## 4 Discussion  
The quality standards for slaughterhouse wastewater discharged into the environment are set out in applicable legislation. In the EU, the issue is regulated by the Urban Wastewater Treatment Directive (91/271/EEC) [28]. Other regulatory measures include the US \"Effluent Limitations Guidelines and New Source Performance Standards for the Meat and Poultry Products Point Source\" [29], and the Chinese \"Integrated Wastewater Disposal Standard (GB 8978-1996)\" [30]. Similarly, guidelines and recommendations to mitigate meat-industry environmental impacts have been released in the form of reference documents on best available techniques [31]. Finally, there are legislative acts such as the Polish Wastewater Regulation (\"Rozporazdzenie w sprawie sciekow\"), intended to transpose the Water Framework Directive into national law [32]. Slaughterhouse wastewater treatment is usually a multistep process and a major technical challenge due to its complex nature, the high organic loadings involved, and the potentially huge impact on the environment. Individual process steps needed to obtain safe-to-discharge effluent include: flotation [33], ultrafiltration, [34; 35], electrocoagulation [36; 37], reverse osmosis [38; 39], electrochemical oxidation [40; 41], the aerobic activated sludge process [42; 43], and combined processes harnessing bacteria-microalgae consortia [44; 45]. Anaerobic digestion is an important and fast-advancing method of treating slaughterhouse wastewater. A major advantage of AD is its capacity to reuse waste by converting it into biomethane and biofertilizer [46]. It is believed that the conditions for AD should be kept within the following limits: the COD concentration should be between 1.5 and 40 gO\({}_{2}\)/dm\({}^{3}\) (to maintain commercial viability), dry matter content should be less than 15%, and the N/C ratio should fall between 1/5 and 1/20 (to ensure COD removal of at least 50%) [47]. The H-LPSW used in the present experiment satisfied these criteria at 15.6 \(\pm\) 1.6 gO\({}_{2}\)/dm\({}^{3}\) COD, 1.04 \(\pm\) 0.28 g/dm\({}^{3}\) TS, and a C/N (TOC/TN) ratio of 11.52 \(\pm\) 1.3. Anaerobic digestion is a fairly popular choice for treating poultry-processing wastewater. However, it is important to note that research results have been nonconclusive and highly variable. Process performance is determined by pollutant load, reactor type, and  
Figure 8: Denaturing gradient gel electrophoresis (DGGE) profile amplicons for 16S rDNA analysis. anaerobic digestion parameters. Mesophilic and thermophytic digestion processes (both with upflow anaerobic filters) have been found to remove 90% and 72% of COD, respectively, at OLR = 9.0 gCOD/dm\({}^{3}\cdot\)day [48]. In another experiment, 79% COD removal rate was achieved with OLR maintained at 10 gCOD/dm\({}^{3}\cdot\)day and 35 \({}^{\circ}\)C. The CH\({}_{4}\) content of the biogas ranged from 46% to 56% [49]. UASB reactors have been used to effectively remove pollutants from poultry-processing wastewater. One group managed to remove 90% of COD under mesophilic conditions from wastewater containing 820 mgO\({}_{2}\)/dm\({}^{3}\) to 12,800 mgO\({}_{2}\)/dm\({}^{3}\)[50]. [PERSON] et al. [51] achieved very high (95%) COD degradation in a UASB reactor despite a very high OLR of 31 gCOD/dm\({}^{3}\cdot\)d at HRTs between 3.5 and 4.5 h and a temperature of 35 \({}^{\circ}\)C. Anaerobic digestion of high-lipid poultry wastewater has been shown to remove 66-70% of COD while providing specific biomethane yields of 562 cm\({}^{3}\)/gCOD\({}_{\rm rem. }\) to 777 cm\({}^{3}\)/gCOD\({}_{\rm rem.}\)[52]. PSW treatment performance has also been tested across various OLRs in UASB reactors. In one study, 90% COD removal was achieved at an OLR of 0.4 gCOD/dm\({}^{3}\cdot\)d, which subsequently dropped to 70% when the OLR increased to 3.0 gCOD/dm\({}^{3}\cdot\)d, decreased further to 65% at an OLR of 10 gCOD/dm\({}^{3}\cdot\)d, and finally fell below 50% when the OLR was increased to 15 gCOD/dm\({}^{3}\cdot\)d [53].