Overland flow systems

Overland flow systems 

 

In overland flow, wastewater is sprayed onto an inclined vegetated terrace and slowly flows to a collection ditch. Purification is achieved by physical, chemical, and biological processes, and the collected water is usually discharged into a nearby stream.

• Overland flow systems are used to achieve secondary treatment effluent quality for effluents coming from primary treatment facilities.
• Overland flow systems ensure high removal of Nitrogen(N), Suspended Solids(SS) and Biochemical Oxygen Demand (BOD)
• Overland flow systems apply previously treated wastewater to a vegetation covered graded land
• Application is done by grated pipes or nozzles at top of slope or by sprinkler systems within the site
• It is best suited for impermeable soils
• Overland flow systems require:
• low permeability soils
• grading within 2 - 8%

• Overland flow systems use the following mechanisms for removal of BOD and SS:
• Biological oxidation
• Sedimentation
• Filtration

• 75 - 90% of Nitrogen is removed by:
• Plant uptake
• Denitrification
• Ammonia volatalization

• 70 - 90% of Phosphorus can be removed by addition of alum of ferric chloride prior to land application. Phosphorus is removed by:
• Fixation processes in soil matrix
• Crop uptake

Effluent is collected in ditches and can be reused or discharged to a surface water body.

The treatment goal of overland flow systems is secondary treatment of wastewater and Nitrogen removal. In order to achieve these objectives the system requires a warm season. At the end of overland flow systems the effluent will have following characteristics:

• BOD < 10 mg/L
• TSS < 10 mg/L
• Total Nitrogen < 10 mg/L