Design Considerations for Evaporative Ponds
In calculating cost factors for evaporative ponds, there’s a common assumption that larger ponds equal greater efficiency and lower cost. “Bigger = Better” can be a good rule of thumb in most industrial operations, but not necessarily in the case of handling brine waste. In fact, in areas with high evaporation rates, a greater number of smaller ponds is ideal. Of course, the effective size of a pond is not simply the volume it contains. Instead, it’s determined by two factors - the total surface area and the average depth.
Pond Size and Depth
In designing a system of evaporation ponds, the first requirement is that the system must have adequate capacity to accept brines at all times, under any condition. From an efficiency perspective, the optimal surface area of a pond is determined by local evaporation rate. In general, evaporation is maximized in ponds ranging from 10-18 inches deep, but the rate of evaporation slows considerably as the concentration of salts increases, reducing efficiency and potentially damaging some types of liners as they dry out and are exposed to increased UV radiation. Shallow ponds also store less water, so care must be made to balance evaporative efficiency with the necessary storage capacity.
On the other hand, ponds that are too deep tend to present problems with controlling wave action from windy conditions in addition to slowing the evaporation process. Ultimately, the optimal depth of an evaporation pond is determined by calculating the necessary surge capacity, how much capacity will be occupied by evaporating water, plus how much volume is expected to be taken up by accumulated salts. Additionally, a freeboard allowance for possible precipitation and wave action must be considered and designers must account for the changing evaporation rates over time as the brine becomes more concentrated.
Placement and Configuration
A series of smaller brine ponds constructed side by side makes for easiest management. With smaller ponds, the volume at risk for any individual spill is considerably easier to handle, and the ponds are not subject to intense wave action in windy conditions. To further mitigate this issue, place your ponds so the long sides are perpendicular to the prevailing wind direction. This gives the wind a shorter distance to build up waves, which can splash brine out of the ponds or damage levees.
With appropriate design and materials, it’s not necessary to always keep the ponds full or wet. While clay liners will degrade and crumble if they dry out, there are better liner options that are fully resistant to dry conditions and even UV degradation. Instead of managing issues associated with problematic liners, it makes more sense to use a high-quality liner and plan your ponds so that the annual evaporation rate exceeds the amount of water stored. This supports a design with a series of shallow concentrating ponds adjacent to crystallization ponds, where brine can be easily piped to the next pond in line with minimal disruption.
Choosing a Liner (part 1)
The liner is the single most important element in constructing an evaporation pond. It is the primary line of defense against the escape of caustic, deadly brine into the environment, where it can poison aquifers, destroy local ecosystems and render the affected soil sterile. Liners must be impermeable, durable and strong enough to withstand friction and wear during salt cleaning. They must also be highly resistant to UV damage over time. They should be easy to install, without risk of tears or punctures that could lead to immediate containment issues. Lightweight and flexible liners are ideal since they require substantially less heavy equipment during installation, which in turn reduces the likelihood of damage.
Some brine ponds may require multiple panels to fully cover the expanse of the pond. Even more likely, secondary containment areas may require a patchwork of panels. There are two considerations here in liner choice, since seams where two panels are joined are the most likely place to experience leaks.
First, eliminating seams wherever possible is the smartest option. This is most easily accomplished by choosing a lightweight, extremely flexible liner. Very heavy liners must be shipped and installed in smaller pieces because they are too unwieldy to handle. They usually require the use of heavy equipment to install. Liners that are relatively stiff also cause problems because some cannot be folded sharply for shipping without permanent damage. Even when you choose a liner that meets these requirements, check with your supplier. Some can produce and ship extremely large single panels in-house (exceeding 150,000 sq ft for some liners), minimizing the need for on-site seaming.
When seams cannot be avoided, the choice of liner can make a huge difference in the reliability of a brine pond. Since seams are the most vulnerable parts for leaks, it’s best to choose a liner that does not rely on tape or glue - those are difficult to apply correctly in the field and they’re simply not as strong as welded seams. RPE liners can be professionally welded either in the factory or on-site to create permanent bonds that stand the test of time.
