Team:Berlin/Project/Plant
3. The Wastewater Treatment Plant
Wastewater treatment itself involves four major steps: primary, secondary, tertiary, and post
treatment.
The primary treatment is a physical process where large debris is separated and sedimented.
The separation happens through screening, skimming, and grinding of sewage. In a separate
tank, called the settling tank, the sedimentation takes place. There, the organic suspended solids
settle to the bottom as the primary sludge and suspended organic matter flows further as the
primary effluent.
Biological processes, containing oxidation of the primary effluent, are involved in the secondary
treatment. These are performed by microorganisms within trickling filters, or aeration tanks
called “activated sludge system”. The trickling filter reactor is filled with solid media, like rocks
and plastic, on the surface of which bacteria grow. When wastewater trickles down, the bacteria
use up the biochemical oxygen demand (BOD) and eventually die, falling off of the media
surface. This filter is open to the atmosphere, so that air flows naturally through the media. After
the treatment, the water leaves from the bottom of the tank and flows into the secondary settling
tank. Bacterial cells settle and are removed from the settling tank as a secondary sludge. Some of
the water is recycled to the filter to maintain moist conditions. This is where we envision our
Enzymatic Flagellulose will be integrated. The water remains in the secondary settling tank for a
couple of hours, which is necessary for the plastic degrading enzymes to have enough time to
work.
Another method would be the activated sludge reactor, where the wastewater is aerated in an
aeration tank, which allows bacteria to consume BOD and therefore grow, multiply, and
flocculate. Here, a high oxygen supply is necessary. The treated water flows into a settling tank,
where the bacteria settle. Then, they are removed as the secondary sludge. Part of the sludge is
recycled back to the activated sludge tank to maintain bacterial population. The remaining high
amount of sludge is anaerobically digested in the post treatment.
In total, the secondary treatment removes carbon, nitrogen, and phosphorus, and the activated
secondary sludge is separated from the secondary effluent in a settling tank.
In the tertiary treatment, the effluent is undertaken disinfection by chlorination.
As for the post treatment, the sludge is processed by anaerobic microbial digestion,
conditioning, and disposing in an anaerobic digester. Here, the purpose is the stabilization of the
organic matter and the reduction of the water content. The latter is for reducing the weight of the
sludge and making it more economical to transport to its final disposal. Stabilizing the organic
matter prevents the formation of odor and decreases the number of pathogens. The sludge
contains cellulose, proteins, lipids, and other insoluble polymers, which are degraded to methane
CH4 in four microbial processes: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In
other words, the polymers of the organic matter are first degraded to sugars, amino- and fatty
acids and then used for the production of short-chain fatty acids, alcohols, CO2 and H2. These
products are used for the formation of acetate, which is followed by the formation of methane
CH4.
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