A model to control intermittent aeration phases.

Dealing with intermittent aeration as a useful practice to improve nitrogen removal efficiency of activated sludge plants, the possibility to plan optimal temporisation during daytime was investigated. A mathematical model (NIDEN) that allows us to manage different situations, with respect to influent load, environmental conditions and operating strategy, was then developed. The model represents a useful tool especially to plan the aeration cycles in small and medium sized plants, where high costs of automatic control through on-line instrumentation might not be justified. Once the input variables have been defined and the set-point values for tank nutrient concentration have been fixed, NIDEN gives an optimal phase temporisation, to obtain either the maximum energy saving or the best total nitrogen removal.

[The audiometric evaluation of Buenos Aires schoolchildren]. / Evaluación audiométrica de escolares bonaerenses.

This cross-sectional study, which was carried out between 3 May and 30 September 1993, involved examination with a pediatric otoscope of a random sample of 100 first-grade schoolchildren in a Buenos Aires hospital, followed by tonal audiometry and tonal sweep in 90 of those children. The objectives of the study were to discover the frequency of hearing disorders and to evaluate the importance of audiometric screening in the child population. The results were classified according to the type (conduction or perception) and degree (mild, moderate, or severe) of hypoacusis, which was detected in 36 children (39%). Of those affected, 35 had mild or moderate conduction hypoacusis (loss of less than 31 decibels [dB] or less than 41 dB, respectively). No case of severe hypoacusis was found. The authors consider that audiometric screening of children entering primary school is justified by the high frequency of preventable hearing problems and by the repercussions that those problems can have on speech and learning.