Modelling of high-rate activated sludge process: Assessment of model parameters by sensitivity and uncertainty analyses

The objective of this study is to develop a mechanistic model to predict the long-term dynamic performance of High-Rate Activated Sludge (HRAS) process, including the removal of carbon (COD), nitrogen (N), and phosphorus (P). The model was formulated with inspiration from Activated Sludge Models No. 1 and 3 (ASM1 and ASM3) to incorporate essential mechanisms, such as adsorption and storage substrate, specific to HRAS systems. A stepwise protocol was followed for calibration with dynamic data from a pilot-scale HRAS plant. Sensitivity analysis identified influential model parameters, including maximum specific growth rate (μ), growth yield (YH), storage yield (YSTO), storage rate (kSTO), decay rate (b), and half saturation of the readily biodegradable substrate for growth (KS1). The calibrated model achieved prediction efficiencies above the normalized Mean Absolute Error (MAE) of 70 % for mixed liquor suspended solids (MLSS), total chemical oxygen demand (TCOD), soluble COD (SCOD), particulate COD (XCOD), total nitrogen (TN), ammonia nitrogen (SNH), total phosphorus (TP), soluble TP (STP), and particulate TP (XTP). Uncertainty analysis revealed that SCOD was underestimated. Based on the dynamic profiles of uncertainty bands and observed data, there is potential for improving the estimation of dynamic behavior in STP. The observed discrepancies may be attributed to variations in wastewater characteristics during the monitoring period, particularly concerning the phosphorus (P) fractions of the readily biodegradable substrate (SS) and soluble inerts (SI), which were not considered as dynamically changing parameters in the model.

Use of water treatment plant sludge in high-rate activated sludge systems: A techno-economic investigation

Coagulants such as aluminum sulfate (Al2(SO4)3 (alum)) and ferric chloride (FeCl3) used in water treatment plants (WTPs) led to the generation of sludge that is usually disposed to landfills. However, the utilization of WTP sludge is being encouraged by authorities to achieve sustainable development. This study aims to investigate WTP sludge utilization in a pilot-scale high-rate activated sludge (HRAS) system as a substitute for conventional coagulants. Based on jar tests, the iron sludge was selected for pilot-scale testing due to its superior ability to enhance the treatment efficiency of the HRAS process compared to alum sludge. Iron sludge addition (20.1 ± 1.6 mg dry sludge/L wastewater) slightly improved the removal efficiency of particulate chemical oxygen demand (pCOD) from 74 % to 81 % (p-value: 0.014). Iron sludge addition had a distinct effect on the sludge characteristics of the HRAS process. The average median particle size (d50) increased from 96 ± 3 to 163 ± 14 μm (p-value<0.00) with the addition of iron sludge, which improved the settleability of the HRAS process sludge. However, the biochemical methane potential (BMP) of the HRAS process sludge decreased by 8.9 % (p-value<0.00) after iron sludge addition. In a scenario analysis of WTP sludge use in a hypothetical HRAS plant, the effluent quality index (EQI), an indicator of environmental impact, was calculated and the cost related to the operation (the transfer and landfill disposal of WTP and HRAS process sludge, energy and chemical consumption of the HRAS plant) was estimated. As a result, using WTP sludge in the HRAS plant did not significantly affect the EQI of the plant but decreased overall cost by 11 %. The results showed that the use of WTP sludge as a coagulant in wastewater treatment could achieve mutual benefits for WTPs and WWTPs and have the potential to realize the circular economy model.