Deployment of fenton and photo-fenton processes for the treatment of municipal and industrial wastewater

Abstract

Effective wastewater treatment is a key aspect in safeguarding a healthy aqueous environment, however many treatment technologies have limitations of either cost or generation of additional waste, which ends up further polluting the environment. Hence, this work focuses on the deployment of iron mediated processes, such as photo-Fenton oxidation, for wastewater treatment and seeks to provide environmentally sustainable solutions to technical challenges associated with these processes. Challenges in the application of photo-Fenton process include the (a) high energy consumption and generation of environmental impacts attributed to the use of artificial UV irradiation during treatment, (b) reduction of the efficiency of the process due to the formation of fouling on quartz sleeves, and (c) depletion of natural resources because of the need to add iron in the wastewater to initiate the Fenton reaction. To address these challenges the following solutions were explored.

First, a solar photo-Fenton process based on a circular economy concept was developed as to minimise the use of energy and chemicals addition in the wastewater. For this purpose, acid mine drainage (AMD), which is very acidic pH˂3 and rich in iron, was used to deliver iron catalyst in the Fenton/photo-Fenton treatment process for the treatment of municipal wastewater. At the best conditions assayed, it was observed that COD removal reached 99% after 90 min of photo-Fenton treatment under simulated solar light in the presence of 30 mg/L Fe. The removal efficiency of the treatment system indicated that this circular economy concept can be very promising and should be considered for wastewater treatment on large-scale.

Secondly, lab-scale experiments were performed to further understand the effect of basic operating parameters on the formation of quartz fouling during the photo-Fenton treatment of municipal wastewater. It was observed that fouling formation was higher in phosphorous-rich followed by iron-rich wastewater. At the same time 30% and 59% COD removal was observed during UVA photolysis and photo-Fenton (UVA/Fe/H2O2) processes, respectively. Moreover, it was observed that iron-rich wastewaters are more suitable for UV-assisted treatment, since fouling formation on quartz sleeves can be minimized by the formation of oxidizing agents, such as hydroxyl radicals during treatment.

Furthermore, this study aims to discover the potential of iron-based processes (i.e., Fenton and coagulation) to remove persistent organic pollutants, such as polymers, from wastewater. Chemical industrial wastewater can seriously affect the receiving environment since it contains various highly dangerous and toxic organic compounds for which conventional treatment is not sufficient. PW1 (COD around 13,000 mg/L) was treated with Fenton reagent i.e., Fe+2 from FeSO4.7H2O and H2O2. The experiments were done for two different ratios of Fe:H2O2:COD 1:1:1 and 2:1:1. The 2:1:1 was performed with dilution of the raw low polymer effluent and was successful in achieving about 90% of COD removal efficiency. Moreover, the BOD5/COD ratio for the treated as well as raw low polymer effluent was calculated based on the BOD5 and COD results.

The iron coagulations process was then applied to treat higher polluted polymer effluent, PW2 (COD around 58,000 mg/L). Iron was used as a coagulant in this case. The effect of various operating factors and the amount of sludge formation after treatment were examined. The effect of Fe:COD ratio as well as of pH was studied on PW2 coagulation treatment. Best removal rate (85.87%) was observed at pH 7 for Fe:COD of 1:1 since PAA are deprotonated at neutral pH. Sludge produced during the coagulation treatment was also measured and analysed.

All in all, iron-based oxidation processes can be a promising environmentally sustainable process for wastewater treatment.