Bruce R. Johnson

Wastewater treatment modelling: dealing with uncertainties

This paper serves as a problem statement of the issues surrounding uncertainty in wastewater treatment modelling. The paper proposes a structure for identifying the sources of uncertainty introduced during each step of an engineering project concerned with model-based design or optimisation of a wastewater treatment system. It briefly references the methods currently used to evaluate prediction accuracy and uncertainty and discusses the relevance of uncertainty evaluations in model applications. The paper aims to raise awareness and initiate a comprehensive discussion among professionals on model prediction accuracy and uncertainty issues. It also aims to identify future research needs. Ultimately the goal of such a discussion would be to generate transparent and objective methods of explicitly evaluating the reliability of model results, before they are implemented in an engineering decision-making context.

Modeling integrated fixed-film activated sludge and moving-bed biofilm reactor systems. I: Mathematical treatment and model development.

A mathematical model for integrated fixed-film activated sludge (IFAS) and moving-bed biofilm reactor wastewater treatment processes was developed. The model is based on theoretical considerations that include simultaneous diffusion and Monod-type reaction kinetics inside the biofilm, competition between aerobic autotrophic nitrifiers, non-methanol-degrading facultative heterotrophs, methanol-degrading heterotrophs, slowly biodegradable chemical oxygen demand, and inert biomass for substrate (when appropriate) and space inside the biofilm; and biofilm and suspended biomass compartments, which compete for both the electron donor and electron acceptor. The model assumes identical reaction kinetics for bacteria within suspended biomass and biofilm. Analytical solutions to a 1-dimensional biofilm (assuming both zero- and first-order kinetics) applied to describe substrate flux across the biofilm surface are integrated with a revised and expanded matrix similar to that presented as the International Water Association (London, United Kingdom) Activated Sludge Model Number 2d (ASM2d) stoichiometric and kinetic matrix. The steady-state mathematical model describes a continuous-flow stirred-tank reactor.

Modeling integrated fixed-film activated sludge and moving-bed biofilm reactor systems II: Evaluation

A steady-state model presented by Boltz, Johnson, Daigger, and Sandino (2009) describing integrated fixed-film activated sludge (IFAS) and moving-bed biofilm reactor (MBBR) systems has been demonstrated to simulate, with reasonable accuracy, four wastewater treatment configurations with published operational data. Conditions simulated include combined carbon oxidation and nitrification (both IFAS and MBBR), tertiary nitrification MBBR, and post denitrification IFAS with methanol addition as the external carbon source. Simulation results illustrate that the IFAS/MBBR model is sufficiently accurate for describing ammonia-nitrogen reduction, nitrate/nitrite-nitrogen reduction and production, biofilm and suspended biomass distribution, and sludge production.

Integrated nutrient removal design for very low phosphorus levels.

The State of Washington has found that the Spokane River is DO impaired, and is requiring dischargers to reduce phosphorus inputs to the river. Spokane County elected to build a new water recovery facility with a target effluent total phosphorus level of 50 microg/L on a seasonal average basis. Spokane County elected to use a private company to design/build and operate their facility. The very low nutrient requirements, and lack of historical operating information, necessitated an integrated approach to the nutrient removal design while considering the risks and benefits of the various treatment options. The process selection evaluated membrane bioreactors and tertiary membranes for the primary liquids process in combination with chemical and/or biological phosphorus removal. The final process selection included chemically enhanced primary treatment, membrane bioreactor with metal salts, and dewatering liquor treatment with an innovative post aerobic digestion step.

Biofilm carrier migration model describes reactor performance

The accuracy of a biofilm reactor model depends on the extent to which physical system conditions (particularly bulk-liquid hydrodynamics and their influence on biofilm dynamics) deviate from the ideal conditions upon which the model is based. It follows that an improved capacity to model a biofilm reactor does not necessarily rely on an improved biofilm model, but does rely on an improved mathematical description of the biofilm reactor and its components. Existing biofilm reactor models typically include a one-dimensional biofilm model, a process (biokinetic and stoichiometric) model, and a continuous flow stirred tank reactor (CFSTR) mass balance that [when organizing CFSTRs in series] creates a pseudo two-dimensional (2-D) model of bulk-liquid hydrodynamics approaching plug flow. In such a biofilm reactor model, the user-defined biofilm area is specified for each CFSTR; thereby, Xcarrier does not exit the boundaries of the CFSTR to which they are assigned or exchange boundaries with other CFSTRs in the series. The error introduced by this pseudo 2-D biofilm reactor modeling approach may adversely affect model results and limit model-user capacity to accurately calibrate a model. This paper presents a new sub-model that describes the migration of Xcarrier and associated biofilms, and evaluates the impact that Xcarrier migration and axial dispersion has on simulated system performance. Relevance of the new biofilm reactor model to engineering situations is discussed by applying it to known biofilm reactor types and operational conditions.

Full-scale assessment of the nutrient removal capabilities of membrane bioreactors.

Operating results from two full-scale membrane bioreactors (MBRs) practicing biological and chemical phosphorus and biological nitrogen removal to meet stringent effluent nutrient limits are analyzed. Full-scale results and special studies conducted at these facilities resulted in the development of guidelines for the design of MBRs to achieve stringent effluent nutrient concentrations--as low as 0.05 mg/L total phosphorus and 3 mg/L total nitrogen. These guidelines include the following: (1) direct the membrane recirculation flow to the aerobic zone, (2) provide intense mixing at the inlets of the anaerobic and anoxic zones, (3) maintain internal recirculation flowrates to maintain the desired mixed liquor suspended solids distribution, and (4) carefully control supplemental metal salt addition in proportion to the phosphorus remaining after biological removal is complete. Staging the various process zones and providing effective dissolved oxygen control also enhances nutrient removal performance. The results demonstrated that process performance can be characterized by the International Water Association (London, United Kingdom) (IWA) activated sludge model number 2d (ASM2d) and the Water Environment Federation (Alexandria, Virginia) chemical phosphorus removal model. These models subsequently were used to develop unique process configurations that are currently under design and/or construction for several full-scale nutrient removal MBRs.

Applications of Mobile Carrier Biofilm Modelling for Wastewater Treatment Processes

One-dimensional (1-D) biofilm models have been demonstrated reliable for specific types of biofilm reactor design. Limitations using mechanistic biofilm models for engineering design do not rely on improved biofilm models, but rely on improved biofilm reactor models. This is important when considering that biofilm reactors containing submerged, free-moving biofilm carriers are the most widely applied biofilm system(s) for municipal wastewater treatment. This paper presents a new biofilm reactor model that considers the impact of submerged free-moving biofilm carrier (Xcarrier) movement on system performance. The model accounts for a hydrodynamic condition characterized as plug flow with back mixing (to model axial dispersion). The relevance of this new biofilm reactor model to engineering situations is evaluated by applying it to relevant scenarios and comparing model results.