Abstract
The desired approach at safeguarding the environment both in control and effective monitoring of chemical discharges is the use of mass balances to account for inflow/outflow of pollutants. Whereas the previous studies were based on several assumptions, the batch studies enabled the construction of mass balances for the Sewage Treatment Work (STWs) using the removal rate data. This study, for the first time measures the rates of removal of cocaine in an STW, and the calculated mass balances were obtained from the removal rate data that were generated. The result of cocaine initial influent of 50 mg L-1 after after 2 hour hydraulic retention times (HRT) produced the final effluents of 110 mg L-1. Projected influent concentrations of cocaine (14, 471 ng L-1) were derived from back-calculation from final effluent concentrations. A useful tool that accounts for the mass-flow of trace drugs in the aquatic environment with minimal errors often due to sampling logistics and desludging process has been provided in this model of mass balance calculations in STWs. 
 Keywords: removal rate, effluent, influent, mass balance, sewage treatment work, cocaine
Highlights
The inherent assumptions by most authors in the literature with the mass balances of organic pollutants which include heavy metals in the sewage treatment works (STWs) have been highlighted (Liu et al, 2017; Manitcharoen and Pinpunchat, 2020: SulejSuchomska et al, 2020: Yang et al, 2017; Zuccato et al, 2008a)
The apparent discrepancies in the literature (Kumura et al, 2007) differ on the actual amount (%) of excreted benzoylecgonine (BZE) and cocaine from cocaine dose were not clear as many batch data were not made available in the assessment quality, more information were needed to address the discrepancies to account for the overall mass flow of drugs
Mass balance calculation of cocaine: The removal data (%) in form of degradation and adsorption to solids from the sludge experiment were shown in Table 1 with the processing sewage units of primary sludge (PS), submerged aerated filter-1 (SAF-1), mixed submerged aerated filter (MSAF) and humus sludge (HS)
Summary
The inherent assumptions by most authors in the literature with the mass balances of organic pollutants which include heavy metals in the sewage treatment works (STWs) have been highlighted (Liu et al, 2017; Manitcharoen and Pinpunchat, 2020: SulejSuchomska et al, 2020: Yang et al, 2017; Zuccato et al, 2008a). In calculating the cocaine load (g day-1), the molar fraction, concentration of flow rate of cocaine and BZE (ng L1), and molar mass ratio were all estimated. The apparent discrepancies in the literature (Kumura et al, 2007) differ on the actual amount (%) of excreted benzoylecgonine (BZE) and cocaine from cocaine dose were not clear as many batch data were not made available in the assessment quality, more information were needed to address the discrepancies to account for the overall mass flow of drugs.
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