The Panggang and the Wonosari-Baron Hydrogeological Subsystems are part of the Gunungsewu karst landscape in Gunungkidul Regency, Yogyakarta, Indonesia. This landscape has many dolines as one of the epikarst of the phenomenon. These dolines usually classified as solution dolines. This study aims to determine the presence or absence of groundwater flow in doline based on hydrochemical characteristics. Hydrochemical data were collected during the dry and rainy seasons, supplemented by water doline sampling on the ten selected dolines. Data were taken from the field and laboratory, both in physical (color, taste, odor, and turbidity) and chemical (pH, EC, and ionic content) properties of water. The hydrochemical analysis was performed using Stiff and Gibbs diagrams. The doline’s water has Ca and Na-bicarbonate facies during the dry season but tends to change to Ca-bicarbonate facies in the rainy season. The water-rock interaction process and precipitation found strongly influence the hydrochemistry of dolines water. The hydrogeological system involves conduit-type groundwater flow. The groundwater is interpreted to contribute to the water doline based on its hydrochemistry, both locally and regionally.

Copper (Cu) toxicity is an increasing concern in marine environments; however, its effects are species- and area-specific. In this study, area-specific Cu toxicity test was conducted on the sensitive early life stages (i.e., embryonic and larval development) of two important fisheries species, namely, Pacific oyster (Crassostrea gigas) and sea squirt (Halocynthia roretzi) using natural sea waters collected from three sites in the coastal area of Miyagi Prefecture, Japan. Measured concentrations for C. gigas and nominal concentrations for H. roretzi were used to calculate effect concentrations for each species. The 10% effect concentration (EC10) for Cu were 12.8–17.0 and 15.0–22.0 µg L⁻¹, and 50% effect concentration (EC50) for Cu were 20.3–22.6 and 45.6–47.2 µg L⁻¹, respectively. Notably, this study is the first to determine the toxicity of Cu to H. roretzi, and our results can provide additional information to strengthen the Cu species sensitivity distribution of saltwater species, which can support bioavailability correction. Moreover, the results of this study can help policymakers to develop more realistic Cu water quality standard while considering the effects of Cu toxicity on important species and the water chemistry in specific regions.

In an electrolytic water treatment, electrode reactions for pollutant removal can be easily controlled by changing an electrode potential. Thereby, it has a potential to realize an efficient water treatment process. However, its coulombic efficiency, η, strongly depends on reactant concentrations near electrodes. In a continuous electrolysis, the reactant concentrations near an electrode is lower than that in the bulk due to electrolytic consumption of reactant, which gives a negative impact on the η. Therefore, an introduction of intermittent electrolysis instead of the conventional continuous one was discussed in this research, where the reactant concentration near the electrode is recovered during the electrolysis-off stage. The intermittent electrolysis of 1 mM formic acid solution revealed that the η of formic acid removal under electrolysis-on/off time cycle of 1/10 s was 3.3 times higher than that under the continuous electrolysis. A popular technique to keep the reactant concentration near the electrode is an increase in the linear velocity of solution on the electrode. However, the η at a linear velocity of 20 cm/s was only ≤ 25% higher than that at 10 cm/s. Thus, the intermittent electrolysis was very effective in enhancing the energy-efficiency of electrolytic water treatment.

Microplastics adversely affect organisms through physical damage, inhibition of food assimilation, and/or toxicity of chemical leachates. We investigated the influence of feeding mode on microplastic ingestion by using polystyrene microbeads (diameter: 4.1 and 20.6 μm) and the estuarine benthic amphipod Grandidierella japonica, which can switch between filter-feeding and deposit-feeding modes. When provided with sediment, amphipods burrowed and were in the filter-feeding mode; they ingested 4.1 and 20.6 μm beads in the ratio at which the two sizes were suspended in the water. Without sediment, however, the amphipods were mainly in the deposit-feeding mode and ingested more 20.6 μm beads, which tended to be deposited on the bottom, compared with 4.1 μm beads. In addition, the number of microbeads ingested by the amphipods in sediment increased as the amount of food provided (i.e., fish food TetraMin) increased, whereas no such increase was observed for the amphipods without sediment. These results indicate that the microbead ingestion was dependent on feeding mode (i.e., presence/absence of sediment), amount of food, and distribution of microbeads (i.e., sizes of microbeads). To better understand the ingestion, accumulation, and toxicity of microplastics in aquatic environments, we recommend that more attention be paid to behavioral changes in benthic organisms.

With the recent development of ultraviolet light emitting diodes (UV-LEDs), it is now possible to apply UV disinfection, previously used only for medium-to-large sized sewage treatment plants, to Johkasou wastewater, a household wastewater treatment system. However, it is known that coliforms that have been inactivated by UV irradiation regain their activity after of sunlight irradiation, etc. Therefore, in small Johkasou, where discharge channels are often open, photorecovery may occur immediately after UV disinfection. However, no studies have been conducted to investigate photorecovery by sunlight after UV disinfection of wastewater from small Johkasou. In this study, the relationship between the photorecovery of coliforms and the dose of sunlight after UV-LED irradiation of effluent in Johkasou was investigated. As a result, we identified the amount of sunlight that will maximize photoreactivation effect. We also clarified the suppressing effect of an excessive amount of sunlight on the photoreactivation phenomenon. Additionally, since it was confirmed that the photoreactivation effect was suppressed by a certain period of darkness following UV-LED irradiation, it was considered that the photoreactivation effect can also be suppressed by allowing wastewater to reside in darkness for the currently estimated residence time in a disinfection tank.

Operating temperature significantly affects biogas output, process stability, and microbial communities involved in anaerobic digestion. There are several unanswered questions regarding how microbial communities adapt in correlation with biogas production performance, especially when a digester fails to maintain thermophilic conditions. In this study, long-term lab-scale anaerobic digestion was carried out using two fed-batch reactors at 55°C, with subsequent decreases in temperature to 48°C and 45°C. Within the first month of incubation, methane (CH₄) production increased by approximately 11.18% following a reduction in temperature from 55°C to 48°C. However, the methane production decreased by 33% after the temperature was downshifted to 45°C. Despite the difference in methane production, the thermophilic methanogen population in both reactors declined significantly in the first month with a temperature decrease. After two months of incubation, these methanogenic communities recovered faster at 48°C than at 45°C, which was highlighted by the rapid colonization of Methanosaeta, Methanobacterium, and Methanothermobacter. Notably, Methanosaeta was the most abundant methanogen under all temperature conditions, indicating its thermotolerance.