## 2 Materials and Methods  
### AOX Analytical Method  
The standard procedure for the determination of AOXs is specified in UNI EN ISO 9562:2004. The AOX measurement method consists of three steps [24; 25; 26]:  
1. Enrichment of organic halogen compounds on activated carbon with contemporaneous isolation of unwanted organohalogen byproducts from acidified samples;
2. Use of a nitrate solution to wash the activated carbon in order to remove interfering inorganic halides;
3. High-temperature--at least 950 \({}^{\circ}\)C [1; 4]--combustion in order to mineralize the adsorbed organic halogens; reaction gases are conveyed to a drier and then to the Ag-coulometric titration cell for the determination of released hydro-halogenides. Here the halogen content is calculated based on the number of electric charges using [PERSON]'s law. The result of the AOX analysis is expressed as \(\upmu\)g(Cl) L\({}^{-1}\). UNI EN ISO 9562:2004 allows the use of two different operating modes:
* In the _column method_, the sample is pressed over at least two interconnected columns filled with granular activated carbon (GAC) placed inside an adsorption column;
* In the _shaking (or batch) method_, the sample is shaken, for a specific period, with powdered activated carbon (PAC), which is subsequently separated from the matrix to be analyzed using a polycarbonate filter.  
Standard-compliant preparation of the sample is a key aspect for a reliable AOX analysis. There are many factors that can distort the results [4]:* A concentration of inorganic chloride ions above 1000 mg L\({}^{-1}\), which may cause interference or false-high readings;
* The presence of bacteria and algae containing chloride ions;
* The presence of inorganic bromides and iodides;
* A dissolved organic carbon (DOC) content that is too high, which can lead to an underestimation;
* A poor affinity of the halogenated compounds for activated carbon.  
Therefore, before the AOX analytical procedure starts, several parameters have to be inspected carefully. All the recommendations to be followed to meet all boundary conditions are listed in UNI EN ISO 9562:2004. Briefly,  
* The standard method is applicable to samples with concentrations of inorganic chloride ions of less than 1 g L\({}^{-1}\); samples with higher concentrations should be diluted before enrichment on activated carbon.
* The standard method is also applicable to samples containing insoluble halides, i.e., halogens that are adsorbed on suspended solids; filtration of the sample before the analysis allows for the separate estimation of dissolved and particulate AOXs.
* The pH value for the adsorption of the sample should be \(\leq\)2 [4].
* Oxidizing agents in the sample must be rendered ineffective by adding sodium sulfite [4].  
In this work, in order to evaluate the presence of any critical issues in the measurement of AOXs, two samples containing different concentrations of AOXs were taken:  
* Sample S1, which was effluent from a thermophilic oxidative biological plant;
* Sample S2, which was a mixture of aqueous waste in which the presence of organochlorine compounds was ascertained.  
Sample S2 was expected to have a high concentration of AOXs, while sample S1 was expected to have a low concentration of AOXs because it was taken from downstream of a thermophilic oxidative biological treatment.  
Then, four different laboratories--namely Lab-1, Lab-2, Lab-3 and Lab-4--were asked to measure the AOX content of the two samples according to UNI EN ISO 9562:2004.  
The AOX concentration values reported in the tables and graphs for the OUR tests came from the measurements carried out by Lab-1.