Bridging the gap between sewer status and new
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 Viklander Department of Civil, Environmental and Natural resources engineering Urban Water
Objectives of present study • 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 (P 90 and EN 752 -2008) Ordinary deposit FWD-deposit
Interdependent factors Two 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 (m 2):
”Two out of three” • 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 buildups of deposits (students t-test, conf int. 95%). • Will B-pipes exhibit more deposits?
Method - Data used Three sewer systems serving single family housing were inspected using CCTV. FWDs were installed in ca 75% of hlds. 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 Larger build-ups of finer sediments and finer sediments together with sanitary waste were documented for each pipe.
Selection of settings, A or B? • Initial settings (based on literature values): Inclination: 0. 9%; Sag area: 0. 07 m 2; No. Households: 8 Pipe name Inclination (%) Sag area (m 2) No. Hlds Class Deposit level SNB 1636 -37 3. 3 0. 02 6 A Low? ? ? SNB 1640 -51 0. 8 0. 13 29 B High? ? ?
Result - Calibration • P-values from the outcome of the calibration: Sagging (m 2) ; N 0 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. 071 (70) Values in parenthesis are % of A-pipes
Applying the established settings Settings: Inclination: 1. 0%, Sag area: 0. 07 m 2 N 0 of households: 8 Tool Calibration set Validation set 1 Validation set 2 Proposed tool 0. 007 (66) 0. 536 (49) 0. 018 (58) P 90 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
- Slides: 10