With the latest IWA Specialist Group conference on the topic in September, Miklos Patziger reviews the current status and coming challenges on the design, operation and economics of large wastewater treatment plants.
The aim of the IWA’s Specialist Group on the Design, Operation and Cost of Large Wastewater Treatment Plants (LWWTPs) is to contribute to a continuous improvement of design, operation and economics of large wastewater treatment plants by sharing knowledge and experience. To assure proper sanitation, especially in densely populated areas, reduce resource consumption (e.g., materials and energy), and recover resources in wastewater (e.g., treated water, energy and nutrients), the core issues with which the group is concerned include:
- design, operation and economics of large wastewater treatment plants
- reuse options on large wastewater treatment plants
- treatment and energy efficiency
- optimisation of existing facilities
- sludge management
- upgrading options to meet new effluent requirements (e.g., removal of micropollutants).
There are many sub-topics covered by the Specialist Group, some of them being key developments that will be transformational in this field – for example, aerobic granular sludge; elimination of micropollutants via Polyaluminium Chloride (PAC) and/or ozone; water, energy and nutrient recovery, etc.
Every four years, the group organises a specialised conference, rotating between Vienna, Budapest and Prague. The conferences provide a platform for researchers, scientists and practitioners to discuss the actual status and further developments in the field. Practitioners, in particular, are encouraged to present progress reports on case histories. The conferences also provide a platform for promoting successful implementation of research findings into practical usage.
Existing large plants
The activated sludge process celebrated its 100th birthday in 2014 and a lot of attention has been paid to LWWTPs since they came into service. A huge number of comprehensive research programmes and innovations have progressed worldwide in the past decades. Biological wastewater treatment went through rapid development.
“measuring methods and process modelling have developed rapidly in the past two decades”
But while most LWWTPs across Europe, North America, Australia and certain parts of Asia fulfill quite strict requirements and high standards regarding treatment efficiency and economics, there are still many challenges that need to be solved.
Mechanical treatment step
The satisfactory operation of the mechanical treatment stage is crucial for the safety and effectiveness of the entire treatment process. This includes screening or sieving, grit removal and, if advantageous, primary settling.
While measuring methods and process modelling, especially computational fluid dynamics (CFD), have developed rapidly in the past two decades, little attention has been paid to the mechanical treatment step. The design and operation principles of grit chambers and primary settling tanks certainly need to be improved based on detailed hydrodynamic investigations comprising fine-scale in situ measurements and CFD modelling.
The main function of grit chambers is to protect the biological and sludge treatment processes from coarse material (screenings), inorganic material (gravel, sand) and grease and oil. The attempt to remove the possibly highest rate of inorganic materials, consisting of a wide range of particle sizes (from 0.1 up to 1-2 mm) without removing too much particulate organic content is extraordinarily challenging. This requires improved design and operation guidelines, including recommendations regarding an optimised geometry, especially cross-section design and length. The key point of the operation is an appropriate aeration control for different geometries and inflow conditions (dry weather inflow, wet weather inflow, and peak flows due to storm events).
Primary settling tanks (PST) are an integral part of the entire wastewater treatment process, sludge treatment and digester gas production. With new developments emerging in wastewater and sludge treatment over the past three decades, especially since biological nutrient removal has been required, their function and operation has become quite complex.
The detailed consideration of primary clarification as an integral part gives the opportunity to optimise the wastewater treatment process regarding maximum nitrogen elimination by providing a sufficient amount of biodegradable chemical oxygen demand (COD) for denitrification or minimising the required volume for nitrogen elimination in the biological treatment step. It also allows energy optimisation of the plant by removing as much particulate COD in the PST as possible without impairing the biological nitrogen removal. Approved design procedures and boundary condition driven control strategies (capacity used, scraper mechanism, sludge removal) need to be developed to contribute to a satisfactory PST function.
Physical and chemical treatment is important in the case of industrial wastewaters (e.g., membrane bioreactors); however, it is also frequently used in municipal wastewater treatment. Chemicals are widely applied for precipitation of phosphorus, flocculation, and to increase the efficiency of sludge dewatering. Chemically enhanced primary treatment (CEPT) is especially used in coastal cities and at LWWTPs with low temperature wastewaters. CEPT increases the effectiveness of the primary treatment quite cost-effectively and leads to a reasonably enhanced digester gas production. The main bottleneck of CEPT is that it strongly lowers the carbon to nitrogen proportion at municipal LWWTPs, decreasing the efficiency of denitrification. Therefore, current development needs of CEPT cover the enhancement of the effectiveness of the subsequent biological treatment.
Biological treatment step and energy efficiency
The field of biological wastewater treatment has developed dramatically. The main driver of this enormous development has been the global problem of eutrophication. While in earlier times only carbon removal was required, for some 30 years the enhanced removal of carbon and nutrients (nitrogen and phosphorus) has been obligatory throughout Europe and many other parts of the world. Reactor volumes, aeration demand and costs of wastewater treatment have increased considerably. This has led to a number of activated sludge-based wastewater treatment technologies and reactor configurations. To support all these developments and innovations, the science has evolved rapidly in areas such as microbiology and microbial ecology, process modelling, measurements, control and automation, membrane technology, aeration and mixing, planning and design, and process economics, with an emphasis on energy considerations.
“A classical challenge of the activated sludge process is the problem of sludge morphology and settleability”
A classical challenge of the activated sludge process is the problem of sludge morphology and settleability (foam and bulking). These depend on many factors. To avoid them is still often highly complicated. A relatively new way of fighting sludge bulking and foam is based nowadays on microbiology. Recent approaches (Nielsen et al, 2010) that probably will result in a breakthrough in many aspects of activated sludge process should allow scaling from the level of individual genes/genomes up to whole communities and ecosystem-level processes.
One of the main bottlenecks of the activated sludge process is its high energy demand. Activated sludge plants are often among the highest energy consumers of a municipality. The annual mean electrical power demand of conventional municipal wastewater treatment with nutrient removal is in the range of 15 to 30 W/population equivalent.
Improving the energy situation of LWWTPs is one of the most important research fields that the SG is dealing with. This is relying on two main points: on one hand producing as high a rate of digester gas as possible (if possible, in an enhanced way, by co-digestion of sludge from smaller WWTPs and other co-substrates), on the other hand decreasing the energy demand of the plant. It is already possible to reduce external energy supply under special circumstances (e.g., special process configurations, co-digestion to produce more biogas) to zero, or to produce more electrical energy than is consumed by the plant. Nevertheless, such plants have still to be connected to an electrical grid to satisfy peak electrical energy demand. Surplus electric energy from the plant can be fed back to the grid. However, zero ‘carbon footprint’ will not be affected by this solution.
It is important, though, to note that energy optimisation should not negatively affect treatment efficiency.
Secondary settling (SS) is an integral part of the biological treatment unit, and usually the last step of municipal wastewater treatment. Performance and behaviour of secondary settling tanks (SSTs) have been investigated intensively in the past decades. Ekama et al (1997) shows the questions arising and a series of research programmes ongoing in the 1980s and early 1990s. Since then, a number of comprehensive hydrodynamic investigations have led to a breakthrough in many aspects of SS. The new edition of the German design guideline “DWA A 131” (DWA 2016) already includes the most important design and operation related improvements and detailed recommendations on the appropriate design and operation of SSTs. However, an up to date IWA publication summarising the full range of new findings and the state of the art in theory and praxis is needed.
In recent years, membrane technology (MBR) has become quite widely applied at LWWTPs. The advantages of MBR are their reduced space demand compared with activated sludge systems equipped with secondary settling tanks and their efficiency in producing a highly clarified effluent. Consequently, MBRs can be applied at LWWTPs with limited space conditions and/or extraordinarily high requirements for treated wastewater quality. The bottlenecks of MBRs are their high operation costs because of their high energy demand and fouling. However, considerable effort is needed to overcome these problems. Moreover, new generation membranes tend to incorporate nanomaterials, such as zeolites, carbon nanotubes, silver nanoparticles and others, to improve properties and performance (Liu et al, 2022).
New challenges, new concepts
Recent global challenges, such as rapid population growth, climate change, water availability and water quality problems in many countries, as well as increasing energy costs, strongly influence concepts in wastewater treatment and prompt new innovations and developments.
“LWWTPs no longer solely focus on wastewater treatment. A lot of them are already complex resource recovery facilities”
The global population is increasing rapidly. For the near future, a dramatic population growth is forecast. The current urban population will nearly double before the end of the century. Consequently, fresh water availability in urban areas per inhabitant is decreasing continuously. Climate change causes an additional decrease of availability of healthy water in many regions. At the same time, increasing food production leads to increasing water demand. These problems are connected with a scarcity of natural sources for agriculture (Grison et al, 2023).
Therefore, in many regions suffering from these problems, new concepts, new solutions and a new approach to integrated water resource management are needed.
Nowadays, LWWTPs no longer solely focus on wastewater treatment. A lot of them are already complex resource recovery facilities. Water resource recovery aims to use all resources in wastewater, such as nutrients, soil enhancers, biogas, heat, and chemicals. Furthermore, integrated water reuse/energy concepts are evolving, with energy footprints becoming more and more important. Some key themes to watch are:
- The reuse of treated wastewater, both ‘potable’ and ‘non-potable’, and/or in a ‘direct’ (for example, Singapore) and an ‘indirect’ way
- The control of nutrient loads and ‘loss’ of valuable wastewater compounds for agriculture (esp. phosphorus).
- The linking of these innovations with improved rainwater management, transfer of freshwater from one river basin to another, and seawater desalination – the only additional source of water beyond precipitation.
- Combating the effects of micropollutants on the environment becomes increasingly relevant with reuse and recycling of wastewater. Removal of micropollutants will generally be standard in European countries.
- Further attention is paid to decreasing greenhouse gas emissions.
- The digital revolution at LWWTPs is highly relevant, spanning digital twins, artificial intelligence, novel monitoring methods and concepts.
- Introducing leading edge technologies at full-scale LWWTPs, including granular sludge, nanobubbles and further advanced methods.
With such opportunities ahead, the forthcoming IWA Specialised Conference on the Design, Operation and Economics of Large Wastewater Treatment Plants, taking place in Budapest, Hungary on 8-12 September 2024, offers an important opportunity to exchange insights on the further development of large treatment plants. •
More information
- DWA (2016): Dimensioning of single-stage activated sludge plants, DWA-A 131. Hennef, in press.
- Ekama G. A., Barnard J. L.Günthert F.W., Krebs P., McCorquodale J. a., Parker D. S., Wahlberg E.J. (1997). Secondary Settling Tanks: Theory, Modelling, Design and Operation. IAWQ scientific and technical report No. 6. International Association on Water Quality, London, England
- IWA (2015): Activated Sludge – 100 years and counting. Edited by David Jenkins and Jiri Wanner, IWA Publishing, 2014, London
- C Grison, S Koop, S Eisenreich, J Hofman (2023) Integrated water resources management in cities in the world: global challenges Volume 37, pages 2787–2803
- W Liu, X Song, Z Na, G Li, W Luo (2022), Strategies to enhance micropollutant removal from wastewater by membrane bioreactors: Recent advances and future perspectives, Bioresource Technology Volume 344, Part B, January
- Stefano Longo 1, Almudena Hospido, Miguel Mauricio-Iglesias (2023), Energy efficiency in wastewater treatment plants: A framework for benchmarking method selection and application, Journal of Environmental Management, Volume 344, 15 October 2023, 118624
The author: Miklos Patziger, Hungary, is secretary of the IWA Specialist Group on Design, Operation and Costs of Large Wastewater Plants
Forthcoming IWA Specialist Group conference
The 14th IWA Specialised Conference on the Design, Operation and Economics of Large Wastewater Treatment Plants will take place in Budapest, Hungary, on 8-12 September 2024. For more information, see: https://lwwtp2024.org
