Sewage treatment is used to process waste products, so they are safe enough to be released into the ocean. The first step in this process is to remove all the large particles.
First the effluent (or sewage) is filtered so that all large, solid particles are removed.
After that comes the primary treatment. In this step, grease and oil will float to the surface and all the weighty sludge will sink to the bottom. The water then flows to the secondary treatment plant, where pools of living bacteria digest the organic matter. The water is now clean enough to be discharged into the ocean.
But the sewage purification process doesn't necessarily stop here. Sometimes, extra steps are added to turn that water into toilet-flushable quality. This is called water reclamation.
Firstly, a disc filter will remove any relatively big particles from the processed water. Then, it flows through an ultra-filtration membrane which removes even smaller particles and organic matter. In this process, most bacteria and viruses will also be removed or destroyed.
Last but not least, using very high pressure, a system of reverse osmosis kills the remaining organisms. This water heads off to toilets all over Hong Kong.
Secondary sewage treatment booster
During the secondary sewage treatment, sewage is retained in several aeration tanks. Compressed air is pumped continuously into each tank through pipes installed at the bottom of the tanks.
Because the process involves bacteria digesting all organic matter, pumping in oxygen encourages the growth of helpful micro-organisms, making the process more efficient. The process is called activated sludge and the sludge it produces helps sanitise water. The sewage will stay in aeration tanks for nine hours before it moves on.
Final sedimentation pool
After visiting the slightly stinky aeration pool, we headed to the final sedimentation pool, a strikingly different place with clearer and less smelly water.
The water is clear because the activated sludge settles at the bottom of the tank.
This sludge is then sent back to the return activated sludge pumping station, with some of it reused in the aeration tanks and the rest disposed of in landfills.
The water in the final sedimentation tanks will finally leave via different channels. A particular species of plant is planted in the pool to prevent algae growing and blocking these outflows.
The water pumped from here is then ready to make its way into Victoria Harbour through the downstream pumping station.