Using bacteria to convert wastewater sludge into valuable resources

Today, about two thirds of the world’s population are guaranteed clean drinking-water. Fortunately, this percentage is rising. However, global threats such as pollution through fossil fuels, climate change with droughts and floods, increasing population, increasing urbanisation and lack of investment in infrastructure, including wastewater treatment, are threatening this trend. The demand for water has never been greater, and water has therefore never been higher on the political agenda. The focus in wastewater treatment for decades has been on developing even better physical and chemical methods, but Dutch microbiologist Mark von Loosdrecht has focused on the biological aspects. 

“Development in environmental technology is almost exclusively the preserve of engineers, and they are not that concerned about what all the microorganisms in the water do,” explains Mark van Loosdrecht, Professor in Environmental Biotechnology at Delft University of Technology, The Netherlands. 

“They call it sludge and just want to get rid of all the impurities. What I am trying to do is understand the bacteria and their interactions instead and, with their help, try to recover all the valuable nutrients from the wastewater. Wastewater treatment can therefore actually become good business rather than an increasing expense for countries globally.” 

Come and work for me – but somewhere else 

Most people think of sludge as a disgusting sticky mass that needs to be discarded. But this semi-solid material from industrial processes, from wastewater treatment and from our toilets actually contains many valuable substances, and with a little help from microorganisms, it can become a valuable resource. For Mark van Loosdrecht, the dream of transforming it started more than 30 years ago – not as a dream of saving the world but through fascination about the often hidden valuable properties of bacteria. 

“During my very first studies of bacteria, while completing my education in Environmental Engineering at Wageningen University & Research, I investigated how bacteria interact with surfaces. It rapidly dawned on me that when engineers study bacteria, they often do not have very good control over the conditions under which the bacteria were grown or the actions of the bacteria themselves. Conversely, as a microbiologist, I did not want to study bacteria alone. I wanted to do both,” says Mark van Loosdrecht. 

Actually, Mark van Loosdrecht got somewhat tired of academia and had agreed with Sef Heijnen, whom he had met while studying for his PhD degree, that he would apply for a job at GistBrocades, where Heijnen worked. However, in the meantime Heijnen had returned to academia. “He said: ‘Yes, you can still come and work with me, but somewhere else,’” he recalls.

Somewhere else was Delft University of Technology, and the topic was developing water and wastewater treatment processes. Ever since he changed directions in 1988, Mark van Loosdrecht has worked in the same place and on the same interdisciplinary topics: integrating microbiology and engineering and integrating research and practice. 

“Wastewater treatment is ideal for studying microbial ecology because it is a diverse microbial ecosystem in which many properties can be studied. Further, when you discover things in the lab, you can often evaluate them easily in practice. This often generates new questions that are taken back to the lab. So this is where science and technology can help each other,” explains Mark van Loosdrecht. 

Combining science and engineering 

Researchers in the Netherlands quickly realised the advantages combining fundamental knowledge of the capabilities of bacteria and their interaction with wastewater treatment. 

“The field was back then essentially very conservative. Those who studied wastewater treatment optimised filters to remove all the impurities. Those who studied bacteria usually studied a specific type of bacterial culture to determine how it behaves. We decided instead to focus the research on the bacterial communities naturally found in wastewater that help to purify it,” explains Mark van Loosdrecht. 

The use of microorganisms to purify water dates back more than a century, when British researchers found that oxygenating wastewater could remove harmful nitrogen-containing compounds such as ammonia. They called this activated sludge, and only much later realised that it was the living organisms in the wastewater that had been activated. The discovery kick-started an interest in understanding how these organisms function and help to remove nitrogen and phosphorus compounds along with other harmful compounds from wastewater. 

“We realised in earlier studies that cultivating pure cultures of bacteria in the laboratory tends to change their properties so that they no longer resemble their naturally occurring ancestors. So even if bacteria could be isolated that could remove a specific substance, other aspects of their behaviour change. The organism is not behaving anymore as it was when it was mixed with the wastewater, where the conditions were completely different and where they coexisted and interacted with many other organisms,” recalls Mark van Loosdrecht. 

Very efficient method 

Mark van Loosdrecht and colleagues therefore decided to study the bacteria under their natural conditions in the wastewater in order to foster and cultivate the relevant organisms. A more dynamic and selective enrichment technique was therefore required – and this became very successful within a few years. 

“The discharge of phosphates into surface water leads to eutrophication and blooming of algae, which was a huge problem at the time. We refined, adopted and further developed an existing process into an efficient technology by integrating chemical and biological removal. This resulted in an optimal combined system for efficient nutrient removal with minimal energy usage to be obtained,” says Mark van Loosdrecht. 

In 1998, this BCFS® process for phosphorus recovery became the first of a series of new wastewater treatment processes developed by Mark van Loosdrecht, his team in Delft and Dutch water utilities. In the same year, the researchers also developed a particularly efficient method to remove ammonia – one of the main sources of nitrogen pollution, alongside nitrogen oxides.

“Ammonia pollution can have substantial effects on both human health worsening cardiovascular and respiratory diseases and the biodiversity through the accumulation of nitrogen in plant species. Another major nitrogen source, nitrous oxide, remains in the atmosphere for 114 years before being removed naturally. One tonne of N2 O affects global warming almost 300 times that of 1 tonne of C2 O,” explains Mark van Loosdrecht. 

A perfect match 

Over the years the researchers had searched for biological means to solve the nitrogen issues. In the early 1990s Mark van Loosdrecht’s research colleagues in Gijs Kuenen’s research group had surprisingly found bacteria that could convert ammonia and nitrite directly into nitrogen gas – the ANAMMOX® (anaerobic ammonium oxidation) process. 

 “By skipping several steps in the nitrogen cycle, this bacterial process saves a lot of time, chemicals and energy. However, for the process to take place, an acceptor of an electron is needed, so the bacteria can donate electrons from ammonium to nitrite,” adds Mark van Loosdrecht. 

By studying the bacterial communities in detail, Mark van Loosdrecht and his colleagues became instrumental in gaining a better biological understanding of the ANAMMOX® process, and in 1998 the development of the SHARON (single-reactor system for high-activity ammonium removal over nitrite) process became the launching pad for a huge breakthrough, which solved the central issue with the ANAMMOX® process, the lack of an electron acceptor. 

“In the SHARON process we take advantage of the growth rate differences between Nitrosomonas, which oxidise ammonium to nitrite and Nitrobacter, which can further oxidise the nitrite to nitrate. By favouring Nitrosomonas, the product from the SHARON process is nitrite, an electron acceptor, so the SHARON process can then efficiently be used to scale up the ANAMMOX®  process and directly convert nitrite into nitrogen gas,” explains Mark van Loosdrecht.

Training bacteria 

In the following years, while SHARON and ANAMMOX® plants were implemented worldwide, Mark van Loosdrecht took wastewater treatment to the next level by studying other natural microbial processes in sludge, combining his experience in wastewater treatment with his knowledge about the physical interactions between bacteria on which he had focused at the start of his career.

“When bacteria grow together, they can make very fluffy structures, as in conventional wastewater treatment, or compact structures that much more easily separate from the cleaned water. The structure formation appeared to be more a physical than biological process, where slow-growing bacteria enable the right conditions to be created for compact structures. Coincidentally, the phosphate-removing bacteria are such bacteria. We thereby converted the activated sludge into small granules that contain various layers of these bacteria trained to perform a specific function,” says Mark van Loosdrecht. 

By cultivating granular sludge on wastewater, Mark van Loosdrecht and colleagues could now not only improve the treatment of wastewater. Once the water is pure, the granules settle at the bottom of the tanks and can be reused. The big advantage here was time. Conventionally activated sludge settles at about 1 metre per hour, whereas the smoother and denser bacterial granules managed 1 metre in 4 minutes. The researchers dubbed the technology Nereda® – derived from the Nereids, mythical Greek water nymphs. 

“Making these processes mechanically simpler and cheaper requires deep understanding of the complex interaction between biology and process technology. Nereda® requires much less space and uses half as much energy as conventional methods. The principles of forming granular sludge are based on physics, but in the end the bacteria do the job,” explains Mark van Loosdrecht. 

It again emphasise the importance of interdisciplinary research. 

Jewellery from the toilet 

The fascination for new knowledge has been absolutely crucial in driving Mark van Loosdrecht and his colleagues towards their new remarkable discoveries. An example of how curiosity, economics and social responsibility can often be integrated is vivianite, a hydrated iron phosphate mineral. In 2016, scientists discovered that it could help solve one of the greatest challenges in wastewater treatment – phosphate discharge, which is an environmental problem but phosphorus is also a limited resource. 

“Phosphorus recycling is crucial for creating a sustainable circular economy and balance in nature. Iron binds phosphate as vivianite, which can then be removed with a magnetic separator. Vivianite can be used as a fertiliser. The curiosity towards how iron behaves in a wastewater treatment process has led to an interesting innovation stimulating a circular economy. We need to be more curious and think outside the box,” says Mark van Loosdrecht. 

Although the vision of Mark van Loosdrecht and his colleagues in Delft of turning sludge into economically profitable resources has not yet been achieved, they are now closer to realising their long-standing dream. They have discovered several substances with a possible market potential by analysing the Nerada® granules, such as Kaumera Nerada® Gum, a sustainable biopolymer. 

“There is already a market for this biopolymer, which in combination with clay has proven to be comparable to some of the existing fibre-reinforced plastics – and with even better fireretardant and heat-resistant properties. The material is actually so attractive that a colleague has started making jewellery from Kaumera Nerada® Gum. Its beautiful pearly lustre can remind us of things that can be made from what we flush down the toilet today,” concludes Mark van Loosdrecht.