Black & Veatch's Frank Rogalla looks at a water taste and odour plan
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 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. When unusual events such as droughts occur, communities that previously escaped T&O problems need to consider developing a contingency plan. Communities with a history of annual short-term problems may then experience events at unexpected times and for longer durations than previously. 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 (for example, effectiveness, operator involvement and cost) can point communities to optimal solutions, and avoid most, if not all, my water tastes funny phone calls. Three steps allow to effectively control T&O and maximise customer satisfaction are:
- know the causes, timing, duration and severity of T&O problems, and monitor raw and finished water quality,
- learn how others handle T&O events, review and evaluate available options with specific needs and budget in mind.
Off-tastes and odours 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 sulphur compounds, such as hydrogen sulphide, associated with rotten egg odours. While sulphide is the most common source of groundwater T&O complaints, other contaminants can cause metallic or salty tastes.
For surface water supplies, 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 characterised by earthy, musty tastes and smells are geosmin and methylisoborneol (MIB).
Those compounds are most often the result of excessive growths of cyanobacteria (blue/green algae), which proliferate during the hot summer months but can appear at any time.
They are easily identified by floating or benthic (bottom) mats, water discolouration or microscopic examination. Actinomycetes - small, filamentous bacteria - can also 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 plant matter in the surrounding watershed. Algae also are related to other odourous compounds, as common byproducts of select members of true algae can produce fishy, cucumber, grassy, floral and other troublesome odours. It is also 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. T&O events often follow rainfall that produces watershed run-off.
Rain during a drought may raise reservoir water level but also wash MIB and geosmin produced by actinomycetes in soil and decaying vegetation into the reservoir. Tracking and recording what is happening now is one of the first steps in preparing for future episodes, such as 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
There are many ways to handle T&O problems. Utilities should at least 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 the water is safe to drink if not aesthetically pleasing. 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 odours 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, as typically done at the beginning of a T&O problem to gain an understanding of the causes and affected areas,
- logging information and providing a standard response about water safety and quality, which helps educate and reassure callers after enough information has been collected to delineate the problem,
- altering treatment to eliminate the problem in water that is being produced.
Concerned customers commonly ask, what are you doing about it? and answering that question requires knowledge of treatment options. Treatment can range from controlling the source of T&O compounds to providing a removal step at the water treatment plant. 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), preventing them to flourish (through mixing and aeration), or killing a major portion of their population with algicides such as copper sulphate 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 a common method of controlling T&O events. Activated carbon removes many of the compounds that cause taste and odour complaints and comes in two forms: powdered (PAC) and granular (GAC).
Many facilities have the ability to add PAC for removal of T&O compounds. Because PAC is an expensive additive, its cost 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 20-50mg/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.treatment of odours
However, because GAC is typically used year-round, adsorption of other organics that use up capacity require the carbon to be replaced typically every one to five years. Many T&O-causing compounds can be readily oxidised to a less offensive form by commonly-used oxidants including 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 odours while chlorine dioxide can be used for phenolic, or medicinal, swampy and decaying-vegetation odours. A stronger oxidant such as ozone is usually needed to treat earthy/musty odours in addition to the other odour types. Another option is using hydroxyl radicals, potent oxidants that can react quickly to partially destroy odour-causing chemicals. They can be produced on-site by combinations of ozone, ultraviolet light and hydrogen peroxide. Several communities have faced problems with their potable water supplies and each has found a solution that works in their individual circumstances. For example, Scottsdale, Arizona knew T&O control would have to be part of its water treatment plans when the city decided to treat Salt River Project (SRP) water instead of continuing to purchase water from Phoenix.
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 odour by removing geosmin and MIB, and control disinfection byproducts by removing total organic carbon. Construction of the 30Ml/d 120 Ml/d Chaparral WTW facility is scheduled to begin this September. Selection of an approach for controlling taste and odour in a particular system must be made on a case by case basis.
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 colour, 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 next time the phone rings.