3/7/08

Ecology of San Francisco Bay, Part 1

Notes and quotes on salt production in San Francisco Bay.

200M tons of salt is produced worldwide every year. North America produces more than one-quarter of it. The U.S. is the world's second largest salt producer, producing 46 million tons a year (China is number one, producing 48M tons--and climbing). Brines for use by chemical companies account for nearly half of this salt production. The remaining is "dry salt" produced by one of three basic technologies: solar evaporation of seawater or saline lake water, solution mining and vacuum pan evaporation and conventional deep-shaft (rock salt) mining. Link.

(Note: "Table salt is typical of the fine, granulated-evaporated salt produced in vacuum pan evaporators. Nearly all food grade salt in the United States is produced by vacuum pan evaporation of brine.” Link.

Sales of dry salt jumped 37.2% in 2007 to 31.7 million tons, according to the annual Salt Institute Statistical Report of US Salt Sales released in February, 2008. Salt industry revenues rose 11.9% to $1.68 billion, excluding transportation costs. Link.

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From Save The Bay's Turning Salt into Environmental Gold [PDF]:
"Salt production in the San Francisco Bay began during the 1860s. The current network of South Bay salt production ponds has been operated for approximately 50 years, with Cargill acquiring the ponds from Leslie Salt in the late-1980s. Annually, Cargill currently produces 650,000 tons of salt per year on a total of 26,190-acre solar evaporation pond complexes using approximately 40 million tons of Bay water."
Cargill's total potential salt production capacity in the South Bay is over 1 million tons.

(Note: The world’s largest solar salt works in Guerrero Negro, in Baja California. Link, Link).

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"In chemical terms, salt is the combination of a sodium ion with a chloride ion, making it one of the most basic molecules on earth. It's also one of the most plentiful: it has been estimated that salt deposits under the state of Kansas alone could supply the entire world's salt needs for the next 250,000 years." Link.

Seawater contains about 3.5% (by weight) dissolved minerals. Sodium chloride is 2.7% of seawater (w/w). The other 0.8% consists of calcium, magnesium and sulfate ions. As seawater evaporates, its volume decreases and the concentration of sodium chloride in the resulting brine increases. Thus, saltworks generally extract as sodium chloride a bit over 2% of the weight of the influent seawater. This means that solar saltworks are quite extensive in area.

The concentrating ponds often have distinct coloration, a pink or red, depending on the salt concentration and what species of plants and animals find it habitable. Link.

Salt pond colors reflect a complex interaction of plants, animals, and varying salinity.
• Low to mid-salinity ponds: Green algae creates the color.
• Moderate salinity ponds: Dunaliella algae proliferates and turns the ponds a lighter shade of green.
• High salinity ponds: High salt concentrations cause the Dunaliella to produce a red pigment. Halophilic bacteria contribute to the red and purplish-red hues. Millions of tiny brine shrimp in mid-salinity ponds add an orange cast.
• In choppy conditions, the colors appear murkier. Heavy rain can even turn the water clear.
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From Save the Bay's Turning Salt into Environmental Gold [PDF]:
Solar salt production begins when Bay water flows into intake ponds via pumps or tide gates. Bay water is less salty than seawater owing to dilution by freshwater from the Sacramento and San Joaquin Rivers, local streams, creeks, and wastewater treatment discharges. This is especially true in winter and early spring when rain and melting snow increase freshwater flows into the Bay. Cargill normally takes Bay water into its system during the dry season when the Bay's salinity is highest--beginning in April or May and continuing in to fall.

In Stage 1 ponds, water volume is reduced by 70 percent, with salinity increasing accordingly. In Stage 2 ponds, salinity increases further and gypsum (calcium sulfate) begins to precipitate. The final Stage 2 evaporator pond, called the "pickle pond", distributes concentrated brine to the crystallizer ponds. By the time pickle leaves the pickle pond each spring, 95 percent of the intake pond's original water has evaporated. The pickle water undergoes its final evaporation in the crystallizer ponds. Sodium chloride precipitates at a rate of approximately 40 tons per acre. By September, the salt bed is five to eight inches deep. Salt harvesting begins in October and continues 24 hours per day until the end of December.
"It takes five years for the sun and wind to evaporate Bay water and produce salt. During this time, the water is moved through a series of ponds. Each successive pond is saltier than the last and each supports different kinds of organisms." (Visitors Center educational sign at the Don Edwards San Francisco Bay National Wildlife Refuge.)

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In 1924, the Leslie California Company began acquiring an empire in bay-side lands suitable for harvesting salt by buying up smaller companies or defunct operations: Plummer, Cyrstal, Turk Island, Arden Salt. Few other climates in the world afforded recovery of salt by solar evaporating of sea water--the cheapest and most efficient salt recovery method of all. But salt wasn’t the entire motive for the swift rise of tidal land sales and new incorporations. Sea water is rich in other chemicals: bromine, magnesium, chlorine, sodium, and potassium. The need for most of these chemicals had been non-existent before 1900, but during World War I, San Francisco Bay’s giant salt concentration ponds provided a source of chemicals useful in the manufacture of explosives. By 1925, a small chemical plant was in operation in Newark using bittern--waste water from salt concentration ponds--to manufacture explosives component magnesium chloride. They called themselves the California Chemical Company. Link.

Bittern ponds reach salinities of 447 parts per thousand--nearly 13 times more saline than seawater. Bittern’s high salinity and "ionic imbalance" is toxic to aquatic species. Once bittern is produced, few options exist for its disposal. Prior to 1970, bittern that was not sold was discharged into San Francisco Bay. By the early 1970s, the federal Clean Water Act and the state Porter-Cologne Water Quality Control Act prohibited bittern discharge in to the Bay. Thus began ongoing, long-term bittern storage onsite. While some bittern continues to be sold for use in dust suppressants and de-icers, much of the bittern produced since the 1970s is stored within the South Bay salt pond complex. Recent operational changes have reduced bittern production, but the backlog of stored bittern remains.

Bittern disposal is an important consideration when assessing the feasibility of salt pond restoration. As part of its operations, Cargill conducts numerous maintenance activities in the Bay salt pond complex, but levee maintenance is the most common. This is required due to erosion, subsidence, and soil compaction.

Incidentally: "Kosher salt is characterized by its big crystals with large surface areas and its lack of additives, such as iodine. This size and shape allows it to absorb more moisture than other forms of salt, and this makes kosher salt excellent for curing meats."


Introduction and TOC
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2 comments:

lisa said...

There's a lot more to salt than I'd ever imagined!

Hannah said...

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