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. |