The effect of high solid retention times in activated sludge reactors on operational costs at Mitchells Plain WWTP

Pollard, Rudolph

The effect of high solid retention times in activated sludge reactors on operational costs at Mitchells Plain WWTP

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Civil Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Many wastewater treatment plants (WWTPs) in South Africa, which use activated sludge (AS) processes, experience difficulty managing solid retention times (SRT), also referred to as sludge age. To avoid dealing with large quantities of sludge waste, some of these WWTPs operate the AS reactors at extended SRTs, far beyond what is required for sufficient nutrient removal or to produce a stable waste activated sludge. Hence, the strategy to operate at high SRTs could be due to a variety of factors, such as insufficient or faulty sludge handling facilities, insufficient means of sludge disposal, high cost of flocculant agents, lack of operational skills, etc. At higher SRTs, waste flow rates are lower, which leads to a lower demand on a WWTPs sludge handling facilities and lower pumping rates. In return, a higher SRT will lead to a higher oxygen utilization rate (OUR) in the AS reactor. This will lead to higher electricity consumption by the aeration equipment or possible poor effluent quality, where (i) the aeration equipment is limited in potential to supply oxygen required to meet the increased demands for organic removal and nitrification and (ii) the thickened sludge production resulting in poor oxygen transfer within the reactor and sludge overflow from secondary settling tanks that have reached their limits in the surface area required for the given solids thickness. Rising cost in electricity and fuel are a daily reality for all in South Africa (SA), resulting in acute energy awareness in all sectors of business, both public and private. This awareness and the need for energy efficiency will continue to escalate, especially with the SA energy supplier (ESKOM) continuing to request and apply escalation rates far above the inflation rate. With this in mind, the aim of this study is to determine the effect of high solid retention times in activated sludge reactors on operational costs at wastewater treatment plants by developing a plant wide steady state model, based on previous work carried out by Marais and Ekama, (1976), Ekama and Wentzel, (2008a), Ekama and Wentzel, (2008b), Wentzel et al., (2008) and Ekama 2009. The model simulated various scenarios with different SRT's at different influent flow rates to find the correlation between electrical energy consumption cost and the operational cost of sludge handling at various SRT's. The various scenarios were evaluated using the EQI and OCI performance indices developed by the IWA benchmark simulation task group (Copp, 2002, Nopenset al., 2010), and modified by (De Ketele, Davister & Ikumi, 2018). These performance indices were then further modified, as described in Chapter Error! Reference source not found., to be improve the relevance to this study. The study found that the average electrical energy (kWh) required for aeration in the Modified Ludzack-Ettinger (MLE) reactor increased by 13.8% from a SRT of 10 days to a SRT of 30 days. The average electrical energy (kWh) for aeration in the University of Cape Town (UCT) reactor increase by 14.7% over the same SRT periods (refer to Section Error! Reference source not found.). It was further found the average electrical energy required for pumping, decreased between a range of 2.1% and 1% from a SRT of 10 days to a SRT of 30 days over the range of flows considered. Electrical energy required for dewatering was also found to decrease with an increase in SRT (22% from 10 days to 30 days), while mixing energy remains constant. Since the majority of the electrical consumption can be attributed to aeration, the increase of electrical energy consumption with SRT of aeration is not offset be the decrease with SRT of the other OCI drivers. The study found that the electrical energy consumption cost increases by 11.6% from an SRT of 10 days to an SRT of 30 days. The rate of increase, however, decreases at higher SRT's. The study further confirmed that the operational costs of sludge handling, which includes dewatering energy consumption, chemical usage, and sludge disposal, decreases with an increase in SRT. The study found that the operational costs of sludge handling decreased by 23% from an SRT of 10 days to an SRT of 30 days. As with electrical energy, the rate of decrease in operational costs, decreases at higher SRT's. In conclusion, study found that although most of the OCI drivers became more cost effective at higher SRT's the increased cost of electricity consumption of aeration at higher SRT's leads to higher overall operating costs at higher SRT's. This confirms the hypothesis that the increase in electrical energy consumption cost will outweigh the savings in operational costs of sludge handling, and therefore it is more economical to operate AS reactors at lower SRTs.

Keywords

Mitchells Plains

WWTP

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