Abstract: The use of antiscalants in reverse osmosis (RO) systems is essential to prevent the crystallization and precipitation of Ca²⁺ and Mg²⁺ ions, which can lead to undesirable consequences such as an increase in the required dosages of hydroxide and carbonate for hardness removal, increase operational costs, and negatively impact the evaporation–crystallization of salt. This study aimed to investigate the industrial antiscalant ingredients and antiscaling mechanism in RO systems, which is crucial for achieving thorough hardness removal and no industrial wastewater discharge. We comprehensively analyzed the water quality of RO concentrate and fractionated it using ultrafiltration based on molecular weights. Fourier transform infrared spectroscopy (FT-IR), UV-visible spectrophotometry (UV-Vis), and excitation–emission matrix fluorescence spectroscopy (EEM) results revealed that the primary constituents of the dissolved organic matter (DOM) were microbial metabolites and humic acid substances with a molecular weight of <3 kDa. These substances comprised functional groups such as carboxyl, alcohol/phenol hydroxyl, and unsaturated hydrocarbon structures. We further analyzed the main composition and structure of antiscalants using ¹H, ¹³C, and ³¹P nuclear magnetic resonance (NMR), confirming that the dominant component is hydroxyethylidene-1,1-diphosphonic acid (HEDP) based on the chemical shift characteristics of methyl carbon, quaternary carbon (19.02×10^–6 and 69.96×10^–6 in ¹³C NMR), and C—P structures (19.94×10^–6 in ³¹P NMR). Anion exchange adsorption experiments were performed for HEDP removal in RO concentrate to evaluate the effectiveness of the antiscalants. Approximately 88.55% HEDP and 38.86% COD removal substantially reduced the amount of carbonate required for complete Ca²⁺ precipitation, with the needed concentration decreasing from 2845.8 to 826.8 mg·L^–1. This reduction demonstrates the dominant role of HEDP in hindering Ca²⁺ crystallization rather than Mg²⁺. Interestingly, even upon the reintroduction of HEDP, the required carbonate dose only increased to 1626.3 mg·L^–1, indicating that DOM removal through adsorption exerted a dispersion effect, which led to the effective inhibition of Ca²⁺ crystallization and aggregation. The study findings elucidate the synergistic antiscaling mechanism between HEDP and DOM, providing valuable insights into the methods for enhancing hardness removal in RO concentrate. This synergistic effect not only improved the efficiency of hardness removal but also contributed to the overall zero discharge of wastewater in RO systems. By understanding and harnessing this mechanism, more effective strategies and regulations can be developed for engineering applications, contributing to significant advancements in industrial wastewater treatment technologies.