WASTEWATER
The consequences of incorrectly operating wastewater evaporation ponds By Bob Ballantyne
E
vaporation can seem so simple. All that is required is some surface area, latent heat from the sun and air movement. It is a simple phase change of water molecules from a liquid to a gas phase. However, if you factor in any ion content, which creates polluted or partially polluted waters, evaporation becomes a little more challenging. Operating in an outdoor environment, with the need to scale up to multi-acreage ponds, can also make evaporation challenging. The consequences of incorrectly applying the operation and methodology of evaporation becomes very real and very expensive. Resource West Inc. (RWI), a wastewater evaporator manufacturer, wanted to find a way to arrest ion content. We wanted to keep it from converting to a dry aerosol and floating away as sub-10-micron particles, which have zero sedimentation rate. The investigation’s task was to keep contaminants within the liquid in the impoundment pond. Also, to concentrate the solution for more efficient treatment and disposal by reducing volumes. A review of the “state-of-the-art” evaporation machinery ensued. First up for evaluation were pond surface evaporators. We found that there was not much of a difference over the natural evaporation rate of the test site impoundment. Using an infrared camera to explore the problems it became obvious that the major drawback was that no air was being introduced. These types of units were basically just raising the surface of the impoundment up in the air. This allowed only a small increase in natural evaporation by inducing a slightly higher surface area above the area of the impoundment they were operated on. But, it did so by stopping any natural evaporation of the surface area it was covering up. So, only the arc induced by sedimentation rates of droplets really added any evaporation enhancement. The spray itself stopped all evaporation beneath it due to the high humidity area above the pond. This completely eliminated the boundary layer and replaced it with a new one at a higher elevation. Subtracting the natural evaporation rate of that area from the rate these devices were able to achieve meant there was a very large consumption of power for a relatively small increase in evaporation enhancement. As a result of these experiments, we recommend that large volumes of air sweep the boundary layer. Induced draft would be a component of a state-of-the-art natural enhanced evaporator. The next step involved operational testing of our legacy units, which were available from 25 hp to 100 hp. They are mostly high-pressure axial flow fans, which can move large volumes of air, at very high pressures. These machines definitely evaporated a lot of water. However, we found large discrepancies between the volume 22 | August 2021
A PitBoss evaporator from RWI.
of air being moved and the amount of water removed from the pond on our five-acre test site. Lots of water was disappearing, but the amount of air being induced was way off the mark for having the capacity to carry away the liquid that had converted to a gas phase. Looking at psychrometric charting and ambient air conditions, the drop from dry bulb to wet bulb is a very simple calculation. The induced draft could not hold the amount of water that was unaccounted for at the end of a run. We knew dilution into drier air during flight would give us a secondary evaporative loss, but surface area calculations, even when considering thermophoresis effects, were well below a level that could account for the amount of water missing from the experiment impoundment. This led to an investigation into what was causing this additional loss from the experimental site’s 75 m3 pond. Leaks were ruled out and the surface area of the pond and lining system was already cataloged and normalized to the loss predicted by the Davis Pro II weather station that monitored and recorded the weather conditions in 15-second intervals. We turned to an infrared camera for a deeper look into what was going on with the loss. The investigation led us down a very deep rabbit hole as every experiment seeking answers continued overleaf…
Environmental Science & Engineering Magazine