Pond based systems without mechanical mixing or aeration - they still have application

The earliest wastewater treatment facilities consisted of lagoons with no mixing and dissolved oxygen provided by surface diffusion/wave action and algae growth. While they take up much more space than more advanced treatment systems, lagoons/ponds do not have high operational costs and are often the best choice for remote locations or areas where there is not treatment alternative.

Key parameters for lagoon treatment are loading rates per unit surface area. Surface area is important for oxygen diffusion and algae growth. Much like constructed wetlands design, the natural biological decay processes are important for effluent quality. Below are the general design guidelines from Curi & Eckenfelder (1980).