Lee W. Clapp

Activity, structure, and stratification of membrane-attached methanotrophic biofilms cometabolically degrading trichloroethylene

A membrane-attached methanotrophic biofilm reactor was developed for the cometabolic degradation of trichloroethylene (TCE). In this reactor, CH4 and O2 are supplied to the interior of the biofilm through the membrane, while TCE-contaminated water is supplied to the exterior, creating a “counter-diffusional” effect that minimizes competitive inhibition between TCE and CH4. In addition, this novel design provides 100% CH4 and O2 transfer efficiencies, promotes the development of a thick biofilm, and minimizes the negative effects of TCE byproduct toxicity. The reactor sustained 80-90% TCE removals at TCE loading rates ranging from 100-320 μmol/m2/d. Chloride mass balances demonstrated that 60-80% of the TCE removed was mineralized. The maximum TCE transformation yield was 1.8 mmol of TCE removed per mole of CH4 utilized, although higher transformation yields are expected at higher TCE loading rates. The CH4 utilization rate was 0.20 mol/m2/d. Scanning electron microscopy (SEM) revealed a dense biofilm with a thickness of at least 400 μm. SEM and transmission electron microscopy (TEM) analyses indicated that the “holdfast” material associated with rosette formation in planktonic Methylosinus trichosporium OB3b ( M.t. OB3b) cells might also contribute to pure-culture biofilm development. In addition, fimbriae-like structures not commonly associated with methanotrophic bacteria were observed in pure-culture M.t. OB3b biofilms. Finally, fluorescent in situ hybridization (FISH) analyses showed the presence of discrete microcolonies of serine-pathway methanotrophs within mixed-culture biofilms.

Passive dissolution of hydrogen gas into groundwater using hollow-fiber membranes

A new hollow-fiber membrane remediation system has recently been developed to passively supply groundwater with dissolved hydrogen (H2) to stimulate the biodegradation of chlorinated solvents. Understanding the mass transfer behavior of membranes under conditions of creeping flow is critical for the design of such systems. Therefore, the objectives of this research were to evaluate the gas transfer behavior of hollow-fiber membranes under conditions typical of groundwater flow and to assess the effect of membrane configuration on gas transfer performance. Membrane gas transfer was evaluated using laboratory-scale glass columns operated at low flow velocities (8.6-12,973 cm/d). H2 was supplied to the inside of the membrane fibers while water flowed on the outside and normal to the fibers (i.e. cross-flow). Membrane configuration (single fiber and fabric) and membrane spacing for the fabric modules did not affect gas transfer performance. Therefore, the results from all of the experiments were combined to obtain the following dimensionless Sherwood number (Sh) correlation expressed as a function of Reynolds number (Re) and Schmidt number (Sc): Sh = 0.824Re(0.39)Sc(0.33) (0.0004<Re<0.6). This correlation is useful for predicting the rate of transfer of any gas from clean membranes to flowing water at low Re. This correlation provides a basis for estimating the membrane surface area requirements for groundwater remediation as illustrated by a simple example.

Evaluation of polyethylene hollow-fiber membranes for hydrogen delivery to support reductive dechlorination in a soil column

Engineered systems are often needed to supply an electron donor, such as hydrogen (H(2)), to the subsurface to stimulate the biological dehalogenation of perchloroethene (PCE) to ethene. A column study was performed to evaluate the ability of gas permeable hollow-fiber membranes to supply H(2) directly to PCE-contaminated groundwater to facilitate bioremediation. Two glass columns were packed with soil obtained from a trichloroethene-contaminated site at Cape Canaveral, Florida, and were fed a minimal medium spiked with PCE (7 microM) for 391 days. The columns were operated in parallel, with one column receiving H(2) via polyethylene hollow-fiber membranes (lumen H(2) pressure of approximately 1atm) and a control column receiving no H(2). PCE was initially dechlorinated at a similar rate and to a similar extent in both columns, likely due to the presence of soil organic matter that was able to support dechlorination. After 265 days of operation, dechlorination performance declined in the control column and the benefits of membrane-supplied H(2) became evident. Although the membrane-supplied H(2) effectively stimulated PCE dechlorination at the end of the experiment (days 359-391), the system was inefficient in that only 5% of the supplied H(2) was used for dechlorination. Most of the remainder was used to support methanogenesis (94%). Despite the dominance of methanogens, nearly complete dechlorination of PCE to ethene was observed in the H(2)-fed column. In addition to the inefficient use of H(2), operational problems included excessive foulant accumulation on the outside of the membrane fibers and water condensation inside the fibers. Use of alternative membrane materials and changes to the operating approach (e.g. pulsing or supplying H(2) at low partial pressures) may help to overcome these problems so that this technology can provide effective and stable remediation of aquifers contaminated with chlorinated ethenes.

Effect of nitrate and sulfate on dechlorination by a mixed hydrogen-fed culture

A novel hollow-fiber membrane remediation technology developed in our laboratory for hydrogen delivery to the subsurface was shown to support the dechlorination of perchloroethene (PCE) to cis-dichloroethene. In previous research, the presence of nitrate or sulfate has been observed to inhibit biological reductive dechlorination. In this study hollow-fiber membranes were used to supply hydrogen to a mixed culture to investigate whether adequate hydrogen could be added to support dechlorination in the presence of alternative electron acceptors. By continuously supplying hydrogen through the membrane, the hydrogen concentrations within the reactor were maintained well above the hydrogen thresholds reported to sustain reductive dechlorination. It was hypothesized that by preventing nitrate and sulfate reducers from decreasing hydrogen concentrations to below the dehalorespirer threshold, the inhibition of PCE dechlorination by nitrate and sulfate might be avoided and dechlorination could be stimulated more effectively. Enough membrane-fed hydrogen was supplied to completely degrade the alternative electron acceptors present and initiate dechlorination. Nevertheless, nitrate and sulfate inhibited dechlorinating activity even when hydrogen was not limiting. This suggests that competition for hydrogen was not responsible for the observed inhibition. Subsequent microcosm experiments demonstrated that the denitrification intermediate nitrous oxide was inhibitory at 13 µM.

Membrane Gas Transfer for Groundwater Remediation: Chemical and Biological Fouling

Microporous gas-permeable membranes are being investigated as a means to provide in situ delivery of hydrogen to contaminated groundwater to stimulate the biodegradation of chlorinated solvents. Be...

Dechlorination of PCE by Mixed Methanogenic Cultures Using Hollow-Fiber Membranes

The study investigated the use of hollow-fiber membranes for hydrogen (H2) delivery to support the biological reductive dechlorination of tetrachloroethene (PCE) Two experiments were performed in which H2 was supplied through membranes placed in stirred batch reactors containing a mixed methanogenic/dechlorinating culture and PCE (≤10 µM. Reductive dechlorination of PCE to cis-dichloroethene was sustained in the reactors receiving H2 (1% H2 and 50% H2), while negligible dechlorination was observed in control reactors (100% N2). The 1%-H2-fed reactor outperformed the 50%-H2-fed reactor in the first experiment. However, the dechlorinating performance in the two reactors was similar in the second experiment. Despite relatively high H2 concentrations (4.6 to 178 µM) that led to H2 consumption (and CH4 production) by methanogens, dechlorination was effectively maintained for the duration of the experiments (35 to 62 days). The results of this study are significant in that dechlorination was sustained in a minimal medium by membrane-delivered H2. Dechlorination was also maintained at aqueous H2 concentrations that exceeded the thermodynamic thresholds for not only dechlorination (<0.1 to 2 nM, but also methanogenesis (∼10 nM) and homoacetogenesis (94 to 400 nM. The results of these experiments suggest that membranes are a promising H2 delivery technology for stimulating the bioremediation of chlorinated ethene-contaminated aquifers.

Feasibility of Using Brackish Groundwater Desalination Concentrate as Hydraulic Fracturing Fluid in the Eagle Ford Shale

Recent estimates predict over 20,000 wells will be drilled for hydraulic fracturing in the Eagle Ford Shale over the next 15 years, accounting for approximately 5-7% of the total water use within the main 16-county area, and as high as 89% in one rural county. Since each well requires about 10000-25000 m 3 of water, there is significant concern about fresh water consumption in drought-stricken South Texas. Hence, development of the Eagle Ford Shale will require water management strategies that maximize use of non-potable water. The main objective of this study is to evaluate the feasibility of using reject concentrate streams from groundwater RO desalination plants located within the Eagle Ford Shale region as hydraulic fracturing fluid. This could have two synergistic advantages: (1) elimination of brackish desalination concentrate discharges to surface waters, and (2) provision of a source of water for the oil and gas industry that does not consume freshwater supplies. This study will perform comprehensive chemical characterization of both an RO reject concentrate stream and hydraulic fracturing flowback water, and also will perform a geochemical modeling analysis to assess the down-hole scaling potential associated with the RO concentrate if used as hydraulic fracturing fluid in the Eagle Ford.

Characterization of Brackish Groundwater Desalination Concentrate Discharge Impacts on Water Quality in a Texas Coastal Area

Management of concentrate streams is one of the major environmental concerns for desalination operations. This study characterized the impacts of concentrate discharge on receiving waters downstream of the Southmost Regional Water Authority (SRWA) brackish groundwater desalination plant, which is located close to the Gulf Coast near Brownsville, Texas. Water quality was characterized with respect to dissolved oxygen (DO), temperature, total dissolved solids (TDS), pH, total and ortho-phosphorus (P), and arsenic (As). The study also generated baseline water quality data as an initial step for evaluating the potential impacts that could occur if the concentrate stream were used as supplement water for the re-flooding and restoration of the Bahia Grande wetland area, which is located within the Laguna Atascosa National Wildlife Refuge. The study showed that the concentrate stream did not appear to have a significant adverse impact on downstream receiving waters and contributed to dilution of salinity in San Martin Lake. However, given projections that brackish groundwater desalination will meet 20 % of the Lower Rio Grande Valley’s municipal water supply within the next 50 years, a critical assessment of the potential cumulative impacts of region-wide brackish groundwater desalination on surface water quality in the region is needed.