M. C. Wentzel

Activated Sludge Model No.2d, ASM2D

The Activated Sludge Model No. 2d (ASM2d) presents a model for biological phosphorus removal with simultaneous nitrification-denitrification in activated sludge systems. ASM2d is based on ASM2 and is expanded to include the denitrifying activity of the phosphorus accumulating organisms (PAOs). This extension of ASM2 allows for improved modeling of the processes, especially with respect to the dynamics of nitrate and phosphate.

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.

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.

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.

Modelling biological nutrient removal activated sludge systems: a review

The external nitrification (EN) biological nutrient removal (BNR) activated sludge (ENBNRAS) system shows considerable promise for full-scale implementation. As an aid for this implementation, a mathematical simulation model would be an invaluable tool. To develop such a model, a study was conducted to select the most suitable simulation model to serve as a starting point for further development. For this, the existing available simulation models for BNRAS systems are compared with one another and evaluated against experimental observations in the literature and on ENBNRAS systems. One process immediately apparent to be crucially important is the anoxic growth of phosphorus accumulating organisms (PAOs), with associated PAO denitrification and anoxic P uptake for polyP formation. These linked processes are lacking in the earlier kinetic simulation models for BNRAS systems, which were based on aerobic PAO growth and P uptake only, but have been incorporated into the more recent kinetic models. This provides a substantive body of information on modelling this aspect. Other processes of significance identified to require consideration are anaerobic slowly biodegradable COD (SBCOD) hydrolysis to readily biodegradable COD (RBCOD), and COD loss. Both processes have significant impact on the predicted BEPR performance. Due to the uncertainties associated with the mechanisms and quantification of these two processes, it is concluded that the most extensively validated kinetic simulation model should be selected for development, and that the omissions in this model should be addressed progressively, using the relevant information drawn from the existing models, the literature and observations on ENBNRAS systems.

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems

Abstract Laboratory research has indicated that a possible cause of low F/M filament bulking in ND (nitrification-denitrification) and NDBEPR (nitrification-denitrification biological excess phosphorus removal) systems occurs as a result of competition for substrate between filamentous and floc-forming organisms which have different denitrification pathways. In ND and NDBEPR systems alternating anoxic-aerobic conditions prevail and continuous utilization of particulate slowly biodegradable COD (SBCOD) by floc-forming organisms in these systems leads to accumulation of the denitrification intermediates nitrite (NO2−) and nitric oxide (NO) under anoxic conditions. It is proposed that a cause for low F/M filament bulking is that the intermediate NO inhibits the utilization of SBCOD by floc-formers under subsequent aerobic conditions, with high concentrations of NO2− exacerbating this effect, thereby allowing filamentous organisms, which do not accumulate NO, to dominate. Some experimental evidence to support this proposal is presented.

Active biomass in activated sludge mixed liquor

The engineering and technology of the activated sludge system are reasonably well established, with systems implemented worldwide for the biological removal of C, N and/or P. Parallel to this development, significant advances have been made in the microbiological and biochemical areas of activated sludge. These advances have been driven by the development of new analytical techniques that allow microorganisms to be studied in situ in the activated sludge environment. However, there has been little cross-linking and overlap between the engineering and technology and microbiology and biochemistry paradigms. In particular, the information from the microbiology and biochemistry has not been integrated into the engineering and technology paradigm, to enable improved system design and optimization. One area that can form a starting point to build bridges between the two paradigm sets, is measurement of active biomass. The current design and simulation models invariably include active biomass for each organism group as fundamental parameters which define quantitatively the kinetic rates of the relevant biological processes. However, these parameters remain purely hypothetical because to date they have not been quantitatively measured; their acceptance is based on the consistency of model predicted results over a wide range of application. This paper describes developments in quantitative measurement of the heterotrophic and autotrophic active biomass concentrations within the engineering and technology paradigm, and the formulation of a multinational project which will attempt to link these measurements and the defined engineering environment to the new microbiological and biochemical analytical techniques. It is hoped that this project will facilitate integration of the two paradigms sets.

Incorporation of inorganic material in anoxic/aerobic-activated sludge system mixed liquor

In the bioreactor of the nitrification denitrification (ND)-activated sludge system, the mixed liquor is made up of organic and inorganic materials. In the current design procedures and simulation models, the influent wastewater characteristics and biological processes that influence the bioreactor mixed liquor organic solids (as volatile suspended solids, VSS, or COD) are explicitly included. However, the mixed liquor total suspended solids (TSS, i.e. organic + inorganic solids) are calculated simply from empirical ratios of VSS/TSS. The TSS concentration is fundamental in the design of secondary settling tanks and waste activated sludge disposal. Clearly, the empirical approach to obtaining an estimate for TSS is not satisfactory within the framework of a fundamentally based model. Accordingly, the incorporation of the inorganic material present in the influent wastewater into ND-activated sludge system mixed liquor was investigated. From an experimental investigation into the distribution of inorganics in the influent, mixed liquor and effluent of a laboratory-scale ND-activated sludge system, it was concluded inter alia that (i) of the total inorganic solids in the influent, only a small fraction (2.8-7.5%) is incorporated into the mixed liquor, (ii) most of the inorganics in the influent (mean 88%) and effluent (mean 98.5%) are in the dissolved form, the balance being particulate, and (iii) the influent and effluent inorganic dissolved solids concentrations are closely equal (mean effluent to influent ratio 100%). Further, a number of models were developed to quantify the mixed liquor inorganic, and, hence, total solids. From an evaluation of these models against the experimental data, it would appear that the best approach to model the incorporation of inorganics into the activated sludge mixed liquor is to follow the concepts and principles used to develop the existing models for organic materials. With this approach, reasonably close correlation between predicted and measured data for mixed liquor and effluent inorganic concentrations were obtained.

The effect of residual ammonia concentration under aerobic conditions on the growth of Microthrix parvicella in biological nutrient removal plants

It is demonstrated with two parallel single reactor intermittently aerated nitrification denitrification systems fed municipal wastewater as influent, that Microthrix parvicella bulking can be stimulated and cured by manipulating the ammonia concentration in the aerobic period (by inhibiting the nitrifiers) to high and low values respectively. The proliferation or not of M. parvicella is hypothesized to be due to their requirement for ammonia as a nitrogen source for growth. In terms of this hypothesis, if nitrification is rapid and complete, ammonia is not freely available and will limit M. parvicella growth. If nitrification is not complete for whatever reason, ammonia is available for the growth of the slow growing M. parvicella, enabling their proliferation to cause a bulking sludge. This hypothesis does not overturn or replace the anoxic-aerobic (AA, or low Food/Microorganism, F/M, ratio) filament bulking hypothesis of Casey et al. (Water SA 25(4) (1999) 425) but appears to be additional to it. Future research will focus on determining how elements of both hypotheses superimpose on the conditions in BNR systems, to produce an AA filament bulking sludge or not.