Management actions to mitigate the occurrence of pharmaceuticals in river networks in a global change context.

Human consumption of pharmaceuticals leads to high concentrations of pharmaceuticals in wastewater, which is usually not or insufficiently collected and treated before release into freshwater ecosystems. There, pharmaceuticals may pose a threat to aquatic biota. Unfortunately, occurrence data of pharmaceuticals in freshwaters at the global scale is scarce and unevenly distributed, thus preventing the identification of hotspots, the prediction of the impact of Global Change (particularly streamflow and population changes) on their occurrence, and the design of appropriate mitigation actions. Here, we use diclofenac (DCL) as a typical pharmaceutical contaminant, and a global model of DCL chemical fate based on wastewater sanitation, population density and hydrology to estimate current concentrations in the river network, the impact of future changes in runoff and population, and potential mitigation actions in line with the Sustainable Development Goals. Our model is calibrated against measurements available in the literature. We estimate that 2.74 ± 0.63% of global river network length has DCL concentrations exceeding the proposed EU Watch list limit (100 ng L-1). Furthermore, many rivers downstream from highly populated areas show values beyond 1000 ng L-1, particularly those associated to megacities in Asia lacking sufficient wastewater treatment. This situation will worsen with Global Change, as streamflow changes and human population growth will increase the proportion of the river network above 100 ng L-1 up to 3.10 ± 0.72%. Given this background, we assessed feasible source and end-of-pipe mitigation actions, including per capita consumption reduction through eco-directed sustainable prescribing (EDSP), the implementation of the United Nations Sustainable Development Goal (SDG) 6 of halving the proportion of population without access to safely managed sanitation services, and improvement of wastewater treatment plants up to the Swiss standards. Among the considered end-of-pipe mitigation actions, implementation of SDG 6 was the most effective, reducing the proportion of the river network above 100 ng L-1 down to 2.95 ± 0.68%. However, EDSP brought this proportion down to 2.80 ± 0.64%. Overall, our findings indicate that the sole implementation of technological improvements will be insufficient to prevent the expected increase in pharmaceuticals concentration, and that technological solution need to be combined with source mitigation actions.

Position paper - progress towards standards in integrated (aerobic) MBR modelling.

Membrane bioreactor (MBR) models are useful tools for both design and management. The system complexity is high due to the involved number of processes which can be clustered in biological and physical ones. Literature studies are present and need to be harmonized in order to gain insights from the different studies and allow system optimization by applying a control. This position paper aims at defining the current state of the art of the main integrated MBR models reported in the literature. On the basis of a modelling review, a standardized terminology is proposed to facilitate the further development and comparison of integrated membrane fouling models for aerobic MBRs.

Fate of organic microcontaminants in wastewater treatment and river systems: An uncertainty assessment in view of sampling strategy, and compound consumption rate and degradability.

The growing awareness of the relevance of organic microcontaminants on the environment has led to a growing number of studies on attenuation of these compounds in wastewater treatment plants (WWTP) and rivers. However, the effects of the sampling strategies (frequency and duration of composite samples) on the attenuation estimates are largely unknown. Our goal was to assess how frequency and duration of composite samples influence uncertainty of the attenuation estimates in WWTPs and rivers. Furthermore, we also assessed how compound consumption rate and degradability influence uncertainty. The assessment was conducted through simulating the integrated wastewater system of Puigcerdà (NE Iberian Peninsula) using a sewer pattern generator and a coupled model of WWTP and river. Results showed that the sampling strategy is especially critical at the influent of WWTP, particularly when the number of toilet flushes containing the compound of interest is small (≤100 toilet flushes with compound day-1), and less critical at the effluent of the WWTP and in the river due to the mixing effects of the WWTP. For example, at the WWTP, when evaluating a compound that is present in 50 pulses·d-1 using a sampling frequency of 15-min to collect a 24-h composite sample, the attenuation uncertainty can range from 94% (0% degradability) to 9% (90% degradability). The estimation of attenuation in rivers is less critical than in WWTPs, as the attenuation uncertainty was lower than 10% for all evaluated scenarios. Interestingly, the errors in the estimates of attenuation are usually lower than those of loads for most sampling strategies and compound characteristics (e.g. consumption and degradability), although the opposite occurs for compounds with low consumption and inappropriate sampling strategies at the WWTP. Hence, when designing a sampling campaign, one should consider the influence of compounds' consumption and degradability as well as the desired level of accuracy in attenuation estimations.

Generation of synthetic influent data to perform (micro)pollutant wastewater treatment modelling studies.

The use of process models to simulate the fate of micropollutants in wastewater treatment plants is constantly growing. However, due to the high workload and cost of measuring campaigns, many simulation studies lack sufficiently long time series representing realistic wastewater influent dynamics. In this paper, the feasibility of the Benchmark Simulation Model No. 2 (BSM2) influent generator is tested to create realistic dynamic influent (micro)pollutant disturbance scenarios. The presented set of models is adjusted to describe the occurrence of three pharmaceutical compounds and one of each of its metabolites with samples taken every 2-4h: the anti-inflammatory drug ibuprofen (IBU), the antibiotic sulfamethoxazole (SMX) and the psychoactive carbamazepine (CMZ). Information about type of excretion and total consumption rates forms the basis for creating the data-defined profiles used to generate the dynamic time series. In addition, the traditional influent characteristics such as flow rate, ammonium, particulate chemical oxygen demand and temperature are also modelled using the same framework with high frequency data. The calibration is performed semi-automatically with two different methods depending on data availability. The 'traditional' variables are calibrated with the Bootstrap method while the pharmaceutical loads are estimated with a least squares approach. The simulation results demonstrate that the BSM2 influent generator can describe the dynamics of both traditional variables and pharmaceuticals. Lastly, the study is complemented with: 1) the generation of longer time series for IBU following the same catchment principles; 2) the study of the impact of in-sewer SMX biotransformation when estimating the average daily load; and, 3) a critical discussion of the results, and the future opportunities of the presented approach balancing model structure/calibration procedure complexity versus predictive capabilities.

Attenuation of pharmaceuticals and their transformation products in a wastewater treatment plant and its receiving river ecosystem.

Pharmaceuticals are designed to improve human and animal health, but may also be a threat to freshwater ecosystems, particularly after receiving urban or wastewater treatment plant (WWTP) effluents. Knowledge on the fate and attenuation of pharmaceuticals in engineered and natural ecosystems is rather fragmented, and comparable methods are needed to facilitate the comprehension of those processes amongst systems. In this study the dynamics of 8 pharmaceuticals (acetaminophen, sulfapyridine, sulfamethoxazole, carbamazepine, venlafaxine, ibuprofen, diclofenac, diazepam) and 11 of their transformation products were investigated in a WWTP and the associated receiving river ecosystem. During 3 days, concentrations of these compounds were quantified at the influents, effluents, and wastage of the WWTP, and at different distances downstream the effluent at the river. Attenuation (net balance between removal and release from and to the water column) was estimated in both engineered and natural systems using a comparable model-based approach by considering different uncertainty sources (e.g. chemical analysis, sampling, and flow measurements). Results showed that pharmaceuticals load reduction was higher in the WWTP, but attenuation efficiencies (as half-life times) were higher in the river. In particular, the load of only 5 out of the 19 pharmaceuticals was reduced by more than 90% at the WWTP, while the rest were only partially or non-attenuated (or released) and discharged into the receiving river. At the river, only the load of ibuprofen was reduced by more than 50% (out of the 6 parent compounds present in the river), while partial and non-attenuation (or release) was observed for some of their transformation products. Linkages in the routing of some pharmaceuticals (venlafaxine, carbamazepine, ibuprofen and diclofenac) and their corresponding transformation products were also identified at both WWTP and river. Finally, the followed procedure showed that dynamic attenuation in the coupled WWTP-river system could be successfully predicted with simple first order attenuation kinetics for most modeled compounds.

Evaluation of emerging contaminants in a drinking water treatment plant using electrodialysis reversal technology.

Emerging contaminants (EC) have gained much attention with globally increasing consumption and detection in aquatic ecosystems during the last two decades from ng/L to lower ug/L. The aim of this study was to evaluate the occurrence and removal of pharmaceutically active compounds (PhACs), endocrine disrupting chemicals (EDCs) and related compounds in a Drinking Water Treatment Plant (DWTP) treating raw water from the Mediterranean Llobregat River. The DWTP combined conventional treatment steps with the world's largest electrodialysis reversal (EDR) facility. 49 different PhACs, EDCs and related compounds were found above their limit of quantification in the influent of the DWTP, summing up to a total concentration of ECs between 1600-4200 ng/L. As expected, oxidation using chlorine dioxide and granular activated carbon filters were the most efficient technologies for EC removal. However, despite the low concentration detected in the influent of the EDR process, it was also possible to demonstrate that this process partially removed ionized compounds, thereby constituting an additional barrier against EC pollution in the product. In the product of the EDR system, only 18 out of 49 compounds were quantifiable in at least one of the four experimental campaigns, showing in all cases removals higher than 65% and often beyond 90% for the overall DWTP process.

Towards integrated operation of membrane bioreactors: effects of aeration on biological and filtration performance.

Two experimental studies evaluated the effect of aerobic and membrane aeration changes on sludge properties, biological nutrient removal and filtration processes in a pilot plant membrane bioreactor. The optimal operating conditions were found at an aerobic dissolved oxygen set-point (DO) of 0.5 mg O2 L(-1) and a membrane specific aeration demand (SADm) of 1 m h(-1), where membrane aeration can be used for nitrification. Under these conditions, a total flow reduction of 42% was achieved (75% energy reduction) without compromising nutrient removal efficiencies, maintaining sludge characteristics and controlled filtration. Below these optimal operating conditions, the nutrient removal efficiency was reduced, increasing 20% for soluble microbial products, 14% for capillarity suction time and reducing a 15% for filterability. Below this DO set-point, fouling increased with a transmembrane pressure 75% higher. SADm below 1 m h(-1) doubled the values of transmembrane pressure, without recovery after achieving the initial conditions.

Characterization of metoprolol biodegradation and its transformation products generated in activated sludge batch experiments and in full scale WWTPs.

Metoprolol (MTP) is a compound of concern, considered as an emerging contaminant due to its high consumption, pseudopersistence and potential ecotoxicity. Activated sludge batch experiments were performed to evaluate the biological transformation of MTP and the formation of transformation products under different treatment conditions. Total MTP removal was obtained in aerobic conditions, and the formation of MTP known metabolites (metoprolol acid (MTPA), α-hydroxymetoprolol (α-HMTP) and O-desmethylmetoprolol (O-DMTP)) and unknown transformation products (TPs) was investigated. The three known metabolites and two new TPs generated along the experiments were identified by liquid chromatography coupled to high resolution mass spectrometry. For the two new TPs plausible structures were proposed based on the tentative identification. MTPA had the major ratio formation for the TPs identified along the experiments (up to 40% of initial MTP concentration after 96 h treatment) and its persistence through biological treatment was proven. Ecotoxicity studies using Vibrio fischeri bioluminescent bacteria in an acute toxicity test showed that MTP and its known TPs are not toxic with the exception of o-DMTP. Finally, MTP and its TPs were monitored in a full scale membrane bioreactor and in a full scale conventional urban wastewater treatment plant (WWTP) and the results were compared with those obtained in batch experiments. α-HMTP was detected for the first time in a WWTP influent whereas MTPA was detected in influent and effluent WWTP samples at much higher levels (up to 100 folds higher) than MTP itself remarking its high persistence.

Instrumentation, control and automation in wastewater--from London 1973 to Narbonne 2013.

Key developments of instrumentation, control and automation (ICA) applications in wastewater systems during the past 40 years are highlighted in this paper. From the first ICA conference in 1973 through to today there has been a tremendous increase in the understanding of the processes, instrumentation, computer systems and control theory. However, many developments have not been addressed here, such as sewer control, drinking water treatment and water distribution control. It is hoped that this review can stimulate new attempts to more effectively apply control and automation in water systems in the coming years.

Assessment of energy-saving strategies and operational costs in full-scale membrane bioreactors.

The energy-saving strategies and operational costs of stand-alone, hybrid, and dual stream full-scale membrane bioreactors (MBRs) with capacities ranging from 1100 to 35,000 m(3) day(-1) have been assessed for seven municipal facilities located in Northeast Spain. Although hydraulic load was found to be the main determinant factor for the energy consumption rates, several optimisation strategies have shown to be effective in terms of energy reduction as well as fouling phenomenon minimization or preservation. Specifically, modifications of the biological process (installation of control systems for biological aeration) and of the filtration process (reduction of the flux or mixed liquor suspended solids concentration and installation of control systems for membrane air scouring) were applied in two stand-alone MBRs. After implementing these strategies, the yearly specific energy demand (SED) in flat-sheet (FS) and hollow-fibre (HF) stand-alone MBRs was reduced from 1.12 to 0.71 and from 1.54 to 1.12 kW h(-1) m(-3), respectively, regardless of their similar yearly averaged hydraulic loads. The strategies applied in the hybrid MBR, namely, buffering the influent flow and optimisation of both biological aeration and membrane air-scouring, reduced the SED values by 14%. These results illustrate that it is possible to apply energy-saving strategies to significantly reduce MBR operational costs, highlighting the need to optimise MBR facilities to reconsider them as an energy-competitive option.

Pharmaceuticals occurrence in a WWTP with significant industrial contribution and its input into the river system.

Occurrence and removal of 81 representative Pharmaceutical Active Compounds (PhACs) were assessed in a municipal WWTP located in a highly industrialized area, with partial water reuse after UV tertiary treatment and discharge to a Mediterranean river. Water monitoring was performed in an integrated way at different points in the WWTP and river along three seasons. Consistent differences between therapeutic classes were observed in terms of influent concentration, removal efficiencies and seasonal variation. Conventional (primary and secondary) treatment was unable to completely remove numerous compounds and UV-based tertiary treatment played a complementary role for some of them. Industrial activity influence was highlighted in terms of PhACs presence and seasonal distribution. Even if global WWTP effluent impact on the studied river appeared to be minor, PhACs resulted widespread pollutants in river waters. Contamination can be particularly critical in summer in water scarcity areas, when water flow decreases considerably.

Benchmark simulation models, quo vadis?

As the work of the IWA Task Group on Benchmarking of Control Strategies for wastewater treatment plants (WWTPs) is coming to an end, it is essential to disseminate the knowledge gained. For this reason, all authors of the IWA Scientific and Technical Report on benchmarking have come together to provide their insights, highlighting areas where knowledge may still be deficient and where new opportunities are emerging, and to propose potential avenues for future development and application of the general benchmarking framework and its associated tools. The paper focuses on the topics of temporal and spatial extension, process modifications within the WWTP, the realism of models, control strategy extensions and the potential for new evaluation tools within the existing benchmark system. We find that there are major opportunities for application within all of these areas, either from existing work already being done within the context of the benchmarking simulation models (BSMs) or applicable work in the wider literature. Of key importance is increasing capability, usability and transparency of the BSM package while avoiding unnecessary complexity.

Effects on activated sludge bacterial community exposed to sulfamethoxazole.

The bacterial community shift on a lab scale Sequencing Batch Reactor (SBR) fed with synthetic wastewater and exposed to 50µgL(-1) of sulfamethoxazole (SFX) for 2months was investigated in this study. The impact on biological nutrient removal performance and SFX removal efficiencies were also studied. Satisfactory biological nutrient removal was observed as regards to COD and Nitrogen. SFX removal efficiencies ranged between 20% and 50% throughout the experimental period, enhanced within the aerobic phases of the SBR cycle, with no evident signs of biomass acclimation. Nevertheless, denaturing gradient gel electrophoresis (DGGE) analysis showed significant variance leading to not only the fading, but also the emergence of new species in the bioreactor bacterial community after SFX dosage. According to the phylogenetic analysis, bacteria belonging to Betaproteobacteria and Gammaproteobacteria classes were the dominant species, among them, the Thiotrix spp. (Gammaproteobacteria) cell number increased due to its tolerance to the antibiotic. On the other hand, the classes Sphingobacteria, Actinobacteria, Chloroflexi and Chlorobi were found to be more vulnerable to the antibiotic load and disappeared. The sulphonamide resistance gene sulI was also quantified and discussed, as there are very few studies on bacterial resistance in lab-scale treatment reactors.

Ragging phenomenon characterisation and impact in a full-scale MBR.

Although there are few studies about clogging phenomenon in the peer-reviewed literature, it is considered one of the main operational challenges by membrane bioreactor (MBR) practitioners. This study presents data from the performance of a full-scale MBR affected by clogging, and ragging in particular. An evaluation of the efficiencies of different applied cleaning methods revealed the acid recovery cleaning to be more efficient than the basic recovery cleanings, although all maintenance cleanings were largely ineffective in recovering membrane permeability. Only declogging cleaning through the manual removal of the accumulated solids was found to be efficient, indicating that such solids were substantially unremoved by chemical cleaning. Moreover, reclogging following manual cleaning demonstrated a propensity for rapid clogging - within a period of 10 days over which the permeability returned to 68 and 88% of the pre-cleaned state. The analysis of the feedwater indicated suspended textile fibres (>70% cotton) to be present at a concentration of more than 40 mg·L(-1), ∼90% being smaller than 1 mm (0.06-0.4 mm). These small lengths of filaments evidently pass through pre-treatment and are retained on the membrane surface, forming 'rags' within the membrane module, notwithstanding the routine high quality of sludge reflected in the capillary suction time and filterability measurements. Pre-treatment improvement, manual cleaning and permeate flux reduction are the only options to minimise ragging impact over MBR performance.

The application of microfiltration-reverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse.

The fate of organic micropollutans (MPs) in a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been investigated for the case of wastewater reuse. Both an operating full-scale water reuse plant and a pilot plant were employed, with 22 individual organic compounds at their ambient concentrations studied for the former and the latter employing two target compounds over a range of feed concentrations. Results revealed removal efficiencies higher than 75% for most compounds in the full-scale plant, though mass flow studies on all streams revealed a significant imbalance of material for some compounds. Rejection efficiencies measured for candidate commercial NF and RO membranes tested at pilot scale challenged with a pharmaceutically active compound (ibuprofen, IBU) and an endocrine-disrupting chemical (nonylphenol, NP) exceeded 99%. Permeate concentrations were 0.005-0.14 microg/L for IBU and below the limit of detection for NP. A mass balance of the MPs for the full-scale plant across the MF and RO stages revealed a significant imbalance associated with the challenge of accurate determination of low concentrations. Differences in pilot plant and full-scale data were otherwise attributed to the impact of membrane ageing (and specifically hydrolysis) on RO rejection of the MPs examined.

Removal of ibuprofen and its transformation products: experimental and simulation studies.

Pharmaceutically active compounds (PhACs) deserve attention because of their effect on ecosystems and human health, as well as their continuous introduction into the aquatic environment. Classification schemes are suggested to characterise their biological degradation, e.g., based on pseudo-first-order kinetics, but these schemes can vary significantly, presumably due to pharmaceutical loads, sludge characteristics and experimental conditions. Degradation data for PhAC transformation products (TPs) are largely lacking. The present work focuses not only on the biodegradation of the pharmaceutical compound ibuprofen but also on its best-known TPs (i.e., carboxyl ibuprofen and both hydroxyl ibuprofen isomers). Ibuprofen is one of the most commonly consumed PhACs and can be found in different environmental compartments. The experiment performed consisted of a set of aerated batch tests with different suspended solid and ibuprofen concentrations to determine the influence of these parameters on the calculated biodegradation constant (K(biol)). Sampling of the liquid phase at different scheduled times was assessed, removal efficiencies were calculated and pseudo-first-order kinetics were adjusted to obtain experimental K(biol) values for the parent compound and its TPs. The experimental data were successfully fitted to ASM-based models, with K(biol) values for the target compounds ranging from almost 1 to 17 L gSST(-1) d(-1), depending on the concentrations of the biomass and ibuprofen. This work provides innovative knowledge not only regarding the removal of TPs but also the formation kinetics of these TPs.

A knowledge management methodology for the integrated assessment of WWTP configurations during conceptual design.

The current complexity involved in wastewater management projects is arising as the XXI century sets new challenges leading towards a more integrated plant design. In this context, the growing number of innovative technologies, stricter legislation and the development of new methodological approaches make it difficult to design appropriate flow schemes for new wastewater projects. Thus, new tools are needed for the wastewater treatment plant (WWTP) conceptual design using integrated assessment methods in order to include different types of objectives at the same time i.e. environmental, economical, technical, and legal. Previous experiences used the decision support system (DSS) methodology to handle the specific issues related to wastewater management, for example, the design of treatment facilities for small communities. However, tools developed for addressing the whole treatment process independently of the plant size, capable of integrating knowledge from many different areas, including both conventional and innovative technologies are not available. Therefore, the aim of this paper is to present and describe an innovative knowledge-based methodology that handles the conceptual design of WWTP process flow-diagrams (PFDs), satisfying a vast number of different criteria. This global approach is based on a hierarchy of decisions that uses the information contained in knowledge bases (KBs) with the aim of automating the generation of suitable WWTP configurations for a specific scenario. Expert interviews, legislation, specialized literature and engineering experience have been integrated within the different KBs, which indeed constitute one of the main highlights of this work. Therefore, the methodology is presented as a valuable tool which provides customized PFD for each specific case, taking into account process unit interactions and the user specified requirements and objectives.

A knowledge-based control system for air-scour optimisation in membrane bioreactors.

Although membrane bioreactors (MBRs) technology is still a growing sector, its progressive implementation all over the world, together with great technical achievements, has allowed it to reach a mature degree, just comparable to other more conventional wastewater treatment technologies. With current energy requirements around 0.6-1.1 kWh/m3 of treated wastewater and investment costs similar to conventional treatment plants, main market niche for MBRs can be areas with very high restrictive discharge limits, where treatment plants have to be compact or where water reuse is necessary. Operational costs are higher than for conventional treatments; consequently there is still a need and possibilities for energy saving and optimisation. This paper presents the development of a knowledge-based decision support system (DSS) for the integrated operation and remote control of the biological and physical (filtration and backwashing or relaxation) processes in MBRs. The core of the DSS is a knowledge-based control module for air-scour consumption automation and energy consumption minimisation.

Knowledge-based system for automatic MBR control.

MBR technology is currently challenging traditional wastewater treatment systems and is increasingly selected for WWTP upgrading. MBR systems typically are constructed on a smaller footprint, and provide superior treated water quality. However, the main drawback of MBR technology is that the permeability of membranes declines during filtration due to membrane fouling, which for a large part causes the high aeration requirements of an MBR to counteract this fouling phenomenon. Due to the complex and still unknown mechanisms of membrane fouling it is neither possible to describe clearly its development by means of a deterministic model, nor to control it with a purely mathematical law. Consequently the majority of MBR applications are controlled in an "open-loop" way i.e. with predefined and fixed air scour and filtration/relaxation or backwashing cycles, and scheduled inline or offline chemical cleaning as a preventive measure, without taking into account the real needs of membrane cleaning based on its filtration performance. However, existing theoretical and empirical knowledge about potential cause-effect relations between a number of factors (influent characteristics, biomass characteristics and operational conditions) and MBR operation can be used to build a knowledge-based decision support system (KB-DSS) for the automatic control of MBRs. This KB-DSS contains a knowledge-based control module, which, based on real time comparison of the current permeability trend with "reference trends", aims at optimizing the operation and energy costs and decreasing fouling rates. In practice the automatic control system proposed regulates the set points of the key operational variables controlled in MBR systems (permeate flux, relaxation and backwash times, backwash flows and times, aeration flow rates, chemical cleaning frequency, waste sludge flow rate and recycle flow rates) and identifies its optimal value. This paper describes the concepts and the 3-level architecture of the knowledge-based DSS and details the knowledge-based control module. Preliminary results of the application of the control module to regulate the air flow rate of an MBR working with variable flux demonstrates the usefulness of this approach.

On-line estimation of suspended solids in biological reactors of WWTPs using a Kalman observer.

The total amount of solids in Wastewater Treatment Plants (WWTPs) and their distribution among the different elements and lines play a crucial role in the stability, performance and operational costs of the process. However, an accurate prediction of the evolution of solids concentration in the different elements of a WWTP is not a straightforward task. This paper presents the design, development and validation of a generic Kalman observer for the on-line estimation of solids concentration in the tank reactors of WWTPs. The proposed observer is based on the fact that the information about the evolution of the total amount of solids in the plant can be supplied by the available on-line Suspended Solids (SS) analysers, while their distribution can be simultaneously estimated from the hydraulic pattern of the plant. The proposed observer has been applied to the on-line estimation of SS in the reactors of a pilot-scale Membrane Bio-Reactor (MBR). The results obtained have shown that the experimental information supplied by a sole on-line SS analyser located in the first reactor of the pilot plant, in combination with updated information about internal flow rates data, has been able to give a reasonable estimation of the evolution of the SS concentration in all the tanks.