Excess Sludge Reduction Research Articles

Behavior of tetracycline and macrolide antibiotics in activated sludge process and their subsequent removal during sludge reduction by ozone

Ozonation is a promising means for the reduction of excess sludge in wastewater treatment plants. However, little information is available on the removal of antibiotics during sludge ozonation. Therefore, this study investigated first the behavior of four commonly-used hydrophobic antibiotics, including three tetracyclines (tetracycline, oxytetracycline, and doxycycline) and one macrolide (azithromycin) in activated sludge process and then their removal during sludge reduction by ozone. Results indicate that the studied antibiotics were primarily transferred into the solid phase of activated sludge via sorption, which was a reversible, spontaneous, and exothermic process governed by surface reactions. Sludge ozonation could effectively remove 86.4–93.6% of the antibiotics present in the sludge at an ozone dose of 102 mg per gram of mixed liquor suspended solids and pH 7.2. The removal of studied antibiotics mainly proceeded through desorption and subsequent oxidation. Increasing the initial pH from 5.0 to 9.5 obviously enhanced the antibiotic removal during sludge ozonation. This study demonstrated that the activated sludge process coupled with sludge ozonation can simultaneously reduce excess sludge and eliminate antibiotics.

Reduction and Heavy Metals Removal of Excess Sludge by Radio Frequency Discharge Plasma

Excess sludge is an unavoidable by-product in wastewater treatment. The conventional methods(landfill, incineration etc.) may cause secondary pollution. In this study, both reduction and heavy metals removal of excess sludge by radio frequency discharge plasma(RFDP) were investigated. The experimental results showed that RFDP was an effective removal method on total chemical oxygen demand (TCOD) and mixed liquid suspended solids(MLSS). Discharge process could also damage the cell structure of sludge, so that organic matter and heavy metals within the cell structure could enter the liquid phase. At the same time, combined with spectral diagnosis, the radical reactions was discussed.

Evaluating the excess sludge reduction in activated sludge system with ultrasonic treatment.

Ultrasonic treatment for enhancing biological processes has recently attracted considerable attention in wastewater treatment. In this study, we systematically investigated the mixed liquor properties of activated sludge under ultrasonic treatment. The sludge samples were collected from the aerobic tank of a full-scale membrane bioreactor (MBR) treating municipal wastewater, and the volatile suspended solids (VSS) concentration was approximately 6.0 g/L. The results showed that ultrasonic treatment induced floc disintegration, organics release, temperature increase, microbial activity and pH variation. The maximum mg soluble chemical oxygen demand (COD) per mg VSS released was estimated to be 0.147 using the Monod equation. The exponential increase in the concentration of dissolved organic matter is related to the loss of relative heterotrophic bacterial activity. A sonolysis-cryptic growth model was demonstrated to be capable of describing ultrasonic sludge reduction, which would support the further development of ultrasonic treatment technology in activated sludge systems.

Biological minimization of excess sludge in a membrane bioreactor: Effect of plant configuration on sludge production, nutrient removal efficiency and membrane fouling tendency

Excess sludge minimization was studied in a MBR with pre-denitrification scheme. Sludge minimization, nitrogen removal performance and membrane fouling tendency were investigated in two configurations, characterized by a different position of the sludge retention reactor (SRR). In particular, the SRR was placed: i) in the return activated sludge line (Anaerobic Side-Stream Reactor – ASSR configuration) and ii) in the mainstream between the anoxic and aerobic reactor (Anaerobic Main-Stream Reactor – AMSR configuration). The achieved results demonstrated that the ASSR enabled a higher excess sludge reduction (74% vs 32%), while achieving lower biological nitrogen removal (BNR) (TN = 63% vs 78%) and membrane fouling tendency (FR = 2.1 · 1012 m−1 d−1vs 4.0 · 1011 m−1 d−1) than the AMSR. It was found that metabolism uncoupling, destruction of EPS and endogenous decay simultaneously occurred in the ASSR. Conversely, selective enrichment of bacteria population with low biomass yield was found the main mechanism affecting sludge minimization in the AMSR.

Performance and mechanism of free nitrous acid on the solubilization of waste activated sludge.

Free nitrous acid (FNA) is a promising chemical reagent for excess sludge reduction. The distinctive properties of FNA treatment on waste activated sludge (WAS) disposal have previously been demonstrated, however, the cellular response, permeabilization, and disruption caused by low-concentration FNA and the direct cell solubilization of WAS using concentrated FNA should be better understood. In this study, the parameters that influence the sludge solubilization efficiency were optimized over a wide range of FNA concentrations. The sludge solubilization efficiency was found to be superior when the sludge was exposed to FNA (when the dosage of NaNO2 was 0.12 g g−1 TSS and the pH was 3.0, FNA = 20.94 mg L−1) for 10 h at 25 °C, and the TSS removal and COD dissolution efficiencies were found to be prominent at 38% and 7%, respectively. In the FNA treatment of WAS, some FNA-tolerable cells increased the K+, Ca2+, and H+ effluxes under low concentrations of FNA, and finally achieved ion homeostasis based on the results using a scanning ion-selective electrode measurement technique. This could cause the cells in WAS to maintain cytoactivity and integrity under a low-concentration FNA treatment. Furthermore, flow cytometry was used to assess the permeabilization and disruption of sludge cells toward a concentration gradient of FNA. Flow cytometry results indicated that cells in sludge flocs were disrupted within 30 minutes when the FNA concentration was above 8 mg L−1.

Bacterial inactivation and regrowth in ozonated activated sludges

Ozonation of the return activated sludge (AS) flow is an emerging option for excess-sludge reduction. This study aimed to evaluate the potential changes suffered by some kinetic parameters of the activated sludge models (ASMs) in the combined ozone-AS process. The heterotrophic maximum specific growth rate (μHmax) was determined by respirometry in three model-sludges (S1 to S3) treated in batch with different O3 doses. S1 was a fresh synthetic biosolid composed by only two particulate fractions. S2 was a digestate of S1 almost made by the endogenous residues. S3 was from a municipal wastewater treatment plant. μHmax increased significantly from 3.5 d−1 originally, to more than 10 d−1 in the ozonated sludges. Ozonation promoted the selection of fast-growing bacteria in the activated sludges, after transitory inactivation and long lag times. Some microorganisms survived to 3 months of digestion and subsequent ozonation, and then regrow faster than before, once fed again with acetate. The research is of interest from the point of view of the application of the ASM models to the ozone-AS process, but also for wastewater disinfection in general.

Efficacy of a high-pressure jet device for excess sludge reduction in a conventional activated sludge process: Pilot-scale demonstration

Application of a high-pressure jet device (HPJD) is promising for the reduction of excess activated sludge from aeration tanks in wastewater treatment plants. Nevertheless, proving the feasibility of an HPJD installed in the return line of a conventional activated sludge (CAS) process remains a challenge. We investigated differences in two pilot-scale CAS processes with or without an HPJD to demonstrate its influence on waste excess sludge reduction and wastewater treatment during operation for 108days. The application of the HPJD (designated CAS-HPJD) reduced waste activated sludge by 65% compared with the standard CAS. Despite a slight increase in suspended solids concentration from the CAS-HPJD effluent, the total amount of excess sludge from CAS-HPJD was 23% lower than that of CAS. These two CAS processes exhibited comparable performances in the removal of organic carbon, ammonium and phosphate, indicating that sludge solubilization by the HPJD did not undermine microbial activities in activated sludge. In addition, the HPJD allowed oxygenation of the aerobic tank because of intensive mixing and subsequently produced fine bubbles, resulting in a reduction in the air flow rate of 35%. High-throughput Illumina sequencing of prokaryote 16S rRNA genes and Sanger-sequencing of cloned eukaryote 18S rRNA genes revealed that microbial community compositions in the two systems were distinct and were related to the HPJD treatment.

Effects of wastewater treatment processes on the sludge reduction system with 2,4-dichlorophenol: Sequencing batch reactor and anaerobic-anoxic-oxic process

The effects of two wastewater treatment processes (sequencing batch reactor, SBR; and anaerobic-anoxic-oxic, A2O) on sludge reduction with metabolic uncoupler 2,4-dichlorophenol (DCP) were studied in laboratory. The experimental results showed that the reduction of cumulative excess sludge in SBR and A2O was 43.7% and 44.2%, respectively, during the stable stage of the test. The two processes had similar average sludge yield and sludge yield reduction, i.e., 0.306 and 0.305mg of SS/mg chemical oxygen demand (COD), and 16.9% and 17.8%, respectively. The effect of DCP on the wastewater treatment efficiencies (namely, removal of COD, total nitrogen, NH4+-N, and total phosphorus) of the two processes were also similar. SBR was more likely to slightly retard the increase of activated sludge SVI with lesser increase in extracellular polymeric substances and protein/polysaccharide ratio. Although DCP did not dramatically affect the microbial communities of sludge, SBR was more favorable for increasing the activated sludge SOUR and maintaining the primary microorganisms of sludge than A2O.

Study of the treatment of domestic sewage using PVA gel beads as a biomass carrier

Abstract Technologies for domestic sewage treatment with low excess sludge yield rate and highly efficient biological treatment methods are needed, and the moving-bed biofilm reactor has great promise for meeting this need. To provide technical support for the treatment of typical domestic sewage, this paper provides an estimate of the bacterial diversity in polyvinyl alcohol (PVA) gel beads determined by a 16S-rRNA gene-based polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) approach, proposes a method for reducing the excess sludge yield rate, and describes the startup and continuous operation of the PVA technology. Domestic sewage was treated by a moving-bed biofilm reactor system using PVA gel beads as a biomass carrier. A significant amount of sphalerite, filamentous bacteria and bacillus were observed on the surface and internal structure of the PVA-gel beads by scanning electron microscope. Clostridiaceae bacterium, Alpha proteobacterium, Phenylobacterium haematophilum and Rhodobacter were identified as dominant bacteria strains using 16S-rRNA PCR-DGGE. The active sludge tanks were found to play a significant role in the reduction of excess sludge.

Efficiency of zero valent iron in the modified Fenton process for the reduction of excess sludge and the key role of citric acid through deflocculation

Efficiency of zero valent iron in the modified Fenton process for the reduction of excess sludge and the key role of citric acid through deflocculation

Comparison of Excess Sludge Reduction Effect of Dichlorophenol Isomers

Differences in sludge reduction effect due to chlorinated sites of dichlorophenol (DCP) isomers used in an uncoupling method were investigated. The study involved two different experiments. A batch experiment conducted to examine the effect of DCPs on SOUR clearly indicated that 3,5-DCP exhibited the strongest uncoupling activity. In the second experiment, in which six different sequence batch reactors were amended with 100 mM DCPs and operated for 2 months, 3,5-DCP also exhibited the strongest sludge reducing effect. The operations examined in this research maintained the concentration of DCPs at 100 mM, and reactors added with 3,5-DCP produced a COD removal rate of >80%, with almost no excess sludge generated in 2 months of continual operation. These results were not obtained with other DCPs. This is the first study to report differences in the sludge reduction effect associated with the chlorination site in the DCP isomer. The results indicate that 3,5-DCP is the most effective option for use in uncoupling methods involving DCPs.

In situ excess sludge reduction in SBBR through uncoupling of metabolism induced by novel aeration modes

To minimize sludge yield (Yobs) in bench-scale sequencing batch biofilm reactors (SBBRs) during wastewater treatment, novel aeration modes were employed: triangular-wave aeration, square-wave aeration, sawtooth-wave aeration and continuous aeration.

Sludge disintegration using a hydrocyclone to improve biological nutrient removal and reduce excess sludge

Abstract Sludge disintegration can simultaneously improve biological nutrient removal and reduce excess sludge. A novel sludge disintegration method is proposed wherein a hydrocyclone is used for internal release of carbon source. The effect of hydrocyclone disintegration on mixed liquor recirculation was studied in a side-stream device of an anoxic/aerobic (A/O) wastewater treatment plant (WWTP). The mechanisms of hydrocyclone disintegration were comprehensively investigated, and its energy consumption was compared with those of other disintegration methods. The sludge disintegration degree (DD) reached 6.66–12.25% in the hydrocyclone processed mixed liquor recirculation, leading to a significant increase in the concentration of soluble chemical oxygen demand (SCOD), protein and polysaccharide. However, the sludge size distribution and the stable structure for microorganism aggregation and cell attachment changes only slightly. The sludge disintegration process using the hydrocyclone involves sheared shedding, rotation desorption and centrifugation lysis which are mainly induced by disordered turbulence. The biological degradation experiments with continuous modes revealed significant improvements in microbial activity in disintegrated mixed liquor recirculation. Similarly, the averaged denitrification rate of disintegrated mixed liquor recirculation increased by 14.43% in batch experiments. The continuous operation indicated that the observed biomass yield and efficiency of excess sludge reduction of hydrocyclone-improved A/O process were 0.34 kg VSS/kg COD and 30.61%, respectively. Compared with other disintegration methods, hydrocyclone disintegration exhibited less intensity but lower energy consumption, which indicated that this method shows promises in advanced nitrogen removal and excess sludge reduction.

Reduction of excess sludge in a sequencing batch reactor by lysis-cryptic growth using quick lime for disintegration under low temperature

ABSTRACTIn the present study, quick-lime-based thermal-alkaline sludge disintegration (SD) under low temperature was combined with cryptic growth to investigate the excess sludge reduction efficiency in the sequencing batch reactor (SBR). The optimized condition of SD was as follows: T = 80℃, pH = 11, t = 180 min, and the SD rate was about 42.1%. With 65.6% of excess sludge disintegrated and returned to the SBR, the system achieved sludge reduction rate of about 40.1%. The lysis-cryptic growth still obtained satisfactory sludge reduction efficiency despite the comparative low SD rate, which suggested that disintegration rate might not be the decisive factor for cryptic-growth-based sludge reduction. Lysis-cryptic growth did not impact the effluent quality, yet the phosphorus removal performance was enhanced, with effluent total phosphorus concentration decreased by 0.3 mg/L (33%). Crystal compounds of calcium phosphate precipitate were detected in the system by Fourier transform infrared spectroscopy and X-ray diffraction, which indicated the phosphorus removal potential of SD using lime. Moreover, endogenous dehydrogenase activity of activated sludge in the lysis-cryptic system was enhanced, which was beneficial for sludge reduction. SD and cryptic growth in the present study demonstrates an economical and effective approach for sludge reduction.

Improvement of an integrated system of membrane bioreactor and worm reactor by phosphorus removal using additional post-chemical treatment.

A membrane bioreactor (MBR) coupled with a worm reactor (SSBWR) was designed as SSBWR-MBR for sewage treatment and excess sludge reduction. However, total phosphorus (TP) release caused by worm predation in the SSBWR could increase the effluent TP concentration in the SSBWR-MBR. To decrease the amount of TP excreted, chemical treatment reactor was connected after the SSBWR-MBR to remove the excess phosphorus (P). The effects of chemical treatment at different time intervals on the performance of the SSBWR-MBR were assessed. The results showed that a maximum TP removal efficiency of 21.5 ± 1.0% was achieved in the SSBWR-MBR after chemical treatment. More importantly, a higher sulfate concentration induced by chemical treatment could promote TP release in the SSBWR, which provided further TP removal from the SSBWR-MBR. Additionally, chemical oxygen demand (COD) removal efficiency of the SSBWR-MBR was increased by 1.3% after effective chemical treatment. In the SSBWR-MBR, the chemical treatment had little effects on NH3-N removal and sludge production. Eventually, chemical treatment also alleviated the membrane fouling in the SSBWR-MBR. In this work, the improvement on TP, COD removal and membrane fouling alleviation was achieved in the SSBWR-MBR using additional chemical treatment.

Single-cell analysis of the disruption of bacteria with a high-pressure jet device: An application of atomic force microscopy

The goal of this study was to reveal the mechanism of bacterial cell disruption by a high-pressure jet device (HPJD) for the reduction of excess sludge produced during wastewater treatment. Escherichia coli and Bacillus subtilis, representative Gram-negative and Gram-positive bacteria respectively, were subjected to HPJD treatment followed by observation of the damaged cells with epifluorescence microscopy and atomic force microscopy (AFM), as well as spectrophotometric quantification of the released intracellular polymeric compounds. The AFM observations demonstrated that the types of cell damage incurred by E. coli and B. subtilis were distinct. B. subtilis showed modest cell disruption comprising partial damage, i.e., the formation of pores and cracks on the cell envelope. In contrast, E. coli showed more extensive damage, i.e., cell cleavage and lysis, in addition to the partial damages observed for B. subtilis. The damaged cells of both bacterial species also exhibited leakage of intercellular compounds such as DNA, proteins and polysaccharides. This loss of intracellular macromolecules resulted in increased cell surface roughness and decreased cell diameter and elasticity, irrespective of the bacterial species tested. The results demonstrated that the degree of cell damage inflicted by HPJD treatment is dependent on the cell wall and membrane structure of the bacterial species. In summary, we propose multiple possible mechanisms of cell disruption resulting from HPJD treatment.

Effects of 3,5-dichlorophenol on excess biomass reduction and bacterial community dynamics in activated sludge as revealed by a polyphasic approach

The effects of 3,5-dichlorophenol (DCP) on excess sludge reduction and microbial community dynamics were studied using laboratory-scale activated sludge reactors. The addition of 3,5-DCP at an interval of 7-8 days of operation resulted in effective reduction of growing biomass without a significant decrease in substrate removal activity. However, this uncoupling effect completely disappeared after 30 days of operation. Quinone profiling showed that a drastic component shift from ubiquinone-8 (Q-8) to Q-10 as the major homolog took place during this period of operation, suggesting that Q-10-containing bacteria, i.e., Alphaproteobacteria, became predominant at the uncoupler-ineffective stage. This result was supported by PCR-aided denaturating gradient gel electrophoresis and clone library analyses of 16S rRNA genes and fluorescence in situ hybridization. Among the gene clones detected, those corresponding to Brevundimonas predominated at the uncoupler-ineffective stage. The uncoupler-added reactor yielded 3,5-DCP-resistant Pseudomonas strains as the predominant cultivable bacteria and non-3,5-DCP-resistant Brevundimonas strains as the second most abundant isolates These results suggest that the disappearance of the uncoupling function of 3,5-DCP during the long-term operation of the reactor is related to the drastic community change with increasing populations of Alphaproteobacteria. Most of these alphaproteobacteria represented by Brevundimonas are not resistant to 3,5-DCP but, by an unknown mechanism, may support the bioprotection of the microbial community from the uncoupling effect.

Anaerobic side-stream reactor for excess sludge reduction: 5-year management of a full-scale plant

The long-term performances of a full-scale anaerobic side-stream reactor (ASSR) aimed at sludge reduction have been monitored for the first time, in comparison with a conventional activated sludge process (CAS). The plant was integrated with an ASSR treatment of 2293–3293 m3. Operational parameters in the ASSR were: ORP -250 mV, interchange ratio of 7–10%, hydraulic retention time of 7 d. No worsening of effluent quality was observed in the ASSR configuration and removal efficiency of COD and NH4 was above 95%. A slight increase in the Sludge Volume Index did not cause worsening in effluent solids concentration. The observed sludge yield (Yobs) passed from 0.44 kgTSS/kgCOD in the CAS to 0.35 in the ASSR configuration. The reduction of Yobs by 20% is lower than expected from the literature where sythetic wastewater is used, indicating that sludge reduction efficiency is largely affected by inert mass fed with influent real wastewater. An increase by 45% of the ASSR volume did not promote a further reduction of Yobs, because sludge reduction is affected not solely by endogenous decay but also by other factors such as interchange ratio and aerobiosis/anaerobiosis alternation.

Degradation of the unbiodegradable particulate fraction (XU) from different activated sludges during batch digestion tests at ambient temperature

One strategy for the management of excess sludge in small wastewater treatment plants (WWTPs) consists in minimizing the excess sludge production by operating the WWTP at very long solids retention times (SRTs > 30 days). A number of recent studies have suggested that sludge minimization at very long SRT results from the degradation of the unbiodegradable particulate fraction (XU) (influent unbiodegradable compounds and endogenous decay products). But the biodegradability of the unbiodegradable particulate fraction has only been evaluated during batch digestion test performed at ambient temperature with sludge fed with synthetic wastewaters. It is not clear to what extent observations made for sludge fed with synthetic influents can be transposed to sludge fed with real influent. The current study thus focused on evaluating the biodegradability of the unbiodegradable particulate fraction for sludge fed with real wastewater. Batch digestion tests (400 days, ambient temperature) were conducted with three different sludges fed with either synthetic or real influents and exposed to aerobic or intermittent aeration conditions. Our results indicate that volatile suspended solids (VSS) decreased even after complete decay of the active biomass (i.e., after 30 days of aerobic batch digestion) indicating that the unbiodegradable particulate fraction is biodegradable. However, very low degradation rates of the unbiodegradable particulate fraction were monitored after day 30 of digestion (0.7–1.7·10−3 d−1). These values were in the lower range of previously published values for synthetic wastewaters (1–7.5·10−3 d−1). The low values determined in our study indicate that the rate could decrease over time or that sludge composition influences the degradability of the unbiodegradable particulate fraction. But our results also demonstrate that extracellular polymeric substances (EPS) have a minor impact on the biodegradability of the unbiodegradable particulate fraction. Overall bound EPS were indeed biodegradable under all conditions and thus did not accumulate in the unbiodegradable particulate fraction. Different bound EPS pools (e.g., cation bound EPS) were associated with specific degradation behaviors. Besides improved mechanistic understanding of sludge degradation processes, our results have implications for the development of decentralized wastewater treatment technologies with on-site reduction of excess sludge.

Alkaline and acid thermal hydrolysis of biological excess sludge in sequencing batch reactor

The efficiency of biological wastewater treatment processes is based on pH sensitivity which leads to the reduction in the amount of sludge production. In order to achieve the best result, thermal hydrolysis is recognized and used, although acid thermal hydrolysis has serious drawbacks (corrosion, required post-neutralization, etc.). Alkaline thermal hydrolysis has been less remarkable, as the subject of the detailed pilot-scale research of two sequencing batch reactors (SBR) reported in this study. Long-term (about 8 months) continuous experiments were conducted to identify the effect of pH increasing from 4 to 11 in controlled temperature of 60°C to achieve the optimum pH for the reduction of excess biological sludge. After providing a steady state in the reactors, sampling and testing parameters including chemical oxygen demand (COD), mixed liquor suspended solids (MLSS), dissolved oxygen (DO), specific oxygen uptake rate (SOUR), sludge volume index (SVI), and yield (Y) coefficient were evaluated. Results showed that in pH of 9, the kinetic yield coefficient decreased from 0.63 to 0.33. Therefore, excess sludge declined to 44%. Moreover, the soluble COD increased slightly in the effluent, whereas the removal percentage decreased to 79 in the reactor while the amount of SVI and SOUR in this pH dropped to 65 mg/l and 12 mgO2/l.g VSS, respectively. However, no sludge was seen in the higher pH or lower pH, whereas effluent soluble chemical oxygen demand (SCOD) and turbidity was increased. Additionally, the wastewater disposal standards were not achieved and had a bad odor.