1. 1
Area 300 - Percolation & Digestion
Area Overview
June 2013
2. 2
UR3R Plant Overview
Waste is separated in area
200 using the two trommels.
Any waste that drops through
both trommels is sent to area
300 as feed.
This is the organic part of the
waste stream and is smaller
than 80mm.
Typically 30-45% of waste is
fed into area 300
5. 5
Percolation
Percolators take a solid organic / mixed waste feed from area
200.
Typically 150 – 250 tonnes / day.
KPI is to accept 90% of available feed every day.
The amount of feed to area 300 depends on the hourly
throughput in area 200 and the performance of the bag
splitters and the trommels.
Waste is mixed with warm irrigation water and is mixed in the
percolator.
The percolator further separates the organics from the waste
stream and converts these solids into a liquid feed for the
digesters.
K P IK P I
7. 7
KPI – Feed to Percolators
Target is 90% of available feed.
Percolators can be bypassed. This puts additional load onto
OGM and reduces the percolate production.
To accept a full days feed, we need two of the three
percolators to be at a low level at the start of processing.
If we don’t have this space, we need to discharge throughout
the day.
If a percolator is fed too quickly, waste will build up at the feed
end and start to restrict the agitator movement.
Maximum level is 60-70% - this is around the top of the
agitator.
8. 8
Benefits of Percolation
Creates a ‘clean’ homogenous
liquid feed from a solid mixture of
organics and other waste.
Allows the digesters to be
operated with a liquid feed, to be
single stage and to have better
throughput for their size.
Allows OGM to be smaller as
percs remove some organics and
give mass reduction.
Perc discharge material is easy
to compost
9. 9
Percolator Discharge / Sand Separation
Waste from the percolators is discharged via screw feeder
into a SNAP press.
This compresses the waste and squeezes out a thick, organic
rich ‘soup’ called percolate.
The remaining solids (known as SNAP) are conveyed to
OGM.
The snap press baskets wear rapidly because of glass/grit in
the waste and are inspected weekly and changed as often as
monthly.
10. 10
Sand Separation / Screening
Percolate is discharged from the snap presses into a sand
separator – this is a settlement tank, with a screw to remove
sand and grit.
Percolate is then pumped over a vibrating screen to sieve out
any fibres and any other remaining grit or solids.
Percolate contains sand and grit, plus fibres and small plastic
particles.
Percolate is ‘cleaned’ in the sand separation process in area
300 basement.
Many of the problems with percolator availability are caused
by failures on screens, which leads to blockages in pumps.
11. 11
Percolation Performance
We want to make as much percolate and have it as rich in
organics (measured as COD) as possible:
Don’t bypass.
Leave waste in percolators for as long as possible – utilise the
space available, but ensure there is space for feed.
Add as much irrigation as possible (but not too much!). The
waste should always be wet, but not waterlogged. For every
tonne of waste feed, add 0.6 – 0.75 tonnes of water.
Add hot irrigation water.
Percolate production is between 500 – 800 tonnes / week.
For each tonne of feed, we should make 0.9 tonnes of
percolate
13. 13
Area 320
Percolate from area 300 is stored in the digester feed tanks.
The digester is continuously mixed with a large recirculation pump – this
pumps the entire digester contents twice per day.
Percolate is dosed into the digester circulation flow based on an hourly
feed rate. This is set based on a number of factors:
How much percolate is available
Digester health and performance
Required gas make
Strength of percolate
Percolate Feeds
Anaerobic Digester
Feed
Tank
14. 14
What goes in, must come out..
As percolate is added to the digester the level will rise. It is vital that we don’t exceed
the maximum liquid level in the digester.
Digestate is removed using the biomass centrifuge. The centrifuge separates solids
from liquid.
The solids are the biomass (the bacteria), that make the digester work. These are
returned to the digester.
The liquid (or centrate) is then treated to remove ammonia and is then used to
irrigate the percolators. It can also be sent off-site, or can be returned directly to the
digesters
Percolate
Anaerobic Digester
Digestate
Biomass
Centrate
Ammonia
Removal
15. 15
Anaerobic Digestion
A digester uses
bacteria to digest
organics.
This happens in the
absence of oxygen (Anaerobic).
This is a living process, so behaves
more like a cow, than a tractor.
The bacteria require a specific set of
conditions to survive and thrive.
The process can not be turned on &
off, it must be run continuously.
The useful product of the process is
methane, which is used as a fuel in
the gas engines
17. 17
Stages of digestion
Digestion happens in two main stages. Each stage is performed by a
separate class of bacteria.
The acetogenic bacteria take feed and break this down into VFA’s (volatile
fatty acids) like acetic acid. They also produce carbon dioxide.
The methanogens use the VFA’s and convert into methane.
The first stage is very robust
and happens very easily.
The second stage is more
delicate and requires more
specific conditions. The
bacteria also take longer to
grow.
Both stages produce by-products
like ammonia and hydrogen
sulphide.
Ammo
nia
Ammo
nia
Aceto-
genesis
Aceto-
genesis
Methano-
genesis
Methano-
genesis
Methan
e
Methan
e
Carbon
Dioxide
Carbon
Dioxide H2SH2S
18. 18
Digester Health
The digesters need specific conditions to operate:
Temperature 35.5 – 38.5 DegC – (ideally 37.0 DegC)
If the temperature is too low, the bacteria will work very slowly
and will become dormant.
If the temperature becomes too high, the bacteria will be
destroyed.
The digesters lose heat to their surroundings. They are also
cooled by adding percolate. The gas producing reaction is
also endothermic, so takes heat energy from the digester.
The digesters are heated continuously using a heat
exchanger, or by returning warm centrate from the
denitrification system.
K P IK P I
19. 19
pH & VFA’s
The digester pH should be between 7.0 – 8.0. (Ideally around
7.7)
VFA’s are acidic and will change the pH. The VFA’s should
be used at the same rate they are produced so the levels
should stay constant.
VFA levels should not exceed 3000 mg/l. (Ideally below
1500mg/l)
High VFA’s are a sign that the methanogens (the delicate
bacteria) are not working properly. This can cause the pH to
change, which will damage these bacteria further. This can
lead to a spiral of increasing VFA’s and decreasing pH, which
if unchecked could kill the digester.
K P IK P I
K P IK P I
20. 20
Ammonia
Ammonia is produced as a waste product when the digester
breaks down proteins.
Too much ammonia is toxic to the digester.
Ammonia is alkaline and will increase the digester pH.
Some ammonia is useful as it provides an alkalinity buffer
against pH changes, which can hold the pH more stable.
Ammonia levels should not exceed 3500 mg/l.
Ammonia is removed by treating centrate in denitrification.
Ammonia levels also tend to plateau and self control due to
losses when controlling digester level.
K P IK P I
21. 21
Solids content
The digester liquid contains solids.
Some of these are beneficial volatile solids – these are biomass
and the organic content of the percolate feed.
Some of these are non-volatile solids. These are mainly sand, glass
& grit in the percolate feed (some are formed in the digester).
These nv-solids are not beneficial and make it more difficult to mix
the digester and will cause damage to pumps, pipework and valves
as they are abrasive.
The NV-Solids content should not exceed 6%.
Solids are removed using the raking system. This draws the heavy
sludge from the bottom of the digester and processes it through a
centrifuge to separate the solids and liquid. The solids are sent to
OGM, whilst the liquid returns to the digester.
K P IK P I
22. 22
Methane Content
The methanogens make methane, whilst the acetogens make carbon
dioxide - biogas is therefore a mixture of both gases.
The methane content is a good guide to the balance between the
separate parts of the digestion process and an early warning of
problems with the methanogens.
The methane content should always
be above 55%.
Methane content will fall as
feed rates are increased and
climb as feed rates fall.
Different feed can reduce the
methane concentration, for
instance molasses.
Ammo
nia
Ammo
nia
Aceto-
genesis
Aceto-
genesis
Methano-
genesis
Methano-
genesis
Methan
e
Methan
e
Carbon
Dioxid
e
Carbon
Dioxid
e
H2
S
H2
S
K P IK P I
24. 24
Area 340 Overview
Biogas is continuously produced from the digesters.
This gas is a mixture of methane and carbon dioxide and also
contains hydrogen sulphide (H2S).
The gas must have the H2S removed using the
desulphurisation plant.
The gas is then stored in the ‘bubble’.
And then used as a fuel in a pair of gas powered engines,
which are used to generate power.
This power is used on site and reduces our reliance on power
from the national grid, which is often generated using gas or
coal.
25. 25
Biogas Safety
Biogas is flammable.
Biogas will asphyxiate.
Biogas contains H2S which is
toxic.
Biogas is often odourless.
Biogas is a powerful
greenhouse gas
We must not vent biogas
to the atmosphere. K P IK P I
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Desulphurisation
To remove the H2S from the biogas it is bubbled through an
iron rich liquid in the gas contactor vessel on Desulph.
The H2S bonds with the iron and dissolves into solution,
leaving clean gas.
This solution is then regenerated by reacting with oxygen.
This produces water and elemental sulphur as a fine powder.
The sulphur is removed from the system by filtration. The filter
press will take 1 – 7 days to fill, depending on the H2S levels
and the gas flow rates.
Clean gas should contain less than 500 ppm H2S.
Desulph should be online >90% of the time.
K P IK P I
K P IK P I
27. 27
Gas Storage
Clean gas is sent via the drier (which removes moisture) into
the gas bubble.
The bubble holds 330m3 of gas, which dependant on gas
make can be as little as 30 minutes storage.
Gas can then be used:
1. In the gas engines to make power and earn ROCS.
2. In the water heater to make hot water.
3. In the flare – to burn off gas, if it can’t be used.
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Gas Engines
We have two 950KW generators.
Each will run between 50 – 100% load. An engine at full load
for a day will make 22.8 MWhr.
We are able to claim 2 ROCs (Renewable Obligation
Certificates) for each MWhr we generate. Together these are
worth twice as much as the actual power.
Biogas must be >50% CH4 to be used in the gensets.
K P IK P I
