School of Water, Energy and Environment (SWEE)

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Browsing School of Water, Energy and Environment (SWEE) by Supervisor "Bajón Fernández, Yadira"

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    ItemOpen Access
    Adaptation of two-stage vertical flow constructed wetlands for treating unscreened sewage in the UK.
    (Cranfield University, 2019-03) Khomenko, Olha; Bajón Fernández, Yadira; Dotro, Gabriela
    The UK water industry has made a priority of embracing low energy sewage treatment, making of two-stage vertical flow constructed wetlands (VFCWs) a plausible alternative to conventional systems. However, for successful implementation of the technology in the UK a further understanding of any required adaptations in its design and operation is required, in particular linked to the UK use of spot sampling regulation of treated effluents and differences in rain profile affecting hydraulic loads in combined sewers. There is also a paucity of information regarding required time for full maturation of the technology and of performance during early operation. In response, the first UK full-scale WWTP based on two-stage VFCWs was built in Derbyshire, UK, and closely monitored during the initial three years of operation, allowing investigation of the system performance and robustness. The efficacy of the system for removal of total suspended solids (TSS), BOD₅ and NH₄ +- N has been evaluated by composite and spot samples analysis. The technology was found to efficiently remove pollutants to the level of 6 ± 3 mg·L⁻ ¹ , 5 ± 2 mg·L⁻ ¹ and 5.8 ± 3.8 mg·L⁻ ¹ for TSS, BOD₅ and NH₄+- N, for 24 hour composite samples. After one year of operation the system was retrofitted with forced aeration on the second stage, which enabled a higher ammonia removal with concentrations in final effluent of 95th percentile concentration of 5.3 mg∙L⁻ ¹comparing to 9.6 mg∙L⁻ ¹achieved in the system with passive aeration. The first stage filters were observed to be hydraulically limited during the first year of operation, with a hydraulic conductivity (HC) of <5.7× 10⁻⁶ m∙s⁻ ¹ which caused prolonged ponding occurrence. This in turn led to poor re-oxygenation of the first stage filter beds and low mineralization of the sludge layer deposited on its surface, which was characterized by TS and VS content of 11.5% ± 1.9% and 64.5% ± 7.3% respectively. A more mineralised sludge layer was present after three operational years, with HC <4.9 × 10⁻⁴ m∙s⁻ ¹ and sludge properties of 20.4% ± 3.5% TS and 62.3% ± 3.5% VS. These values are comparable to data reported for two-stage VFCWs operated in France and ponding was minimal after three years, suggesting this as the required length for system maturation. Close monitoring of the system during the first three years of operation evidenced the need to adapt rotating period lengths according to system age and sludge layer properties. Resting periods of up to 14 days were required for the young system to encourage sufficient hydraulic conductivity of the sludge layer, while the conventional 3.5 days feed/ 7 days rest cycle is appropriate after three years of operation. The influence of sludge properties on permeability was assessed in laboratory experiments, as well as artificial modifications of the sludge surface that could sustain increased permeability and accelerate sludge layer mineralisation. A positive impact of increased VS on permeability was found, with an increase in VS from 60% to 75% improving permeability from 6.14 × 10⁻ ¹⁷ m∙s⁻ ¹ to 2.00 × 10⁻ ¹⁶ m∙s⁻ ¹ in samples with TS of 9% and from 2.06E × 10⁻ ¹⁷ m∙s⁻ ¹ to 1.25 × 10⁻ ¹⁶ m∙s⁻ ¹ in samples with TS 15%. A greater relative contribution to permeability of preferential flow pathways (PFPs) present in the sludge layer than of sludge properties (TS, VS) was observed. PFPs were scarce in the young full scale primary VFCWs, which was believed to be the reason for their poor hydraulic acceptance. A positive effect on PFP formation and sludge drying rate was observed when artificially modifying the sludge layer by application of secondary activated sludge or by making initial cuts on its surface. Artificial modifications of the sludge layer could be implemented in full-scale systems to improve hydraulic acceptance and re-oxygenation, to shorten maturation of primary filters and to maintain prolonged operational periods without desludging.
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    ItemOpen Access
    Maximising the value of trade waste.
    (Cranfield University, 2020-12) Berzal De Frutos, Olivia; Bajón Fernández, Yadira; Pidou, Marc
    Anaerobic digestion (AD) of sewage sludge is a typical treatment method used before its disposal or land application as it has been shown to reduce pathogens, stabilise the sewage sludge and reduce sludge volumes, generating at the same time biogas for energy generation. Anaerobic co-digestion (Aco-D) is used to balance the characteristics of two or more waste in AD to improve performance and increase biogas yield. Aco-D of sewage sludge and trade wastes (TWs) has been seen to have a beneficial effect by balancing nutrients and toxicity. Currently, TWs typically with a lower chemical oxygen demand (COD) concentration of below 50g/L are accepted on large wastewater treatment for treatment with the municipal wastewater in the biological nutrients removal process (BNR). This may have the benefit of balancing the organics/nutrients ratio in the systems but the increased load in both organics and nutrients generally leads to an increase in the cost of treatment which is expected to be compensated by the charge (gate fee) applied to take these TWs. Alternatively, TWs, especially with greater COD content (> 50 g/L) could be considered for Aco-D with sewage sludge on those same sites but this approach requires careful control of TWs loads as they are known to contain toxic compounds that could affect the AD process. Furthermore, the lower strength TWs could also be considered for Aco-D, if concentrated to reach suitable organics content for addition to the AD reactor. However, there is very limited literature on the Aco-D of sewage sludge and TWs, especially when considering the wide range of wastes accepted on sites. Therefore, the potential to use Aco-D on high and low strength TWs was considered in this study. The aim of this project was to understand the effect of TWs in Aco-D with sewage sludge to maximise the benefits and reduce the negative impact of TWs. To achieve this goal, batch and semi-continuous AD experiments with sewage sludge were carried out to investigate the impact of addition of key components of TWs, identified through the study of a data containing the characteristics of 160 TWs and ultimately identify concentrations range in which the compounds may be beneficial or inhibitory. In the batch experiments, methane yield and lag phase inhibitions were observed from 1764 mg TAN/L, 592 mg Zn/L, 22 mg Cu/L, 464 mg Al/L, 4535 mg Cl/L, 162 mg SO₄/L, 92 mg NO₃/L, 25 mg Hg/L and 17 mg As/L using Ammonia sulphate, Zinc chloride, Copper chloride, Aluminium sulphate, Sodium chloride, Sodium sulphate, Sodium nitrate, Mercury sulphate and Sodium arsenate, all highlighting the importance of managing loads on the AD plant. Zinc sulphate and sodium nitrate were further studied in semi-continuous experiments to understand the possible long- term impacts of the compounds in the reactor over long-term operation. Inhibitory values were identified for Zn from 700 mg Zn/L and from nitrate from 2000 mg NO3/L decreasing biogas production to 0 and 20-38 mL/% VS red/d compared with the value for control reactors with only sewage sludge of 80-100 mL/% VS red/d. Accumulation in the solid fraction was observed in the zinc experiments, while nitrate showed an inhibition of the methanogenesis because of the generation of by-products from the denitrification process. The concept of pre- concentrating low organic content TWs in a dead-end filtration cell using microfiltration (MF) and ultrafiltration (UF) membranes was studied, with the concentrates produced and then tested in a batch AD test as 10% of the feed with sewage sludge. The results showed that most of the TWs studied, including wastes from the food, chemical, leather, personal care products and paper industries showed non-effect or beneficial in the Aco-D with sewage sludge except for a resin TWs. Finally, it was demonstrated that an economic benefit can be expected from the co-digestion of the concentrates with sewage sludge from most of those TWs based on the avoided cost of treating the TWs in the BNR and the increase in methane production. Overall, this work demonstrated that it is possible to maximise the value of trade wastes through co- digestion with sewage sludge, but careful addition will be required to ensure optimum operation of the AD.
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    ItemOpen Access
    Strategies for controlling nucleation and crystal growth in membrane distillation crystallisation.
    (Cranfield University, 2023-06) Ouda, Alaa Samir; McAdam, Ewan; Bajón Fernández, Yadira
    Membrane distillation crystallisation (MDC) is a promising technology that can address the primary restriction of regulating supersaturation rate in conventional crystallisation technologies, thus affording good control over nucleation and crystal growth. The supersaturation rate (R’) was used as a key parameter to govern the onset of nucleation (induction time) instead of irreversible flux decline, which is typically used in previous MDC studies. In this work, in-line turbidimetry enabled the precise determination of the induction time and metastable zone width (MSZW), which have been correlated to the supersaturation rate for a detailed characterisation of the nucleation kinetics and growth mechanism. The supersaturation at which nucleation occurred was measured within the interfacial boundary layer and bulk solution to differentiate between two scaling mechanisms (crystal adhesion or deposition) associated with the underlying nucleation mechanism based on classical nucleation theory. This allowed the MDC to decouple surface scaling from bulk nucleation at specific MSZW regions where the achieved crystallisation trajectory could balance between crystal quality and reduced membrane scaling. The increased supersaturation rate independent of the boundary layer enabled the transition towards a homogeneous nucleation mechanism where scaling is minimised, which aligns with classical nucleation theory but contradicts previous MDC studies. The consistent and reproducible nucleation kinetics obtained in this study while controlling multiple supersaturation factors suggested crystallisation to be inherently scalable, which is a unique facet compared to conventional crystallisers. Furthermore, the ability to sustain the supersaturation profile following nucleation enabled the system to reposition the crystallisation trajectory close to the MSZW threshold, providing high potential for nucleation kinetic control while regulating crystal size, size distribution and yield. Therefore, the kinetic framework established in this work offers advanced control over the nucleation mechanism and growth phase, which can be employed to fulfil commercial requirements for zero liquid discharge and more valuable crystalline products.
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    Vertical flow constructed wetlands for treating unscreened sewage in the UK
    (Cranfield University, 2016-09) Pereira Gomez, Ledicia; Bajón Fernández, Yadira; Jefferson, Bruce
    The use of two stage vertical flow constructed wetlands (VFCWs) for sewage treatment post coarse screening is an established option in France. The need to reduce the energy and maintenance requirements associated with small sewage works remains a key objective to the UK water industry. Two stage VFCWs have been identified as a candidate technology to meet these aspirations. However, there is a paucity of information concerning operation and performance during the start-up period which could last up to two years as well as knowledge transfer relating to differences in hydraulic and organic loading patterns. Accordingly, the UK’s first two stage VFCWs for municipal sewage treatment has been recently built and operated to assess its suitability. Overall, the site performed similar to values reported in the literature regarding total suspended solids, biological demand and ammonium-N being respectively 6.2 ± 3.4 mg·L-1 , 5.6 ± 2.6 mg·L-1 and 5.8 ± 3.8 mg·L-1 compared to literature values of 10 ± 10 mg·L-1 , 6 ± 4 mg·L-1 and 5 ± 6 mg·L-1 , based on composite sampling. However, a key difference compared to operating systems in France was sustained operating periods beyond the design hydraulic load leading to long periods of surface ponding. This had two major impacts: a limiting ability to re- oxygenate the filter body affecting the nitrification performance and retardation of the sludge mineralisation rate reducing the operating infiltration rate and hydraulics of the filters. This highlights the hydraulic limitations of the young filter (5 months of operation) especially in winter conditions. Future work has been suggested in order to adapt the technology to UK conditions such as extending first stage, optimising feeding strategy, using a storm and first stage overflow constructed wetland, aeration of the second stage or design based on infiltration rate.