Sludge Density Research Articles

Forming densified activated sludge in a modified simultaneous nitrification–denitrification and phosphorus removal (SNDPR) system by introducing shift work mode

Forming granular-like sludge in the Simultaneous Nitrification-Denitrification and Phosphorus Removal (SNDPR) process would promote nutrient removal and improve the sludge settleability. This study introduced the “shift work mode” into an SNDPR system, which featured that 50 % of sludge was temporarily transferred into and “rested” in a side-stream reactor a cyclic time rather than let it keep “working” in the mainstream SNDPR reactor. Results showed that 77.6 % of inorganic nitrogen and 80.6 % of orthophosphate were removed well even after the combination, which attributed to the co-work of Candidatus_Competibacter (10.1 %–29.0 %), nitrifiers, and phosphate accumulating organisms. The 30-min settled sludge volume and sludge volume index were 8 % and 45.8 mL/g, respectively, the sludge density was 1.045 g/mL, and cumulative 90 % of the particle diameter was less than 80 µm; these indicated the densified activated sludge was formed. After analyses, the increase in the food-to-microorganisms ratio and temporary enrichment of filamentous bacteria Thiothrix might be the causes. Such shift work mode promisingly serves as a simple and practical way to improve settleability and promote granulation.

The impact of perfluorooctanoic acid shock on hydrogen-driven nitrate and arsenate removal

Perfluorooctanoic acid (PFOA) is a type of toxic per- and poly-fluoroalkyl substance (PFAS) commonly found in groundwater due to its use in firefighting and industrial applications. The main purpose of this study was to investigate the influence of PFOA shock on the biological performance of a hydrogen-driven bioreactor for nitrate and arsenate removal. Four hydrogen-driven removal reactors (HdBRs) used for the simultaneous removal of nitrate and arsenal were operated with concentrations of either 0, 1, 5, and 10 mg/L of PFOA to induce shock on the systems and examine the corresponding bacterial response. Our results showed that PFOA shock inhibited and decreased the maximum hydrogen-driven arsenate removal rate. Principal Component Analysis (PCA) confirmed that this performance decrease occurred due to a bacterial strike triggered by PFOA shock. PFOA toxicity also led to protein secretion and sludge density decreases. Bacterial analyses showed shifts in the community population due to PFOA shock. The dominant bacteria phylum Proteobacteria became more abundant, from 41.24% originally to 48.29% after exposure to 10 mg/L of PFOA. Other phyla, such as Euryarchaeota and Bacteroidetes, were more tolerant to PFOA shock. Although some of the predominant species within the sludge of each HdBR exhibited a decline, other species with similar functions persisted and assumed the functional responsibilities previously held by the dominant species.

Two-Dimensional Modelling of Sludge Heat and Mass Transfer in a Paddle Dryer

In this paper, we propose a concise but general two-dimensional model based on the penetration model in order to simulate the heat and mass transfer and drying kinetics of sludge in the paddle dryer. The sludge control equations were developed to simulate the heat and mass transfer of sludge and the penetration model was introduced to describe the mixing of sludge. Compared to the experimental results in the existing literature, the drying kinetics simulated simulated by the present model were in good agreement with the experimental data under various operation conditions. The effects of four key parameters on drying kinetics in the paddle dryer were studied. The results showed that the water content increased with the increase of sludge flowrate and the decrease of wall temperature and sludge density. Furthermore, the stirring velocity had little effect on the drying kinetics of the sludge. The present study contributes to the better understanding of the sludge drying kinetics and provides guidance for the optimal design of industrial paddle dryers.

Closed aerobic biodegradation kinetics test with activated sludge and low concentration chemical mixtures

The activated sludge process at wastewater treatment plants is important to prevent discharge of organic pollutants to the environment. Determination of biodegradation kinetics in activated sludge is challenging for mixtures that cover a diverse range of structures. The aims of this study were to (1) design a closed aerobic biodegradation batch test with activated sludge and (2) develop a sample preparation procedure that is compatible with LC-MS and Solid Phase Microextraction (SPME) coupled to GC-MS. A headspace:sludge ratio of 4:1 was sufficient to ensure aerobic conditions in activated sludge for 7 days at co-solvent concentrations <0.01%. Ethanol was added to sub-samples (50%) to stop biodegradation, extract sorbed chemicals and allow storage at −18 °C without ice formation. The ethanol extracted the chemicals from the sludge before filtration (0.2 μm). The filtrate was diluted in ultrapure water to <12% ethanol before analysis by SPME GC-MS/MS and was suitable for direct injection on LC-MS/MS. Biodegradation was distinguished from sorption through abiotic controls using autoclaved poisoned sludge. Linalool, naphthalene, α-isomethylionone, phenanthrene, citronellol, drometrizole, 2-ethylhexyl 4-methoxycinnamate, dicyclohexyl phthalate, BP-1, BP-3, methyl-, ethyl-, propylparaben, alkyl sulfates and isethionates degraded within 48 h in activated sludge, while musk ketone, tonalide and 1,3,5-trichlorobenzene did not. A 10 times reduction of sludge density did not markedly affect the microbial diversity but slowed biodegradation kinetics (partly explained by theory). This study demonstrated a ‘cold’ alternative to an OECD 314b test and how biodegradation kinetics can be determined for mixtures of diverse chemicals in closed batch tests with activated sludge.

Inorganic composite coagulant for wool scouring wastewater treatment: Performance, kinetics and coagulation mechanism

Wool is perceived to be a clean and green natural fibre. However, raw wool contains substantial amounts of contaminants that must be removed in a wool scouring process before the wool can be used as a textile fibre. Coagulation is the first step for scouring wastewater treatment. But the role of inorganic coagulants in increasing sludge density for scouring wastewater treatment remains unclear. This work systematically prepared a series of inorganic composite coagulants (polyaluminium ferric silicate (PAFS)) and investigated their efficiency for treating wool scouring wastewater. Compared with conventional coagulants, polyaluminium chloride (PAC) and polyferric sulfate (PFS), PAFS showed improved coagulation performance over a wider pH range (3–7), achieving higher removal rates of 97 % and 99 % for chemical oxygen demand (COD) and turbidity, respectively. In addition, PAFS showed its advantages for practical wastewater treatment, including fast floc settling rate, increased sludge density (with a reduced sludge volume) and low floc moisture content. This optimized PAFS was successfully obtained in a scaled-up process. Zeta potential tests revealed that PAFS coagulated colloidal particles by the synergistic action of electro-neutralization and adsorption bridging. This work has practical implications for increasing the efficiency of wastewater treatment and reducing the environmental impact of wool scouring.

Preparation of sludge–cyanobacteria composite carbon for synergistically enhanced co-removal of Cu(II) and Cr(VI)

Traditional sludge disposal is currently restricted by the risk of secondary pollution. Sludge carbon material has gained widespread attention because of its low cost and environmentally sustainable properties. However, owing to the high ash content and low–energy density of sludge, sludge pyrolysis alone has certain limitations, and the performance of carbon materials needs to be improved. Herein, a sludge–cyanobacteria composite carbon (SCC) was easily synthesized, and the adsorption process of Cu(II) and Cr(VI) by SCC was examined. SCC-700-2-50% exhibited a high SBET (1047.54 m2/g) and developed pore structure rich in functional groups (such as –NH, –OH, and C–O). The combination of pore structure and functional groups improved the adsorption performance of SCC. The adsorption processes exhibited a synergistic effect in a binary system: the qm of Cu(II) and Cr(VI) were 386 mg/g and 341 mg/g, respectively, and the selectivity of Cu(II) adsorption by SCC was greater than Cr(VI). The adsorption process, examined by SEM-EDS, FTIR, and XPS analysis, indicated that Cu(II) as a cationic interface strengthens Cr(VI) adsorption through electrostatic interaction, and the anion Cr(VI) created a valid electrostatic shield against the electrostatic repulsion between H+ and Cu(II), facilitating Cu(II) adsorption. SCC had great reusability: Cu(II) and Cr(VI) adsorption capacity were 90% and 84%, of the initial adsorption capacity, respectively, after six cycles. This study demonstrates the prospect of SCC as a valid adsorbent for multiple heavy metal contaminations removal.

Stabilization of Sludge in Zakho Municipal Wastewater by Anaerobic Digestion for Biogas Production in Kurdistan Region, Iraq

Raw wastewater sludge samples were collected from two sanitary point sources in Zakho city wastewater pipes. Anaerobic digestion is utilized sludge stabilization as it was effective in reducing the volume of sludge and cheap source of clean biogas. the sludge of municipal wastewater of Zakho district is worth to be used for the generation of biogas and just 15 -17 kg sludge produced 233 to 350 m3 gas in solid retention time of 18 and 24 day respectively. Suspended solid is close related to the amount of COD that removed from sludge and approximately 1 kg COD generate 0.5-0.55 Kg suspended solid in the water. Volatile solid (VS) /total solid (TS) ratio is 0.46 after anaerobic digestion which locate in the range of anaerobic lagoon stabilization. Anaerobic digestion lead to reduction in total solid of wastewater especially organic or volatile suspended solid. Specific gravity of the sludge solid is1.5 which indicate that the sludge is stabilized by anaerobic digestion. Sludge volume index (SVI) indicate that the settleable biosolids are old, good quality and low turbidity. Sludge density index (SDI) increased from 1.83 to 2.9 ml/g after stabilization by anerobic digestion. CO2 and H2S are reduced to zero after stabilization by anaerobic digestion. Nutrients N, P, Ca, Mg, Cl are elevated significantly in final stabilized sludge to be used as organic fertilizer, except sulfate is decreased to acceptable ranges after stabilization.

The Efficiency of Air-lift Bioreactor Using PVA-gel Carrier Materials to Treat Organic Components in Artificial Wastewater

An air-lift bioreactor using PVA-gel was used to remove organic components in wastewater treatment. The wastewater treatment system using Sequencing Batch Reactor with carrier material volume of 5%, activated sludge density of 2.5 g/L to treat artificial wastewater made from cat food (Whiskas) with initial COD content of 2,000 mg/L. After 10 days of operation, the removal efficiency of total COD, soluble COD, and achieved 92, 89, and 84%, respectively. Despite the high removal efficiencies of organic components, the output wastewater did not achieve QCVN 40:2011/BTNMT (Column B) discharge standards. Due to high concentrations of TSS and slow biodegradable organic components. The experimental system will be necessary to study the operating conditions to treat wastewater more efficiently and achieve the discharge standards to the environment.&#x0D; &#x0D; &#x0D;

Modeling and optimization of an upflow anaerobic sludge blanket (UASB) system treating blackwaters

In the present work, a novel mathematical dynamic model describing the upflow anaerobic sludge blanket (UASB) has been developed. The model incorporates both the biological processes and the transport of sludge particles specifically related to the UASB treatment of blackwater. The processes occurring in the reactor are modeled as a sequence of two completely-mixed reactive tanks, one representing the sludge bed where most solids concentrate and the other representing a less solid-concentrated liquid volume. Solids from the sludge bed are assumed to be dragged to the upper zone of the reactor by the methane gas produced. The time variation of the sludge bed volume is included by assuming a constant sludge density. The model is calibrated against published data obtained from a stable operation of a lab-scale UASB system treating at different organic loading rates blackwater collected from real vacuum toilets. Good model prediction compatibility with the measurements of emitted biogas and methane gas flow rates, effluent COD, volatile fatty acids, pH and sludge bed volume was revealed. Furthermore, the model was successfully validated against another independent set of data from a UASB reactor treating a more diluted type of blackwater collected from conventional toilets. A scenario analysis performed using such a validated model reveals higher methane production yield but also higher effluent COD concentration at increasing organic loading rates and sludge bed volumes. It is found that longer HRTs can improve COD removal efficiency and methane production in blackwater treatment processes.

Prediction of sludge settleability, density and suspended solids of aerobic granular sludge in the presence of pharmaceutically active compounds by quantitative image analysis and chemometric tools

Steroid estrogens namely 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) and antibiotics including sulfamethoxazole (SMX) are pharmaceutically active compounds (PhAC) of emerging concern due to their environmental and human health impacts even at ppb range concentrations. These compounds usually flow to wastewater treatment plants (WWTP) and are released to the aquatic systems due to inefficient removal in conventional biological systems. In this work, a sequencing batch reactor (SBR) with aerobic granular sludge (AGS) was operated in the presence of E2, EE2 and SMX. SVI5, SVI30/SVI5 ratio, VSS, and TSS of mature AGS (in absence of PhAC), as well as in the presence of PhAC (0.221 mg L−1 of E2, 0.278 mg L−1 of EE2 and 0.290 mg L−1 of SMX), were successfully predicted with multilinear regression (MLR) using morphological and structural parameters of floccular and granular fractions of AGS obtained from quantitative image analysis (QIA). Good prediction models were obtained for the SVI5 (R2 of 0.976), floccular VSS (R2 of 0.949) and TSS (R2 of 0.934), granular VSS (R2 of 0.930) and TSS (R2 of 0.916), SVI30/SVI5 ratio (R2 of 0.917) and density (R2 of 0.889). These results emphasize the usefulness of this methodology for monitoring dysfunctions in AGS in the presence of the studied PhAC.

Experimental, statistical and financial analysis of the treatment of organic contaminants in naphthenic spent caustic soda using electrocoagulation process modified by carbon nanotubes

This study aims for the treatment of organic contaminants in naphthenic spent caustic soda from petroleum refinery by electrocoagulation (EC) process, in which graphite cathodes are modified by carbon nanotubes (CNTs). Studied variables include electrode configuration, anode materials (iron/aluminum), perforated anode effect, pH, EC time, current density, type and volume of sludge, and electrode mass loss. Eventually, the total operating cost is estimated to assess the commercial practicability of the process. Cyclic voltammetry methodology (CVM) is used to investigate the effect of coated graphite electrodes with carbon nanotubes, using wet chemical coating with/without bonder, and CVD methods shows that the CVD method has the best performance. The central composite design (CCD) method and analysis of variance (ANOVA) are used to evaluate and optimize the variables and response, COD removal, and to determine significance and adequacy of mathematical models proposed by response surface methodology (RSM). The estimated optimal condition is 8, 120 min, and 18.750 mA cm−2 for pH, EC time, and current density respectively, in which the reactor successfully remove 76% of COD. The results show the quadratic model is adequately significant for the response and in coated samples not only hydrogen releasing happens sooner, but also by consuming lower energy, the higher current intensity will produce which is shown the importance of CNTs on the cathode electrode. Overall, financial and energy consumption analyses authenticated that this process is perfectly satisfactory as a commercial method to pretreat industrial wastewaters such as petroleum refinery plants in the future.

Effect of long-term low concentrations of TiO2 nanoparticles on dewaterability of activated sludge and the relevant mechanism: the role of nanoparticle aging.

Nanoparticles can undergo aging phenomena in sewage treatment systems, which alter their physical and chemical properties. However, the effect of aged nanoparticles on the dewatering performance of activated sludge under long-term low concentrations is yet to be reported in sewage treatment systems. Here, we compared the chronic effects of pristine and aged TiO2 nanoparticles on the sludge dewatering index, which includes specific resistance to filtration (SRF) and bound water (BW) in a sequencing batch reactor (SBR) at μg/L concentration levels, and analyzed the relevant mechanisms. The results indicated that aging in the sludge supernatant altered the photosensitivity and water stability of nanoparticles, which was mainly due to the changes in the zeta potential and energy band of the particle and was ultimately attributed to the combined effect of particle surface inclusions such as organic matter and inorganic salt. At 10 μg/L, nanoparticles reduced the sludge dewaterability, which observed an improvement at 100 μg/L. This is because 10 μg/L promoted the secretion of extracellular polymeric substances (EPS), which regulated the structure of sludge flora and increased the abundance of secreted quorum sensing-acyl-homoserine lactones (QS-AHL) and EPS genera, while the corresponding exposure results for 100 μg/L were the opposite, owing to the damage and necrosis effects caused by exposure under long-term light, which reduced EPS production and increased sludge density. Interestingly, aging could alleviate the effects of two exposure concentrations on sludge dewatering, mainly because of the decrease in the photoactivity of the nanoparticles. The results of this study show that environmental aging could delay, but not reverse the results of exposure to specific concentrations of nanoparticles. However, the significantly different ecological effects of photosensitive nanoparticles with two environmentally relevant concentration should be refined and confirmed again in freshwater environments to provide a basis for subsequent scientific management and control of photosensitive nanoparticles.

Granular activated carbon promoting re-granulation of anammox-hydroxyapatite granules for stable nitrogen removal at low phosphate concentration

Anaerobic ammonium oxidation (anammox) coupled with hydroxyapatite (HAP) crystallization not only achieves simultaneous nitrogen removal and phosphorus recovery, but also cultivates excellent anammox granules. However, a floatation and wash-out of anammox-HAP granules was occurred at low phosphate concentrations. In this study, a reactor inoculated with mature anammox-HAP granules and fed with low phosphate (5 mg P/L) was added with granular activated carbon (GAC) to maintain sludge granulation and nitrogen-removing stability. At influent total nitrogen >800 mg/L and nitrogen loading rate ~ 9.8 kg/m3/d, a satisfactory nitrogen removal of around 88% was maintained during 140 days of operation. Insufficient phosphate supplement resulted in a sludge bulking, with suspended solid and sludge density decreased whereas sludge water content and expansion ratio increased due to HAP loss. Nevertheless, the sludge re-granulation was found at the later stage as the proportion of granules in 2.8– 3.35 mm went up to 37.4% after large granules disintegrated into small pieces at the initial stage. The settling velocity was finally ranging from 129.8 to 182.2 m/h. In addition, Candidatus Brocadia was increased from 2.1% to 20.1% and dominated in the microbial community. These findings suggest GAC was able to promote re-granulation of anammox-HAP granules at low phosphate concentration, which avoids sludge flotation and widens their application as an inoculum.

Swelling behaviours of compacted lime-softening sludge for application in landfill liners

The objective of this study was to investigate the swelling potential of compacted lime-softening sludge for application in landfill liners. The study involved the assessment of the effect of compaction and moulding moisture content (30–40%), corresponding to the Proctor standard compaction test. One-dimensional oedometer swell tests were conducted using distilled water, tap water, and municipal landfill leachate, resulting in the determination of the expansion indices. Moreover, changes in the moisture content and dry density during the swelling process were investigated. The expansion index was significantly influenced by the initial moisture content and liquid chemistry. Subsequently, these factors also affected the sludge dry density decrease, and its moisture content increase, whereas the impact of the initial dry density on expansion was of low importance. An increase in the sludge moulding moisture content, limited swelling in all liquids used. The highest expansion, dry density, and moisture content changes due to swelling were identified for leachate at w < wopt. It should be underlined that the effect of liquid on the swelling potential faded away along with a further increase in the moisture content w > wopt. The novelty of the work lies in identifying a significant plunge of the expansion index at w ≈ wopt for the leachate swelling test. The lime-softening sludge non-swelling moisture content was defined as wnon ≈ (wopt + 4.0%) − (wopt + 4.5%). For practical engineering implications, the moisture content between (wopt + 2.0%) and (wopt + 4.0%) was provided for the most suitable sludge application in landfill liners.

Operational mode affects the role of organic matter in granular anammox process

In the presence of organic matter, the granular anammox system under sequencing batch mode showed more robust anammox performance than that under completely mixed mode, which was attributed to the better biomass retention with high settling ability and stability of granular sludge. Based on the specific anammox activity test, stratified and mixed distribution of heterotrophic bacteria was found under completely mixed and sequencing batch mode, respectively. The stratified microbial distribution resulted in low enzyme activity of anammox bacteria and sludge disintegration by hindering substrate transfer with a large accumulation of EPS on the granular surface. Whereas the heterotrophic bacteria mixed in granules (mixed microbial distribution) act as a “skeleton”, which increased the particle size, density, and stability of granular sludge. Compared with biokinetic-based selection, diffusion-based selection with high substrate penetration depth more likely resulted in the mixed granular structure and strong resistance to organic inhibition under sequencing batch mode.

Effect of calcification on anaerobic granular sludge: Micro-morphological structure and microbial community

In this study, the effect of calcification on methanogenic activity, basic properties, microbial community structure, and microstructure of anaerobic granular sludge (AnGS) was investigated. Results indicated that inorganic crystals were deposited in the sludge in the form of calcite, which increased sludge density and led them to deposit at the bottom of the anaerobic reactor. Calcification markedly reduced the values of volatile suspended solids (VSS) / total suspended solids (TSS) and extracellular polymeric substances (EPS) content by 35.0 % and 27.1 %, respectively. Chloroflexi, Actinobacteria, Firmicutes, and Bacteroidetes were the most dominant functional species in both types of samples, accounting for 69.1 % of the calcified samples and lower than 71.8 % of the uncalcified samples. Compared to uncalcified sludge, the abundance of Methanosaeta (9.7 %) in the calcified sludge decreased by 34.0 %. And calcification reduces the abundance of gene pathway and methane-related enzymes. In addition, the proportion of inorganic crystals in calcified sludge was 180.6 % higher than that in uncalcified sludge, while the proportion of biomass and pores decreased by 11.7 % and 73.8 %, respectively. And the proportion of larger pores was significantly reduced. These factors lead to the cumulative methane production in calcified AnGS decreased by 26.8 %, and the specific methanogenic activity (SMA) reduced by 16.2 %.

Catalytic co-pyrolysis of oil sludge with HDPE to obtain paraffinic products over HUSY zeolites prepared by dealumination and desilication

Oil sludge is a waste from the oil industry with a complex composition and its treatment is still a challenge. In addition, polyolefin waste is becoming a problem due to its accumulation and slow degradation. In this study, catalytic co-pyrolysis was used to treat oil sludge and high density polyethylene (HDPE) at 450 °C and at an atmospheric pressure for 15 min to generate paraffin-rich oily products. Y zeolites were obtained by dealumination and desilication processes which were characterized by XRD, N2 adsorption, FTIR, 29Si and 27Al NMR, TG and NH3 TPD. Pristine and modified zeolites were evaluated in the co-pyrolysis in order to assess their performances through the quality of the obtained pyrolytic oil (GC–MS and SimDis). The modificated zeolites presented higher mesoporosity than the original solid as so lower total acidity. Amidst the tested catalysts, Mod-Si-10 and Mod-Al presented the best performances showing high yield of liquid fraction, rich in parafinic light hydrocarbons, probably due to their moderate acidity and improved mesoporosity. Consequently, these two zeolites presented outstanding catalytic characteristics alongside high selectivity to paraffinic hydrocarbons in the diesel fraction from co-pyrolisis of the oil sludge waste with HDPE.

Shock effects of monovalent cationic salts on seawater cultivated granular sludge

Saline wastewater is commonly encountered in various industries, posing challenges to biological treatments. The application of seawater as a seed source provides a media of diverse halophilic organisms for rapid startup. However, effects of transitioning from a mixed salt source to monovalent salt solutions prevalent in industry remains unexplored. Hence, seed sludge was cultivated using seawater and later granulated under a mixed-salt synthetic medium comprising a mixture of NaCl, KCl and Na2SO4 at a combined concentration of 0.8 M (0.27 M each). The stable, acclimated granules were then tested against single salt media of 0.8 M NaCl, KCl, or Na2SO4. Shift to single salt media resulted in granule disaggregation, poor settling, sludge washout and development of fluffy or slimy flocs. Changes in exopolysaccharides composition after the single salt shift was the predominant reason for the large changes in sludge morphology. The impacts of KCl and Na2SO4 were more significant than the shift to NaCl. The resulting impacts also had a major influence on the treatment performance. A complex mechanism involving monovalent cation stimulation of proteins; ionic strength impacts on exopolysaccharides and morphology; solution density influence on sludge density and settling; and tonicity impacts on cell viability and treatment is described.

Microwave treatment of municipal sewage sludge: Evaluation of the drying performance and energy demand of a pilot-scale microwave drying system

Sewage sludge management and treatment can represent up to approximately 30% of the overall operational costs of a wastewater treatment plant. Microwave (MW) drying has been recognized as a feasible technology for sludge treatment. However, MW drying systems exhibit high energy expenditures due to: (i) unnecessary heating of the cavity and other components of the system, (ii) ineffective extraction of the condensate from the irradiation cavity, and (iii) an inefficient use of the microwave energy, among others issues. This study investigated the performance of a novel pilot-scale MW system for sludge drying, specifically designed addressing the shortcomings previously described. The performance of the system was assessed drying municipal centrifuged wasted activated sludge at MW output powers from 1 to 6 kW and evaluating the system's drying rates and exposure times, specific energy outputs, MW generation efficiencies, overall energy efficiencies, and specific energy consumption. The results indicated that MW drying significantly extends the duration of the constant rate drying period associated with the evaporation of the unbound sludge water, a phase associated with low energy input requirement for evaporating water. Moreover, the higher the MW output power, the higher the sludge power absorption density, and the MW generation efficiency. MW generation efficiencies of up to 70% were reported. The higher the power absorption density, the lower the chances for energy losses in the form of reflected power and/or energy dissipated into the MW system. Specific energy consumptions as low as 2.6 MJ L−1 (0.74 kWh L−1) could be achieved, well in the range of conventional thermal dryers. The results obtained in this research provide sufficient evidence to conclude that the modifications introduced to the novel pilot-scale MW system mitigated the shortcomings of existing MW systems, and that the technology has great potential to effectively and efficiently drying municipal sewage sludge.

Enhancement of activated sludge wastewater treatment with hydraulic selection

The slow settling rate of activated sludge (AS) in secondary clarifiers and batch reactors results in increased hydraulic retention time, AS with a higher water content pumped to the digesters, and higher suspended solids concentration in the clarified supernatant. Settling rates of AS can be improved through the removal of poor settling floc with a diameter less than 100 µm. Currently, the main technology used for the enhancement of AS settling is aerobic granular sludge reactors—very few of these reactors have been adopted into the United States market due to their cost and difficulty to integrate the technology into current wastewater treatment infrastructure. The objective of this study was to develop a technology that could enhance the settling properties of AS while maintaining nitrification/denitrification and that could easily be integrated into current wastewater treatment infrastructure. We have developed a hydraulic selection process that removes poor settling floc directly from the secondary clarifiers or batch reactors. The hydraulic selection process is a vacuum system that is suspended in the AS, the system then develops a unique pressure difference based on poor settling sludge density, floc diameter, and terminal settling velocity. The selection process captures and removes poor settling floc allowing the remaining floc to grow and increase the settling rate of AS. The hydraulic selection process was tested for 120 days in a 120 L batch reactor. Results from the experiment showed that the hydraulic selection process was successful, in 63 days the settling velocity increased from 0.2 to 20 m/hr, mean floc diameter increased from 62 to 430 µm, and sludge volume index decreased from 280 to 45 mL/g. Nitrification was maintained for nearly the entire duration of the experiments with the exception of a few days due to drastic changes in influent wastewater concentrations. Denitrification was only maintained for a limited period during the experiment due to an unfavorable carbon to nitrogen ratio.