Traditionally, chlorine and related compounds, or hydrogen peroxide / peracetic acid, have been used. Electrochemical and photocatalytic methods offer alternatives for water system disinfection. 

Electrochemical disinfection

Electrochemical disinfection is a physical-chemical method by which chemical oxidants are generated in situ via redox reactions on the surface of an electrode ². This method does not lead to the transport and storage of hazardous materials ³. Furthermore, it introduces economic efficiencies since it can be readily scaled.

Multiple oxidants can be considered for electrochemical disinfection. The process needs to be modified as per the oxidant, electrode type, water composition and system operating voltage. Examples of oxidants include free chlorine, hydroxyl radicals or sulphate radicals. Electrogenerated chlorine is the most common, where breakpoint chlorination is used to assess for how long chlorine needs to be generated (which is until all available ammonia has reacted and free chlorine is detected in the water system) ⁴.

The effect is seemingly superior to conventional chemical treatment. A recent review found the synergetic impact of radicals and chlorine, together with the contribution of high chlorine concentration at acidic pH near the anode surface, were the main factors enhancing the disinfection performance of in situ electrochlorination, based on a single pass of water through the electrochemical cell ⁵.

In terms of design, there are several variations. These include electrode cell geometries in the form of separator-divided or undivided cells with immersed electrodes, parallel plate electrodes, 3D-flow-by and flow-through electrodes, rods, and tubular electrodes in monopolar, bipolar, or mixed arrangements. For the killing of microorganisms, the most important consideration is with the inactivation kinetics and the associated quantification of disinfection results. Inactivation kinetics and modelling frameworks are necessary to elucidate the disinfection performance and to help optimise operational disinfection conditions.

There is a trade-off between those systems that operate at higher overpotentials - which consume more energy per dose - and contact times. The more energy exerted, the faster the process. At the same time, the more energy exerted, the greater the cost.

Photocatalytic disinfection 

Photocatalytic disinfection is the reaction between a photocatalyst (the initiator of reaction) and an aqueous media (the target of disinfection). Photocatalysis can inactivate microorganisms and degrade organic pollutants contained in wastewater. It can also reduce many inorganic pollutants into harmless substances. 

In terms of the mechanism, the adsorption of a photon with sufficient energy by TiO2 promotes electrons to be released. This creates reactive oxygen species. In water, this creates hydrogen peroxide, hydroxyl and hydroperoxyl radicals, which cause the loss of membrane respiratory activity within bacterial cells. 

The primary mode for killing bacteria is due to membrane and cell wall damage, caused through lipid peroxidation products and the leakage of intercellular components like cations, RNA and protein. The efficacy against Gram-Negative Bacteria (GNB) has been widely reported ⁶. 

The process of photocatalysis has the advantage of being activated by s

olar energy and not producing any toxic by-products. However, optimisation can be complex as water characteristics influence the surface chemistry of the photocatalyst materials. This highlights the need to understand the nature of the water to be treated, the impact of seasonality and the types of chemicals present. Photocatalytic disinfection is also dependent on different operational parameters like photocatalysts dosage, pH, light source and temperature. 

In a related application, TiO2-coated filters have been tested out for the disinfection of air with some success, causing oxidation of bacteria suspended on microbial carrying particles ⁷. Other catalytic processes that have the potential to decontaminate water include electrocatalysis and ozone catalysis.

Optimisation with artificial intelligence

As with most areas of life, Artificial Intelligence (AI) is presenting opportunities to improve processes. For example, in China, researchers at the Shanghai Institute of Pollution Control and Ecological Security have used AI (fuzzy logic-based neuro systems) to assess complex variations and to accurately adjust outputs for wastewater disinfection ⁸. This extends to ensuring public safety by applying AI to control disinfection by-products and residues (and to meet water discharge standards).

Summary

The treatment of wastewater, especially from hospitals and pharmaceutical plants, is very important, both as part of a process to reduce pollutants but also for the control of pathogens. 

Attention must also be paid to microbial reduction when making water suitable for drinking. Environmental concerns are directing treatment processes away from high volumes of chemicals to alternative measures. This article has considered electrochemical and photocatalytic alternatives.