Browsing by Author "Barak, Jan"

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    Analysis of phosphorus flux in reed beds at chemically dosed wastewater treatment works
    (Cranfield University, 2014-05) Barak, Jan; Dotro, Gabriela; Jefferson, Bruce
    Phosphorus (P) as a macronutrient contributes to eutrophication. In the UK, iron (Fe) and aluminium (Al) salt dosing is a well-established wastewater treatment strategy for its removal. Small works with a population equivalent (p.e.) under 2,000 often employ horizontal subsurface flow constructed wetlands (HSSF CWs) as a means for subsequent tertiary (3°) treatment. Although these significantly improve the final effluent’s quality, P release has, on occasions, been observed. This study attempts to contribute to a better understanding of P flux mechanisms in reed beds and to outline a mitigation strategy countering P release. The literature review identified that, in given circumstances, the P concentration gradient, redox conditions, pH and Fe cycling are key potential factors governing P flux. The field survey revealed that secondary (2°) P effluent concentration negatively correlated with P release from the reed bed. In laboratory scale sludge reactor series, P concentration in wastewater was observed to be buffered by molecular diffusion driven by a concentration gradient in the sludge-wastewater interface. The instantaneous equilibrium point appeared to lie in 0.1 to 0.5 mg/L interval in the first 10 minutes, shifting to 1 mg/L in the next 8 hours and higher in the later stages. In biologically active systems, the shift of the equilibrium point seemed to be dominated by changes in redox potential linked to simultaneous microbial utilisation of oxygen (O2) and nitrate (NO3 - ), eventually leading to a reduction of Fe (III) and sulphate (SO4 2- ), with subsequent P release. The start of Fe (III) reduction coincided with reductive depletion of nitrate-nitrogen (NO3-N) below 1 mg/L. In systems with limited biological activity, P release was linked to disassociation from Fe-P compounds under decreasing pH. In an experiment assessing hydrodynamics, an increase was recorded in Fe and P flux fluctuation due to convection. Based on the findings, maintaining hydraulic residence time (HRT) under 24 hours and reed bed influent in concentrations above 0.5 mg/L total phosphorus (TP) and 15 mg/L NO3-N is proposed as a means to prevent or delay P release.
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    The role of concentrations gradients on phosphorus and iron dynamics from chemically-dosed horizontal flow wetlands for tertiary sewage treatment
    (IWA Publishing, 2019-06-24) Barak, Jan; Dotro, Gabriela; Jefferson, Bruce
    This study examined the dynamics of iron (Fe) and phosphorus (P) transformations from the surface sludge accumulated in tertiary horizontal flow (HF) treatment wetlands (TW) chemically dosed for P removal. Site surveys showed P was stored in HF TW with and without artificial aeration on average, with instances of P release in the non-aerated site. Controlled experiments revealed storing TW surface sludge for over 24 hours resulted in limited oxygen and nitrate concentrations, resulting in both P and Fe release. The rate of P release increased with increasing water-sludge P concentration gradients, and the reaction could take as little as 10 minutes. Convection had no impact on P transformation rates. The findings suggest mitigation strategies could include the manipulation of the biogeochemical environment by managing oxygen and nitrate concentrations within the wetlands. A better understanding of links between Fe, P, and nitrate is needed to test proactive mitigation strategies for small wastewater treatment plants.