Most wastewater management projects involve a mix of different holding units depending on which stage of treatment or storage is needed. Tanks may be used as temporary storage or for holding leftover sludge, while larger ponds are preferred for processing and treatment. When choosing between the two options for each part of your project, consider these major differences between tanks and ponds for wastewater containment.
Above ground tank volume is much more limited than that of an excavated pond or in-ground tank. At a certain point, the weight and force of the wastewater held in the tank will require such thick walls that it will make the design unfeasible to produce. If you need to hold millions of gallons of wastewater at a time from a hydraulic fracturing operation or a power plant, only ponds and other in-ground basins can accommodate this level of flow. Ponds can be built to practically any size as long as soil conditions are taken into consideration.
All storage vessels, regardless of design or size, run the risk of overflowing if the supply of wastewater outstrips its capacity. Oversizing ponds and tanks is a good method for building in protection against surges and flood conditions. However, no amount of oversizing if enough if it’s not based on 25, 50, and 100-year flood records for your area. Secondary overflow protection methods, like backup containment pads and emergency drains, help you cope with an unexpected natural disaster or system failure. Tanks are harder to size for overflow control than ponds due to the fixed nature of prefabricated sizing. Custom built tanks are easier to design for flood protection, but they also tend to cost more to construct than in-ground lined ponds. In-ground designs do less damage to surrounding property when the flow overtops the banks since it rises up gradually rather than spilling over and down from the edge of a raised tank.
Cost Per Gallon
The cost of fabrication, installation, and design varies greatly between individual tanks and ponds. Yet, there are many opportunities to make lined ponds more affordable than tanks per gallon of storage or containment space. First, pre-existing excavation or even natural depressions can be used as the structure of the pond. It’s rare to have tanks capable of handling wastewater on hand already. Second, affordable and lightweight geomembrane liners allow you to create much larger ponds with little change in shipping costs for the materials. Third, pond liners with long-term warranties, tend to require less maintenance than metal, plastic, and even fiberglass tanks. Consider the cost of ownership over the course of a decade or more, not just the upfront costs of installation, when comparing tanks vs ponds for wastewater storage and containment.
Adding Pumps and Drains
If you plan to treat and process your wastewater rather than simply storing it for a certain period, you’ll likely need to integrate various filters, pumps, drains, and other additions into the system. Unless the tank is designed to work with these units, a lot of work is required to cut new openings and then seal them tightly against leaks. Geomembranes are easy to cut on the job site for quick installation of any penetrations. Sealing goes quickly as well thanks to the availability of matching boots that bond directly to the RPE liner material. Why struggle to connect pumps and filters to rigid tanks when you can work with a flexible material that conforms to any curve or shape?
As with large penetrations like pumps and bottom drains, wastewater treatment and processing stages also tend to require extensive plumbing. Some pipes may deliver steady supplies of additives to speed along the clarification process, while others are required to pipe off dangerous gases or remove thick sludge from the bottom of a tank. Ponds are much easier to plumb since the soil around the banks can be excavated at any time. If you choose to bury pipes under and around a lined pond, you’ll still be able to access them more easily than if they were under a heavy tank or solid concrete.
Surface Area and Depth
Finally, ponds offer a much higher surface area to depth ratio in most cases. It’s also possible to design deep and small ponds that mimic the aphotic and anaerobic depths of tall and narrow tanks. In most cases, large amounts of surface area are needed for safe storage and processing of wastewater. The larger surface area speeds up evaporation in drying stages where solids must be condensed through the loss of liquid. Tanks lose strength when built shallow and wide rather than tall and narrow, so they tend to cost more and more as they offer greater surface area. This is rarely true for custom built ponds.
Careful consideration of your project’s needs will reveal if tanks or ponds are better for you. In all likelihood, you’ll probably need both to accomplish all of your containment goals. Whether you need liners for full-sized ponds or just for making tank containment pads, you’ll find what you need at BTL Liners.