Trends in Drinking Water Treatment
The drinking water industry is currently responding to a wave of new regulations. Disinfection byproduct (DBP) control, microbial removal, the treatment of inorganic contaminants, security concerns, and improved analytical methods that detect potentially harmful contaminants at part-per-trillion levels create additional demands on the operators of water treatment plants.
At the same time, the wave of regulation is breaking on the rocks of cost control. Consumers are hesitant to pay more for their water.
These pressing needs have spurred the development of a number of new water treatment technologies that may be more effective, less costly or both.
While conventional treatment (coagulation, sedimentation, filtration with chlorine disinfection) remains the “king of the hill,” other treatments are gaining popularity. Alternative disinfection strategies are being used to control DBPs; disposable media have been developed to treat arsenic; and low-pressure membrane systems improve microbial removal and filtration performance while reducing or eliminating the need for coagulation. If there is a general trend in treatment, it is to replace some or all of the conventional treatment processes with alternative technologies. This article discusses two processes, river bank filtration (RBF) and ultraviolet (UV) disinfection, which are frequently being considered by utilities.
River bank filtration
RBF is a treatment technique widely practiced in Europe and the subject of much interest in the U.S. Approximately 16% of the drinking water supply in Germany is treated via RBF. The fundamental concept is to draw surface water from a stream or river into an aquifer and use the porous material in the aquifer as a natural filter. When implementing the RBF process, an extraction well is located close enough to a river so the river is within the well’s zone of influence. Surface water is drawn from the river toward the extraction well, passing through riverbed sediment and aquifer material acting as a filter, and mixing with water already in the aquifer.
As used in Germany, post-RBF treatment can include granular activated carbon to remove persistent synthetic organic contaminants or aeration/ozonation for iron and manganese removal. German water treatment professionals have such high confidence in the ability of a properly designed RBF system to control pathogens that disinfection is viewed as unnecessary.
The RBF process offers several advantages. It requires no chemicals, is simple to operate and needs little maintenance. RBF can reduce concentrations of many pollutants and DBP precursors through a combination of mechanisms, including filtration, biodegradation and dilution. In addition, RBF can attenuate shock loads of contaminants and equalize temperature fluctuations.
Disadvantages of RBF include an inability to control infiltration rates and flow paths, the risk of breakthrough of pathogens or other contaminants accumulated in the river sediment or aquifer, and the inability to backwash the river sediments or aquifer.
Two factors are fundamental to the successful implementation of RBF design: selecting the appropriate aquifer material and allowing sufficient time for the river water to be filtered by the aquifer material.
UV disinfection of drinking water is becoming more widespread throughout the U.S. Although it is viewed as an advanced technology, the first drinking water application of UV disinfection was at Marseilles, France in 1910. Three factors have combined to accelerate interest in UV. These include the need to treat chlorine-resistant protozoa such as Cryptosporidium, concern regarding the formation of DBPs when using conventional disinfectants and advances in the reliability of UV systems.
Three types of UV lamps are currently available, low pressure; low pressure/high output; and medium pressure. Low-pressure lamps produce monochromatic radiation. These lamps are the most energy efficient and inactivate the most protozoa per unit of energy consumed. Yet, these lamps only provide relatively low-intensity radiation, so many lamps are needed to treat large volumes of water. Medium-pressure lamps produce polychromatic radiation. These lamps are less efficient but can produce much greater intensity than low-pressure lamps. Hence, fewer medium-pressure lamps than low-pressure lamps are required to treat a given flow rate.
As the name indicates, low-pressure/high-output lamps provide monochromatic radiation at greater intensity than low-pressure lamps.
The key water quality parameter influencing the design of a UV system is the UV transmittance. This is a measure of how well the water transmits UV radiation. For a given size reactor opening at a constant flow rate, water with a higher UV transmittance will require fewer lamps than water with a lower UV transmittance. Because UV transmittance can vary seasonally, accurate UV transmittance data is required to design a UV reactor.
Validation of the performance of a UV system is the most significant factor in obtaining certification from a regulatory agency. Because no residual is left in the water to demonstrate adequate disinfection performance, challenge tests are required to demonstrate reactor performance. Surrogate organisms are used to challenge the UV reactor over the range of possible operating conditions.
Challenge tests are complex, costly and difficult to perform onsite. In the future, as vendors standardize reactor design and improve computational fluid dynamic models, regulators may no longer require challenge test data for this treatment option.