Water Services Training Group

WRc FOG Project

Engineering/Operations Issues

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

Active separator (skimmer)

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

Inadequate grease separators

Examples of two small and poorly maintained separators

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 heads

Jetting heads

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

Before and after using recommended

procedure

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

Jetting and cleaning practice

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

Problem pumping station

Problem pumping station

Problem pumping station

Problem pumping station

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

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