A sewage treatment lagoon is constructed by excavation of a large shallow basin with earth embankment. Wastewater entering such lagoon treatment systems receive little pre-treatment other than solids screening. The most basic of lagoon treatment systems treat raw wastewater by bacteriological or algal processes only.
Often lagoons are divided into a series of interconnected ponds. Such interconnected smaller ponds as part of a single lagoon has been demonstrated to be more efficient than separate larger ponds.
Each of these ponds treats the sewage using different types of processes as outlined below.
There are three types of ponds:
- Aerobic ponds have virtually no dissolved oxygen (DO), low algae population and are suitable for high organic loading. Detention times are long and counted in days
- Anaerobic ponds
- Facultative ponds treat the sewage by two processes, aerobic at the surface and anerobic at the bottom after the sludge has settled.
- Oxidation or Maturation ponds are shallow ponds to encourage an aerobic state and to permit the passage of ultraviolet light which decreases the amount of fecal pathogens and viruses present in the entering wastewater from the facultative pond. This wastewater may then be suitable for agricultural use.
Lagoons for sewage treatment are a very low cost option, but is subject to odour development and requires relatively large areas of land, depending on loading. Such systems are popular in rural areas, where loading is small due to low population and lagoons can be constructed away from population centres.
The effectiveness of lagoons is dependent on the following factors:
- pond depth
- shape and layout of the ponds within the lagoon to optimise hydraulic efficiency
- Organic loading of the entering raw waste
- Ambient temperature, rainfall, sunlight and wind velocity
The advantages of lagoons are
- low cost of construction
- easy and low cost of operation and maintenance
- very little to no use of chemicals