## 1 Introduction  
Various analyses of wastewater show how the growth of the world's population goes hand in hand with an increase in the concentration of organic micropollutants, such as perfluoroalkyl substances [1], cyanobacteria [2], mycotoxins [3], hormones [4], psychoactive substances [5], pesticides [6], cosmetics, and industrial additives and drugs [7; 8; 9], and demonstrate how this type of pollution reflects the increase in population growth [10; 11]. Hence, there is a need to remove these pollutants present, especially in the most industrial areas, from China to South Africa and from North and South America to Europe, wherever human activity presents significant developments [12; 13]. The concentration of these pollutants, compared to traditional ones (conventional and unconventional), is much lower (on the order of ppm or ppb), and this characteristic has led to their classification as emerging contaminants (ECs) or persistent organic pollutants (POPs). There are no administrative regulations for environmental protection from this type of pollutant because its danger to the environment is not yet well understood. Despite the low concentrations, these are persistent and potentially toxic to microorganisms due to their complicated molecular structures, creating problems for the environment and humans. They can also be present in the water that is used to irrigate cultivation fields, even bringing the problem to the land and aquifers. Some have been identified in drinking water, consequently increasing the risk of these pollutants coming into contact with the human population [14]. Most of the pharmaceutical pollutants are introduced into the environment through people's use of them who, failing to metabolize them completely, expel them into wastewater treatment processes that are not yet able to manage their removal. Other sources from which pharmaceutical pollutants come are hospitals, intensive livestock farming, and industrial waste [15]. The existing treatments used for the purification of wastewater do not currently guarantee the removal of ECs, which are degraded into less harmful compounds and only partially mineralized into inorganic substances. One of the ECs is olmesartan acid (OLM) [16], a hypertensive that is estimated to have an average daily consumption of almost 12 DDD per 1000 inhabitants (it was 3.7 in 2007), with a defined daily dose (DDD) value of 20 mg. In Italy alone, an expenditure of about EUR 150 million per year is estimated, almost five times more than the EUR 31 million in 2011 (it is among the top thirty active ingredients agreed upon for expenditure agreed by the national health system). It is marketed as olmesartan medoximil, the prodrug from which the active pharmaceutical is derived [17]. An excretion of 90% and a percentage of removal in wastewater treatment plants (WWTPs) not exceeding 20% are estimated for this drug. The simulation of the chlorination process in WWTP, which ensures the reduction of emerging pollutants, has allowed to study the degradation of OLM [18; 19; 20; 21; 22; 23], leading to the isolation of nine degradation byproducts (DPs), eight of these are new (Figure 1) [24]. The aim of this work was to analyze the less abundant and more polar fractions that had been overlooked in previous studies, arriving at the determination of another 13 DPs, six of them isolated for the first time. All the DPs were identified by combining nuclear magnetic resonance (NMR) and mass spectrometry (MS), using matrix-assisted-laser desorption/ionization as a source and a time-of-flight analyzer (MALDI-TOF) for mass analysis. For all 22 isolated products, a formation mechanism has been proposed that integrates the one previously reported, having isolated and identified some products whose existence had only been hypothesized as possible intermediates. Finally, we evaluated the  
Figure 1: Olmesartan acid and the already known breakdown products.