Browsing by Author "Khomenko, Olha"
Item Open 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, GabrielaThe 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.Item Open Access Influence of sludge layer properties on the hydraulic behaviour of gravel-based vertical flow constructed wetlands for primary treatment of sewage(Elsevier, 2019-07-09) Khomenko, Olha; Dotro, Gabriela; Jefferson, Bruce; Coulon, Frederic; Bajón Fernández, YadiraSludge accumulation on the first stage of French design vertical flow constructed wetlands has been reported to improve treatment performance by favouring even sewage distribution on the beds' surface and increasing water retention time. However, due to its relatively low permeability, sludge layer can restrict the hydraulic capacity of the wetlands, requiring careful consideration of the feeding and resting strategy in order to avoid system ponding. This study aimed to elucidate the impact that sludge layer properties have on its permeability and investigate artificial modifications that could enhance it. A positive impact of increased organic matter content on sludge permeability was observed, with a 3-times permeability increase for a 15 percentage points higher volatile solids content. The opposite effect was observed for total solids, where an increase of 19 percentage points led to a drop of 1.6 × 10−16 m2 on permeability. The impact of different surface modifications on drying kinetics and sludge layer properties was studied as a means to accelerate sludge layer mineralisation. Artificial modifications that modify surface tension of the sludge layer have been proved to achieve greater permeability and faster mineralisation of the sludge, with potential to achieve higher hydraulic acceptance and reduced operational costs (lower sludge accumulation) if implemented in full scale vertical flow constructed wetlands.