Browsing by Author "Graham, Nigel"

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    Evaluating the impact of underwater skimming on slow sand filter performance and operation
    (Elsevier, 2025-05-15) Elemo, Tolulope; Chipps, Michael; Graham, Nigel; Turner, Andrew; Bretagne, Sophie; Jefferson, Bruce; Hassard, Francis
    Conventional cleaning of slow sand filters (SSFs) requires the beds to be drained before a layer of media and the Schmutzdecke are removed, called ‘dry skimming’ (DS), which can result in significant downtime. An alternative is proposed whereby the filter is skimmed whilst still submerged, called ‘underwater skimming’ (UWS). Previous attempts to avoid draining the bed have led to concerns about the risks of UWS in terms of headloss development, particle penetration, and microbial water quality. In this study, pilot scale SSFs, cleaned by either DS or UWS, were operated concurrently, to compare and assess the potential risks of UWS in terms of filtrate quality, microbial removal, and recovery following skimming. While all filters exhibited effective turbidity removal (0.04–0.9 NTU turbidity from outlet), UWS filters had improved performance immediately after cleaning compared to DS in terms of recovery of filtrate microbial water quality. Specifically, total coliforms in the UWS filter outlets, in the first seven days post-skimming, ranged from 1 to 109 most probable number (MPN)/mL, compared to 1 to 1414 MPN/mL for DS filters. Both methods yielded satisfactory headloss recovery, indicative of limited particle penetration at depth, and effective cleaning. Exploring different sweetening flow rates during UWS revealed no observable differences in headloss, turbidity reduction, or microbial quality between the flow rates tested. Schmutzdecke microbial community was similar irrespective of cleaning method and was governed instead by seasonal changes and the ripening process. The improved microbial removal afforded by UWS provides a means of significantly improving water productivity and enabling options for much better utilisation of SSFs.
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    Predicting the impact of underwater skimming on dissolved oxygen consumption in slow sand filters for potable water treatment
    (Elsevier, 2024-12-01) Elemo, Tolulope; Chipps, Michael; Graham, Nigel; Turner, Andrew; Jefferson, Bruce; Hassard, Francis
    In a well-functioning slow sand filter (SSF), dissolved oxygen (DO) is crucial for enabling aerobic processes and microbiota growth. Given that DO supply is predominantly via the feed water, flow pauses (e.g., during cleaning) may trigger anoxic/anaerobic conditions in the stagnant filter bed. Underwater skimming (UWS) is an advanced cleaning technique that employs a skimmer with a shrouded blade, mounted on a mobile platform, to remove the fouling layer composed of sand and particles in order to improve the efficiency of slow sand filtration. As UWS results in changes to the flow pattern of the SSF, a mathematical model was developed to predict DO utilization after a flow perturbation associated with UWS operation. The model was based on a depth resolved measurement of specific oxygen utilization derived from a full scale SSF. Pilot plant experiments monitored DO in the feed and filtrate of SSFs cleaned using underwater and conventional dry skimming techniques. The highest oxygen utilization was in the Schmutzdecke layer, with additional demand imposed by the presence of a granular activated carbon (GAC) sandwich layer. It was observed that pseudo-steady state conditions occurred following filter ripening, where DO utilization, driven by biological activity, remained relatively constant regardless of filter cleaning technique. For flow pauses between three and 24 h, the pause duration's importance decreased, while the hydraulic loading rate became the critical factor for DO recovery in the filter. Additionally, introducing a 'sweetening flow' during UWS ensured a continuous DO supply, facilitating quicker DO replenishment post-cleaning. The model reliably predicted filtrate DO within ±0.6 mg/L, demonstrating its operational utility, especially in the optimisation of UWS methodology. As such, UWS can be applied to clean SSFs with the methodology modified to prevent any detrimental effects to DO management within the filter. This study predicted DO dynamics in SSFs, advancing UWS techniques and could be applied for enhancing water treatment strategies by filtration.
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    Understanding the risk of enhanced particle penetration into slow sand filter beds when using underwater skimming techniques
    (Elsevier, 2025-06-01) Elemo, Tolulope; Chipps, Michael; Graham, Nigel; Turner, Andrew; Jefferson, Bruce; Hassard, Francis
    This study evaluated abiotic slow sand filters (SSFs) to understand the risk of particle penetration during underwater skimming (UWS), focusing on clogging, headloss development, and particle breakthrough. Pilot-scale filters containing clean sand were challenged with dispersed kaolin particles to simulate surface accumulation, and the sand surface was agitated to mimic UWS procedures. The study was undertaken with no maturation period to consider the worst-case scenario corresponding to the period just after filter skimming. Agitating the surface and restarting flow released captured particles, some moving downward through the filter. Shallow filter depths resulted in particles appearing in the filtrate, but increasing the media depth beyond 500 mm minimized this effect. Since 90 % of headloss occurred in the upper layers, deeper particle penetration was insignificant. Increasing the hydraulic loading rate from 0.3 to 0.5 m/h reduced particle retention by 0.72 log, yet all abiotic SSFs achieved over 2 log particle capture. Small particles (2–10 μm) were removed by 2 logs, indicating sufficient non-viral pathogen retention under routine conditions. Effective capture of particles sized 2–125 μm suggested minimal risk to water quality and public health during UWS on full-scale SSFs. Using clean sand and kaolin represented a worst-case scenario, excluding biological maturation and particles. The findings suggest that under normal conditions, UWS does not increase deep particle penetration or breakthrough, supporting its safe implementation to enhance filter maintenance without compromising water quality.
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    Underwater remote skimming of slow sand filters for sustainable water production
    (American Chemical Society, 2022-08-16) Hassard, Francis; Elemo, Tolulope; Chipps, Michael; Turner, Andrew; Jefferson, Bruce; Graham, Nigel
    Slow sand filters (SSF) are a simple water treatment technology providing an important alternative to conventional drinking water treatment. SSF are extensive in terms of carbon cost and chemical use but require a large land area and are complex to operate, as periodic cleaning is required to prevent filter clogging. Therefore, redundant SSF beds are required to enable water production to occur during long cleaning downtimes. Underwater skimming (UWS) is a cleaning innovation where the foulant layer (containing sand and particles) is removed using a skimmer consisting of a shrouded blade mounted on a vehicle platform. Sand, particles, and biofilm are skimmed prior to ex situ washing of the recovered sand. In this Viewpoint, we posit that the introduction of an in situ underwater skimmer operated remotely can substantially help to offset the aforementioned challenge of downtime, with its associated loss of production, enabling the technology to operate more efficiently and remain a pertinent and advantageous process option within modern water treatment facilities or possibly resource constrained settings. Otherwise, this resilient biotechnological process could be replaced by chemical and energy-intensive processes which increase the entropy of water treatment more than SSF. The anticipated benefits and challenges of UWS of SSF are discussed.