Evaluation of the optimal sewage sludge pre-treatment technology through continuous reactor operation: Process performance and microbial community insights.

This study investigated the impact of two low-temperature thermal pre-treatments on continuous anaerobic reactors' performance, sequentially fed with sludge of different total solids content (∼3 % and ∼6 %) and subjected to progressively increasing Organic Loading Rates (OLR) from 1.0 to 2.5 g volatile solids/(LReactor⋅day). Assessing pre-treatments' influence on influent sludge characteristics revealed enhanced organic matter hydrolysis, facilitating sludge solubilization and methanogenesis; volatile fatty acids concentration also increased, particularly in pre-treated sludge of ∼6 % total solids, indicating improved heating efficiency under increased solids content. The reactor fed with sludge pre-treated at 45 °C for 48 h and 55 °C for an extra 48 h exhibited the highest methane yield under all applied OLRs, peaking at 240 ± 3.0 mL/g volatile solids at the OLR of 2.5 g volatile solids/(LReactor⋅day). 16S rRNA gene sequencing demonstrated differences in the reactors' microbiomes as evidence of sludge thickening and the different pre-treatments applied, which promoted the release of organic matter in diverse concentrations and compositions. Finally, the microbial analysis revealed that specific foam-related genera increased in abundance in the foam layer of reactors' effluent bottles, dictating their association with the sludge foaming incidents that occurred inside the reactors during their operation at 2.0 g volatile solids/(LReactor⋅day).

Optimization of supercritical carbon dioxide explosion for sewage sludge pre-treatment using response surface methodology.

The present work was conducted to assess whether the implementation of Supercritical Carbon dioxide Explosion (SCE) is an efficient approach for sewage sludge pre-treatment. In this context, SCE was optimized with the aim to develop a method attempting to increase the biodegradability of sewage sludge's organic matter content, and thus, to enhance the subsequent anaerobic digestion and methane production. The statistical tool of response surface methodology was applied to evaluate the effects of the main pre-treatment parameters (i.e. temperature and time) and their interactions on methane yield, which was defined as the response. Temperature was found to be the most significant variable, having the greatest effect on methane yield. Following this, an optimum set of pre-treatment conditions corresponding to a temperature of 115 °C and time of 13 min, was determined. Under these optimum conditions, the predicted response value was 300 mL CH4/g of volatile solids. The corresponding experimental value obtained from the validation experiment fitted well with this value, clearly demonstrating the effective use of response surface methodology in optimizing SCE. Additionally, under optimum conditions, the methane yield presented a statistically significant increment of 8.7%, compared to untreated sludge. This revealed the impact of SCE as an effective and alternative way for the efficient pre-treatment of sewage sludge. Finally, thermal pre-treatment, alkaline and acidic hydrolysis were also applied to the already pre-treated sludge. It was concluded that the combined pre-treatment techniques contributed to a further increase of methane production compared to raw (untreated) substrate.

Biomass Characteristics and Their Effect on Membrane Bioreactor Fouling.

Biomass characteristics are regarded as particularly influential for fouling in Membrane Bio-Reactors (MBRs). They primarily include the Mixed Liquor Suspended Solids (MLSS), the colloids and the Extracellular Polymeric Substances (EPS). Among them, the soluble part of EPS, which is also known as Soluble Microbial Products (SMP), is the most significant foulant, i.e., it is principally responsible for membrane fouling and affects all fundamental fouling indices, such as the Trans-Membrane Pressure (TMP) and the membrane resistance and permeability. Recent research in the field of MBRs, tends to consider the carbohydrate fraction of SMP (SMPc) the most important characteristic for fouling, mainly due to the hydrophilic and gelling properties, which are exhibited by polysaccharides and allow them to be easily attached on the membrane surface. Other wastewater and biomass characteristics, which affect indirectly membrane fouling, include temperature, viscosity, dissolved oxygen (DO), foaming, hydrophobicity and surface charge. The main methods employed for the characterization and assessment of biomass quality, in terms of filterability and fouling potential, can be divided into direct (such as FDT, SFI, TTF100, MFI, DFCM) or indirect (such as CST, TOC, PSA, RH) methods, and they are shortly presented in this review.

Fluoride removal from water by composite Al/Fe/Si/Mg pre-polymerized coagulants: Characterization and application.

Fluoride, an anionic pollutant, is possibly to be found in excessive concentrations especially in groundwaters and can show detrimental effects on human health, in concentrations higher than the commonly applied legislation limit of 1.5 mg/L The most commonly applied method for water de-fluoridation is performed by Al-based coagulants, which however presents some important limitations, such as the applied relatively high dosage, producing rather excessive amounts of chemical sludge. In this study, the use of novel pre-polymerized Al-based coagulants was examined, regarding their efficiency towards fluoride removal, as compared with the conventionally applied AlCl3. The novel coagulants were characterized by measuring the main physico-chemical properties, the aluminum species distribution, the zeta potential, the particles' size distribution and the produced flocs' sizes. The results showed that the Mg-containing coagulant (PSiFAC-Mg30-10-15) was the most efficient, when applied in pH values relevant to fluoride-containing groundwaters; it was also the only coagulant, which increases its efficiency at pH values > 7. The uptake capacity of coagulants, regarding fluoride, to reach the residual/equilibrium concentration limit of 1.5 mg F/L (Q1.5-value) at the pH value 7.0 ±â€¯0.1 were found 170, 134 and 94 mg F/g Al for the cases of PSiFAC-Mg30-10-15, AlCl3·6H2O and PSiFAC-Na1.5-10-15, respectively. Accordingly, at the pH value 7.8 ±â€¯0.2 the Q1.5-values were found 189, 118 and 41 mg F/g Al for the same coagulants; whereas considering the residual aluminum concentration this was ranged at 15 ±â€¯5, 25 ±â€¯5 and 30 ±â€¯5 µg Al/L, respectively. In addition, (beneficial) increase of residual magnesium concentration, when applying the coagulant PSiFAC-Mg30-10-15 was 15 ±â€¯5 mg/L.

Application of a ceramic membrane contacting process for ozone and peroxone treatment of micropollutant contaminated surface water.

This study investigates the performance of membrane-based ozonation and peroxone processes, regarding the transformation of carbamazepine (CBZ), benzotriazole (BZT), p-chlorobenzoic acid (pCBA) and atrazine (ATZ) in natural surface waters, as well as the formation of bromates. Ozonation, performed with the use of ceramic membrane contactor, was able to diminish CBZ concentration below 0.1 µM at 0.4 mg O3/mg DOC, i.e. presenting >90% removal rate, whereas the transformation of BZT, pCBA and ATZ was not exceeded 70%, 57% and 49%, respectively, under the same experimental conditions. The addition of H2O2 reduced the removal efficiency of CBZ, since up to -8% transformation values were observed at 0.1 mg O3/mg DOC. In contrast, the transformation of ozone-resistant compounds pCBA and ATZ was slightly improved by approximately 5-10%, at 0.8 mg O3/mg DOC. Membrane-based oxidative treatment of surface water resulted to high bromate concentrations (49 µg/L and 28 µg/L for ozone and peroxone process, respectively, at 0.8 mg O3/mg DOC). The results obtained by using the membrane contactor were also compared with the corresponding from conventional batch experiments. These results suggest that the implementation of membrane contactors with the highest possible inner surface per volume along with the use of low ozone gas concentration are required to improve the removal of micropollutants and diminish bromate formation.

Stabilization of Cr-rich tannery waste in fly ash matrices.

In the present work, the stabilization/solidification of a Cr-rich ash obtained from the anoxic incineration of tannery hazardous wastes was studied. Chromium in the starting waste was exclusively in amorphous form and in trivalent state. The waste was embedded in fly ash-based cementitious material matrices. Calcium and sodium hydroxides, as well as sodium silicate, were used as activators. The proposed process combines mechanical activation with hydrothermal curing. Successful immobilization of chromium was achieved, as attested by standard leaching tests. Backscattered electron images revealed the existence of the C-S-H gel, and elemental mapping by energy dispersive X-ray spectroscopy showed a good interdispersion of chromate and aluminosilicate species, verifying that chromium was well distributed in the final amorphous cementitious matrix. X-ray diffraction confirmed the absence of Cr-rich crystalline phases of calcium aluminosilicates, where chromium can enter in hexavalent state. The stiffness of the stabilized samples was reduced with increasing the amount of added Cr-rich ash, as attested by measurements of the dynamic Young's modulus.

Impact of O3 or O3/H2O2 treatment via a membrane contacting system on the composition and characteristics of the natural organic matter of surface waters.

The present study aims to evaluate changes in the structure-composition of natural organic matter (NOM) that occur after the application of bubbleless ozonation or peroxone treatment of surface waters. The oxidation experiments (using 0.5-2 mg O3/mg DOC, or 2:1 O3:H2O2 molar ratio) were performed in a continuous mode, using a tubular ceramic membrane contactor. Fluorescence spectroscopy (emission-excitation matrix) and liquid chromatography-organic carbon detection (LC-OCD) were mainly used for the detailed DOC characterization. In brief, the application of single ozonation resulted to high reduction of humic-like peak fluorescence intensities (50-85%) and also to the formation of two new peaks in the region of protein-like components. The co-addition of H2O2 did not present the anticipated increase in the reduction of fluorescence intensity; however, it resulted to the further oxidation of protein-like fluorophores. LC-OCD measurements confirmed the decrease of average molecular weight of NOM during ozone treatment, due to the gradual degradation of biopolymers (14-23%) and humic substances (11-17%) towards building blocks and low molecular weight (LMW) neutrals. Advanced oxidation process (AOP) treatment by the mixture O3/H2O2 resulted in the simultaneous decrease of building blocks and LMW neutral concentrations. Conventional batch ozonation and AOP experiments were conducted using ozone-saturated solutions to investigate the effect of different contacting patterns. The results revealed that the different reaction pathways followed during bubbleless and conventional batch experiments may also influence the formation of NOM oxidation intermediates.

Production of demineralized water for use in thermal power stations by advanced treatment of secondary wastewater effluent.

The operation and efficiency of a modern, high-tech industrial full-scale water treatment plant was investigated in the present study. The treated water was used for the supply of the boilers, producing steam to feed the steam turbine of the power station. The inlet water was the effluent of municipal wastewater treatment plant of the city of Bari (Italy). The treatment stages comprised (1) coagulation, using ferric chloride, (2) lime softening, (3) powdered activated carbon, all dosed in a sedimentation tank. The treated water was thereafter subjected to dual-media filtration, followed by ultra-filtration (UF). The outlet of UF was subsequently treated by reverse osmosis (RO) and finally by ion exchange (IX). The inlet water had total organic carbon (TOC) concentration 10-12 mg/L, turbidity 10-15 NTU and conductivity 3500-4500 µS/cm. The final demineralized water had TOC less than 0.2 mg/L, turbidity less than 0.1 NTU and conductivity 0.055-0.070 µS/cm. Organic matter fractionation showed that most of the final DOC concentration consisted of low molecular weight neutral compounds, while other compounds such as humic acids or building blocks were completely removed. It is notable that this plant was operating under "Zero Liquid Discharge" conditions, implementing treatment of any generated liquid waste.

Effects of ozonation pretreatment on natural organic matter and wastewater derived organic matter - Possible implications on the formation of ozonation by-products.

The aim of this study was to investigate possible implications of natural and wastewater derived organic matter in river water that is subsequently used following treatment for drinking purposes. River water was subjected to lab-scale ozonation experiments under different ozone doses (0.1, 0.4, 0.8, 1.0 and 2.0 mgO3/mgC) and contact times (1, 3, 5, 8 and 10 min). Mixtures of river water with humic acids or wastewaters (sewage wastewater and secondary effluents) at different proportions were also ozonated. Dissolved organic carbon and biodegradable dissolved organic carbon concentrations as well as spectroscopic characteristics (UV absorbance and fluorescence intensities) of different types of dissolved organic matter and possible changes due to the ozonation treatment are presented. River water, humic substances and wastewater exhibited distinct spectroscopic characteristics that could serve for pollution source tracing. Wastewater impacted surface water results in higher formation of carbonyl compounds. However, the formation yield (µg/mgC) of wastewaters was lower than that of surface water possibly due to different composition of wastewater derived organic matter and the presence of scavengers, which may limit the oxidative efficiency of ozone.

Enhanced As(III) oxidation and removal by combined use of zero valent iron and hydrogen peroxide in aerated waters at neutral pH values.

The oxidation and removal of As(III) by commercially available micro-scale zero-valent iron (mZVI) was studied in aerated synthetic groundwater with initially 6.7 µM As(III) at neutral pH values. Batch experiments were performed to investigate the influence of ZVI and H2O2 concentrations on As(III) oxidation and removal. Oxidation and removal kinetics was significantly increased by increasing ZVI concentration or by adding H2O2 in micromolar concentrations slightly higher than that of initial As(III). Observed half-lifes for arsenic removal without added H2O2 were 81-17 min at ZVI concentrations of 0.15-2.5 g/L, respectively. X-ray absorption spectroscopy (XAS) confirmed that almost all As(III) was converted to As(V) after 2 h of reaction in the pH range 5-9. Addition of 9.6 µM H2O2 to 0.15 g/L ZVI suspensions diminished half-lifes for arsenic removal from 81 to 32 min and for As(III) oxidation from 77 to 8 min, i.e., by approximately a factor of 10. The increased rate of As(III) oxidation is attributable to enhanced formation of oxidants by the Fenton reaction with higher initial concentrations of H2O2. In practice, results of this study suggest that addition of small amounts (<1 mg/L) of H2O2 in various forms (e.g. stable and widely available Na-percarbonate) to water prior to treatment could significantly enhance As(III) oxidation and removal with ZVI.

The effect of influent temperature variations in a sedimentation tank for potable water treatment--a computational fluid dynamics study.

A computational fluid dynamics (CFD) model is used to assess the effect of influent temperature variation on solids settling in a sedimentation tank for potable water treatment. The model is based on the CFD code Fluent and exploits several specific aspects of the potable water application to derive a computational tool much more efficient than the corresponding tools employed to simulate primary and secondary wastewater settling tanks. The linearity of the particle conservation equations allows separate calculations for each particle size class, leading to the uncoupling of the CFD problem from a particular inlet particle size distribution. The usually unknown and difficult to be measured particle density is determined by matching the theoretical to the easily measured experimental total settling efficiency. The present model is adjusted against data from a real sedimentation tank and then it is used to assess the significance of influent temperature variation. It is found that a temperature difference of only 1 degrees C between influent and tank content is enough to induce a density current. When the influent temperature rises, the tank exhibits a rising buoyant plume that changes the direction of the main circular current. This process keeps the particles in suspension and leads to a higher effluent suspended solids concentration, thus, worse settling. As the warmer water keeps coming in, the temperature differential decreases, the current starts going back to its original position, and, thus, the suspended solids concentration decreases.

Comparative evaluation of conventional and alternative methods for the removal of arsenic from contaminated groundwaters.

The present paper intends to summarize the recent findings regarding the development of alternative treatment methods applicable to small municipal drinking water systems. Small systems are frequently affected by the new permissible arsenic concentration rules, as imposed by several international organizations-World Health Organization, European Commission, United States Environmental Protection Agency. The innovate treatment methods reviewed are (a) adsorptive filtration, using iron oxide coated sand or polymeric materials; (b) zero-valent iron; (c) solar disinfection, solar oxidation and removal of arsenic (SORAS); (d) iron-based adsorbents, such as granular ferric hydroxide (GFH); (e) biological oxidation and removal of arsenic, accomplished simultaneously with the biological iron oxidation. In addition, other more conventional methods for arsenic removal are also discussed, such as iron or alum coagulation, lime softening, ion exchange, activated alumina and membrane separation processes. The aforementioned methods have been comparatively evaluated and the relevant conclusions have been drawn with respect to the applicability of arsenic treatment methods, depending on certain parameters, such as locally varying water quality characteristics, sustainability, and economic feasibility.

Recent advances in the bioremediation of arsenic-contaminated groundwater.

The biological treatment of groundwater is used primarily to remove electron donors from water sources, providing (biologically) stable drinking water, which preclude bacterial regrowth during subsequent water distribution. To the electron donors belong also the dissolved metal cations of ferrous iron and manganese, which are common contaminants found in most (anaerobic) groundwater. The removal of iron and manganese is usually accomplished by the application of chemical oxidation and filtration. However, biological oxidation has recently gained increased importance and application due to the existence of certain advantages, over the conventional physicochemical treatment. The oxidation of iron and manganese is accelerated by the presence of certain indigenous bacteria, the so-called "iron and manganese oxidizing bacteria." In the present paper, selected long-term experimental results will be presented, regarding the bioremediation of natural groundwater, containing elevated concentrations of iron and arsenic. Arsenic is considered as a primary pollutant in drinking water due to its high toxicity. Therefore, its efficient removal from natural waters intended for drinking water is considered of great importance. The application of biological processes for the oxidation and removal of dissolved iron was found to be an efficient treatment technique for the simultaneous removal of arsenic, from initial concentrations between 60 and 80 microg/l to residual (effluent) arsenic concentrations lower than the limit of 10 microg/l. The paper was focused on the removal of As(III) as the most common species in anaerobic groundwater and generally is removed less efficiently than the oxidized form of As(V). To obtain information for the mechanism of As(III) removal, X-ray photoelectron spectroscopy (XPS) analyses were applied and it was found that As(III) was partially oxidized to As(V), which enabled the high arsenic removal efficiency over a treatment period of 10 months.

The application of bioflocculant for the removal of humic acids from stabilized landfill leachates.

The evaluation of bioflocculant, in comparison with traditional inorganic coagulants, for the removal of humic acids from landfill leachates stabilized by biological treatment, was performed using conventional jar-test coagulation experiments. The optimized conditions (pH and coagulant dosage) were identified for the treatment of synthetic solutions as well as for biologically pre-treated landfill leachates. It was found that the application of bioflocculant was quite efficient in the removal of humic acids from synthetic solutions as well as in the reduction of COD content from real landfill leachates. The optimal pH value was found to be between 7 and 7.5, while a 20 mg/l bioflocculant dosage was sufficient in providing more than 85% humic acid removal. The results were comparable with those obtained by the application of conventional coagulants such as alum or polyaluminum chloride; therefore, bioflocculant can be considered as a viable alternative in the treatment of landfill leachates applying coagulation.

Biological treatment of Mn(II) and Fe(II) containing groundwater: kinetic considerations and product characterization.

In the present article, the treatment of groundwater containing Mn(II) and Fe(II) has been investigated. The biological oxidation of Mn(II) and Fe(II) in upflow filtration units comprised the applied experimental technique. The oxidation processes were mediated by specific bacteria, namely the Leptothrix ochracea and Gallionella ferruginea, which belong to the general category of manganese and iron oxidizing bacteria. This work was focused on the characterization of the products of biological oxidation and to the examination of the kinetics of Mn(II) removal as compared with Fe(II) removal from groundwaters. The products of biological oxidation were characterized using the spectroscopic techniques XRD, XPS and SEM-EDS and comprised a mixture of biogenic hydrous manganese and iron oxides. The oxidation state of manganese in the precipitates was found to be between 3 and 4. Iron oxides were mainly in the form of amorphous ferrihydrite. The kinetic results indicated that the rates of manganese and iron oxidation were several orders of magnitude greater than the respective for abiotic oxidation. The bacterially mediated oxidation of iron was faster than manganese oxidation, presenting half-lives of reaction 0.9 and 3.98 min, respectively.

Application of flotation for the separation of metal-loaded zeolites.

Several industrial wastewater streams may contain heavy metal ions, which must be effectively removed, before the discharge or reuse of treated waters could take place. Different bonding materials, presenting selectivity and fast reaction kinetics for the removal of metals, have been examined for this purpose. The objective of the present paper was to investigate the application of dispersed-air flotation for the separation of metal-loaded sorbents. Two similar zeolite samples were applied as effective bonding agents for the removal of zinc, a toxic metal commonly found in many industrial wastewaters. This combined process, termed sorptive flotation, involves the preliminary scavenging of metal ions, by using the appropriate sorbent particles (usually present as ultrafine particulates), followed by flotation for the effective separation of them. The obtained results were very promising, as both metal and sorbent were effectively removed/separated from the dispersion.

Application of biological processes for the removal of arsenic from groundwaters.

Bacteria are widespread, abundant, geochemically reactive components of aquatic environments. In particular, iron-oxidizing bacteria, are involved in the oxidation and subsequent precipitation of ferrous ions. Due to this property, they have been applied in drinking water treatment processes, in order to accelerate the removal of ferrous iron from groundwaters. Iron also exerts a strong influence on arsenic concentrations in groundwater sources, while iron oxides are efficient adsorbents in arsenic removal processes. In the present study, the removal of arsenic (III and V), during biological iron oxidation has been investigated. The results showed that both inorganic forms of arsenic could be efficiently treated, for the concentration range of interest in drinking water (50-200microg/L). In addition, the oxidation of trivalent arsenic was found to be catalyzed by bacteria, leading to enhanced overall arsenic removal, because arsenic in the form of arsenites cannot be efficiently sorbed onto iron oxides. This method comprises a cost competitive technology, which can find application in treatment of groundwaters with elevated concentrations of iron and arsenic.

Removal of As(V) from wastewaters by chemically modified fungal biomass.

Biosorption has been demonstrated to be a useful alternative to conventional treatment systems for the removal of toxic metals from dilute aqueous solution. The objective of this paper was to examine the main aspects of a possible strategy for the removal of arsenates, employing P. chrysogenum biomass. The pretreatment of biomass with common surfactants (as hexadecyl-trimethylammonium bromide and dodecylamine) and a cationic polyelectrolyte was found to improve the biosorption efficiency. The initial biomass showed a relative low affinity for metallic anions, whereas with the application of modified samples a significant uptake of arsenic was observed. Sorption data were well described by typical Langmuir and Freundlich adsorption isotherms. Promising results were obtained in laboratory experiments and effective As(V) removals were observed.

Removal of arsenic from contaminated water sources by sorption onto iron-oxide-coated polymeric materials.

The modification of polymeric materials (polystyrene and polyHIPE) by coating their surface with appropriate adsorbing agents (i.e. iron hydroxides) was investigated in the present work, in order to apply the modified media in the removal of inorganic arsenic anions from contaminated water sources. The method, termed adsorptive filtration, has been classified as an emerging technology in water treatment processes as it presents several advantages towards conventional technologies: the production of high amounts of toxic sludge can be avoided and it is considered as economically more efficient; whereas it has not yet been applied in full-scale treatment plants for low-level arsenic removal. The present experiments showed that both modified media were capable in removing arsenic from the aqueous stream, leading to residual concentration of this toxic metalloid element below 10 microg/L, which is the new maximum concentration limit set recently by the European Commission and imposed by the USEPA. Though, among the examined materials, polyHIPE was found to be more effective in the removal of arsenic, as far as it concerns the maximum sorptive capacity before the filtration bed reaches the respective breakthrough point.