Article

A Comprehensive Aeration System Model for WRRF Design and Control

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

During the design of a water resource recovery facility (WRRF), it is becoming industry practice for process engineers to use simulation software to assist with the design of the plant and its aeration system. The aeration process is one of the key components of the activated sludge process, and as such, is one of the most important aspects of modeling wastewater treatment systems. A comprehensive aeration system model has been developed in SIMBA# that can model both the air supply and demand. The model includes sub-models for centrifugal and positive displacement blowers, pipes and fittings, valves, and diffusers. Both compressible and incompressible flow can be modelled. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning. This will allow more detailed system-wide testing before commissioning.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The air supply distribution was modeled similarly to other studies with equipment components such as the blower battery, piping system, valves and diffusers (Amaral et al., 2017;Juan-García et al., 2018;Schraa et al., 2017). As thoroughly presented in Schraa et al. (2015) and Schraa et al. (2017), the air supply system equipment sub-models are linked together by using physical gas models block diagrams, which, although use a different simulation environment, are based on the same physical equations. However, specific equipment curves, such as blowers, diffusers and valves, were alternatively modeled by implementing directly the manufacturer curves into the Simulink model by using a 2-D look-up table block. ...
... When valve flow coefficient curves (Cv/Kv curves) are available, redistribution of air mass flow rates can be estimated as a function of valve stem opening (Schraa et al., 2015;Skousen, 1998). In this study, the Cv curves are used to estimate the pressure drop as a result of different valve openings. ...
Article
Diffused aeration is the most implemented method for oxygen transfer in municipal activated sludge systems and governs the economics of the entire treatment process. Empirical observations are typically used to regulate airflow distribution through the adjustment of manual valves. However, due to the associated degrees of freedom, the identification of a combination of manual valves that optimizes all performance criteria is a complex task. For the first time a multi-criteria optimization algorithm was used to minimize effluent constituents and energy use by parametrizing manual valves positions. Data from a full-scale facility in conjunction with specific model assumptions were used to develop a base-case facility consisting of a detailed air supply model, a bio-kinetic model and a clarification model. Compared to the base-case condition, trade-offs analysis showed potential energy savings of up to 13.6% and improvement of effluent quality for NH4+ (up to 68.5%) and NOx (up to 81.6%). Based on two different tariff structures of a local power utility, maximum costs savings of 12800 USD mo-1 to 19000 USD mo-1 were estimated compared to baseline condition.
Article
During the design of a water resource recovery facility, it is becoming industry practice to use simulation software to assist with process design. Aeration is one of the key components of the activated sludge process, and is one of the most important aspects of modelling wastewater treatment systems. However, aeration systems are typically not modelled in detail in most wastewater treatment process modelling studies. A comprehensive dynamic aeration system model has been developed that captures both air supply and demand. The model includes sub-models for blowers, pipes, fittings, and valves. An extended diffuser model predicts both oxygen transfer efficiency within an aeration basin and pressure drop across the diffusers. The aeration system model allows engineers to analyse aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, controller design and tuning, and energy costs. This enables engineers to trouble-shoot the entire aeration system including process, equipment and controls. It also allows much more realistic design of these highly complex systems.
Article
Aeration is an essential component of aerobic biological wastewater treatment and is the largest energy consumer at most water resource recovery facilities. Most modelling studies neglect the inherent complexity of the aeration systems used. Typically, the blowers, air piping, and diffusers are not modelled in detail, completely mixed reactors in a series are used to represent plug-flow reactors, and empirical correlations are used to describe the impact of operating conditions on bubble formation and transport, and oxygen transfer from the bubbles to the bulk liquid. However, the mechanisms involved are very complex in nature and require significant research efforts. This contribution highlights why and where there is a need for more detail in the different aspects of the aeration system and compiles recent efforts to develop physical models of the entire aeration system (blower, valves, air piping and diffusers), as well as adding rigour to the oxygen transfer efficiency modelling (impact of viscosity, bubble size distribution, shear and hydrodynamics). As a result of these model extensions, more realistic predictions of dissolved oxygen profiles and energy consumption have been achieved. Finally, the current needs for further model development are highlighted.
Article
Full-text available
This review covers automatic control of continuous aeration systems in municipal wastewater treatment plants. The review focuses on published research in the 21st century and describes research into various methods to decide and control the dissolved oxygen (DO) concentration and to control the aerobic volume with special focus on plants with nitrogen removal. Important aspects of control system implementation and success are discussed, together with a critical review of published research on the topic. With respect to DO control and determination, the strategies used for control span from modifications and developments of conventional control methods which have been explored since the 1970s, to advanced control such as model-based predictive and optimal controllers. The review is supplemented with a summary of comparisons between control strategies evaluated in full-scale, pilot-scale and in simulations.
Article
Full-text available
Aeration consumes about 60% of the total energy use of a wastewater treatment plant (WWTP) and therefore is a major contributor to its carbon footprint. Introducing advanced process control can help plants to reduce their carbon footprint and at the same time improve effluent quality through making available unused capacity for denitrification, if the ammonia concentration is below a certain set-point. Monitoring and control concepts are cost-saving alternatives to the extension of reactor volume. However, they also involve the risk of violation of the effluent limits due to measuring errors, unsuitable control concepts or inadequate implementation of the monitoring and control system. Dynamic simulation is a suitable tool to analyze the plant and to design tailored measuring and control systems. During this work, extensive data collection, modeling and fullscale implementation of aeration control algorithms were carried out at three conventional activated sludge plants with fixed pre-denitrification and nitrification reactor zones. Full-scale energy savings in the range of 16–20% could be achieved together with an increase of total nitrogen removal of 40%.
Conference Paper
During the design of a water resource recovery facility (WRRF), it is becoming industry practice for process engineers to use simulation software to assist with the design of the plant and its aeration system. The aeration process is one of the key components of the activated sludge process, and as such, is one of the most important aspects of modeling wastewater treatment systems. A comprehensive aeration system model has been developed in SIMBA# that can model both the air supply and demand. The model includes sub-models for centrifugal and positive displacement blowers, pipes and fittings, valves, and diffusers. Both compressible and incompressible flow can be modelled. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning. This will allow more detailed system-wide testing before commissioning.
Article
The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.
 The Effect of Membrane Diffuser Design and Geometry on Fine-Bubble Performance. Technical White Paper, US Filter -Envirex Diffused Aeration Products
  • R D Sproull
  • M L Doyle
  • L B Ratzlow
Sproull, R. D., Doyle, M. L. & Ratzlow, L. B.  The Effect of Membrane Diffuser Design and Geometry on Fine-Bubble Performance. Technical White Paper, US Filter -Envirex Diffused Aeration Products.