Evaporation of waste wash water occurs during normal operation of the pressure, steam or automatic vehicle wash system. It is generally assumed that 10-30% of wash water is lost to evaporation. Accelerated evaporation can be used to dispose of excessive wastewater, highly contaminated wastewater or to remove recycled water with high dissolved solid (TDS) build up.

General Definitions:

Evaporation The process in which a liquid dissipates or emits vapor, fumes or invisible minute particles.
Distillation A process that consists of driving gas or vapor from liquids or solids by heating and condensing to liquid products once again.
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 and can be measured by electric current.
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.

Efficiency:

Water evaporation efficiency is based on some basic laws of physics:

Based on this very basic formula, it should cost about $.05 of fuel to evaporate one gallon of water. However, the following efficiency factors must be considered:

The fuel cost of bringing the entire wastewater reservoir 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.

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.

Problems

  • Very slow

  • 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 airflow. This design has no method of removing dried solids. Cost estimates depend on the temperature of the waste stream.

Problems

  • Source water should be preheated by process generating waste.
  • Efficiency is dependent on the relative humidity and water temperature.
  • Air permitting may be required
Boiling Water 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.

Problems

  • 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°
  • 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 and melt down
Steam Tube with Water Exhaust 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%.

Problems

  • 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 extra energy to overcome the insulation caused by suspended solids build up on vessel bottom
  • Risk of system running dry and melt down
Heat Exchanger 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%.

Problems

  • Air permitting may required
  • 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
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%.

Problems

  • No method of collecting solids, droplet carryover
  • VOC’s will vaporize and could create air pollution
  • Not suitable for distillation or water reuse
Steam Tube 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.

Problems

  • 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
  • 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 and melt down