There is no question that unpleasant taste and/or odor in drinking water can create negative water quality perception challenges for water utilities. Though taste and odor are not health concerns, consumers still are likely to notice and voice their concerns.
That’s why capital investments in treatment systems and processes to address taste and odor (T&O) compounds in drinking water are increasingly becoming a priority for many water utilities.
Two biological constituents that frequently cause taste and odor problems — geosmin and methyl isoborneol (MIB) — are present in surface water sources that feed many facilities across the Rocky Mountain Front Range. Geosmin and MIB are byproducts of excessive algae growth and subsequent die-off of both algae and other biological constituents. They are usually found in shallow reservoirs or other surface water bodies with relatively little movement.
A number of treatment options are available to address T&O, ranging from mitigation measures at source water locations, to pretreatment injection of powdered activated carbon or certain types of filtration further along in the treatment train.
Many utilities are opting for an advanced treatment strategy consisting of ozone and hydrogen peroxide injection combined with biofiltration at a later stage in the treatment cycle.
Liquid oxygen requires a significant volume of storage on-site, along with vaporizers to convert the liquid oxygen into a gaseous state. Once converted to gas, the oxygen flows through a generator where it undergoes electrolysis to add a third oxygen atom to the oxygen molecules (O2) to form ozone molecules (O3). Hydrogen peroxide is then mixed with the ozone gas to form hydroxyl radicals — a strong and highly effective oxidant that provides much faster reaction speeds and high levels of removal for a wide range of constituents that can cause taste or odor issues.
Ozone injection provides a great deal of flexibility because it can be introduced at multiple stages throughout treatment cycles and combined with chemical treatment. Often the ozone/hydrogen peroxide mixture is dosed into the water just downstream of the pretreatment stage. At this point, large volumes of settleable solids and other impurities have already been removed, which boosts the effectiveness and efficiency of the oxidation.
Following this oxidation stage, water flows downstream through a biofiltration cycle. This second treatment layer consists of conventional media filters utilizing granular activated carbon (GAC). This is a biologically active process enabled by the oxidation that converts the total organic carbon present in the water to a form of carbon called assimilable organic carbon (AOC). The AOC is basically a short-chain form of carbon that encourages biogrowth on the media within the filters. This growth results in a film of biologically active constituents that consume most of the remaining compounds that cause taste and odor.
The GAC filters are irregularly shaped, which encourages absorption of T&O compounds into the media and promotes growth of biological material on the surface. Because biofiltration requires prolonged growth of biological matter on the filter media for maximum effectiveness, it is recommended that filters remain in place for relatively long periods of time, sometimes up to several years. In contrast, conventional GAC filtration without the ozone injection cycle requires spent filters to be replaced on a routine basis.
Though the biofilters are primarily intended for removal of T&O compounds, they also function as conventional filtration — removing turbidity, suspended solids and other constituents that must be removed before the water moves into the clearwell.
This effective two-step process of ozone and biofiltration results in 99% removal of T&O compounds. Jar testing, pilot testing and full operational testing have verified the effectiveness of this two-stage system, demonstrating consistently high removal rates from source water and high purity of finished water conveyed to consumers.
Weighing Pros and Cons
Ozone treatment or pretreatment processes are highly regarded for effectiveness in producing high-quality finished water but can require a relatively high level of capital investment. The system requires a large footprint to accommodate on-site oxygen storage as well as vaporizers and ozone generators. Ozone injection processes can vary as well, with some plants opting for a basin system for diffusion that requires additional piping, while others utilize pipe-loop reactors that enable side-stream injection into the water stream. The combination of chemicals like hydrogen peroxide for additional treatment via filtration or other means can increase costs.
Facilities that treat source water containing lower percentages of T&Os may be candidates for less intensive treatment solutions such as powdered activated carbon or standalone GAC treatment filtration.
Another strategy that could be viable for some water utilities or public water authorities is to address T&O constituents directly at the raw water sources. Though many utilities and water districts rely on multiple sources of raw water, upstream mitigation utilizing ultrasonic technologies or various forms of aeration to promote oxidation and water movement can be a part of the solution.
Getting the Right Bang for Your Buck
Challenges in achieving the water quality consumers expect can vary greatly, depending on geographical region, weather and drought patterns, and many other factors that can impact sources of raw water.
Advanced treatment systems like the ozone and biofiltration system installed at the new Thornton Water Treatment Plant in Colorado are certainly complex but can pay dividends in terms of customer satisfaction. This combination of ozone-induced oxidation and biofiltration is proving to be a highly effective and viable solution that has addressed a challenging raw water supply.
Learn more about how a new 20-MGD water treatment facility serving Thornton, Colorado, was designed with the flexibility to address multiple source water challenges.