Effect and mechanism of induced crystallization softening treatment on water quality in drinking water distribution system with high hardness water source

Abstract

Induced crystallization softening is one of the main methods for softening drinking water at present. It can also affect dynamic balance between water and pipe wall sediments in drinking water distribution system (DWDS) while reducing the total hardness. This paper takes HS water treatment plant (HSWTP) and DWDS as examples to study the influence of drinking water formed by homogeneous mixing of softened and non-softened drinking water on water quality and original sediments in DWDS. The results show that when the dosage of softening agent is about 300 mg/L, the effluent water quality of HSWTP was the best, and the scaling potential was basically eliminated. Continuous operation results show that there is no significant change in water quality index of HSWTP effluent in DWDS. However, the electrochemical characteristics assays results show that at the initial stage of softening treatment (0–10d), there is a material transition between water and pipe wall sediments in the DWDS. The original dynamic equilibrium stability of the DWDS is broken, and the DWDS is in a state of “instability” which can last about 30 days. Dynamic stability can be achieved in the DWDS again after 30–50 days of continuous operation. The degree of “instability” in DWDS is positively related to the extension of water supply pipeline and the increase of dosage of softening agent. The operation mode of adjusting dosage of softening agent by small increments for several times can alleviate the impact on drinking water quality caused by “instability” of DWDS.