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Permeable Reactive Barriers - ORC Column, Batch and Bench Scale Modeling Case Study


The following is a summary of the findings by Yeh, Lin, and Wu: (Yeh et al., 2010)

Background and site description

The authors performed a series of column, batch, and bench scale tests to determine if oxygen releasing compounds (ORC) in a permeable reactive barrier can create enough dissolved oxygen to promote microbial growth. The microbes can then be used to decompose BTEX (benzene, toluene, ethylbenzene, and p-xylene).

Testing Design


A column test was first created to justify the use of ORC. 100 g of ORC was used in the column tests and consisted of cement, sand, H2O, KH2PO4, NaNO3, and varying concentrations of CaO2 (10-50%). Cylindrical columns were packed from the bottom as follows: 10 cm of Ottawa sands, 5-10 cm of ORC,  and 40 cm of Ottawa sands on top. Perforated stainless steel plates supported the packing media (top and bottom) and bottom of the column. Both K2HPO4 and KH2PO4 regulated the pH and served as nutrient components in the medium.

A batch test was created to determine the inhibitory concentrations of benzene and toluene, and the required DO needed for microbial growth. The batch test used 5 mL of inoculum in a 100 mL nutrient solution. 5 mL were added to varying concentrations of benzene and toluene, ranging from 20-320 mg/L. The microbial concentration was determined spectrophotometrically initially and after mixing. The DO required was also measured using BOD5 testing guidelines from NIEA.

Bench-scale PRB system
The bench-scale PRB is shown in the figure below:

Case study ORC Bench scale figure


The closed tank was filled with Ottawa sand to a bed depth of 20 cm. The ORC was 2 cm thick and situated 20 cm from the water inlet. The ORC composition consisted of cement, sand 40% CaO2, KH2PO4, K2HPO4, NaNO3, and H2O. Perforated stainless steel plates were placed on either side of the ORC and at the outlet to prevent water channeling. 12 monitoring wells were placed throughout the tank to measure DO, CFU (colony forming units), pH, oxidation-reduction potential (ORP), and BTEX concentrations. The inlet concentrations of benzene, toluene, ethylbenzene and p-xylene were maintained each at approximately 30mg/L. Two BTEX shock loadings were also applied by increasing the concentration to 60 mg/L for 4 hours to test the stability and the recovery capability of the PRB system. According to oxygen-releasing rate estimated in the column experiments, the initial amounts of ORCs were determined. Water samples were collected for BTEX analysis daily, and for DO, CFU, pH and ORP analysis weekly. The microbial communities were analyzed and can be found in the case study.


Column Tests
Varying the CaO2 concentrations allowed for different oxygen releasing rates of the ORC beads, and the results of the tests found that 40% CaO2 provided the greatest oxygen-releasing rate. This concentration was used in the bench scale.

Batch Tests
From benzene and toluene concentrations ranging from 20-80 mg/L, the organisms decomposed the contaminants to less than or equal to 5% of residual. The microbes used these compounds as carbon sources to build their own biomass. The highest measured biomass was at 80 mg/L. At high concentrations (320 mg/L) the benzene and toluene inhibited microorganism growth, so 90% residual still remained. The results also indicate that the amount of ORC used provides sufficient oxygen for bacterial growth, and increase the efficiency in decomposing the pollutants.

Bench Scale
The test was performed over a 100 day period.  There was no reduction in the removal of BTEX during the first shock loading. After the second shock-loading, however, the removal efficiencies of benzene, toluene, ethylbenzene and p-xylene from this system were reduced to 32, 44, 75 and 75%. The pre-shock removal efficiencies were between 10 to 21% greater than the following shock loading, but slowly increased to a stable level.

Case study ORC table 1

The results showed that aerobic degradation did occur, due to differences between the DO and BTEX concentrations in the influent and effluent.

To provide sufficient oxygen, the ORC was replaced on days 38 and 94.  

Discussion and Conclusions
The ORC PRB was successful in providing DO necessary to sustain microbial growth, and successfully reduced BTEX concentrations. The authors further concluded that an ORC can be used for approximately 40 days, but must be replaced after that time period to ensure a sufficient DO. 


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