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.
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.
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 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:
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
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