Since the 1980s, China’s urban water infrastructure has grown and developed steadily, with wastewater treatment coverage and capacity reaching up to 97.5% and around 190 million m3/d as of 2020. However, China’s wastewater treatment plants (WWTPs) are also facing challenges, including increasingly strict discharge standards and the urgent need to reduce carbon footprints (as published in the article ‘Reducing emissions – China’s plan to go green’, published in The Source in December 2022). For this reason, the upgrading of existing WWTPs and the planning of future WWTPs are of great importance, especially with regards to the current global progression towards a circular/blue economy with low-carbon, high-efficiency operations leading to the ‘clean’ development of resources. 

Increasingly, serious environmental problems and resource depletion are forcing us to recognise the importance of the circular economy. It is now apparent that wastewater, rather than simply being a waste substance in need of treatment is, in fact, a carrier of valuable resources and energy. In municipal wastewater, organic matter (COD/BOD), nitrogen (N) and phosphorus (P) are often regarded as ‘pollutants’ of water environments. This may be so, but they are also valuable resources with high potential as nutrients for crops, for example. Moreover, wastewater also contains a great deal of chemical/thermal energy that can be used to reduce the carbon footprint of WWTPs. It is a significant step towards the creation of a circular economy to recover and recycle these resources, rather than allowing them to be wasted. It is also important that the methods used in such a recovery should have a minimal footprint, including investment, operation, and energy consumption costs.  

In consideration of these factors, and of the necessity for innovative techniques to enable this transformation, it is proposed that existing municipal WWTPs be upgraded and future WWTPs be designed as Blue Water Factories (BWFs) following a framework developed through collaboration between China and the Netherlands. 

As an initial measure, Beijing University of Civil Engineering & Architecture (BUCEA) in China invited Delft University of Technology (TU Delft) in the Netherlands to establish in 2016 the Sino-Dutch R & D Centre of Future Wastewater Treatment Technologies (hereinafter referred to as the ‘Sino-Dutch Centre’) with the Beijing Capital Eco-Pro Group (BCEPG), with the aim of developing sustainable techniques of wastewater treatment. After a fruitful collaboration of six years, the design of BWFs was completed. Figure 1 illustrates how existing WWTPs can be upgraded and future WWTPs built to support the circular economy. 

By special invitation from the China International Fair for Trade in Services (CIFTIS), which was held on 3 September 2022, Professor Mark van Loosdrecht of TU Delft made a keynote presentation entitled: ‘Blue Water Factories (BWFs): A Future Becomes Visible’, on behalf of the Sino-Dutch Centre.  

Recycling methods 

The principles of BWFs lead us to focus on four methods of recycling: nutrients (mainly phosphorus recovery), biomaterials (EPS: extracellular polymeric substances; ALE: alginate-like extracellular polymers; and PHA: polyhydroxyalkanoate), heat and power (sludge-incinerated combined heat and power and effluent thermal energy), and water (from reclaimed effluent).  

From a technical perspective, five key elements must be considered:  

1. separation and recovery of celluloses from influent; 
2. aerobic granular sludge (AGS) for simultaneous COD and N/P removal, followed by the recovery of valuable biomaterials such as EPS/ALE and/or PHA, from waste sludge;
3. sludge drying and self-sustained incineration for recovering chemical energy via combined heat and power (CHP), as well as phosphorus and metals (to produce coagulants in combinations of desalinated brine) from sludge-incinerated ashes;  
4. recovery of thermal energy from effluent by water source heat pumps (WSHP) for sludge in-situ drying inside and/or district heating/cooling outside; 
5. effluent recycling for non-drinking purposes. 

Finally, carbon neutral operations (based on both sludge-incinerated CHP and effluent thermal energy) and smart control (based on the modelling of processes) are both important features of BWFs, helping to recover energy, stabilise operations and ensure low consumption of energy and chemicals.  

Waste sludge is now known to be a rich source of both energy and phosphates. Conventionally, anaerobic digestion (AD) is used to recover the chemical/organic energy contained in waste sludge. However, there are two key points that are rarely appreciated. First, chemical energy is only one-ninth of the thermal energy contained in effluents; and second, methane is produced from AD through an entropy-increasing process. For these reasons, AD seems ultimately unsuitable for the disposal of sludge in a circular economy. Instead, the best approach would seem to be first drying the sludge and then incinerating it. In this way, thermal energy recovered from effluents could be applied as a low-grade heat source for drying sludge.  

Moreover, with regards to entropy, organic materials should be regularly recycled with a particular focus on high value-added biomaterials. Thus, it is important that ALE is recovered prior to sludge incineration. As the content of organic materials in sludge is reduced after ALE extraction, some agricultural biomass could be added to residual sludge to support self-sustained incineration. This illustrates a novel concept of ‘low value in exchange for high value’, which includes low-grade thermal energy used for drying sludge to generate high-grade electricity via incineration. In fact, sludge incinerated ashes are also ideally suited for both phosphate recovery and coagulant production with removed metals. Finally, residual ashes can be reused for building materials.  

These are all important points, in fact vital ones. Carbon neutrality is now a global issue in the fight against climate change, and China has shown that it is leading the way with the announcement of its dual carbon goals for 2030 and 2060, supported by the Guidelines for Carbon Accounting and Emission Reduction Pathways in the Urban Water Sector. •

More information 

Hao, X., Li, J., van Loosdrecht, M.C.M., Jiang, H., Liu, R., 2019. Energy recovery from wastewater: Heat over organics. Water Res., 161, 74-77. https://doi.org/10.1016/j.watres.2019.05.106 

Hao, X., Chen, Q., Loosdrecht, M.C.M. van, Li, J., Jiang, H., 2020. Sustainable disposal of excess sludge: Incineration without anaerobic digestion. Water Res. 170, 115298 https://doi.org/10.1016/j.watres.2019.115298 

Hao, X., Wu, D., Li, J., Liu, R., Loosdrecht, M. Van, 2022b. Making Waves: A sea change in treating wastewater – Why thermodynamics supports resource recovery and recycling. Water Res. 218, 118516. https://doi.org/10.1016/j.watres.2022.118516 

Wang, X.Y., Shi, C., Hao, X.D., van Loosdrecht, M. C. M., Wu, Y.Y., 2023, Synergy of phosphate recovery from sludge-incinerated ash and coagulant production by desalinated brine. Water Res., 231, 119658. https://doi.org/10.1016/j.watres.2023.119658