The effect of PO43--P concentration on sludge settleability and nutrients removal performance.

In order to explore the effect of influent PO43--P concentration on the relationship of phosphorus remove performance and sludge settleability, three sequencing batch reactors (SBRs) operated as anaerobic/aerobic mode were used to treat wastewater with different PO43--P concentrations (same COD), and the organic loading rate (OLR) was changed through adjusting the anaerobic (aerobic) duration. The sludge settleability, nutrients removal, and microorganism species were investigated. The results showed that when the influent PO43--P were 4.3 mg·L-1 and 8.6 mg·L-1, increasing the OLR through decreasing aerobic duration could significantly improve sludge settleability, while decreasing anaerobic duration could not. It was found that increase the OLR could promote the denitrification compete for carbon sources with phosphorus release process by inhibiting the accumulation of NO2--N, leading to the decrease of phosphorus removal ability. When the influent PO43--P was 17.2 mg·L-1, sufficient nitrification was beneficial to enrich denitrifying phosphorus accumulating bacteria (DPAO), and the activities of Thauera and Flavobacterium (DPAO) were stronger. Therefore, increasing influent PO43--P concentration and reducing aerobic duration could help phosphorus accumulating bacteria (PAO) compete with denitrification for COD and enrich DPAO, thus reducing carbon source consumption.

Nitrogen removal characteristics of biofilms in each area of a full-scale AAO oxidation ditch process.

Plastic carriers were installed in different areas of a full-scale anaerobic/anoxic/aerobic (AAO) oxidation ditch process, and the dynamics of nitrogen removal, biofilm morphologies, and microorganism species were investigated. The results showed that the biofilm at the front of the aerobic area (dissolved oxygen [DO] = 0.93 mg L-1) provided the best denitrification, with specific nitrate and nitrite reduction rates of 10.16 and 3.78 mg·(g·h)-1, respectively. The biofilm in the middle of the aerobic area (DO = 1.27 mg L-1) exhibited the best nitrification performance, with a maximum specific ammonia oxidation rate of 3.21 mg·(g·h)-1. The biofilm at the end of the aerobic area (DO = 0.01 mg L-1) exhibited the highest anammox potential with a maximum specific anammox rate of 0.67 mg·(g·h)-1. No correlation was observed between the specific nitrogen removal rates and abundance of nitrogen-removing microorganisms at the genus level. Biofilm denitrification during the process was primarily completed by heterotrophic denitrifying bacteria (Thauera, Acinetobacter, Hyphomicrobium, and Thermomonas). Aerobic denitrifying bacteria (0.19% Thauera and 0.34% Hyphomicrobium) were identified as the main denitrifying bacteria in the middle of the aerobic area. The dominant nitrifying microorganisms in the middle of the aerobic area were Nitrosomonas (0.50%) and Nitrospira (1.04%). A biofilm in the end of the aerobic area exhibited specific anammox potential, which may have been related to the dominance of 0.024% Candidatus Brocadia. Kinetic analysis revealed that adding plastic carriers to the front and middle of the aerobic area contributed to stable nitrogen removal efficiency.

Characteristics of nutrients removal under partial denitrification initiated by different initial nitrate concentration.

The partial denitrification (PD) is a very promising process developed in the last decade, to study the comprehensive influence of influent carbon to nitrogen (C/N) on the activated sludge system under PD, six sequencing batch reactors (SBRs) were operated in parallel at C/N of 2.75, 3.30, 4.13, 5.50, 8.25 and 16.50, the nitrogen removal, phosphorus removal and sludge settleability of PD were investigated. The results showed that PD was observed treating synthetic wastewater in all the six SBRs, and the nitrite accumulation rate (NAR) was highest at C/N of 5.50 (NAR of 82.30%). However, due to the alternate inhibition of NO2--N and free nitrous acid (FNA) produced by a limited carbon source, both the sludge settleability and phosphorus removal deteriorated. The average SVI at C/N of 8.25 was 130% lower than C/N of 3.30, and the average amount of PO43--P released at C/N of 16.5 was 189% higher than C/N of 2.75. Kinetic analysis showed that the denitrification kinetics of PD and complete denitrification were similar, and the nitrite accumulation was caused by the difference between nitrate reduction rate and nitrite reduction rate. Variations of on-line parameters (pH and ORP) revealed that nitrite accumulation could be indicated by judging the nitrate turning point and nitrite turning point on pH and ORP curves, which provided guidance for the setup of PD.

Enhanced denitrification of secondary effluent using composite solid carbon source based on agricultural wastes and synthetic polymers.

Solid-phase denitrification is a promising approach to enhance nitrate removal. In this work, polybutylene succinate (PBS) and peanut shell (PS) (with crosslinked polyvinyl alcohol-sodium alginate (PVA-SA) as carrier) were used to prepare a composite solid carbon source (3P) to denitrify the secondary effluent. The results showed that for carbon release performance, 3P had not only a large release of organics, like PS, but also the excellent sustainability of PBS. Among the short chain fatty acids released by PBS, PS, PVA-SA and 3P, the percentages of acetic acid were 59.42%, 72.54%, 72.29% and 92.11%, respectively. When 3P was used as external carbon source, denitrification performance could be enhanced with effluent dissolved organic carbon lower than 20 mg/L. The prepared 3P could improve denitrification, from both microbial and kinetic aspects. The relative abundance of Gammaproteobacteria increased from 39.32% to 43.58%, and the half saturation constant of the fitting Monod equation was 21.28 mg/L. The prepared 3P is an ideal carbon source for secondary effluent denitrification. Using multiple crosslinking methods to produce carrier is an effective way to show the properties of each material.

Nitrification characteristics of long-term idle aerobic activated sludge during domestication.

Nitrite accumulation usually occurred when domesticating the idle aerobic activated sludge. A sequencing batch reactor (SBR) was used to investigate whether the short-cut nitrification sludge could be cultivated using the idle sludge as inoculated sludge. The results showed that the nitrification process consisted of three stages. In the first stage, the activity of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were very low with almost no nitrification performance. In the second stage, the activity of AOB started to recover with the effluent NH4+-N gradually decreased to 0.29 mg L-1, while NOB was alternately inhibited by free ammonia (FA), free nitrous acid (FNA), and nitrite. The effluent NOx--N was mainly NO2--N with an average nitrite accumulation ratio of 74.00%. In the third stage, the nitrification altered from short-cut nitrification to complete nitrification, and the nitrification kinetics of AOB and NOB were both well-fitted to the Monod equation (R2 > 0.92). The variations of effluent pH and ORP between cycles could indicate the recovery stage of the nitrifying ability. Through monitoring the curves of effluent pH and ORP, when the domestication process is between the pH peak and ORP plateau, the short-cut nitrification sludge could be cultivated. This study revealed the mechanism of nitrite accumulation during the domestication of long-term idle aerobic activated sludge, and established a control strategy to accelerate the domestication.