What Happens at the Sewage Treatment Works? - Part 2

Thanks for this report to Duncan Berry, Assistant Director Liquid Waste who gave me a guided tour around the plant, and for Deputy Kevin Lewis, who facilitated my tour. Part 1 deals mainly with liquid waste treatment, and part 2 with solid waste treatment.

What Happens at the Sewage Treatment Works? - Part 2

Biosolids

Having seen what happens to the water and how the particulate matter is filtered out, and the water treated to become chemically pure, in 2015 in improved process was introduced for dealing with the sludge removed from the process.

As stated before, the larger waste matter is filtered out at the start of the process and burnt at the energy from waste plant.

But further along the process the sludge is removed in the primary settlement tanks. Before 2015 this the digested and dewatered sludge was conditioned with imported lime, but matters have improved considerably since then.

Rather like milk, sludge also goes through a process of “pasteurisation” when it is heated to destroy harmful pathogens. There are three tanks which work in sequence. As one tank fills, one of the two other tanks empties while the third tank is isolated and in ‘pasteurising’ mode, cooking the sludge at 55C for a minimum of four hours.

There are three “sludge digestors” which take this material - they are solid concrete and have a capacity of 1,650m3 and this is kept for 18 to 20 days at 37 0C to further break down any pathogens.

The resulting “sludge cake” at the end of the process can be used as a slow-release fertiliser on farmer’s fields, although it cannot be used on all fields, depending on what is being grown. It is supplied free of charge, and by doing this, the States are helping farmers reduce their dependence on imported quick release inorganic nitrate fertilizers. This is a much better way which also recycles one product from waste back into the land.

Biogas

An end product of the digestion process is biogas – mainly methane, which is stored in a spherical tank which holds 500 M3. The tank is also surrounded by lightening conductors to minimise any risk as it is inflammable.

A final stage of processing the gas is to remove gases which would disrupt the burning process. Untreated biogas contains high levels of sulphides, such as hydrogen sulphide, which many people will know as the “rotten eggs” smell. This is done via a “biogas scrubber”. The hydrogen sulphide removal protects the next part of the process (combined heat and power unit). If the hydrogen sulphide was not removed it would cause severe corrosion within the CHP engine and greatly reduce its lifespan.

The methane gas can then be used to power a generator – a Combined Heat and Power Unit (CHP)) - where it produces heat and electricity as it is burnt.

A CHP generates electricity whilst also capturing usable heat that is produced in this process. This contrasts with conventional ways of generating electricity where vast amounts of heat are simply wasted.

The hot water produced in this process can be used to heat the raw sludge feed to the required pasteurisation temperature.

The electricity can be used to reduce dependence on the JEC for running the plant, and saves around £1,000 a day.

Other Useful Facts about the Plant

In case of power cuts in the JEC supply, there are backup generators, which also power the First Tower pumping station.

Computer systems check the plant’s operations and the pumping stations. If there is a problem the operator’s mobile phone will receive an alarm. Operators are on call 24 / 7 to fix any problems ​straight away.

Finally, here is part of the area which will be used for the new sewage treatment plant.

The new facility will treat the sewerage from a connected population equivalent to 118,000 which is not estimated to come about until 2035.

The site clearance includes the removal of the Clinical Waste Incinerator (CWI) which is still on site. The new clinical waste incinerator can be seen next to the green waste recycling facilities at La Collette.

The Household Recycling Centre has already been relocated to La Collette and the hillside at Bellozanne is being excavated and stabilised providing more room for the new plant.

It is worth noting that one feature of the old plant was that because it grew by stages, it had to move up the valley, which meant pumping waste water uphill.

In contrast, because the new plant is designed “from the ground up”, the tanks in the processing of the liquid waste have been arranged in an order so that the “stages” go downhill, and can use gravity for flow.

Supplementary Photos and Descriptions:

Archimedes screw pumps

These are the Archimedes screw pumps which return the biomass to the beginning of the activated sludge plant. This Biomass contains the microorganisms to treat the sewage and is recycled again and again.

Below is seen the activated sludge return to anoxic zones:

This is the return activated sludge channel which the Archimedes screw pumps discharge into

Sludge storage tanks

More pictures of the Biosolids digesters

What Happens at the Sewage Treatment Works? - Part 1

Thanks for this report to Duncan Berry, Assistant Director Liquid Waste who gave me a guided tour around the plant, and for Deputy Kevin Lewis, who facilitated my tour. Part 1 deals mainly with liquid waste treatment, and part 2 with solid waste treatment.

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). 

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.

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.

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.