Slow sand filters (SSFs) are essential for ensuring microbial quality and biological stability of drinking water in the Netherlands. However, gaps exist in understanding of removal processes of dissolved organic carbon (DOC) and ammonium and the effects of grain size, loading rate, and backwashing on removal in SSFs.
Four lab-scale SSF columns filled with fine (0.4-0.6 mm) and coarse (0.85-1.25 mm) sand were constructed and operated in two phases with a total of 165 days. In phase I, SSFs operated at a flow rate of 0.5 m/h to investigate the influence of grain size. After stabilization, higher loading rate of 2 m/h and backwashing procedure (20% expansion for 5 min) was applied during the phase II experiment. Various physicochemical and biological parameters, including DOC, ammnoium, phosphate, and ATP were analyzed in water along the filter depth. Additionally, biomass development on sand was quantified suing ATP measurement.
Results showed the stable SSF operation after 90-100 days, removing 100% of dosed 1.5 mg/L of DOC and 1.0 mg/L of ammonium. Compared with fine sand, coarse sand had similar removal performance but better backwashing effectiveness and lower clogging risk. Increased loading rate led to faster microbial growth, reducing operational lifespan, and poor removal performance. Backwashing showed minimal impact on DOC and ammonium removal capacity and microbial activity, which were recovered after backwashing within 7-14 days, indicating the potential for backwashing to prolong SSF’s operational lifespan.
This research investigated the DOC and ammnoium removal processes and the influence of grain size, loading rate, and backwashing on filter performance. Providing insights for optimized SSF design and operation. Future studies could delve into mechanisms using isotope analysis or metagenomics, along with more comprehensive sand sample analysis.

Sewage systems hold a critical function within the fabric of urban infrastructure, primarily by ensuring efficient wastewater management. Within this network, sewage pumping stations emerge as important components. The performance of such stations is susceptible to being undermined by system failures and malfunctions, which can cause significant implications for both public health and occurrences of Combined Sewer Overflow (CSO) events. Consequently, discerning the patterns and root causes of these failures is of great importance.
The present study explores and seeks to understand the reliability of sewage pumping stations, particularly in the setting of the Department of Public Works in Rotterdam. The scope of this study is confined to data collected between 2016 and 2020. The research encompasses a diverse range of interconnected aspects, such as examining discrepancies in failure data, categorizing types of failures, tracking changes in failure patterns over time, and selecting a fitting statistical model to accurately represent the findings. The core focus of this study is to formulate an analysis framework to assess the reliability and failure patterns in wastewater pumping stations. In these patterns, various trends can be discerned through interarrival time - the period between two failures. For instance, an increasing trend indicates less frequent failures, suggesting improved reliability of the pumps. The devised framework combines both objective and subjective trend analyses, multiple trend tests, and employs a range of models such as the Homogeneous Poisson Process (HPP), Renewal Process (RP), and Non-Homogeneous Poisson Process (NHPP). This methodological approach is structured to facilitate a better understanding of the patterns of system behavior and shifts. Analyzing pump performance patterns over five years revealed various trends among 447 pumps studied. Of these, 254 had consistent failure patterns; 98 showed a stable trend, 146 improved over time with fewer failures, and 10 experienced more failures, indicating declining performance. The remainingpumps were divided into various trend groups or segments, demonstrating different patterns in their performance over the five years. The application of statistical methods elucidated these failure patterns, contributing to the evaluation of sewage pump station efficiency. Notwithstanding these progressions, my investigation has identified certain domains that could derive advantages from additional enhancements. The current methodology, which entailed manual configuration of kernel parameters and dependence on piecewise linear regression, was deemed insufficient for about 50% of the pumps. The study proposes the integration of sophisticated parameter estimation techniques, including Bayesian optimization or grid search, and alternative modeling methodologies to tackle this issue. It is recommended that forthcoming research expands its scope beyond Rotterdam and investigates a wider variety of pump mechanisms to validate the generalizability of the findings presented in this study. In conclusion, this thesis develops an analytical framework based on examination of failure patterns and system dynamics. It establishes a platform for future progress by outlining a distinct pathway for prospective investigations pertaining to the failures in sewage pumping stations.

In this research, the use of spatially applied adaptive pathways for rainwater resilient redesign of urban areas is investigated. It is explored how adaptive pathways can contribute to the integration of the climate stress of pluvial flooding in the planning process for the redesign of existing urban areas. This is done by developing adaptive pathways for the rainwater resilient design for a case-study in Amsterdam and evaluating their possible contribution to the planning process for redesign in general and specifically in Amsterdam.

The use of spatially applied adaptive pathways for rainwater resilience was explored by first developing pathways for a redesign for a case study in Amsterdam, the neighbourhood Bosleeuw, following the Dynamic Adaptive Policy Pathways (DAPP) approach. It was found that when considering pluvial flooding as the system stressor, the number of parallel design alternatives that could be included in the map are limited because rainwater resilience is considered at neighbourhood scale. Moreover, the shortage of the rainwater storage capacity in the current situation compared to the set objective for 2050 results in the inability to express the adaptation tipping points in terms of storage capacity and a sell-by date in terms of time. In addition, because of the number and size of the required measures to reach the objective for rainwater resilience in Amsterdam, the need to consider the implementation time in the adaptive pathways arose, to ensure the feasibility of the proposed pathways.

To this end, a set of modifications were made to the set-up of the pathways for a single design alternative, that enhanced the functionality of the pathways to illustrate the feasibility of the implementation of a solution. The results showed that the modified pathway set-up enable the incorporation of the implementation time of measures, the definition of adaptation tipping points and their sell-by dates and reviewing the effect of partially implemented measures combined.
These functionalities are demonstrated by developing pathways for the implementation of the redesign for the case-study under the temporal limit of the renewal strategy in Amsterdam: the strategy to plan all projects in the public space simultaneously and at the moment of planned maintenance or renewal of infrastructure. The results show that the modified set-up of the pathways enable the visualization and comparison of different implementation strategies and allow for the back casting of required actions to attain a set objective, without going into time-consuming or incomprehensible detail. However, the beneficial feature of the DAPP approach to postpone decisions to the future by reviewing parallel solutions and keeping future options open, is eliminated in the modified set-up, that reviews the implementation of a single solution. Similarly, the focus on the duration of implementation time and the consideration of actions in a timeline reduced the feature of the DAPP approach to express moments of decision and action in terms of how far the uncertain development has progressed rather than in time, which is a key attribute of allowing uncertainty in decision-making and design. The modified pathway set-up presented in this research, therefore, should be considered as complementary to the DAPP approach rather than a replacement. Overall, it is concluded that the adaptive pathways can contribute to rainwater resilience planning by providing a guiding spatial and temporal overview of required actions to support decision-making and the monitoring of adequate implementation of spatial adaptations.

Globally, drinking water sources are polluted with poly- and perfluoroalkyl substances (PFAS). The toxicity and persistent properties of these industrial chemicals raised concerns about environmental and public health. As a result, drinking water companies are removing PFAS from drinking water using separation technologies. Anion exchange and nanofiltration membranes have been proven to be effective drinking water treatment methods for the removal of PFAS. However, these drinking water technologies produce large volumes of PFAS-containing waste streams, which poses new challenges for the drinking water industry. To prevent toxic PFAS from re-entering the environment, these waste streams must be treated. This can be done by PFAS destruction, however, due to the large volumes of the waste streams, this is very expensive and energy-intensive. Therefore, concentrating the drinking water waste streams before destruction is desired. This research examined existing PFAS-concentration technologies and compared their PFAS removal efficiency, volume reduction, and cost-effectiveness in concentrating the PFAS waste streams produced during drinking water treatment. These waste streams include the concentrate of nanofiltration and the brine from anion exchange. The analysed concentration technologies are foam fractionation, adsorption of PFAS onto DEXSORB+ and all-silica BEA zeolites, and nanofiltration.
Foam fractionation removes PFAS from the waste stream by injecting air bubbles. Two laboratory setups were made for the injection mechanism of the air bubbles. First, by passing pressurized air through an air stone, and second, by adding pressurized water (i.e. white water) to the waste stream. The adsorbents were tested in the laboratory by conducting equilibrium batch experiments with different adsorbent dosages. The laboratory experiments were performed on both drinking water waste streams. The performance of concentrating the anion exchange brine solution with nanofiltration membranes was evaluated with the use of IMS Design models.

In this study simultaneous and continuous inline dosing of PAC and coagulant in UF was investigated. Surface water was directly treated with PAC-UF and performance of the system was assessed by looking at membrane fouling and organic micropollutants removal. Combining adsorption and membrane processes in one technique, hybrid membrane processes, enhances performance of OMP removal by membrane processes. Powdered activated carbon (PAC) combined with coagulation and ultrafiltration (UF) is a possible treatment technique for surface water.
Permeability in UF-membranes remains steady, when dosing coagulant. Only a small increase in irreversible fouling of 0.2-0.7*109 m-1h-1 is visible. However, absence of coagulant dosing causes irreversible fouling to increase to 8.7*1010 m-1h-1 and can not be easily reversed with a chemically enhanced backwash (CEB). Inline dosing of PAC alone causes an irreversible fouling of 11.4*1010 m-1h-1 and highest increase in reversible fouling of 11.7*1010 m-1h-1. Addition of coagulant (1.2 mg/l) lowers reversible fouling compared to no dosing, on the other hand addition of PAC to coagulant shows no clear increase or decrease in reversible fouling. Coagulation has a negative effect on capillary blocking, an increase of 4-8% compared to no coagulant or adsorbent. PAC and no dosing did not influence pore blocking.
Highest removal of low to good adsorbable OMP was 10-63% for continuous and simultaneous dosing of 12 mg PAC/l and 1.2 mg FeCl3/l. Increasing filtration time (30 to 60 min) showed highest removal efficiency of 75% of Sotalol and 5-Methyl-1H-Benzotriazole, both good adsorbable OMP. Removal of PFAS varies between a few percent and 37%. Continuous dosing of coagulant (1.2 mg/l) with PAC (10 mg/l) has a negative impact on OMP adsorption including PFAS. With a higher PAC dose (15 mg/l) removal efficiency was not affected.
Addition of coagulant with PAC-UF showed to be effective to prevent irreversible fouling, with direct surface water treatment. However, coagulation is responsible for blocking of capillaries even resulting in a small decrease of permeability. Removal of OMP including PFAS was much lower compared to other PAC-UF systems and therefore PAC-UF with simultaneous and continuous inline dosing of PAC is not a good set-up. Adjustments in set-up should be made in order to make this configuration work. Possible improvements are increase of filtration time between backwash and dosing sequence of PAC and coagulant.

Currently, organic micropollutants (OMPs) are continuously and uncontrollably released into the water environment worldwide, as the reason for their special properties, OMPs removal has been a global challenge. This study focuses on acetaminophen degradation by photo-electrolysis (PEC) activities, which is one of the promising advanced oxidation processes (AOP) technologies. First, we report the fabrication methods of the BiVO4/BiOI heterojunction on FTO glass, then characterised the prepared photoanodes with XPS, XRD, SEM, EDS, UV-vis and IPCE. The results demonstrated the BiVO4/BiOI p-n heterojunction had been successfully electrodeposited on the FTO glass. Further, the LSV and EIS analysis in this study showed the BiVO4/BiOI photoanode had less photocurrent density than BiVO4 when carried out in the solution of acetaminophen. Even if the heterojunction did not improve the photocurrent, it significantly enhance the acetaminophen removal efficiency in the PEC degradation process. BiVO4/BiOI photoanode achieved 99% degradation efficiency in 3 hours and obtained 0.019 mi n−1 of the reaction rate constant. Overall, these results indicate that BiVO4/BiOI heterojunction has a great application potential for the degradation of OMPs in the wastewater treatment plants secondary effluent.

Small islands that support urban communities provide a unique opportunity to study the urban water cycle, its energy needs, and possible links to renewable energy. The aim of this paper is to explore to what extent an island’s urban water cycle and the renewable electricity production system required to satisfy the urban water cycle’s demand can become sustainable using Caye Chapel (Belize) as a study case. For this research, the water-energy system is the urban water cycle and the renewable electricity production for the urban water cycle. Twelve alternatives were proposed for the water-energy system. The different alternatives are divided among those that consider the reuse of wastewater, rainwater harvesting, and the use of wind turbines, PV panels, or both for the renewable electricity production. Then, those alternatives were optimized to produce the minimum water demand shortage, minimum amount of treated water that is not reused, and renewable electricity shortage. Later, the optimized alternatives are evaluated using multi-criteria decision analysis (MCDA). It was observed that the alternatives that only consider one renewable source for the electricity generation and do not consider the reuse of wastewater, are outperformed by the alternatives that consider more than one renewable source and reuse the treated wastewater.

Being one of the oldest water treatment systems in the world, slow sand filters (SSFs) have played a big role in the water treatment sector due to their reliability and their effective physico-chemical and biological purification capabilities. They are still widely used as a disinfection step in many countries, especially those that do not use chlorine for disinfection. SSFs are gaining popularity in recent times, especially due to their ability to deactivate pathogens like Giardia lamblia and Cryptosordidium parvum, which have been posing significant challenges to the public drinking water systems for years.
Over time, researchers have mainly focused on understanding the importance of the schmutzdecke layer of the SSFs, as it is understood that the schmutzdecke is essential for their functioning. This has limited the research done in the deeper layers of the sand bed, resulting in a lack of understanding of the activities here.
The aim of this study is to investigate the physico-chemical and biological functions happening in the deeper layers of the SSF, and to determine whether parameters like nitrate, turbidity, DOC, FEEM, ATP and particle cell counts can evaluate the ripening of a scraped bed. Furthermore, the ability of these parameters to sufficiently indicate biological stability of the sand bed is to be assessed.
In order to determine this, two full scale filters were sampled and analysed. One was a scraped bed and the other was a mature bed. They were also compared to judge how differently they behaved throughout the ripening phase.
The results of the tests showed that nitrate, turbidity and ATP are good indicators of ripening. ATP and particle cell counts, when used in parallel, are good assessors of biological stability. FEEM also serves as a useful method to determine why the DOC concentrations in the water samples were similar for both the filter beds. ATP tests conducted on the schmutzdecke sand samples provide evidence of ripening over time for the scraped bed. Further research using parameters like iron, manganese, ammonia and oxygen is required to strengthen the understanding of the workings of the deeper layers of the sand bed and the various physico-chemical and biological mechanisms happening within.

Globally, water demand is rising and resources are diminishing. In the context of climate change and a growing world population, a further increase in water scarcity seems inevitable. Aiming towards a sustainable future, water should be used as efficiently as possible by minimizing water losses, which can be higher than 50% in some drinking water networks. To minimize water losses it is crucial to detect, localize and repair leaks as soon as possible. Leaks can be efficiently and automatically tracked down in the early stages by using leak detection and localization techniques. These techniques are based on coupling information from flow and pressure measurements with the hydraulic model of the drinking water network. The success of such methods depends to a great extend on the estimation of the uncertain water demand in the area. The water demand within a hydraulic model is usually estimated in a deterministic fashion, which lacks the ability to realistically describe the fluctuations in water demand. To include realistic fluctuations in water demand, this study proposes a novel approach by estimating the water demand in a stochastic way by simulating it with SIMDEUM. This stochastic demand model uses information of water users and water-use appliances from Dutch statistics to simulate realistic domestic drinking water demands and its stochastic variations. With this approach, this study aims to assess the influence of a realistic stochastic demand model on the robustness of leak detection and localization algorithms. The applied case study is a residential area in the Netherlands, consisting of a drinking water network with an inflow and six pressure sensors. The corresponding hydraulic model uses stochastic demand loading conditions as inputs. The model represents the network reliably when the SIMDEUM software with local statistics is used instead of average Dutch household statistics. By conducting a Monte Carlo analysis, the influence of stochastic demand on the variability of simulated flow and pressures is determined. Artificial leaks are simulated and the influence of stochastic demand on the performance of leak detection and leak localization is analyzed. The leak detection method consists of setting up confidence intervals per sensor from Monte Carlo simulations and checking whether data falls within these intervals. The leak localization method is based on simulating artificial leaks on all possible locations in the model and comparing the resulting simulated pressures for each simulation to the observed data by using Pearson’s correlation coefficient. The position of the leak in the simulation most similar to the observations is most likely to be near the real leak position. The results show that the stochastic demand variability is strongly linked to the performance of leak detection and localization. The leak detection and localization is most sensitive during the night due to low nocturnal demand fluctuations and least sensitive during the morning peak, when diurnal demand fluctuations are highest. Moreover, the results show that the position and size of the leak significantly influence the performance of leak detection and localization. This study shows that stochastic water demand can be used to quantify the influence of realistic demand variations on the performance of leak detection and localization. Hence, it is recommended to assess the robustness of more leak detection and localization techniques by using stochastic water demand. Furthermore, it gives insight into the expected variability in pressure throughout the network, hence, can prove to be useful in optimal sensor placement.

Biofouling and scaling are ongoing challenges for reverse osmosis (RO) membranes application in wastewater reclamation. Adequate RO feed pretreatment is necessary for biofouling and scaling control. The objective of this thesis was evaluated the effectiveness of ion exchange treatment with weak acid cation (WAC) and strong base anion (SBA) resins columns, in series, after ultrafiltration (UF) treatment for the pretreatment of municipal wastewater treatment plant effluent in order to be used for RO desalination. Specifically, the performance of two SBA resins, the Amberlite SCAV4 Cl (SCAV4) and the Amberlite IRA458 Cl (IRA458) was assessed at three regeneration levels (120, 100 and 80 g NaCl/Lr). The effect of the different regeneration levels on anions removal (sulfate, phosphate and nitrate), total organic carbon (TOC) removal and the operational exchange capacity was researched. Moreover, the subsequent effect on RO biofouling and scaling potential was investigated based on the product water quality of the two SBA resins with bio-growth potential tests and software tests (WAVE design, PHREEQC 3 and Avista Ci), respectively. Phosphate and sulfate removal was above 97% for both resins at all regeneration levels. TOC removal of about 70% was achieved in all cases. It was also observed that sulfate, phosphate and TOC removal remained the same up to the point where product nitrate concentration reached its feed concentration. The removal of nitrate was influenced by the regeneration level. For both SBA resins, a lower regeneration level caused a higher nitrate baseline leakage in product water, hence to a lower removal. The macroporous SCAV4 found to have higher selectivity towards nitrate compared to the gel IRA458. Nevertheless, the anion selectivity order for both SCAV4 and IRA458 was HCO3- < NO3- < HPO42- < SO42-. A minor increase in the operational exchange capacity for both SBA resins was observed at higher regeneration levels. Overall, the influence of the different regeneration levels was found to be limited towards the product water quality and the operational exchange capacity. Also, both SBA resins resulted to similar product water quality. The operational exchange capacity of IRA458 at each regeneration level was higher than that of SCAV4, due to the former’s greater total exchange capacity. The RO feed water quality produced without ion exchange pretreatment (only UF) supported bacterial growth up to 54 ± 1.5 × 106 cells/mL. The RO feed water qualities produced with ion exchange pretreatment in the cases of SCAV4 and IRA458 supported bacterial growth up to 3.4 ± 0.3 × 106 cells/mL and to 1.25 ± 0.2 × 106 cells/mL, respectively. The resulted reduction in the bacterial growth potential was above 90% after ion exchange treatment with either one of the tested SBA resins. The growth-limiting nutrient for the RO feed water qualities produced by either SBA resin was phosphorus. The bacterial growth supported by the two RO feed water qualities produced with ion exchange pretreatment after extra phosphorus addition was 80% lower than that supported by the RO feed water quality produced without ion exchange. This difference suggests that both SBA resins removed a considerable fraction of assimilable organic carbon (AOC). It was concluded that ion exchange pretreatment with either one of the tested SBA resins resulted in nutrients removal (P and C) in the RO feed that lowers the biofouling potential compared to RO feed pretreatment with only UF. The software results suggest lower scaling potential for several scalant types in the RO feed after ion exchange treatment with either one of the tested SBA resins. Among them was calcium phosphate, which was found to have high scaling potential in RO feed without ion exchange treatment (only UF) either with or without anti-scalant dosing. However, the scaling potential of some silica and iron based minerals was high, thus anti-scalant dosing might be required.

Resource recovery from waste streams is indispensable to shifting from a linear economy to a circular economy. Industrial concentrate is a waste stream that is actively researched to explore and expand the prospects of resource recovery. Strontium (Sr), an alkaline earth metal, while found regularly in concentrate streams has not been a focus for recovery. Through this study, the recovery of Sr as strontium carbonate (SrCO₃) via precipitation from concentrate streams has been investigated. The objective of this study is to characterize SrCO₃ precipitation in terms of factors that influence it. Specifically, the effect of multiple factors has been investigated on the amount of Sr precipitated (as % relative to the initial amount of Sr) as well as the purity of the corresponding SrCO₃ precipitate (%). Factors that could affect either the amount of Sr precipitated or the purity of the precipitated SrCO₃ were identified based on theoretical knowledge. Equilibrium-based simulations were performed using OLI studio to confirm their impact. The conditions associated with the identified influencing factors were investigated in concentrate from a reverse osmosis unit during drinking water production. A Box-Behnken Design was implemented to quantify the relationship between the identified factors and two responses (i) Amount of Sr precipitated and, (ii) Purity of SrCO₃ precipitated. The range of the factors in the design was established based on their ranges as observed in the concentrate. The value of pH and concentrations of strontium, inorganic carbon (C), calcium (Ca), barium (Ba) and inorganic sulfur (S) were hypothesized to have quantitative and qualitative effects on the SrCO₃ precipitated. Magnesium (Mg) was hypothesized to have a qualitative effect on the SrCO₃ precipitated. Direct SrCO₃ precipitation from concentrate was not practical due to considerably higher concentration of other ions. To facilitate SrCO₃ precipitation from concentrate, the concentration of the other ions was reduced by recovering them as precipitates. The recovery of these precipitates was investigated through variation of process conditions via OLI simulations. Theoretically, it is possible to recover 0.0014g of SrCO₃ (100% purity) per liter of concentrate along with 0.684g CaCO₃ (99.95% purity), 0.161g Mg(OH)₂ (96% purity), 0.0083g Ca(OH)₂ (99.99% purity) and  NaCl solution (4.65 M). Using Box-Behnken Design, the mathematical relationship between the amount of Sr precipitated (% relative to initial concentration) and purity of SrCO₃ precipitated (%) subject to the identified factors was established. The equations had a R² of 98.75% and 97.38% respectively. S was found to not have any significant impact on SrCO₃ precipitation while a positive interaction effect between Sr and Ca was also identified. %Sr precipitated=0.82098+0.01319(pH)-0.04004(Sr)+0.11581(C)-0.07343(Ca)-0.01314(pH*pH)-0.04299(Sr*Sr)-0.04073(C*C)-0.02041(Mg*Mg)+0.01818(pH*Ca)+0.05369(Sr*C)+0.07436(Sr*Ca)-0.05763(C*Ca)  SrCO₃ purity(%)=0.5155-0.20923(pH)+0.12593(Sr)-0.03178(C)-0.06115(Ca)-0.07187(Mg)+0.0617(pH*pH)-0.0388(Sr*Sr)-0.0295(C*C)+0.0456(Mg*Mg)-0.0485(pH*Sr)+0.0595(pH*C)+0.0405(pH*Ca)-0.0903(pH*Mg)+0.0481(Sr*C)+0.0614(Sr*Ca)-0.0453(C*Ca) The predictive R² of these equations are 93.45% and 86.32% respectively. This is representative of their prediction accuracy and hence their applicability to replace experiments and simulations. These equations, however, are built on outputs from OLI simulations. Therefore, their accuracy is subject to the predictive capability of OLI software. Experimental results based on the chosen Box-Behnken Design are needed to either replace the OLI simulations results entirely or account for the inaccuracy of OLI simulations.

UV/chlorine, as an emerging advanced oxidation process, is able to degrade organic micropollutants in water via the generation of reactive oxidant species and direct reaction with HOCl/OCl- as well. In this study, light-emitting diodes (LEDs) were applied as the UV sources to investigate the efficacy of UV/chlorine process for the degradation of trimethoprim (TMP, a frequently detected antibiotic in waters), at slightly alkaline pH.

This thesis proposes an approach for tackling the problem of data-shortage in hydrological modelling. A hydrological model, in the context of this thesis, translates meteorological data to stream-flow of a river, for a specific catchment. A model incorporates parameters that describe the dynamics of a chosen model, in this case a FLEX (Flux-Exchange) based model. All parameter together form a set. These parameters are unknown and therefore need to be determined, which is done via a calibration process. In the calibration process, the modelled flow will be evaluated against the observed stream-flow over a period of time for a certain hydrological signature. A hydrological signature is defined as the quantification of specific information concerning the rainfall-runoff dynamics of a catchment, e.g. the mean stream-flow. The evaluation for a certain hydrological signature is known as an evaluation criterion. Literature deems the use of one or two criteria in combination with multiple years of data enough to ensure a good performance from the parameter-sets that exit the calibration process. As this amount of data is not available everywhere, this thesis proposes to still ensure good model performance for less observed data. For this purpose, a selection was made of evaluation criteria to be applied in the calibration process. These criteria were selected in such a way that various different characteristics of the hydrological response were covered. A benchmark was created for comparison purposes in which 10 years of data was used during calibration and one evaluation criterion. After the benchmark, the observed data was shortened and evaluation criteria were added. The results showed that 6 months worth of data in combination with all criteria would create benchmark-level performances without extra information. The same level of performance can be reached with 3 months of data but this would require extra information. This information would be e.g. during which period of time observed data should be collected.

The Dutch drinking water benchmark has been carried out every three years since 1997. In 2011, the benchmark was incorporated into the Drinking Water Act. As a result the benchmark switched from a voluntarily benchmark executed by the Association of Dutch water companies to a mandatory benchmark executed by the Human Environment and Transport Inspectorate. In addition, the drinking water companies have to hand in an improvement plan 6 months after the benchmark publication. Based on interviews with benchmark coordinators of drinking water companies the influence of the inclusion of the benchmark into the Drinking Water Act on the improvement driven part of the benchmark is studied. For this purpose, the following four issues are discussed: changes experienced as a consequence of the shifting from a voluntarily to a mandatory benchmark, influence of mandatory character of the benchmark on the drivers, identified by de Goede et al. (2016), the role of the improvement plans and a new instrument to stimulate improvement.

The differences between the voluntarily and mandatory benchmark are identified: the benchmark switched from being only an improvement driven instrument to an instrument for accountability as well and the drinking water organizations lost control over the development of the benchmark. The influence of the mandatory character on the drivers for performance improvement have been determined: only the driver 'enhanced transparency' is (positively) influenced. In order for the improvement plans to be able to have a positive effect on the stimulation of improvement, a feedback system should be implemented and the publications of the benchmark and improvement plans have to be faster. The benchmark is still thought to be useful, although an adaptive benchmark could stimulate improvement again.