Most likely, this morning some of us went to the bathroom, brushed our teeth, took a bath, and did our things in the toilet, maybe followed by washing the dishes that have been sitting on the sink since last night dinner. Little did we know, the water (or better we call it wastewater) that we used is flushed and drained to a collection pipe. In the case of EM-41 class, our wastewater is collected by sewer pipes in Dresden which has a total length of 1700 km with diameter of up to 2 meters.
These pipes are laying under the roads of Dresden and deliver our wastewater to a specific plant in Kaditz, an area northwest of Dresden. Occupying an area as large as 25 football fields, the plant was constructed in 1910 to treat the wastewater produced by Dresden’s population. Currently, everyday approximately 120.000 m3 of wastewater coming from 700.000 people is reaching the treatment plant. Since Dresden’s wastewater system applies a combined system, the incoming wastewater is collected together with rainwater and treated together in the wastewater treatment plant (WWTP).
To give you some illustration about the smell that we inhaled that afternoon, here is a picture of raw wastewater which just arrived in the WWTP; a dark-brown water.
Dirty, hence the dark color. Full of organic materials, which could lead to oxygen depletion in the aquatic ecosystems. Full of microorganisms, which some are harmful to humans, and of course smelly. And these are the shades of brownish colors of the same wastewater during the treatment processes:
More or less 24 hours later, we will receive this not-even-close-to brown water, which is the treated water from the WWTP.
Almost like clear water, it is still not safe for drinking, but is now safe to be discharged into receiving water bodies, in this case the famous Elbe River. With only 5 mg/l of Biochemical Oxygen Demand (BOD5), and 41 mg/l of Chemical Oxygen Demand (COD), NH4-N of 0,7 mg/l, N total of ±13 mg/l, and P Total of 1 mg/l, this water won’t be a harm to humans and neither to the environment once it is discharged into the river.
How could the wastewater be turned into clear water? The answer is because of several physical, biological, and chemical processes that were taking place in the plant. At first, the wastewater was treated with physical and mechanical treatment with bar screens and grit chamber to remove the trash and to remove sand subsequently from the wastewater. The water then underwent a sedimentation process in which its floating and settle-able solids sink to the bottom of the tank. Afterwards, the water was transported to aeration tanks. In these tanks, air bubbles were introduced to the water to be used by the microorganisms which “eat” the organics. The water then again settled in secondary sedimentation tanks, from which it can then be safely discharged to receiving water bodies.
The sludge coming from the primary and secondary sedimentation tanks was also treated in a series of sludge treatment units. Unfortunately, we did not have the chance to visit all the units, but we could see from afar the two giant egg-shaped anaerobic digestion tanks.
We had fun during the visit, especially since the weather was rather nice today. And since 81% of the energy used is produced by the plant (mainly from anaerobic sludge treatment units and small amount produced by solar systems), we proudly took a picture in between the solar panel systems.
by Vika Ekalestari (Indonesia)
special credit to Tamara Karp (CIPSEM) for the title idea 😉