Process engineering: Wastewater treatment plant goes the distance

A mobile reverse osmosis (RO) system from USFilter effectively treated water at an abandoned fertilizer plant. This allowed system effluent to be discharged into Tampa Bay without environmental impact.

By Bill Perpich, Jr.

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The Piney Point fertilizer plant in Palmetto, Fla., was shut down in 1999 after its parent company declared bankruptcy. The state of Florida in 2001 started managing the facility, which had made phosphoric acid. A series of ponds at the site held 1.2 billion gal. of acidic, ammonia-laden process water. These ponds were in danger of overflowing, thereby spilling contaminated water into Tampa Bay.

To prevent a disaster, the state organized a team to manage the site and select wastewater treatment and offsite disposal methods to handle the contaminated ponds. They chose a mobile reverse osmosis (RO) system from USFilter, Warrendale, Pa., to treat about 800,000 gpd so the water could be discharged into Tampa Bay without adversely affecting the environment.

Why Piney Point was a problem
During phosphoric acid manufacturing, phosphate rock reacts with sulfuric acid and forms calcium sulfate, or gypsum, as a waste byproduct. This waste gypsum is known as phosphogypsum. Each ton of phosphoric acid produced generates about 4.5 tons of phosphogypsum. The phosphogypsum is mixed with hot acidic process water to create a slurry that is pumped to a disposal area on site. This area is called a phsophogypsum stack, known locally as gyp stacks, whereas the pumps, ditches, surge ponds and other means for collecting and conveying the phosphogypsum and process water are called the phosphogypsum stack system (Figure 1).

When the Piney Point facility was abandoned, its phosphogypsum stack system contained about 1.2 billion gal. of acidic process water, about half of which was stored in ponds on top of the gyp stacks or contained in adjacent above-grade cooling ponds. The process water was contaminated with a variety of pollutants, such as ammonia nitrogen, phosphates, fluoride, radioactive compounds and trace metals, in concentrations that exceed the Florida Department of Environmental Protection (FDEP) water quality standards.

According to FDEP, the main environmental risk posed by Piney Point was a catastrophic failure of the more than 30-year-old gypsum stack dikes. Such a failure would release acidic process water into an estuary along Tampa Bay, adversely affecting sea grasses and marine organisms.

Once rainwater comes into contact with process water or another contaminated part of the plant, it adds to the total volume of wastewater at the site. Due to the size of the Piney Point facility, more than 12 million gal. of process water are collected for every inch of local rainfall. The year the site was shut down, the area experienced heavy rains and tropical storm Gabrielle.

After the area received 19 in. of rain in September 2001, a group of local engineers developed a list of alternatives to quickly reduce the quantity of process water at Piney Point.

The team and FDEP investigated the feasibility of transferring the water offsite. They found, however, that the daily capacity of other fertilizer plants or Tampa wastewater treatment plants was insufficient to handle the volume of process water at Piney Point. Therefore, discharge options had to be developed for both the short-term recovery from Gabrielle and the long-term closure of the site.

For short-term recovery from the storm, the U.S. Environmental Protection Agency (EPA) issued a permit allowing fully treated wastewater to be discharged 100 miles offshore into the Gulf of Mexico. About 248 million gal. were discharged offshore using two 10-million-gal. tankers between July and November 2003. The long-term closure required reliable and sustainable options to remove between 1.0 Mgpd and 1.5 Mgpd from the site during a three-year period.

The promise of success
The FDEP considered RO technology as the most promising treatment method for producing a high-quality effluent with low concentrations of ammonia while avoiding the neutralization and disposal costs associated with the existing two-stage lime process at Piney Point. There were two concerns about using RO, however. The first was the absence of historical data on RO performance under acidic conditions. The second was the necessity of disposing the RO reject by sending it back to the gypsum stack. This would eventually cause constituents in the ponds to build up; this water would have to be treated before the site could be closed.

RO technology had never been used for any length of time to treat process water for direct discharge from a phosphate plant. Earlier trials, some at Piney Point, proved the efficiency of RO, but the long-term impact on operation, maintenance and materials of construction were unknown.

In spite of this, FDEP decided to phase in RO technology at Piney Point using mobile equipment to treat water 24/7. The feed water characterization and effluent quality specifications are listed in Table 1.

Table 1. Discharge effluent limits
Constituent Typical Feed Concentration Contract Specification
Calcium, mg/l

551

NA

Magnesium, mg/l

229

NA

Sodium, mg/l

1,290

NA

Potassium, mg/l

196

NA

Aluminum, mg/l

8.4

NA

Barium, mg/l

0.02

NA

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