Surahammar: a case study of the impacts of installing food waste disposers in 50% of households

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.