Biogas has improved due to technological advancements, environmental awareness, policy support, and research innovation, making it a more cost-effective and environmentally friendly renewable energy source. The Generalized Linear Model (GLM) was employed to examine the relationship between purchased and generated energy from 2007 to 2023. Metrics such as deviance, log likelihood, and dispersion phi were examined to assess model fit. The Mann–Kendall test was utilized to detect trends in energy datasets. Biochemical Oxygen Demand (BOD5) and Chemical Oxygen Demand (COD) reduction was significant, exceeding 97% from 2014 to 2023. However, treated sewage displayed limited susceptibility to biological degradation, with COD to BOD5 ratios increasing from 2.28 to 6.59 for raw sewage and from 2.33 to 7.05 for treated sewage by 2023. Additionally, the efficiency of sewage purification processes was calculated, and multivariate regression analysis was conducted on gas composition data. Principal Coordinate Ordination (PCO) and k-means clustering were used for dimensionality reduction and biogas component clustering, respectively. This research showed that biogas from the waste water treatment process can be used, particularly in methane production. Technological advancements have made biogas production more efficient, enhancing energy generation within a circular economy framework.

The expansion of sewage networks and treatment facilities results in considerable amounts of municipal sludge, which is essential for biogas production as part of energy diversification efforts. Principal Component Analysis (PCA) demonstrated a strong correlation between biogas production and its utilization in power generation units. Modernization efforts led to an increase in biogas utilization in power units but a decrease in boiler utilization, independent of the overall biogas production levels. The general linear model (GLM) further confirmed that biogas production was positively influenced by the amount of waste digested, while utilization in power units increased post modernization. A repeated measures ANOVA (Analysis of Variance) indicated significant increases in both dry matter and mineral content in digested sludge compared to raw sludge. SIMPER (Similarity Percentage) analysis revealed that the addition of glycerin water significantly reduced the nitrogen, ammonium nitrogen, and calcium content, while modernization increased these elements and slightly decreased the magnesium concentration. Multivariate dispersion analysis showed that samples treated with glycerin water exhibited less variability in metal content. Regression models explored the factors influencing mineral elements and dry mass in fermented sludge. The zinc content was positively associated with mineral content, while copper showed a negative correlation. The addition of glycerin water increased the mineral content, whereas modernization had the opposite effect. The nitrogen content was negatively correlated with dry mass. These findings provide valuable insights into optimizing sewage sludge treatment and biogas production processes by underlining the approaches for enhancing sludge properties to support efficient biogas production.