In-tank system eliminates THMs for rural consecutive system
Stanly County, in central North Carolina, is typical of many smaller rural water utilities in the U.S. Due to excess capacity in the city water system that supplies the surrounding county, Stanly County receives finished water that is often at or above the maximum contaminant level (MCL) for total trihalomethanes (TTHMs). Without a treatment plant of its own, Stanly County is limited to few options for bringing its water into compliance.
Stanly County purchases its water from the City of Albemarle and distributes it to approximately 4,700 customers in six service areas spread out over 119 sq miles. Before the late 1980s, the City of Albemarle had a number of large textile mills and an active manufacturing economy, which translated to high demand for water. Fearing water shortages, Albemarle expanded its treatment and distribution capacity to 18.5 mgd in the early 1990s. But with the steady offshoring of U.S. textile manufacturing, Albemarle’s manufacturing base has all but disappeared, leaving behind a large amount of excess capacity in the city’s distribution system. By the time this water reaches the city limits and is sold to Stanly County, it is already very old.
The Millingport service area in Stanly County often received the oldest and poorest-quality water from the City of Albemarle. This, combined with historically low demand in this service area, resulted in high TTHM levels, exceeding the Stage 1 MCL 50% of the time over the last six years.
Managers at Stanly County had made multiple attempts to improve the water quality in this part of their service territory. A small floating solar-powered draft tube mixer was installed in the only water tank in the area, a 200,000-gal pedisphere. There is unclear data as to the effectiveness of the mixer due to a number of factors, including problems with the device itself. One possible but highly controversial and costly option left to the county and the city was one nobody liked: a switch from chlorine to chloramine residual disinfectant.
PAX Water Technologies has developed an approach for efficient and effective removal of THMs in distribution storage tanks and reservoirs using a combination of aeration hardware, laboratory and computer software and modeling. In partnership with Utility Service Co., the largest and most experienced tank-maintenance company in the U.S., tanks in the distribution system can be quickly retrofitted with aeration technology to remove targeted quantities of THMs.
An optimization algorithm, along with water quality data and operational conditions, determines configuration for each facility. THM reduction can be predicted for any configuration of aeration equipment. Additionally, aeration systems can be custom-designed to deliver targeted levels of THM reduction. The system is comprised of a pumped circulation loop that utilizes sprayers and other air-handling equipment to optimize removal efficiency and energy consumption.
The 200,000-gal tank in the Millingport area was an ideal target for this system. Initial computer modeling showed that a small aeration system would be able to dramatically lower THM levels in the water. A system targeting a peak removal of 90% between the tank inlet and outlet conditions was designed and installed in August 2011.
Installation and startup was completed on Aug. 22, 2011. THM samples were collected at the tank on Aug. 25 both for the water filling the tank and for the water leaving the tank. TTHM levels entering the tank were 75 μg/L. TTHM levels leaving the tank were 0 μg/L (non-detect). A second pair of measurements taken on Aug. 28 showed incoming TTHM levels had risen to 142 μg/L, but outgoing TTHM levels were still at 0 μg/L. A third set of measurements on Sept. 6 showed incoming TTHM levels falling to 59 μg/L, with concentrations leaving the tank at 0 μg/L. A final set of measurements showed that inlet TTHM concentrations had fallen to 4 μg/L, with exiting water remaining free of TTHMs.
The rapid decline in TTHM concentrations in water leaving the Millingport tank is indicative of the effects of a highly efficient aeration system in a relatively small tank with low turnover. Calculations of the expected efficiency of the aeration system showed that greater than 90% removal was expected, assuming higher-than-average turnover. For conditions of low turnover, higher levels of TTHM removal were predicted.
The decrease in TTHM concentrations in the water going into the tank is not surprising, given the hydraulics of the distribution system in this area. Water enters and leaves the Millingport tank and dilutes with water coming from the City of Albemarle. During periods of low turnover, the TTHM-free water from the tank dilutes the small amount of fresh water coming from the City of Albemarle, resulting in very low TTHM levels.
In order to sample water that is more representative of the water entering the Millingport service area from the City of Albemarle, sample sites were moved. Instead of sampling water just before it entered the Millingport tank, a sample was taken at the vault entry point several miles closer to the City of Albemarle, where water enters the Stanly County system. Similarly, in order to understand the effect of the PAX TRS on water further away from the tank, a sample was taken 1.3 miles downrange of the Millingport tank at the furthest point in the Millingpoint service area.
The data from these more distant areas shows that, while the county continued to receive water near or above the MCL for TTHMs, the aeration system in the Millingport tank was able to depress levels of TTHMs far enough that, even in the most distant parts of the service area, TTHM levels remained within compliance.
With their TTHM levels now under control in the Millingport service area, managers of the Stanly County Utilities Department are now weighing how to apply this technology to other parts of their distribution system. While adding aeration to each of their remaining storage tanks is one option, managers are considering how to apply aeration strategically both to a select number of their own storage tanks and in the storage tanks of their water supplier. For the time being, concerns of a mandatory conversion from chlorine to chloramine have been put to rest.