G. A. Ekama

Biological wastewater treatment: Principles, modelling and design

For information on the online course in Biological Wastewater Treatment from UNESCO-IHE, visit: http://www.iwapublishing.co.uk/books/biological-wastewater-treatment-online-course-principles-modeling-and-design Over the past twenty years, the knowledge and understanding of wastewater treatment have advanced extensively and moved away from empirically-based approaches to a first principles approach embracing chemistry, microbiology, physical and bioprocess engineering, and mathematics. Many of these advances have matured to the degree that they have been codified into mathematical models for simulation with computers. For a new generation of young scientists and engineers entering the wastewater treatment profession, the quantity, complexity and diversity of these new developments can be overwhelming, particularly in developing countries where access is not readily available to advanced level tertiary education courses in wastewater treatment. Biological Wastewater Treatment addresses this deficiency. It assembles and integrates the postgraduate course material of a dozen or so professors from research groups around the world that have made significant contributions to the advances in wastewater treatment. The book forms part of an internet-based curriculum in biological wastewater treatment which also includes: * Summarized lecture handouts of the topics covered in book * Filmed lectures by the author professors * Tutorial exercises for students self-learning Upon completion of this curriculum the modern approach of modelling and simulation to wastewater treatment plant design and operation, be it activated sludge, biological nitrogen and phosphorus removal, secondary settling tanks or biofilm systems, can be embraced with deeper insight, advanced knowledge and greater confidence.

A GENERAL MODEL FOR THE ACTIVATED SLUDGE PROCESS

A comparison of experimental data with those predicted by the mathematical model of Ekama and Marais (1978) describing the dynamic behaviour of the completely mixed activated sludge (CMAS) process, indicated that certain hypotheses in this model needed replacement. The following principal changes were made: (1) The energy requirement for adsorption was replaced by a bi-substrate hypothesis. In this hypothesis municipal wastewaters are taken to consist of two substrates - a rapidly biodegradable soluble and a slowly biodegradable particulate substrate. The growth behaviour of the former is in accordance with the classical Monod (1950) formulation; while the growth behaviour of the latter is controlled by the rate of extracellular enzymatic breakdown of the large complex molecules to simpler ones before transference through the cell walls. (2) The extracellular growth-limiting reaction is modelled by an active site theory and replaces the modified Blackwell (1971) equation for synthesis of stored particulate substrate. (3) The classical endogenous respiration concept is replaced by a death-regeneration one. Incorporation of these concepts in the CMAS process equations give excellent description of the process behaviour (oxygen utilization rate, volatile solids production, COD, TKN, nitrate) under constant and cyclic flow and load conditions for single and multiple reactor configurations. The model allows extension to the single sludge nitrification-denitrification and the contact stabilization processes although these are not discussed in this paper.

Integrated chemical–physical processes modelling—II. simulating aeration treatment of anaerobic digester supernatants

Abstract A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model is developed to simulate the physical and chemical processes that occur on aeration of anaerobic digester supernatant. Included in the model are the kinetic reactions for (i) weak acid/base dissociations (water, carbonate, ammonium, phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite, amorphous calcium phosphate, calcium and magnesium carbonate and (iii) stripping of CO 2 and NH 3 gasses. A preliminary validation of the model is done using data available in the literature. The model was then applied to simulate batch aeration tests on two anaerobic digester supernatants from (i) a spent wine upflow anaerobic sludge bed digester and (ii) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO 4 -P, free and saline ammonia (FSA) and H 2 CO 3 ∗ Alkalinity (from which inorganic carbon is calculated) were measured. After establishing (i) the minerals most likely to precipitate viz. struvite (MgNH 4 PO 4 ), newberyite (MgHPO 4 ), amorphous calcium phosphate (ACP), CaCO 3 and MgCO 3 and trial and error determination of (ii) the solubility products, (iii) specific precipitation rate constants and (iv) the specific gas stripping rates, a good correlation was obtained for all the parameters for both supernatants. The solubility product values for the minerals that precipitated were the same in both supernatants, and fall in the range of values quoted in the literature but the specific precipitation rate constants of the minerals were different in the two supernatants. Compared to the equilibrium chemistry approach to modelling three phase mixed weak acid/base systems, the kinetic approach facilitates (i) integration of the weak acid/base model with biological kinetic models and (ii) determination of solubility products and precipitation rates in an integrated manner for a number of minerals simultaneously from a single batch test.

The activated sludge process—3 single sludge denitrification

Abstract The general aerobic bi-substrate active-site death-regeneration activated sludge model including nitrification of Dold et al. ( Prog. Wat. Technol. 12, 47–77, 1980) is extended to include the kinetic behaviour of the denitrification process in single sludge systems. The extension requires a change in the value of only one of the kinetic constants ( K ′ mp ) in the expression for the particulate substrate utilization rate when the environment becomes anoxic. The extended model simulates very closely the response of the multi-reactor nitrification-denitrification process configurations under both constant and cyclic flow and load conditions. Under constant flow and load conditions, the denitrification response predicted can be reduced to that approximated by a zero order reaction dN/d t = − KX a with two rates in the primary and one in the secondary anoxic reactor respectively.

Modelling multiple mineral precipitation in anaerobic digester liquor

Mineral precipitation problems have been experienced with the conveyance and treatment of anaerobically digested primary and waste activated sludge blends. This paper describes an experimental investigation into mineral precipitation in anaerobic digester liquor (ADL) from the Cape Flats (CF) Wastewater Treatment Plant (WWTP) (Cape Town, South Africa), and application of the three-phase (aqueous/solid/gas) physical and chemical processes kinetic model developed by Musvoto et al. (Water Res. 34 (2000) 1857; Water Res. 34 (2000) 1868; Water SA 26(4) (2000) 417) to the experimental data. From the experimental investigation and theoretical modelling, it is concluded inter alia that: (i) there is a close correlation between experimental measured and theoretically predicted data, (ii) the dominating mineral that precipitates is struvite, with small amounts of amorphous calcium phosphate and negligible newberyite, calcite and magnesite, (iii) the precipitation of struvite is governed by the increase in pH when CO2 is lost from the ADL, (iv) the ADL is initially undersaturated with respect to struvite, but becomes supersaturated at pH > 7.3-7.7, (v) the rate and mass of struvite precipitation are controlled by the rate of pH increase and the initial Mg concentration and (vi) the three-phase kinetic model is able to simulate accurately the time dependent precipitation data for multiple minerals competing for the same species and allows determination of specific precipitation rates for a number of minerals simultaneously in an integrated manner from a single batch test. Some operational strategies to minimise struvite precipitation are proposed.

Heterotroph anoxic yield in anoxic aerobic activated sludge systems treating municipal wastewater.

As input to the steady state design and kinetic simulation models for the activated sludge system, the correct value for the heterotroph anoxic yield is essential to provide reliable estimates for the system denitrification potential. This paper examines activated sludge anoxic yield values in the literature, and presents experimental data quantifying the value. In the literature, in terms of the structure of ASM1 and similar models, theoretically it has been shown that the anoxic yield should be reduced to approximately 0.79 the value of the aerobic yield. This theoretical value is validated with data from corresponding aerobic OUR and anoxic nitrate time profiles in a batch fed laboratory scale long sludge age activated sludge system treating municipal wastewater. The value also is in close agreement with values in the literature measured with both artificial substrates and municipal wastewater. Thus, it is concluded that, in ASM1 and similar models, for an aerobic yield of 0.67mg COD/mg COD, the anoxic yield should be about 0.53 mg COD/mg COD. Including such a lower anoxic yield in ASM1 and similar models will result in a significant increase in denitrification potential, due to increased denitrification with wastewater RBCOD as substrate. In terms of the structure of ASM3, for the proposed substrate storage yields and the aerobic yield of 0.63 mg COD/mg COD, experimental data indicate that the corresponding anoxic yield should be about 0.42 mg COD/mg COD. This is significantly lower than the proposed value of 0.54 mg COD/mg COD, and requires further investigation.

Greenhouse gases from wastewater treatment - A review of modelling tools

Nitrous oxide, carbon dioxide and methane are greenhouse gases (GHG) emitted from wastewater treatment that contribute to its carbon footprint. As a result of the increasing awareness of GHG emissions from wastewater treatment plants (WWTPs), new modelling, design, and operational tools have been developed to address and reduce GHG emissions at the plant-wide scale and beyond. This paper reviews the state-of-the-art and the recently developed tools used to understand and manage GHG emissions from WWTPs, and discusses open problems and research gaps. The literature review reveals that knowledge on the processes related to N2O formation, especially due to autotrophic biomass, is still incomplete. The literature review shows also that a plant-wide modelling approach that includes GHG is the best option for the understanding how to reduce the carbon footprint of WWTPs. Indeed, several studies have confirmed that a wide vision of the WWPTs has to be considered in order to make them more sustainable as possible. Mechanistic dynamic models were demonstrated as the most comprehensive and reliable tools for GHG assessment. Very few plant-wide GHG modelling studies have been applied to real WWTPs due to the huge difficulties related to data availability and the model complexity. For further improvement in GHG plant-wide modelling and to favour its use at large real scale, knowledge of the mechanisms involved in GHG formation and release, and data acquisition must be enhanced.

New framework for standardized notation in wastewater treatment modelling.

Many unit process models are available in the field of wastewater treatment. All of these models use their own notation, causing problems for documentation, implementation and connection of different models (using different sets of state variables). The main goal of this paper is to propose a new notational framework which allows unique and systematic naming of state variables and parameters of biokinetic models in the wastewater treatment field. The symbols are based on one main letter that gives a general description of the state variable or parameter and several subscript levels that provide greater specification. Only those levels that make the name unique within the model context are needed in creating the symbol. The paper describes specific problems encountered with the currently used notation, presents the proposed framework and provides additional practical examples. The overall result is a framework that can be used in whole plant modelling, which consists of different fields such as activated sludge, anaerobic digestion, sidestream treatment, membrane bioreactors, metabolic approaches, fate of micropollutants and biofilm processes. The main objective of this consensus building paper is to establish a consistent set of rules that can be applied to existing and most importantly, future models. Applying the proposed notation should make it easier for everyone active in the wastewater treatment field to read, write and review documents describing modelling projects.

Design and start-up of a high rate anaerobic membrane bioreactor for the treatment of a low pH, high strength, dissolved organic waste water.

A Submerged Membrane Anaerobic Reactor (SMAR) is being developed for the treatment of waste water originating in Sasols coal to fuel synthesis process. The laboratory-scale SMAR uses A4-size submerged flat panel ultrafiltration membranes to induce a 100% solids-liquid separation. Biogas gets extracted from the headspace above the anaerobic mixed liquor and reintroduced through a coarse bubble diffuser below the membranes. This induces a gas scour on the membranes that avoids biomass immobilization and membrane fouling. The substrate is a high strength (18 gCOD/l) petrochemical effluent consisting mostly of C2 to C6 short chain fatty acids with a low pH. Because of this, the pH of the reactor has to be controlled to a pH of 7.1. Organic Loading Rates of up to 25 kgCOD/m3 reactor volume/d has been observed with effluent COD normally <500 mgCOD/l and FSA <50 mgN/l with no particulates >0.45 microm at hydraulic retention times of 17 hours. 98% of the COD is converted to methane and the remainder to biomass. Mixed Liquor (MLSS) concentrations >30 gTSS/l can be maintained without deterioration of membrane fluxes, even though the Diluted Sludge Volume Index (DSVI) indicates that the sludge cannot be settled. No noteworthy deterioration in membrane performance has been observed over the 320 day operational period.

Using bioprocess stoichiometry to build a plant-wide mass balance based steady-state WWTP model.

Steady-state models are useful for design of wastewater treatment plants (WWTPs) because they allow reactor sizes and interconnecting flows to be simply determined from explicit equations in terms of unit operation performance criteria. Once the overall WWTP scheme is established and the main system defining parameters of the individual unit operations estimated, dynamic models can be applied to the connected unit operations to refine their design and evaluate their performance under dynamic flow and load conditions. To model anaerobic digestion (AD) within plant-wide WWTP models, not only COD and nitrogen (N) but also carbon (C) fluxes entering the AD need to be defined. Current plant-wide models, like benchmark simulation model No 2 (BSM2), impose a C flux at the AD influent. In this paper, the COD and N mass balance steady-state models of activated sludge (AS) organics degradation, nitrification and denitrification (ND) and anaerobic (AD) and aerobic (AerD) digestion of wastewater sludge are extended and linked with bioprocess transformation stoichiometry to form C, H, O, N, chemical oxygen demand (COD) and charge mass balance based models so that also C (and H and O) can be tracked through the whole WWTP. By assigning a stoichiometric composition (x, y, z and a in C(x)H(y)O(z)N(a)) to each of the five main influent wastewater organic fractions and ammonia, these, and the products generated from them via the biological processes, are tracked through the WWTP. The model is applied to two theoretical case study WWTPs treating the same raw wastewater (WW) to the same final sludge residual biodegradable COD. It is demonstrated that much useful information can be generated with the relatively simple steady-state models to aid WWTP layout design and track the different products exiting the WWTP via the solid, liquid and gas streams, such as aerobic versus anaerobic digestion of waste activated sludge, N loads in recycle streams, methane production for energy recovery and green house gas (CO(2), CH(4)) generation. To reduce trial and error usage of WWTP simulation software, it is recommended that they are extended to include pre-processors based on mass balance steady-state models to assist with WWTP layout design, unit operation selection, reactor sizing, option evaluation and comparison and wastewater characterization before dynamic simulation.