Chemical coagulation and flocculation are conventional methods for the treatment of pollutant wastewater. Electrocoagulation (EC) is an experiential water treatment technology that presents an innovative and novel alternative in which sacrificial metal anodes dissolve controllably to dose wastewater electrochemically. Its principle advantage is its ability to provide active cations necessary for coagulation without chemical addition.
The technology however suffers from sporadic pollutant-centred studies scattered throughout the literature that focus only on optimising EC for the removal of a particular contaminant. The purpose of the work presented here is to provide a holistic study of EC for the benefit of understanding the effect of various parameters on the fate of floc particles and coagulation. It is hoped that this will aid in advancing the technology beyond the current pollutant-centred studies and allow future researchers to base operational parameters on the trends and patterns presented in this work.
In order to understand the impact of EC parameters on flocculation two wastewater sources were used. Trickling filter bed effluent from a sewage treatment plant containing 0.2 mmol/L phosphate pollution and a synthetic turbid system containing 1 g/L suspended colloids were used. This was due to their difference in pollutant nature, commonality in wastewater treatment applications, and their availability at the time of study.
The parameters investigated were; pollutant loading, inter-electrode distance, coagulant concentration, current density, agitation, electrolysis time, coagulant retention time, pH, and conductivity. Studies were also undertaken to compare EC to conventional chemical coagulation. Phosphate concentration, total suspended solids and turbidity pollutant removal efficiencies were used to infer the impact of the various chemical and process parameters on flocculation.
It was found that initial EC experiments could not compete similarly to chemical
coagulation due to iron speciation differences between the two separate coagulant methods. It was found that non-EC parameters such as dissolved oxygen content and floc seeding played an important part in affecting flocculation along with coagulant concentration and influent pH. Characterisation of floc aggregates formed revealed differences in floc sizes
related to the various pollutant removal mechanisms of charge neutralisation and sweep-flocculation. Zeta potential measurements carried out delineated pH conditions in which either removal mechanism would prevail, whilst floc retention time studies provided information on the mechanisms involved in converting active iron hydroxide floc particles to deactivated magnetite particles. Ultimately, trends were delineated for all parameters studied revealing how changes to operational conditions affected; (i) coagulant removal mechanism, (ii) efficacy of produced flocculant, and (iii) size of floc particles.