Biochar as Alternative Material for Heavy Metal Adsorption from Groundwaters: Lab-Scale (Column) Experiment Review.

The purpose of this manuscript is to present a review of laboratory experiments (including methodology and results) that use biochar, a specific carbon obtained by a pyrolysis process from different feedstocks, as an alternative material for heavy metal adsorption from groundwater. In recent years, many studies have been conducted regarding the application of innovative materials to water decontamination to develop a more sustainable approach to remediation processes. The use of biochar for groundwater remediation has particularly attracted the interest of researchers because it permits the reuse of materials that would be otherwise disposed of, in accordance with circular economy, and reduces the generation of greenhouse gases if compared to the use of virgin materials. A review of the different approaches and results reported in the current literature could be useful because when applying remediation technologies at the field scale, a preliminary phase in which the suitability of the adsorbent is evaluated at the lab scale is often necessary. This paper is therefore organised with a short description of the involved metals and of the biochar production and composition. A comprehensive analysis of the current knowledge related to the use of biochar in groundwater remediation at the laboratory scale to obtain the characteristic parameters of the process that are necessary for the upscaling of the technology at the field scale is also presented. An overview of the results achieved using different experimental conditions, such as the chemical properties and dosage of biochar as well as heavy metal concentrations with their different values of pH, is reported. At the end, numerical studies useful for the interpretation of the experiment results are introduced.

The Sensitivity of a Specific Denitrification Rate under the Dissolved Oxygen Pressure.

The biological denitrification process is extensively discussed in scientific literature. The process requires anoxic conditions, but the influence of residual dissolved oxygen (DO) on the efficiency is not yet adequately documented. The present research aims to fill this gap by highlighting the effects of DO on the specific denitrification rate (SDNR) and consequently on the efficiency of the process. SDNR at a temperature of 20 °C (SDNR20°C) is the parameter normally used for the sizing of the denitrification reactor in biological-activated sludge processes. A sensitivity analysis of SNDR20°C to DO variations is developed. For this purpose, two of the main empirical models illustrated in the scientific literature are taken into consideration, with the addition of a deterministic third model proposed by the authors and validated by recent experimentations on several full-scale plants. In the first two models, SDNR20°C is expressed as a function of the only variable food:microrganism ratio in denitrification (F:MDEN), while in the third one, the dependence on DO is made explicit. The sensitivity analysis highlights all the significant dependence of SDNR20°C on DO characterized by a logarithmic decrease with a very pronounced gradient in correspondence with low DO concentrations. Moreover, the analysis demonstrates the relatively small influence of F:MDEN on the SDNR20°C and on the correlation between SDNR20°C and DO. The results confirm the great importance of minimizing DO and limiting, as much as possible, the transport of oxygen in the denitrification reactor through the incoming flows and mainly the mixed liquor recycle. Solutions to achieve this result in full-scale plants are reported.

Evaluation of automotive shredder residues (ASR) landfill behavior through lysimetric and traditional leaching tests.

With regards to European waste catalog, automotive shredder residues (ASR) can be classified both as a hazardous or non-hazardous waste according to its hazardous properties (H1-H14). It is thus important to carry out an adequate chemical-physical characterization to identify the presence and concentration of those substances able to give, to this extremely heterogeneous material, the hazardousness character of. The issue of waste characterization, to identify the proper site for appropriate waste disposal, is based, according to the relevant laws, to the use of leaching tests. The analysis of the potential effects of landfilled waste in laboratory, however, run into several difficulties in reproducing phenomena depending both on the characteristics of small, heterogeneous quantity of waste and on the local boundary conditions. These difficulties are much more significant as the waste is heterogeneous at the small scale of the laboratory. This is one of the main problems often leading to scattered results even when starting from the same waste parcel. Present research aimed to overcome the above-mentioned difficulties deriving from waste heterogeneity and was based on a lysimetric simulation. Experimentation with lysimeter has shown it effectiveness in the comparison between leachate from the lysimeter and an ASR landfill leachate, from which similar distribution of metal mass ratios, close values for both BOD5 and COD, as well as the absence in both the fluids of organochlorinated compounds, emerge.

Remediation of Soil Polluted by Organic Compounds Through Chemical Oxidation and Phytoremediation Combined with DCT.

Soils contaminated with organic substances is an important issue across Europe: In some areas, these are the main causes of pollution, or the second after contamination from waste disposal. This paper included an experimental application that compared three methods of remediation of contaminated sites, based on electric fields: A single treatment (electroremediation); and two combined treatments, phyto-electrochemical and electrooxidation (a combination of chemical treatment and a DCT-direct current technology). The contaminated soil was taken from a former industrial area devoted to oil refining, located between two roads: The one national and the other one for industrial use. Nine soil samples were collected at two depths (0.2 and 0.4 m). The initial characterization of the soil showed a density of 1.5 g/cm³ and a moisture of about 20%; regarding grain size, 50% of the soil had particles with a diameter less than 0.08 mm. The electrochemical treatment and electrooxidation had an efficiency of 20% while the two combined methods had efficiencies of 42.5% for electrooxidation (with H2O2) and 20% for phyto-electroremediation (phyto-ER) with poinsettias.

Experimental and numerical evaluation of Groundwater Circulation Wells as a remediation technology for persistent, low permeability contaminant source zones.

Contaminants removal stoked inside low permeability zones of aquifers is one of the most important challenge of groundwater remediation process today. Low permeability layers can be considered persistent secondary sources of contamination because they release pollutants by molecular diffusion after primary source of contamination is reduced, causing long plum tails (Back-Diffusion). In this study, the Groundwater Circulation Well (GCW) system was investigated as an alternative remediation technology to the low efficient traditional pumping technologies to restore contaminated low permeability layers of aquifers. The GCW system creates vertical groundwater circulation cells by drawing groundwater through a screen of a multi-screen well and discharging it through another screen. The suitability of this technology to remediate contaminated low permeability zones was investigated by laboratory test and numerical simulations. The collected data were used to calibrate a model created to simulate the Back-Diffusion process and to evaluate the effect of different pumping technologies on the depletion time of that process. Results show that the efficiency of the GCW is dependent on the position and on the geometry of the low permeability zones, however the GCW system appears more suitable to restore contaminated low permeability layers of aquifers than the traditional pumping technology.

Life Cycle Assessment of waste disposal from olive oil production: Anaerobic digestion and conventional disposal on soil.

Extra virgin olive-oil (EVO) production is an important economic activity for several countries, especially in the Mediterranean area such as Spain, Italy, Greece and Tunisia. The two major by-products from olive oil production, solid-liquid Olive Pomace (OP) and the Olive Mill Waste Waters (OMWW), are still mainly disposed on soil, in spite of the existence of legislation which already limits this practice. The present study compares the environmental impacts associated with two different scenarios for the management of waste from olive oil production through a comparative Life Cycle Assessment (LCA). The two alternative scenarios are: (I) Anaerobic Digestion and (II) Disposal on soil. The analysis was performed through SimaPro software and the assessment of the impact categories was based on International Life Cycle Data and Cumulative Energy Demand methods. Both the scenarios are mostly related to the cultivation and harvesting phase and are highly dependent on the irrigation practice and related energy demand. Results from the present study clearly show that the waste disposal on soil causes the worst environmental performance of all the impact categories considered here. Important environmental benefits have been identified when anaerobic digestion is chosen as the final treatment. It was consequently demonstrated that anaerobic digestion should be a feasible alternative for olive mills, to produce biogas from common olive oil residues, reducing the environmental burden and adding value to the olive oil production chain.

An empirical model for the evaluation of the dissolution rate from a DNAPL-contaminated area.

This paper investigates dynamic variation in the morphologic distribution of dense non-aqueous phase liquids (DNAPLs), which take into account the coupled mass transfer. Experiments were carried out in a 2D tank representing a reconstructed aquifer model. DNAPL dissolution rates were investigated over a wide range of DNAPL saturations, several source configurations, and different hydraulic conditions. Morphometric indexes are presented that take into consideration further factors affecting the dissolution process. Local information regarding transport parameters related to the characteristics of the medium was obtained through a neural network and an optimization algorithm applied to experimental tracer tests. The history of DNAPL source architecture, in terms of saturation, indentation grade, and orientation, was determined by image analysis. Dissolved concentrations were registered and mass transfer rate coefficients were obtained for a wide range of source-zone configurations. A statistical analysis was performed to develop a constitutive equation that is descriptive of the mass transfer rate as a function of source-zone metric characteristics. A new empirical dissolution model using the proposed morphometric parameters is presented and compared with other models. The mass transfer correlation reported incorporates morphometric parameters and considers the complex and variable architecture of non-miscible contaminants. The proposed correlation can be used for an initial assessment of non-aqueous phase liquid (NAPL) dissolution rates over a wide range of saturation (residual and non-residual) conditions and different aqueous phase velocities within the NAPL source zone.

Contaminant back-diffusion from low-permeability layers as affected by groundwater velocity: A laboratory investigation by box model and image analysis.

Low-permeability lenses represent potential sources of long-term release when filled from contaminant solute through direct contact with dissolved plumes. The redistribution of contaminant from low to high permeability aquifer zones (Back-Diffusion) was studied. Redistribution causes a long plume tail, commonly regarded as one of the main obstacles to effective groundwater remediation. Laboratory tests were performed to reproduce the redistribution process and to investigate the effect of pumping water on the remediation time of these contaminated low-permeability lenses. The test section used is representative of clay/silt lenses (k≈1∗10-10m/s/k≈1∗10-7m/s) in a sand aquifer (k≈1∗10-3m/s). Hence, an image analysis procedure was used to estimate the diffusive flux of contaminant released by these low-permeability zones. The proposed technique was validated performing a mass balance of a lens saturated by a known quantity of tracer. For each test, performed using a different groundwater velocity, the diffusive fluxes of contaminant released by lenses were compared and the remediation times of the low-permeability zones calculated. For each lens, the obtained remediation timeframes were used to define an analytical relation vs groundwater velocity and the coefficients of these relations were matched to grain size of the low-permeability lenses. Results show that an increase of the velocity field is not useful to diminish the total depletion times as the process mainly diffusive. This is significant when the remediation approach relies on pumping technology.

Pore-scale simulations of concentration tails in heterogeneous porous media.

The retention of contaminants in the finest and less-conductive regions of natural aquifer is known to strongly affect the decontamination of polluted aquifers. In fact, contaminant transfer from low to high mobility regions at the back end of a contaminant plume (i.e. back diffusion) is responsible for the long-term release of contaminants during remediation operation. In this paper, we perform pore-scale calculations for the transport of contaminant through heterogeneous porous media composed of low and high mobility regions with two objectives: (i) study the effect of permeability contrast and solute transport conditions on the exchange of solutes between mobile and immobile regions and (ii) estimate the mass of contaminants sequestered in low mobility regions based on concentration breakthrough curves.

The modelling of odour dispersion as a support tool for the improvements of high odours impact plants.

Two scenarios in terms of odour impact assessment were studied during the phase of upgrading of an existing waste treatment plant: CALPUFF was used for the simulation of odour dispersion. Olfactometric measures, carried out over different periods and different positions in the plant, were used for model calibration. Results from simulations were reported in terms of statistics of odour concentrations and isopleths maps of the 98th percentile of the hourly peak concentrations, as requested from the European legislation and standards. The excess perception thresholds and emissions were utilized to address the plant upgrade options. The hourly evaluation of odours was performed to determine the most impacting period of the day. An inverse application of the numerical simulation starting from defining the odour threshold at the receptor was made to allow the definition of the required abatement efficiency at the odours source location. Results from the proposed approach confirmed the likelihood to adopt odour dispersion modelling, not only in the authorization phase, but also as a tool for driving technical and managing actions in plant upgrade so to reduce impacts and improve the public acceptance. The upgrade actions in order to achieve the expected efficiency are reported as well.

Predictive model of limestone scaling in ammonia stripping towers and its experimental validation on a treatment plant fed by MSW leachate-polluted groundwater.

Groundwater pollution by municipal solid waste (MSW) landfill leachate is a global concern. Stripping towers are one of the most implemented techniques for the removal of ammonia pollution. This study presents a predictive computational model to estimate calcium carbonate precipitation in ammonia stripping towers. The model considers the Ca2+ super-saturation condition due to the water pH, temperature and salinity. The results have been validated through experimental data obtained from a plant fed with MSW landfill leachate-polluted groundwater. The plant consisted of two parallel lines composed of a coagulation-flocculation stage at high pH followed by a stripping tower. Six combinations of water pH and temperature conditions were tested. Maximum precipitation was 1,400 kgCaCO3 after a period of 120days, observed at inlet pH and temperatures of 10.5 and 38 °C The maximum removal efficiency of ammonia was reported as 91%, 87% and 80% respectively. Finally, a good relationship between the loss of efficiency in ammonia removal and the increase of precipitating CaCO3 to the tower plain area ratio, valid for all water pH and temperatures, has been found.

Image analysis procedure for studying Back-Diffusion phenomena from low-permeability layers in laboratory tests.

In this study, the long-term tailing derived from the storage process of contaminants in low-permeability zones is investigated. The release from these areas in the groundwater can be considered a long-term source that often undermines remediation efforts. An Image Analysis technique is used to analyze the process and evaluate the concentrations of a tracer at different points of the test section. Furthermore, the diffusive flux from the low-permeability lenses is determined. To validate the proposed technique, the results are compared with samples, and the diffusive fluxes resulting from the low-permeability zones of the reconstructed aquifer are compared with a theoretical approach.

Airborne toluene removal for minimizing occupational health exposure by means of a trickle-bed biofilter.

The paper presents the experimental results on a biotrickling pilot plant, with a water scrubber as pre-treatment, finalized to the treatment of an airborne toluene stream in a working place. The air stream was characterized by a very high variability of the inlet concentrations of toluene (range: 4.35-68.20 mg Nm(-3)) with an average concentrations of 16.41 mg Nm(-3). The pilot plant has proved its effectiveness in toluene removal, along a 90-day experimentation period, in steady-state conditions. The scrubbing pre-treatment has achieved an average removal efficiency of 69.9 %, but in particular it has proven its suitability in the rough removal of the toluene peak concentrations, allowing a great stability to the following biological process. The biotrickling stage has achieved an additional average removal efficiency of 75.6 %, confirming the good biodegradability of toluene. The biofilm observation by a scanning confocal laser microscope has evidenced a biofilm thickness of 650 µm fully penetrated by toluene degrading bacteria. Among the micro-population Pseudomonas putida resulted the dominant specie. This bacterium can therefore be considered the responsible for most of the toluene degradation. The whole experimented process has determined an average 92.7 % for toluene removal efficiency. This result meets the most stringent limits and recommendations for occupational safety, given by authoritative organizations in the USA and EU; it also meets the odorous threshold concentration of 11.1 mg Nm(-3).

Influence of denitrification reactor retention time distribution (RTD) on dissolved oxygen control and nitrogen removal efficiency.

Low concentrations of dissolved oxygen (DO) are usually found in biological anoxic pre-denitrification reactors, causing a reduction in nitrogen removal efficiency. Therefore, the reduction of DO in such reactors is fundamental for achieving good nutrient removal. The article shows the results of an experimental study carried out to evaluate the effect of the anoxic reactor hydrodynamic model on both residual DO concentration and nitrogen removal efficiency. In particular, two hydrodynamic models were considered: the single completely mixed reactor and a series of four reactors that resemble plug-flow behaviour. The latter prove to be more effective in oxygen consumption, allowing a lower residual DO concentration than the former. The series of reactors also achieves better specific denitrification rates and higher denitrification efficiency. Moreover, the denitrification food to microrganism (F:M) ratio (F:MDEN) demonstrates a relevant synergic action in both controlling residual DO and improving the denitrification performance.

Experimental treatment of a refinery waste air stream, for BTEX removal, by water scrubbing and biotrickling on a bed of Mitilus edulis shells.

The paper presents the results of a two-stage pilot plant for the removal of benzene, toluene, ethylbenzene and xylene (BTEX) from a waste air stream of a refinery wastewater treatment plant (WWTP). The pilot plant consisted of a water scrubber followed by a biotrickling filter (BTF). The exhausted air was drawn from the main works of the WWTP in order to prevent the free migration to the atmosphere of these volatile hazardous contaminants. Concentrations were detected at average values of 12.4 mg Nm(-3) for benzene, 11.1 mg Nm(-3) for toluene, 2.7 mg Nm(-3) for ethylbenzene and 9.5 mg Nm(-3) for xylene, with considerable fluctuation mainly for benzene and toluene (peak concentrations of 56.8 and 55.0 mg Nm(-3), respectively). The two treatment stages proved to play an effective complementary task: the water scrubber demonstrated the ability to remove the concentration peaks, whereas the BTF was effective as a polishing stage. The overall average removal efficiency achieved was 94.8% while the scrubber and BTF elimination capacity were 37.8 and 15.6 g BTEX d(-1) m(-3), respectively. This result has led to outlet average concentrations of 1.02, 0.25, 0.32 and 0.26 mg Nm(-3) for benzene, toluene, ethylbenzene and xylene, respectively. The paper also compares these final concentrations with toxic and odour threshold concentrations.

Automotive shredder residue: a survey of the hazardous organic micro-pollutants spectrum in landfill biogas.

In this study, we report an extensive set of analytical results on the quality of the biogas produced by a landfill of automotive shredder residues. In particular, the investigation was directed towards the identification of a spectrum of polycyclic aromatic hydrocarbons (16 compounds) and a wide range of volatile organic compounds (35 compounds). This article highlights the most important indications of toxicological concern for the detected compounds. Among the polycyclic aromatic hydrocarbons, chrysene shows the highest concentration, followed by pyrene and benzo(b)fluoranthene. Dibenz(a,h)anthracene, the most carcinogenic of the tested compounds, displayed results below the limit of analytical detectability. Benzo(a)pyrene, another typical carcinogenic compound, was detected at low concentrations. With regard to volatile organic compounds, the survey revealed a relevant concentration of toluene (found in fuels and paint thinner) significantly higher than the other compounds. Noticeable amounts of hexane, trichloromethane, and acetone were also found.

Calculating specific denitrification rates in pre-denitrification by assessing the influence of dissolved oxygen, sludge loading and mixed-liquor recycle.

This article presents the results of an experimental study on the correlation among the specific denitrification rate (SDNR), the dissolved oxygen concentration (DO), the F:M ratio (F:M) and the mixed-liquor (ML) recycle in the pre-denitrification reactors fed by domestic sewage. The experimental curves reveal a 28.8-32.0% reduction in the SDNR at 20 degrees C (SDNR(20 degrees C)) with DO equal to 0.1 mgO2 L(-1) and F:M in the range 0.2-0.4 kgBOD5 kgMLVSS(-1) d(-1). The SDNR reduction increases to 50.0-55.9% with DO = 0.3 mgO2 L(-1). A mathematical correlation of these results and an equation for calculating SDNR(20 degrees C) as function of the F:M as well as the average DO and BOD5 in the total flow rate fed in the denitrification stage are proposed. The conducted experience gives useful suggestions for practical usage, in particular regarding the denitrification reactor design, and represents a good starting point for future applications with the aim to optimize the biological process in domestic sewage treatment plants.

Use of a reactive transport model to describe reductive dechlorination (RD) as a remediation design tool: application at a CAH-contaminated site.

In this paper, a numerical model is presented that is capable of describing the complex set of biochemical processes that occur in chlorinated aliphatic hydrocarbon (CAH)-contaminated groundwater when an exogenous electron donor is added. The reactive pattern is based on the degradation pathways of both chlorinated ethanes and ethenes, and it includes electron donor production (H2 and acetate) from the fermentation of an organic substrate as well as rate-limiting processes related to electron acceptor competition. Coupling of the kinetic model to a convection-dispersion module is described. The calibration phase was carried out using data obtained at a real CAH-contaminated site in the north of Italy. Model simulations of different application scenarios are presented to draw general conclusions on the effectiveness of reductive dechlorination (RD) as a possible cleanup strategy. Early outcomes indicate that cleanup targets can only be achieved if source longevity is reduced. Therefore, metabolic RD is expected to produce beneficial effects because it is known to induce bioenhanced degradation and transformation of CAHs.

An integrated wastewater treatment system using a BAS reactor with biomass attached to tubular supports.

This paper describes laboratory experiments aimed to develop a new wastewater treatment system as an alternative to a conventional domestic wastewater plant. A modified Biofilm Airlift Suspension reactor (BAS), with biomass attached to tubular supports, is proposed to address low organic loads (typical of domestic sewage) and low residence time (typical of compact reactors technology). Attached and suspended biomasses, coupled to the high dissolved oxygen (DO), allow high removal efficiencies (90% and 56% for COD and N-NH(4)(+) removal respectively) and high effluent quality to be reached. The experimental activity, divided into three parts, demonstrates the good efficiency of the process, and the reduction of the removal kinetics for the high operating pressure used in the technology. The occurrence of simultaneous nitrification-denitrification (SND) was also observed. When compared with the conventional BAS system, the present treatment shows comparable removal efficiencies and higher specific removal rates (80 mg COD/g VSS and 2.60 mg N-NH(4)(+)/g VSS). The experimental results were coupled with the development of a numerical model to aid in designing a full-scale treatment plant in Italy.

Laboratory investigation of DNAPL migration in porous media.

Laboratory experiments have been carried out with and without groundwater flow in a two-dimensional laboratory-scale tank to assess the influence of layered media and hydraulic gradient on DNAPL infiltration and redistribution processes. Hydrofluoroether has been used as DNAPL and glass beads have been utilized as porous medium. An image analysis procedure has been used to determine saturation distribution during infiltration and redistribution processes. This method allows quantitative time dependent full fields mapping of the DNAPL saturation, as well as the monitoring of DNAPL saturation variation. By means of performed experiments important information were obtained about the migration and redistribution process, the infiltration and migration velocity, the characteristics of migration body. The experimental results show that the hydraulic gradient promotes the infiltration process, increasing the infiltration rate. It hampers DNAPL spread and fingering bringing to a reduction of residual DNAPL and it also promotes the DNAPL redistribution, and it reduces the amount remaining at residual saturation. Furthermore the hydraulic gradient promotes downward and down-gradient migration. DNAPL migration in the direction of water flow, can be considered important due to significant errors in the location of sources in the case of high gradients and high aquifer thicknesses, and for high water flow velocities, such as those which can be expected during pumping actions in water supply or in remediation activities.