Forced Evaporation of Wastewater
What is Forced Evaporation? Forced evaporation is heating wastewater with a heat source, causing it to boil and vaporize. The vapor is vented to the atmosphere or recaptured through a condenser/chiller. A volume reduction of up to 95% can be achieved, leaving only the residual solids or sludge to disposed of. Evaporation rates range from 3 gallons per hour to over 100 gallons per hour depending on the size of the evaporator, type of heat source and humidity levels.
Components of Forced Evaporators: There are four main components to most forced evaporation systems: a heat source, a wastewater reservoir, a steam vapor evacuation system, and a control system.
Heat source options include natural gas, liquid propane, electric or steam heat.
The wastewater holding reservoir is usually insulated, and constructed of mild or stainless steel. Holding capacities range from 50 gallons to over 500 gallons.
The steam evacuation system is a blower or inline draft-inducing fan. It is critical for maximum evaporation rates to remove the vapor quickly and efficiently.
The control system operates the evaporator. This system controls the level of the liquids, the temperature, and provides a series of safety controls to prevent to high of temperature or over-flow conditions.
What Can You Evaporate? Many waste streams are mainly water. Disposal costs can be cut by as much as 95% by reducing the volume with evaporation. The list of what can be evaporated is almost endless. Aqueous cleaning solutions, rinse water, water based machining coolants, acid and caustic baths, air compressor condensation, floor scrubber water, and mop bucket water, etc. are some common waste streams currently being evaporated.
What Should Not Be Evaporated? Most water testing laboratories of evaporators offer a Material Stream Assessment (MSA). This test, along with your Material Safety Data Sheets (MSDS), can tell you if your waste stream is a candidate for evaporation. Effluent that should be avoided are high concentrations of flammable liquids with a low flash points, waste streams with high levels of Volatile Organic Compounds (VOC’s) and any wastewater stream that contains more than trace amounts of Mercury. Because of Mercury’s low boiling point, it should not be evaporated because of air pollution emissions. Waste streams with high amounts of suspended or dissolved solids should only be evaporated in units designed for easy clean out. Highly corrosive effluent should only be evaporated in units constructed of high nickel content chloride resistant alloys such as SM0254 or ALGXN.
Recovery and Disposal of Residual Solids: Your individual waste stream will determine the amount of residual solids or sludge left after the evaporation process. The water content of these solids will vary by unit design and manufacturer. Unwanted recovered solids should be tested to determine the proper method of disposal. A Toxicity Characteristic Leaching Procedure (TCLP) test will tell if your residual solids are landfill able or must be hauled by a licensed waste hauling contractor. Non-leaching materials such as HB-7 spill grabber, peat moss or other adsorbents may be added to your residual solids to make them non-leach able and landfill able. You will need to contact your local sanitary refuse company on their disposal handling requirements.
The feasibility of recovering certain substances from the wastewater is determined by the value of the recoverable substance. Recovery of gold, silver, copper or titanium and other valuable metals can aid in the repayment of your investment.
Permitting: The need to permit evaporators varies from state to state and from local and county air sheds. You will need to contact your local Air Quality Control Board to see if permitting is necessary. The Material Stream Assessment will help you in the application portion of permitting. In many cases no permitting is necessary or can come under existing permits. Sometimes a permit for wastewater evaporation is not necessary, but a permit for a gas fired appliance is required.
Construction Materials: Several factors dictate the type of construction materials used. Wastewater, when heated, can be very corrosive. When pH levels are very high or very low or the chloride content is high, selecting the proper construction materials is critical. For a waste stream with low corrosively, high calcium, or oily water, mild steel is used successfully. As the corrosively increases, the use of stainless steel or exotic chloride resistant alloys is required. It may be possible to lower the corrosively in pretreatment by lowering or raising the pH level of the waste stream.
Types of Forced Evaporators: The five most popular wastewater evaporation systems are listed below. There are other systems available but not presented in this paper.
Under Floor Heating-This design transfers the heat from an insulated heat chamber through out the entire floor. The heat source options include natural gas, liquid propane, or electric heat.
Immersion Tube-This type of design uses a heated tube under the solution as a heat exchanger. The heat source is usually natural gas, liquid propane or steam.
Submerged Combustion-Submerged combustion is similar to an immersion tube. The difference is the small holes in the top of the heat bubble, allowing bubbles of hot gases to come in contact with the solution. The heat sources are usually natural gas or liquid propane.
Direct Injection-Evaporation is achieved by spraying wastewater directly into a natural gas or liquid propane flame.
Thermal Oxidation-This is also known as an incinerator. The thermal oxidizer burns the wastewater more than evaporating it. Operating temperatures are as high as 1400 degrees Fahrenheit.
Sizing and Design Considerations: The first thing to consider when sizing an evaporator system is how much wastewater you generate per hour. It is easy to make the mistake of sizing your systems evaporation rate too close to your actual waste stream flow.
There are several factors to take into consideration, which directly affect overall evaporation rates. Most manufactures rate their equipment using clean water as a standard when setting their maximum evaporation rates. As the solids content of the wastewater increases, the evaporation rate will decrease. When evaporating high solids waste streams, extra time is needed for more frequent clean outs. If you are going to evaporate high solids waste streams, choose a unit that is ergonomically easy to clean.
Another factor to consider is heat up time between evaporation cycles. Heat up times can vary due to the ambient temperature of the waste stream and the type and thermal efficiency of the unit. As the thermal efficiency of the evaporator goes up, so does the purchase price of the unit. Average costs per gallon to evaporate start at 3 cents per gallon and may go as high as $1.00 per gallon, depending on the design and energy costs.