Recovery of nitrification in cadmium-inhibited activated sludge system by bio-accelerators

Metabolomic reveals the responses of sludge properties and microbial communities to high nitrite stress in denitrifying phosphorus removal systems

Nitrite, as an electron acceptor, plays a good role in denitrifying phosphorus removal (DPR); however, high nitrite concentration has adverse affects on sludge performance. We investigated the precise mechanisms of responses of sludge to high nitrite stress, including surface characteristics, intracellular and extracellular components, microbial and metabolic responses. When the nitrite stress reached 90 mg/L, the sludge settling performance was improved, but the activated sludge was aging. FTIR and XPS analysis revealed a significant increase in the hydrophobicity of the sludge, resulting in improve settling performance. However, the intracellular carbon sources synthesis was inhibited. In addition, the components in the tightly bound extracellular polymeric substances (TB-EPS) of sludge were significantly reduced and indicated the disturb of metabolism. Notably, Exiguobacterium emerged as a new genus when face high nitrite stress that could maintaining survival in hostile environments. Moreover, metabolomic analysis demonstrated strong biological response to nitrite stress further supported above results that include the inhibited of carbohydrate and amino acid metabolism. More importantly, some lipids (PS, PA, LysoPA, LysoPC and LysoPE) were significantly upregulated that related enhanced membrane lipid remodeling. The comprehensive analyses provide novel insights into the high nitrite stress responses mechanisms in activated sludge systems.

Removal of sulfamethoxazole and Cu, Cd compound pollution by arbuscular mycorrhizal fungi enhanced vertical flow constructed wetlands

The combined pollution of antibiotics and heavy metals (HMs) has a serious impact on the water ecological environment. Previous researches mainly focused on the removal of antibiotics or HMs as single pollutants, with limited investigation into the treatment efficiencies and underlying mechanisms associated with their co-occurring pollution. In this study, 16 micro vertical flow constructed wetlands (MVFCWs) were constructed to treat composite wastewater consisting of sulfamethoxazole (SMX), copper (Cu) and cadmium (Cd), involving two different inoculation treatments (arbuscular mycorrhizal fungi (AMF) inoculated and uninoculated) and eight kinds of pollutant exposure (Control Check (CK), SMX, Cu, Cd, SMX + Cu, SMX + Cd, Cu + Cd, SMX + Cu + Cd). The findings of this study demonstrated that the inoculation of AMF in MVFCWs resulted in removal efficiencies of SMX, Cu, and Cd ranging from 18.70% to 80.52%, 75.18% to 96.61%, and 40.50% to 89.23%, respectively. Cu and CuCd promoted the degradation of SMX in the early stage and inhibited the degradation of SMX in the later stage. Cd did not demonstrate a comparable promotive impact on SMX degradation, and its addition hindered Cu removal. However, comparatively, the presence of Cu exerted a more pronounced inhibitory effect on Cd removal. Furthermore, the addition of Cu augmented the abundances of Proteobacteria, Bacteroidetes (at the phylum level) and Rhodobacter, Lacunisphaera and Flavobacterium (at the genus level), and Cu exposure showed a substantially stronger influence on the microbial community than that of Cd and SMX. AMF might confer protection to plants against HMs and antibiotics by enriching Nakamurella and Lacunisphaera. These findings proved that AMF-C. indica MVFCW was a promising system, and the inoculation of AMF effectively enhanced the simultaneous removal of compound pollution.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

The response and anti-stress mechanisms of nitrifying sludge under long-term exposure to CdSe quantum dots

The wide application of CdSe quantum dots (QDs) increases its stress risk to sewage treatment systems. This study evaluated the response of nitrification performance, floc characteristics and microbial community of nitrifying sludge under long-term exposure to CdSe QDs. Results showed CdSe QDs (≥1 mg/L) would decrease the activity of ammonia monooxygenase (AMO). Under the stress of 30 mg/L CdSe QDs, the activity of AMO was reduced by 66%, while the activities of hydroxylamine oxidase and nitrite oxidoreductase were enhanced by 19.1% and 26%, respectively. Thus, the final nitrification effects were not adversely affected, and the production rates of NO2--N and NO3--N were accelerated. Additionally, CdSe QDs improved biomass concentration in sludge and maintained the stability of sludge settleability. High throughput sequencing analysis showed that CdSe QDs evidently reduced the abundance and diversity of microbial community in nitrifying sludge. The abundances of amino acid metabolism and lipid metabolism were enriched. Moreover, CdSe QDs decreased the fluorescence intensity of tryptophan-like protein from 2,326 to 1,179 a.u. in loosely bound extracellular polymeric substances (EPSs) and from 3,792 to 3,117 a.u. in tightly bound EPSs. To relieve CdSe QD stress, the polysaccharide content increased from 0.31 to 0.61 mg/g MLSS and intracellular antioxidant defense was activated. With CdSe QD level increasing to 30 mg/L, the total antioxygenic capacity and the activities of catalase were enhanced up to 411% and 143.2%, respectively. Thereby, CdSe QDs had little adverse effects on cell membrane integrity, microbial metabolism and the abundance of Nitrospirae.

Positive responses and mechanisms of nitrifying sludge to carbon quantum dots: reactor performance, microbial behavior, and antioxidant defense

Extensive application of carbon quantum dots (CQDs) enlarges its concentration in sewage treatment system. The response of nitrifying sludge to CQDs after long-term exposure was investigated. Results showed that CQD concentrations of 0-100 mg/L presented positive effect to enzymes involved in nitrification, accelerating NH4+-N degradation and NO2--N transformation. The oxidation rate of NO2--N was significantly improved from 3.14 to 7.91 mg/(L h) under the stress of 100 mg/L CQDs. Besides, CQDs stimulated the production of sludge biomass and kept the stability of sludge settleability. Additionally, CQDs were mainly captured by loosely bound extracellular polymeric substances, reducing aromatic-like protein. Microbes alleviated CQD stress by secreting tryptophan-like protein and polysaccharides. After few CQDs entered cells, intracellular antioxidant defense was activated. Total antioxidant capacity level was heightened at least 31%. The activities of superoxide dismutase and catalase were enhanced at relatively low and high CQD concentration levels. Hence, microbial metabolic pathways, microbial community, and nitrifying bacteria were not significantly affected by CQDs. The findings of this work provide new insight for understanding the environmental implication of CQDs in the biological treatment system.

Recovery Strategies for Heavy Metal-Inhibited Biological Nitrogen Removal from Wastewater Treatment Plants: A Review

Biological nutrient removal is an integral part of a wastewater treatment plant. However, the microorganism responsible for nutrient removal is susceptible to inhibition by external toxicants such as heavy metals which have the potential to completely inhibit biological nutrient removal. The inhibition is a result of the interaction between heavy metals with the cell membrane and the deoxyribonucleic acid (DNA) of the cell. Several attempts, such as the addition of pretreatment steps, have been made to prevent heavy metals from entering the biological wastewater systems. However, the unexpected introduction of heavy metals into wastewater treatment plants result in the inhibition of the biological wastewater treatment systems. This necessitates the recovery of the biological process. The biological processes may be recovered naturally. However, the natural recovery takes time; additionally, the biological process may not be fully recovered under natural conditions. Several methods have been explored to catalyze the recovery process of the biological wastewater treatment process. Four methods have been discussed in this paper. These include the application of physical methods, chelating agents, external field energy, and biological accelerants. These methods are compared for their ability to catalase the process, as well as their environmental friendliness. The application of bio-accelerant was shown to be superior to other recovery strategies that were also reviewed in this paper. Furthermore, the application of external field energy has also been shown to accelerate the recovery process. Although EDTA has been gaining popularity as an alternative recovery strategy, chelating agents have been shown to harm the metal acquisition of bacteria, thereby affecting other metabolic processes that require heavy metals in small amounts. It was then concluded that understanding the mechanism of inhibition by specific heavy metals, and understanding the key microorganism in the inhibited process, is key to developing an effective recovery strategy.

Exploiting Biofilm Characteristics to Enhance Biological Nutrient Removal in Wastewater Treatment Plants

Biological treatments are integral processes in wastewater treatment plants (WWTPs). They can be carried out using sludge or biofilm processes. Although the sludge process is effective for biological wastewater systems, it has some drawbacks that make it undesirable. Hence, biofilm processes have gained popularity, since they address the drawbacks of sludge treatments, such as the high rates of sludge production. Although biofilms have been reported to be essential for wastewater, few studies have reviewed the different ways in which the biofilm properties can be explored, especially for the benefit of wastewater treatment. Thus, this review explores the properties of biofilms that can be exploited to enhance biological wastewater systems. In this review, it is revealed that various biofilm properties, such as the extracellular polymeric substances (EPS), quorum sensing (Qs), and acylated homoserine lactones (AHLs), can be enhanced as a sustainable and cost-effective strategy to enhance the biofilm. Moreover, the exploitation of other biofilm properties such as the SOS, which is only reported in the medical field, with no literature reporting it in the context of wastewater treatment, is also recommended to improve the biofilm technology for wastewater treatment processes. Additionally, this review further elaborates on ways that these properties can be exploited to advance biofilm wastewater treatment systems. A special emphasis is placed on exploiting these properties in simultaneous nitrification and denitrification and biological phosphorus removal processes, which have been reported to be the most sensitive processes in biological wastewater treatment.

Comparative Study on Using Various Recovery Stimulation Methods to Boost Nitrification Recovery in SBRs Inhibited by Hazardous Events

A system consisting of six SBR units was operated in parallel for three phases to investigate the impacts of salinity shock and anaerobic and aerobic starvation on the activated sludge process stability and effects of various recovery stimulation methods on the subsequent recovery period. Different recovery strategies were applied in each SBR unit, including natural recovery, adding bio-accelerators, a stepwise increase feed strategy, a stepwise strategy coupled with bio-accelerators dosing, extended aeration time, and extended aeration time coupled with bio-accelerators dosing. It was concluded that the combination of stepwise strategy and dosing bio-accelerators showed the most efficiency in boosting system recovery after being subjected to NaCl shock and starvation. The boosting effect of the stepwise strategy alone was slightly better in recovery after NaCl shock. Furthermore, extending the aeration rate could bring more positive effects when resuscitating the system after long-term anaerobic starvation. For the unit that only received dosing of bio-accelerators during the recovery period, it could be concluded that there was a specific time requirement for the bio-accelerators to take effect significantly, as the impact of bio-accelerators on the beginning days of recovery periods was very slight. In contrast, adjusting operational regimes such as stepwise increased feed volume or extending aeration time could significantly boost the SBRs from the first recovery days. Hence, highly effective recovery efficiency could be achieved by coupling dosing bio-accelerators with other operational adjustment methods, especially stepwise strategies.

Dynamic modeling of the activated sludge microbial growth and activity under exposure to heavy metals

The presence of heavy metals in the environment can lead to ecological and health problems. The evolution of biological systems, such as activated sludge, exposed to heavy metals is still underexplored. Therefore, this study sought to develop a model of microorganism activity and growth in activated sludge and used it to investigate the toxicity of five metals: Cu, Cd, Ni, Zn, and Ag. Patterns in the evolution of the toxic effects caused by these metals were similar at the beginning of exposure. Differences in toxicity between metal ions were noted for longer exposure times. Changes in model parameters indicate the influence of metal ions on the mass and energy balance of living cells. Decreases in new enzyme units and biomass production yields in contaminated activated sludge indicate a shift from anabolic reactions to metal homeostasis and resistance.

Application of different redox mediators induced bio-promoters to accelerate the recovery of denitrification and denitrifying functional microorganisms inhibited by transient Cr(VI) shock

The transient hexavalent chromium (Cr(VI)) shock may directly inhibit the denitrification process of municipal wastewater treatment plants (WWTPs), which is difficult to recover in a short time. This study developed four nontoxic bio-promoters (combination of L-cysteine, flavin adenine dinucleotide (FAD), biotin, cytokinin and different redox mediators) to quickly restore the denitrification performance after high-loading Cr(VI) suppressing. After feeding with 100 mg/L of Cr(VI) for 42 cycles (T, 4 h), the removal efficiency of nitrate was reduced by 85.00%, and nitrite was accumulated simultaneously. The denitrification performance was recovered quickly with the addition of bio-promoters, introducing redox mediators showed noticeable superiority on the bio-inhibition release. Compared with sodium humate and riboflavin, the AQDS induced bio-promoter achieved the best nitrate removal recovery performance within only 28 T, and the recovery rate was 2.16 times faster than the natural recovery. Microbial analysis showed that Cr(VI) specially inhibited napA-type denitrifiers, and the OTU numbers sharply dropped by 48.74%. Redox mediators induced bio-promoters could effectively recover the abundance of napA-type and nirS-type denitrifying microorganisms, which was consistent with the change of nitrate removal efficiency. This study offers a cost-effective approach to deal with Cr(VI) shock problem, which may promote the development of bio-promoters for WWTPs.

Cadmium Pollution Impact on the Bacterial Community Structure of Arable Soil and the Isolation of the Cadmium Resistant Bacteria

Microorganisms play an important role in the remediation of cadmium pollution in the soil and their diversity can be affected by cadmium. In this study, the bacterial community in arable soil samples collected from two near geographical sites, with different degrees of cadmium pollution at three different seasons, were characterized using Illumina MiSeq sequencing. The result showed that cadmium is an important factor to affect the bacterial diversity and the microbial communities in the high cadmium polluted area (the site H) had significant differences compared with low cadmium polluted area (the site L). Especially, higher concentrations of Cd significantly increased the abundance of Proteobacteria and Gemmatimonas whereas decreased the abundance of Nitrospirae. Moreover, 42 Cd-resistant bacteria were isolated from six soil samples and evaluated for potential application in Cd bioremediation. Based on their Cd-MIC [minimum inhibitory concentration (MIC) of Cd²⁺], Cd²⁺ removal rate and 16S rDNA gene sequence analyses, three Burkholderia sp. strains (ha-1, hj-2, and ho-3) showed very high tolerance to Cd (5, 5, and 6 mM) and exhibited high Cd²⁺ removal rate (81.78, 79.37, and 63.05%), six Bacillus sp. strains (151-5,151-6,151-13, 151-20, and 151-21) showed moderate tolerance to Cd (0.8, 0.4, 0.8, 0.4, 0.6, and 0.4 mM) but high Cd²⁺ removal rate (84.78, 90.14, 82.82, 82.39, 81.79, and 84.17%). Those results indicated that Burkholderia sp. belonging to the phylum Proteobacteria and Bacillus sp. belonging to the phylum Firmicutes have developed a resistance for cadmium and may play an important role in Cd-contaminated soils. Our study provided baseline data for bacterial communities in cadmium polluted soils and concluded that Cd-resistant bacteria have potential for bioremediation of Cd-contaminated soils.

Rapid recovery of inhibited denitrification with cascade Cr(VI) exposure by bio-accelerant: Characterization of chromium distributions, EPS compositions and denitrifying communities

Hexavalent chromium (Cr(VI)) may inhibit denitrification in biological wastewater treatment systems, and the inhibited denitrification process is difficult to recover in a short time. This study explored Cr(VI) cascade impact (20-125 mg L^-1) on denitrification and developed one nontoxic biological accelerant (combination of L-cysteine, flavin adenine dinucleotide, biotin and cytokinin) for denitrification recovery. The results showed that NO₃^--N removal efficiency decreased from 75.7% to 21.5% when Cr(VI) concentration increased from 80 to 125 mg L^-1. Addition of accelerant could effectively promote the removal of NO₃^--N, and observably reduce the recovery time (42 T) compared with natural recovery (63 T). Furthermore, the main site of Cr(VI) reduction and Cr(III) immobilization was located in the intercellular compartment of the biofilm. Microbes produced more tightly bound extracellular polymeric substances (TB-EPS) to protect them from toxicity under the low Cr(VI) concentrations, while low EPS was secreted when Cr(VI) concentration was higher than 60 mg L^-1. Compared to natural recovery system, bio-accelerant addition was beneficial to the recovery of denitrifiers activities, especially for the bacteria containing nirS gene. The results facilitated an understanding of Cr(VI) impact on denitrification, and the proposed bio-accelerant can be potentially applied to heavy metal shock-loading emergency situations.

Effect of Cobalt, Cadmium and Manganese on Nitrogen Removal Capacity of Arthrobacter arilaitensis Y-10

The aim of this study was to investigate the possibility of a simultaneous nitrification–denitrification hypothermic bacterium for applying in Cd(II), Co(II), and Mn(II)-contaminated wastewater. The influence of Cd(II), Co(II), and Mn(II) on the inorganic nitrogen removal capacity of the hypothermia bacterium Arthrobacter arilaitensis Y-10 was determined. The experimental results demonstrated that low concentration of Cd(II) (2.5 mg/L) exhibited no significant impact on bioremediation of ammonium. The nitrate and nitrite removal activities of strain Y-10 were enhanced by 0.1 and 0.25 mg/L of Cd(II), but hindered by more than 0.25 and 0.5 mg/L of Cd(II), respectively. However, the cell growth and denitrification activity ceased immediately once Co(II) was supplemented. In terms of Mn(II), no conspicuous inhibitory impact on ammonium bioremediation was observed even if Mn(II) concentration reached as high as 30 mg/L. The bioremediation of nitrates and nitrites was significantly improved by 0.5 mg/L of Mn(II), and then dropped sharply along with the increase of Mn(II). The order of the degree of inhibitory influence of the three heavy metal ions on the nitrogen bioremediation ability of strain Y-10 was Co(II) > Cd(II) > Mn(II). All the results highlighted that the heterotrophic nitrification was less sensitive to the inhibitory effects of Cd(II), Co(II), and Mn(II) relative to aerobic denitrification.

Evaluation of partial nitrification efficiency as a response to cadmium concentration and microplastic polyvinylchloride abundance during landfill leachate treatment

The partial nitrification efficiency response to the presence of cadmium (Cd²⁺) and microplastics was investigated. Microplastics polyvinylchloride (PVC) abundance was 0-10,000 particles/L, and Cd²⁺ concentration was 0-10 mg/L. Cd-only inhibited the NH₄⁺-N oxidation rate 1.21, 1.23, and 1.18 times with concentrations at 1, 5, and 10 mg/L, respectively. PVC-only inhibited NH₄⁺-N oxidation rate 1.01, 1.21 and 1.05 times with PVC abundance at 1000, 5000 and 10,000 particles/L, respectively. The ammonia oxidation rate was improved with the co-existence of PVC and Cd²⁺ at the conditions PVC1000 and PVC5000, which could be attributed to the PVC. PVC at 1000 particles/L could act as carrier and mitigate the negative effect of Cd²⁺ to the partial nitrification process. Moreover, the partial nitrification process was largely inhibited with PVC abundance at 10,000 particles/L. First-order kinetic models could simulate the NH₄⁺-N, NO₂^-N, and NO₃^--N changes in the partial nitrification process.

Diversity evolution of functional bacteria and resistance genes (CzcA) in aerobic activated sludge under Cd(II) stress

An activated sludge sequencing batch reactor (SBR) was used to treat divalent cadmium (Cd(II)) wastewater for 60 d to investigate the overall treatment performance, evolution of the bacterial community, and abundance of the Cd(II) resistance gene CzcA and shifts in its potential host bacteria. During stable operation with a Cd(II) concentration of 20 mg/L, the average removal efficiencies of Cd(II) and chemical oxygen demand (COD) were more than 85% and that of total phosphorus was greater than 70%, while the total nitrogen (TN) was only about 45%. The protein (PN) content in the extracellular polymeric substances (EPS) increased significantly after Cd(II) addition, while polysaccharides displayed a decreasing trend (p < 0.05), indicating that EPS prefer to release PN to adsorb Cd(II) and protect bacteria from damage. Three-dimensional fluorescence spectral analysis showed that fulvic acid-like substances were the most abundant chemical components of EPS. The addition of Cd(II) adversely affected most denitrifying bacteria (p < 0.05), which is consistent with the low TN removal. In addition, quantitative polymerase chain reaction analysis revealed that CzcA gene abundance decreased as the Cd(II) concentration increased, possibly because expression of the CzcA gene was inhibited by Cd(II) stress. The majority of CzcA gene sequences were carried by Pseudomonas, making it the dominant genus among Cd(II)-resistant bacteria.

Adaption and restoration of anammox biomass to Cd(II) stress: Performance, extracellular polymeric substance and microbial community

Cadmium (Cd) can cause the deterioration of biological systems through inhibiting the enzymes activity and disturbing the microbial metabolism. Although the influence of Cd on conventional wastewater treatment process has been studied, the response of anammox to Cd exposure still remains unclear. This study firstly investigated the adaption and restoration of anammox biomass to Cd(II) stress. Results showed that long-term exposure of anammox bacteria to 2 mg L^-1 Cd(II) was beneficial for the reactor performance, while 5 mg L^-1 Cd(II) would cause the decline of SAA, extracellular polymeric substance content and relative abundance of Candidatus kuenenia by 40%, 25% and 31%, respectively. Furthermore, these indexes could approximately recover to the initial status after withdrawing Cd(II) from the influent. Overall, the anammox biomass exhibited a certain adaption and restoration ability to the suppression of Cd(II). This study may provide key valuable information for the biological treatment of wastewater containing Cd(II).

Insights into the effect of nickel (Ni(II)) on the performance, microbial enzymatic activity and extracellular polymeric substances of activated sludge

The performance, nitrogen removal rate, microbial enzymatic activity and extracellular polymeric substances (EPS) of activated sludge were assessed under nickel (Ni(II)) stress. The organic matter and NH₄⁺-N removal efficiencies were stable at less than 10 mg/L Ni(II) and subsequently decreased with the increment of Ni(II) concentration from 10 to 30 mg/L. The specific oxygen uptake rate and dehydrogenase activity kept stable at less than 5 mg/L Ni(II) and then declined at 5-30 mg/L Ni(II). Both specific ammonia-oxidizing rate (SAOR) and specific nitrite-oxidizing rate (SNOR) decreased with the increment of Ni(II) concentration. The changing trends of ammonia monooxygenase and nitrite oxidoreductase activities were matched those of SAOR and SNOR, respectively. The nitrite-reducing rate and nitrate-reducing rate illustrated a similar variation tendency to the nitrite reductase activity and nitrate reductase activity, respectively. Ni(II) impacted on the production, chemical composition and functional group of EPS. The relation between the sludge volume index and the EPS production exhibited a better linear function with a negative slope, demonstrating that Ni(II) improved the sludge settleability despite of the increase of EPS production.

Speciation of nickel and zinc, its short-term inhibitory effect on anammox, and the associated microbial community composition

This study provides insight into the short-term effects of nickel and zinc on anammox. The impacts of these heavy metals are evaluated based on their potentially bioavailable fractions, including the intracellular, surface-bound, soluble, free-ion, and weak (labile) complexes of heavy metals, in the presence of certain inorganic/organic species. Results showed that the IC50 values for soluble, intracellular, cell-associated, surface-bound, and free-ion Ni concentrations are 5.99, 0.250, 0.930, 0.680, and 1.36 mg/L, respectively. The inhibitory effect of Zn is found to be lower with respect to Ni, with IC50 values of 6.76, 11.9, 15.1, and 2.71 mg/L for the soluble, intracellular, cell-associated, and free-ion Zn concentrations, respectively. This is the first detailed evaluation of anammox inhibition based on the fractionation of heavy metals. Metagenomic analysis reveals that Candidatus Kuenenia constitute approximately 89% of the entire Planctomycetes population, whereas Candidatus Brocadia are detected in relatively low fractions (3%).

Evaluation of robustness of activated sludge using calcium-induced enhancement of respiration

Robustness of an activated sludge system, describing uncertainty and operational risk, was evaluated using the absence or presence of calcium-induced enhancement of respiration (CaER) effect. Generally, the fast-growing system was susceptible to external environmental variations, of which the sludge exhibited significant CaER effect under normal operational conditions, while the slow growing system showed less significant CaER effect. However, sludge in both systems exhibited CaER effect under stressed conditions of decreasing temperature or ammonia shocking. Therefore, the absence of CaER effect indicates a more robust system, while the presence of CaER effect indicates a susceptible system. Additionally, a method to identify safe and dangerous shocking was established by a hybrid usage of absence or presence of CaER effect and recovery index (RI) curve type. The evaluation of robustness could help determining when adjustment should be really taken to cope with the uncertainty, and thus holds a high promise for field application.

Respiration adaptation of activated sludge under dissolved oxygen and hypochlorite stressed conditions

In this work, the relationship between environmental disturbance and the recovery of activated sludge at a low dissolved oxygen of 0.5mg/l or in the presence of sodium hypochlorite of 1-15mg Cl₂/gSS/d was examined. When microorganisms entered their physiological adaptation period, a sharp increase in endogenous respiration rate was observed. The activity recovery potential of sludge depended on the ratio of the endogenous respiration rate to the maximum respiration rate. A subsequent decrease in this ratio after a sharp increase indicates that the disturbance was recoverable. An increase in this ratio to a certain value, e.g., 0.35, suggests that the sludge system could not adapt to the new environments and thus was unrecoverable. In addition, the recoveries of sludge respiration and effluent quality were asynchronous, which was impacted by both sludge activity and operating conditions. These results provide a useful approach for the operation of activated sludge systems.