Sewage sludge production poses a significant global challenge, with anaerobic digestion (AD) being the most widely employed method to manage this issue. However, since hydrolysis is the rate-limiting step in AD, pretreatment of sludge can aid in enhancing the process.
The current study aimed to evaluate sludge solubilization and gas production—particularly bio-hydrogen (bio-H₂) production—following low-temperature thermal pretreatment (LTTP) at 55°C. This thesis replicates the full-scale setup of the Themista® system, which employs a two-stage thermal pretreatment process with 55°C and 70°C as the heating stages, operating in a semi-continuous mode. Four distinct pretreatment experiments were conducted to assess sludge solubilization and gas production: LTTP , LTTP with H₂O₂ at 55°C, LTTP with FeCl₂ at 55°C, and LTTP with both H₂O₂ and FeCl₂ at 55°C.
The results demonstrated that both thermo-chemical and thermal pretreatments exhibited similar soluble chemical oxygen demand(sCOD) release patterns. A shift from tightly bound to more soluble fractions of extracellular polymeric substances (EPS) was observed post-pretreatment. Gas production was noted only at 55°C across all conditions, with bio-H₂ being produced under each pretreatment. Notably, thermal pretreatment with FeCl₂ resulted in an average H₂ COD/tCOD ratio that was 24 times higher than that of thermal pretreatment alone. However, the average sludge to gas COD percentage for all pretreatments remained below 0.06%, with the highest of 0.1% observed in the thermal treatment with FeCl₂. Additionally, significant increases in BMP and a sludge COD to CH₄ conversion percentage of 75% were observed post-treatment with thermal pretreatment combined with FeCl₂

Non-sewered sanitation systems have emerged as a valuable complement to sewered systems over the past two decades. Their importance is pronounced in urban areas of low- and middle-income countries, where they could prevent ~600,000 deaths annually by supplementing inadequate sanitation infrastructure. Additionally, in high-income countries, non-sewered systems are gaining traction due to sewer capacity limits and the growing emphasis on resource recovery.

Despite these advantages, the treatment and reclamation of blackwater (toilet wastewater) in non-sewered systems remains challenging. Approximately 20,000 liters/year of freshwater are used per person for toilet flushing, equivalent to an individual's drinking water needs for 15 years. Blackwater reclamation offers a promising solution to reduce freshwater extraction and prevent environmental pollution. However, research in this area has gained prominence recently, and there remains a gap in designing treatment trains. Current research focuses on individual technologies rather than proposing holistic frameworks that could aid urban planners and researchers in selecting technologies along the treatment train. Furthermore, the carbon footprint of complete non-sewered systems has not been quantified.

Addressing this, the thesis aims to: (1) develop a methodology to select technologies along the treatment train, (2) quantify the carbon footprint of a non-sewered system, and (3) assess the relevance of blackwater reuse in urban communities worldwide. The NEST building in Switzerland serves as a case study to demonstrate the application of the proposed approaches.

The methodology involves collecting data on blackwater quality and quantity, compiling a list of suitable technologies through literature review, and refining this list using pre-selection criteria. A pre-selection matrix groups technologies according to these criteria, assisting researchers in selecting context-specific technologies. Configured treatment trains are evaluated using 14 decision-making criteria to ensure social, technical, economic, and environmental sustainability. Applying this methodology, the most suitable treatment train for non-potable reuse (e.g., toilet flushing) includes a moving bed biofilm reactor, chemical precipitation, ultrafiltration, granular activated carbon, and UV disinfection. For indirect potable reuse (e.g., recharge of drinking water reservoirs), nanofiltration should be included.

Process designs are developed for the treatment trains using BioWin and used for the carbon footprint analysis. Indirect emissions from electricity consumption, particularly in the urine concentration unit, are identified as the largest contributors. Transitioning to renewable energy could significantly reduce the carbon footprint, emphasizing the need for comprehensive life cycle cost and benefit evaluations.

Finally, the thesis assesses the feasibility of applying the developed methods and results in urban communities across various income levels. For low-income countries, the selection of simple, robust technologies is emphasized due to frequent power cuts and high maintenance costs. Middle-income countries require a balance between efficiency and operational ease, while high-income countries benefit from resource recovery, necessitating legislative support for non-sewered systems.

In conclusion, this thesis establishes a framework for designing safe, reliable, and sustainable non-sewered sanitation systems for urban blackwater treatment and reuse, addressing a critical gap in current literature. It calls for pilot testing to validate these theoretical frameworks, advancing the operational knowledge of non-sewered sanitation to match the maturity level of traditional sewered systems.

WASH interventions are essential to support human health, prosperity, and dignity, as they provide the base for an adequate standard of living. In many low- and middle- income countries, especially in rural and low-income areas, decentralized wastewater treatment systems (DEWATS) can offer a solution to convey, treat, and dispose of or reuse wastewater closer to the source and through smaller conveyance networks. In Indonesia, and as such in the Brantas basin in East Java, focus area of this study, the government has recognized DEWATS as their best available option for improving sanitation in dense low-income urban settings. Although the percentage of households with access to proper sanitation in the province of East Java has been increasing steadily, service coverage and the quality of sanitation systems still need to be increased to reach the desired coverage by 2024. Similar to other fields of application, within WASH and concerning DEWATS, stakeholder engagement is key to develop and strengthen integrated and sustainable approaches. Nonetheless, it is challenging to formulate targeted interventions in the watershed since they depend on the willing support of various stakeholders who may have different priorities (even within their own institutions), having diverse (and sometimes conflicting) viewpoints. This may result in stakeholders strongly contesting the appropriateness of various solutions. An exploration of stakeholder priorities is therefore needed to facilitate the application of wastewater treatment technologies. Due to its participatory approach and the type of interpretation that the method allows, Q-methodology was selected to explore this situation. Q-methodology is a set of techniques which allow for the study of ‘subjectivity’, combining statistics with the depth provided by qualitative data. It is composed of the data collection technique (called Q-sorting) and a data analysis step via correlation and factor analysis. In this research, the perspectives and priorities of various stakeholders regarding decentralized wastewater treatment solutions are explored to assess the applicability and acceptability of DEWATS in the Brantas river basin. This aims to identify context-based criteria and challenges to the implementation of DEWATS in the Brantas watershed. Although findings from Q-methodology cannot be generalized for a general population, the findings from this study show strong indication that Surabaya government workers tend to prioritize similar criteria, that Malang community members think more similarly than their Surabaya counterparts, and that if a respondent prioritizes the elements describing the 4-F1 perspective (Basic current needs; Long-term sustainability; Socialization and collaboration; High awareness; Economically conscious) then they are likely to be some type of specialist. Additionally, four distinctive perspectives were identified: a focus on long-term sustainability of the system, although with less defined stakeholder responsibilities; a pragmatic, need-based approach; a preference for community-led approaches with user-friendly systems and greater subsidy availability; and a collaborative, but still community-run, approach with a high feeling of ownership. Further research is needed nonetheless to identify how the levels of agreement and disagreement among the different groups for the various statements presented could be integrated into the design and decision-making process for DEWATS in the region.

This research explores the feasibility of implementing ceramic microfiltration (CMF) treatment in Maputo, Mozambique, to reclaim wastewater for industrial reuse, addressing the city's pressing water scarcity challenges. As rapid urbanization increases Maputo's reliance on potable water for industrial and agricultural needs, this study evaluates reclaimed wastewater as a sustainable alternative to alleviate demand on the city's limited freshwater resources. Using a CMF pilot plant, the project tested wastewater from the recently upgraded Infulene Wastewater Treatment Plant (WWTP) to assess whether CMF treatment could achieve quality standards suitable for applications such as cooling, concrete production, car washes, agricultural irrigation, and municipal park irrigation. Furthermore, the opportunity of scalability was tested through a water balance, while relevant stakeholders were interviewed and costs estimated to complete the feasibility assessment.

Laboratory results indicated that CMF treatment effectively reduces turbidity, chemical oxygen demand (COD), and biological pollutants like E. coli and coliforms. However, dissolved particles and heavy metals were not removed, limiting its efficacy for high-specification uses. While the treated effluent met quality standards for lower-specification applications, such as local car washes and park irrigation, it did not reach the stricter requirements needed for cooling water or concrete production. This underscores a need for process optimization, particularly through coagulation, to expand CMF's application range.

To assess sustainable water availability, a water balance analysis of the Infulene WWTP considered seasonal flows and local agricultural demands. The findings suggest that although the current water supply is insufficient during dry months, full capacity utilization and improved sewer network connections in the future could support CMF-based water reuse consistently across seasons, with potential scalability for additional users.

Economic analysis compared CMF's capital and operational costs with revenue from reclaimed water sales, showing that while considerable initial investment is required, direct piping could potentially make CMF-treated water competitively priced against potable supplies under the condition of reaching maximum treatment capacity at a scaled up CMF plant. High costs associated with truck-based delivery, however, present a barrier to adoption for potential users. Stakeholder interest was strong across industrial users and developers, though contingent on achieving cost parity with the existing water network.

This study concludes that, while integrating CMF technology into Maputo's water management strategy offers promise, challenges remain in achieving quality standards for certain industrial applications and in lowering costs. Addressing these technical and economic barriers could open avenues for CMF's broader adoption, especially with future assessments that include alternative suppliers and configurations.

Elevated levels of ammonia in discharged water leads to eutrophication and potential toxicity in water bodies. To combat nitrogen pollution, strict ammonium discharge limits of 1 mgNH₄-N/L standard is set by the water framework directory. In many cases, current wastewater treatment facilities struggle to consistently meet this standard, particularly during peak concentration periods. For years now, the influent's peak pattern has been mirrored in the biologically treated water and currently, there lacks an effective solution to address this issue. This study explores use of zeolite in a biological filter, a potential technology to remove ammonia peaks from effluent wastewater. The fundamental concept of this technology involves combining zeolite with a nitrifying biofilm. In this process, the zeolite dampens the ammonia peak by adsorption, while the nitrifying biofilm regenerates the zeolite. Clinoptilolite, a natural zeolite of 1mm particle size and synthetic wastewater were chosen in this study. The impact of competing cations was investigated due to clinoptilolite's affinity for cations. It was found that the presence of other cations decreases ammonium removal capacity by 6.2 times of which potassium is the main competitor. But, percentage of potassium removed is much less than ammonium though it's concentration is 5 times higher. 44.2% of the total potassium was removed in 60 minutes compared to 70.9% ammonium removal. In the experiments with the nitrifying biofilm, it was found, that it does
not affect rate of adsorption by zeolite, but enhances ammonium removal. Experimental results indicate that zeolite can be bio-regenerated effectively, and rate of conversion is faster than rate of desorption of adsorbed ammonium. The system of zeolite and biofilm has some buffer capacity, but cannot compensate for the bicarbonate anions needed for H⁺ released. A BioWin model
was designed to simulate the survival of biomass during extended periods of low concentration, and the results indicated that conversion capacity of the system reduces after 7 days of DWF concentrations. But the biomass can sustain longer periods of DWF concentrations, but it takes some time and exposure to higher substrate concentrations to revive it's capacity. In conclusion this study
confirms the potential in this technology and confirms effective bio-regeneration capabilities. The results from this research can built upon to answer questions regarding knowledge gaps with reactor operation and design. This paves the way for future studies to make it an industrially viable technology. 

Aerobic granular sludge (AGS) is an innovative biotechnology extensively applied for treating municipal wastewater, and it can potentially treat sugar industry wastewater. Glucose is a prevalent substrate in sugar industry wastewater; nevertheless, the effect of glucose on AGS systems remains unexplored. In this study, an AGS reactor using glucose as the sole carbon source was operated in anaerobic-aerobic cycles. The system maintained a solids retention time (SRT) of 10 days, resulting in good granulation and enhanced biological phosphorus removal (EBPR) performance. The glucose fed was rapidly taken up within 10 minutes, with a portion stored as intracellular polymers such as glycogen and poly-hydroxy-alkanoates (PHAs), while another portion underwent anaerobic fermentation to lactate and formate. The carbon balance was not completely closed, with 16% of the carbon speculated to be utilized for the production of an unidentified polymer. The microbial community consisted of diverse organisms, with Micropruina identified as the most abundant genera and Ca. Accumulibacter (a typical type of PAOs) as the second most abundant genera based on metagenomic analysis. A batch test was conducted by adding an excess of glucose, lactate, and formate, revealing that lactate was the probable substrate utilized by PAOs. Additionally, Micropruina was hypothesized to be involved in glucose consumption, glycogen storage, and lactate production. Micropruina and Ca. Accumulibacter collaborate in utilizing glucose, providing them with a significant competitive advantage within the system. Due to their slow growth rate, these bacteria play a crucial role in achieving favorable granulation when supplied with glucose. Promoting the growth of these organisms can be a valuable strategy in engineering applications.

Poly-hydroxy-alkanoate (PHA) is an intracellular polymer that can be used as an energy and carbon source by microorganisms. Measuring PHA is important for understanding the microbial metabolism of enhanced biological phosphorus removal (EBPR) and aerobic granular sludge (AGS) systems. There is a commonly used method to measure PHA, which is based on organic solvent extraction and gas chromatography (GC). However, there are different versions of the same method with different parameters, but the role of some of these parameters is unclear. When different types of biomass are analyzed, there is a requirement to understand the parameters and obtain an optimal protocol. In this study, the effect of various digestion times, different alcohols and organic solvents, and acid concentrations were tested to obtain the optimal protocol. The results showed that a minimum digestion time was required to get the maximum yield of PHA, and the time might differ when using different types of biomass. Methanol was shown to be better for GC separation than propanol. Using different organic solvents didn’t affect the final concentration, and an optimal acid concentration was required to determine by comparison. The GC temperature program optimization showed that lower oven temperature in GC is more beneficial for peak separation. From the analysis, it would be suggested to use methanol and chloroform for digestion and keep the digestion time for 24 hours.

Improving circularity and reducing greenhouse gas emissions can be achieved by utilizing secondary streams like Fischer-Tropsch biosludge, which is typically disposed of in landfills. Biochar, which has accumulated increased interest recently, has significant potential for decreasing carbon emissions and can be created from biosludge. This master thesis was set up as an opportunity-based research project to understand the utilization of Fischer-Tropsch biosludge by the hydrothermal carbonisation process and characterise its product phases; biochar, aqueous phase and gaseous phase. A central composite surface response design was used to conduct experiments to analyse the impact of temperature and time on the characteristics of the phases. Three different levels of the factors have been used. The high ash content and low surface area of the biochar made it difficult to determine a specific use case for the biochar. Yet, the aqueous phase has low ash and possible potential for biogas creation by anaerobic digestion. The gas phase is quantitatively not of significance. Stockpiling the biochar could sequestrate carbon credits and more research can be conducted to find a post- or pre-treatment to improve the biochar for a social-economically benefiting application.

Growing concerns about elevated nitrate levels in natural springs on the southern slope of the Irazu Volcano, Cartago province in Costa Rica, were the driving force for a multi-disciplinary study on the problem. The region is characterized by its high agricultural output and steep slopes. More than sixty springs located in the area are managed by a large number of local water authorities, ASADAS. The study focused on determining the main sources of nitrate pollution. Anthropological activities such as agricultural practices and domestic actions are found to contribute the most. A multivariate polynomial regression model was used with a large set of parameters. From the results it can be seen that human activities within the by law determined 200 meter radius protection zone around the springs, are most influential on high nitrate concentrations in the springs. Furthermore a stakeholder analysis, fieldwork, financial analysis of alternatives and farmer interviews were performed to produce a comprehensive list of recommendations for the various stakeholders. The recommendations are compiled to ensure a future with clean drinking water for all the citizens in the region.

For this thesis a hydrochemical groundwater survey was carried out on the Caribbean island of Curaçao in 2020 as part of the NWO SEALINK project. In order to be well-adapted against anthropogenic and natural pressures, Curaçao needs representative data, yet thorough hydrochemical datasets were only sparsely collected in the wet seasons of 1977 and 1992. The aim of this thesis was to determine the current chemical state of the groundwater and analyze for long-term pollution trends with an extended database spanning four decades, also including data that was collected in another fieldwork campaign in 2021.

This research addresses the critical issue of the poor implementation of sanitation and its effect on community and environmental health in the Global South. To overcome the social, cultural, technical, and financial complexities underlying poor sanitation, the participation of local community members in sanitation planning processes is deemed crucial for the successful development and implementation of sanitation systems.
To engage local stakeholders as decision-makers and improve design freedom, it is proposed to engage stakeholders in determining their demands, needs, and priorities by combining the Fuzzy Analytical Hierarchy Process (FAHP) and Focus Group Discussions (FGD) as a participatory decision-making method. The applicability and suitability of this method are assessed through a case study in Hagonoy, the Philippines, in which two parallel design cycles are carried out: one with only local sanitation experts as decision-makers and one with only local community members as decision-makers. A systematic literature review of design criteria applicable to sanitation design processes was conducted to overcome selectors' bias regarding the decision criteria assessed in the FAHP and the FGD. The FAHP data was collected among 60 community members evenly distributed over six neighbourhoods in Hagonoy and six local sanitation experts. Three FGDs were held with local community members, whilst one FGD was held with local sanitation experts. The design cycle based on the defined design scope was limited to the development of a conceptual design, proposing a treatment train capable of treating the produced domestic wastewater such that the effluent satisfies the local General Effluent Standards, specifically for the parameters water volume, Total Phosphorus (TP), Chemical Oxygen Demand (COD), Ammonia (NH3), and Nitrate (NO32-). Wastewater treatment technologies aligning with the demands and needs specified by the stakeholders were obtained from scientific literature, which were combined into a sanitation system configuration that could treat the water sufficiently.
The completion of a sanitation design process without predefining the possible solutions, but by integrating the FAHP + FGD in the systematic derivation of the design scope, demonstrated how a demand- and need-driven design approach allows increased flexibility and sensitivity for the context in the development of a design. The parallel design cycles showed how the FAHP + FGD method could be applied as a systematic procedure to understand how design criteria in sanitation design are preferred and interpreted differently between local community members and local sanitation experts and how this can influence the conceptual design. Altogether, this research contributes to the body of knowledge in community-engaged sanitation design by demonstrating the effectiveness of the FAHP and FGD method in engaging community members in a need-driven design process, overcoming selectors’ bias for technologies and trade-offs, and producing relevant design proposals. However, acknowledging that the FAHP + FGD method was limited to engaging stakeholders only in defining the needs and demands, verification of the designs’ alignment with the perspectives of stakeholders and continuous engagement throughout the entire design process is essential to be further researched in the future.

Since its production concentrations of the manmade chemical compounds PFAS found in the environment has increased significantly. The properties created by incorporating these chemicals, make them attractive for industrial usage and for many consumer products. These chemicals bioaccumulate and are very persistent. With known adverse health effects, its treatment to prevent release in the environment has become more and more important. To remediate PFAS in aquatic environments, such as groundwater and surface water helps protect our drinking water sources and reduces exposure to these harmful compounds. With this in mind the treatment technology, electrochemical oxidation of PFAS is applied here in various water matrices to study the destruction of these compounds. Through galvanostatic EC experiments, water types in 10L continuously stirred reactor tanks, where pumped through an electrochemical cell consisting of boron-doped diamond electrodes at a constant current of either 30 or 40 A for a duration of either 3 or 6 hours. Samples were processed and then analysed for 29 PFAS through HPLC-MS/MS. Remediation of these compounds proved to be less efficient than in other studies.

Due to the increasing impact of terrestrial agriculture on climate change, the attention of a myriad of industries is shifting towards the use of alternative, low-emission resources. Seaweed cultivation has presented itself as a contribution to the mitigation of the increased pressure on current resources. However, coastal and offshore marine areas are often unfit for seaweed cultivation due to increasing maritime activity. As wind farm areas are increasing, offshore seaweed aquaculture in multi-use platforms at sea (MUPS) has been proposed as one of the possibilities for smart use of ocean space. Apart from providing a multitude of benefits through the many applications of seaweed, it is also widely suggested that seaweed could offer ecosystem services during its growth by means of nutrient bioextraction of eutrophied waters. In this thesis, the critical nutrient flows of cultivation of S. latissima in MUPS at the North Sea are quantified using a dynamic mathematical nutrient model, and the impact on the marine vicinity is assessed. The assessment is performed for two scenarios: (1) a seaweed farm producing for a high-value chemicals factory, and (2) a seaweed farm producing for a fuel biorefinery. Both these scenarios are modelled over the course of one cultivation season on four offshore wind farm locations in the North Sea. Moreover, an analysis is performed on the potential role of monitoring technologies in offshore seaweed aquaculture in MUPS. The results of this study are combined to assess whether it is possible to establish ecosystem services through large-scale offshore seaweed cultivation in MUPS at the North Sea using monitoring technologies and nutrient analyses. The analysis in this study showed that offshore seaweed cultivation has a promising potential for nutrient bioextraction in the North Sea. However, nitrate depletion could occur during the last months of cultivation, when primary productivity is naturally lower. It is recommended that further research on the ecological effects of this nitrate depletion is conducted, and measures are taken to minimise the risk of detrimental effects. It is concluded that a combination of nutrient analyses and monitoring technologies could provide a more comprehensive understanding of the impact of large-scale offshore seaweed cultivation. Subsequently, this can create a solid foundation for the development of ecosystem services and the further development of the offshore seaweed sector in the near future.

Fine sieved fraction (FSF) is a solid waste generated from wastewater treatment plants (WWTPs). Cellulose fiber is the most important component of FSF, which can be fermented into volatile fatty acids (VFAs) and then be used as carbon sources in biological nutrient removal (BNR). This thesis measured and evaluated the aerobic & anaerobic biodegradability of solids in fermented FSF (fFSF) and the BNR performance of FSF-derived VFAs. In addition, this research tested the feasibility of using secondary sieving as a potential downstream processing technology. The products of secondary sieving (liquid: secondary filtrate, solid: secondary FSF) were compared with fFSF in terms of biodegradability and BNR performance. The aerobic biodegradability of raw FSF, fFSF and secondary
filtratewere 52.3 ± 1.4, 60.4 ± 4.7 and 72.1 ± 6.4 %, respectively. The anaerobic biodegradability of raw FSF, fFSF and secondary FSF are 56.8 ± 8.4, 68.2 ± 6.4, 51.5 ± 7.8 %, respectively. The results of biodegradability revealed that secondary sieving increased the proportion of readily biodegradable solids and reduced the overall organic load of activated sludge systems. The residual solids after secondary sieving could be converted into biogas or recovered as building materials. FSF-derived VFAs could substantially increase denitrification rates and phosphate release rates. fFSF and secondary filtrate could increase the denitrification rate by 40 - 50%and increase the phosphate release rate by around 200 % compared to blank. Theoretically, the effluent nitrogen load could be reduced by up to 53.7%if FSF-derived carbon sources are fully used for denitrification atWWTP Aarle-Rixtel. If used for phosphorus removal, the capacity provided by FSF-derived VFAs could nearly remove all phosphate from the effluent. To sum up, FSF-derived VFAs could effectively increase BNR rates
and significantly reduce the effluent nitrogen and phosphorus load. The main advantages of FSFderived VFAs over conventional carbon sources are sustainability and waste-reduction. Secondary sieving could reduce the solid load of activated sludge tank and achieve a combined recycling of VFAs and fibers, but the total COD available for BNR will be reduced as a compensation. The results in this thesis can facilitate the application of fFSF in WWTPs and the improvement of FSF-derived VFAs production.

Environmental guidelines (EU WFD) impose a strict effluent phosphate target on wastewater treatment plants (WWTP). An abundance of phosphate (and other nutrients) in a water body is called eutrophication. Eutrophication can lead to algal blooms which result in very low oxygen concentrations in the water. Other species, like fish and aquatic plants, can die because of the lack of oxygen. Most WWTPs in the Netherlands are performing well and reach phosphate effluent concentrations below 1 mg/l. Nevertheless, Dutch water authorities are looking for ways to improve nutrient removal to perform even better then the WDF guidelines. At the same time they are anticipating on the expectation of more strict phosphate effluent targets in the future.

This research is focusing on VFA production from the sieved fraction of activated sludge. Sieves with a mesh size of 0.5 mm were used to sieve cellulose fibers from the sludge. Microbial conversion processes are stimulated in the VFA production reactors. A pilot plant that was performed resulted in very high phosphate concentrations in the reactor. The first phase of this research was devoted to identifying the source of the high phosphate concentration. The second phase was focused on phosphate removal and recovery strategies.

It was found that 20% of the sievings consisted of PAO sludge. The conditions in the VFA production pilot lead to extreme phosphate release by PAO. This resulted in high phosphate levels ranging up to 200 mg/l.

The released phosphate potentiates recovery methods. It was not able to precipitate struvite at the low pH levels at which the VFA production reactor was operated. Calcium carbonate addition could decrease phosphate levels (50%) and can be used to control pH in the reactor. The applied method does not allow for easy recovery because the precipitate ends up in the sievings. Finally vivianite precipitation was achieved by adding FeCl2. 90% phosphate removal was achieved. The para-magnetic property of vivianite allows for easy recovery with a magnetic separator. This results in the production of 2 valuable streams from a waste stream: VFA and vivianite.

Anaerobic digestion (AD) is an effective and economical in secondary sludge management. Thermal hydrolysis process (THP) is a reliable pre-treatment method to improve the biodegradability of sludge. Partial nitritation and anammox process (PN/A) is a suitable method used to remove nitrogen in reject water which has a low COD/N ratio. However, some organic compounds produced in THP process, such as melanoidins, have negative effects on AOO and anammox bacteria. Considering that humic substances occupy a large part of the soluble organics in reject water of THP-AD process. Therefore, this study aimed to investigate the acute and long-term toxicity of three types of humic substances (commercial humic acid, melanoidins and reject water) on partial nitritation step of PN/A process. The biodegradability of three types of humic substances under aerobic and anaerobic conditions, and the evolution of humic substances during the biodegradability assays were also analysed. The results showed that all three types of humic substances had higher biodegradability under aerobic condition than that under aerobic condition, and melanoidins had more biodegradable compounds than reject water and humic acid. During the AD process, humic substances acted as electron acceptors, decreasing the cumulative methane production of all three types of humic substances compared to the blank. Also, small MW fractions were transformed to large MW fractions in all three types of humic substances during both aerobic and anaerobic digestion. Moreover, during aerobic and AD, both fulvic acid-like and humic acid-like substances were formed. In addition, the results of acute toxicity tests showed commercial humic acids had a stronger inhibitory effect than reject water and melanoidins on AOO biomass. In addition, a positive correlation between humic substances concentration and AOO activity loss was observed. The long-term operation of PN reactor was failed due to the formation of granular sludge in the reactor and the growth of AOO biomass in the tubing and culture medium.

Thermal hydrolysis (TH) has been widely applied to sewage sludge treatment to improve downstream anaerobic digestion (AD) by enhancing organic solids hydrolysis and subsequent biogas productivity. However, TH also has several drawbacks, including the formation of recalcitrant compounds, which are anaerobically non-biodegradable and even inhibitory to the downstream AD. Melanoidins produced from the Maillard reaction have been regarded as a representative of such compounds. Theoretically, the production of melanoidins increases with increasing TH temperature.
This study collaborated with TORWASH®, a patented TH technology developed by TNO, aiming to characterise the recalcitrant compounds derived under TORWASH® conditions from 180 - 210 °C. In addition, we investigated the effects of anaerobic pre-digestion on the TH performance, attempting to increase the biogas productivity of digestate. Moreover, we also sought pretreatment methods that could mitigate the production of recalcitrant compounds. By reviewing the studies of Zhang et al. (2020) and other researchers in this field, we found that acidic pretreatments could be the solution. Then, we assumed that the addition of acetic acid and CO2 gas could limit the progression of the Maillard reaction. A laboratory simulation of the TH process was performed on waste activated sludge (WAS) at 180 - 200 °C and anaerobically pre-digested WAS at 180 - 210 °C. The following TH tests with acidic pretreatments were only conducted on WAS at 190 °C.
We found that the production of recalcitrant compounds (soluble humic substances) increased with increasing reaction temperature, corresponding to increased UVA254 and colour formation. Proteins were primarily hydrolysed or denatured during TH, and competition for carbohydrates between the Maillard and caramelisation reactions could occur at 190 °C and above. In addition, TH disintegrated high molecular weight DOM > 1 kDa into low molecular weight DOM < 0.1 kDa, and stronger sludge disintegration occurred at higher reaction temperatures. Moreover, BMP decreased after anaerobic pre-digestion but increased after TH treatment. The liquid fraction of pre-digested WAS could reach an equivalent degree of anaerobic biodegradability as that of WAS when applying a higher reaction temperature.
All acidic pretreatments applied decreased the production of soluble humic substances by 10 %, corresponding to a 13 % decrease in UVA254. CO2 pressurisation at 10 bar could effectively reduce the formation of fulvic-acid like and humic-acid like matter, compared with the untreated sample. The effect of 20 bar CO2 was less pronounced than that of 10 bar CO2, whether or not the samples were pre-added with HAc.

The hydrolysis of sludge solids especially for difficultly degradable sludges such as WAS is not fully understood, yet. The first-order hydrolysis rate was shown to function well for most easily degradable sludges and soluble substrates. This description for substrate hydrolysis in the context of anaerobic digestion has the benefit of being very simple and therefore applicable for many engineering applications where little data is available. On the other hand, in the last decades many studies reported that the first-order hydrolysis would need a modification to better describe the degradation of difficulty degradable solids. Guo et al. (2021) developed a cascade system for anaerobic digestion of WAS that does not seem to follow first-order hydrolysis kinetics when lowering the applied SRTs from 22 to 15 and 12 days, respectively. Based on observations by Guo et al. (2021) and a statistical analysis of the cascade system performed in the study at hand it seems that the first order hydrolysis rate constant is in fact a coefficient and that the first-order hydrolysis rate is not solely dependent on sludge characteristics and substrate concentrations. This hypothesis is tested in the thesis at hand. In Guo et al.’s study the cascade system was always compared to a reference system. To test this hypothesis and understand the kinetics of the cascade system in more detail a statistical analysis was performed for both systems from which an empirical hydrolysis model was derived. This model was implemented in ADM1 to replace the existing hydrolysis rate expression and was tested for the mentioned cascade system and the reference system. The empirical model was compared to the results of the standard ADM1 which uses a first-order hydrolysis expression. The empirical model assumed a dependency of the hydrolysis rate based on load and residence time along the cascade system to achieve a change in hydrolysis rate coefficients along the cascade system. The models were compared based on visual inspection and quantitative analysis of the simulated results. Both models showed low R² values which is likely due to the high level of detail implemented in ADM1 that does not fit to the resolution of the experimental data. However, calculated RMSE values agreed with the standard deviations of the experimental results. Therefore, the overall predictive capability for both models is given. The ADM1 managed to model the reference system with reasonable agreement to the experimental data. The performance of the empirical model for the reference was comparable. For the cascade system however the ADM1 could not fully describe the experimental at the applied low SRTs of 15 and 12 days. The empirical model in this case showed better predictive capabilities. This is an indication that a hydrolysis rate which is made dependent on system characteristics such as load and residence time might indeed have its justification and be better applicable to anaerobic digestion systems that show a concentration profile along the reactor as it is in the case with plug-flow and cascade systems.

Groundwater is a major source of drinking water containing various elements out of which arsenic(As) is one of the toxic elements present. It is present in the form of arsenite, Conventionally, As(III) can be effectively removed if it is pre-oxidized to arsenate, As(V), thereby not involving any chemical dosage. There are various techniques to remove arsenic from drinking water like membrane filtration, electro-coagulation, filtration, adsorption and ion exchange. Among these the iron electro-coagulation technique of arsenic removal is one such technique which can be done by electrochemically generating oxidizing compounds like
hydrogen peroxide, H2O2. Instead of dosing H2O2 anaerobically, it can be generated before the aeration step and can improve the As(III) oxidation with the groundwater native Fe(II).

The in-situ electrochemical generation of H2O2 was done by means of an air-cathode reactor setup, which reduces atmospheric oxygen O2 to H2O2, under anoxic water. This H2O2 then reacts with ferrous iron, Fe(II) to produce ferric iron Fe(III) and reactive oxidizing species(ROS)/intermediate products/fenton products. These ROS mainly form poorly ordered solids, which have higher adsorption capacities than the products of aeration. The oxidation of
As(III) is 4 times more by H2O2, than the oxidation by O2.

By varying the applied charge dosage (CD) and the rate of dosage (Charge Dosage Rate, CDR),the faradaic efficiencies of both Fe and H2O2 were analyzed. It was found that as the CDR increased, the overall faradaic efficiency of H2O2 generation also increased from 76.32% to 92.07%. However, there might have been discrepancies in the faradaic efficiencies of Fe due to acid & base dilutions, human errors or the difference in the operational values of the current. In theory, 1 mole of H2O2 oxidizes 2 moles of Fe(II), and in the absence of O2 the main Fe(II)-oxidant is the generated H2O2.

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.