Full Scale Ozone Performance for Direct Filtration –

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.