Search Results (3)

Biogas digestive effluent (BDE) is a nutrient-enriched source that can be utilized as an organic fertilizer for rice cultivation without synthetic fertilizer (SF) application. However, a primary concern is the stimulation of methane (CH₄) emissions due to the enrichment of the labile organic carbon, a favorite substrate of methanogenic archaea. Methanotrophs potentially reduce greenhouse gas (GHG) emissions from rice fields owing to metabolizing CH₄ as a carbon source and energy. We therefore examined the effect of the application of methanotroph-inoculated BDE to the rice cultivated paddy soil on GHG emissions and rice productivity under a pot experiment. Methanotrophs (Methylosinus sp. and Methylocystis sp.), isolated from the Vietnamese Mekong Delta’s rice fields, were separately inoculated to the heated BDE, followed by a 5-day preincubation. Methanotroph-inoculated BDE was supplied to rice cultivation to substitute SF at 50% or 100% in terms of nitrogen amount. The results showed that the total CH₄ emissions increased ~34% with the application of BDE. CH₄ emissions were significantly reduced by ~17–21% and ~28–44% under the application of methanotroph-inoculated BDE at 100% and 50%, respectively. The reduction in CH₄ was commensurate with the augmentation of pmoA transcript copy number under methanotroph-inoculated BDE. In addition, methanotroph-inoculated BDE application did not increase nitrous oxide (N₂O) emissions and adversely affect rice growth and grain productivity. This study highlighted the BDE-recirculated feasibility for a lower CH₄ emission rice production based on methanotrophs where high CH₄-emitting fields were confirmed. Full article

Biological settling ponds are a practicable approach for treating super-intensive shrimp aquaculture wastewater for almost all shrimp producers in the Vietnamese Mekong Delta (VMD). The optimization of the hydraulic retention time (HRT) of biological settling ponds plays a crucial role in establishing the stability of outflow wastewater quality and suitability of the settling pond area (SPA). This study aims to suggest appropriate HRT and SPA for super-intensive shrimp wastewater treatment systems based on the National Standard (QCVN 02-19:2014/BNNPTNT) and the best aquaculture practices (BAP) standards and guidelines. We investigated 20 typical super-intensive shrimp farms in the VMD and collected effluent samples from siphoning process, daily water exchange, and outflow of biological effluent-treatment settling ponds. The results showed that the average of each super-intensive shrimp farm produced wastewater at approximately 218 m³ ha⁻¹ day⁻¹. The contaminant loads of TSS, COD, TKN, and TP were commensurate to 177, 113, 9.86, and 4.19 kg ha⁻¹ day⁻¹, respectively. Based on the relationship between outflow COD, TSS concentrations, and HRT of biological-surveyed settling ponds, a 13.4-day HRT and 1934-m² SPA were suggested to optimize the super-intensive shrimp wastewater treatment systems. Our recommendation for further work is to continuously optimize the HRT and SPA rates of functional ponds (anaerobic, facultative, and maturation) to ameliorate the engineering configuration of the recommended biological settling pond. Full article

Chlorpyrifos ethyl is a popular insecticide widely used in agriculture within the Vietnamese Mekong delta, including for rice farming. Here, local farmers often apply pesticides at very high rates which leads to contamination of the surrounding environment. Silver barb (Barbonymus gonionotus) is a fish species indigenous to the delta, which resides in a variety of water bodies and is also commonly cultivated in rice–fish systems. As a result, this species is at high risk of exposure to chlorpyrifos ethyl. This study aims to determine the lethal concentration (LC₅₀) of chlorpyrifos ethyl, as well as the effects of sub-lethal concentrations on the activity of cholinesterase and growth of Silver barb. Lethal concentration testing was conducted in a static non-renewed system. Three concentrations of chlorpyrifos ethyl (1%, 10% and 20% LC50-96 h) were conducted in triplicate to assess the effects of chlorpyrifos ethyl on the brain cholinesterase (ChE) of fingerling fish for 15 days, and on their growth for 60 days. Results showed that chlorpyrifos ethyl was highly toxic to fingerling Silver barb with a LC₅₀-96 h of 0.119 ppm. The lowest observed effect concentration (LOEC) was 1%LC50-96 h for ChE and 10%LC50-96 h for growth. No observed effect concentration (NOEC) of chlorpyrifos ethyl for growth was 1%LC50-96 h. The result from this study suggests that ChE activity is significantly inhibited at environmentally realistic concentrations in the Vietnamese Mekong delta and can be used as a biomarker of pesticide exposure. Further study in the rice fields as well as in the canals or rivers is required. Full article