Low Sludge Volume Index Research Articles

Synergistic role of powdered activated carbon (PAC) for aerobic granular sludge (AGS) formation and organic micropollutant (OMPs) removal

Aerobic granular sludge (AGS) is a spherical aggregate formed by the self-aggregation of activated sludge. AGS consists of diverse microbial groups, not only shortening the settling time but also enhancing treatment efficacy within a shorter reaction time. However, it necessitates a longer start-up time for granule formation. This study aims to investigate the synergistic effects of powdered activated carbon (PAC) on AGS formation in a sequencing batch reactor (SBR) and its potential for maintaining biodegradability in the presence of inhibitory organic micro-pollutants (OMPs). PAC substantially supported the formation of faster and more stable granule (PAC-AGS) with a very low sludge volume index (SVI < 50 mL/g). Morphological observation indicated that PAC-AGS is more compact than AGS without PAC addition, and PACs were broadly present inside and on the surface of PAC-AGS granules. Respirometric bioassay demonstrated that biodegradation of organic matter in the presence of OMP, e.g., diclofenac and isoproturon, was enhanced when PAC-AGS was used as seed instead of AGS due to the alleviated inhibition of those OMPs by PAC. The overall result illustrated that PAC has dual positive contributions to AGS reactor for wastewater treatment by accelerating the formation of AGS and enhancing removal performance even in the presence of inhibitory OMPs.

Achieving mainstream partial nitritation with aerobic granular sludge treating high-rate activated sludge effluent

New configurations for wastewater treatment plants (WWTPs) are under development to increase their self-sufficiency and sustainability. One is the A-B process, in which the A-stage is designed to maximize the redirection of influent chemical oxygen demand (COD) to the production of biogas. A promising A-stage alternative is the high-rate activated sludge (HRAS). B-stage, on the other hand, can consist of the combined partial nitritation (PN)-anaerobic ammonium oxidation (anammox). In this study, PN with aerobic granular sludge (AGS) was attempted under mainstream conditions at pilot scale. The effluent of a HRAS system – ideally operating at 2 g/L of total suspended solids, 0.5 mg/L of dissolved oxygen and 1 h of hydraulic retention time (52 ± 13 % COD removal) – was fed to the PN-AGS reactor initially inoculated with floccular sludge. Granulation and PN were studied with daily/seasonal fluctuations and without adding external reagents. Selective wasting from the top of the settled sludge bed was found a successful strategy to trigger granular sludge formation. The maintenance of a low sludge volume index (SVI < 100 mL/g) was helpful enough for improved operation of the system (i.e., COD removal and PN). Stable PN was achieved due to the suppression of the nitrite oxidizing bacteria activity by inhibitory levels of free nitrous acid. Finally, working with fast settling biomass at long settling times reduced the influent-biomass-nitrite contact and, thus, denitritation.

Assessment of Cu(II) impact on aerobic sludge biomass and its post-exposure self-recovery potential

Heavy metals are released from various sources, including metal plating, potentially at a small-scale industrial level, and from automobile service and repair workshops, paint works, etc., spread across the city. Being part and parcel of the city, the wastewater from these metallic sources finds its way into the sewage system and can potentially disrupt biological sewage treatment plants. Therefore, conducting health and quality assessments of the sludge biomass becomes crucial to ensure the effective operation of these treatment plants and to maintain environmental and public health standards. In this study, impacts of Cu(II) (10, 20, 40, and 80 mg/L) exposure on sludge biomass have been investigated, and the potential of self-recovery of sludge biomass was explored when Cu(II) exposure is stopped. Reactors have been operated for 0–25 d with Cu(II) (termed as stressed phase) and then another 29 d without Cu(II) (termed as recovery phase) in the feed. During stressed phase operation, Cu(II) stressed reactors have shown 39–75 % chemical oxygen demand (COD) removal, compared to 91 % for control. Biomass activity and ECatalase activity have been inhibited under Cu(II) stress with changes in microbial community structure. During the recovery phase, Cu(II) stressed reactors have shown 41–82 % COD removal, whereas the control reactor has shown 91 % COD removal. A low sludge volume index (SVI) and high biomass interface settling velocity (ISV) with increased biomass activity have been obtained for biomass exposed to 10 and 20 mg/L of Cu(II). Lower Cu(II) doses have led to partial recovery of enzyme activity and microbial community structure, but higher doses have shown incomplete recovery. The study shows the impact caused by Cu(II) metal in a biological wastewater treatment reactor. Through comprehensive analysis of basic and advanced physico-chemical and biological parameters, the study recommends preliminary and confirmatory tests to know metallic ingress through wastewater influent in a treatment system. Advanced knowledge about metallic ingress will be useful for operators to take necessary steps for the sustainable operation of wastewater treatment systems.

2D and 3D electrochemical degradation (ECD) of raw cotton industry wastewater (CIWW) using stainless steel and aluminium electrodes.

Two-dimensional (2D) and three-dimensional (3D) batch electrochemical degradation (ECD) of raw cotton industry wastewater (CIWW) was adopted using stainless steel (SS) and aluminium (Al) electrodes. ECD as a treatment option was aimed at removing priority quality parameters, viz. chemical oxygen demand (COD), colour, chloride, nitrate, etc. COD removal of 85 and 80% were achieved by using 3D SS and 2D SS electrodes operated at 6 V (0.9 A) for a maximum electrolysis time (ET) of 30 min. Similarly, 76 and 70% COD removal were achieved for 3D Al and 2D Al electrodes, respectively. Simultaneous colour removal in the 2D ECD system using SS and Al electrodes was low by 12 and 11% compared to the 3D ECD system. Water quality parameters, viz. total dissolved solids, chloride, nitrate, phosphates, and sulphate were also removed by 3D (SS and Al) and 2D (SS and Al) electrodes. Higher pollutant removal efficiencies were observed at 30 min ET for 3D SS electrodes compared to 2D SS, 3D Al, and 2D Al. Post-ECD slurry showed good settling characteristics for SS electrodes generating dense and sturdy flocs giving a low sludge volume index values for 2D SS electrodes compared to other electrode options.

Densification of activated sludge for better settling performances: experimental characterization in batch column and model parameters calibration.

Conventional activated sludge (CAS) and densified sludge obtained using hydro-cyclone selective wasting were compared at a full-scale water resources recovery facility. The densified tested sludge, containing around 30-50% of aerobic granules, showed enhanced settleability with low and stable sludge volume index (SVI) compared to CAS, which suffered recurrent filamentous bulking. Further in-depth batch settling tests were carried out using a 40 cm diameter column fitted with ultrasonic transducers to monitor both sludge blanket height and vertical velocity profiles. Hindered settling and compression parameters were calibrated from the experiment for latter modelling use. Test sludge displayed more than doubled settling velocities compared to CAS, with hindered settling velocities remaining >3 m·h-1 even at high solids concentrations of 6.85 g·L-1. The compression regime was attained at much higher critical concentration for the test sludge. It also displayed enhanced thickening properties, with concentrations obtained after 30 min of settling being 20.9 and 8.5 g·L-1 respectively for test and control sludge. This allows for a substantial reduction of recirculation rates in practice. These results open perspectives in optimizing existing plant operation as well as clarifier design and modelling using densified sludge.

Gravimetric selection of activated sludge for settling properties improvement and granular sludge formation – full scale case study

Abstract With an increasing frequency of extreme weather events wastewater treatment plant operators face a need to increase plant hydraulic capacity while continuing to comply with effluent quality requirements. An economically attractive way to meet this challenge is to significantly improve secondary settling tank performance and therefore assure safe wet weather plant operation. The objective of this study was to evaluate gravimetric selection technology conducted in hydrocyclones as a way for improving activated sludge settling characteristics in continuous flow BNR with a long sludge retention time and assess its ability to form granular sludge. Long term operational data and sludge morphology monitoring data were analysed. A significant drop in sludge volume index (SVI) (to values under 50 ml/g) and increase in return activated sludge solids concentration (to values above 20 g/L) was observed. Hydrocyclone installation consistently, selectively retained denser, larger flocs in the system and after a start-up phase large granules begin to appear. Gravimetric selection did not prevent the seasonal filamentous biomass outgrowth and temperature drop at the beginning of winter resulted in rapid rise of SVI and SST sludge blanket height especially after heavy rain. Technology under study proved to be effective under certain process conditions but it needs further research to consistently maintain low SVI values throughout the whole year.

Improvement of biomass aggregation in sludge bulking by magnetic field application

AbstractHigh biomass aggregation is commonly associated with good solid–liquid separation that can inhibit the occurrence of sludge bulking. The application of magnetic fields in the removal of pollutants has been studied previously but the relation between a magnetic field with biomass aggregation and its effect on pollutant removal efficiency receives very little attention. This present study was conducted to analyze the effect of the magnetic field toward biomass aggregation that can eventually contribute to the good removal performance of the pollutants. Two continuous laboratory‐scale sequencing batch reactors known as Reactor A and Reactor B were set up. Reactor A was equipped with an 88 mT magnetic field while Reactor B acted as the control. Reactor A recorded higher biomass aggregation with the range of 75.0–95.2% compared to Reactor B which possessed mostly medium aggregation with an average of 65.5%. As a result, lower sludge volume index (SVI) was recorded in Reactor A (25.8 ml/g) compared to Reactor B (73.1 ml/g). Likewise, Reactor A showed significantly higher chemical oxygen demand (COD) and ammonia‐nitrogen removal compared to Reactor B. These findings imply that the magnetic field successfully improves biomass aggregation, therefore improving the removal performance under the sludge bulking occurrence.

Organic matter and total nitrogen removal from wastewater using a pilot-scale membrane-aerated biofilm reactor

A pilot-scale membrane-aerated biofilm reactor (MABR) modified with Polyvinyl Alcohol (PVA) gel bio-carriers containing denitrifying bacteria was operated with the purpose of enhancing total nitrogen removal efficiency from actual domestic wastewater. MABR system was operated under 6, 9 and 12 h of hydraulic retention times (HRT) in different time periods. Addition of the bio-carriers increased nitrification and total nitrogen removal efficiencies by maximum of 11% and 18% at 12 h of HRT respectively. Maximum of 88% of nitrification and 79% of total nitrogen removal efficiencies were reported in the PVA gel modified MABR. Organic matter removal performance was mostly maintained above 90% with insignificant improvements. In addition to this, natural pH increase observed after PVA gel addition paves a new way for potential cost reduction in industrial scale MABR applications. Low sludge volume index around 80 mL/g indicated rapid settling characteristics of produced sludge.

Effect of the famine phase length on the properties of aerobic granular sludge treating greywater

Abstract Sequencing batch reactors (SBR) treating high-strength greywater need an aerobic granular sludge (AGS) with good properties, such as a low sludge volume index (SVI) and high settling velocities and substrate uptake rates to yield short settling and aeration stages. To promote the formation of stable granular sludge, the length of the famine phase could be a key factor. In this regard, the effect of the duration of this variable on the AGS properties was assessed by comparing a gradual versus an abrupt reduction of the famine phase in two SBR treating greywater. The initial average famine phase of 3.3 h was gradually reduced to 0.3 h over 20 weeks in one reactor, and abruptly in another one. This condition induced filamentous outgrowth, as well as the deterioration on the properties of the sludge; the effect was more accelerated when the famine periods were abruptly shortened. In both cases the reduction on the famine periods induced increased organic loading rates, which led to degranulation events when it was higher than 2.5 g-chemical oxygen demand g-volatile suspended solids−1 d−1. Afterwards, the biomass adapted to this situation, by forming new small-filamentous aggregates with similar SVI to that of the stable AGS formed with the longest famine period.

Rapid start-up of an aerobic granular sludge system for nitrogen and phosphorus removal through seeding chitosan-based sludge aggregates

This study presents a novel strategy to accelerate the start-up of aerobic granular sludge (AGS) system and ensure the nutrient removal during cultivation. This new method consists of preparing the chitosan-based sludge aggregates outside the reactor and then seeding the reactor with such sludge aggregates. To prepare chitosan-based sludge aggregates, chitosan was dissolved in acetic acid solution acting as a cationic flocculant to bind negatively charged sludge together, and then the dissolved chitosan was in situ precipitated by readjusting pH to form stable sludge aggregates. The chitosan-induced charge neutralization and water-insolubility of chitosan were the two main reasons for the super-rapid formation of chitosan-based sludge aggregates. The as-prepared chitosan-based sludge aggregates had a much lower sludge volume index at 30 min (SVI30) (90.1 mL/g) than the original sludge (SVI30 = 328.0 mL/g). They also had some AGS-like characteristics such as large particle size (1300 μm) and fast settling velocity (23.8 m/h). Consequently, short settling time can be achieved and excessive biomass wash-out can be avoided in the rapid start-up of AGS system with chitosan-based sludge aggregates as inoculant, which was beneficial to accelerating sludge granulation while maintaining nutrient removal. Additionally, the abundances of filamentous bacteria and Candidatus Accumulibacter and the content of extracellular polymeric substances increased during cultivation, which could also contribute to the AGS formation. By seeding chitosan-based sludge aggregates in the anaerobic/oxic sequencing batch reactor, complete granulation was rapidly achieved in 10 days, and good removals of nitrogen and phosphorus was obtained after 14–18 days of cultivation.

Membrane fouling suppression using intermittent electric current with low exposure time in a sequencing batch membrane bioreactor

Abstract This study assessed for the first time the use of intermittent electric current with low exposure time in a sequential batch membrane bioreactor (SB-MBR) aiming at reducing the membrane fouling and improving the treatment performance. The SB-MBR was initially operated without electric current application (control period) and, subsequently, using the electric current density of 10 A m−2 and intermittent exposure mode of 6 min ON/30 min OFF (1.2 h d−1). Compared to the control period, the mixed liquor filterability improved substantially when the electrocoagulation was applied, resulting in a membrane fouling rate 65% lower. This better filterability condition was attributed to the improvement of the floc formation process, which resulted in larger floc size and lower sludge volume index. The biomass growth was positively impacted by the electrochemical process, resulting in a higher sludge yield coefficient. The reactor performance regarding COD and NH4+-N removal was not influenced by the electric current application, exhibiting efficiencies above 99% in both experimental periods. On the other hand, the phosphorus concentration in the reactor effluent decreased as the electrocoagulation was applied, increasing its average removal efficiency from 62% (control period) to 89%. The anodic dissolution rate was estimated at 1.28 g d−1, considerably lower than the values reported in other studies regarding EBRM running in continuous mode. Thus, it can be inferred that the integration of the electrochemical process with SB-MBR using low exposure mode is an interesting alterative to reduce the membrane fouling, extend the anode lifespan and improve the reactor performance.

Flocs in disguise? High granule abundance found in continuous-flow activated sludge treatment plants

To date, high performance of full-scale aerobic granular sludge (AGS) technology has been demonstrated on a global scale. Its further integration with existing continuous flow activated sludge (CFAS) treatment plants is the next logical step. All granular sludge reactors operated in sequencing batch reactors (SBR) mode with anaerobic feeding conditions select for growth of phosphorus and glycogen accumulating organisms (PAO and GAO, respectively), which are known to enhance sludge settling characteristics. Therefore, we hypothesized that AGS are commonly present at full-scale CFAS processes with enhanced biological phosphorus removal (EBPR) and low sludge volume index (SVI). This hypothesis was confirmed at 13 EBPR plants, where granules were found present (at plants where SVI was lower than 100 ml/g) with a strong correlation between high granule abundance and low SVI. A wide range of granule abundance was found among the plants, ranging from 0.5% to as high as 80%. Evaluations of the EBPR plant process configurations showed that high granule abundances may be related to selector design features such as high anaerobic food to mass (F/M) ratios, unmixed in-line fermentation, and high influent soluble COD fraction. Granules were also observed at a non-EBPR plant with an aerobic selector receiving high F/M feeds. Quantitative PCR and 16S rRNA gene sequencing analyses revealed higher relative gene abundance of Accumulibacter PAO and Competibacter GAO in the granules over flocs, as well as a correlation between granule abundance and some possible EPS producers such as Flavobacterium and Competibacter. Our results indicated that process configurations that select for slow-growing or EPS-producing heterotrophs play an important role for granule formation in full-scale CFAS systems as previously shown in SBR configurations.

Elucidating sludge characteristic, substrate transformation and microbial evolution in a two-sludge denitrifying phosphorus removal system under the impact of HRT

Granule formation has been recognized as a promising biotechnology in denitrifying phosphorus removal (DPR) systems by facilitating phosphorus accumulation organisms (PAOs) especially denitrifying PAOs (DPAOs), and hydraulic selection made this a more difficult task in continuous operation. This study aimed at exploring the microscopic mechanism and putting forward an effective strategy for DPR granulation under the impact of hydraulic retention time (HRT) (12 h, 10 h, 8 h) in a novel Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor (A2/O - MBBR) system. With the reduction of intracellular carbon storage (CODintra) efficiency (88.58%–78.53%), nitrogen (N) (85.45%–79.11%) and phosphorus (P) (96.55%–92.47%) removals both dropped, but it exhibited a growth of anoxic phosphorus uptake rate (PURA) (3.79–5.68 mg P/(gMLVSS·h)). The batch tests associating with substrate transformation of poly-β-hydroxyalkanoates (PHA), glycogen (Gly) agreed well with the corresponding stoichiometry of phosphorus release rate (PRR) (4.83–7.53 mg P/(gMLVSS·h)), PURA (3.55–5.43 mg P/(gMLVSS·h)), oxic phosphorus uptake rate (PURO) (6.08–6.21 mg P/(gMLVSS·h)), and DPAOs/PAOs ratios (57.17%–89.31%), indicating a shift of microbial community. DPR granules gradually stabilized with low sludge volume index (SVI5/SVI30 ratio = 1.1–1.2), dense and compact structure, higher P content (11.63%), more extracted extracellular polymeric substances (EPS) (111.40–160.31 mg/gMLVSS) as proteins/polysaccharides (PN/PS) ratios (1.70–3.47) increased, leading to better sludge settleability and cell hydrophobicity. Fluorescence in situ hybridization (FISH) results showed that PAOs (mainly Cluster I: 20.20%) were the dominant bacteria in the A2/O reactor although a small amount of Defluviicoccus (3.18–3.48%) was responsible for nitrite accumulation, while ammonium-oxidizing bacteria (AOB) (mainly Nitrosomonas: 10.75%) and nitrite-oxidizing bacteria (NOB) (mainly Nitrospira: 15.06%) were enriched in the MBBR.

Cost-effective treatment of sludge conditioning using supernatant fluid polyelectrolyte.

Treatment of activated sludge using pure polyelectrolyte and its supernatant fluids for sludge dewatering was examined. The supernatant was used for treatment of fresh sludge for reducing the total cost of sludge conditioning by almost 50% and had great impact on wastewater treatment especially on that with large flow rate. The utilization of supernatant can be considered as a sustainable and economical method for pollution reduction especially in the developing countries. This would overcome the problem of imperfect mixing, difficulties in operated sludge conditioning, and utilized wasted polyelectrolyte. A slightly high positively charged polyelectrolyte was selected during this study. The results showed this polymer and the supernatant had significant influence on the sludge volume index (SVI) and sludge settling. The SVI and settling were improved by 50% and 60%, respectively. We found the sludge treated with supernatant showed a highest sludge settling properties compared with using pure polyelectrolyte especially for sludge samples with low total suspended solid (TSS) and SVI. Using mixed supernatant with pure polyelectrolyte showed higher settling compared to second treatment. Zeta potentials were measured, and increases in zeta potentials were observed for use of both pure polymer and supernatant fluids. The experiments of using supernatants for sludge treatment were found to reflect the use of waste for waste and principle of circulation cleaning; thus it can be a valuable reference for the researchers working in the field of sludge treatment.

Multivariate data analysis of full-scale sludge dewatering

A multivariate data analysis (MVDA) was conducted to identify operating parameters that affect the sludge dewatering efficiency in a Canadian pulp and paper mill. The sludge is a mixture of waste activated sludge (WAS) and three different types of primary sludge. The dewatering efficiency was defined by three target parameters: the cake solids content after dewatering, the solids capture efficiency, and the costs of dewatering aids (chemicals) used per ton of dewatered sludge solids. The results show that the most influential parameters affecting the dewatering efficiency are 1) the type and amount of primary sludge added to the sludge mix, 2) the operating parameters of the dewatering equipment and 3) the sludge solids throughput. A high proportion of fiber-rich primary sludge in the sludge mix substantially enhances dewaterability. Operating the screw press at a low torque and/or at a high screw press speed lowers the dewatering performance. The pH adjustment of the sludge prior to dewatering also significantly affects dewaterability. The effectiveness of the dewatering aids decreases notably when the pH is not within the target range. Besides the processes in the dewatering plant, the activated sludge process and the associated amount and quality of the WAS also affect the dewaterability of the sludge mix. In particular, the dewatering efficiency seems to be seasonal. The higher temperature during the summer produces less WAS, and thus increasing the proportion of primary sludge in the sludge mix and improving dewaterability. Also, WAS with high microbial activity and firm flocs, as well as a low sludge volume index enhances dewaterability.

Performance of Aerobic Granular Sludge for Domestic Wastewater Treatment

Aerobic granular sludge can be used to treat various types of wastewater, such as industrial, municipal and domestic wastewater. This study investigated the treatment of low-strength domestic wastewater while simultaneously developed aerobic granular sludge in a sequencing batch reactor (SBR). Activated sludge was used as the seeding for granulation. The results indicated good COD and ammoniacal nitrogen removal at 72% and 73%, respectively. Aerobic granular sludge was successfully developed with low sludge volume index (SVI30) of 29 mL/g, which demonstrated an excellent settling property of aerobic granular sludge. Biomass concentration increased significantly compared to the seed sludge, indicating high biomass density in the SBR system. Settling velocity of aerobic granular sludge was significantly higher compared to the conventional activated sludge. This study showed the feasibility of aerobic granular sludge to be developed using low-strength domestic wastewater. Moreover, this study demonstrated the long-term application of aerobic granular sludge in domestic wastewater treatment.

Comparison of rapid granulation developed from the same industrial sludge with two different substrates

The investigation of rapid and stable granule formation seeded with the refinery sludge with two different substrates (phenol and sodium acetate) was carried out in two different sequencing batch reactors (SBRs); R1 and R2. R1 and R2 were operated with phenol; a recalcitrant contaminant, and sodium acetate as a substrate, respectively. Faster granule formation was observed in R1. Mature granules were formed in R1 after 40 days of reactor operation, whereas, in R2, granules took a longer time to develop (50 days). Better granular characteristics were observed in R1 with larger granule size (1123.60 ± 113.55 μm), higher biomass concentration (volatile suspended solids of 3.65 ± 0.09 g L−1), higher extracellular polymeric substances (EPS) (77.26 ± 1.13 mg gVSS−1), higher granule settling velocity and lower sludge volume index (38.02 ± 1.58 m h−1 and 36.42 ± 1.85 mL gTSS−1, respectively). 94% chemical oxygen demand (COD) removal was noticed in R1 as compared to 91% in R2. Nearly complete removal of phenol was obtained in R1. Ammonia nitrogen was almost completely transformed in both of the SBRs. Therefore, this study provides an experimental explanation for faster and better granulation for the treatment of recalcitrant compounds by selecting such an inoculum to which the recalcitrant substrate is previously exposed.

Effect of air flow rate on development of aerobic granules, biomass activity and nitrification efficiency for treating phenol, thiocyanate and ammonium

The impact of air flow rate on aerobic granulation was evaluated for treating toxic multiple pollutants; phenol (400 mg L−1), thiocyanate (100 mg L−1) and ammonia nitrogen (100 mg L−1) by using three lab scale sequencing batch reactors (SBRs) (R1, R2 and R3). Larger granules (2938.67 ± 64.91 μm) with higher biomass concentration (volatile solids of 4.17 ± 0.09 g L−1), higher granule settling velocity (55.56 ± 1.36 m h−1) and lower sludge volume index (35.25 ± 1.71 mL gTSS−1) were observed at optimal air flow rate of 2.5 L min−1 (R2). Confocal laser scanning microscopic images illustrated the extended fluorescence for extracellular polymeric substances in R2. In R2, partial nitrification was achieved. Phenol was completely removed in all the reactors while partial removal of SCN− and no nitrification were observed with a decrease (1.5 L min−1) and an increase (3.5 L min−1) in air flow rates (R1 and R3, respectively). This study provides an experimental contribution to examine the effect of optimal combination of aeration and toxic multiple pollutants, governing characteristics and nitrification efficiency of granules along with SBR performance in an economic way in terms of optimal air supply.

A low sludge generated anode by hybrid solar electrocoagulation for the removal of lead

In this work, perforated zinc is proposed as a new anode for lead removal by hybrid solar electrocoagulation. The characteristics of the sludge were investigated to understand the behaviour of lead removal during electrocoagulation. Sludge products formed were characterised using X-ray diffraction (XRD), X-ray fluorescence (XRF) and Field Emission Scanning Electron Microscopy (FESEM). In addition, the pH variation during electrocoagulation and effects on the sludge products were examined. At optimum conditions showed that the perforated zinc electrode produced better performance with high removal efficiency, low sludge volume index and less energy consumption.

Laboratory-scale assessment of vacuum-degassed activated sludge for improved settling properties

ABSTRACTVacuum degassing of activated sludge was tested at eight different Swedish wastewater treatment plants with laboratory-scale equipment in batch mode in order to evaluate its efficiency on improvement of sludge compaction and settling properties. The results show that the efficiency of the degassing technique is mainly dependent on the initial sludge volume index (SVI) of the target sludge which was found to be related to its process configuration. Facilities with full activated sludge-based nitrogen removal processes, including both nitrification and denitrification, had high SVIs (>300 mL g−1) and were strongly affected by vacuum degassing with reduction of SVI up to 30%. Nitrogen removal facilities also including biological phosphorus removal showed better compaction and settling properties with relatively lower SVIs and were affected to a lesser extent by degassing with SVI reduction of 10–20%. Wastewater treatment plants without full biological nitrogen removal, lacking either nitrification or denitrification (or both) processes in the activated sludge had the lowest SVIs observed with almost no effect of vacuum degassing.