Primary sedimentation modelling with characterized setting velocity groups.

Within a plantwide water and resource recovery facility context, an important requirement for a primary sedimentation unit model is the correct fractionation of the settleable portion (primary sludge - PS) of the raw wastewater total suspended solids (TSS) according to the (i) unbiodegradable particulate organic (UPO), (ii) biodegradable particulate organic (BPO), and (iii) inorganic settleable solid (ISS) components. This paper focuses on improving a current TSS- based primary settling tank (PST) model to account for correct proportions of these three components, with characterized settling velocity groups. The steps taken towards development of the primary sedimentation unit model involved the development of a discrete particle settling model in Microsoft Excel and the utilisation of well characterised municipal wastewater data from previous studies in the discrete particle settling model, to reproduce PS and settled wastewater outputs in settling fractions of UPO, BPO and ISS, via steady state and dynamic calculations and under strict material mass balances. Finally, the insights obtained from discrete particle settling model calculations were implemented in the development of a dynamic University of Cape Town primary sedimentation unit (UCTPSU) model. This dynamic model was rigorously verified to be internally consistent with regards to material mass balances and utilised to simulate plantwide scenarios, under steady state conditions, whereby the impact of incorrect characterisation of TSS components (UPO, BPO and ISS) fractions was evaluated. From these evaluations, it was noted that the incorrect disaggregation of the TSS components of primary sludge can lead to incorrect predictions with regard to parameters such as the settled wastewater composition and the activated sludge system capacity. Thus, the investigation revealed the need to measure key wastewater parameters such as particle settling velocities and the UPO fraction, towards realistically modelling the primary sedimentation unit operations.

Kinetics of biological and chemical processes in anoxic-aerobic digestion of phosphorus rich waste activated sludge.

Because the functions of these water and resource recovery facilities (WRRFs) stretches beyond simply meeting effluent requirements (i.e., also includes optimisation of products to be generated as recovered resources), a high level of accuracy is required in using mathematical models that virtually replicate (hence predict) WRRF system responses to dynamic conditions. The currently developed mathematical models embrace the majority of advances made towards tracking nitrogen (N) and phosphorus (P) through the entire WRRF, and significant effort has been made towards calibrating them to predict realistic outcomes. This paper presents the stepwise calibration of the PWMSA model (Ikumi et al., 2015) for aerobic (AerD) and anoxic-aerobic digestion (AAD) processes, through predictions of (i) mineral precipitation potential, in isolation to biological reactions (ii) AerD bioprocesses (including nitrification, orthophosphate (OP) release, and endogenous respiration), in isolation to mineral precipitation (iii) predicted interaction of the mineral precipitation and the biological processes of organic removal and nitrification, excluding P accumulating organisms (PAOs) and polyphosphate (PP) release during AerD, (iv) replicated interaction of mineral precipitation and bioprocesses of P release and nitrification kinetics (v) predicted PAO behavioural kinetics of anaerobic OP release with acetate uptake and aerobic PP uptake, in isolation to nitrification and (vi) predicted nitrate denitrification and anoxic OP release. The calibrated kinetic parameters allowed for the model capability of reproducing the data from the key biological, physical and chemical processes occurring in the various environments of sludge treatment (aerobic, anoxic and anaerobic) within satisfactory level of accuracy.

Plant-wide modelling of phosphorus transformations in wastewater treatment systems: Impacts of control and operational strategies.

The objective of this paper is to report the effects that control/operational strategies may have on plant-wide phosphorus (P) transformations in wastewater treatment plants (WWTP). The development of a new set of biological (activated sludge, anaerobic digestion), physico-chemical (aqueous phase, precipitation, mass transfer) process models and model interfaces (between water and sludge line) were required to describe the required tri-phasic (gas, liquid, solid) compound transformations and the close interlinks between the P and the sulfur (S) and iron (Fe) cycles. A modified version of the Benchmark Simulation Model No. 2 (BSM2) (open loop) is used as test platform upon which three different operational alternatives (A1, A2, A3) are evaluated. Rigorous sensor and actuator models are also included in order to reproduce realistic control actions. Model-based analysis shows that the combination of an ammonium ( [Formula: see text] ) and total suspended solids (XTSS) control strategy (A1) better adapts the system to influent dynamics, improves phosphate [Formula: see text] accumulation by phosphorus accumulating organisms (XPAO) (41%), increases nitrification/denitrification efficiency (18%) and reduces aeration energy (Eaeration) (21%). The addition of iron ( [Formula: see text] ) for chemical P removal (A2) promotes the formation of ferric oxides (XHFO-H, XHFO-L), phosphate adsorption (XHFO-H,P, XHFO-L,P), co-precipitation (XHFO-H,P,old, XHFO-L,P,old) and consequently reduces the P levels in the effluent (from 2.8 to 0.9 g P.m-3). This also has an impact on the sludge line, with hydrogen sulfide production ( [Formula: see text] ) reduced (36%) due to iron sulfide (XFeS) precipitation. As a consequence, there is also a slightly higher energy production (Eproduction) from biogas. Lastly, the inclusion of a stripping and crystallization unit (A3) for P recovery reduces the quantity of P in the anaerobic digester supernatant returning to the water line and allows potential struvite ( [Formula: see text] ) recovery ranging from 69 to 227 kg.day-1 depending on: (1) airflow (Qstripping); and, (2) magnesium ( [Formula: see text] ) addition. All the proposed alternatives are evaluated from an environmental and economical point of view using appropriate performance indices. Finally, some deficiencies and opportunities of the proposed approach when performing (plant-wide) wastewater treatment modelling/engineering projects are discussed.

A new general methodology for incorporating physico-chemical transformations into multi-phase wastewater treatment process models.

This paper introduces a new general methodology for incorporating physico-chemical and chemical transformations into multi-phase wastewater treatment process models in a systematic and rigorous way under a Plant-Wide modelling (PWM) framework. The methodology presented in this paper requires the selection of the relevant biochemical, chemical and physico-chemical transformations taking place and the definition of the mass transport for the co-existing phases. As an example a mathematical model has been constructed to describe a system for biological COD, nitrogen and phosphorus removal, liquid-gas transfer, precipitation processes, and chemical reactions. The capability of the model has been tested by comparing simulated and experimental results for a nutrient removal system with sludge digestion. Finally, a scenario analysis has been undertaken to show the potential of the obtained mathematical model to study phosphorus recovery.

Biodegradability of wastewater and activated sludge organics in anaerobic digestion.

The investigation provides experimental evidence that the unbiodegradable particulate organics fractions of primary sludge and waste activated sludge calculated from activated sludge models remain essentially unbiodegradable in anaerobic digestion. This was tested by feeding the waste activated sludge (WAS) from three different laboratory activated sludge (AS) systems to three separate anaerobic digesters (AD). Two of the AS systems were Modified Ludzack - Ettinger (MLE) nitrification-denitrification (ND) systems and the third was a membrane University of Cape Town (UCT) ND and enhanced biological P removal system. One of the MLE systems and the UCT system were fed the same real settled wastewater. The other MLE system was fed raw wastewater which was made by adding a measured constant flux (gCOD/d) of macerated primary sludge (PS) to the real settled wastewater. This PS was also fed to a fourth AD and a blend of PS and WAS from settled wastewater MLE system was fed to a fifth AD. The five ADs were each operated at five different sludge ages (10-60d). From the measured performance results of the AS systems, the unbiodegradable particulate organic (UPO) COD fractions of the raw and settled wastewaters, the PS and the WAS from the three AS systems were calculated with AS models. These AS model based UPO fractions of the PS and WAS were compared with the UPO fractions calculated from the performance results of the ADs fed these sludges. For the PS, the UPO fraction calculated from the AS and AD models matched closely, i.e. 0.30 and 0.31. Provided the UPO of heterotrophic (OHO, fE_OHO) and phosphorus accumulating (PAO, fE_PAO) biomass were accepted to be those associated with the death regeneration model of organism "decay", the UPO of the WAS calculated from the AS and AD models also matched well - if the steady state AS model fE_OHO = 0.20 and fE_PAO = 0.25 values were used, then the UPO fraction of the WAS calculated from the AS models deviated significantly from those calculated with the AD models. Therefore in plant wide wastewater treatment models the characterization of PS and WAS as defined by the AS models can be applied without modification in AD models. The observed rate limiting hydrolysis/acidogenesis rates of the sludges are listed.