Primääripuhdistuksessa

Primääripuhdistus, known in English as primary treatment, is the initial stage of physical wastewater treatment. Its primary goal is to remove settleable solids and floating materials from raw sewage before it undergoes further treatment processes. This stage is crucial for reducing the overall organic load and preventing clogging of subsequent equipment.

The process typically involves passing the wastewater through a series of screens or grit chambers. Screens are used to remove larger debris such as rags, plastics, and other solid objects that could damage pumps or other machinery. Grit chambers, on the other hand, are designed to slow down the flow of wastewater, allowing heavier inorganic materials like sand, gravel, and eggshells to settle to the bottom. These settled solids, known as grit, are then removed and disposed of.

Following screening and grit removal, the wastewater flows into sedimentation tanks, also called primary clarifiers. In these large tanks, the flow velocity is further reduced, allowing finer suspended organic solids to settle out as sludge. Simultaneously, lighter materials like oil and grease float to the surface and are skimmed off. The settled sludge is then collected and sent for further processing, often anaerobic digestion, while the partially clarified wastewater, known as primary effluent, moves on to secondary treatment. Primääripuhdistus significantly reduces the concentration of suspended solids and biochemical oxygen demand (BOD) in the wastewater.