Wastewater Treatment Challenges

In developed areas of the world, each person produces about 150 l of wastewater per day. That is about 50,000 l (12,500 gallons) per person per year, or about 10 million cubic meters for a city of 100,000 people. We use water to flush the toilet, wash our dishes, or have a shower. Normally we do not think about what happens to the water when it goes down the drainpipe—out of sight is out of mind! To avoid damaging the environment, wastewater must be treated before it is released into oceans, lakes, and rivers. Most wastewater worldwide is not treated, and even treated wastewater can still contain pollutants. The scale of the wastewater problem is enormous, and it gets worse each day as the world’s population grows. Industrial wastewater is also a concern, because water coming from manufacturing can be contaminated with dangerous chemicals and heavy metals.

Wastewater treatment plants do a good job of removing most contaminants, but nutrients like phosphorus and nitrogen are not removed. You might wonder why nutrients can be bad, but nutrient pollution can damage oceans, lakes, and rivers. How? The nutrients released into the environment cause lots of microalgae to grow in the waterways where wastewater is released. Microalgae are tiny, single-celled plants and, like all plants, they use sunlight to perform photosynthesis and produce oxygen. However, when these large masses of algae die, the bacteria that break them down use up lots of oxygen. This can lead to vast regions of the world’s oceans that have too little oxygen (Figure 1). These areas are called low-oxygen zones, and they can be deadly to the fish and shellfish we eat, along with many other organisms [1]. When some species of algae grow excessively, they can also produce certain chemicals that are toxic to humans and animals.

But what if microalgae could be part of the solution instead of the problem? Specifically, what if these organisms could be used to improve wastewater treatment? First, we will tell you about how wastewater is commonly treated, and then we will explain how microalgae can help!

Wastewater Treatment Overview

Treatment of wastewater is essential to avoid pollution of lakes, oceans, and rivers (Figure 2). The first step of treatment is screening. Screening removes large particles such as wood, grease, rags, plastic, and gravel. Then comes the removal of smaller, dense particles. This is done using gravity—by letting the wastewater sit in large tanks so the particles can settle to the bottom. The wastewater is now ready for the next step of treatment, which normally uses bacteria to remove contaminants from wastewater. How is this possible? Well, the bacteria eat the pollutants and clean up the water. There are several technologies that use bacteria to treat wastewater, and one example is called the activated sludge system, which has been around for over a century. This system contains an aeration tank, where bacteria are supplied with oxygen so they can clean the wastewater in a process called biodegradation. Supplying oxygen to bacteria is expensive, and the bacteria still do not eat all the phosphorus and nitrogen. After treatment, the bacteria are separated from the treated water in a settling tank. The third step in wastewater treatment is to kill any remaining bacteria to produce treated wastewater that is released to the environment.

What about the stubborn pollutants that remain in the water? The water may need further treatments, including filtration, ultraviolet light, and a chemical called ozone, to sterilize it and remove the remaining pollutants—but even these treatments do not sufficiently remove all nutrients.

How Can Microalgae Improve Wastewater Treatment?

Scientists have shown that microalgae can be used to improve the efficiency of the wastewater treatment process. Along with breaking down contaminants, microalgae also produce oxygen, consume carbon dioxide, and remove nutrients like phosphorus and nitrogen from the wastewater more completely than traditional wastewater treatment does. And it is less expensive, too!

Even better, the large amounts of algae, called algae biomass, grown at wastewater treatment plants could then be used to produce products including biofuels (replacements for fossil fuels like gas or oil) and bioplastics (replacements for traditional plastics) [2, 3]. The biomass that is leftover could be composted and converted into fertilizer to support farming. This technique basically allows the nutrients removed from wastewater to be recycled.

Production of biofuels from microalgae is usually expensive, with the cost and availability of chemical nutrients, especially phosphorus, being a key limitation [4]. The cost of producing biofuels can be decreased if wastewater is used as a source of nutrients for microalgae. Figure 1 shows that massive amounts of algae can grow using nutrients in wastewater [1]. So, instead of releasing nutrients from wastewater into to the environment and creating massive amounts of algae biomass in the oceans, this algae biomass could be created at future wastewater treatment plants that then recycle the nutrients. This technology has not yet been widely used [5], but scientists and engineers could make it happen, and future wastewater treatment plants could instead be called resource recovery facilities.

The Future of Wastewater Treatment

To protect the planet, one of the 17 goals set out by the United Nations is to ensure safe water and safe wastewater disposal for all. Making valuable products while effectively treating wastewater is a good way forward! Wastewater treatment technologies of the future will not only produce clean water, but will also capture the precious nutrients, that were in poo, in the form of biomass. Captured biomass can then be used to produce biofuels, bioplastics, and fertilizers.

The conversion of wastewater treatment plants to resource-recovery facilities will be a major challenge requiring technological advances in many fields, including engineering, robotics, biology, chemistry, and public health. Wastewater treatment facilities at universities could provide ideal sites for developing the necessary technologies and could also provide educational and training opportunities. All developed areas on Earth need wastewater treatment. Might your future job be at a resource recovery facility?

Glossary

Wastewater: ↑ Water containing pollutants such as human poo and any water that goes down the drain is called wastewater.

Pollutants: ↑ Pollutants are unwanted materials found in wastewater like poo, toxic chemicals, and nutrients like nitrogen and phosphorus.

Wastewater Treatment Plants: ↑ Facilities that are designed to remove pollutants from wastewater are called wastewater treatment plants.

Microalgae: ↑ Microalgae are single cell plants that grow in water using sunlight and/or pollutants.

Biodegradation: ↑ Bacteria and microalgae can eat many pollutants in a process known as biodegradation.

Biomass: ↑ Biomass is renewable organic matter obtained from plants and animals. It holds the stored energy from the sun that plants capture through photosynthesis. Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

Biofuels: ↑ Microalgae and bacteria can make chemicals that can be used as fuel, such as diesel, ethanol, and methane. Fuels made by microalgae and bacteria are called biofuels.

Bioplastics: ↑ Some chemicals made by microalgae and bacteria can be used as substitutes for plastics made from petroleum. Bioplastics are plastic-like materials made by living things.

Resource Recovery Facility: ↑ An improved wastewater treatment plant that not only cleans wastewater, but also recovers and recycles nutrients is a resource recovery facility.

Conflict of Interest

Author AK was employed by the company KUKK K&F Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors acknowledge the value of collaboration to understand and address environmental problems.