Unlocking the biological heart of sewage treatment

Installation of HKUST’s SANI process © HKUST

Biological processes have long provided the living heart of sewage treatment. Keith Hayward spoke to co-editors of the second edition of Biological Wastewater Treatment, a leading reference book in this area, about recent progress with unlocking even more potential.

The activated sludge process, used so widely in sewage treatment, celebrated its 100-year anniversary in 2014. Seen against that heritage, the past decade or so is not so long. But it is a decade in which sewage treatment built around biological processes has continued to evolve. This progress is reflected in the updates and new chapters included in the second edition of the book Biological Wastewater Treatment, the first edition of which was published by IWA Publishing in 2008.

One area that has evolved is the use of granular activated sludge processes, covered by a new chapter dedicated to this topic. Professor Mark van Loosdrecht at the Delft University of Technology has been instrumental in the development of the Nereda process, a commercialised granular activated sludge process. Nereda was developed by a consortium of Delft University of Technology, Dutch water authorities, and consultancy Royal HaskoningDHV, with the latter registering the Nereda trademark.

The formation of granular sludge, which settles rapidly, was originally exploited in anaerobic treatment. Its aerobic equivalent for biological nutrient removal emerged in the early 2000s.

Bulking of sludge flocs can be a problem in activated sludge treatment plants – indeed, the book includes a chapter on managing this. Van Loosdrecht describes granular sludge technology as “not much more than preventing bulking sludge and bringing it into the extreme”.

That seemingly straightforward premise has clearly brought progress. Royal HaskoningDHV’s website lists more than 60 municipal and industrial plants around the world. According to Van Loosdrecht, there are approximately another 40 under construction. “As an alternative to the activated sludge process, it has entered into the market relatively rapidly,” he says.

Van Loosdrecht explains that, when compared with traditional treatment, Nereda requires fewer pumps and requires a single reactor and no clarifier. “People perceive it as more complex, but actually it is simpler,” he says.

For Van Loosdrecht, this progress is all the more significant given that countries such as the Netherlands already largely have the wastewater treatment infrastructure they need and so will tend to work with existing assets. In the case of the Netherlands, he notes that only 12 new sewage treatment plants have been built in the country in recent years. “Of the plants that have been built in the last seven years, they are all granular sludge,” he says.

This indicates that there is a willingness in the sector to adopt new technologies, but that the speed of adoption has to be viewed in the light of the long life of existing assets. “I think the perception and reception is very positive,” adds Van Loosdrecht.

Extracellular materials

Another interesting dimension to aerobic granular sludge is that the granule formation involves gel-forming extracellular polymers produced by the microbes. In the case of Nereda, this has been pursued as a commercial opportunity through the development of a gum called Kaumera.

Van Loosdrecht explains that this started with work some 10 years ago to better understand the formation of the granules, with the formation of a consortium starting four or five years ago to explore opportunities more closely.

“We still don’t know exactly what the polymer is, but we know what it can do,” he says, adding: “We are busy developing products.”

Van Loosdrecht says that there has already been the first commercial shipping of Kaumera, for use as a binder in the making of fertiliser pellets. The demonstration plant can produce up to 300 tonnes a year. Other potential uses include production of composite materials, including for use in construction, of non-flammable materials, and of flocculants.

“We know how to extract it – that can be done at a reasonable scale,” says Van Loosdrecht. “Currently the focus is on product design.”

Resource recovery and reuse

This development with Kaumera reflects the escalation over the past decade of interest in resource recovery and reuse, and in opportunities around the circular economy.

Professor Damir Brdjanovic of IHE Delft Institute for Water Education notes that biological treatment has evolved in this time across its various dimensions, covered in the many chapters of the book. “We made sure that, where we show the latest developments, we incorporated, wherever we could, elements of recycling and reuse of materials,” he says.

Production and capture of biogas is long established in the sector, but he notes the more intensive recent drive towards viewing sewage treatment plants as ‘energy factories’. For example, use of the anammox process improves the net energy production at treatment plants, so extra attention is given to this in the second edition, he adds.

Brdjanovic cites other examples, such as the recovery of phosphate as vivianite or struvite, the potential to harvest biopolymers such as alginate, and the production of bioplastics.

Brdjanovic mentions also the growing recognition of the potential around the vast quantity of toilet paper flushed into sewers each day. “We get tonnes of cellulose ending up in the sewage,” he says. According to Brdjanovic, about 20-30% of particulate COD (chemical oxxygen demand) is related to cellulose fibres, and it can be recovered from sewage. “This cellulose can be reused in industrial processes, or it can be hydrolysed into sugars and fermented into volatile fatty acids, which can be reused again in the treatment plant for better removal of phosphorus and nitrogen,” he explains.

“There are a number of areas where we address the possibility to recover water, energy and chemicals, which contributes not only to biological wastewater treatment efficiency, but allows for closing of cycles and reuse of resources,” he adds.

Sulphur solutions

A further such area where there has been much development over the past decade is around the use of sulphur-based wastewater treatment. This makes use of microorganisms able to use inorganic sulphur rather than organic carbon, offering the prospect of new bioprocesses for removing nitrogen from water and wastewater streams. Process options include sulphur-driven autotrophic denitrification (SdAD).

A technology of particular interest is the SANI process – a registered trademark meaning Sulphate reduction, Autotrophic denitrification, Nitrification Integrated process. Development work on this has been under way in Hong Kong, where most of the population uses seawater for toilet flushing. This brings huge savings in freshwater demand but means that the wastewater is saline and there can be odour issues for the sludge treatment because of the sulphur levels present, although these can be resolved in practice.

The SANI process integrates anaerobic organic removal by sulphate reduction and anoxic nitrogen removal by autotrophic denitrification, which reduces the biological sludge production significantly. Studies began in 2004, with laboratory work at the Hong Kong University of Science and Technology (HKUST) and a pilot-scale study in Hong Kong from 2004-2007. Work has included full-scale demonstration at a wastewater treatment plant of the Hong Kong Drainage Services Department during 2013-2017.

Professor Guanghao Chen is at HKUST, and has worked on the development of SANI, as has Mark van Loosdrecht. He notes that coastal municipal sewage in any case generally contains relatively high levels of sulphur, because of seawater intrusion and infiltration into coastal water systems. He adds that there is also a general issue of sulphur levels increasing in groundwaters beneath inland cities. This indicates that there are wider opportunities for the application of sulphur-based bioconversion.

“A current challenge for the application of SANI/SdAD is its limited use due to the short history of sulphur biotechnology development in municipal wastewater treatment,” says Chen. He adds that the technology is not as well connected with resource recovery as, say, anammox, and there is the potential for further increase energy efficiency. “These are issues we have been addressing in our laboratory for a few years,” he adds.

Modelling development

These various areas of progress rely on different aspects of sewage treatment microbiology, but the progress has come thanks to developments in different fields, not least in mathematical modelling.

Damir Brdjanovic notes that modelling started in the early 1980s. “It was IWA, at that time under a different name, that instigated the first real global developments, which resulted in the first major publication and the first consensus model – ASM1 (Activated Sludge Model No.1),” he says. “From that moment on, modelling took off really rapidly.”

The speed and capacity of computing have of course transformed things, and models
are used extensively by ‘real’ modellers and in tools for engineers, designers and scientists. “You don’t have to be a modeller to use models,” says Brdjanovic.

Brdjanovic sees that the progress with understanding around biological treatment and microbiology means models can now capture more of a metabolic approach. “We are moving very rapidly from black box approaches to transparent box approaches, where empirical approaches to design are replaced by more insightful metabolic approaches,” he says.

Many different aspects have been brought into models over the past decade, include glycogen accumulating organisms, pH and biologically induced phosphorus precipitation, the anammox process, and sulphur conversions, says Brdjanovic. Alongside this, there is also progress with physical modelling and use of computational fluid dynamics, which he sees are being coupled to provide better predictions. “What is important is that the modelling is really moving into plant-wide modelling, including not only biological but also chemical and physical units, as well as moving into treatment plants being seen as water resource recovery facilities,” he says.

“Scientists are pushing the boundaries further, and models are becoming even more interesting and versatile,” adds Brdjanovic. He anticipates that, alongside tools for wide use, more companies specialising in modelling will emerge offering services to engineering companies and even universities.

With that prospect in mind, Brdjanovic notes that an important aspect of the book was to aim for each chapter to represent a consensus view on the subject, and that the chapter on modelling stands out in this respect. “In the beginning, the different modelling groups came together and left their egos outside of the room and came up with a consensus model, which was ASM1,” he recalls. The task for the modelling chapter was to provide a space to all those involved with the field, but to put developments in perspective, recognising that there is no one-size-fits-all solution. “It is important that readers understand where certain developments fit in and where they can be used,” he adds.

Multidisciplinary teams

The case of modelling highlights how different areas of expertise contribute to progress, underlining the importance of a multidisciplinary approach to research.

“I don’t believe that everybody should be multidisciplinary, because it is simply not how people are,” says Mark van Loosdrecht. There is though a need for a few people in every field able to move between that field and another. “In my research group, I strive for this multidisciplinarity – not by having people who can do everything, but partly having specialists and partly generalists and seeing that they communicate and work together,” he says.

Van Loosdrecht illustrates this with the example of aerobic granular sludge. “You cannot understand granule formation from a microbial perspective. If you purely study the microorganism, you will not understand it,” he says. Granule formation is related to reaction diffusion processes, to which bacteria then respond. To understand this well requires a good interaction of microbiology and chemical engineering, he explains.

Van Loosdrecht sees that breakthroughs can come when generalists move around. “For good teams, search for people who can do that, alongside having specialists,” he says, adding that he currently sees a particular need for this to exploit the potential of artificial intelligence in the wastewater treatment field.

Unlocking the potential

Biological wastewater treatment has delivered great benefits over the past 100 years, despite not having access to the far greater understanding of these systems that has been gained in recent years. “Unlocking of biological potential has always been done without exactly knowing what is there,” says Van Loosdrecht. This new knowledge promises further advances. “Now we know what’s there,” he continues, adding: “That sometimes helps to bring improvements.” By this, he means that in many cases progress has not come as a result of specifically targeting end needs. “What is missing in the environmental field is a kind of upfront market research,” he says.

Even so, there are many opportunities ahead. “An array of new possibilities is opening up that will change the planning and design of wastewater treatment facilities in the near future through development of a sustainable low carbon society,” says Guanghao Chen. These will come from application of AI, big data, machine learning, sensors, material science, and so on, he says.

“I believe the design and operation of wastewater treatment plants is still quite traditional,” he adds. Opportunities include dynamically handling variations of influent quality to secure effluent quality while at the same time increasing energy efficiency. Multidisciplinary research teams that include water utility staff need to be formed and work together very closely to make progress in this area of smart biological wastewater treatment plants,” he says.

Recalling the history of activated sludge, as a part of biological wastewater treatment, Damir Brdjanovic concludes: “Biological treatment was here 100 years ago, and it will stay for at least another 100 years. Together with chemical and physical treatment, it will remain a cornerstone of wastewater treatment.” •

More information

Biological Wastewater Treatment – Principles, Modeling and Design, 2nd edition

Editors: Guang-Hao Chen, Mark CM van Loosdrecht, GA Ekama and Damir Brdjanovic

ISBN13: 9781789060355

eISBN: 9781789060362

See: www.iwapublishing.com/books/9781789060355/biological-wastewater-treatment-2nd-edition