Optimal and suboptimal control of anaerobic digesters

Anaerobic digester failure due to entry of inhibitors or sudden changes in the feed substrate concentration may be encompassed beneficially by applying optimal control theory. An almost proportional relationship between the dilution rate and the methane production rate leads to a simple suboptimal control law with only minor loss in performance, after the occurrence of the above mentioned events.     

Transient modeling of trickling filters for biological ammonia removal

A dynamic model for nitrifying trickling filters is developed, based on material balances in the biofilm and the bulk liquid. The model predicts the profile of ammonia as a function of the operating parameters (volumetric flow rate and feed ammonia concentration) and the biofilm thickness as a function of filter depth and time. This revised version was published online in July 2006 with corrections to the Cover Date.     

Driving in mild cognitive impairment: The role of depressive symptoms

Objectives: Previous studies indicate a negative association between depression and driving fitness in the general population. Our goal was to cover a gap in the literature and to explore the link between depressive symptoms and driving behavior in individuals with mild cognitive impairment (MCI) through the use of a driving simulator experiment.

Methods: Twenty-four individuals with MCI (mean age = 67.42, SD = 7.13) and 23 cognitively healthy individuals (mean age = 65.13, SD = 7.21) were introduced in the study. A valid driving license and regular car use served as main inclusion criteria. Data collection included a neurological/neuropsychological assessment and a driving simulator evaluation. Depressive symptomatology was assessed with the Patient Health Questionnaire (PHQ-9).

Results: Significant interaction effects indicating a greater negative impact of depressive symptoms in drivers with MCI than in cognitively healthy drivers were observed in the case of various driving indexes, namely, average speed, accident risk, side bar hits, headway distance, headway distance variation, and lateral position variation. The associations between depressive symptoms and driving behavior remained significant after controlling for daytime sleepiness and cognition.

Conclusions: Depressive symptoms could be a factor explaining why certain patients with MCI present altered driving skills. Therefore, interventions for treating the depressive symptoms of individuals with MCI could prove to be beneficial regarding their driving performance.     

Dynamics of a chemostat with three competitive hydrogen oxidizing denitrifying microbial populations and their efficiency for denitrification

In this work, competition for two nitrogen resources (nitrate-, nitrite-nitrogen) between three hydrogen oxidizing denitrifying populations (Acidovorax sp. strain Ic3 (X₁), Paracoccus sp. strain Ic1 (X₂), and Acinetobacter sp. strain Ic2 (X₃)) was examined. The dynamics of three systems of microbial populations (system I: X₁ − X₃, system II: X₂ − X₃, and system III: X₁ − X₂ − X₃), grown in a chemostat, was studied using bifurcation analysis. The chemostat is the most common type of biological reactor used for the study of microbial growth under controlled conditions. The effect of the operating parameters (i.e., dilution rate and feed nitrate nitrogen concentration) on the long-term behavior of the systems showed that X₃ was the predominant population for a wide range of combinations of dilution rate and feed nitrate nitrogen concentration. Also, coexistence of two populations (X₂X₃, X₁X₃) was observed. The results of the bifurcation analysis were also used to determine the denitrification rate and the nitrite nitrogen accumulation for each of the three systems as a function of the dilution rate (up to 0.17 h⁻¹) and the feed nitrate nitrogen concentration (up to 300 mg/L). The highest denitrification rate was achieved by system I (28 mg/Lh). A comparison between the three systems showed that the nitrite nitrogen concentration in system I was less than the one in system III, while the two systems gave similar denitrification rates. The second system had the greatest accumulation of nitrites with the lowest denitrification rate.     

Human mortality in Cyprus: the role of temperature and particulate air pollution

Climatic change results in increased occurrence of heat waves, and the thermal stress caused by such phenomena is leading to higher levels of heat-related mortality worldwide. This study is the first to examine the effect of extreme weather on mortality in Cyprus. It investigates the individual effect of meteorological indicators on mortality, as well as the role of particulate air pollution (PM₁₀). A generalized linear model (GLM) with quasi-Poisson regression was implemented. GLM included a temperature function and was adjusted for relative humidity and seasonality. The temperature function was developed under a newly developed framework of distributed lag nonlinear models, which capture nonlinearities and delayed effects of heat simultaneously. GLM was extended to examine the confounding effect of air pollution. All the results on heat effects are presented. High temperatures had a significant effect on mortality with increased mortality rates, independent of humidity and seasonality. Mortality risk increased steeply above a temperature threshold. A direct heat effect was shown, with higher risk on the current and next day of a severe heat event. PM₁₀ was not found to have a confounding effect on the temperature–mortality relationship, since the strength of this relationship remained after the inclusion of PM₁₀ in the model. Differences existed between urban and coastal areas.     

Release,distribution, and impacts of polychlorinated biphenyls (PCB) induced by dredged material disposal activities at a deep-water estuarine site

The aquatic disposal field investigation initiated in Elliott Bay, Puget Sound, WA, in February 1976, was designed to evaluate the ecological effects of open-water disposal of dredged material. The experimental disposal site was located at a depth of 60 m in marine estuary with generally weak circulation. Approximately 114,000 m³ of dredged material contaminated with polychlorinated biphenyls (PCBs) were dumped at the site from split-hull barges. The material presented in this paper is limited to a discussion of PCB impacts. It imcludes (a) a summary of results from the studies conducted between 1976 and 1977 to assess short-term impacts during and after the disposal of PCB contaminated sediments at the Elliott Bay disposal site; (b) a presentation of preliminary results from the continuation studies initiated in February 1979 to determine long-term impacts of the disposal operations. The preliminary data indicate that both dredged-material deposit and the associated PCBs appear to be stable. No major long-term impact on benthic organisms is apparent from this analysis.