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As the industry transitions towards sustainability, the use of newer technologies and approaches will be critical in meeting water quality goals and human health protection.
FREMONT, CA: Drinking water and other uses of water are constantly under threat from a variety of pollutants, making it increasingly challenging to ensure their quality. There are many sources of water pollution which can be broadly divided into two categories: point sources and dispersed sources.
Point-source pollutants are from a single, identifiable source, such as a factory or sewage outfall pipe. These types of pollutants are relatively easy to identify and control, as they flow from a single location and can be treated at the source comprising industrial wastewater, sewage, and oil spills. Dispersed sources constitute agricultural runoff, urban stormwater drainage, and atmospheric deposition. These types of pollutants are particularly challenging to control as they stem from multiple locations and can be arduous to identify and manage.
The types of water pollutants are diverse and can include pathogenic organisms, oxygen-demanding wastes, plant nutrients, synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, and heat. Domestic sewage is a significant source of many of these pollutants, particularly pathogenic organisms, plant nutrients, and oxygen-demanding wastes.
There are three types of wastewater, or sewage: domestic sewage, industrial sewage, and storm sewage. Domestic sewage is wastewater from homes and apartments and contains a wide range of impurities, including putrescible organic materials and plant nutrients. Industrial sewage contains specific and identifiable chemical compounds, depending on the nature of the industrial process. Storm sewage is runoff from precipitation that picks up a range of substances as it travels over the ground.
Principal pollutants
• Organic material
The biochemical oxygen demand, or BOD, is a measure of the amount of putrescible organic material present in sewage; the greater the BOD, the more oxygen is needed by microbes to break down the organic materials in sewage. It is among the most crucial factors in sewage treatment plant design and operation. BOD levels in industrial sewage can be several times higher than in-home sewage. When storm sewage is combined with household sewage in combined sewerage systems, the BOD of the combined sewage is particularly of concern.
For lakes and rivers, dissolved oxygen is a significant indicator of water quality. The greater the water quality, the higher the dissolved oxygen content. The organic compounds start to decompose when sewage enters a lake or stream. Since bacteria use oxygen in their metabolism, it is consumed, leading to the death of aquatic creatures. The water turns septic as oxygen content falls to zero and induces an unpleasant odour that is connected with the decomposition of organic substances in the absence of oxygen.
• Suspended solids
Suspended particles are a significant aspect of sewage. The total amount of suspended particles in the sewage directly affects the quantity of sludge generated in the treatment facility. Compared to household sewage, industrial and storm sewage have larger quantities of suspended particles. The effectiveness of the treatment process at a treatment plant is determined by the thoroughly suspended particles and BOD being removed.
• Plant nutrients
Compounds of nitrogen and phosphorus, two elements that are fundamental nutrients necessary for plant development, can be found in domestic sewage. Excessive nitrate and phosphate levels in lakes can hasten the growth of algae. In a process known as eutrophication, algal blooms increase the pace at which lakes naturally age.
• Microbes
Each gallon of domestic sewage includes several millions of bacteria. The majority are coliform bacteria from the human gastrointestinal system, while other germs may also be present in residential sewage. The presence of coliforms is a sign of sewage contamination. Typically, a high coliform count denotes recent sewage contamination.
Emerging technologies
Latest technologies are revolutionising the wastewater treatment industry, providing solutions to existing and future challenges. With stricter water quality standards, limited land areas, and climate change, there is a need for improved treatment methods, automation, and environmental considerations.
Improved treatment methods, such as the membrane bioreactor process, ballasted floc reactor, and integrated fixed-film activated sludge (IFAS) process, allow for increased treatment efficiency using less land. These methods provide both secondary and tertiary treatment in a small land area, making them ideal for upgrading older wastewater treatment facilities.
Automation is also a critical component of advanced wastewater purification processes. The use of online analytical instruments, programmable logic controllers (PLC), supervisory control and data acquisition (SCADA) systems, human-machine interface (HMI), and various process control software enables the automation and computerisation of treatment processes with the provision for remote operations. Such innovations improve system operations significantly, thus minimising supervision needs.
Environmental considerations are an alternative key aspect of emerging technologies in wastewater treatment. Natural treatments, energy conservation, and carbon footprint reduction are some of the key considerations for communities facing energy and electricity challenges. Green technologies and the use of renewable energy sources, including solar and wind power, for wastewater treatment, are evolving and will aid to minimise the environmental impacts of human activities. Ecological and economical natural wastewater treatment and disposal systems, such as constructed wetlands, lagoons, stabilisation ponds, soil filters, drip irrigation, groundwater recharge, and other similar systems, have already gained importance in many places, especially in smaller communities. These systems provide potential applications for environmentally friendly technologies.
Finally, newer technologies and approaches have continued to improve the efficiency by which energy, nutrients, and other chemicals are recovered from treatment plants, helping create a sustainable market and becoming a revenue generation source for wastewater processing facilities. Concepts such as nutrient trading have also emerged, which can help to minimise nutrient pollution effects as well as reduce financial burdens on societies for costly treatment plant upgrades. Improved treatment methods, automation, and environmental considerations are critical components that will continue to evolve and shape the future of wastewater treatment.
