Conference Paper

Modelling of Food Waste Disposer particle transport through a sewer network

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Managing domestic food waste is challenging due to potential environmental and financial impacts. One solution is to divert this waste from landfill by using domestic food waste disposers to grind this energy rich material so it can be removed via traditional sewer networks and subsequently making this resource available for anaerobic digesters. Water utilities however wish to understand the risks associated with this additional loading of food waste particles within their sewer systems. This work outlines the development of a novel modelling approach that has been empirically calibrated to predict transport and settling of food waste in sewers and to contribute to understanding if there are any risks associated with increased sewer settlement. The work has focused on dry weather flows as this poses the highest risk in terms of settlement and with specific attention on evening mealtimes (6 – 8 pm) when food is most likely to be added to the sewer. The improved understanding and modelling capability is important for policy makers to help make informed decisions on whether food waste disposers are a viable, and potentially beneficial solution, for food-waste management.

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This paper reviews 15 years of sewage works' monitoring data to assess the effect of installing in-sink food waste disposers (FWD) and how these effects compare with the published scientific literature. For the first time, it has been possible to assess at full scale the load/cost transfer from solid-waste to wastewater management. Within a period of 10 years, 50% of households in the town of Surahammar in Sweden chose to have FWD installed as their means of managing their kitchen food waste. The drainage from the households feeds a single wastewater treatment works (WwTW) that comprises primary settlement, activated sludge, followed by chemical phosphate precipitation and mesophilic anaerobic digestion. The sewer system is separate but with overflow between foul and surface water in times of surcharge; the diameters and gradients of the sewers are unexceptional. This paper reviews the influent and biogas monitoring data for the years before installation started and the 10 years after the first peak of installations (by which time they had been installed in 30% of households). This provides a unique opportunity to verify the published research on FWD. The operational monitoring data are consistent with the already published research that FWD have little or no impact on water use, sewer blockages, vermin or wastewater treatment. The data are consistent with a hypothesis that in-sewer biological process acclimated to the change in wastewater composition and treated the dissolved and fine particulate load before it reached the WwTW. The digesters produced 46% more biogas than before FWD were installed (P=0.01). There was no significant increase in hydraulic load, or in the loading of BOD7, COD, N or NH4. As a result of Surahammar's overall waste strategy, not just the FWD, but the tonnage of waste to landfill from the municipality has also decreased from 3600 tonnes/year in 1996 to 1400 tonnes/year in 2007.
Conference Paper
Food waste disposers (FWD) offer a potential alternative to curb-side collection of food waste, allowing for energy recovery through anaerobic digestion at wastewater treatment plants. A thorough characterisation of the particles produced by FWD is needed to ascertain the impact on sewer networks. This study presents initial characterisation of particle sizes and settling velocities for particles emitted by a FWD. The results give a particle size distribution for the tested food groups and shows increasing settling velocities with increasing particle sizes.
Food wastes with typical US food composition were analyzed to characterize different constituents in both particulate and soluble phases i.e., solids, chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD), nitrogen (N), phosphorus (P). Relationships between various pollutants were also investigated using 50 samples. One gram of dry food waste generated 1.21g COD, 0.58g BOD5, 0.36g Total SS, 0.025g Total N, and 0.013g Total P. Distribution of constituents between particulate and aqueous phases indicated that 40% of COD and 30% of nitrogen were present in soluble form. Relative mass ratios of COD and nitrogen to solids were three to five times higher in particulates than in aqueous phase. However, COD/N ratios were higher in aqueous form than particulates at 63:1 versus 42:1. Detailed relationships between parameters showed that COD, nitrogen, and phosphorus in particulates are 200%, 3.6%, and 3.5% of the volatile suspended solids. Copyright © 2015 Elsevier Ltd. All rights reserved.
To increase biogas generation and decrease vehicle transportation of solid waste, the integration of food waste disposers (FWDs) into the wastewater system has been proposed. However, concerns have been raised about the long-term impact of the additional load of the FWDs on sewer systems. To examine the said impact, this study has used closed-circuit television inspection techniques to evaluate the status of 181 concrete pipes serving single family housing areas with a diameter of 225 mm, ranging from a 100% connection rate of households with an FWD to none. A minor study was also performed on a multi-family housing area, where mainly plastic pipes (200 mm) were used. The extent and distribution of deposits related to the ratio of FWDs, inclination and pipe sagging (backfalls) were ascertained by using linear regression and analysis of variance. The results showed that FWDs have had an impact on the level of deposits in the sewer, but this has, in turn, been of minor significance. With a high connection rate of FWDs upstream of a pipe, the extent of the total level of deposits, as well as finer sediments, was statistically determined to be greater. However, the majority of the deposits were observed to be small, which would suggest the impact of FWDs on sewer performance to be minor. As food waste not compatible with the FWD was seen in the sewers, educational campaigns could be beneficial to further lower the risks of sewer blocking.
Scouring: hydraulic structures design manual
  • H Breusers
  • A Raudkivi
Breusers, H. & Raudkivi, A., 1991. Scouring: hydraulic structures design manual, A.A.Balkema. Rotterdam