Full Scale Ozone Performance for Direct Filtration – How to Manage Lake Turn-Over Events and Widely Varying Flows and Demands Denise Funk PE BCEE1, Al Sosebee2, Hussein Khorramzadeh PE2, Chris Schulz PE BCEE3 1 CDM Smith - Atlanta Gwinnett County Department of Water Resources 3 CDM Smith - Denver 2 Abstract Since the 75-mgd Shoal Creek Filter Plant began operation in 2005, the ozone facility has encountered widely variable conditions including record low lake levels, very low plant flow rates with low ozone demand, and severe lake turn-over events with unusually high ozone demand. This paper discusses these operational challenges along with the process control approach and lessons learned from each situation. Located in Gwinnett County, Georgia, the Shoal Creek Filter Plant is a direct filtration drinking water facility with pre-ozone utilized for primary disinfection. The ozone system was designed to achieve 1-log Cryptosporidium inactivation and uses a vertical J-tube dissolution system, which improves ozone mass transfer efficiency and ozone residual stability. Additional benefits of the ozone system include oxidation of taste and odor compounds and enhanced microflocculation for the direct filtration process. The source water supply is Lake Sydney Lanier, a U.S. Army Corps of Engineers impounded reservoir on the Chattahoochee River. Raw water quality has historically been excellent with low turbidity and low total organic carbon, although conditions can change rapidly during lake turn-over events which occur in autumn and sometimes spring. Historical plant data clearly depicts how water quality conditions change in a matter of hours during lake turn-over events. In September 2011, raw water temperature increased by 5 oC and ozone demand increased from 1 to 2.8 mg/L during a 24-hour period (Figure 1). The operational approach used during this event required flexibility to move from typical control parameters based upon disinfection requirements to both disinfection and oxidation requirements to meet disinfection, filtration turbidity and taste and odor goals. Shoal Creek Filter Plant Ozone Dose, Demand and Crypto Inactivation Onset of Lake Turnover Fall 2011 12.0 Raw Water Flow Rate Average 30 mgd Range 29 to 35 mgd 25.0 Lake Turnover Raw Water Temp Increase 8.0 Applied dose (mg/L) 20.0 Transferred Dose (mg/L) Ozone Demand (mg/L) 6.0 15.0 Raw Water Temp Log Removal 21-Sep-11 20-Sep-11 19-Sep-11 0.0 18-Sep-11 0.0 17-Sep-11 5.0 16-Sep-11 2.0 15-Sep-11 10.0 14-Sep-11 4.0 Temperature (oC) and Log Crypto Inactivation 10.0 Concentration (mg/L) 30.0 Figure 1. Shoal Creek Filter Plant Operations during Lake Lanier Turn-Over, September 2011. This paper also presents the capital improvements that were made to the ozone system to meet a wider range of operational scenarios. The plant must routinely operate at flows as low as 27 mgd with applied ozone doses as low as 0.5 mg/L and an average of approximately 1.0 mg/L. During high demand events such as lake turn-overs, the ozone facility has needed to provide up to 5.0 mg/L and the ozone system must be capable of providing a dose of 3.0 mg/L for peak plant flows of 98 mgd. To provide better control during very low demand conditions, new ozone gas flow control stations were designed using dual control valves with each dedicated to a particular gas flow range. This dual valve approach has allowed for appropriate control valve sizing at typical low gas flow rates, while providing a high flow line to effectively control the gas feed for the upper range.
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