Mikko Vepsäläinen

Natural organic matter removal by coagulation during drinking water treatment: A review

Natural organic matter (NOM) is found in all surface, ground and soil waters. An increase in the amount of NOM has been observed over the past 10-20 years in raw water supplies in several areas, which has a significant effect on drinking water treatment. The presence of NOM causes many problems in drinking water and drinking water treatment processes, including (i) negative effect on water quality by causing colour, taste and odor problems, (ii) increased coagulant and disinfectant doses (which in turn results in increased sludge volumes and production of harmful disinfection by-products), (iii) promoted biological growth in distribution system, and (iv) increased levels of complexed heavy metals and adsorbed organic pollutants. NOM can be removed from drinking water by several treatment options, of which the most common and economically feasible processes are considered to be coagulation and flocculation followed by sedimentation/flotation and sand filtration. Most of the NOM can be removed by coagulation, although, the hydrophobic fraction and high molar mass compounds of NOM are removed more efficiently than hydrophilic fraction and the low molar mass compounds. Thus, enhanced and/or optimized coagulation, as well as new process alternatives for the better removal of NOM by coagulation process has been suggested. In the present work, an overview of the recent research dealing with coagulation and flocculation in the removal of NOM from drinking water is presented.

Removal of organic matter from a variety of water matrices by UV photolysis and UV/H2O2 method.

A re-circulated flow-through photoreactor was used to evaluate the ultraviolet (UV) photolysis and UV/H(2)O(2) oxidation process in the purification of three different water matrices. Chemically coagulated and electrocoagulated surface water, groundwater contaminated with creosote wood preservative and 1,2-dichloroethane (DCE) containing washing water from the plant manufacturing tailor-made ion-exchange resins were used as sample waters. The organic constituents of creosote consist mainly of harmful polycyclic aromatic hydrocarbons (PAH) whereas 1,2-DCE is a toxic volatile organic compound (VOC). Besides analyzing the specific target compounds, total organic carbon (TOC) analysis and measurement of change in UV absorbance at 254 nm (UV(254)) were performed. Initial TOC, UV(254) and pH varied significantly among treated waters. Initial H(2)O(2) concentrations 0-200 mg/l were used. The UV/H(2)O(2) treatment was efficient in removing the hazardous target pollutants (PAHs and 1,2-DCE) and natural organic matter (NOM). In addition, high removal efficiency for TOC was achieved for coagulated waters and groundwater. Also, the efficiency of direct photolysis in UV(254) removal was significant except in the treatment of 1,2-DCE containing washing water. Overall, UV(254) and TOC removal rates were high, except in case of washing water, and the target pollutants were efficiently decomposed with the UV/H(2)O(2) method.

Electrochemical inactivation of paper mill bacteria with mixed metal oxide electrode

In this study electrochemical inactivation of selected bacteria living in paper mill circulating waters was investigated. Three aerobic bacteria species (Deinococcus geothermalis, Pseudoxanthomonas taiwanensis and Meiothermus silvanus) were inactivated effectively (>2 log) at a mixed metal oxide (MMO) electrode in 3 min. The influence of parameters, such as current density and initial pH or chloride concentration of synthetic paper machine water (SPW) on the inactivation efficiency were studied. Increasing current density and initial chloride concentration of SPW increased the inactivation rate but change of pH value did not have significant influence on the inactivation rate. It was observed that inactivation was mainly due to the electrochemically generated chlorine/hypochlorite. Electrochemical oxidation showed good performance for inactivation these primary biofilm forming bacteria species with improved current efficiency by higher initial chloride concentrations.

Precipitation of dissolved sulphide in pulp and paper mill wastewater by electrocoagulation

The precipitation of dissolved sulphide ions by electrocoagulation was studied at laboratory scale using pulp and paper mill wastewaters. Concentrations of dissolved organic carbon and phosphorus were analysed before and after the electrocoagulation process to examine the suitability of the process for treatment of sulphide odour from pulp and paper mill wastewater. The electrochemical cell used in this study was constructed from monopolar dissolving iron electrodes. The dissolved iron concentration was directly proportional to the applied electric charge (C/L) at the tested current densities. Electrochemically produced ferrous iron (Fe2+) precipitated dissolved sulphide ions efficiently. Electricity consumption of the treatment was 4–8 C/mg S2− while iron consumption was 1.1–2.2 mg/mg S2− during the initial phase of the sulphide precipitation when the applied electric charge was 10–60 C/L. When 60 C/L was applied, 88% of dissolved sulphides and 40% of phosphorus was precipitated. The reduction in DOC was low during the sulphide precipitation. According to these results, electrocoagulation can precipitate dissolved sulphides effectively and thereby reduce sulphide odours of pulp and paper mill wastewaters.

Electrochemical oxidation of sulphides in paper mill wastewater by using mixed oxide anodes.

In this study, the electrochemical oxidation technique was used to oxidize sulphides present in paper mill wastewater. Inactivation of anaerobic bacteria in wastewater was also investigated. Sulphide oxidation was effective during the experiments, and the best efficiency was achieved by the smallest current density used. One of the main oxidants of sulphides during the experiments was oxygen. Anaerobic bacteria were better inactivated with higher initial chloride concentration in wastewater because of electrochemically generated chlorine/hypochlorite. Dissolved oxygen, redox potential and pH values of the wastewater increased because of electrochemically generated oxygen‐based oxidants and oxidation reactions occurring on the anode. In general, it can be said that sulphide removal was successful in the present study.

Influence of Carbon Sources and Concrete on Microbiologically Influenced Corrosion of Carbon Steel in Subterranean Groundwater Environment

Microbiologically influenced corrosion of carbon steel was assessed in a laboratory environment simulating the deep geological repository of radioactive waste. A dense and diverse biofilm was forme...

Effect of Electrochemical Potential on Stress Corrosion Cracking Susceptibility of EN 1.4301 (AISI 304) Austenitic Stainless Steels in Simulated Hot Black Liquor

Stress corrosion cracking (SCC) susceptibility of austenitic EN 1.4301 (AISI 304, UNS S30400) stainless steel was studied as a function of electrode potential at T = 190°C in 15 g/kg NaOH + 150 g/kg Na2S containing caustic environment simulating heavy black liquors (HBL) of the pulp industry. Severe cracking was detected at the corrosion potential and at the cathodic potential of −0.11 VMo/MoS2 reference electrode. On the other hand, at anodic potentials of 0.03 VMo/MoS2 to 0.3 V Mo/MoS2 no cracking was observed. Thus, SCC of EN 1.4301 steel can potentially be mitigated in HBL environment by applying anodic protection. At the corrosion potential, selective dissolution of Fe and slight localized enrichment of Ni and Cr, as well as Na, S, and O, was observed. At anodic potentials, Fe was selectively dissolved and marked enrichment of both Ni and Cr, as well as Na, S, and O, took place in the corrosion product. The simultaneous enrichment of Ni and Cr in the corrosion product film was concluded to be the pre...