The goal of a wastewater treatment facility is to remove visible and invisible material from sewage so that it can be returned to the environment and used again. The elements removed include human waste, food scraps, chemicals, minerals and other hazards, which, if allowed to remain in the water, can break down, use up oxygen and adversely affect residents and wildlife. It’s a process that involves two to three steps, depending on state regulations, and includes filtration and clarification to remove solids, sediment and other pollutants.

To remove, deactivate and eliminate the presence of pathogenic microorganisms and discourage the spread of disease into the surrounding environment, wastewater must then be disinfected. For decades this process was primarily accomplished through the use of chemical chlorine treatments, both liquid and gas. Today municipalities are turning to ultraviolet light as an effective, safe, cost-conscious and eco-friendly way to get the job done.

The new UV disinfection system for Grants Pass Water Restoration Plant in Oregon uses low-pressure, high-output UV units to enhance energy efficiency and reduce long-term operations costs. (Photo provided)

In California, the city of Roseville’s Dry Creek Wastewater Treatment Plant has one of the largest LPHO systems ever designed to produce recycled water. (Photo provided)

Understanding UV disinfection

UV disinfection is a physical process in which harmful bacteria and other microorganisms are neutralized as they pass by submerged mercury arc lamps that are contained in enclosed chambers. According to the Environmental Protection Agency, the system transfers electromagnetic energy from the lamps to an organism’s genetic material, where it penetrates the cell wall and renders it incapable of reproduction. The system’s effectiveness depends on the characteristics of the water, the amount of UV radiation applied, how long an organism is exposed to the light and the configuration of the reactor.

“Science and water chemistry is all about disease response, how many pathogens are in the water and what is the target treatment,” said Andy Salveson PE, water reuse chief technologist for Carrollo Engineers of Walnut Creek, Calif. “We are not always targeting zero, but the level that will protect public health as well as the surrounding environment.”

A better option?

Salveson is a nationally recognized expert in water reuse and disinfection, with more than two decades of experience serving both the public and private sector in the research and design of water and wastewater treatment systems. He not only provides guidance and expertise on state-of-the-art technologies, but remains current on the latest information regarding reuse, planning, design and research in the industry as well.

He said although it may seem like a relatively new trend, UV disinfection is not new at all. It emerged in the early 1900s as a method of creating clean drinking water; however, reliability and cost concerns kept it on the back burner for decades. It wasn’t used as an application for wastewater until the latter part of the 20th century when the EPA rewrote the 1948 Federal Water Pollution Act, reintroduced it in 1977 as the Clean Water Act and began funding municipal projects designed to protect the integrity of water in communities nationwide.

“They knew they needed to help communities take care of the water supply in the ’70s, but the ability to treat huge amounts of water takes a lot of development and that did not occur until the ’80s,” he said. “By the ‘90s utilities started converting to UV, but it is an expensive process. It takes time to raise enough money to make that change. Unlike chlorine, which is an annual cost and budget line item, a UV system is a major, large-scale, upfront investment a community recovers over 20 years.”

Now that costs for UV disinfection systems have become more competitive, communities are seeing it as an option that’s right for them. It’s a multimillion dollar investment with a lot of long-term advantages, is comparable to the cost of hydrochloride — or liquid chlorine — treatment and is less hazardous than chlorine gas.

Where it works

In 1991, Altoona Water Authority in Pennsylvania spent approximately $8,000 on a chlorine gas treatment for wastewater. The chemical was stored on site in 2,000-pound containers that not only threatened aquatic life, after disinfection, but also exposed employees to toxic vapors during connection — forcing the use of protective breathing equipment.

“There are risks with chlorine disinfection that have to be mitigated,” Salveson said. “Chlorine gas is the cheapest way to disinfect wastewater bar none, but it has a safety hazard. When it is stored in containers that hazard is mitigated, but if there is a leak, it can be deadly. With hydrochloride, there is a contact risk. It’s a liquid, so it costs more to transport and store. But it is comparable to the cost of UV.”

Hydrochloride also has to be removed after application, which makes UV disinfection an attractive alternative.

In December of 1991, Altoona Water Authority made the switch. It purchased the All Aquaray closed chamber horizontal system by Ultraviolet Purification Systems Inc., which treats 20 million gallons of water each day and includes two Type 304 stainless steel reactors that operate under pressure up to 30 psi.

By converting to UV, the city of Altoona found an environmentally responsible, easy and cost-effective method of disinfecting the area’s wastewater that works for them. When considering UV disinfection for wastewater, Salveson said officials should look into a system’s performance, safety and use.

“UV has some distinct advantages over other applications. It can disinfect more effectively; it’s not toxic; it’s safer for employees and wildlife; and you don’t have to remove one chemical with another before sending it back into the system. It costs a lot and requires more in terms of operations and maintenance, but it’s solvable. As long as you keep up on the maintenance, your system will keep humming along.”

Advantages & disadvantages of UV disinfection systems for wastewater

ADVANTAGES:

• Effective at inactivating most viruses, spores and cysts
• Eliminates the need to generate, handle, transport and store toxic, hazardous and/or corrosive chemicals
• No residual effect that can be harmful to humans or aquatic life
• Is user-friendly
• Has a shorter contact time compared to other applications (20–30 seconds with low-pressure lamps)
• Requires less space than other methods

DISADVANTAGES:

• Low dosage may not effectively inactivate some viruses, spores and cysts
• Organisms can sometimes repair and reverse the effects of UV through a “photorepair” mechanism
• Requires a preventive maintenance program to control the fouling of tubes
• Turbidity and total suspended solids in wastewater can render UV disinfection ineffective
• Is not as cost-effective as chlorinated gas