The availability of clean water is a major problem facing the world. In particular, the cost and destruction caused by viruses in water remains an unresolved challenge and poses a major limitation on the use of recycled water. Here, we develop an environmentally friendly technology for sterilising water. The technology bubbles heated un-pressurised carbon dioxide or exhaust gases through wastewater in a bubble column, effectively destroying both bacteria and viruses. The process is extremely cost effective, with no concerning by-products, and has already been successfully scaled-up industrially.
Wastewater usually contains human enteric viruses like hepatitis and rotavirus and bacteria like Escherichia coli. If this water is to be reused it has to be disinfected. Collivignarelli et al.1 found that ultraviolet (UV) irradiation and chemical treatments using chlorine, chlorine dioxide, peracetic acid or ozone were the most used technologies for wastewater disinfection. However, all these water disinfection technologies have limitations. For example, chlorine and chlorine dioxide react with organic compounds and form reactive chlorinated organic compounds that are hazardous to humans. In addition, chlorine needs at least 30 min contact time and is not able to eliminate Cryptosporidium. Chlorine dioxide has high management costs and is very unstable. Other disinfection methods such as ozone and UV irradiation are complex to operate and maintain. Rotavirus can be resistant to UV treatments and its efficiency is affected by the dissolved organic and inorganics in the wastewater, as well as its colour and turbidity.2 Paracetic acid increases chemical oxygen demand (COD) and biochemical oxygen demand (BOD) due to the formation of acetic acid.1 Therefore, a major challenge exists to develop new, energy-efficient technologies to address these problems.
Here we report on one such candidate technology for sterilisation that seems to do the job. It uses atmospheric pressure bubbles of CO2 in a new device (ABCD). If this process successfully inactivates MS2 virus (ATCC15597-B1) and E. coli C-3000 (ATCC15597), that are surrogates for enteric pathogens, then this technology will be able to inactivate real waterborne viruses and bacteria for water reuse without the need for (high energy) boiling.
In preceding work3,4 we conducted different experiments where the bubble diameter of 1–3 mm was measured using high speed cameras. An earlier variant we called the hot bubble column evaporator (HBCE) process.5,6,7 It used hot air bubbles of 1–3 mm diameter and was operated in the temperature range of 150–250 °C. The bubbles transferred heat to surrounding water and thermally inactivated dispersed viruses and bacterial cells. At the same time, low, steady-state solution temperatures in the range of 42–55 °C were maintained.8 An instantaneous transient hot surface layer must also form around the rising, initially hot, air bubbles. The inactivation process clearly involves collisions of bacteria or viruses with the hot air bubbles5,6 and the surrounding heated layers.7 Other gases (air, N2, O2 and Argon) achieved similar inactivation results, at 200 °C inlet gas temperatures for viruses and at 150 °C for bacteria.9 However, CO2 gas, at the same inlet gas temperature, is far superior with much higher inactivation rates at lower temperatures than with other gases.9 Hence, we here embark on a more thorough study of the effects of CO2 bubbling on viral and bacterial inactivation in pure sodium chloride solutions, using the HBCE device at atmospheric pressure with the acronym ABCD.
Many waste disposal industries like landfills, bio-gas plants and coal power plants emit large amounts of CO2. Hence, the potential use of CO2 bubbles in water treatment processes to sterilise water at atmospheric pressure offers an attractive new technology at the very least. Earlier we showed9 too that the heat generated in exhaust combustion gases that contain CO2 can also be used to increase the performance of this new sterilisation treatment. That we will also take further.
The process is very different to others that involve CO2. Thus, many authors10 have shown that pressurised CO2 in a range of 5 to 1000 atm can achieve viral and bacterial inactivation.
High-pressure carbon dioxide has been proposed as a cold pasteurisation alternative for more than 25 years.11 The new ABCD reactor, described here, achieves equivalent or better results but without the need for pressurisation, i.e., at just 1 atm. The process has been patented by the University of New South Wales as Australian Patent Application No. 2017904797.