A matter of taste
People expect high pressure from their garden hoses, low totals on their water bills and good-tasting water in their glasses. Nothing elicits complaints more quickly than bad-tasting or foul-smelling water. The time to address taste and odor (T&O) problems is not when the phone starts ringing, but long before water reaches the tap by planning how to address periodic or potential problems.
Details of a T&O plan depend on the water source, frequency and duration of previous T&O problems, existing treatment facilities and customers’ attitudes about how their water tastes. In the case of an annual — or less-frequent — short-term problem, educating the public about the cause of the problem is one option. The utility should explain that the water is safe to drink and estimate how long the event is expected to last. However, that option becomes less satisfactory as T&O problems become more frequent and dramatic.
With the drought that has affected much of the United States, communities that have thus far escaped T&O problems need to consider developing a contingency plan. Communities with a history of annual short-term problems may experience events at unexpected times and for longer durations than in previous years. When a drought exacerbates T&O problems, other options merit consideration, including changes in source water management practices and providing additional treatment at the plant. Treatment options for the most difficult T&O problems include powdered activated carbon (PAC), granular activated carbon (GAC), ozone and advanced oxidation processes.
Understanding the basic issues in selecting appropriate processes (e.g., effectiveness, operator involvement and cost) can point communities to optimal solutions, and avoid most, if not all, “My water tastes funny” phone calls.
To effectively control T&O, utilities should know the causes, timing, duration and severity of the problems; monitor raw and finished water quality; learn how others handle T&O events; and review and evaluate options with specific needs and budgets in mind.
Know the nature of the problem
Off-tastes and odors can be caused by contaminants that vary by water source. Some contaminants are detected primarily by taste (salty, sweet, sour or bitter) while others are obvious by their smell. For instance, groundwater may contain a mix of reduced sulfur compounds, such as hydrogen sulfide, associated with “rotten egg” odors. While sulfide is the most common source of groundwater T&O complaints, other contaminants can cause metallic or salty tastes.
For cities using a surface water supply, byproducts of algae growth are typically the main cause of customer complaints. Algae growth can be accelerated during warm, dry weather, when reservoir water levels are low and nutrient concentrations are high. The main culprits in T&O problems characterized by earthy, musty tastes and smells are geosmin and methylisoborneol (MIB). Those compounds are often the result of excessive growths of cyanobacteria (blue-green algae) in water supplies. Cyanobacteria proliferate during the hot summer months but can appear at any time. They are easily identified by floating or benthic (bottom) mats, water discoloration or microscopic examination.
Actinomycetes — small, filamentous bacteria — also can produce geosmin and MIB but are difficult to identify in water supplies because they are non-pigmented, extremely small and do not produce blooms. Major agents of decay, they inhabit lake sediments and dying aquatic weeds as well as dead plants in the surrounding watershed. T&O problems often follow rainfall that produces watershed runoff. Rain during a drought may raise the reservoir water level but also wash MIB and geosmin produced by actinomycetes in soil and decaying vegetation into the reservoir.
Algae also are related to other T&O characteristics. “Odorous compounds other than geosmin and MIB are common byproducts of select members of ‘true algae’ that can produce fishy, cucumber, grassy, floral and other troublesome odors,” says Bob Hoehn, professor emeritus at Blacksburg, Va.-based Virginia Tech.
It also is important to know what conditions historically contribute to T&O water problems. Examples include low river flows and reservoir levels, upstream runoff and other source-related changes. Tracking and recording what is happening now is one of the first steps in preparing for future episodes.
That means sampling the water supply and distribution system for T&O-causing compounds before a problem occurs. There are three types of monitoring: chemical analysis, sensory analysis and mechanical olfaction (electronic noses). The human nose is still the preferred indicator of potential T&O problems. The correct monitoring approach and frequency must be determined case by case.
Know what works
Utilities should alert the public when T&O issues occur by providing a description of the problem and the affected area, cause and expected duration, as well as assurance that the water is safe to drink if not aesthetically pleasing. Information can be shared through the traditional media as well as the Internet.
The effectiveness of public communication hinges on the knowledge and tenacity of the person receiving complaints. Whoever answers the phone — whether a public relations professional, receptionist, chemist or someone else — must be aware of ongoing events and be able to discern if the problem is part of an ongoing episode or is unrelated and requires individual attention.
With today’s tightened security, any residents reporting strange odors from their taps must be taken seriously. In fact, it is now more likely that customers will call not only if their water tastes bad, but even if it tastes “different.” By arming representatives with standard questions to ask callers, utilities can make informed decisions about the next course of action. Responses may include:
Sending a representative to the complaint site to obtain a sample;
Logging information and providing a standard response about water safety and quality; and
Altering treatment to eliminate the problem in water that is being produced.
Concerned customers commonly ask, “What are you doing about it?” Answering that question requires knowledge of treatment options. Treatment can range from controlling the source of T&O compounds to providing a removal or oxidation step at the water treatment plant. Generally, control methods rely on limiting T&O-producing microbial growth, removing algal byproducts from water by adsorption or converting byproducts into a less offensive form through oxidation.
Microbial growth can be limited by decreasing the microbe’s food supply (nutrients), creating conditions that are not conducive for them to flourish (through mixing and aeration), or killing a major portion of their population with algicides such as copper sulfate or biological predators. Those steps are best taken before a problem occurs by monitoring the watershed for conditions known to produce algae and then applying one or more of the techniques before algae bloom.
Removing the contaminants from the water as it passes through the treatment plant also is commonly used to control T&O events. Activated carbon removes many compounds that cause complaints and comes in two forms: powdered (PAC) and granular (GAC). Many facilities have the ability to add PAC to remove T&O compounds.
Because PAC is an expensive additive, the cost of the treatment can be contained by adding it only when T&O problems are most likely to occur. Using sufficient amounts of PAC — typically in the range of 20 mg/L to 50 mg/L — is key to the success of the operation.
GAC may be installed in existing filters or in post-filtration contactors. It provides a positive barrier to T&O compounds throughout the year. However, because GAC is typically used year-round, adsorption of other organics that use up the GAC adsorption capacity require the carbon to be replaced frequently. For T&O applications, GAC typically must be replaced every one to five years.
Many taste- and odor-causing compounds can be readily oxidized to a less offensive form. Commonly-used oxidants in potable water treatment include chlorine, chlorine dioxide, potassium permanganate and ozone. The effectiveness of each depends on the targeted compound. Chlorine and potassium permanganate can be effective for grassy, fishy and cucumber odors while chlorine dioxide can be used for phenolic, medicinal, swampy and decaying-vegetation odors. A stronger oxidant like ozone is needed to treat earthy/musty odors in addition to other odor types. Using hydroxyl radicals produced onsite by combinations of ozone, ultraviolet light and hydrogen peroxide is another option. Hydroxyl radicals are potent oxidants that can react quickly to partially destroy odor-causing chemicals.
Solutions are individual
Several communities have faced problems with their potable water supplies, and each has found a solution that works in their circumstances. For example, before last summer, warming temperatures in Medford, Ore., increasingly brought complaints from residents about the musty T&O of their water. The city relies on spring water supplemented by Rogue River water from May through October (a period when demand increases).
In the past, PAC applied at its 45 million-gallon-per-day (mgd) Duff Water Treatment Plant was sufficient to reduce the typical 5 to 10 nanograms per liter (ng/L) geosmin levels in the reservoir feeding the Rogue River. Unfortunately, PAC was no match for the unusually high concentrations (30 ng/L) of geosmin.
Medford Water Commission conducted tests that demonstrated that ozonation would be a better way to combat T&O problems than increasing PAC dosage, and it identified the optimal dose. The $4.6 million ozone system precedes conventional treatment — and preempts consumer complaints.
“Since we started up the ozone system July 1, 2002, we haven’t received a single water quality complaint regarding taste or odor,” says Jim Stockton, supervisor of the Duff Water Treatment Plant. “We not only achieved our taste and odor goals, but reaped additional side benefits such as coagulant chemical reduction and enhanced disinfection.”
Scottsdale, Ariz., knew that T&O control would have to be part of its water treatment plans when the city decided to treat the Salt River Project (SRP) water instead of continuing to purchase treated SRP water from Phoenix. “The local surface water supply can have significant taste and odor problems if not treated properly,” says Jim Clune, director of Scottsdale Water Quality and Treatment.
Anticipating that problems would arise, the city evaluated treatment processes for T&O control as part of a study to develop design criteria for its new membrane filtration treatment facility. GAC was evaluated with and without ozone.
A six-month pilot study demonstrated that GAC columns could effectively control taste and odor by removing geosmin and MIB, and control disinfection byproducts by removing total organic carbon. Construction of the 30 mgd Chaparral Water Treatment Plant facility is scheduled to begin this September.
Fishy and grassy tastes and smells characterized a periodic T&O problem experienced by Concord, N.C. The city drew its water from a relatively new source, the Coddle Creek Reservoir. Initially, the problems were effectively controlled by oxidation using potassium permanganate with the addition of PAC.
In 1998, a drought changed the nature of algal activity within the reservoir and spawned MIB- and geosmin-related earthy, musty odors resistant to oxidation. Bench-scale testing indicated that although neither potassium permanganate nor the grade of PAC previously used was effective in addressing the problem, a 7 mg/L dose of a different type of PAC would essentially remove the odor-causing compounds.
“[Our work] highlights the importance of selecting a PAC that is appropriate for the taste and odor compounds that need to be removed,” says Danny Williamson, supervisor of the Coddle Creek Water Treatment Plant. The work also demonstrated that a period of detention in a pipeline enhances the application of PAC.
Selection of an approach for controlling T&O in a particular system must be made case by case. The right fit depends on how long the event lasts, its severity, the sensitivity of the customer to costs versus taste, the physical arrangement of the source and the treatment plant, and the complexity of the system versus staffing levels.
Each approach to T&O control has pros and cons. Benefits can include control of other contaminants, enhanced disinfection or filtration, and improvement in other water quality parameters such as color, disinfection byproducts and pathogen control. Costs for T&O control may be partially offset by reducing costs in other treatment areas. Knowing more about what causes T&O problems and what has worked for others in addressing them will make it easier to know what to do the next time the phone rings.
The authors are members of Black & Veatch’s water and wastewater treatment technology team. Mark Waer is a senior water treatment technologist in the Phoenix office; Lee Harms is a senior water treatment engineer; and Rick Bond and Nick Burns are process engineers in the company’s Kansas City, Mo., office.