Francesca Gialdini

Conventional oxidation treatments for the removal of arsenic with chlorine dioxide, hypochlorite, potassium permanganate and monochloramine

Arsenic is widespread in soils, water and air. In natural water the main forms are arsenite (As(III)) and arsenate (As(V)). The consumption of water containing high concentration of arsenic produces serious effects on human health, like skin and lung cancer. In Italy, Legislative Decree 2001/31 reduced the limit of arsenic from 50 to 10 μg/L, in agreement with the European Directive 98/83/EC. As consequence, many drinking water treatment plant companies needed to upgrade the existing plants where arsenic was previously removed or to build up new plants for arsenic removal when this contaminant was not previously a critical parameter. Arsenic removal from water may occur through the precipitation with iron or aluminum salts, adsorption on iron hydroxide or granular activated alumina (AA), reverse osmosis and ion exchange (IE). Some of the above techniques, especially precipitation, adsorption with AA and IE, can reach good arsenic removal yields only if arsenic is oxidized. The aim of the present work is to investigate the efficiency of the oxidation of As(III) by means of four conventional oxidants (chlorine dioxide, sodium hypochlorite, potassium permanganate and monochloramine) with different test conditions: different type of water (demineralised and real water), different pH values (5.7-6-7 and 8) and different doses of chemicals. The arsenic oxidation yields were excellent with potassium permanganate, very good with hypochlorite and low with monochloramine. These results were observed both on demineralised and real water for all the tested reagents with the exception of chlorine dioxide that showed a better arsenic oxidation on real groundwater than demineralised water.

Influence of drinking water treatments on chlorine dioxide consumption and chlorite/chlorate formation

Disinfection is the last treatment stage of a Drinking Water Treatment Plant (DWTP) and is carried out to maintain a residual concentration of disinfectant in the water distribution system. Chlorine dioxide (ClO2) is a widely used chemical employed for this purpose. The aim of this work was to evaluate the influence of several treatments on chlorine dioxide consumption and on chlorite and chlorate formation in the final oxidation/disinfection stage. A number of tests was performed at laboratory scale employing water samples collected from the DWTP of Cremona (Italy). The following processes were studied: oxidation with potassium permanganate, chlorine dioxide and sodium hypochlorite, coagulation/flocculation with ferric chloride and aluminum sulfate, filtration and adsorption onto activated carbon. The results showed that the chlorine dioxide demand is high if sodium hypochlorite or potassium permanganate are employed in pre-oxidation. On the other hand, chlorine dioxide leads to the highest production of chlorite and chlorate. The coagulation/flocculation process after pre-oxidation shows that chlorine dioxide demand decreases if potassium permanganate is employed as an oxidant, both with ferric chloride and aluminum sulfate. Therefore, the combination of these processes leads to a lower production of chlorite and chlorate. Aluminum sulfate is preferable in terms of the chlorine dioxide demand reduction and minimization of the chlorite and chlorate formation. Activated carbon is the most effective solution as it reduced the chlorine dioxide consumption by about 50% and the DBP formation by about 20-40%.

Metal leaching in drinking water domestic distribution system: an Italian case study

The objective of this study was to evaluate metal contamination of tap water in seven public buildings in Brescia (Italy). Two monitoring periods were performed using three different sampling methods (overnight stagnation, 30-min stagnation, and random daytime). The results show that the water parameters exceeding the international standards (Directive 98/83/EC) at the tap were lead (max = 363 μg/L), nickel (max = 184 μg/L), zinc (max = 4900 μg/L), and iron (max = 393 μg/L). Compared to the total number of tap water samples analyzed (122), the values higher than limits of Directive 98/83/EC were 17 % for lead, 11 % for nickel, 14 % for zinc, and 7 % for iron. Three buildings exceeded iron standard while five buildings exceeded the standard for nickel, lead, and zinc. Moreover, there is no evident correlation between the leaching of contaminants in the domestic distribution system and the age of the pipes while a significant influence is shown by the sampling methods.

Study of the reuse of treated wastewater on waste container washing vehicles

The wheelie bins for the collection of municipal solid waste (MSW) shall be periodically washed. This operation is usually carried out by specific vehicles which consume about 5000 L of water per day. Wastewater derived from bins washing is usually stored on the same vehicle and then discharged and treated in a municipal WWTP. This paper presents a study performed to evaluate the reuse of the wastewater collected from bins washing after it has been treated in a small plant mounted on the vehicle; the advantage of such a system would be the reduction of both vehicle dimension and water consumption. The main results obtained by coagulation-flocculation tests performed on two wastewater samples are presented. The addition of 2 mL/L of an aqueous solution of aluminum polychloride (18% w/w), about 35 mL/L of an aqueous solution of CaO (4% w/w) and 25 mL/L of an aqueous solution of an anionic polyelectrolyte (1 ‰ w/w) can significantly reduce turbidity and COD in treated water (to about 99% and 42%, respectively); the concomitant increase of UV transmittance at 254 nm (up to 15%) enables UV disinfection application by a series of two ordinary UV lamps. Much higher UV transmittance values (even higher than 80%) can be obtained by dosing powdered activated carbon, which also results in a greater removal of COD.

Removal of chlorite from drinking water: Laboratory and pilot-scale studies to predict activated carbon performance at full scale

aUniversity of Brescia, Department of Civil Engineering, Architecture, Land, Environment and of Mathematics, via Branze 43, 25123 Brescia, Italy, Tel. +39 0303711299; Fax: +39 0303711312; email: sabrina.sorlini@unibs.it (S. Sorlini), Tel. +39 0303711302; emails: michela.biasibetti@unibs.it (M. Biasibetti), francesca.gialdini@gmail.com (F. Gialdini) bUniversity of Pavia, Department of Civil Engineering and Architecture, Via Ferrata, 1, 27100 Pavia, Italy, Tel. +39 0382985314; Fax: +39 0382528422; email: michela.biasibetti01@universitadipavia.it (M. Biasibetti), Tel. +39 0382985312; Fax: +39 0382985589; email: mcristina.collivignarelli@unipv.it (M.C. Collivignarelli)

CHAPTER 20:Evaluation of Chlorite and Chlorate Removal by Activated Carbon in a Pilot Scale Drinking Water Treatment Plant

Chlorine dioxide (ClO2) applied in the disinfection of drinking water can produce chlorite (ClO2−) and chlorate (ClO3−). The World Health Organization recommends a guideline value (GV) of 700 µg L−1 for both of these compounds in drinking water, since they can cause oxidative damage to human red blood cells. This work considers a drinking water treatment plant with high chlorine dioxide consumption in disinfection resulting in chlorite concentrations in treated water exceeding the WHO GV. The plant, treating a maximum water flow of 38.9 ML d−1, produces drinking water for the city of Cremona (76,000 inhabitants) in the North of Italy. The treatment train comprises aeration, biofiltration, chemical oxidation with potassium permanganate and ferric chloride, sand filtration, and final disinfection with chlorine dioxide. Regular monitoring was carried out for chlorine dioxide demand, UV absorbance at 254 nm (UV254) and permanganate index (PI) at the inlet and at the outlet of each treatment stage. A pilot scale study was carried out to evaluate chlorite removal with activated carbon. Two columns filled with two different mineral activated carbons were installed at the outlet of the sand filter of the plant. Each column was tested under two conditions: 249 L h−1 flow with a 10 minute empty bed contact time (EBCT) and 124.5 L h−1 flow with a 20 minute EBCT. The chlorine dioxide demand, UV254, PI, and chlorite and chlorate concentration were analyzed at the inlet and at the outlet of each column. The results of this study showed a significant reduction in chlorine dioxide demand, UV254 and PI, close to 70%. With a 10 minute EBCT with both carbons chlorite is reduced by 20–40% during the first two weeks of operation of the columns; from the 3rd to the 6th week the reduction decreases with values of 11–17%, and then below 10%. With a 10 minute EBCT, chlorate concentration at the outlet of both columns was quite variable during the operating period.