Localized recharge processes in the NE Mekong Delta and implications for groundwater quality.

Understanding recharge in the Mekong Delta is critical for the delta's groundwater resources, and requires the investigation of recharge processes at the local scale. In this study of the north eastern area of the Mekong Delta, time-series of environmental tracer data (δ18O, δ2H, major ions and 3H) and markers of rural pollution (NH4 and NO3) were used to highlight localized recharge and impacts on groundwater quality. Results highlighted new hydrological insights into recharge processes, including that the Pleistocene aquifer receives recent recharge (< 60 years), predominantly during high rainfall months (> 100 mm/month). However, due to shallow clay layers there are significant spatial variations in these recharge processes, which were observed in the seasonal fluctuation of groundwater δ18O values in groundwater. Wet season δ18O changes ranged from below analytical uncertainty (≤ 0.10 ‰) to up to 0.56 ‰, and the calculated fraction of rainfall contribution to the aquifer is ≤5 % to 16 %. Rainfall recharge via the acrisol soils results in low groundwater EC (20-55 µS/cm), acidic groundwater (pH 3.6-5.6), and may also have resulted in the low groundwater NO3 concentrations (≤ 5.3 mg NO3/L) at many sites due to adsorption, therefore delaying not reducing NO3 contamination. Site specific variations in nitrogen processes includes increased NO3 (to 29.7 mg/L) from fertiliser transfers or nitrification, and increased NH4 (to 1.4 mg/L) likely due to the recharge of irrigation waters. Unlike other recharge areas across the northern Mekong Delta, this north-eastern region provides a groundwater resource unaffected by arsenic contamination. Therefore, these results should inform on priority areas for protection from further contamination by rural anthropogenic activities.

Comparative evaluation of yeast and bacterial treatment of high salinity wastewater based on biokinetic coefficients.

This paper compares the performance of the aerobic treatment of high organic-high salinity wastewater by yeast and bacterial systems. The biokinetic coefficients for both the systems were determined and used to analyze the behavior of the yeast and bacterial systems under high salinity conditions. It was found that the yeast culture was more efficient compared to the bacterial culture, especially for high salinity conditions that severely inhibit growth and performance of bacterial systems. The values of the biokinetic coefficients obtained from this study are in agreement with the observations. Nutrient removal capacity has also been found to be better for yeast due to higher nutrient uptake in the yeast biomass.

High salinity wastewater treatment using yeast and bacterial membrane bioreactors.

Two laboratory-scale membrane bioreactor systems were investigated to treat high salinity wastewater containing high organic (5,000 mg/L COD) and salt content (32 g/L NaCl), namely: (1) the Yeast Membrane Bioreactor (YMBR) and; (2) Yeast pretreatment followed by Bacterial Membrane Bioreactor (BMBR). In the YMBR system, experimental runs were conducted with a mean biomass concentration of 12 g MLSS/L. Here the maximum COD removal rate of 0.93 g COD/g MLSS x day was obtained at F/M of 1.5 g COD/g MLSS.d. Whereas, the BMBR system was operated with a biomass concentration of up to 25 g MLSS/L, resulting in maximum COD removal rate of 0.32 kg COD/kg MLSS x day at F/M ratio of 0.4. In comparison to BMBR, YMBR could obtain higher COD removal rate at higher organic loading, indicating the potential of a yeast reactor system to treat high salinity wastewater containing high organic concentration. Transmembrane pressure in BMBR was progressively increased from 2 to 60 kPa after 12 d, 6 d and 2 d at a hydraulic retention time (HRT) of 14 h, 9 h and 4 h, with average biomass concentration of 6.1, 15 and 20 g MLSS/L, respectively. Whereas the transmembrane pressure in YMBR has increased from 2 to 60 kPa only after 76 days of operation, with an average biomass concentration of 12 MLSS/L and an operating HRT range of 5-32 h.