Variable or factor reference No.

Variables that influence solid transport and factors identified that can lead to pipe blockages

No. of authors agreeing with the variable or factor

Author and year

Significant variables (PERC 2012)

SV1

Pipe gradient

3

Flat gradients are identified by Arthur et al. [63] , Fenner et al. [59] and Rodriguez et al. [60] as a cause of sewer blockages. This is not supported by Lillywhite and Webster [7] .

SV2

Toilet paper used

4

Toilet paper has been identified by Littlewood [29] and Cummings [28] as a factor in solid transport, by implication toilet paper is a factor in solid deposition and solid deposition is a blockage factor which is identified by Arthur et al. [63] and Fenner et al. [59] .

SV3

Flush volume

3

Flush volume is a factor in solid transport, by implication a factor in solid deposition which is identified by Arthur et al. [63] , Fenner et al. [59] and Littlewood [29] .

Non-significant variables (PERC 2012)

NSV1

% of trailing water

0

In the blockage factors % trailing water has not been explicitly provided as a cause of blockages; however, Table 3.10 which is an update of this table to include rig-based research includes specifics in relation to % trailing water.

NSV2

Flush rate

(velocity)

2

Flush velocity is related to self-cleansing velocity, self-cleansing velocity has been identified by Arthur et al. [63] and Littlewood [29] as a factor in sewer blockages.

Blockage factors from Table 4

BF1

Age of sewer

2

Davies et al. [69] and Rodriguez et al. [60] cite sewer age as a factor in sewer blockages.

BF2

Depth of cover

4

Davies et al. [69] , Fenner et al. [59] , Marlow [66] and Rodriguez et al. [60] agree that depth of cover influences sewer blockages. This is mainly due to two factors including deformation or collapse of shallow sewers when loaded and tree root penetration.

BF3

Fats, oils and grease

4

Fats, oils and grease have been identified by Davies et al. [69] , Fenner et al. [59] , Littlewood [29] , Marlow [66] and Rodriguez [60] as having influence on sewer blockages.

BF4

Joint/material type

3

Davies et al. [69] , Lillywhite and Webster [7] and Marlow [66] all identify joint type and material pipe type as factors in sewer blockages.

BF5

Junctions

3

Junctions have been identified by Arthur et al. [63] , Lillywhite and Webster [7] and Littlewood [29] as a factor that can lead to blockages.

BF6

Ground conditions

3

Davies et al. [69] , Fenner et al. [59] and Marlow [66] agree that ground conditions can cause adverse pipe movement leading to misalignment or back falls leading to sewer blockages.

BF7

Gross solids/misuse

5

Arthur et al. [63] , Lillywhite and Webster [7] , Littlewood [29] , Marlow [66] and Rodriguez et al. [60] all cite misuse of sewers through flushing items such as nappies as a cause of sewer blockages.

BF8

Intermittent flow

3

Arthur et al. [63] , Littlewood [29] and Marlow [66] all cite intermittent flow as a cause of sewer blockages.

BF9

Length of sewer

3

Arthur et al. [63] , Fenner et al. [59] and Rodriguez et al. [60] refer to length of sewer as a cause of sewer blockages.

BF10

Small dia. pipe

4

Arthur et al. [63] , Fenner et al. [59] , Marlow [66] and Rodriguez et al. [60] report that sewer blockages occur more frequently in small diameter pipes, usually in pipes of 150 mm diameter or less.