Impacts on sewer performance due to changes to inputs in domestic wastewater

Abstract: The impacts of changes in domestic wastewater inputs on sewer performance have been debated since the dawn of the great sewer construction movement in the 1850s. Nowadays, typical household wastewater that enters sewers can generally be divided into streams from the WC, shower and/or bathtub, kitchen sink, washing machine and dishwasher. Changes in thecomposition of domestic wastewater entering a sewer will depend on inter alia the properties of the appliances used in the households and householders’ residential habits. Recent such changes with a potentially detrimental impact on sewer performance are increased levels of fat, oil and grease used in kitchen practices; the introduction of kitchen sink food waste disposers;and the installation of domestic water efficient appliances. The overall aim of this thesis was, from an historical perspective, to explore how recent changes to inputs in domestic wastewater can impact performance of sewer systems and how this relates to sewers as future carriers of domestic wastewater.Various methods to examine the aim have been employed. Two questionnaire surveys were distributed. The first was sent out to sewer operators/managers employed at Scandinavian public water utilities to explore their views on the extent of fat, oil and grease in sewer systems.The second was sent to sewer/operators managers employed at public water utilities in Northern Sweden to explore the extent of monitoring of incidents of blockages. Two field studies have also been undertaken. One entailed the investigation of long term impacts on sewer performance from the introduction of food waste disposers in housing areas. The other evaluated which parameters of domestic wastewater could be used to estimate levels of infiltration/ inflow into sewers. In addition, a case analysis was undertaken to assess future impacts on self-cleansing regimes in sewer networks when rural subarctic villages are retrofitted with water efficient appliances. A literature review was performed to identify and analyse studies examining changes to domestic wastewater the impact these changes may have on sewer systems,and their implications for sewer management. Responses from the first questionnaire survey suggested that an increase in blockages due tofat, oil and grease, in particular at pumping stations, was the most reported problem. Responses from the second questionnaire survey indicated that the extent of monitoring reoccurring blockage locations was found to be low, although two water utilities possessed GIS-systems which supported such endeavours. The first field study indicated that the level of in-sewerdeposition was observed to be increased when a pipe received high input of effluents from the kitchen sink disposers. This was observed to a further degree when a pipe had a gentle inclination combined with significant sagging along its length. From the second field study it was found that among compounds normally sampled to determine wastewater characteristic, the parameters Tot-N and Tot-P provided the closest fit to estimated levels of infiltration/inflowbased on water consumption rates. The case analysis indicated that the decrease in in-pipe velocities when all households are retrofitted with water saving appliances could be counteracted by sewer relining when water consumption rates did not drop below 130 l/hd.d. The impacts on sewer performance from these changes seemed to be dictated by many factors,e.g. household practices and the status of separate pipes. Therefore there is a need to put the changes to inputs in the context of larger societal trends to properly understand their full implications regarding future objectives when relying on the sewer as a carrier of domestic wastewater.