To limit costs, consider site-specific parameters if installing off-the-shelf trihalomethane removal systems.
DISINFECTION by-product (DBP) regulatory compliance requires public water systems to achieve total trihalomethane (TTHM) levels of 80 ug/L (ppb) or less for a locational running annual average throughout a community. Often a water treatment plant's most cost-effective approach is to continue using the same treatment processes used in the past but strip excess THMs from the treated water by using a spray aeration system. Such a system may be custom designed or formed from off-the-shelf equipment.
SYSTEM CONSIDERATIONS In comparison, off-the-shelf THM removal spray aeration equipment may offer advantages over a custom-designed system, such as lower cost, faster delivery, and the flexibility to move the entire system to a different tank if city flow patterns change. However, to realize such advantages, important issues (e.g., site and hydraulic concerns) must be considered before applying off-the-shelf equipment to a water system for THM removal.
Clearwell vs. Distribution System. Stripping THMs at a clearwell rather than in the distribution system is a decision that presents the advantage of the whole plant's output being treated, assuring THM compliance throughout the city. Sufficient electric power is usually nearby, too.
A disadvantage to a clearwell site is that the city may have to purchase a larger system. For example, a water treatment plant may produce 7 mgd, but only a part of the city using 0.5 mgd has a THM-compliance problem. Instead of purchasing a clearwell system, it may make more sense for the city to install an in-line THM removal skid to treat just the neighborhood using 0.5 mgd.
Another disadvantage of locating THM stripping equipment in a clearwell is that THMs are usually low there, so THM removal decreases. For example, only 40 ug/L may be in the water at the clear-well, but sufficient formation potential in the water could drive THMs up to 150 ug/L at the end of the distribution line.
A solution is to establish intense mixing at the front of the clearwell to help convert the formation potential into actual THMs, and then strip the higher THMs out at the back of the clearwell. In the aforementioned example, after mixing at the front and stripping at the back of the clearwell, the THMs may leave the clear-well at 35 pg/L but only rise to 50 pg/L at the end of the line, which is well under the compliance limit. A cursory look at the THM reduction at the clearwell in this example would indicate the stripping system removed only 5 pg/L of THMs, reducing the level from 40 pg/L to 35 mg/L. However, a comparison at the end of the line tells the whole story—a reduction of 100 pg/L was achieved, putting the system well under the compliance limit.
Another disadvantage of siting a removal system at the clearwell is that there may not be enough headspace for a spray aeration THM stripping system. However, making a small hole in the clearwell's roof and putting the spray aeration equipment into a small 'dog house' at the top can overcome the problem. Then the water can be pulled up from the clearwell, treated, and drained back into the clearwell.
Distribution Tanks. Most distribution tanks use just one pipe to fill and empty the tank, and that pipe has a 'tee' in the mainline. Sometimes a city will assume, based on supervisory control and data acquisition (SCADA) system averages, that a THM stripping system in a tank can help achieve compliance in a nearby section of the city. But often it turns out that, in the peak THM season, too much water flows down the mainline and past the tank tee—not into the tank where it can be treated. SCADA averages can be misleading for placing a THM stripping system into a one-pipe tank, and overall results will vary significantly from day to day depending on how much water enters the tank.
Two-pipe tanks pose different challenges. With all the water going into, through, and out the tank, bypass problems are avoided. As a result, this type of tank may be a good location for a THM stripping system for a particular section of a city. The problem with these tanks, though, is that the SCADA water level records don't reflect the actual flow going through the tank at any one time; the tank level may not change even when 3,000 gpm of flow is going through it. As a result, other data must be used for determining the tank's flow rate in sizing a THM removal system.