1.1 Role of Hydrodynamics in the Performance Improvement of WWTU
Water and wastewater treatment normally take place in a series of continuous flow units, each designed to perform a step of the purification process – such as the physical removal of suspended material by settling in sedimentation units, or the chemical inactivation of microorganisms by chlorination in contact units – aimed at delivering water or wastewater at a quality that meets the relevant standards.1 For instance, conventional water treatment usually involves coagulation or flocculation, sedimentation or filtration and disinfection. Each of these processes has its own set of constraints in terms of what is required to achieve the specific treatment goal during the passage of water or effluent through the corresponding unit. Typical factors for consideration include:
- The characteristics of the water or effluent prior to entering a given treatment unit.
- The desired characteristics of the water or effluent at the point of leaving a given treatment unit.
- The nature of the treatment process required for such modification(s) to occur and what intervention is needed to accelerate or promote it (such as dosage of chemicals, use of aerators etc.).
- How such intervention takes place (e.g. which reagent should be used, at what dosage rate etc.).
- Local environmental conditions that may affect treatment, such as temperature, light intensity etc.
- The rate at which water or effluent passes through the unit and, if the regime is unsteady, what the critical flow conditions to be taken into account are.
- How water or effluent passes through the unit, in terms of the flow behaviour.
All of the above aspects are usually interconnected, so a change (intentional or not) in one of them may have knock on effects on virtually all the others and on their relative importance. However, usually:
1 Stricter regulation, environmental constraints and public health issues generally call for continued improvements to water and wastewater treatment systems. While it is technically possible to purify waters or wastewaters to the point of removing all suspended and dissolved elements, this is usually neither economically feasible nor desirable, or necessary from a health or environmental viewpoint, being typically achieved only in laboratories and other applications that require extremely pure water in their processes.
- Factor 1 is more or less set, at least within a certain range, as determined by various factors, for example, the quality of source waters or wastewater produced by a particular industrial process or the outcome of a preceding treatment stage.
- Factor 2 is governed by a fixed treatment goal, such as defined by regulation or as required for a subsequent step of the treatment system.
- Factors 3 and 4 are the two key design aspects of the treatment system and are traditionally considered in view of factors 5 and 6 as well.
- Factor 5 is defined by externalities, although certain control or attenuating measures can be taken if necessary (to mitigate deleterious influences on the treatment process, for example).
- Factor 6 is determined by either the rate of wastewater production or by the rate at which water supply is demanded from a given treatment plant, and can be constrained by how much water can be abstracted from a given source.
- Factor 7 is largely neglected or considered only implicitly in conventional water and wastewater treatment design approaches. It is the main goal of this book to highlight the implications of such a reality, and outline how it does not need to be so, through providing the means to assist with making this factor more widely regarded as a key design aspect of water and wastewater treatment units.
It is common practice to assess the degree to which a given treatment goal is achieved by calculating an ‘efficiency’ measure of each treatment process that expresses how close (or otherwise) the outflow characteristics are either to the desired characteristics of factor 2, or, relatively to the inflow characteristics of factor 1. Such a measure represents the combined outcome of all of the above listed factors and can be affected by most design and operation actions. During operation of an existing water or wastewater treatment unit, a typical control action to improve such efficiency is to adjust factor 4, as all other aspects are pretty much set for a given operating condition. Design imperfections and/or difficult operating conditions can thus be compensated for, at least in part, and/or temporarily, for example by increasing reagent dosage for a particular process or altering the frequency of aerator usage2 in a certain unit. However, regular use of such ‘medicine’ can have deleterious side effects of its own, such as enhancing the production of undesirable by-products and increasing operating costs and energy demands of a given treatment process stage.3 Not taking factor 7 into account carries precisely this risk and is a common reason why existing water and wastewater treatment units operating in sub-optimal ways need to undergo a retrofit.
It follows that the process efficiency of a given unit depends as much on the physical, chemical or biological reaction of interest as it does on the flow pattern taking place in its interior. This is because the flow pattern governs, for example, the residence/contact time, turbulence levels, collisions and shear to which different fluid portions are subjected to in their passage through the unit. The combined effect of flow features on process efficiency is often overlooked in teaching materials on the design of water and wastewater treatment units. As a result, it is not uncommon to find treatment units operating in a cost-ineffective way, contributing to health problems and environmental impacts, although other factors can also impair process performance.4
But the topic itself is certainly not new. Research undertaken in the 1950s in the area of chemical reaction engineering (Danckwerts, 1953; Wehner and Wilhelm, 1956) were scientific precursors of hydrodynamic-kinetic models for continuous flow reactors, whereby the process efficiency of a unit and a hydrodynamic parameter associated with the flow pattern in the unit are explicitly related in analytical form. This was followed, in the 1960s and 1970s, by pioneering work on the hydrodynamics of water and wastewater treatment (e.g. Louie and Fohrman, 1968; Sawyer and King, 1969; Thirumurthi, 1969; Watters, 1972; Kothandaraman et al., 1973; Marske and Boyle, 1973; Thirumurthi, 1974; Hart et al., 1975; Trussel and Chao, 1977; Hart and Gupta, 1978; Silva and Mara, 1979), mostly for chlorine disinfection units and wastewater stabilisation ponds. Subsequent scientific research also focused on other applications including, for example, sedimentation units (Adams and Rodi, 1990; Lyn and Rodi, 1990) and, more recently, a wide range of processes and units have been investigated for the effect of hydrodynamic aspects using mostly Computational Fluid Dynamics (CFD) tools (e.g. Khan et al., 2006; Sartori et al., 2015; Karpinska and Bridgeman, 2016; Meister et al., 2017; Li et al., 2018). Despite significant improvements leading to so-called ‘rational’ design approaches (as early as in the 1970s) and modern all-encompassing CFD simulations (mainly since the 2000s), few in-depth applications to other types of water or wastewater treatment units appear to have been translated into design practice. The topic still does not receive too much attention in the remit of typical civil, environmental and water engineering degree schemes and teaching material (apart from a few examples, such as Sykes et al., 2003). Concurrently, significant gaps in knowledge directly applicable to treatment unit design and practice remain to date (e.g. Teixeira et al., 2016; Li et al., 2018).
2 Where and when used, aerators can significantly affect flow patterns in treatment units, to the point of impairing application of generalised models to estimate hydrodynamic and treatment e...
