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WRc FOG Project Work Package 1: Legal Work Package 2: Sociological Work Package 3: Biological/chemical Work Package 4a: Engineering –
designing FOG out (sewers/ pumping stations, grease separators)
Work Package 4b: Operations – cleaning techniques etc (sewers/ pumping stations)
Engineering/Operations Issues
Grease separator selection Products database Jetting and cleaning of FOG from
sewers Sewer design Pumping station design
FOG Work Package 4 - Deliverables
Reports Operational Problems caused by FOG Guidance document on Sewer Cleaning and
Blockage Removal Techniques Factors that Influence FOG Accumulation on
Sewer Pipes Design to Minimise FOG Deposition and
Associated Operational Disruption Review of Use of Grease Separators Grease Separator Advice
FOG Work Package 4 - Deliverables
FOG Strategy – various advice sheets and leaflets, including:
Grease separator advice Operational guidance – improved cleaning
procedures Design guidance for sewers and pumping
stations Guidance for improving existing sewer
systems and pumping stations
Large number of different types of separator are now commercially available
Many are not of the traditional design and, as such, are not covered, by a standard
However, many of the new designs show promise and deserve further investigation
Grease separator selection
Grease separator selection (2)
When considering new types of separator, look at: Their practicality - ease to install,
where to install, ease to maintain/clean etc
Their overall effectiveness Guidance given by manufacturers/
suppliers etc – size/installation etc
The standard (EN 1825) Difficult to understand (information
spread between Part 1 and Part 2) Only considers conventional ‘large’
underground separators – Not practical for most town centre catering outlets
No consideration of more recent separator designs, including skimmers.
Grease separator standard
Grease separator standard (2)
The standard (EN 1825) Does not consider hybrids – Dosing and
separators Most separator suppliers don’t understand EN
1825● Suppliers have a variety of sizing methods● Seems to be little standardisation but plenty
of confusion● Often, sizing is only for the food trap part of
the separator
EN 1825 separator
INLET
Sludge Trap Grease Separation Chamber
A
B
OUTLET
A – Grease separation zone B – Grease collection zone
Grease separator selection
Three main issues to consider: Location/access Size Maintenance
Each property is unique Selection is an iterative process
may need combination of separator/ biological solution
Grease separator selection
Location Close to source – connect all sources
(dishwashers issue)
Far enough away for effluent cooling Upstream of other inputs (rainwater and
foul water) Outside units preferable - Consider internal
units at properties with no external space Enable access for cleaning/maintenance
Grease separator selection
Size To provide adequate retention time
for separation to take place - determined by:
● Flow rate to the separator ● Volume of FOG in discharge● Maintenance frequency v size
Grease separator selection
Maintenance By whom
Staff – small separators, easy access, cleaning record, storage of wastes
Contractor – for larger underground separators, less frequent visits, vactor units remove wastes from site
Frequency – determined by storage capacity of the separator
Products database Database including details of:
Grease separators and Biological treatments
Product/company name Contact details for products Basic product information Guidance on appropriate applications
Available to contributors and their agents through password access
Jetting and cleaning
The most frequent symptom of FOG in the sewer system is a blockage
Sewer blockages may occur at any point on the sewer system, from the point of source to the wastewater treatment works
Sewer blockages can be categorised as a primary or secondary blockage
Jetting and cleaning Primary FOG related blockages are
due to natural deposition of FOG at a specific location as a result of the combination of:
a source and appropriate type of FOG, concentration of FOG in the flow, the natural processes of separation, and hydraulic conditions
Jetting and cleaning Secondary FOG blockages are those that
occur as a direct result of a previous blockage or severe flow restriction.
Flow velocity upstream of the original blockage will reduce - FOG that would otherwise remain in the flow will separate out.
FOG will be high on the pipe wall or in the crown of the upstream pipe, due to the increased depth of flow
Sewer debris, (sanitary towels and other sewage debris) then sticks to the FOG on the crown of small pipes, a secondary blockage subsequently develops
Jetting and cleaning practice
The following may seem obvious BUT experience often shows otherwise !
All FOG related blockages must be effectively cleared and the upstream pipe effectively cleaned to prevent repeat incidents
A recommended procedure has been proposed to ensure effective cleansing
Jetting and cleaning practice (2) General purpose jets are a
compromise Removal of FOG from a sewer should
be carried out as a staged and incremental process
1. General purpose jet to remove soft debris2. Plough to remove hard sediments, 3. Spinning jet to remove FOG4. Survey with a camera
Jetting and cleaning practice (3) Analysis shows that the correct
cleaning practice will be beneficial in terms of:
Reducing the number of operational disruptions
Improving customer satisfaction Cost effectiveness
Jetting and cleaning practice (4)
Should the dislodged FOG be: Removed at the manhole:
or Passed forward ?
No evidence of significant problems if:
Jetting and cleaning practice (5)
Break fragments into small pieces, remove large fragments
Remove large quantities Remove if FOG is mixed with sediment
Jetting and cleaning practice (6)
Remove if downstream problems may occur
Notify operators of pumping stations and works of the possibility of dislodged FOG being passed downstream
Sewer design
Greater likelihood of FOG deposition/ adherence: Higher concentration of FOG Lower the temperatures of the pipe
surfaces Slower the velocity of flow/movement of
effluent
Existing sewer improvements (1) First option – control FOG at source
Good kitchen practice Install grease separators and/or Biological treatment systems
Modest improvements to drainage system to reduce areas with low velocity
Other measures: Improve sewer cleaning/blockage practice Consider off line, as opposed to on-line
tanks
Existing sewer improvements (2) Modest improvements to drainage
system to reduce areas with low velocity: Removal of Buchan traps Relay pipes with sags, to enable positive
gradient Removal of tree roots Make good intruding laterals (use of saddle
connections etc) Improve benching/pipe layout in manholes Install drop pipes to high level connections
Existing sewer improvements (3) Where existing systems need to be
altered to reduce CSO spills etc, ensure that the ‘improvement’ will not result in areas of low velocity/tranquil flow where serious FOG deposition could occur.
Similarly, where storage needs to be added to reduce sewer flooding
(These may seem obvious, but they have happened)
New sewer design to avoid FOG The following should be done as good
practice, to ensure there are no areas with very low velocity: Pipe velocity, 0.6 m/s or greater Use minimum number of pipe joints (joints
dissipate energy in small flows) Use easier to clean pipe materials Good detailing of connections (saddles etc) Good detailing in manholes
New sewer design to avoid FOG
Also: Avoid flow control devices If tanks are necessary, use off-line
systems
Pumping station design
Greater likelihood of FOG build up with: Higher concentration of FOG Low velocity/movement in the wet well:
● More tranquil the effluent in the wet well
● If there are dead zones in the wet well
Existing pumping station improvements (1) Where possible, remove dead zones
Remove redundant/non essential equipment
Remove baffles from incoming flows Consider re-profiling wet well floor Ensure effluent is mixed at start of pump
down
Existing pumping station improvements (2) Refit with easy clean materials Replace fouled float switches with
ultrasonics (located clear of the effluent) Use varying pump on/off levels , to
avoid FOG shelves at ‘pump on’ level Improve access to wet wells, for
cleaning purposes Review pump regimes to reduce FOG
deposition in rising mains
New pumping station design (1)
Greater likelihood of FOG build up: Higher concentration of FOG Low velocity/movement in the wet well
Ensure wet well does not have dead zones
New pumping station design (2) Avoid dead zones:
Use circular wet wells Use deep/small diameter wet wells – better
mixing Avoid excessive use of equipment in the
wet well Minimise use of dividing walls Make use of energy/turbulence caused by
incoming flow Use of non stick materials
FOG Work Package 4 - Deliverables
Reports Operational problems caused by FOG Guidance document on Sewer Cleaning and
Blockage Removal Techniques Factors that Influence FOG Accumulation on
Sewer Pipes Design to Minimise FOG Deposition and
Associated Operational disruption Review of Use of Grease Separators Grease Separator Advice
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