USA - 2010
Sustainable System Treats Stormwater Runoff with Nature’s Help
Tapping into the water-filtering capabilities of nature, Alcoa’s Point Comfort alumina refinery in Texas implemented an engineered natural system for stormwater management that, on an annual basis, diverts 151,416 kiloliters (40 million gallons) of water from treatment and discharge, improves runoff water quality, recovers 5,000 metric tons of bauxite that equates to a US$150,000 savings in raw material, and reduces maintenance costs by US$500,000.
Prior to the new system’s installation, all stormwater runoff from the refinery’s 12-hectare (30-acre) bauxite storage and handling area was discharged into the site’s storm sewer system. There, it was combined with other plant runoff and stored primarily in a 10-hectare (25-acre) onsite lake. The majority of the collected water was reused in the refining process, with the excess treated at an onsite plant and discharged into local water sources.
A major challenge was that stormwater runoff from the bauxite storage and handling area contained particles of bauxite, which is the raw material from which alumina is extracted at the site. These solids would build up, eventually clogging storm sewers and decreasing the depth of the storage lake. The site’s storm sewer pipes had to be cleaned frequently, and the lake had to be dredged to avoid flooding, which could lead to unsafe conditions.
The stormwater runoff from the bauxite storage and handling area does not contain any regulated chemical components that require wastewater treatment. Because of this, as well as the opportunity to recover bauxite and have less maintenance, staff at Point Comfort and the Alcoa Technical Center decided to create a sustainable treatment system for the area. The goals were to capture and recycle the bauxite solids while improving the quality and reducing the volume of the area’s runoff.
The resulting engineered natural system channels the runoff from the bauxite storage area through a series of man-made trenches, serpentine-shaped marshes, and collection ponds to filter out the bauxite and other sediments. The components, in the order in which they occur in the system, are:
- Sedimentation trenches for bauxite recovery and removal of solids;
- A 1.8-hectare (4.5-acre) flow equalization pond for stormwater retention, storage and equalization, and bauxite recovery;
- Shallow marshes totaling 1.4 hectares (3.4 acres) for water quality treatment and habitat creation;
- A 0.2-hectare (half-acre) micropool for water reuse; and
- A 3.2-hectare (8.0-acre) tree plot for water consumption and habitat creation.
The tree plot consists of 7,000 hybrid popular trees, with each capable of consuming 38 to 95 liters (10 to 25 gallons) of water a day. Future plans include using this excess clean water to replace potable water within the plant for process use.
The new system also recovers 5,000 metric tons of bauxite residue annually, reducing the cost for pipe cleaning and lake dredging by US$500,000.