The main purpose of the recently introduced Bürger–Diehl simulation model for secondary settling tanks was to resolve spatial discretization problems when both hindered settling and the phenomena of compression and dispersion are included. Straightforward time integration unfortunately means long computational times. The next step in the development is to introduce and investigate time-integration methods for more efficient simulations, but where other aspects such as implementation complexity and robustness are equally considered. This is done for batch settling simulations. The key findings are partly a new time-discretization method and partly its comparison with other specially tailored and standard methods. Several advantages and disadvantages for each method are given. One conclusion is that the new linearly implicit method is easier to implement than another one (semi-implicit method), but less efficient based on two types of batch sedimentation tests.
Benchmark simulations of entire wastewater treatment plants (WWTPs) are today performed with one-dimensional simulation models of the secondary settling tank (Gernaey et al. 2014; Li & Stenstrom 2014). In the model by Bürger et al. (2011, 2013), sometimes referred to as ‘Bürger–Diehl model’, the physical phenomena of hindered settling, volumetric bulk flows, compression of the sludge at high concentrations and dispersion of the suspension near the feed inlet can be included in a flexible way. Each phenomenon is associated with a separate constitutive function with its model parameters, and can be activated or deactivated at the user's discretion. The possibility to include sludge compression is particularly important, since this improves the predictive power considerably (De Clercq et al. 2008; Ramin et al. 2014; Guyonvarch et al. 2015; Torfs et al. 2015; Van Loosdrecht et al. 2015). However, the inclusion of physical phenomena that result in second-order derivative terms in the model partial differential equation (PDE), e.g., compression, means that straightforward time discretization by easy-to-implement, explicit methods such as the explicit Euler method leads to long computational times.
Bürger et al. (2013) presented implementation details of a numerical algorithm, which gives reliable simulations with respect to the underlying physical principles and can be obtained with a user-defined accuracy. For long-time simulations of entire WWTPs, it is important to keep discretization errors small. This calls for a fine resolution in both space (many layers in the settler) and time (short time steps), which implies long computational times. Conversely, fast computations obtained with a low resolution in space and time come at the cost of poor accuracy.
In scientific computing, the numerical error is a measure of how close a numerical solution is to the exact solution or reference solution (obtained by a very fine discretization) of the model, i.e., the governing differential equation. The efficiency of a numerical method is assessed by relating the numerical errors to the computational (central processing unit; CPU) times necessary to obtain the numerical solutions for different discretizations. It is then said that one numerical method is more efficient than another if it allows a numerical solution with a determined numerical error to be obtained in less CPU time, or equivalently, a given budget of CPU time allows one to obtain a more accurate numerical solution by the first method than by the second.
The simulation model by Bürger et al. (2013) is based on a method-of-lines formulation of the underlying nonlinear PDE. This means a system of time-dependent ordinary differential equations (ODEs), one for each layer of the settler. Simulations of PDEs are stable and reliable if a so-called CFL (Courant–Friedrichs–Lewy) condition is satisfied. This gives a maximal time step for each given layer thickness . If only hindered settling and bulk flows are included, then the CFL condition means that can be chosen proportional to , i.e., , which results in fast simulations. When compression or dispersion is included and standard ODE solvers used, the CFL condition states that , which means very small when the error should be reduced (small is chosen).
The investigated methods are used for the simulation of two different batch settling tests in a vessel with m. While the present study is limited to batch settling in a column, the method proposed herein should be appropriate to handle secondary settling tanks as well (under suitable modifications that would mainly affect the convective flux, but not the compression term for whose discretization the present method has been tailored). It is therefore essential to demonstrate that the method can robustly handle several scenarios, in particular a scenario that exhibits some spatial variation of the solution. For this reason we consider a conventional Kynch test (KT; Kynch 1952), that is, the settling of an initially homogeneous suspension, and a Diehl test (DT; Diehl 2007), where a body of concentrated suspension is initially located above clear liquid, separated by, for example, a membrane. The DT shows a stronger variation of the solution than the KT. Both tests provide complementary information that can be used to identify large portions of the flux function f (Betancourt et al. 2014).
For the discussion of the various time-stepping methods, which will take the numerical solution from to , it will be important to carefully distinguish between numerical fluxes and concentrations that are evaluated at the old time step and those evaluated at , which we mark by the respective upper index n and . The time-integration methods compared herein are the following, where are constants that depend on the choice of the functions f and d.
The explicit Euler method
The semi-implicit method
The linearly implicit method
To advance the solution from to , one proceeds as follows:
For , set .
Other time-stepping methods
Any ODE solver could be used for the method-of-lines system (3)–(5). Possibly competitive methods are adaptive step-size methods and we choose here the ODE solver ode15s in Matlab (2014), which is an implicit multi-step method of variable order with step-size control. Of several standard ODE solvers investigated by Diehl et al. (2015), this was the second most efficient one (after the SI method) in the investigations with both stand-alone settler simulations and benchmark simulations for the entire activated sludge process.
RESULTS AND DISCUSSION
For accurate simulation results, should be chosen small, whereby the CFL condition implies that the explicit Euler method requires much smaller time steps than the other methods. On the other hand, the SI method needs more computations at every time step and requires the evaluation of the Jacobian matrix of the nonlinear algebraic system of equations. However, even when this system is solved by the Newton–Raphson method, the SI method has turned out to be far more efficient than explicit Euler (Diehl et al. 2015). To reduce the implementation complexity and to remove the necessity to solve numerically a system of nonlinear algebraic equations at every iteration in the SI method, whose convergence depends on the Newton–Raphson iterations, the LI method can be used instead. The advantage of solving a linear system only in each iteration, which also means an easier implementation, is paid fort by the price of a larger numerical error.
With the aim of finding an efficient time-integration method for the simulation of sedimentation with compression, we have in this work confined ourselves to two batch sedimentation tests for the comparison of some methods. The ideas of the new LI method and its implementation have been provided. When evaluated on the batch settling tests, the following advantages and disadvantages can be concluded.
Explicit Euler method:
+ Implementation easy.
+ Convergence proof exists also for continuous sedimentation (numerical approximate solutions converge to the PDE solution as ) by Bürger et al. (2005). Robust method.
− Not efficient.
+ Most efficient of the investigated methods. Under the assumption that the Newton–Raphson iterations find a solution each time step, convergence of the numerical method was proved for batch sedimentation by Bürger et al. (2006).
− Implementation more complex than Euler and LI. At each time step, a nonlinear system of algebraic equations is solved, e.g., by Newton–Raphson iterations, which require tolerance parameter to be set. There is no guarantee that these iterations converge.
+ Implementation easy. The ingredient in addition to the Euler method is basically that a linear system of equations is solved at each time step. Robust method.
± Second most efficient for for batch sedimentation. The efficiency can be adjusted to some extent by a parameter. Fastest method for a given , but least accurate. Convergence proof in preparation.
+ Ready-to-use standard time solver.
− Implementation most complex of the investigated methods. Well-established robust ODE solver for stiff problems, but not developed for solutions containing discontinuities.
− Least efficient of the investigated methods.
RB is supported by Fondecyt project 1130154; Conicyt project Anillo ACT1118 (ANANUM); Red Doctoral REDOC.CTA, MINEDUC project UCO1202; BASAL project CMM, Universidad de Chile and CI2MA, Universidad de Concepción; and Fondef ID15I10291. RB and CM are also supported by CRHIAM, Proyecto Conicyt Fondap 15130015. SD acknowledges support from The ÅForsk Foundation, Sweden.