Chemical precipitation in wastewater treatment involves the addition of chemicals to alter the physical state of dissolved and suspended solids and to facilitate their removal by sedimentation. The chemicals used in wastewater treatment include Alum, Ferric chloride, Ferric sulfate, Ferrous sulfate, and Lime. The inherent disadvantage associated with most chemical unit processes (activated carbon adsorption is an exception) is that they are additive processes. (Metcalf & Eddy, Wastewater Engineering Treatment Disposal Reuse, Third Edition, page 301-303). This problem is eliminated in the Electro-coagulation process. These chemicals are not only expensive, but, more importantly, the net increase in the dissolved constituents in the wastewater render it impractical or impossible to reuse.

Electro-coagulation uses electricity to precipitate the dissolved and suspended solids. The total dissolved solids in the liquid usually decrease by 27 to 60 percent. This enables the water to be reused in many applications, such as water reuse in steam cleaning operations. Reuse of the water provides a major advantage because this eliminates all EPA and POTW discharge concerns, to say nothing of the replacement costs of the water itself.

Electro-coagulation produces a cleaner water than either chemical precipitation or sedimentation (Waste water Engineering, page 488 ). As discharge requirements become more stringent EC will become more essential.

Constituent: Percentage of removal by:

	Electro-coagulation	Chemical Coagulation	Sedimentation
TSS	95 to 99%	80 to 90%	50 to 70%
BOD	50 to 98%	50 to 80%	25 to 40%
Bacteria	95 to 99.999%	80 to 90%	25 to 75%

The handling and disposal of the sludge resulting from chemical precipitation is one of the greatest difficulties associated with chemical treatment.

Metal		Range		Median
Arsenic (As)		1.1-230		10
Cadmium (Cd)		1 - 3,410		10
Chromium (Cr)		10 - 99,000		500
Cobalt (Co)		11.3 - 2,490		30
Copper (Cu)		84 - 17,000		800
Iron (Fe)		1,000 - 154,000		17,000
Lead (Pb)		13 - 26,000		500
Manganese (Mn)		32-9,870		260
Mercury (Hg)		0.6 - 56		6
Molybdenum (Mo)		0.1 - 214		4
Nickel (Ni)		2 - 5,300		80
Selenium (Se)		1.7 - 17.2		5
Tin (Sn)	2.6 - 329		14
Zinc (Zn)		101 - 49,000		1,700

EPA concerns about toxic leachable metal build up in soils may cause many sludges to be deposited in hazardous waste land fills. This not only increases the disposal cost several fold, it eliminates the beneficial soil additive effect discussed earlier.

The reality of the Hazardous waste limits can be illustrated by acceptable disposal limits at non hazardous disposal facilities like Forward, Inc. Stockton, CA. Forward, Inc. can only accept waste for disposal with levels of metals below the following leachability listed limits:

Element	Title 22 STLC (mg/l)	Title 22 TTLC (mg/kg)
Antimony (Sb)	15.00	500
Arsenic (As)	5.00	500
Barium (Ba)	100.00	10,000
Beryllium (Be)	0.75	75
Cadmium (Cd)	1.00	100
Chromium (Cr)	560.00	2500
Hexavalent (Cr+6)	5.00	500
Cobalt (Co)	80.00	8,000
Copper (Cu)	25.00	2,500
Lead (Pb)	5.00	1,000
Mercury (Hg)	0.20	20
Molybdenum (Mo)	350.00	3,500
Nickel (Ni)	20.00	2,000
Selenium (Se)	1.00	100
Silver (Ag)	5.00	500
Thallium (Tl)	7.00	700
Vanadium (V)	24.00	2,400
Zinc (Zn)	250.00	5,000

Forward has no fixed limit for petroleum hydrocarbons except:

Benzene at 0.5 mg / l (TCLP).

Electro-coagulation may soon move from the optional treatment method to the essential treatment method as the US EPA begins to enforce the laws protecting the environment from toxic wastes, including heavy metals. Electro-coagulation cleans most wastewater streams better, with less operating cost, producing less sludge, with the sludge being a better quality than chemical precipitation. The reuse opportunities for the water is increased because dissolved solids are not added to the waste water stream; and usable products are harvested because the metal oxides pass leach ability tests, allowing the sludge to be utilized as a soil additive.