA-100 anions exchange membrane.  Bio-side
contained 0.1 N  NaCl.

On
the other hand, experiments with PCA-100 membranes showed a linear dependence
of flux on driving force to much higher levels and much less intereference of
the perchlorate on the transport of nitrate and chlorate (Figure 2).   Tests 
of membrane conductivity  and regenerability supported the improved results
with the PCA-100 membranes.  Another type (AMI-7001) also showed promise based
on conducitivity but may be problematic in terms of polymer structure.  The
membrane selectivity for perchlorate and the other oxyanions over chloride was
clearly lower for the PCA-100 membranes than ACS.  A qualitative model will be
proposed to explain these findings and relate it to the differences in ion
exchange membrane structure. 

If
time allows, the implications of these findings for operating the IEMB as a
plug flow reactor will be outlined.  

[1]      A.D. Fonseca, J.G. Crespo, J.S. Almeida, M.A. Reis,
Drinking water denitrification using a novel ion-exchange membrane bioreactor,
Environ. Sci. Technol. 34 (2000) 1557–1562.

[2]      S.
Fox, Y. Oren, Z. Ronen, J. Gilron, Ion exchange membrane bioreactor for
treating groundwater contaminated with high perchlorate concentrations., J.
Hazard. Mater. 264 (2014) 552–9.