The key difference is that facilities that are designed and managed as lagoons provide treatment in addition to storage. In some areas, people have come to refer to any earthen facility that contains wastewater as a lagoon, so referring to the specially designed systems as “treatment lagoons” may help to appropriately distinguish them.
The most noticeable difference between a storage pond and a treatment lagoon is size. For a given scenario, a treatment lagoon is much larger than a storage pond.
A storage pond is an earthen structure designed to store manure and other biodegradable byproducts of animal production (soiled bedding, wash water, unclean runoff, etc.) over the critical time period when it cannot be applied to farm land. This period could be three to six months or more, depending on geographical location and/or regulatory requirements. Before the next critical storage period begins, all of the storage pond contents that equipment can practically reach must be removed to make room for the manure and wastewater generated during the upcoming storage period. Some states require that additional storage volume be provided so 1 to 2 feet of liquid are left in the bottom to prevent the earthen floor from drying out.
A treatment lagoon provides volume for storage (like a storage pond) but also provides significant additional volume to accommodate the dilution that is necessary for desired biological treatment of collected material. A certain volume of liquid?called a “permanent pool”?must be maintained in a treatment lagoon at all times to ensure that the needed dilution and desired treatment occur. As the contents of a treatment lagoon are biologically broken down, a small fraction settles to the bottom and forms a sludge layer. A lagoon must also provide volume for sludge accumulation, and this sludge must be removed periodically to maintain the design treatment volume. For these reasons, treatment lagoons are much larger than their storage-only cousins and are not normally emptied while in operation.
Biological treatment is highly dependent on temperature. For this reason, lagoons do not work as well in colder midwestern U.S. climates as in more southern and western U.S. locations.
While treatment lagoons are generally more expensive to build, a lagoon stabilizes added organic matter, reducing odor and influencing nutrient content. The resulting liquid contents of a treatment lagoon are much lower in nutrient content. Less land is required annually for application of “lagoon effluent” than for slurry from a storage pond. However, in years when sludge is removed, land requirements may be dramatically higher since phosphorus and other minerals are retained in the sludge. The storage pond, on the other hand, conserves more nutrients. Material taken from a storage pond is generally more nutrient-dense than is lagoon effluent and has more value overall for use in growing crops. Material from a storage pond is also more odorous, so there is greater need for it to be promptly incorporated into the soil than for lagoon effluent.