Ozone pretreatment process decreases membrane fouling, benefits small North Dakota facility
Ozone has been effectively used for disinfection in municipal water treatment applications since the early 1900s. More recently, a growing body of evidence is showing that ozone oxidation is very efficient at removing a whole class of organic compounds related to trihalomethanes (THMs), disinfection byproducts (DBPs) and the emerging endocrine disrupting compounds (EDCs). Despite the treatment benefits, the limitations of traditional ozone technology have restricted its use to mostly the larger—greater than 40 million gal per day (mgd)—treatment plants.
Over the years, smaller systems have explored ozone treatment, only to find its process engineering, maintenance and complexity limitations too costly and difficult to justify. Ironically, given today’s operating challenges, these same small- to medium-sized facilities are the ones who might benefit the most from ozone treatment.
Over the past 10 years, advances in process instrumentation, control and automation have made their way into ozone technology. More recently, a new class of modular ozone generators and smart ozone control technology has evolved. Together, these new technologies are a great fit for small- to medium-sized water and wastewater treatment facilities looking to improve performance while also reducing the cost and complexity of treatment.
North Burleigh WTP
The North Burleigh Water Treatment Plant (WTP) near Bismarck, N.D., is one example of a small facility that achieved significant benefits through the adoption of new ozone technology. By precisely matching ozone production to real-time demand, the North Burleigh plant has increased finished water production by more than 64% while simultaneously reducing total unit operating cost by more than 20%.
Originally constructed in 2005, the North Burleigh WTP uses ozone in both its pretreatment and disinfection process steps. Ozone in the pretreatment step oxidizes dissolved iron, manganese and total organic carbon (TOC) in the raw water to decrease fouling of the microfiltration-reverse osmosis (MF-RO) membranes. This application is particularly challenging due to continuously varying water quality from a mixture of surface and groundwater drawn from sand beds beneath the Missouri River. Overall, the combined ozone demand from both the pretreatment and disinfection steps could vary by 10% to 30% throughout a given day.
In 2010, the Burleigh plant increased its capacity to 2.5 mgd. A key objective of the upgrade was the installation of a new oxygen-based ozone system that could provide real-time process control, reduced maintenance and lower operating cost. After a thorough design review and competitive bidding, a new Pinnacle Ozone Solutions generator and injection system was selected based on operating advantages and lowest evaluated cost.
The newly installed ozone system offers a number of performance advantages, including higher output (up to 120 lb per day), higher ozone concentration (6% to 10%) and 0 to 100% turndown control. The modular design of the ozone generator provides built-in redundancy and a 35% smaller footprint than competing systems. Integral power supplies within each ozone module operate at 23 kHz, eliminating audible noise and large external transformer sets and significantly reducing electrical harmonics.
The new ozone system also includes a dedicated vacuum swing adsorption oxygen concentrator. Onsite oxygen supply is significantly lower in cost than liquid oxygen options and eliminates concerns about oxygen delivery and other maintenance. Most importantly, every aspect of ozone generation and process control is completely integrated into the Pinnacle control system, allowing for real-time optimization of all operating parameters to match actual process conditions.
Installation and startup of the new ozone system was completed in April 2011. Shortly thereafter, a series of devastating floods inundated much of the upper Great Plains and Bismarck area from May to September 2011. Despite the significant operating challenges, the new ozone system has remained online for more than 15,000 hours of continuous service with virtually no maintenance.
To date, the performance benefits of the ozone system have exceeded all expectations. Evaluation of operating data from the same seven-month period before and after the floods show that the Burleigh WTP has increased treatment capacity by 64% with a simultaneous 56% decrease in total unit operating cost over the same period. As a result of the increased capacity, the Burleigh WTP has reduced its wholesale purchases of water from the city of Bismarck by 50% to 80%. These results are especially remarkable because the plant experienced a net 8% increase in unit chemical cost, and 10% increase in unit power costs over the same period.
Overall, the performance and operating cost improvements from the new ozone system at the Burleigh WTP are ideal. The results revealed that carefully selected ozone equipment and advanced controls systems can deliver a host of operating benefits:
Based on the experience at the North Burleigh WTP, the South Central Regional Water District installed a second Pinnacle Ozone system for its new sister WTP in Emmons, N.D.