Multi-Stage Approach Makes Water Disinfection More Efficient, Effective
A multiple-stage approach to water treatment, though it may involve increased design and capital costs, can render water disinfection technologies - including chlorination, ultraviolet (UV) and ozone - more efficient and effective.
By Jim Lauria
A multiple-stage approach to water treatment, though it may involve increased design and capital costs, can render water disinfection technologies - including chlorination, ultraviolet (UV) and ozone - more efficient and effective. Minimizing the chance of blinding downstream systems can reduce labor and materials costs, energy and chemical consumption, and waste stream production, improving both return on investment (ROI) and return on environment (ROE).
|Brent Ferguson of California Water Service Company checks pressure on an automatic self-cleaning screen filter that removes solids before river water enters a rack of MF membranes.|
Disinfection efficacy can be improved through better pretreatment removals by minimizing the chlorine or other oxidant demand of raw water. Reduced diffusion or absorption of germicidal wavelengths of UV light can also be realized in certain applications. Reducing the amount of organic solids in the water can reduce the production of disinfection byproducts (DBPs) in reactions with ozone or halogenated disinfectants such as chlorine or bromine.
California Water Service Co., in partnership with the City of Bakersfield, commissioned the $16 million, 8 mgd Northwest Water Treatment Plant in February 2007, allowing the growing city to enhance its groundwater drinking water supply through conjunctive use of surface water from the Kern River. Developing surface water supplies reduces demand on local groundwater, which serves some 70 municipal wells and is already in severe overdraft. The cost of drilling a new municipal well has quadrupled over the course of a few years to more than $1 million, straining both organizations' budgets.
The partnership chose microfiltration (MF) technology, followed by chlorination for residual disinfection. The microfilters are protected by three Amiad EBS automatic self cleaning filters with 300-micron weavewire screens. A traveling water screen strainer catches debris from the intake structure supplying the plant.
The screen filters initiate a self-cleaning process when a target pressure differential is achieved between the dirty and clean sides of the screen. A discharge valve opens to atmospheric pressure. Pressure from inside the filter pushes water - and filter cake - through small nozzles that scan the screen in a spiral pattern, removing trapped solids from the entire surface with a minimum of back flush water and energy consumption. Back flushing takes place without interrupting filtration.
Flush water from the membrane system is directed through a pair of 300-micron Amiad SAF filters - smaller units that employ the same self-cleaning technology.
Efficiency remains paramount - vital in the semi-arid Bakersfield environment. Recovery rates of nearly 98 percent are achieved mainly through using an efficient process and returning filter backwash streams to the head of the plant, said plant operator Brent Ferguson.
High Water, Big Challenges
Ferguson said water entering the plant at 10 to 15 NTUs triggers the back flush process about every half-hour. When high river flows increase the sediment load and debris in the intake water, the filters clean themselves as often as every 5 minutes.
The screen filtration system was challenged throughout much of 2010 by flooding upstream.
"Lake Isabella Dam had to release a lot of water - it hadn't been that high in years," Ferguson said. "It was scouring the riverbanks, and we were ending up with a lot of landscape bark-type of material. It was getting past our traveling water screen and into the intake. We had to replace strainer motors in the strainers and flush out our intake pipeline.
"There was no way around it," Ferguson said. "The traveling water screen did everything it was supposed to do. The strainers did what they were supposed to do. But the debris would pile up on the intake grating and fall into the intake, and the strainers aren't designed to handle material that big."
|The Amiad Automatic Micro Fiber (AMF) system traps solids and turbidity with cassettes of tightly wound polyester fibers. Automatic self-cleaning eliminates the economic and ecological costs of cartridge disposal.|
Cal Water ended up stationing workers at the intake 24 hours a day for a few weeks, manually cleaning the screen of debris until a conveyor belt system could be installed. The EBS screen units were able to get back to normal operation filtering out suspended solids instead of chunks of woody debris. It was a clear illustration of how every stage in the system impacts the one downstream.
Better UV Performance
UV disinfection systems rely on the transmission of specific wavelengths of energy to kill pathogens. Solids in the water can reduce the transmission in various ways, from fouling the quartz sleeve that protects the UV lamp, shielding or scattering the rays, or absorbing germicidal wavelengths. According to one source in the UV industry, water treatment professionals can expect a one-percent drop in transmissivity at a wavelength of 254 nm per 2 NTUs of turbidity.
At Israel's Beer Sheva 13,200 gpm wastewater treatment plant, a study is underway to review the effects of a multi-stage filtration system before UV disinfection. An Amiad SAF automatic self-cleaning screen filter with a 25-micron screen is positioned upstream of a 7-micron Amiad Automatic Micro Fiber (AMF) filtration system.
Inside the AMF system, an array of cassettes, each one a grooved plastic core wound tightly with fine polyester fibers, captures particles, dramatically reducing fine sediment load and turbidity. When a target pressure differential is reached, a high-pressure stream of water is directed through the fibers and deflected by the grooves in the core, dislodging trapped solids.
Water enters the system from the clarifier containing 20 mg/L or more of total suspended solids (TSS). The 25-micron SAF reduces the load to 4 mg/L, allowing the AMF to focus on smaller particles and clarify the water to 1 mg/L. By contrast, sand media systems were unable to reduce the TSS below 2 mg/L. Sand media systems also require as much as four times the amount of fresh water for back flushing than the automatic self-cleaning screen filters - a vital environmental consideration in the parched Negev Desert.
Protecting ozone disinfection systems must take into account several kinds of fouling. Large particles can interfere with an ozone system's venturi injector. Oxidizable organic solids - which can be measured as chemical oxygen demand (COD) - can tie up ozone. Given the amount of energy required to generate ozone, it is much more effective to simply remove the particle than to create enough ozone to oxidize it.
Because ozone may often be placed upstream or downstream of UV disinfection systems, the sequence of the disinfection processes can help guide the selection of the most appropriate pretreatment system.
If UV is the first disinfection step, followed by ozone, a fine filtration system such as cartridge or AMF may be the most appropriate choice. AMF units with a filtration degree of 2 microns can deliver a 3-log (99.9%) removal of Giardia and Cryptosporidium cysts, as well as removal of other solids, making the UV step more effective.
|Inside the automatic self-cleaning filters, scanning nozzles concentrate the force of back flush to dislodge filter cake from the screen as water flows from inside pressure to an outlet open to atmospheric pressure.|
If ozone is installed upstream of the UV system, an automatic self-cleaning screen filter can provide removal of sediment, soil organic matter and biological solids.
In developing any multi-stage filtration system, several simple concepts must be kept in mind. The first is the sequence - ensuring that a given system isn't being relied upon to filter out materials too large for efficient operation. Plugging a costly 1-micron cartridge or an even finer membrane system with sediment is an expensive and destructive process, just as challenging a screen filter with chunks of bark or gravel. Work your way from coarse to fine, seeking economical and efficient solutions at every step.
Study the water you are treating. A particle size distribution (PSD) test of raw influent water can guide the selection of the proper degree of filtration necessary to ensure efficient and effective operation.
Consider seasonal changes in influent water quality. Cal Water in Bakersfield experienced a dramatic shift in particle size when floodwaters scoured banks that are generally out of reach of the river. Drinking water treatment plants that draw from reservoirs often face algal blooms in the summer, adding high levels of deformable biological solids to their list of contaminants.
Last, consider not just the return on investment provided by another level of treatment, but also the return on the environment. Reducing the sediment load on fine cartridge filters commonly used before membranes can dramatically lower the number of cartridges that need to be replaced and disposed of - in turn lowering materials used and landfill space needed to house the spent units, as well as energy required to transport them to the landfill. Protecting membranes effectively can reduce the use of cleaning chemicals. Choosing efficient systems can lower the amount of produced water from back flushing and the consumption of energy.
In all, adding the right layer of protection in the right place - whether it's a conveyor belt for debris removal at a flooded intake or a high-tech microfiber filter that boosts the efficacy of a UV disinfection unit - can improve the efficiency and return of a multi-million-dollar system. WW
About the Author: Jim Lauria is a writer and chemical engineer with over 20 years of global experience as an executive in the water treatment industry. He is Vice President, Sales and Marketing, for Amiad Filtration Systems in Oxnard, Calif.