BeneVap evaporators are capable of handling wide variations in water chemistry but each installation requires some customisation. While wastewater types share some common characteristics, each individual source has a unique chemical signature that must be considered before attempting to treat or eliminate it. We start with a thorough lab analysis of the wastewater and process that with a computational model to predict how it will behave in the evapo-concentration process.
Performance factors affected by chemical constituents in the water include:
- Reduction of volume
- Degree of concentration
- Residual concentrate chemistry
- Scaling potential
- Corrosion potential
- Foaming potential
- Air emissions and odours
Salt content of wastewater limits how much volume reduction is possible. For example, a saturated brine of pure sodium chloride holds about 350 g/L of NaCl before salts begin to precipitate (form solids). Seawater holds about 35 g/L of salt, so theoretically it can be evaporated until it concentrates ten times for 90% volume reduction. Whereas a water with 3.5 g/L salt might be concentrated 100 times for 99% volume reduction.
However, it is a bit more complicated than that because there are not many wastewater sources that contain pure sodium chloride. They typically contain a variety of inorganic constituents that will precipitate well before sodium chloride does. Several divalent cations, such as calcium and barium, will combine with carbonates, silicates, sulphates and phosphates to precipitate. The newly formed compounds can be present as fine suspended solids but they also can form scale that collects on the equipment.
Scale and corrosion
The Langmuir saturation index (LSI) is calculated in the pre-analysis and is a good place to start in predicting whether the evaporating solution will cause corrosion of the equipment or cause scaling. LSI takes into account the measured pH and hardness of the water. If the index is less than zero it will tend to cause corrosion of metal if it is above zero, scaling is likely.
However, these values change as the wastewater is reacted in the concentration vessel. For example, an alkaline solution may become more acidic when ammonia is driven off or calcium carbonate is precipitated. So it is necessary to consider the effects throughout the evapo-concentration cycle.
We use geochemical models to estimate these effects as the solution concentrates. However, these are not able to fully estimate effects across the wide range of temperature inside the concentration vessel. The submerged combustion process introduces extremely hot gas bubbles into the liquid pool and these actually do the work of vaporisation. While the temperature in the main pool of liquid is in the range of 85-90°C, the real action happens at the bubble interface which can be 800-1000°C.
We have developed and continue to develop a library of water chemistries in order to compare them against performance in BeneVap evaporators. This is an important tool in evaluating the feasibility of treating a given wastewater.
Wastewaters inevitably carry an organic carbon component and it is important to understand the nature of those compounds before embarking on a project. Some organics are more volatile than others and some decompose in the process and some are quite recalcitrant to transformation. Some cause odours or cause foaming.
Proteins and fatty acids in wastewater tend to produce foam which can inhibit evaporation efficiency. This is common with landfill leachate or pond water that has algal growth. We can often predict the effects of foam in benchtop studies. We like to know the total organic carbon (TOC) or dissolved organic carbon (DOC) when evaluating a wastewater.
The amount of organic carbon may limit the degree of concentration. The extremely hot gas bubbles will decompose some organic compounds like VOCs which are often destroyed in the process.
In some jurisdictions there are concerns about the presence of persistent pollutants such as pesticides, dioxins and PFAS. So it is very helpful to review a thorough analyses of these in the wastewater. It is possible that trace amounts of these would escape in emissions.
The BeneVap process tends to drive off ammoniacal-nitrogen from wastewater. This is particularly effective when the solution is alkaline. As gaseous ammonia volatilises from the vessel it combines with water vapour and carbon dioxide in the stack to form ammonium hydroxide and ammonium carbonates. As such there is little or no ammonia odour associated with the process.