Integrated urban water management is a well-accepted concept for managing urban water. It requires efficient and integrated technological solutions that enable system-wide gains via a whole-of-system approach. Here, we create a solid link between the manufacturing of an iron salt, its application in an urban water system, and high-quality bioenergy recovery from wastewater. An iron-oxidising electrochemical cell is used to remove CO₂ (also H₂S and NH₃) from biogas, thus achieving biogas upgrading, and simultaneously producing FeCO₃. The subsequent dose of the electrochemically produced FeCO₃ to wastewater and sludge removes sulfide and phosphate, and enhances sludge settleability and dewaterability, with comparable or superior performance compared to the imported and hazardous iron salts it substitutes (FeCl₂, and FeCl₃). The process enables water utilities to establish a self-reliant and more secure supply chain to meet its demand for iron salts, at lower economic and environmental costs, and simultaneously achieve recovery of high-quality bioenergy.

Magnesium hydroxide [Mg(OH)₂] is a non-hazardous chemical widely applied in sewer systems for managing odour and corrosion. Despite its proven effectiveness in mitigating these issues, the impacts of dosing Mg(OH)₂ in sewers on downstream wastewater treatment plants have not been comprehensively investigated. Through a one-year operation of laboratory-scale urban wastewater systems, including sewer reactors, sequencing batch reactors, and anaerobic sludge digesters, the findings indicated that Mg(OH)₂ dosing in sewer systems had multifaceted benefits on downstream treatment processes. Compared to the control, the Mg(OH)₂-dosed experimental system displayed elevated sewage pH (8.8±0.1vs 7.1±0.1), reduced sulfide concentration by 35.1%±4.9% (6.7±0.9mgSL^-1), and lower methane concentration by 58.0%±4.9% (19.1±3.6mgCODL^-1). Additionally, it increased alkalinity by 16.3%±2.2% (51.9±5.4mgCaCO₃L^-1), and volatile fatty acids concentration by 207.4%±22.2% (56.6±9.0mgCODL^-1) in sewer effluent. While these changes offered limited advantages for downstream nitrogen removal in systems with sufficient alkalinity and carbon sources, significant improvements in ammonium oxidation rate and NOx reduction rate were observed in cases with limited alkalinity and carbon sources availability. Moreover, Mg(OH)₂ dosing in upstream did not have any detrimental effects on anaerobic sludge digesters. Magnesium-phosphate precipitation led to a 31.7%±4.1% reduction in phosphate concertation in anaerobic digester sludge supernatant (56.1±10.4mgPL^-1). The retention of magnesium in sludge increased settleability by 13.9%±1.6% and improved digested sludge dewaterability by 10.7%±5.3%. Consequently, the use of Mg(OH)₂ dosing in sewers could potentially reduce downstream chemical demand and costs for carbon sources (e.g., acetate), pH adjustment and sludge dewatering.

Acidic nitritation driven by acid-tolerant ammonia-oxidizing bacteria (AOB) has gained wide attention due to its potential in sustainable wastewater and sludge treatment. However, limited knowledge of initiating acidic nitration using conventional activated sludge hindered the wider studies and application of this technology at lab- and field-scale. This study evaluates three strategies for initiating acidic nitritation: a constant low hydraulic retention time (HRT); an extended initial HRT followed by manual HRT reduction; and pH-controlled HRT. All strategies successfully started acidic nitritation using seed sludge from a local wastewater treatment plant (WWTP) containing undetectable acid-tolerant AOB. Among the three strategies, pH-controlled HRT was the most efficient, with a smoother (minimal fluctuations) and faster (around 30 days) start-up process than the other two strategies. This was attributed to an initial redundancy in ammonia oxidation capacity (i.e. making the proton generation rate caused by ammonium oxidation exceed the alkalinity supply rate by influent), allowing AOB to overcome the activity valley during the transition from neutral to acid pH Level. Using pH as a real-time proxy of AOB activity also leveraged the unique low buffer capacity at acidic pH. Based on these findings, a pilot-scale acidic nitritation reactor treating diluted sidestream wastewater was initiated for the first time using the pH-controlled strategy. The pilot reactor immediately achieved nitrite accumulation and reached the target hydraulic loading rate quicker than the lab reactor, indicating higher influent nitrogen concentration may facilitate NOB suppression and a higher growth rate of acid-tolerant AOB. Based on those results, the versatile start-up strategies using both mainstream or sidestream wastewater were further discussed. Overall, this work greatly expands potential applications of acidic nitritation and paves the way for future field-scale applications.

The waste activated sludge (WAS) of wastewater treatment system is often rich in phosphorus (P), which is a basic element of human life and could use up in the near future. This study proposed an integrated approach to efficiently recover P as vivianite from WAS and simultaneously enhance the sludge dewaterability. The raw WAS was first acidified using FeCl₃, which was then fed to anaerobic fermenter for Fe³⁺ reduction. After fermentation, a technology named acid-elutriation was introduced to convert Fe and P from solid phase to liquid phase and concomitantly enhance the liquor-solid separation. Finally, vivianite was obtained via sludge eluate neutralization. The enhanced sludge dewaterability not only increases the recovery efficiency of Fe and P but also decreases the cost of sludge disposal.