Bridging the gap between sewer status and new sewer inputs

A decision support tool to predict the effect of food waste disposers (FWDs) effluent on small-diameter sewers

Jonathan Mattsson, Annelie Hedström & Maria ViklanderDepartment of Civil, Environmental and Natural resources engineering

Urban Water

• To develop an easy-to-use tool to identify pipes vulnerable to increased deposition by FWDs.

• To compare the developed tool to methods to assess self cleansing conditions (P90 and EN 752-2008)

Objectives of present study

Ordinary deposit FWD-deposit

Interdependent factorsTwo unfavorable pipe-settings out of these three reported to cause build-ups:

•Gentle inclination (%)

•Few households (i.e. low wastewater load)

•A large sag area (m2):

• Pipes with at least two unfavorable settings Class B

• Pipes with at most one unfavorable setting Class A.

• A-pipes compared to B-pipes in terms of build-ups of deposits (students t-test, conf int. 95%).

• Will B-pipes exhibit more deposits?

”Two out of three”

Area Accumulated Length (m)

Number of pipes

Material Diameter

Calibration set 2239 50 Concrete 225

Validation set 1 1934 45 Concrete 225

Validation set 2 1941 36 Plastic 200

Method - Data used

Three sewer systems serving single family housing were inspected using CCTV. FWDs were installed in ca 75% of hlds.

Larger build-ups of finer sediments and finer sediments together with sanitary waste were documented for each pipe.

• Initial settings (based on literature values):

Inclination: 0.9%; Sag area: 0.07 m2; No. Households: 8

Selection of settings, A or B?

Pipe name Inclination (%)

Sag area (m2)

No. Hlds Class Deposit level

SNB1636-37

3.3 0.02 6 A Low???

SNB1640-51

0.8 0.13 29 B High???

• P-values from the outcome of the calibration:Result - Calibration

Sagging (m2) ; N0 of households

Inclination (%)

0.8 0.9 1.0

0.06; 5 0.524 (72) 0.484 (70) 0.012 (64)

0.07; 8 0.418 (74) 0.385 (72) 0.007 (66)

0.07; 11 0.887 (76) 0.887 (76) 0.071 (70)

Values in parenthesis are % of A-pipes

Applying the established settings

Settings: Inclination: 1.0%, Sag area: 0.07 m2 N0 of households: 8

Tool Calibration set Validation set 1 Validation set 2

Proposed tool 0.007 (66) 0.536 (49) 0.018 (58)

P90 0.336 (76) 0.247 (53) 0.064 (58)

EN 752-2008 0.301 (78) 0.103 (62) 0.147 (56)

Concluding remarks

• The tool could be used for homogenous sewer networks.

• Many pipes classified as ”B”

• Improvements: Extreme values (e.g. negative inclination), manholes.

• Cause for concern: Intermittent flow, deposits really stuck?

Thank you