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Coventry, United Kingdom

Nivala J.,University of Aarhus | Nivala J.,Helmholtz Center for Environmental Research | Knowles P.,Natural Systems Utilities LLC | Knowles P.,Aston University | And 4 more authors.
Water Research | Year: 2012

This paper reviews the state of the art in measuring, modeling, and managing clogging in subsurface-flow treatment wetlands. Methods for measuring in situ hydraulic conductivity in treatment wetlands are now available, which provide valuable insight into assessing and evaluating the extent of clogging. These results, paired with the information from more traditional approaches (e.g., tracer testing and composition of the clog matter) are being incorporated into the latest treatment wetland models. Recent finite element analysis models can now simulate clogging development in subsurface-flow treatment wetlands with reasonable accuracy. Various management strategies have been developed to extend the life of clogged treatment wetlands, including gravel excavation and/or washing, chemical treatment, and application of earthworms. These strategies are compared and available cost information is reported. © 2012 Elsevier Ltd.

Dotro G.,Cranfield University | Dotro G.,Waste Water Research and Development | Jefferson B.,Cranfield University | Jones M.,Waste Water Research and Development | And 3 more authors.
Environmental Technology | Year: 2011

Intermittent aeration of activated sludge plants (ASPs) is a potential strategy that may help deliver reduced operational costs while providing an adequate effluent quality. This review paper critically assesses the implications of temporary turning aeration off in continuous flow nitrifying ASPs, including impact on dissolved oxygen concentrations, process biology and operational parameters. The potential savings and pitfalls of the approach are further illustrated through an example scenario. Findings from this review indicate rapid dissolved oxygen depletion times of 1-60 minutes and a significant reduction of nitrification rates from 0.12 to less than 0.04 g NH 4-N/g VSS/d. Further negative impacts include a potential increase in nitrous oxide emissions from 0.07% to 27% N 2O-N per mole of NH 4-N oxidized; enhanced filamentous bacteria growth; a noticeable increase in effluent turbidity developing within one hour of air supply interruption; and, if no mechanical mixing is in place, risk of mixed liquor suspended solids settling in the bioreactor within short times (23-53 min). However, the potential savings in terms of aeration costs could amount to 33%-45% if instrumentation adequacy and impact on process biology and carbon equivalent emissions are excluded from the economic analysis. Further research on the areas of nitrous oxide emissions and the use of hybrid systems to provide resilience and robustness to the intermittent operation of continuous flow nitrifying ASPs is recommended. © 2011 Copyright Taylor and Francis Group, LLC.

Aboobakar A.,Cranfield University | Cartmell E.,Cranfield University | Stephenson T.,Cranfield University | Jones M.,Waste Water Research and Development | And 3 more authors.
Water Research | Year: 2013

This paper reports findings from online, continuous monitoring of dissolved and gaseous nitrous oxide (N2O), combined with dissolved oxygen (DO) and ammonia loading, in a full-scale nitrifying activated sludge plant. The study was conducted over eight weeks, at a 210,000 population equivalent sewage treatment works in the UK. Results showed diurnal variability in the gaseous and dissolved N2O emissions, with hourly averages ranging from 0 to 0.00009 kgN2O-N/h for dissolved and 0.00077-0.0027 kgN2O-N/h for gaseous nitrous oxide emissions respectively, per ammonia loading, depending on the time of day. Similarly, the spatial variability was high, with the highest emissions recorded immediately after the anoxic zone and in the final pass of the aeration lane, where ammonia concentrations were typically below 0.5 mg/L. Emissions were shown to be negatively correlated to dissolved oxygen, which fluctuated between 0.5 and 2.5 mgO2/L, at the control set point of 1.5 mgO2/L. The resulting dynamic DO conditions are known to favour N2O production, both by autotrophic and heterotrophic processes in mixed cultures. Average mass emissions from the lane were greater in the gaseous (0.036% of the influent total nitrogen) than in the dissolved (0.01% of the influent total nitrogen) phase, and followed the same diurnal and spatial patterns. Nitrous oxide emissions corresponded to over 34,000 carbon dioxide equivalents/year, adding 13% to the carbon footprint associated with the energy requirements of the monitored lane. A clearer understanding of emissions obtained from real-time data can help towards finding the right balance between improving operational efficiency and saving energy, without increasing N2O emissions. © 2012 Elsevier Ltd.

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