## 2 Materials and Methods
The measurement of the petroleum product (PP) and anionic surfactant (AS) contents in the samples was conducted using the Fluorat-2M (LLC \"Lume\", Saint Petersburg, Russia). This device is based on the fluorimetry method, which enables the determination of petroleum hydrocarbon and AS concentrations in water and soil samples by measuring the fluorescence intensity of organic compounds. The determination of petroleum products using a fluorimeter involved the preliminary extraction of petroleum products from the sample using an organic solvent, typically hexane or chloroform. The obtained extract was then subjected to fluorescence measurements at specific wavelengths characteristic of the aromatic hydrocarbons present in petroleum  
Figure 2: The examined sludge: (**a**) collection after dewatering; (**b**) collection from the disposal site; (**c**) sludge that underwent freeze–thaw–drying cycles. products. GOST R 52160-2003 \"Water. Method for determining petroleum products by the fluorimetric method\" [35] was used as the standard. The measurement of anionic surfactants was carried out in accordance with the standard GOST 31861-2012 \"Water. Method for determining anionic surfactants by the fluorimetric method\" [36]. Anionic surfactants themselves do not exhibit fluorescence; however, they can form fluorescent complexes with certain reagents. One such method is based on the interaction of AS with fluorescent dyes, leading to changes in their fluorescence. Fluorescent dye sensitive to the presence of AS (Rhodamine B) was used. All other analytical measurements were performed using standard methods, including the use of an ARL OPTIM'X 200W wavelength-dispersive X-ray fluorescence spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA), a pH sensor (Hanna Instruments, Woonsocket, RI, USA), and a Hach Lange DR6000 spectrophotometer (Hach, Loveland, CO, USA). Wavelength-dispersive X-ray fluorescence (WDXRF) operates on the principle of X-ray fluorescence, where a primary X-ray beam excites the atoms in a sample, causing them to emit secondary (fluorescent) X-rays characteristic of the elements present. In WDXRF, the emitted X-rays are dispersed by wavelength using a crystal spectrometer, allowing for high-resolution separation of overlapping spectral lines. The analysis was carried out using the provisions of ISO 18227:2014--Soil quality--Determination of elemental composition by X-ray fluorescence [37]. The analysis procedure was as follows:  
* Drying: The sewage sludge samples were dried, typically at 105 \({}^{\circ}\)C, to remove moisture. Drying ensures consistent weight measurements and prevents interference from water content. * Grinding and pulverizing: the dried sludge was ground to a fine powder (particle size < 75 \(\upmu\)m) using a mechanical grinder. * Fusion or pressed pellets: the powdered sample is mixed with a binder (boric acid) and pressed into a pellet under high pressure. * Calibration curves are established for each element of interest by plotting the intensity of characteristic X-rays against known concentrations. * The characteristics of the sludge are presented in Section 3. Composing of the sludge was conducted in separate reaction vessels equipped with steam condensation devices. The reaction reactors were placed in water baths to facilitate initial heating of the sludge in the case of inadequate heating during the initial phase. Figure 3 illustrates a single reaction vessel containing the composting mixture, as well as the water baths in heating mode with the reaction vessels installed within them. Figure 3: Laboratory reactor used in the wastewater treatment efficiency evaluation phase: (**a**) reaction vessel containing the composting mixture; (**b**) water baths in heating mode with the reaction vessels. The cyclic freezing-thawing-drying process of the wastewater sludge was conducted in a Weiss WT 240/70 climate chamber (Weiss Technik GmbH, Reiskirchen, Germany).