Benito J. Mariñas

Science and technology for water purification in the coming decades

One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.

Inactivation of Cryptosporidium parvum oocysts with ozone

Ozone inactivation rates for Cryptosporidium parvum (C. parvum) oocysts were determined with an in vitro excystation method based on excysted sporozoite counts. Results were consistent with published animal infectivity data for the same C. parvum strain. The inactivation kinetics for C. parvum were characterized by a lag phase followed by pseudo-first order kinetics. Both the magnitudes of the lag phase and the pseudo-first order rate constant were found to be different for C. parvum oocysts from two different sources, and to obey Arrhenius law for the experimental temperature range of 5–30°C. CT requirements for C. parvum inactivation were found to increase by an average factor of approximately three for every 10°C decrease in temperature.

Kinetics of Escherichia coli inactivation with ozone

Abstract The kinetics of Escherichia coli inactivation with ozone were investigated with semi-batch and continuous-flow tubular reactors in solutions buffered with phosphate at pH 6 and 8, temperatures ranging from 5 to 25°C, and in the presence (0.01 m ) and absence of radical scavenger tert-butanol. Ozone concentrations and contact times investigated ranged from 6 to 41 μg/l, and 1.8–33 s, respectively. Inactivation kinetics were found to be first-order with respect to both ozone concentration and active microorganism density. The overall second-order rate constant was 1301/(mg s) at 20°C, and the corresponding activation energy was 37,100 J/mol. Experimental results corresponding to tests performed at pH 6 and 8, and in the presence (0.01 m ) and absence of radical scavenger tert-butanol, confirmed that dissolved molecular ozone was primarily responsible for E. coli inactivation in the range of experimental conditions investigated.

Inactivation of Escherichia coli with ozone: chemical and inactivation kinetics

Abstract The apparent chemical and inactivation reactions taking place during the disinfection of Escherichia coli with ozone in the presence of humic acid were investigated with continuous-flow tubular reactors. The apparent decomposition of dissolved ozone in the presence of humic acid and E. coli cells was modeled successfully with mixed second-order rate expressions within a time scale relevant to E. coli inactivation with ozone. The rate for the ozone inactivation of E. coli in the presence of humic acid was slower than that in the absence of natural organic matter due to the faster decomposition of dissolved ozone and thus the lower exposure of E. coli cells to the disinfectant in the former case. However, the second-order inactivation rate constant was approximately the same in the presence and absence of humic acid confirming that molecular ozone rather than radicals was the species generally responsible for inactivation. The overall rate of reaction between ozone and organic matter associated with E. coli cells was slower that the rate of E. coli inactivation by ozone. Only 25% of the initial relatively fast ozone demand were satisfied by the time that practically all micro-organisms present in solution were inactivated. TEM micrographs revealed that noticeable changes in the interior of E. coli cells did not take place until most of the cells present in the sample were non-viable.

Sequential inactivation of Cryptosporidium parvum oocysts with ozone and free chlorine.

The objective of this study is to investigate the synergy involved in the sequential inactivation of C. parvum oocysts with ozone followed by free chlorine at 1-20 degrees C. Primary ozone and free chlorine inactivation curves are characterized by an initial lag-phase, followed by one or two post-lag-phase segments, the first segment at a faster rate than the second, of pseudo-first-order inactivation. The kinetics of primary inactivation with ozone and free chlorine has a relatively strong temperature dependence, and vary both with oocyst lot and oocyst age. Synergy is observed for the sequential inactivation of C. parvum oocysts with ozone/free chlorine. Ozone pre-treatment results in the disappearance of the lag-phase and the occurrence of a secondary free chlorine inactivation curve with generally two pseudo-first-order segments, the first segment at a faster rate than the second. The kinetics of both secondary segments is significantly faster than the post-lag-phase rate of inactivation with free chlorine alone. The temperature dependence for both phases of the secondary free chlorine inactivation kinetics is weaker compared to that for primary inactivation with ozone or free chlorine. As a result, the level of synergy in sequential disinfection with ozone/free chlorine increases with decreasing temperature within the range relevant to drinking water utilities. Good agreement is found between the kinetics determined using the modified in-vitro excystation method of viability assessment and animal infectivity data recently reported in the literature for both primary inactivation with ozone, and sequential disinfection with ozone/free chlorine.

Inactivation of Cryptosporidium parvum oocysts with ozone and monochloramine at low temperature

The rate of Cryptosporidium parvum inactivation decreased with decreasing temperature (1-20 degrees C) for ozone and for monochloramine applied alone as well as after pre-treatment with ozone. Synergy was observed at all temperatures studied for the ozone/monochloramine sequential disinfection scheme. The synergistic effect was found to increase with decreasing temperature. The inactivation rate with monochloramine after ozone pre-treatment was 5 times faster at 20 degrees C and 22 times faster at 1 degree C than the corresponding post-lag phase rates of inactivation with monochloramine at these temperatures when no ozone pre-treatment was applied. The CT required for achieving 2-logs of inactivation ranged from 11,400 mg min l-1 at 20 degrees C to 64,600 mg min l-1 at 1 degree C when monochloramine was applied alone. In contrast, the CT required for an overall sequential inactivation of 2-logs ranged from 721 mg min l-1 at 20 degrees C to 1350 mg min l-1 at 1 degree C when applying monochloramine after ozone pre-treatment. The presence of excess ammonia in the monochloramine solutions was not responsible for the synergy observed in ozone/monochloramine sequential disinfection.

Depth Heterogeneity of Fully Aromatic Polyamide Active Layers in Reverse Osmosis and Nanofiltration Membranes

We studied the depth heterogeneity of fully aromatic polyamide (PA) active layers in commercial reverse osmosis (RO) and nanofiltration (NF) membranes by quantifying near-surface (i.e., top 6 nm) and volume-averaged properties of the active layers using X-ray photoelectron spectrometry (XPS) and Rutherford backscattering spectrometry (RBS), respectively. Some membranes (e.g., ESPA3 RO) had active layers that were depth homogeneous with respect to the concentration and pK(a) distribution of carboxylic groups, degree of polymer cross-linking, concentration of barium ion probe that associated with ionized carboxylic groups, and steric effects experienced by barium ion. Other membranes (e.g., NF90 NF) had active layers that were depth heterogeneous with respect to the same properties. Our results therefore support the existence of both depth-homogeneous and depth-heterogeneous active layers. It remains to be assessed whether the depth heterogeneity consists of gradually changing properties throughout the active layer depth or of distinct sublayers with different properties.

Inactivation of Bacillus subtilis spores with ozone and monochloramine.

The inactivation kinetics of Bacillus subtilis spores with ozone and monochloramine was characterized by a lag phase followed by a pseudo-first-order rate of inactivation. The lag phase decreased and the post-lag phase rate constant increased with increasing temperature within the range investigated (1-30 degrees C for ozone, 1-20 degrees C for monochloramine). The corresponding activation energies were 46820 J/mol for ozone and 79640 J/mol for monochloramine. The CT concept was found to be valid within the concentration range investigated of 0.44-4.8 mg/l for ozone, and 3.8-7.7 mg/l as Cl(2) for monochloramine. The inactivation kinetics of B. subtilis spores with both ozone and monochloramine varied with pH within the range of pH 6-10 investigated. The fastest ozone and monochloramine inactivation rates were observed at pH 10 and 6, respectively. Different stocks of the same strain of B. subtilis spores had different resistance to ozone and monochloramine mainly because of discrepancies in the extent of the lag phase. B. subtilis spores might not be conservative surrogates for C. parvum oocysts for ozone disinfection at relatively low temperature mainly due to the spores having a lower activation energy compared to that for the oocysts. In contrast, the activation energy for monochloramine was comparable for both microorganisms but differences in the extent of the lag phase might result in the spores being overly conservative surrogates for the oocysts at relatively low temperature.

Synergy in sequential inactivation of Cryptosporidium parvum with ozone/free chlorine and ozone/monochloramine

Abstract The main objective of this study was to investigate the inactivation kinetics of Cryptosporidium parvum oocysts with sequential disinfection schemes involving ozone as a primary disinfectant, and free chlorine or monochloramine as a secondary disinfectant. Two types of synergistic effects were observed. Ozone pre-treatment resulted in the removal of the relatively more pronounced initial lag phases observed for monochloramine and hypochlorous acid. An additional and more important synergistic effect was an enhancement in the rate of secondary inactivation with both hypochlorous acid and monochloramine after complete removal of the lag phase by ozone pre-treatment. A stronger synergy was observed at a lower temperature. The secondary inactivation rate was 1.1–2.8 (hypochlorous acid) and 2.4–9.2 (monochloramine) times faster than the corresponding post lag-phase primary inactivation rate at respective temperatures of 30–10°C. Consistency between the two viability assessment methods, modified in-vitro excystation and animal infectivity, was demonstrated or shown for both primary inactivation with ozone and secondary inactivation with ozone/monochloramine.

Inactivation of bacillus subtilis spores and formation of bromate during ozonation

Inactivation of B. subtilis spores with ozone was investigated to assess the effect of pH and temperature, to compare the kinetics to those for the inactivation of C. parvum oocysts, to investigate bromate formation under 2-log inactivation conditions, and to assess the need for bromate control strategies. The rate of B. subtilis inactivation with ozone was independent of pH, decreased with temperature (activation energy of 42,100 Jmol(-1)), and was consistent with the CT concept. B. subtilis was found to be a good indicator for C. parvum at 20-30 degrees C, but at lower temperatures B. subtilis was inactivated more readily than C. parvum. Bromate formation increased as both pH and temperature increased. For water with an initial bromide concentration of 33 microgl(-1), achieving 2-logs of inactivation, without exceeding the 100 microg l(-1) bromate standard, was most difficult at 30 degrees C for B. subtilis and at midrange temperatures (10-20 degrees C) for C. partum. pH depression and ammonia addition were found to reduce bromate formation without affecting B. subtilis inactivation, and may be necessary for waters containing more than 50 microgl(-1) bromide.