Efficiency of dechlorination

This section summarizes the field studies performed to compare the efficiencies of dechlorination chemicals under identical conditions to evaluate the chemical of choice for various dechlorination applications (18,19). The field tests were conducted at Tacoma Waters, WA, Portland Bureau of Water Works, OR, and East Bay Municipal Utility District (EBMUD), CA. Six dechlorination chemicals were used in solution, tablet, or powder form in these tests (Table 3). In the Tacoma and Portland studies, a 1 % solution of the dechlorination chemicals were introduced into water released from a hydrant. The field studies evaluated the rate of dechlorination, effect of overdosing, and concurrent water quality impacts when stoichiometric or twice the stoichiometric amounts of dechlorination agents were added. In the EBMUD dechlorination studies, bags, or dispensers containing tablets or powders of dechlorination chemicals were placed in the flow path of hydrant water. At all three sites, the water used for the test originated from surface water sources rather than from groundwater sources. Table 3 summarizes the chemicals, forms and dosing rates used in these studies. 

When 12 tablets were placed across the flow of 100 gpm, the chlorine concentration decreased below the detection limit (0.1 mg/L) within 5 min. It remained below the detection limit even after 60 min. In the next test, initially a flow rate of 300 gpm was maintained and 16 tablets were placed across the flow. Within 5 min the chlorine concentration decreased to below detection limit. After 10 min, the flow rate was increased to 450 gpm. At this increased flow rate, the residual chlorine concentration increased to values of 0.6-0.8 mg/L, well above the detection limit of 0.1 mg/L (which is the allowable discharge hmit in many locations), within 25 min (Fig. 3). This indicates that the flow rate of chlorinated waters can significantly impact the efficiency of dechlorination operations. Higher flow rates may not provide sufficient contact time for dissolution of tablets into the stream. After approx 40 min, the number of tablets was increased to 20. This decreased the residual concentration to below detection limit within 5 min. The increase in the number of tablets probably provided an enhanced contact area and better dissolution of the tablets into the flow, resulting in a decrease in the residual chlorine concentrations. 

When used in powder or crystal form, dechlorination chemicals (ascorbic acid and sodium thiosulfate) dissolved rapidly causing water-quality concerns, although physical methods (tablets) have been developed since to slow down dissolution rates. Sodium sulfite, when used in tablet form, was very effective in dose control. One tablet was sufficient to dechlorinate 2 mg/L of chloraminated water to below 0.1 mg/L for 45 min when water was released at 100 gpm. Finally, these field tests also indicated that the flow rates of chlorinated waters can significantly impact the efficiency of dechlorination operations. 

Lowry and Johnson (43) explored the efficiency of dechlorination of dissolved PCBs by Fe° (particle size = < 150,000 nm) and Fe p> (particle size = 30 to 50 nm) particles in water/methanol solutions. With commercial Fe , no PCB dechlorination was observed even after 180 days, suggesting that this form of iron is not reactive. On the other hand, Fe p) resulted in effective dechlorination of PCBs within 45 days. Wang and Zhang (34) also studied the dechlorination of PCBs in an ethanol-water mixture under ambient conditions using Fe p> and Pd coated Fe p). Over a 17 h experiment, partial degradation ( 25%) of PCB to biphenyl was observed with Fef P) while Pd/Fe resulted in complete dechlorination. 

Liu, Y., Majetich, S.A., Tilton, R.D., Sholl, D.S. and Lowry, G.V. (2005) TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environmental Science and Technology, 39, 1338—1345. 

The use of electrochemical methods for the destruction of aromatic organo-chlorine wastes has been reviewed [157]. Rusling, Zhang and associates [166, 167] have examined a stable, conductive, bicontinuous surfactant/soil/water microemulsion as a medium for the catalytic reduction of different pollutants. In soils contaminated with Arochlor 1260, 94% dechlorination was achieved by [Zn(pc)] (H2pc=phthalocyanine) as a mediator with a current efficiency of 50% during a 12-h electrolysis. Conductive microemulsions have also been employed for the destruction of aliphatic halides and DDT in the presence of [Co(bpy)3]2+ (bpy=2,2 -bipyridine) [168] or metal phthalocyanine tetrasulfonates [169]. 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Which metal is best suited for remediation work Both Ca and Na have been examined extensively. Commodore has built recently the SET and SoLV processes technology around Na, based on extensive commercial experience. Pittman et al. [24] demonstrated recently that the order of metal efficiencies for the dechlorination of aliphatic and aromatic model compounds at 25°C was Na > K > Ca > Li in dry ammonia and Na> K,Ca,Li in the presence of a 50-mol excess of water. These laboratory studies were carried out in the absence of soil [24], The presence of water significantly reduced the efficiency of both Ca and Li, whereas Na and K efficiencies were only modestly reduced. 

The environmental problems associated with chlorinated aromatics continue to stimulate Interest in reactions involving photochemical replacement of chlorine by hydrogen. The efficiency of photochemical dechlorination of some... 

The chloramine removal efficiency of catalytic carbon is reported to be an order of magnitude greater than that of conventional activated carbons used for dechlorination. Various factors such as empty bed contact time (EBCT), influent chloramine concentration, particle size, and temperature influence treatment efficiency using catalytic carbon (11,12). In a study using water containing 2 mg/L influent chloramine concentration, an increase in EBCT from 10 to 30 s increased the volume of water treated to below 0.1 mg/L chloramine from 250 bed volumes to 11,000 bed volumes. In a different study, reducing the mesh size from 20 x 50 to 30 x 70 increased the bed volumes treated from 11,000 to 28,000 at a 30 s EBCT and 2 mg/L influent chloramine... 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

That dechlorinations w re accelerated by the ZV-metal particles was demonstrated by replacing the metal particles with silica (acid washed sea sand). For the silica column operated under dechlorinating conditions that had been optimal for ZV metal (400 C, 31.0 MPa), the recovery of organically bound chlorine from the eluate was virtually quantitative. In further trials, acetone-hexane extract of a sandy loam soil (spiked with 600 ppm Aroclor 1254) was fed to the reactor at 0.1 mLymin and dechlorinated efficiently. Chlorinated residues were not detected in the reactor effluent by GC-MS. More importantly, soil co-extractives in the PCB solution did not seem to affect the course or the efficiency of the reaction perceptibly. 

Chlorinated ethenes are subject to a variety of microbial degradation processes that include reductive dechlorination (Vogel et al., 1987 Maymo-Gatell et al., 1997), aerobic oxidation, anaerobic oxidation (Bradley and Chapelle, 1996), and anaerobic cometabolism (McCarty and Semprini, 1994). Both, laboratory studies (Bradley and Chapelle, 1998), and field studies (Chapelle and Bradley, 2000) show that the efficiency of chlorinated ethene biodegradation depends on ambient redox conditions. Therefore, reliable tools to measure the redox conditions are crucial to imderstand and even predict chlorinated ethene degradation. 

Lithium or sodium dissolving in tetrahydrofuran-t-butanol can accomplish efficient reductive dechlorination of chlorine compounds. This synthetic method has been important in obtaining hydrocarbons via halogenated intermediates. Important examples are conversion of dichlorocyclopropanes (prepared by dichlorocarbene... 

There are two problems for dechlorination relating to washing by deionized water. The first one is the ruthenium ions in the pore will be dissolved again, which leads to the loss of noble metal ruthenium as active components. The other one is that washing time is too long and the efficiency of the dechlorination is not ideal. 

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