Effect of filling ratio and backwash on performance of a continuous-flow SPD reactor packed with PCL as carbon source

The intrinsic relevance of nitrogen removal pathway to varying nitrate loading rate in a polycaprolactone-supported denitrification system

Up to date, the intrinsic association of nitrate loading rate (NLR) with treatment performance of solid-phase denitrification (SPD) systems is still ambiguous. To address this issue, three continuous up-flow bioreactors were configured. They were packed with polycaprolactone (PCL) under a filling ratio of 30%, 60% or 90% and were operated under a varying NLR of 0.34 ± 0.01-3.99 ± 0.12 gN/(L·d). Results showed that the denitrification efficiency was high (RE > 96%) and stable except the case with the highest NLR, which was mainly attributed to the lack of available carbon sources. At the phylum or genus level, most of the detected dominant bacterial taxa were either associated with organics degradation or nitrogen metabolism. The difference in bacterial community structure among the three stages was mainly caused by NLR rather than the filling ratio. Moreover, as the NLR got higher, the Bray-Curtis distance between samples from the same stage became shorter. By the results of gene or enzyme prediction performed in PICRUSt2, the main nitrogen metabolism pathways in these reactors were denitrification, dissimilatory nitrate reduction to ammonium (DNRA), assimilatory nitrate reduction to ammonium (ANRA) and nitrogen fixation. Moreover, aerobic and anaerobic nitrate dissimilation coexisted in the systems with the latter playing a dominant role. Finally, denitrification and DNRA occurred under both high and low NLR conditions while nitrogen fixation and ANRA preferred to occur under low NLR environments. These findings might help guide practical applications.

Microbial explanation to performance stratification along up-flow solid-phase denitrification column packed with polycaprolactone

In this study, the fluctuating profiles of physicochemical and microbial characterizations along different filling heights of continuously up-flow solid-phase denitrification (SPD) columns packed with polycaprolactone (PCL) were investigated. It was found both the PCL filling area and non-filling area made significant contributions to treatment performance and denitrification mainly occurred near the bottom of the filling column. Nitrate displayed a high proportional removal (≥98.7%) among all the cases except the one with the lowest filling ratio (FR30) and highest NLR (3.99 ± 0.12 gN/(L·d)), while nitrite and ammonium displayed a weak accumulation in final effluents (nitrite ≤ 0.40 mg/L; ammonium ≤ 0.98 mg/L). The intensity of PCL hydrolysis in the top substrate was stronger than those in the middle or bottom. Both dissimilatory nitrate reduction to ammonium (DNRA) and microbial lysis contributed to ammonium accumulation, and nitrate was mainly removed via traditional denitrification and DNRA. JGI_0000069-P22_unclassified and Gracilibacteria_unclassified might contribute to denitrification.

Influence mechanism of filling ratio on solid-phase denitrification with polycaprolactone as biofilm carrier

In this study, three up-flow fixed-bed bioreactors were constructed with three different filling ratios (filling volume/effective volume: 30%, 60% and 90%) of polycaprolactone (PCL). Above 98% of nitrate removal efficiency was achieved with low accumulations of nitrite and ammonium for each filling ratio. Low filling ratio of PCL had extensive folds and pores that favored the attachment and growth of microorganisms; however, excessive biomass restrained nitrate specific reduction rate (NaSRR). The most dominant genera were Comamonas (0.80-57.64%), Stenotrophomonas (2.59-54.39%), Acidovorax (7.32-23.55%), Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium (0.30-19.74%) and Thermomonas (0.12-14.58%). Nitrate reductase (EC 1.7.99.4), nitrite reductase (EC 1.7.2.1) and nitric oxide reductase (EC 1.7.2.5) predicted by PICRUSt2 were abundant in high influent nitrate load (NaL). According to the analysis of carbon balance model, the utilization rate (η) of PCL showed a highly positive correlation with influent NaL, indicating reducing filling ratio or HRT might be an effective measure to save cost for nitrate removal.

Research progress in solid carbon source-based denitrification technologies for different target water bodies

Nitrogen pollution in water bodies is a serious environmental issue which is commonly treated by various methods such as heterotrophic denitrification. In particular, solid carbon source (SCS)-based denitrification has attracted widespread research interest due to its gradual carbon release, ease of management, and long-term operation. This paper reviews the types and properties of SCSs for different target water bodies. While both natural (wheat straw, wood chips, and fruit shells) and synthetic (polybutylene succinate, polycaprolactone, polylactic acid, and polyhydroxyalkanoates) SCSs are commonly used, it is observed that the denitrification performance of the synthetic sources is generally superior. SCSs have been used in the treatment of wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface water, and groundwater; however, the key research aspects related to SCSs differ markedly based on the target waterbody. These key research aspects include nitrogen pollutant removal rate and byproduct accumulation (ordinary wastewater); water quality parameters and aquatic product yield (recirculating aquaculture systems); temperature and hydraulic retention time (agricultural subsurface drainage); the influence of dissolved oxygen (surface waters); and nitrate-nitrogen load, HRT, and carbon source dosage on denitrification rate (groundwater). It is concluded that SCS-based denitrification is a promising technique for the effective elimination of nitrate-nitrogen pollution in water bodies.