Effect of agitation, turbidity, aluminium foil reflectors and container volume on the inactivation efficiency of batch-process solar disinfectors.

We report the results of experiments designed to improve the efficacy of the solar disinfection of drinking water, inactivation process. The effects of periodic agitation, covering the rear surface of the container with aluminium foil, container volume and turbidity on the solar inactivation kinetics of Escherichia coli (starting population = 10(6) CFU ml(-1)) were investigated. It was shown that agitation promoted the release of dissolved oxygen from water with subsequent decrease in the inactivation rates of E. coli. In contrast, covering the rear surface of the solar disinfection container with aluminium foil improved the inactivation efficiency of the system. The mean decay constant for bacterial populations in foil-backed bottles was found to be a factor of 1.85 (std. dev. = 0.43) higher than that of non-foil-backed bottles. Inactivation rates decrease as turbidity increases. However, total inactivation was achievable in 300 NTU samples within 8 h exposure to strong sunshine. Inactivation kinetics was not dependent on the volume of the water container for volumes in the range 500-1500 ml.

Solar disinfection of drinking water contained in transparent plastic bottles: characterizing the bacterial inactivation process.

A series of experiments is reported to identify and characterize the inactivation process in operation when drinking water, heavily contaminated with a Kenyan isolate of Escherichia coli, is stored in transparent plastic bottles that are then exposed to sunlight. The roles of optical and thermal inactivation mechanisms are studied in detail by simulating conditions of optical irradiance, water turbidity and temperature, which were recorded during a series of solar disinfection measurements carried out in the Kenyan Rift Valley. Optical inactivation effects are observed even in highly turbid water (200 ntu) and at low irradiances of only 10 mW cm-2. Thermal inactivation is found to be important only at water temperatures above 45 degrees C, at which point strong synergy between optical and thermal inactivation processes is observed. The results confirm that, where strong sunshine is available, solar disinfection of drinking water is an effective, low cost method for improving water quality and may be of particular use to refugee camps in disaster areas. Strategies for improving bacterial inactivation are discussed.

Inactivation of fecal bacteria in drinking water by solar heating.

We report simulations of the thermal effect of strong equatorial sunshine on water samples contaminated with high populations of fecal coliforms. Water samples, heavily contaminated with a wild-type strain of Escherichia coli (starting population = 20 x 10(5) CFU/ml), are heated to those temperatures recorded for 2-liter samples stored in transparent plastic bottles and exposed to full Kenyan sunshine (maximum water temperature, 55 degrees C). The samples are completely disinfected within 7 h, and no viable E. coli organisms are detected at either the end of the experiment or a further 12 h later, showing that no bacterial recovery has occurred. The feasibility of employing solar disinfection for highly turbid, fecally contaminated water is discussed.

Net positives: conservative approach to measurement of proportions positive in substerilization process studies.

A method (designated the net positive approach) is described for scoring results of sterility tests done with more than one recovery condition on product items treated with inactivating agents at levels less than those used in sterilization processes proper (subprocess treatments). Three tests of sterility, one per recovery condition, constitute a single sample. A growth in one or more recovery conditions is scored as a net positive for that sample. The net positive approach reduces the occurrence of false-negatives and assigns a value to the proportion of items reported as nonsterile (proportions positive) that is conservative.

Precurrent self-prompting operants in children: "Remembering".

IN EXPERIMENT I, ONE OF THREE FORMS OF COLLATERAL BEHAVIOR WAS TRAINED: Differential collateral behavior specific in form to one of two discriminative stimuli; Common collateral behavior of a single form regardless of the stimulus; or Nondifferential collateral behavior of either form regardless of the stimulus. Children were next given a short-delay matching-to-sample task in which the discriminative stimuli served as samples, and the children's previously trained collateral behavior terminated the delay and presented the comparison stimuli. Subjects engaging in sample-specific collateral behavior immediately acquired matching. Subjects engaging in sample-nonspecific collateral behavior failed to acquire matching or did so gradually. In Experiment II the minimal delay in the matching task was varied in a mixed sequence, first with collateral behavior required, and then with collateral behavior prohibited. When emitting collateral behavior Common and Nondifferential subjects showed delay-related decrements in matching while Differential subjects did not. When not emitting collateral behavior all subjects showed delay-related decrements in matching. Common and Nondifferential subjects matched more accurately when prohibited from emitting collateral behavior. Differential subjects matched more accurately when emitting collateral behavior. The results accord with Skinner's (1953, 1968) analysis of precurrent operants.