What Happens at the Sewage Treatment Works? - Part 1
The sewage treatment works at Bellozane was built in 1959, and while it has been added to and substantially upgraded over the years, it is really now at the end of its lifespan. At its maximum, it can cope with sewage from 100,000 people, and the Island’s population is now in excess of that. Maintenance and repair has become increasingly more difficult and costly.
The new plant, which will be coming on line by around 2022, can cope with up to 118,000 people, with a leeway for an 10% extra if needed. It will also factor an 18% reduction in odour, and one of the main treatment stages, the primary settlement tanks will also be covered to reduce smells further.
For those interested, there is a device called an eNose, or electronic nose, which originated in the Netherlands, which uses sensor de detect the density of odours. (http://www.enose.nl/rd/technology/)
It is amazing to think that within an Island of 45 square miles, there are 354 miles (570 km) of sewers taking our waste along to be treated. Some of the waste comes naturally downhill, via Queen’s road from the North of the Island, but gravity cannot do all the work over the Island, and there are 110 pumping stations in operation, of which the main one is at First Tower, where sewers from West and East meet and are pumped uphill to Bellozane.
Stage 1: Screening
The first stage is when the sewage comes in from east and west, and via a pipe (visible at the back) from the north, and passes through a fine 6 mm grill to remove the larger particulates. This larger waste matter is burnt at the Energy from Waste Plant at La Collette.
Stage 2: Grit Separation Tanks
What remains is fine grained, and goes into the separation tanks. Within those tanks, the fat rises to the surface, and is swept off, while the grit sinks to the bottom (which is V-shaped) and is scraped out. This scraping across happens every thirty minutes. The process removes the fat (on top) and the grit (sinking to the bottom).
There are three of the separation tanks, which provide redundancy in case any one of them needs to be taken out of use for repair or servicing.
Stage 3: Primary Clarifiers
The liquid is then taken through to one of four primary clarifiers, where an arm sweeps around and this removes all the suspended solids from the sewage.
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum. As the skimmer slowly rotates around the clarifier, skimmed floating material is pushed into the trap above the fenced enclosure. This process removes around 60% of the solids (grit).
The liquid is then taken through to one of four primary clarifiers, where an arm sweeps around and this removes all the suspended solids from the sewage.
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum. As the skimmer slowly rotates around the clarifier, skimmed floating material is pushed into the trap above the fenced enclosure. This process removes around 60% of the solids (grit).
Stage 4: Anoxic Zone
This stage is where the sewage begins biological treatment. It is a bacteriological treatment, and the first tank is devoid of oxygen, so the micro-organisms break down the nitrates (NO3) in the water to take the oxygen. The technical term for this is an “anoxic zone”.
This is important because Nitrogen concentrations in treatment plant effluents must be controlled in order to avoid adverse effects in receiving waters. High effluent organic matter concentrations may result in depletion of dissolved oxygen in receiving waters, thereby having a negative impact on aquatic life.
Micro-organisms in this operation include ones like Pseudomonas which is a common bacteria found all over the world in soil, water, and plants. The plant itself generates the bacteria in the next stage of the process.
This stage is where the sewage begins biological treatment. It is a bacteriological treatment, and the first tank is devoid of oxygen, so the micro-organisms break down the nitrates (NO3) in the water to take the oxygen. The technical term for this is an “anoxic zone”.
This is important because Nitrogen concentrations in treatment plant effluents must be controlled in order to avoid adverse effects in receiving waters. High effluent organic matter concentrations may result in depletion of dissolved oxygen in receiving waters, thereby having a negative impact on aquatic life.
Micro-organisms in this operation include ones like Pseudomonas which is a common bacteria found all over the world in soil, water, and plants. The plant itself generates the bacteria in the next stage of the process.
Stage 5: Aerobic Zone
The tanks in this area are bubbling away with air pumped through them, hence the term “aerobic”. This is a breeding ground for the bacteria. Aerobic wastewater treatment is a biological treatment that uses oxygen to break down organic matter and remove other pollutants like ammonia.
There is a feedback mechanism so that air is pumped through as needed to maintain but not exceed levels of micro-organisms, which makes the system very cost efficient.
Stage 6: Final Settlement Tanks
The water reaching these tanks is very much clearer, and ducks can be seen swimming in it. It still contains concentrations of microorganism suspended in the treated water, so if swallowed, would probably give you gastroenteritis.
The micro-organisms move to a sludge in the bottom of the tanks – the rotating arms scoop it up, and this “activated sludge” is passed back to the Anoxic Zone to start the cycle again. The clarified water is now ready for the final stage.
There were originally 4 of these tanks but there are now 8 extra ones, as this is a slow part of the process, and the extra tanks were needed as the population grew to cope with demand.
The water reaching these tanks is very much clearer, and ducks can be seen swimming in it. It still contains concentrations of microorganism suspended in the treated water, so if swallowed, would probably give you gastroenteritis.
The micro-organisms move to a sludge in the bottom of the tanks – the rotating arms scoop it up, and this “activated sludge” is passed back to the Anoxic Zone to start the cycle again. The clarified water is now ready for the final stage.
There were originally 4 of these tanks but there are now 8 extra ones, as this is a slow part of the process, and the extra tanks were needed as the population grew to cope with demand.
Looking at any of these tanks when full, it is hard to imagine how large they are, but they go down some distance into the ground. This is an empty tank currently being serviced. The extra number of tanks means that it is possible to take them out of service for routine maintenance.
Stage 6: Ultra Violet Treatment Plant
The final stage of treatment is by ultra violet light, which kills off any microorganisms and renders the water pure of bacteria, viruses and other pathogens. This was brought in at the end of the 1990s and ensures that the process does not need chemical treatment (such as chlorine) for disinfecting the water.
Unlike other parts of the treatment process, this part takes bare seconds as the water flows past. A specific wavelength of UV ensures best results from irradiation.
Ultra-violet does not penetrate water deeply for more than about 2 cm, without losing its efficacy so there are grids each with 1,000 UV lamps to ensure the treatment is effective.
Although a UV grid can be seen in the photo, the flowing water with the live UV disinfectors pass beneath the floor of this room.
Once treated, this water can be discharged via the outfall into St Aubin’s Bay near the First Tower Pumping station at a distance of 0.5 km from the sea wall. The outfall also contains water from streams passing down the valley.
Storm Surges
At times when there is heavy rainfall, water is passed through the inlet works and primary settlement tanks t to remove most of the particulate matter, and from there via the UV treatment to the bay. This is partially treated, as it may still some smaller grains matter, but it has still been disinfected by the UV process.
At times when there is heavy rainfall, water is passed through the inlet works and primary settlement tanks t to remove most of the particulate matter, and from there via the UV treatment to the bay. This is partially treated, as it may still some smaller grains matter, but it has still been disinfected by the UV process.
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1 comment:
The new plant, which will be coming on line by around 2022, can cope with up to 118,000 people, with a leeway for a 10% extra if needed
The new one will be nowhere near large enough, with the present population of say 115,000, add to that an annual growth of 1,500, bringing it up to 131,500 in only eleven years, that is seven years after completion…..They had better start building the next one immediately.
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