Evaporators for Waste Water
Evaporation of waste water has special problems: Scaling, corrosion and emissions. Energy costs can also be substantial. Waste water from a variety of sources is usually an even bigger challenge. To achieve long sustained runs, it is necessary to implement "input control" and feed preparation. It is usually easier to prevent problems. This is where input control is so important. Know what can be processed without problems, and only accept waste waters that meet the acceptance criteria. Feed preparation is about removal of solid matter and VOC type contaminants. pH of the water may have to be adjusted to prevent corrosion and emission of certain types of chemicals.
- Know how a waste is generated
- Establish a waste profile using screen tests
- Sample all shipments and perform screen tests. Compare to established profile
- Receive waste water and process the water to remove solids and VOCs. Adjust pH if required
- Determine volume reduction factor to apply by evaporation of a sample
Volume reduction factors depend on initial concentrations of TDS (total dissolved solids) and the nature of the TDS. Concentrates from the evaporation process can be solidified for land disposal, or they may have components that can be recovered. It is also possible to evaporate more water using chrystalizers/dryers. Pre-treatment of waters is best done using methods that do not add TDS.
The Direct Heat Transfer Evaporator uses a hot gas as energy for evaporation. One of the great advantages of using fuels such as Natural Gas, Propane or Fuel Oil is that the evaporation equilibrium temperature can be kept low, usually in the 50°C-60°C range. This prevents volatilization of many types of chemicals. Waste Oils may be used to lower energy costs. There will be a somewhat higher capital cost for the waste oil burner part. Hot gases from incinerators or biomass burners are also suitable energy sources. The best economics can be achieved when liquid waste is incinerated and the hot combustion gases are used for evaporation. New fuel is only used for primary ignition. Direct Heat Transfer evaporators also function as scrubbers. Hot combustion gases are quenched very rapidly with the added benefit that temperatures do not remain in the Dioxin formation range.
Equilibrium temperatures depend on water content and temperature of the hot gas used. The more water, the higher the temperature. It is possible to recover some energy by pre-heating air or water. This will condense water.
Direct Heat Transfer Evaporators are very simple to operate:
- Start re-circulation pump
- Start feed pump
- Start blower
- Start burner
- Set feed tank cutout level based on volume reduction factor
Shutdown procedure starts with burner shutdown. Blower is shut down last.