Clean water is crucial for human survival and health. For thousands of years, many civilizations have risen and fallen on the ability to secure sources of water and use them to their advantage. To this day, there are parts of the world that have low water security and have to rely on water that may be contaminated with pathogens, chemicals or both. There are two major ways to produce water that is fit for human consumption. The first is to extract it from the environment, such as drawing it from wells, storing and using rainwater, or by directly treating seawater to remove the salt. The second, which is becoming more and more common as population growth increases and the amount of readily available surface water decreases, is by recycling the wastewater that we produce in the course of our everyday lives, both through filtration and through more advanced techniques like electrocoagulation.
(Re)cycles of History
Water recycling has a history that parallels the growth of large cities, and started with the introduction of mass sewerage in the mid-19th century, following the development of indoor plumbing. What is now called primary treatment of sewage was developed at the time and involved draining it to a lagoon and separating out the solids to be disposed of separately. However, this process still left problems with odours, until it was found at the end of the 19th century that introducing oxygen into the decomposing sewage reduced them significantly. This breakthrough was the beginning of the biological aerobic and anaerobic treatments (secondary treatment) that are fundamental in modern wastewater processing.
By the early 20th century, the amount of human and industrial liquid waste that required treatment had grown exponentially. Such waste was normally piped directly into rivers and oceans, causing various environmental problems. As a result, the first large-scale water waste treatment plans began to be built by the 1930s in industrialized nations around the world, but it was not until mid to late in the century that laws began to be passed mandating the treatment of such water up to certain regulatory standards. Currently, after primary and secondary treatment, oxidation and/or chlorination are used to increase water quality in modern treatment plants.
This Bit’s Mostly Filter
A key component of wastewater treatment is filtration, used to separate solids (also known as sludge) from liquid waste. Various types of filters exist, and have differing success rates depending on the materials used. For example, ceramic filters are often used in developing countries as they are very cost-effective, remove the majority of pathogens and can improve water quality by 60–70%. However, these filters are not adequate in the treatment of wastewater, which often contains not only larger pathogens but also toxic chemicals that can slip through the filter pores.
To combat this problem, modern industrial water filtration systems use a variety of techniques, mainly focused around membrane filtration: the tinier the pores in the filter, the more undesirable content that is filtered out. Reverse osmosis, which operates by applying pressure to a solution with a high concentration of contaminants to force water through the pores and leave the contaminants behind, can also help. However, this process isn’t perfect, and some contaminants are extremely hard to remove with standard filtering techniques alone.
An Electrifying Approach
One kind of technology, known as electrocoagulation (also known as radio frequency diathermy or short wave electrolysis) can help to produce water that is cleaner and clearer than that produced by standard filtration. The technique can remove contaminants that filtration has a hard time with, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. It works by using electrodes to induce charged particles in the liquid that cause the contaminants to react and become less soluble, or coagulate together, allowing their easier removal. There are several reactions that take place independently, each contributing to removing different types of impurities:
• Seeding: creating large, stable insoluble complexes of metal ions that can be filtered out
• Emulsion breaking: charged hydrogen and oxygen particles break apart oils
• Halogen complexing: metal ions bind to chlorines in pesticides etc, making them less soluble
• Bleaching: oxygen ions destroy bacteria, viruses, cyanides and other biohazards
Also, oxidation-reduction reactions that would normally happen in nature are sped up, further breaking down harmful compounds, and the pH of the water swings toward neutral as the process continues.
Better Living Through Chemistry
Electrocoagulation is starting to be used more and more in water treatment, and has the advantages of being cheap to use as well as reducing the reliance on other chemicals to treat water (cutting down on the risk of secondary contamination). Coupled with its ability to remove more contaminants than standard filtration, it is little wonder that the technique’s popularity is increasing. As the technology continues to improve, the increase in potential water security and the reduction in environmental harm should ensure that electrocoagulation is at the forefront of industrial water treatment in the future.