The use of nutrient-rich feed in shrimp farming in Central Vietnam has led to high nitrogen (N) and phosphorus (P) contents in the pond sediment. The objectives of the study were to assess the effectiveness of biochar-sediment microbial fuel cells (BC-SMFCs) in suppressing P and N release from two types of sediment in intensive (Int) and extensive (Ext) pond drainages in Central Vietnam. Single chamber SMFCs were set up and operated under open or closed-circuit (no SMFC or SMFC) conditions. Coconut shell biochar (BC) was amended to sediments at 1%. For Int-sediment, total phosphorus (TP) release was reduced by no BC-SMFCs through co-precipitation with Fe. On the other hand, BC-SMFCs did not suppress TP release because P was released from BC and organic matter decomposition was enhanced in the sediment. Application of BC enhanced organic N mineralization in the sediment. Nitrification and denitrification occurred in the overlying water, reducing mineral N concentrations. For Ext-sediment, BC addition and SMFC conditions did not affect TP and total nitrogen (TN) release because of low initial organic matter content, and less reductive condition. Our study suggested that the effect of SMFCs was masked by BC which released more P from Int-sediment to the water.

The anaerobic degradability of dewatered sludge cake collected from the municipal wastewater treatment plant was investigated in the continuous lab-scale digester under varied agitation intensities, where the system received the total volatile solids (TVS) of ca. 155 g-TVS kg-cake⁻¹ and was operated in a partial liquor displacement using water to control ammonium concentration at about 2,200 mg-N L⁻¹. When the agitation was strengthened to 2.0 m s⁻¹, a soluble total organic carbon (sTOC) was accumulated up to 3,400 mg-C L⁻¹ in association with a lowered methane production. The limited accumulation of acetate and propionate (11% of sTOC) during the event suggested the hydrolysis and/or acidogenesis were inhibited in the operation. The reduced agitation intensity ameliorated the inhibition and attained an almost comparative process performance to that of the control experiment fed with a diluted cake of 80 g-TVS kg-cake⁻¹. The offline respirometry tests revealed the inhibitory substance generated in the digester was not present in the soluble portion of digestate but in the colloidal form. The tests also indicated the inhibitory substance led to a methanogen’s decay. The source of the inhibitory substance was speculated to be polyacrylamide used for the sludge dewatering process in the plant and/or high humic substances present in the cake.