Electrocoagulation in water treatment:continuous versus batch processes and sludge utilization

Abstract This thesis deals with electrocoagulation (EC) in water treatment. The EC method has been in use with continuous development since the late 19th century, although there is a need for improved technical solutions. A review of the literature reveals design needs for sludge control, current feed, and maintenance. This thesis introduces a novel continuous EC system and compares it to a commonly used batch EC system for the removal efficiency of suspended solids, such as viable microbial cells, organic compounds, and nutrients. Operational costs of processes are also compared. The continuous EC process shows potential in the treatment of paint industry wash water for reusable conditions and nutrient recovery via struvite precipitation during the humic acid water treatment. The second part of the thesis focuses on EC sludge in selected applications. One publication from this thesis is a review of EC sludge utilization in different applications. Nutrient recovery is important to avoid eutrophication in water bodies, and it can be achieved in the EC process by precipitating struvite, a low-releasing fertilizer. During struvite precipitation, a sacrificial Mg anode is used, and the nutrients (NH4-N and phosphate) are recovered from wastewater. Further, EC-based sludges were explored as adsorbents (recovery of humic acid) and catalysts (catalytic wet peroxide oxidation of bisphenol A). Calcined EC sludge was found to be a suitable adsorbent and catalyst for the examinated applications.Tiivistelmä Väitöstutkimuksessa tarkastellaan elektrokoagulaatiota (EC) veden käsittelyssä ja siinä muodostuvan sakan hyödyntämistä. Aluksi vertaillaan jatkuvatoimista ja panostoimista EC-menetelmää. Vaikka EC-menetelmä on kehitetty jo 1800-luvun lopulla, on menetelmässä ja siihen liittyvässä laitteistokehityksessä edelleen parannettavaa. Kehittämistarpeet liittyvät mm. sakan muodostumisen hallintaan ja talteenottoon, virran syöttöön sekä laitteiston ylläpitoon. Väitöstutkimuksessa esitellään uusi jatkuvatoiminen EC-laitteisto ja sitä verrataan yleisesti käytössä olevaan panoslaitteistoon. Vertailua tehdään erilaisten muuttujien kuten mikrobien, kiintoaineksen ja ravinteiden poistamisen suhteen EC prosessissa. Lisäksi vertailussa käytetään myös taloudellisia tunnuslukuja, jotka prosessista ovat laskettavissa. Uusi jatkuvatoiminen EC-laitteisto on potentiaalinen puhdistettaessa maalinpesuvesiä uudelleen käytettäväksi ja struviitin muodostamisessa liukoisen Mg-anodin avulla. Väitöstyön toisena tavoitteena on EC-prosessissa muodostuvan sakan hyödyntäminen. Kirjallisuusselvityksen avulla kartoitettiin erilaisia EC-sakan hyödyntämismahdollisuuksia. Ravinteiden talteenotto on tärkeää, jotta vältetään vesien rehevöitymistä. Ravinteiden (NH4-N ja fosfaatti) talteenottoa tutkittiin saostamalla struviittia (niukkaliukoinen lannoite) EC-prosessissa liukoisen Mg-anodin avulla. Lisäksi tutkittiin EC-sakan hyödyntämistä humushapon adsorptiossa sekä katalyyttisessa bisfenoli A:n peroksidihapetuksessa. Kalsinoidun EC-sakan todettiin soveltuvan hyvin adsorbentiksi ja katalyytiksi em. sovelluksissa

Electrocoagulation Sludge Valorization—A Review

In the field of electrocoagulation (EC), various studies on pollutant removal and on the use of different EC technologies have already been made. An EC process generates sludge, which is considered waste, resulting in increased operational costs that come from waste disposal. Sludge contains valuable materials, such as the nutrients or metals removed during water purification, along with metals, such as aluminum or iron, which come from the electrodes used in an EC system. Based on the principles of circular economy or based on existing legislations, reducing the production of valuable wastes, and increasing the valorization rate of as many materials as possible are important endeavors. This study is mainly a review of the existing sludge valorization studies. This review highlights the valorization of sludge as a fertilizer (mainly as struvite), pigment, construction material (mainly as blocks), adsorbent, and catalyst. While it has already been found that EC sludge is valorizable, more studies on EC sludge valorization and on the quality of sludge produced from the effluent of EC processes are warranted

Electrocoagulation sludge valorization:a review

Abstract In the field of electrocoagulation (EC), various studies on pollutant removal and on the use of different EC technologies have already been made. An EC process generates sludge, which is considered waste, resulting in increased operational costs that come from waste disposal. Sludge contains valuable materials, such as the nutrients or metals removed during water purification, along with metals, such as aluminum or iron, which come from the electrodes used in an EC system. Based on the principles of circular economy or based on existing legislations, reducing the production of valuable wastes, and increasing the valorization rate of as many materials as possible are important endeavors. This study is mainly a review of the existing sludge valorization studies. This review highlights the valorization of sludge as a fertilizer (mainly as struvite), pigment, construction material (mainly as blocks), adsorbent, and catalyst. While it has already been found that EC sludge is valorizable, more studies on EC sludge valorization and on the quality of sludge produced from the effluent of EC processes are warranted

Phosphate and Ammonium Removal from Water through Electrochemical and Chemical Precipitation of Struvite

Batch electrocoagulation (BEC), continuous electrocoagulation (CEC), and chemical precipitation (CP) were compared in struvite (MgNH4PO4·6H2O) precipitation from synthetic and authentic water. In synthetic water treatment (SWT), struvite yield was in BEC 1.72, CEC 0.61, and CP 1.54 kg/m3. Corresponding values in authentic water treatment (AWT) were 2.55, 3.04, and 2.47 kg/m3. In SWT, 1 kg struvite costs in BEC, CEC, and CP were 0.55, 0.55, and 0.11 €, respectively, for AWT 0.35, 0.22 and 0.07 €. Phosphate removal in SWT was 93.6, 74.5, and 71.6% in BEC, CEC, and CP, respectively, the corresponding rates in AWT were 89.7, 77.8, and 74.4%. Ammonium removal for SWT in BEC, CEC, and CP were 79.4, 51.5, and 62.5%, respectively, rates in AWT 56.1, 64.1, and 60.9%. Efficiency in CEC and BEC are equal in nutrient recovery in SWT, although energy efficiency was better in CEC. CP is cheaper than BEC and CEC

Comparison of batch and novel continuous electrocoagulation processes in the treatment of paint industry wash water

Abstract Water is crucial to all life forms on earth. Still, millions of people are suffering because of lack of fresh water. One of the most important reasons is industrial pollution. That is why more effective and economical water treatment systems must be developed and studied worldwide. In this study, a new design of continuous electrocoagulation systems is introduced and compared with widely studied batch process systems. This novel design improves the controllability of water flow and electricity. In the treatment of paint industry wash water, a batch system and a novel continuous electrocoagulation system with an aluminum (Al) anode and an iron (Fe) cathode were used. Two parallel analyses of both processes were conducted, and the average efficiency of chemical oxygen demand and Al removal were 68% and 79.8% in batch and 69.7% and 62.1% in the continuous system, respectively. The number of microbes decreased by 99.6% in the batch and by 99.8% in the continuous system. The calculated operational costs in this experiment were 1.63 €/m³ for the batch and 1.19 €/m³ for the continuous system. The novel continuous electrocoagulation process was as efficient as the batch process in this study

Use of Fe and Al Containing Electrocoagulation Sludge as an Adsorbent and a Catalyst in Water Treatment

In this study, three different electrocoagulation (EC) sludges were studied as an adsorbent (removal of humic acids) and as a catalyst [catalytic wet peroxide oxidation (CWPO) of bisphenol A (BPA)]. The sludges originated from electrocoagulation process in which aluminum (Al) and iron (Fe) electrodes were used for the treatment of mining industry wastewater. All the materials were used as dried sludge and calcined material. The stability of these materials was studied in neutral and alkaline conditions with analysis of the leached iron content in solution. Based on the EC sludge characterization with X-ray fluorescence (XRF), X-ray diffractometer (XRD), and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy different forms of Fe occurring in EC sludges were found. The Brunauer–Emmett–Teller (BET) method showed reduced surface area after calcination process. Stability of the sludges was studied in neutral conditions, and the amount of iron leaching was low (<1.4  ppm). Adsorption experiments showed that the removal of humic acids (measured as total organic carbon) was over 50% in all tested materials in the pH range of 3–9, and over 92% with the S3 calcined material in all studied pH range. The calcined samples were catalytically more active than raw material in CWPO of BPA. The highest removal of BPA was 85% over calcined sludge. Therefore, calcined EC sludges are suitable materials for catalyst and adsorbent use. The main novelty of this paper was the finding of sludge modification in the EC process of water treatment through different electrode material and current density. This modification can be made in EC water treatment process, and it may provide low-cost materials to different utilization of EC sludge.peerReviewe

Sulphate removal from mine water by precipitation as ettringite by newly developed electrochemical aluminium dosing method

Abstract Precipitation as ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) is an effective method for sulphate removal from mine water. The addition of calcium hydroxide and aluminium to sulphate-containing mine water in stoichiometric amounts induces an increase in pH to approximately 12.5, leading to the precipitation of ettringite. Typically, aluminium salts are used as the source of aluminium; however, in this research, an electrochemical dosage of aluminium was used, and the results were compared with the results of chemical ettringite precipitation as well as the results of computational simulations of sulphate removal. The mine water sulphate concentration was reduced 99.0% and 98.6% from the initial 1,060 ± 20 mg L⁻¹ using a current density of 28 mA cm⁻² in electrochemical aluminium dosing and chemical aluminium dosing, respectively, which was close to the theoretical 100% sulphate removal. When using the current density of 28 mA cm⁻² in the electrochemical aluminium dosing, the recovered ettringite purity was 92.5%, which was almost the same as the 92.6% purity in the chemical aluminium dosing characterised with X-ray diffraction and Rietveld analysis. The results indicate that the electrochemical dosing of aluminium could be an alternative to aluminium salt application in ettringite precipitation

Phosphate and ammonium removal from water through electrochemical and chemical precipitation of struvite

Abstract Batch electrocoagulation (BEC), continuous electrocoagulation (CEC), and chemical precipitation (CP) were compared in struvite (MgNH₄PO₄·6H₂O) precipitation from synthetic and authentic water. In synthetic water treatment (SWT), struvite yield was in BEC 1.72, CEC 0.61, and CP 1.54 kg/m³. Corresponding values in authentic water treatment (AWT) were 2.55, 3.04, and 2.47 kg/m³. In SWT, 1 kg struvite costs in BEC, CEC, and CP were 0.55, 0.55, and 0.11 €, respectively, for AWT 0.35, 0.22 and 0.07 €. Phosphate removal in SWT was 93.6, 74.5, and 71.6% in BEC, CEC, and CP, respectively, the corresponding rates in AWT were 89.7, 77.8, and 74.4%. Ammonium removal for SWT in BEC, CEC, and CP were 79.4, 51.5, and 62.5%, respectively, rates in AWT 56.1, 64.1, and 60.9%. Efficiency in CEC and BEC are equal in nutrient recovery in SWT, although energy efficiency was better in CEC. CP is cheaper than BEC and CEC

Use of Fe and Al containing electrocoagulation sludge as an adsorbent and a catalyst in water treatment

Abstract In this study, three different electrocoagulation (EC) sludges were studied as an adsorbent (removal of humic acids) and as a catalyst [catalytic wet peroxide oxidation (CWPO) of bisphenol A (BPA)]. The sludges originated from electrocoagulation process in which aluminum (Al) and iron (Fe) electrodes were used for the treatment of mining industry wastewater. All the materials were used as dried sludge and calcined material. The stability of these materials was studied in neutral and alkaline conditions with analysis of the leached iron content in solution. Based on the EC sludge characterization with X-ray fluorescence (XRF), X-ray diffractometer (XRD), and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy different forms of Fe occurring in EC sludges were found. The Brunauer–Emmett–Teller (BET) method showed reduced surface area after calcination process. Stability of the sludges was studied in neutral conditions, and the amount of iron leaching was low (&lt;1.4  ppm). Adsorption experiments showed that the removal of humic acids (measured as total organic carbon) was over 50% in all tested materials in the pH range of 3–9, and over 92% with the S3 calcined material in all studied pH range. The calcined samples were catalytically more active than raw material in CWPO of BPA. The highest removal of BPA was 85% over calcined sludge. Therefore, calcined EC sludges are suitable materials for catalyst and adsorbent use. The main novelty of this paper was the finding of sludge modification in the EC process of water treatment through different electrode material and current density. This modification can be made in EC water treatment process, and it may provide low-cost materials to different utilization of EC sludge