91´óÉñ

The City of Helena, MT, operating the Ten Mile Water Treatment Plant and Missouri River Treatment Plant, faces seasonal water quality changes. It¡¯s two sources create treatment challenges in meeting regulations and maintaining quality effluent.

Low alkalinity and varying source water quality at the Ten Mile Plant make it difficult to meet required Total Organic Carbon (TOC) % removal, to comply with the EPA Disinfectant/Disinfection Byproduct (DBP) Rules within current plant processes and infrastructure. Seasonal organic, taste & odor compounds and classification as a Zero Discharge Facility make operational costs of the Missouri River Plant a concern.

Summer demand can reach 15 MGD while winter demand drops to 3 MGD typically 100% supported by the Ten Mile Plant. The Missouri Plant is used for peaking or backup, such as when plant upgrades are needed. Recently the Ten Mile Plant has upgraded clearwell covers, PLC systems, filters, and media. The unique water quality challenges of these two plants and two sources are shown in Table 1.

The Missouri River Plant pumps water from the Missouri River to the Helena Valley Regulating Reservoir then to the plant. Since the reservoir provides water for irrigation and fish & wildlife use, they cannot treat the reservoir. As a result, the plant treats any organic, taste, and odor compounds that are present. Classified as a Zero Discharge Plant, the Missouri River Plant uses its backwash water for managed irrigation and recycles it back to the plant to be retreated. While recycling backwash water may allow for water conservation and savings on permits, due to seasonal quality changes, total operational costs must be evaluated to determine overall impact.

The Ten Mile Plant is gravity fed from a reservoir at the top of the Rocky Mountain Divide via Ten Mile Creek. It was built in 1991 when the city lost its filtration waiver for turbidity compliance. Additionally, the use of contact absorption clarifiers reclassified the facility from direct filtration to conventional treatment which requires compliance with TOC % removal. With low alkalinity (typically 1525 mg/L), the plant falls into elevated TOC % removal classification during spring runoff when source TOC exceeds 4 mg/L TOC as highlighted in Tables 2 and 3.

?

The challenge of meeting required TOC % removal at the Ten Mile Plant led the City of Helena to partner with state regulators. Compliance was achieved with a treatment plan combined with real-time water quality monitoring.

Not only did TOC data help them understand real-time TOC % removal but also seasonal changes in source water TOC. The value of the data led them to acquire a Sievers* M5310C TOC Laboratory Analyzer for the Ten Mile Plant to pilot and enact treatment changes that led to more consistent water quality and better compliance. Prior to on-site TOC analysis, the city relied on sending samples to third party contract labs. These results took 1-2 weeks leaving the plant blind to both changes in water quality and process effectiveness.

The following plant changes were put into effect with the help of on-site TOC analysis:

• Moved chemical addition point (PAC & coagulants) to the front of the plant as opposed to directly before the filters.
• Piloted different chemicals to test TOC % removals as well as color and turbidity removals including polyaluminum chloride, aluminum sulfate, and reduction in use of cationic polymers.

With relatively high alkalinity and low source TOC, the Missouri River Plant easily meets its 25% TOC removal requirement, but it benefits from TOC data to properly help dose PAC to overcome taste & odor issues. Future consideration of ozone for taste & odor management would benefit from optimized ozone to TOC dosing for cost savings and process efficiency.