Evaluating the impact of plant-wide WWTP control strategies on energy consumption and greenhouse gas emissions

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Modelling gas-liquid mass transfer in wastewater treatment : when current knowledge needs to encounter engineering practice and vice versa

Abstract Gas–liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades from both academia and industry. Indeed, improvements in modelling gas–liquid mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid–base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas–liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas–liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas–liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas–liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners

Performance Assessment of Wastewater Treatment Plants : Multi-Objective Analysis Using Plant-Wide Models

As the knowledge about anthropogenic impacts of climate change has grown, the awareness of the contributions from treatment of wastewater has widened the scope for wastewater treatment plants (WWTPs). Not only shall ever stricter effluent constraints be met, but also energy efficiency be increased, greenhouse gases mitigated and resources recovered. All under a constant pressure on costs. The main objective of this research has been to develop a plant-wide modelling tool to evaluate the performance of operational strategies for multiple objectives at the plant and for off-site environmental impact. The plant-wide model platform Benchmark Simulation Model no. 2 (BSM2) has been modified to improve the evaluation of energy efficiency and include greenhouse gas emissions. Furthermore, the plant-wide process model has been coupled to a life cycle analysis (LCA) model for evaluation of global environmental impact. For energy evaluation, a dynamic aeration system model has been adapted and implemented. The aeration model includes oxygen transfer efficiency, dynamic pressure in the distribution system and non-linear behaviour of blower performance. To allow for modelling of energy recovery via anaerobic co-digestion the digestion model of BSM2 was updated with a flexible co-digestion model allowing for dynamic co-substrate feeds. A feasible procedure for substrate characterisation was proposed. Emissions of the greenhouse gases CO2, CH4 and N2O were considered. The bioprocess model in BSM2 was updated with two-step nitrification, four-step denitrification and nitrifier denitrification to capture N2O production. Fugitive emissions of the three gases were included from digestion, cogeneration and sludge storage. The models were tested in case studies for the three areas of development: aeration, co-digestion and greenhouse gas production. They failed to reject the hypothesis that dynamic process models are required to assess the highly variable operations of wastewater treatment plants. All parts were combined in a case study of the Käppala WWTP in Lidingö, Sweden, for comparison of operational strategies and evaluation of stricter effluent constraints. The averaged model outputs were exported to an LCA model to include off-site production of input goods and impact of discharged residues and wastes. The results reveal trade-offs between water quality, energy efficiency, greenhouse gas emissions and abiotic depletion of elemental and fossil resources. The developed tool is generally applicable for WWTPs and the simulation results from this type of combined models create a good basis for decision support

Optimization of Aeration Diffuser System Design : A Simulation Study

The influence of aeration diffuser system design on electricity usage, effluent water quality, and life-cycle cost in biological wastewater treatment was investigated. A plant-wide model was implemented, and simulations were carried out with different process configurations and aeration systems. Model-aided design of new aeration diffuser systems could significantly decrease electricity usage and life-cycle cost while at the same time avoiding negative effects on the treatment performance. The optimum distribution of diffuser systems in tanks in series was found to be influenced by process configuration, volumetric loading rate, temperature, and the internal recirculation flow rate. Compared with a conventional design approach, increasing the number of diffusers, up to a critical point, led to higher energy efficiency and lower life-cycle cost. This was despite an increasing limitation of the minimum airflow rate, leading to dissolved oxygen levels significantly exceeding control targets. Aeration systems optimized by simulations were found to, independently of process configuration, exhibit 20% lower electricity usage and 16%-18% lower life-cycle costs compared with systems designed based on a more conventional approach typically applied in practice

Balancing effluent quality, greenhouse gas emissions and operational cost - developing dynamical models for intergrated benchmarking of wastewater treatment plants

The scope for wastewater treatment plants (WWTPs) widens to consider not only water quality and cost, but also energy efficiency and greenhouse gas (GHG) emissions. The on-going research project, “Development and dynamic analysis of operational strategies for enhanced energy efficiency of wastewater treatment systems” aims to develop a tool for integrated evaluation of energy efficiency and greenhouse gas (GHG) emissions with effluent quality and operational costs. The tool will build on the comprehensive Benchmark Simulation Model no. 2. It will be extended with models for: i) production and emission of CO2, N2O and CH4 and ii) calculation of energy consumption and production at the plant. Here, a first attempt is presented together with a case where different set-points of dissolved oxygen, DO, in the aeration-tanks are simulated. The simulation results show that lowering the DO set-point from 2 mg l–1 to 1 mg l–1 reduces the CO2-emissions by 570 kg d–1, a reduction from 30,500 kg CO2 d–1 emitted from the plant at 2 mg DO l–1. However, at the same time this slightly increases the effluent discharge. Moreover, due to increased nitrite accumulation in the aeration tank, the N2O emissions increase by 12,500 kg carbon dioxide equivalents, CO2e d–1, which dramatically exceeds the reduction due to lowered power generation. Accordingly the total GHG-emissions are reduced by an increase of the DO set-point to 3 mg l–1, but the reduction is insignificant and the increased operational cost disproportional to the improvements in the emissions. The conclusion is that, to meet multi-objective goals, integrated benchmarking of the three criteria is essential