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Reducing Energy Footprint of a Waste Water Treatment Plant by Increasing Harvesting Efficiency of Solids During Primary Clarification
Conferences & Talks
23 October 2013
This presentation describes a novel hydrodynamic separation (HDS) technology that has the potential to dramatically reduce the energy footprint of a wastewater treatment plant (WWTP) by reducing the energy required for aeration and by increasing biogas production to offset plant energy demand. These goals are achieved because of the ability of HDS to effectively harvest from primary effluent substantial amounts of those organic solids, which are nearly neutrally buoyant and do not sediment out, before they enter the secondary treatment step. Most WWTPs include a primary treatment consisting of clarifiers to remove settleable solids before biological treatment. This process is sometimes enhanced by chemical precipitation to remove a greater portion of the suspended solids. Biodegradable solids not removed in primary treatment translate into greater oxygen demand in the downstream biological processes. In addition, organic solids harvested in primary treatment have higher energy content than the biomass in the waste activated sludge, both of which are often anaerobically digested to produce biogas for energy. Therefore, improved primary treatment performance can yield energy benefits not only from the increased mass of organic solids for biogas production but also from reduced oxygen demand (aeration) in secondary treatment. Hydrodynamic Separation (HDS) is a purely fluidic approach to concentrate suspended solids in a liquid. By carefully controlling the geometry and the flow through a curved channel the resulting hydrodynamic forces will cause particles beyond a certain cut-off size to focus near one side wall, independent of their density (Figure 1). By having two outlets near the exit, with for example an 80:20 split ratio, particles can be concentrated into the stream with 20% of the flow (concentrate stream), while the other 80% of the flow (effluent stream) has a greatly reduced concentration of suspended solids. Further concentration can be achieved by cascading several HDS stages and using the concentrate stream of one stage as the input for the next stage. Depending on the type of suspended solids concentrations in excess of 1 wt % for loose organic matter and aggregates to concentrations in excess of 10 wt % for dense solid particles have been achieved. Using an 80:20 laboratory HDS prototype, we have demonstrated successful separation of 50-70 % of the suspended solids in primary effluent samples from two different WWTPs. These samples have gone through primary clarification and represent the plant streams that have the most difficult-to-settle solids. An average plant’s primary clarification only removes 40% of the influent solids, the settleable portion with higher specific density, undesirably overflowing much of the suspended solids to the bioreactor. We anticipate by connecting multiple HDS in series for the concentrate streams, primary solids can be dewatered to reach a sludge concentration similar to that of a primary clarifier (Figure 2). In this paper, introduction to the basic physical concept behind HDS and the impacts of geometry and channel design on harvesting efficiencies will be presented. Besides discussing separation results using single channels in the laboratory, we will present first data on a pilot system that is scheduled to be moved to a local (Silicon Valley, California) WWTP in late summer of 2013 for tests in a real environment. Construction of this trailer-mountable pilot system capable of unattended operation at a flow rate upward of 10,000 gallons per day with complete fluidic actuation, sensing, pumping, controls and data recording is underway. This California State Energy Commission-sponsored pilot study will be furnished with third–party engineering Measurements and Verification that include HDS energy consumption and calculations for biogas production, installed cost, payback time, and economic impacts when and if HDS-based water treatment systems are widely deployed in wastewater treatment plants for primary solids harvesting in the State of California and beyond.
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