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Treating algal blooms in source water with advanced oxidation

Treating algal blooms in source water with advanced oxidation

According to Anderson Regional Joint Water System officials, the effectiveness of the preoxidation process is bringing operational efficiencies to its media filtration system and reducing the amount of chemicals needed in the process.

Taste and odor issues in South Carolina drinking water are resolved through a formulaic approach.

By Steve Green and Stephanie Smith

After conventional treatment methods did not resolve ongoing taste and odor issues in their finished potable water, officials of the Anderson Regional Joint Water System (ARJWS) explored advanced treatment technologies to ensure they could continue to provide palatable water to their 200,000 customers in Upstate South Carolina.

The ARJWS pumps about 48 million gallons per day (mgd) from Lake Hartwell to 15 municipalities and agencies. Although the drinking water met drinking water quality standards, ARJWS was receiving hundreds of complaints each week of musty-smelling and bad-tasting water caused by nontoxic compounds in the lake. For many utilities like ARJWS that rely on surface water, the presence of unpleasant taste and odors in drinking water is a serious and increasingly frequent concern, requiring sustainable and holistic solutions.

ARJWS officials were intent on solving these issues and engaged engineering consultants Goodwyn, Mills and Cawood (GMC) to implement a $13.5 million treatment system upgrade at the facility. GMC teamed up with water technology provider Xylem Inc. to begin the multiphase process of determining the most viable treatment option in terms of treatment objectives, life-cycle costs, and total cost of ownership.

Water quality issues at ARJWS were the result of the increasing occurrence of harmful algae blooms (HABs) in Lake Hartwell. Heavy rains in 2013 led to elevated runoff of algae-stimulating nutrients into the lake, and with warm summer temperatures, conditions were perfect for an algal bloom. A larger bloom in 2014 — and evidence that algal blooms were increasing in duration and frequency throughout the region — made it clear that the problem was unlikely to subside in the foreseeable future.

Across the United States, many types of algae can cause HABs in both freshwater and marine systems. In freshwater drinking water sources, HABs are typically an overgrowth of cyanobacteria, also known as blue-green algae. Cyanobacteria are best known for their production of potentially harmful toxins, which can be released in even heavier doses as the algae die and their cells break open. The danger presented by the toxins depends on the type and amount produced, and toxins have been an area of intense focus for state agencies and the EPA, tasked with overseeing the provision of safe drinking water.

Cyanobacteria also produce nontoxic compounds like geosmin and 2-Methylisoborneol (MIB), the source of the “dirty-tasting” water that had plagued ARJWS from 2013 to 2017. In 2014 alone, geosmin and MIB levels increased from about 50 to 2,000 parts per trillion, very high levels considering most people can sense the smell or taste at concentrations of 10 to 20 parts per trillion.

Conventional treatment systems can be overwhelmed by these algal compounds during an intense bloom. Prevention of algal growth is a global challenge due to climate change and intensive agricultural practices, things that water managers have little control over. Removing the algae upon intake is possible with coagulation or filtration, but the sheer volume of biomass is difficult for water treatment plants to manage, and processes that cause the cells to die can worsen the situation because the toxins and odorous compounds are released into the water. Once released into the water, treatment options become fairly narrow.

Officials at the ARJWS were familiar with all of these challenges. They tried various recommended best practices such as copper- and peroxide-based algaecides for in-lake treatment. Adding powder activated carbon and chlorine dioxide within the treatment plant to adsorb and oxidize the objectionable compounds also met with limited success due to the high concentrations being generated by the HABs.

The goals of the water treatment plant upgrade were to eliminate seasonal taste and odor events and color associated with naturally occurring iron and manganese and establish resilience against algae-linked compounds and other contaminants of emerging concern (CECs).

A construction management at risk project delivery approach was chosen to ensure success, meet the aggressive schedule, and provide the most value through a collaborative relationship among the designer, builder, and equipment supplier.

Xylem provided treatability testing and analysis of four treatment options: ozone along with various combinations of three advanced oxidation processes (AOPs) — ozone, hydrogen peroxide, and/or ultraviolet (UV) light — to meet the treatment goals.

Ozone is generated by means of a silent electrical discharge in an oxygen-containing gas. As soon as ozone is introduced into water, any hazardous pollutants present are effectively degraded through oxidation without creating harmful chlorinated byproducts or significant residues. By decomposing into oxygen as it reacts, ozone provides a cost-effective and environmentally responsible alternative to oxidation with chlorine, absorption (activated carbon), or separation processes (reverse osmosis).

AOP is the combination of two or more processes to generate hydroxyl (OH-) radicals. Compared to other oxidants, OH- radicals have considerably higher oxidation potential, and once formed in water they immediately attack virtually all existing oxidizable substances. The high degradation performance and the quick reaction kinetics of AOP provide the formula for success when it comes to eliminating numerous persistent contaminants.

The processes used in AOP — ozone (O3), hydrogen peroxide (H2O2), UV radiation, and chlorine — are powerful treatment technologies by themselves. The key to selecting the best AOP solution is to find the right combination of these processes to most efficiently generate OH- radicals that reduce the seemingly nondegradable contaminants, rendering them harmless.

Many of the treatments to adsorb or destroy toxins and taste and odor compounds are also effective for CECs. HAB impacts can be seasonal in nature, so a complete analysis considers turndown capabilities and the associated savings for an accurate life-cycle cost. In addition, the overall analysis should consider future treatment objectives and anticipate future regulation to the extent possible.

At ARJWS, four rounds of rigorous bench-scale testing at varying oxidant doses and MIB concentrations were performed to provide critical information on contact time, chemical dose, equipment dimensions, equipment head loss, and energy requirements. All technologies proved effective in removing MIB from an influent concentration of 400 ng/L to 4 ng/L — below the threshold detectable to humans.

This comprehensive and formulaic approach, combined with the capabilities of Xylem’s AOP treatability testing lab, led to selection of the ozone + peroxide process for ARJWS due to its ability to feed ozone alone and augment with peroxide for quenching ozone residuals. This process was considered the most viable option to address current and future taste and odor, color, and CEC impacts for the lowest combined capital and operating costs over the expected project life. Over a 20-year period, Ozone AOP was the least costly at an estimated capital cost of $12.5 million and an annual operations and maintenance cost of $378,000.

As the project moved into the design phase, GMC selected Xylem to provide the full-scale system because of its ability to provide a cost-effective, reliable solution on an accelerated timeline. Xylem also ensured optimization of the overall plant upgrade design concurrently with the equipment manufacturing process.

The project was completed ahead of schedule and under budget by general contractor Brasfield & Gorrie, coming online in the spring of 2018, just in time for the beginning of the seasonal algae blooms. ARJWS officials report they are completely satisfied with the solution and say the effectiveness of the preoxidation process is bringing operational efficiencies to its media filtration system and reducing the amount of chemicals needed in the process.

At the start of the algae bloom season, the system is measuring MIB of 11.2 and 17.7 ng/L and geosmin of 0.5 to 4 ng/L in the inlet of the system and non-detect in the outlet. Xylem and utility managers will continue close monitoring of the system as higher inlet levels of MIB and geosmin are expected through the summer months.

For other utilities experiencing threats to their source water due to nutrient pollution, AOP treatment technology is an increasingly attractive solution. While there are no general rules regarding which processes are the most cost effective due to the unique circumstances at each facility, lab or pilot testing provides valuable information on the most viable treatment options so utilities can confidently provide high-quality, clean, safe, and reliable drinking water to their customers.

Steve Green is the Water Utilities business development manager for Xylem Inc. (www.xylem.com/en-us), based in Portland, Ore. He is an advisor member of the Water Design Build Council (https://waterdesignbuild.com), a not-for-profit organization dedicated to evolving best practices in water design-build delivery. Stephanie Smith is product manager, Environmental Monitoring Systems, Xylem Inc. She is based in Columbus, Ohio.