Anaerobic digestion
is an important way for maintaining sewage
sludge stability, reduction, and resource recovery. However, the low
organic content generally limits methane production. Recently, thermal
hydrolysis has been widely used for sludge pretreatment to improve
the anaerobic digestion efficiency. Generally, an increased temperature
is preferred to enhance the solubility of organic matters in the sludge.
However, high energy requirement comes with increased temperature.
Application of low-temperature thermal treatment could overcome this
drawback. However, the appropriate low-temperature pretreatment time
is still uncertain. In this study, an extended contact time with low
thermal pretreatment (90 °C) was chosen to realize a more efficient
and economical digestion process of low-organic content sludge. The
results demonstrated that the solubilization of proteins and carbohydrates
was significantly promoted by the contact time-extended thermal hydrolysis
pretreatment. The following anaerobic digestion efficiency of low-organic
content sludge was also dramatically improved with the prolonged contact
time. The maximum methane production could reach around 294.73 mL/gVS
after 36 h of 90 °C treatment, which was 5.56 times that of the
untreated groups. Additionally, based on the energy balance calculation,
extending the thermal hydrolysis time resulted in a more economically
feasible anaerobic digestion than increasing the temperature. The
dewatering properties and the stability of the heavy metals were also
reinforced, implying the advanced deep utilization of the digested
low-organic content sludge. In conclusion, sludge pretreated by low-temperature
thermal hydrolysis with a prolonged contact time could be more effective
for low-organic content sludge treatment and disposal.
Salinity is one of the most important factors for tidal-affected water bodies including estuaries and tidal river reaches. However, due to the limitations of simultaneous manual monitoring in long-distance areas, studies of estuaries are preferred to tidal reaches. Hence, in this study, we investigated the tidal reach of the Minjiang River and five sets of field observations were used to determine the influence of saltwater intrusion in different seasons. During the dry and wet season with low discharge, the longitudinal salinity profiles showed that a station located about 45 km from the river mouth still suffered from saltwater intrusion especially when the upstream discharge was under 754 m s, where the periodical fluctuation in the salinity remained the same with the water level, but there was a time lag of approximately 4 h compared with the discharge process. However, during the wet season in April and May 2016, the monitored salinity was approximately at the detectable limit of 0.02-0.04 ppt at the station close to the river mouth, which indicated that there was no saltwater exchange into the river, although dual flow directions were observed in the survey periods. The major differences among five survey periods were mainly related to upstream discharge rather than the seasons, the tidal range, and tidal excursion. The conclusions of this study have strategic importance for protecting water sources by guiding the government to assess the optimal freshwater release time and discharge rate to prevent saltwater entering the important tidal-affected river reaches.
The distribution variation in chromophoric dissolved organic matter (CDOM) content in mid-latitude subtropical drinking water source reservoirs (MDWSRs) has great significance in the security of aquatic environments and human health. CDOM distribution is heavily influenced by biogeochemical processes and anthropogenic activity. However, little is known regarding the impact of component variation and phytoplankton growth on CDOM distribution variation in MDWSR. Therefore, samples were collected from a representative MDWSR (the Shanzai Reservoir) for analysis. CDOM absorption and fluorescence coupling with parallel factor analysis were measured and calculated. The results indicated that only two CDOM components were found in the surface water of Shanzai Reservoir, fulvic acid, and high-excitation tryptophan, originating from terrestrial and autochthonous sources, respectively. The types of components did not change with the season. The average molecular weight of CDOM increased in proportion to its fulvic acid content. The distribution variation in CDOM content mainly resulted from the variation in two CDOM components in summer and from high-excitation tryptophan in winter. Phytoplankton growth strongly influenced the distribution variation of CDOM content in summer; the metabolic processes of Cyanobacteria and Bacillariophyta consumed fulvic acid, while that of Cryptophyta produced high-excitation tryptophan.
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