City of Devils Lake,
Arsenic Treatment Bench-Scale Evaluation
- Relatively high ferric chloride (20 mg/L) necessary due to high levels of arsenic, organics, and phosphates.
- Acidified ferric coagulant product reduced optimum coagulant dose to 5 mg/L FeCl3.
Carollo Engineers conducted a bench-scale evaluation for the City to determine baseline performance capabilities. The coagulation/filtration process typically represents the lowest cost alternative for moderately sized facilities (e.g., > 1 mgd). Equipment is readily available from a variety of suppliers eliminating the concern of “locking” yourself into a single supplier for the life of the equipment. Many of the proprietary media systems are appropriate for smaller systems and in situations where operator staffing is an issue. However, the limited number of proprietary media systems and their proprietary nature limit the ability of these systems to create a bid environment as competitive as standard coagulation/filtration equipment.
The C/F process was tested on two groundwater from the City’s water supply. Both supplies were similar in water quality and indicated little variation in terms of the ability of a C/F process to remove arsenic. From bench-scale testing, the following results were discovered:
- Raw water quality included high hardness (>300 mg/L CaCO3), organics (>4.5 mg/L TOC), manganese (0.8 mg/L Mn) and arsenic (30 to 35 ppb As).
- Competition from phosphates and organics along with a relatively high arsenic content resulted in an optimum coagulant dose of 20 to 35 mg/L FeCl3, depending on water source.
- “Acidified” ferric coagulant decreased optimum coagulant dose by 88%.
- Arsenic removed by the C/F process required less than 10 minutes of reaction time.
- Increasing flash mix intensity from 3,000 to 6,000 (GT-value) provided an additional 40% removal of arsenic.