Carlo Merli

Recovery of EDTA and metal precipitation from soil flushing solutions

This work studies the effectiveness of a process proposed for the recovery of ethylenediaminetetraacetic acid (EDTA) and metal precipitation from soil flushing solutions. Two series of experimental tests were carried out on two samples of a soil artificially contaminated with copper or lead. The metals were extracted from the soil by flushing with a 0.05 M aqueous solution of EDTA sodium salt (E-Na(2)). Cu or Pb extraction efficiencies of about 95 and 98% were observed, respectively. The two extracted solutions were then treated to obtain EDTA recovery and metal precipitation from the aqueous solution. EDTA recovery was achieved in two steps. An initial evaporation treatment lead to reduce the solution volume by about 75%. This was followed by the acidification of the residue solution, which precipitated more than 93% of the used EDTA. The precipitated EDTA was removed by filtration, and was suitable for reuse after adding an alkaline agent. Metal precipitation from the filtered solution was performed using two different methods: an almost total (99.5%) Pb precipitation in alkaline conditions was achieved after complex destabilization through the sequential addition of ferric ions and sodium phosphate, while 93.6% copper precipitation was achieved with ferrous sulfate as a destabilization agent.

Treatment of industrial landfill leachate by means of evaporation and reverse osmosis.

In this paper a process for the treatment of landfill leachate involving evaporation and reverse osmosis was proposed. Experimental tests were performed on an industrial landfill leachate. The leachate was subjected to evaporation so as to obtain a distillate containing a small amount of organic material and a substantial amount of inorganic substances (consisting primarily of metals and ammonium salts). The distillate of the evaporation treatment was then subjected to reverse osmosis. The reverie osmosis tests were performed using two different membranes: the AD membrane (thin two-ply film of polyamide) and the SC membrane (thin three-ply film of polyamide). Tests carried out at different values of pH showed a reduction of organic content of about 88% when AD membranes were used and about 80% with SC membranes independently of pH. As regards ammonium, comparable reductions of over 97% were registered for both types of membrane in the optimal conditions of pH = 6.4 (97.1% for AD membranes and 97.7% for SC).

Influence of Soil Organic Matter on Copper Extraction from Contaminated Soil

Column experiments of copper extraction from four contaminated soils characterized by a content of Soil Organic Matter (SOM) ranging from 1% to 25% are presented and discussed. The extraction was performed by flushing the soil with an aqueous solution of a sodium salt of ethylene diamminotetraacetic acid (EDTA). Preliminary tests were performed on a soil containing 25% of organic matter, to investigate the influence of pH, concentration and volumes of EDTA on its chelant action and on the dissolution of SOM. Having selected the optimal conditions for the extraction process, a further series of tests was conducted on the four soils to evaluate the influence of organic content on copper extraction yields. EDTA solutions at 0.01 M, 0.05 M, 0.1 and 0.2 M were injected at 0.33 ml/s; copper and organic matter extraction yield were determined. At a pH of 5, 15 pore volume (PV) of a solution containing 0.05M EDTA, extracted about 99% of copper contained by the soil with the higher organic matter content. Under the same conditions, and for soil with > 6% SOM, extraction yields over 80% were achieved, while at lower organic content, copper extraction was dramatically reduced. This was attributed to the formation of highly stable copper-humate complexes and to their increasingly dissolution that occurred in the soils with higher organic matter level. Experimental tests performed at different contamination levels (1200 mg/kg, 2400 mg/kg) showed that EDTA extraction effectiveness also depended upon initial soil Cu concentration.

A steady-state model for the evaluation of disk rotational speed influence on RBC kinetic: model presentation.

The physical and biological mechanisms of attached-biomass growth were analyzed and a steady-state model was proposed to determine the soluble carbonaceous removal in an RBC unit for different organic loading rates in the reactor. The objective of the model was the prediction of the organic loading rate corresponding to the maximum removal capacity in the system. A system of equations was solved where the influent soluble carbonaceous substrate concentration was the main variable. Monods rate law was used for the growth of microorganism: the soluble carbonaceous substrate was the limiting substrate. Endogenous decay was neglected. The influence of disk rotational speed on the RBC removal capacity was investigated, the disk rotational speed being a parameter acting on oxygen transfer in the biofilm. The criteria for the evaluation of the kinetic parameter in the model were proposed.

A Real Time Toxicity Bioassay for Activated Sludge Reactor

Activated sludge reactor is commonly employed for the secondary treatment of industrial wastewater, showing a high purification yield, operating simplicity and cost-effectiveness. Nevertheless, industrial wastewater generally contain numerous organic and inorganic compounds which are not biodegradable; heavy metals are often found in effluents from electroplating and metal-processing industries, manufacturing of paints, plastics, scientific instruments, salts are found in wastewater from production of pesticides, pharmaceutical and food industry, waste-dump percolating water. Primary treatments are often designed to reduce heavy metal and salt contents in wastewater, nevertheless trace concentrations persist and may reach the biological reactor, resulting in biomass inhibition and dramatic loss in purification efficiency. Respirometry through Oxygen Uptake Rate measurement is one of the most rapid and reliable methods to assess the inhibition of activated sludge. In previous studies toxicity tests for specific compounds have been performed following different procedures and attaining heterogeneous results. In this work a simple modification of the configuration is proposed, in order to assess in real time the toxicity and the inhibiting effect of the incoming wastewater before it reaches the reactor. A sample is drawn from the influent and a respirometric test is performed on mixed liquor collected from the reactor itself. If the inhibition exceeds the limit, the influent is sent to storage and to an alternative treatment. Preliminary experiments were carried out with bench-scale CSTR, supplied with synthetic wastewater. Copper sulphate and sodium chloride were chosen as inhibiting compounds, supplied separately to batch samples of activated sludge in shock load at different concentrations. Copper concentration in the mixed liquor was measured by atomic absorption and salt concentration was determined through conductance measures. Inhibition up to 90% was evaluated by OUR test with short response time, demonstrating the suitability of this tool to a real time toxicity bioassay for activated sludge reactor.

Sequential extraction analysis provides decision-making tools for the use of contaminated sediments

To construct a new tourist harbour in Marina di Ragusa (Sicily), the dredging of 300,000 m3 of marine sediment is required. To evaluate potential disposal options, a detailed physico-chemical characterisation of this material is needed. A very cost-effective solution is represented by the re-use of the dredged material for beach nourishment. To this aim, compliance with environmental standards and a strong homogeneity between the receiving beach and the source sediments must be proved. Analytical determinations for the main parameters show an arsenic content exceeding the Italian quality standard (12 mg·kg−1). In order to evaluate arsenic mobility and availability, and therefore the actual release into the environment, a three-step sequential extraction procedure was applied to a limited number of samples. The results obtained seem to exclude the risk, showing that arsenic is almost totally bound to the resistant fraction and thus does not represent an impediment to the beneficial re-use of sediment.

Treatment of Wastewater with High Inorganic Salts Content

The study deals with the waste disposal of an aqueous solution with a high salt content (about 5% sodium sulfate) and an organic fraction consisting primarily of naphthalenesulfonic acids. First, a process based on crystallization for the removal of sodium sulfate from the wastewater was developed, leading to a recovery of about 70% of the sodium sulfate. The wastewater with the residual salts content was fed to a batch mesophilic anaerobic digestion process. The acclimatization of a methanogenic consortium to different mixtures of the desalted wastewater and a growth substrate was studied. Complete acclimation of the biomass to the solution was observed over a broad range of wastewater amount in a growth medium (from 3.5 to 224 mL of wastewater per liter of feeding solution). After a suitable acclimation period of time the biomass was able to digest the pure wastewater up to an amount of 57.1 mL per liter of the feeding solution, obtaining total organic carbon (TOC) (mg/LC) removal rates of 0.04 kg TOC/k...

EXPERIMENTAL VALIDATION OF A MODEL DESCRIBING THE CYCLE OF NITROGEN IN A STEP SLUDGE RECIRCULATION ACTIVATED SLUDGE SYSTEM WITH DENITRIFICATION

Abstract In this paper, a model describing the cycle of nitrogen in a Step Sludge Recirculation (SSR) reactor, was developed. The SSR reactor is a multistage, continuous stirred tank reactor where the recycled biomass is distributed over all stages. The SSR system provides a uniform treatment of the wastewater and high purification efficiency for both carbon and nitrogen removal. The objective of this work was to develop a mathematical model of the SSR system, based on the analysis of various parameters that are present in the system. The simplified form of the IWA‐ASM model was adapted to include the carbon removal, nitrification and denitrification processes. For model validation, a SSR pilot plant fed with synthetic wastewater was operated for approximately four months. The experimental results show that the SSR system seems to be appropriate to attain maximum TOC and nitrogen removal with minimum sludge production. The proposed model seems to be capable of expressing the behavior of carbon removal, nitrification, denitrification and various microorganism species in a SSR configuration of a nitrogen cycle. When the experimental results were compared with those estimated by the proposed model, the model predictions matched well with the experimental results.

ACTIVATED SLUDGE MULTISTAGE PLANT WITH STEP SLUDGE RECIRCULATION: PROCESS OPTIMIZATION

Abstract Results of experimental tests carried on a multistage step sludge recirculation activated sludge plant are presented. The main objective of this experimentation regards the characterization of the plant configuration that presents the maximum removal rate for carbonaceous and azotate substrate. The removal efficiency and the kinetics constants, as a function of the sludge recirculation flow in each stage, were also determined.

Optimisation of batch membrane processes for the removal of residual heavy metal contamination in pretreated marine sediment

In this article, an innovative process that uses membranes to purify marine sediments contaminated with heavy metals is presented. The proposed process is composed of a centrifugation step, performed with a hydrocyclone, followed by a batch membrane step. The initial separation of sand from the raw stream appears to be very efficient and is important to avoid plugging the membrane separation step. The remaining stream, called silt, is then processed by a batch membrane separation step, capable of separating organic and inorganic pollutants from the purified water. Pilot tests were carried out to estimate membrane performance, including the maximum recovery value achievable, typical rejection values, the permeate and critical fluxes.