Abstract
In a wastewater treatment plant (WWTP), several sludge streams exist and the composition of their liquid phase varies with time and place. For evaluating the potential for formation of precipitates and equilibria for weak acids/bases, the ionic strength and chemical composition need to be known. This information is often not available in literature, and even neglected in chemical model-based research. Based on a literature review, we proposed three ranges of concentration (low, typical and high) for the major constituents of the liquid phase of the different streams in a WWTP. The study also discusses the reasons for the concentration evolution, and the exceptional cases, to allow readers to consider the right range depending on their situation. The ionic strength of the different streams and the contribution of its constituents were calculated based on the ionic composition. The major contributors to the ionic strength for the wastewater-based streams (influent, effluent and mixed sludge) were Na+, Cl-, Mg2+ and Ca2+, representing 50-70% of the ionic strength. For digestate, NH4+ and HCO3- accounted for 65-75% of the ionic strength. Even though the ionic strength is recognized to impact several important wastewater treatment processes, its utilization in literature is not always adequate, which is discussed in this study.
Highlights
The last decades saw a great development in the amount of wastewater treated
Sulphate and chloride are commonly added in wastewater treatment plant (WWTP) as counter-ion of iron or aluminium, and present in industrial wastewater (Rubio Rincon 2017)
The ionic composition of the liquid in the different sludge streams of a WWTP largely depends on the influent wastewater and on the process scheme of the WWTP
Summary
The last decades saw a great development in the amount of wastewater treated. China bears the world’s largest municipal wastewater infrastructure, and over 90% of the country wastewater was treated in 2018 (Qu et al 2019). The wastewater composition can vary strongly depending on the location of the wastewater treatment plant (WWTP) and the type of influent streams. Even higher salt loads can be found in specific places like Hong-Kong, where seawater is directly used to flush toilets (Wright & Colling 1995; Yu et al 2002; Liu et al 2019). It is common for WWTPs to process some industrial wastewater, which can bear important loads of diverse elements, depending on the type of industry
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