After arriving in China in July 2017, I was encouraged to work on two topics: improving water quality in river basins (at all scales) and improving the performance of WasteWater Treatment Plants (WWTPs), now known as Water Resource Recovery Facilities (WRRFs), to emphasise the fact that all water is a resource, and the purpose of a WRRF is to improve the quality of water before it is released to receiving waters or otherwise reused.

In 2014, while working with CDM Smith in Perth, I helped to prepare a bid by CDM Smith to treat wastewater at Beenyup, to a level suitable for injection in the Surficial Aquifer, so it could later be pumped and retreated to drinking water standards. This was my first involvement in an indirect potable reuse project, but it was not my first introduction to wastewater treatment. Professor Don Harleman (MIT) tried to help me to work at a Frito Lay factory in Texas USA in the summer of 1978, studying circulation in wastewater treatment ponds, but the US government refused to let me work on a student visa, while in transition between Caltech and MIT, so I spent the summer in Pasadena reviewing Hugo Fisher's manuscript on "Mixing in Inland and Coastal Waters". I now smile at the thought that this was "sliding door" moment, that could have changed and did inflluence my career. In about 1984, the Water Authority of Western Australia commissioned the Centre for Water Research at UWA to study odour problems in the sewer system in Perth; Prof. Ian Wood introduced me to Dr Tony McNulty, who we hired to work on laboratory and field studies injecting air into sewer pipes to prevent anaerobic conditions. So after many years in other fields, I find myself working in a new field.

In 2018, in a period of about eight months, I worked with environmental engineer Jiang Huanhuan and software engineer Tang Jinquan, and used GoldSim simulation software to write our own versions of the International Water Association's (IWA's) BSM1 and BSM2 benchmark models for simulating biological nutrient removal (BNR). I had been challenged to find ways to improve the performance of wastewater treatment plants. I quickly learned about ASM1, ASM2d, ASM3 etc. and other component models such as the model of a secondary clarifier developed by Imre Takács et al. I then learned about the open source MATLAB code shared by IWA's Modelling and Integrated Assessment (MIA) Specialist Group, about commercial codes BioWin, GPS-X, SIMBA#, Sumo and WEST (in alphabetical order) and about a few others like ASIM, and decided I wanted to have control of my own code. Since that time we have completely re-engineered WRRM1 and WRRM2, as we called them, but we still use GoldSim as a development environment, largely because of the ease of visualising changes dynamically.

After attending Watermatex 2019 in Copenhagen and WEFTEC 2019 in Chicago, and meeting the developers of all of the commercial software, I was invited to join the Management Committee of MIA. I contributed to two IWA papers together with other members of the IWA MC, both on the topic of Digital Twins of WRRFs: a white paper and a more complete journal paper.

Our work is continuing towards implementation of Digital Twins.