Legionella occurrence in municipal and industrial wastewater treatment plants and risks of reclaimed wastewater reuse: Review

Wastewater treatment plants (WWTPs) have been identified as confirmed but until today underestimated sources of Legionella, playing an important role in local and community cases and outbreaks of Legionnaires' disease. In general, aerobic biological systems provide an optimum environment for the growth of Legionella due to high organic nitrogen and oxygen concentrations, ideal temperatures and the presence of protozoa. However, few studies have investigated the occurrence of Legionella in WWTPs, and many questions in regards to the interacting factors that promote the proliferation and persistence of Legionella in these treatment systems are still unanswered. This critical review summarizes the current knowledge about Legionella in municipal and industrial WWTPs, the conditions that might support their growth, as well as control strategies that have been applied. Furthermore, an overview of current quantification methods, guidelines and health risks associated with Legionella in reclaimed wastewater is also discussed in depth. A better understanding of the conditions promoting the occurrence of Legionella in WWTPs will contribute to the development of improved wastewater treatment technologies and/or innovative mitigation approaches to minimize future Legionella outbreaks.

Occurrence of Legionella in wastewater treatment plants linked to wastewater characteristics

In recent years, the occurrence of Legionella in wastewater treatment plants (WWTP) has often been reported. However, until now there is limited knowledge about the factors that promote Legionella's growth in such systems. The aim of this study was to investigate the chemical wastewater parameters that might be correlated to the concentration of Legionella spp. in WWTP receiving industrial effluents. For this purpose, samples were collected at different processes in three WWTP. In 100 % of the samples taken from the activated sludge tanks Legionella spp. were detected at varying concentrations (4.8 to 5.6 log GU/mL) by the quantitative real-time polymerase chain reaction method, but not by the culture method. Statistical analysis with various parameters yielded positive correlations of Legionella spp. concentration with particulate chemical oxygen demand, Kjeldahl nitrogen and protein concentration. Amino acids were quantified in wastewater and activated sludge samples at concentrations that may not support the growth of Legionella, suggesting that in activated sludge tanks this bacterium multiplied in protozoan hosts.

[Contribution of time of flight and point spread function modeling to the performance characteristics of the PET/CT Biograph mCT scanner]

OBJECTIVE

To characterize the performance of the Biograph mCT PET/CT TrueV scanner with time of flight (TOF) and point spread function (PSF) modeling.

MATERIAL AND METHODS

The PET/CT scanner combines a 64-slice CT and PET scanner that incorporates in the reconstruction the TOF and PSF information. PET operating characteristics were evaluated according to the standard NEMA NU 2-2007, expanding some tests. In addition, different reconstruction algorithms were included, and the intrinsic radiation and tomographic uniformity were also evaluated.

RESULTS

The spatial resolution (FWHM) at 1 and 10cm was 4.4 and 5.3mm, improving to 2.6 and 2.5mm when PSF is introduced. Sensitivity was 10.9 and 10.2 Kcps/MBq at 0 and 10cm from the axis. Scatter fraction was less than 34% at low concentrations and the noise equivalent count rate (NECR) was maximal at 27.8 kBq/mL with 182 Kcps, the intrinsic radiation produced a rate of 4.42 true coincidences per second. Coefficient of variation of the volume and system uniformity were 4.7 and 0.8% respectively. The image quality test showed better results when PSF and TOF were included together. PSF improved the hot spheres contrast and background variability, while TOF improved the cold spheres contrast.

CONCLUSIONS

The Biograph mCT TrueV scanner has good performance characteristics. The image quality improves when the information from the PSF and the TOF is incorporated in the reconstruction.