1. WASTEWATER TREATMENT IN LAHTI There are 2 wastewater treatment plants in Lahti - Kariniemi - Ali-Juhakkala In these slides we examine waste water treatment in Kariniemi, where the daily intake was 21 500 m3 (2011)

2. WASTEWATER TREATMENT IN LAHTI (Kariniemi) WWTP was built in 1975 Biological treatment – activated sludge process WWTP is underground in a cave (volume = 120 000 m3) Sludge treatment on the ground Operation of the WWTP happens from control room

3. SCREENING Screening removes big particles (e.g. paper) from wastewater. Screenings are washed, dryed and transported to a landfill.

4. GRIT REMOVAL Grit is removed by sedimentation, which means separation of solid, insoluable, particles into a concentrated slurry. Grit is washed, dryed and transported to a landfill.

5. PRIMARY CLARIFICATION In primary clarification sludge is removed by sedimentation. Settled sludge is pumped to a sludge treatment. Effluent goes to the aeration (biological treatment).

6. AERATION Aeration + secondary sedimentation = biological treatment In aeration, the activated sludge concentration is 3 g/l. Alltogether about 35000 kg of bacteria.

7. SECONDARY CLARIFICATION In secondary clarification the activated sludge sinks down and is pumped back to aeration. Effluent goes to the Nikula basin via tunnel, from which to river Porvoonjoki.

8. NIKULA BASIN Volume of the basin is 50 000 – 60 000 m3

9. PROCESS RECAP WATER TREATMENT Screenings Grit removal Primary sedimentation Aeration Secondary sedimentation Tunnel Nikula basin Porvoonjoki SLUDGE TREATMENT Thickening Anaerobic digestion Sludge dryed with centrifuges Composting

10. PROCESS RECAP BIOGAS Biogas is collected and used to heat wastewater treatment plant. The surplus gas is sold to power company. CAVE VENTILATION Cave is ventilated by two blowers -> 20 m3 / second Air is changed once per hour in the cave.

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