Non-Thermal Plasma for Chemical-Free Water Treatment

Before water reaches the tap, it must go through a range of treatment steps to make it wholesome and fit for human consumption. This involves the usage of large amounts of chemicals to remove dissolved and suspended contaminants and disinfection of the water. One of the important characteristics of water that has significant effect on the effectiveness of the treatment process is pH (a measure of the acidity of water). This parameter is so important to the point that if it is not properly controlled within a certain range, the treatment process fails. pH control is required for example during water clarification and disinfection and also during removal of phosphorus or heavy metals in wastewater. The requirement for pH control during water and wastewater processing is wide and touches almost all steps in the treatment process. However, accurate control of pH requires the use of a significant amount of chemicals such as lime, caustic soda, hydrochloric acid, and sulphuric acid. Until now, addition of chemicals to control the pH is the "norm" and this practice has not been challenged or disrupted. With increasing pressure on natural resources and stringent environmental and safety legislations, there is a need for new sustainable processes to reduce the use of chemicals and the associated carbon footprint with their transportation. With this in mind, radical thinking in this ignored area is required.

A new sustainable and innovative approach for pH control is the subject of this proposal. The basic concept of the new idea is that when water is exposed to plasma, its pH changes to either acidic or alkaline values. This project will use this concept to develop a new sustainable method for the control of pH in water and wastewater treatment processes and this technique is expected to lead to a chemical-free treatment method with a range of additional benefits.

Plasma plays an important role in a wide variety of industrial applications such as material processing and semiconductor manufacturing. More recently, interest has increased significantly in liquid plasma discharge as an alternative to conventional water treatment techniques. Out of the types of plasmas, non-thermal plasma (NTP) is of interest since the power used is mainly to generate the plasma without heating the bulk volume. NTP is produced by a high voltage discharge between two electrodes where a large amount of high energetic electrons, various reactive molecular and radical species, ions and photons are generated (e.g. OH-radicals, ozone, hydrogen peroxide, UV, shock waves). When water is exposed to this highly reactive environment, a range of useful chemistries for the oxidation of contaminants and disinfection of water resulting from the synergetic effects of these take place. Removal of contaminants in water has been the main focus of research carried out on plasma application in water treatment so far. However, when applied to water, NTP does not only produce oxidants and disinfectants, but also it has potential to change the pH of the water hence it can be used as a method to control pH in water. The primary focus of this research is to determine the optimal conditions for controlling pH in water under conditions similar to those expected in a real treatment process. This will involve the design and fabrication of three plasma prototypes with different electrode configurations, then determine the most effective NTP operating conditions that enables production of water at a given alkaline and acidic pH set values. The system will then be tested using real drinking waters and real wastewaters. The project will also provide an appreciation of the additional benefits that the system is expected to offer such as removal of pesticides, metals and endocrine disrupting chemicals and based on the outcomes of this study, the key issues that will determine the commercial potential of NTP for pH control will be defined.

Large volumes of chemicals such as lime, caustic soda, and acids are traditionally used to control the pH of water and wastewater to adequate values required in the treatment process. This practice has not been changed or challenged since its first use but with the increasing demand for natural resources, which are finite, increasing costs, and stringent environmental and safety regulations there is a need for alternative methods. The technology proposed in this proposal is unconventional and is expected to disrupt the methods currently used by water utilities to control the pH of the water and wastewater they process. The longer term aim of this research is to provide an effective sustainable technology for pH control in water which is expected to provide additional benefits in the treatment process such as removal of metals and water contaminants as well as water disinfection. So far there has been little attention to this area of the treatment process despite the significant amounts of chemicals used for pH control, which are in most cases transported to the treatment site. Transport and use of resources is unsustainable and the proposed method avoids both. The proposed method will develop into a major integral unit operation in the water and wastewater treatment processes. The work proposed in this proposal will lead to a commercially valuable and transformative technology suitable for implementation world-wide. The outcome of this research will benefit the UK economy as well as enhance its position in innovation in the water sector. The main beneficiary of this technology will be the water industry that will benefit a sustainable pH control technique offering no purchase and storage of chemicals, safer operating sites since there will be no handling of chemicals for pH control, reduction of impact on the environment, reduction in sludge volumes due to reduction in chemical use, reduction of the environmental impact resulting from the haulage of chemicals to the treatment sites, and reduction of the water microbial and contaminant loads. The latter benefit also leads to reductions in other chemicals used for disinfection and oxidation purposes (e.g. chlorine and ozone) as well as potential for the reduction of disinfection by-products. These together could lead to enormous cost savings on water and wastewater processing beneficial to industry and customers. The proposed technology would have a vast international market not only in the water sector but also other industries and niche areas requiring pH control would also benefit from the proposed technique.