Containment
Since wastewater storage facilities often serve for extended periods, engineers must consider long-term risks. Over many years of service, the safety of municipal and natural waterways and the air, soil, wildlife, and humans are the primary concerns. That's a pretty tall order, and reliable, effective solutions can represent a significant operational expense.
Regardless of the source, wastewater should be impounded according to its specific properties and contaminants. In some cases, wastewater might be stored in above-ground tanks such as drums, holding tanks, and structures made of concrete or steel. These above-ground solutions may be the most practical option if you're working in a municipal area where space is limited, and the location of underground utilities reduces your ability to create an in-ground impoundment. Because of the proximity to human populations in these situations, you'll need to carefully consider factors such as the quantity of water, length of storage, the security of the site, applicable regulations, and finally, potential threats to the public, environment, and first responders in the case of a breach.
In other cases, water is stored in-ground, often in specially constructed ponds. These solutions apply when the wastewater is produced on-site in facilities located outside of urban centers. Many agricultural and livestock businesses, mining operations, and chemical processing facilities use in-ground ponds to manage their wastewater in situations like these.
Other in-ground storage options include pre-formed vaults and tanks, which drop into a hole in a single piece. While it seems that this may be a relatively fast and easy solution compared to pond construction, the cost and time required for installation are roughly equivalent. Moreover, whether pre-cast or built-in place, tanks and vaults are considerably more limited in storage volume than in-ground basins. When you need to store millions of gallons of wastewater at a time, a pond quickly becomes more economical to build and maintain than dozens of individual tanks or vaults. Don't forget to consider the long-term ownership costs when comparing prices for these two different water storage methods.
Processing
All sewage lagoons rely on bacteria to break down waste. While some of these bacteria can be harmful, they're still valuable in their ability to reduce the nutrient levels of wastewater. Different bacteria flourish if a sewage lagoon has plenty of oxygen mixed into the water rather than the anaerobic conditions of a still and deep pond. Planning the digestion and primary treatment of a steady supply of sewage often requires the use of more than one of three distinct types of lagoons.
Processing is the primary treatment stage for sewage and mixed wastewater. Most significantly, in-ground pools found at a water treatment facility are processing ponds. Any first-in-line methods (digestion) employed to passively or actively improve wastewater is considered a processing treatment.
Polishing is the next stage, where fine particles that remain after processing settle out. Once TSS and BOD measurements meet standards and any final disinfection requirements are satisfied, the fully treated wastewater is ready for release.
In some small water treatment setups, processing and polishing are handled sequentially in the same pond, and occasionally they even proceed concurrently in a facultative (mixed) pond, where both aerobic (oxygenated) and anaerobic (oxygen-free) processes are maintained.
Anaerobic lagoons process animal waste in most cases. The relatively uniform mixture of organic waste coming from livestock concerns is well suited to slow, steady breakdown throughout the layers of water and sludge. "Anaerobic" means "without oxygen," so this process is appropriate for deep ponds with less surface area than other lagoons. The less surface water is in contact with the air, the less oxygen can exchange, and the better conditions are for anaerobic digestion. To this end, covers are often used with anaerobic lagoons to limit oxygen exchange even more.
Anaerobic digestion is a slow process, so these processing ponds have long retention times. It can take months to process manure wastewater to the point where it's ready to advance. Still, anaerobic lagoons are entirely passive and require no pumps and little maintenance other than sludge removal, making them a good fit for individual farms and smaller livestock holdings.
Aerobic Lagoons are usually naturally aerated. Natural aeration can come from something as simple as a drop from the inlet pipe in a small residential lagoon, but most larger plants achieve this by keeping their ponds as shallow as possible. Shallow ponds maximize surface area, allowing for natural gas exchange enhanced by wind and water movement. Naturally aerobic ponds process water substantially faster than anaerobic lagoons and are less likely to release unpleasant odors or dangerous gases. However, they require a much larger footprint, and substantial commercial facilities or livestock farms may need more space than is practical. If aerobic processing is needed, but natural aeration requires too much space and time, powered aeration is the only choice.
