The lagooning technique is a natural treatment technique that consists in the accumulation of wastewater in ponds or basins, known as biological or stabilization ponds, where a series of biological, biochemical and physical processes take place. Lagoon is a stationary system having a continuous flow: several ponds working in parallel in which the inlet flow and the outlet flow are equals form lagoon plants. The ponds depth range from 1 to 5 m. Wastewater stays in the ponds from 2 to 10 days in order to be purified.

Lagoon purification is very efficient as wastewater treatment method to purify wastewater from small or medium communities and from communities having variable population. Some lagoon plants could be built to purify wastewater produced by food seasonal industries.
 

Biological ponds types

There are two main categories of biological ponds: anaerobic and oxidation (aerobic) ponds. The ponds are different one from the other in shape, depth, organic rate, level of treatment. A lagoon system generally includes an anaerobic pond and one or more oxidation ponds. The first realizes primary treatment, the second secondary or tertiary treatment. The system is designed according to wastewater features, number of inhabitants served, and quality requirements of the outlet stream, climatic conditions. Before lagooning, wastewater pre-treatment is always provided: grid filtration, oil removal and possibly sand removal.

Anaerobic ponds

In the anaerobic ponds the conditions are anaerobic, due to the lack of oxygen and light. The anaerobic processes that take place in them destroy and stabilize the organic matter: the organic material removal is about 70%, the suspended solid removal 30% and the bacterial removal in quite low.
Anaerobic ponds are deeper than oxidation ones (4-5 m) and are usually the first step in a lagoon system. Bad smells are often produced in an anaerobic pond, but the forming of a superficial shell can avoid their emission in ambient air.

Oxidation ponds

In the oxidation ponds the purification processes are principally aerobic and the oxygen is provided by photosynthetic activity of algae, which grow in them and by the exchange between the pound surface and the ambient air.
In optional ponds the processes are aerobic on the surface, where the sunlight can penetrate allowing the photosynthetic activity of algae. The processes are anaerobic on the bottom, due to the lack of oxygen. These ponds are up to 2 m deep. They usually are the second step in a lagoon system.
In aerobic ponds the processes are aerobic for the whole pond depth, usually of about 0.5 – 1 m. They are placed after the optional ponds generally.
The maturation ponds are 1.0 – 1.5 m deep and have a retention time of 2-3 days. They can be used as the last stage in a lagoon system, to remove the last organic compounds, suspended solid particles, nitrates, phosphate and pathogenic microorganisms.

Purification yields

Lagooning purification is more reliable than traditional purification plants: organic compound removal is up to 90%, nutrient substances removal is around 70-90%, the suspended solid particles are lower, due to the presence of micro algae.
High interest is addressed to these systems because of their very strong removal of pathogenic microorganisms.

Building and managing features

Lagooning systems are simple to build, not requiring the operation of specialized stuff. They consist of one or more ponds laid out in parallel or in series; the plant scheme can be easily varied to be adapted to different demands.
Before oxidation pond sedimentation or griding is needed. In some cases it is useful to recirculate the outlet stream to dilute and oxygenate the wastewater. Unfortunately this can increase maintenance and building costs.
The maintenance operations are very simple: vegetation trimming, cleaning of inlet and outlet pipes, algae and foam removal, banks damages reparation. Mud removal is quite simple: the mud layer grows of 2-3 cm per year, then mud removal can be carried on every 5-10 years.
The lagoon is efficient and cost effective, thus quite competitive. Often the only limiting factor is the land availability.

Check also the other natural wastewater treatment techniques: phytodepuration and storage in tanks.