Getting To the Bottom of Scum – A New Process for the Separation and Dewatering of Clarifier Scum
Annual Conference
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New Plymouth District Council (NPDC) have recently completed a major upgrade of their sludge thickening facility at New Plymouth Wastewater Treatment Plant (WWTP), to substantially improve the performance of the existing Picket Fence Thickeners. This includes the introduction of a new process to remove water from the scum (consisting of fats, filamentous microbes, floating solids, much from trade waste sources), which is collected and removed from the surface of the secondary clarifiers, and introduced into the sludge stream.
Prior to the upgrades, scum was collected and incorporated directly into the Waste Activated Sludge (WAS) stream, so that it could be treated through the sludge thickening, dewatering and drying processes. This led to significant quantities of carry-over water being introduced into the solids stream, which resulted in poorer sludge thickening and dewatering performance, and ultimately greater energy use in the thermal drying process. It also caused carry-over of scum to the surface of the Picket Fence Thickeners (PFTs), which resulted in the scum being reintroduced to the headworks.
As part of the upgrades, the scum was separated from the WAS pumping process and a Baleen Filter was installed to remove free water from the scum, prior to the solids being incorporated into the thickened sludge storage tank (prior to dewatering). The Baleen Filter is typically used in the food industry to dewater products or waste streams, but was identified as a novel cost-effective approach to remove free water from scum at New Plymouth WWTP. Based on trials undertaken by NPDC, further enhancement of the Baleen Filter process has been made by retrofitting a flocculation system.
This paper presents the project from inception to operation, including the outcomes of early trials and the key matters and lessons learnt that influenced the design and construction, recognising the increasing importance of effective scum management in WWTP operations.