The Deer Island Wastewater Treatment Plant in the United States is the second largest wastewater treatment plant in the world, located on a small island in Boston Harbor.
Once the dirtiest port in the United States, the factory is now open to visitors for great views.
The Kashima Sewage Treatment Plant has two major features, one is its "giant egg" digester, and the other is its odor treatment system.
Today, I will take you all into this sewage plant built on an island.
1. Factory overview
Factory history
The Kashima Sewage Treatment Plant was built in the late 1800s with only one large steam-powered pumping station for the primary treatment of sewage.
By the 1960s, Kashima introduced more elaborate primary treatment facilities.
In the 1990s, secondary treatment technology for sewage gradually matured and was implemented in Kashima, and a sewage treatment plant was built today serving 43 areas of Boston.
Scale and effect
The Kashima Sewage Plant covers an area of about 910.5 acres and treats wastewater from 43 areas of Boston, including domestic sewage, rainwater, groundwater and industrial wastewater.
The plant treats approximately 1.3 million tonnes of wastewater per day and has a maximum design capacity of 5.1 million tonnes per day.
On average, more than 94% of solids and organic matter are removed from incoming wastewater, and the treated wastewater can be released into the marine environment.
Kashima currently generates about 26 percent of its own electricity demand, and more than half of the island's energy demand is provided by the plant's energy recovery
2. Process flow
Wastewater from Boston is collected through underground pipes and pumped by pumps from deep rock tunnels beneath the harbor to Deer Island. After entering Kashima, the sewage is treated secondarily and discharged into the seawater. The following figure shows the process flow diagram of Kashima Sewage Treatment Plant.
Kashima sewage plant process flow chart
Simply put, after the sewage is pumped into Kashima, it first enters the sand grit tank for preliminary treatment. The sewage then enters sedimentation tanks for primary and secondary treatment, where the mud and sand are concentrated in a gravity concentrator and then transferred to the giant egg digester for anaerobic treatment.
Primary processing
Kashima has a total of 48 sedimentation tanks of about 57*13*7 meters. Sludge and scum are separated from the wastewater by gravity. This step removes about half of the contaminants from the wastewater (50-60% of the total suspended solids and up to 50% of pathogens and toxic substances).
Secondary processing
The wastewater then enters the secondary treatment facility, where it consumes residual organic matter from the wastewater using gravity and microorganisms through reactors and sedimentation tanks and settles to the bottom of the sedimentation tank. After secondary treatment, about 80-90% of pollutants and toxic substances can be removed.
Low temperature pure oxygen facility
Kashima uses a unique low-temperature pure oxygen facility, and the cryogenic plant is capable of producing 130-220 tons of pure oxygen per day to support the secondary biological treatment process. (Principle: Separate liquid air, pressurize at low temperatures, make the air into a liquid state, and then evaporate, because the boiling point of liquid nitrogen is ‐196 ° C, lower than the boiling point of liquid oxygen (‐183 ° C), so nitrogen first evaporates from the liquid air, and the rest is mainly liquid oxygen. ）
Disinfection and discharge
After primary and secondary treatment, the wastewater enters the disinfection tank. Wastewater is disinfected with sodium hypochlorite to kill bacteria, and sodium bisulfite is added to dechlorinate the water so that the chlorine content in the final discharge does not threaten marine life.
To protect the marine environment, wastewater will be transported from the disinfection tank through an effluent tunnel to a 100-foot deep water in a nearby bay (which takes about 10 hours), and then the wastewater will be quickly and thoroughly mixed into the surrounding water through more than 50 special diffusers (pictured below, each with 8 drains).
Drainage diffuser
3. Giant egg digester
If you're lucky enough to fly to Boston Logan Airport, you'll see 12 "giant eggs" standing by the sea. You might be wondering if this is an oil tank from an oil processing plant or a gas tank from a natural gas processing plant? But their practical use is the digestion process in sewage plants.
Giant egg digester
The sludge in the giant egg digester comes from the sludge separated from the primary and secondary treatments. Each digester is about 27.4 meters in diameter and about 40 meters high.
Anaerobic digestive process
Its principle is to mimic the natural digestive process of the stomach, keeping the fluid temperature at 36.67 ° C. Naturally occurring microorganisms in the sludge can decompose the sludge into methane gas, carbon dioxide, solid organic by-products and water. The process significantly reduces the amount of remaining sludge.
Energy use
The remaining sludge (94% water) is transported from the site through pipes, dehydrated and turned into fertilizer particles, and methane gas by-products are collected and transported to the boiler.
This collected methane leads to the steam cogeneration system: steam is generated through a boiler unit that provides hot water and heat to the facility, and the steam also flows through a steam turbine generator that generates electricity to deliver electricity to the water plant.
The cogeneration facility saved the Kashima wastewater plant approximately $15 million in fuel costs and $2.8 million in electricity costs per year.
4. Odor control
Many domestic sewage plants have odor problems, but in the Kashima sewage plant, we can't smell a trace of odor, how do they do it?
In order to solve the odor problem, the Kashima plant has established a special odor control and treatment system.
Kashima Gas Emission Treatment Process
Kashima uses a continuous monitoring system to control odors from primary and secondary treatment processes as well as sludge treatment, plant pumping and sand removal facilities.
The exhaust gases produced during the treatment process (mainly anaerobic digesters) are transported through pipes to the odor treatment unit.
Here, the exhaust gases are first chemically leached in the air scrubber and then pushed into the carbon adsorption device by the fan to remove odors and volatile organic compounds.
5. Summary
As the second largest sewage treatment plant in the world, Kashima Sewage Treatment Plant has advanced sludge treatment and energy utilization technology, and strictly controls odors.
At the same time, Kashima Sewage Plant is committed to creating a green ecological landmark, opening the factory area to the public free of charge and setting up a tour route. Visitors can visit the beautiful harbor along the way and have picnics, fishing, and cycling here.
This long-term development concept is worth learning from large domestic sewage plants.
Recently, I am going to do a series of articles on the inventory of global sewage plants, and will continue to update some valuable sewage treatment projects at home and abroad. Of course, if you have an interest in the sewage plant, you can also leave a message or private message me below.
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