wastewaterevap101

WASTEWATER EVAPORATION 101
By Ronald G. Fink

A review of wastewater evaporators from the Romans to the latest technology, including water evaporation history, various technologies, and associated concerns and advantages.

History
Water evaporation was first used by the Phoenicians, Romans and Chinese to obtain salt from seawater. Large flats were filled with seawater and natural evaporation from the sun evaporated the water and left behind dry salt. The first boiling water evaporators in the U.S. are traced back to the Onondaga Indians from the Syracuse, NY area in 1654. The Onondagas used iron pots to boil local brine water down to a dry salt. Syracuse still has the nickname of "Salt City". Hence, the first U.S. boiling water evaporators. Without realizing it, they also discovered "server corrosion" and "evaporator meltdown", a problem that plagues traditional boiling water reactors to this day. The combination of water, salt and iron just doesn't mix well. Boiling water evaporators have not changed much over the last 350 years. Industrial wastewater evaporators are using boiling water technology and are still plagued with corrosion problems.
Many applications utilizing evaporators assume and plan for a less than one-year life. Corrosion and meltdown are anticipated and accounted for. A new technology could change the history of evaporators. It is a total novel approach: no steel vessel, no pot of boiling water, no slurry concentrate to dispose of, no corrosion, and no meltdown.

Definitions:
            Evaporation
The process in which a liquid dissipates or emits vapor, fumes or invisible minute particles into the air.

            Distillation
The process of boiling a liquid then condensing and collecting the vapor; used to purify liquids and to separate liquid mixtures.

            BTU's (British Thermal Unit)
Amount of Energy required to raise a 1 lb. mass of water 1°F @ 1 atmosphere.

            VOC's (Volatile Organic Compound)
Organic compound, which readily dissipates into the air at room temperature, i.e., benzene, gasoline.

            TDS (Total Dissolved Solids)
The amount of ionic matter dissolved in a fluid that can be measured by electric current. Dissolved solids in water can be deceiving. For example: seawater contains 35,000 ppm of salt, yet it will appear crystal clear. 35,000 ppm is equivalent to 3.5% of contaminants.

            Thermal Oxidation
High temperature breakdown of contaminants to carbon dioxide and water.

            TSS (Total Suspended Solids)
Substances suspended in a fluid large enough to be visible by the human eye and small enough to be kept in suspension by the movement of the fluid molecules.

            Condenser
An apparatus in which gas or vapor is condensed to liquid form.

            Slurry Concentrate
Boiling water evaporators leave a concentrated liquid slurry of the contaminants consisting of dissolved and suspended solids.

Efficiency:
Boiling water evaporation efficiency is based on some basic laws of physics:
•It takes 8,092 BTU's to evaporate one gallon of water
•Natural gas has a heating value of 1,000 BTU's per cubic foot (1 Therm=100,000 BTU's)
•Approximate cost of natural gas is $0.50 per Therm
Based on this very basic formula, it should cost about $.04 of fuel to evaporate one gallon of water under ideal conditions.

Lab Analysis:
It is very important to completely understand the waste stream so the proper technology, system and materials can be utilized. The waste streams need to be analyzed for pH, heavy metals, chlorides, dissolved solids and suspended solids. Chlorides and pH will affect the corrosion rate of boiling water evaporators, heavy metals and VOC's could affect emissions, and dissolved and suspended solids will affect pre-treatment and clean-out schedules.

Solids:

Dry Ash Residue from

Heavy Equipment Dealer

Dry Ash Residue from

Phosphertizing Plant

Dry Ash Residue from

Salt Truck cleaning

Both dissolved and suspended solids can create numerous problems for a boiling water evaporator:
•First, dissolved salts will raise the boiling temperature. The higher the salt content, the more heat and energy required to bring the waste stream to a boil.
•Second, suspended solids tend to settle to the tank bottom, creating a barrier factor that insulates the heat source from the wastewater, causing overheating of the steel vessel and creating carbide precipitation or carbon depletion of the steel. This results in a general weakening and eventual failure or meltdown.
•Third, dissolved and suspended solid volume. Clean out schedules will be dictated by the volume of solids in the waste stream. A waste stream containing 50,000 ppm of suspended and dissolved solids is equivalent to 5% solids. One thousand gallons of wastewater weighs approximately 8,400 lbs. If 5% is solids, that is the equivalent of 420 lbs of solids. Even using advanced Thermo Oxidation Dry Chamber Flash Evaporation, that will process the solids to a completely dry ash, you will have a pile of 420 lbs of solids as a dry ash. This is usually easily disposed of through normal waste channels. With a boiling water evaporator, these solids must be removed as slurry, usually three parts of water to one part of solids, to create a flowable slurry mixture. This slurry contains all the contaminants in the waste stream and usually must be handled by a licensed hauler to a licensed waste processer.

Traditional Wastewater Evaporation Methods:
Natural Evaporation Ponds
Rely on the combination of solar heat and wind to naturally evaporate water. This method requires a great deal of area, is slow and is subject to the weather.
Concerns
•Very slow
•Takes up a lot of space
•Requires large land mass and pond liner
•Odor problems

Forced Air Evaporators
Utilize blowers to force air in a counter current to a spray of water pumped to the top of a column and free falling downward. The evaporation rate is largely dependent on the water temperature and the dew point. This method is cost effective and highly efficient when the waste stream to be evaporated is preheated by another process and the wastewater does not contain volatile compounds (VOC's) that would be
readily transferred to the air, and thereby create air pollution. Not recommended for water high in dissolved or suspended solids as they will deposit on the internals and block the water and airflow. This design has no method of removing dried solids. Cost estimates depend on the temperature of the waste stream.
Concerns
•Source water should be preheated by process generating waste
•Efficiency is dependent on the relative humidity and water temperature
•Air permitting may be required. VOC's will create air pollution

Boiler Blow-Off Evaporators
Can evaporate large volumes (2-4 gpm) at a low cost and are effective on pretreated waste streams. Have no method of collecting or removing of suspended solids. Most of the dissolved solids are vaporized in the steam at 212°F and blown off into the atmosphere. VOC will be vaporized and will create air pollution. Therefore, all VOC's and solids must be removed from this system. Efficiency is 70%-85%.

Concerns
•No method of collecting solids, droplet carryover
•VOC's will vaporize and create air pollution
•Not suitable for distillation or water reuse

The fuel cost of bringing the entire wastewater reservoir (50-300 gal.) up to boiling (usually over 212°F).

The higher the salts or chloride in the dissolved solids, the higher the temperature must go before boiling occurs; the higher the TDS, the higher the temperature

Suspended solids are usually heavier than water and tend to settle, forming an insulation barrier, which tends to overheat the steel vessel while depriving the wastewater of the heat. Excessive heat builds up in the vessel bottom resulting in carbide precipitation, which is the carbon scale seen on overheated steel. This results in loss of strength, buckling, and eventually failure of the tank bottom, fire tube or melt down.

The ability to transfer as much heat to the water as possible. Systems that vent flu gas are generally less efficient than systems that utilize the flu gases for additional heating of the wastewater.

The key to fuel efficiency is to maximize the BTU value of the heat source. A poor efficiency system can still have good fuel efficiency if the waste heat is used for another source, such as water or air heating. A safety concern with overuse of waste heat is the cooling of the waste heat gases. If they are cooled to the point they no longer vent properly, a back up of flu gases can occur. A flu gas exhaust fan can be used to alleviate this problem.

Basic Boiling Water Tank Evaporators
Basically heating the water to its boiling point 212°F and exhausting the steam via an exhaust pipe. This method has no way to remove dried solids other than baking the tank contents down to a cake, which insulates the heat and holds it into the steel, causing early tank failure. Efficiency is usually 65%-75% depending on design. With the addition of a condenser you can distill the water for reuse.
Concerns
•VOC's vaporize to atmosphere as air pollution
•Acids and salts will attack the steel
•Cost of energy to heat the entire tank of wastewater to over 212°F
•Cost of extra energy to bring high salt content water to a boil
•Dissolved solids and suspended solids are periodically drained off in a slurry solution and this highly
concentrated liquid waste must be disposed of
•Cost of extra energy to overcome the insulation caused by suspended solids build up on vessel bottom
•Risk of system running dry resulting in melt down

Steam Tube with Water Exhaust Boiling Water Evaporator
Basically the same concept as a Boiling Water Evaporator. However, the hot exhaust gases are bubbled through the wastewater to improve heat transfer efficiency. VOC's will be vaporized and air pollution will result. Dissolved and

suspended solids removed periodically by draining a slurry. Efficiency is 75%-85%.
Concerns
•VOC's will be exhausted as air pollution
•Acids and salts will attack the steel, shortened vessel, and steam tube life
•Cost of extra energy to bring high salt content water to a boil
•Dissolved solids and suspended solids are periodically drained off in a slurry solution and this highly concentrated liquid waste must be disposed of
•Cost of energy to bring the entire tank of wastewater to over 212ºF
•Cost of extra energy to overcome the insulation caused by suspended solids build up on vessel bottom
•Risk of system running dry resulting in melt down

Heat Exchanger Boiling Water Evaporators
Heat a coil filled with a high temperature oil, which is pumped to another coil inside a tank containing the waste. The advantage to this system is that the tank can be made of non-corrosive polypropylene with no direct flame contact. The vapor can be distilled, efficiency is 70%-80%.Concerns
•VOC's will be exhausted as air pollution
•Coil failure due to corrosion
•Acids and salts will attack the steel, shortened coil life
•Cost of extra energy to bring high salt content water to a boil
•Dissolved solids and suspended solids are periodically drained off in a slurry solution and this highly concentrated liquid waste must be disposed of.
•Cost to bring the entire tank of wastewater to over 212ºF

Steam Tube Boiling Water Evaporators
Utilize a hot tube as a heat source. The flame is directed inside a steel tube thereby saving the tank from flame impingement and early failure. The tube will fail, however, it is easily replaceable and considerably less expensive than the tank. Efficiency is 70%-80%. The water is heated to 212ºF. The vapor can be distilled and reused.
Concerns

•VOC will be vaporized and will create air pollution
•Acids and salts will attack steel vessel
•Low tube life expectancy
•Cost of extra energy to bring high salt content water to a boil
•Cost of energy to heat the entire tank of wastewater to over 212ºF
•Dissolved solids and suspended solids are periodically drained off in a slurry solution and this highly concentrated liquid waste must be disposed of
•Cost of extra energy to overcome the insulation caused by suspended solids build up on vessel bottom
•Risk of system running dry resulting in melt down

New Technology:
Thermo Oxidizer - Flash Evaporation
Utilizes a ceramic chamber to flash evaporate atomized wastewater in a dry chamber. The atomized wastewater is heated in a chamber of hot gases to 800ºF - 1400ºF resulting in a complete flash evaporation of the water, leaving behind all the contaminants as a dry ash. All volatiles in the wastewater are burned and actually add BTU value. The heat source can be oil, gas, diesel, or used oil. A secondary chamber thermally oxidizes the flu gases to eliminate any air pollution.

Considerations
•No steel for acids and salts to corrode
•No steel vessel to replace
•No cost associated with bringing high salt content water to a boil
•No cost associated with energy needed to overcome slurry solution and suspended solids insulating heat source from the water
•No cost associated with disposal of concentrated slurry
•No VOC air pollution worries
•No cost of energy to heat an entire vessel of wastewater to 212ºF
•No risk of system running dry, resulting in meltdown
•No cost to haul off waste oil if waste oil burner is utilized

Future Technology:

The future may be in microwave technology!

Author:
Ronald G. Fink, President/CEO of RGF Environmental Group, West Palm Beach, Florida. Mr. Fink is a Mechanical Engineer who has published over 50 technical papers and holds numerous patents. RGF Environmental Group is a technology company with over 500 environmental products and the inventor and patent holder of Thermo Oxidation Dry Chamber Flash Evaporators. Visit www.rgf.com.