Low infiltration sewer systems and their role in reducing wet weather sewage overflows within the Sydney Basin

Chris Harris - Sydney Water Corporation , 30 October, 2014

Rain induced sewer infiltration can increase flows in a sewerage system by ten times, far exceeding the designed capacity. The increased flows result in wet weather sewage overflows into receiving waterways. Wet weather sewage overflows are detrimental to the environment and water authorities around the world have been trying for decades to reduce or eliminate sewer infiltration. During wet weather induced sewage overflows oxygen depletion occurs due to the biodegradation of organic matter as the sewage is untreated which is a major cause of the phenomenon known as fish kills. An increase in the concentration of some organic micro-pollutants, metal concentrations and pathogenic/ faecal indicator micro-organisms and turbidity also occurs which leads to a reduction of photosynthetic primary production. The negative effects on the rivers health are limited in duration due to processes such as sedimentation, dilution and decay.

The aim of this paper is to establish sewer infiltration as an environmental issue and to ascertain whether low infiltration sewer systems within the Sydney basin are operating as intended. Sydney Water has recently implemented a technical specification for low infiltration sewer (LIS) systems with the intent of minimising the effect infiltration has on a sewer network. LIS systems are constructed of polyvinylchloride, polypropylene or glass reinforced plastic with rubber ring joints or solvent weld joints. Due to the expectation of negligible amounts of inflow and infiltration the sewer pipes are constructed to accommodate only as much as three times peak dry weather flow which makes the pipes smaller than pipes within a sewerage system constructed to comply with the Water Services Association Australia, Sewerage Code of Australia, Sydney Water Edition. A three part monitoring plan was formulated to firstly establish sewer infiltration as an issue and to secondly assess the performance of Sydney Water’s LIS system prior to any dwellings being constructed and thirdly after all dwelling construction had finished. The preliminary results confirm sewer infiltration is an issue within the Sydney basin, and suggest the LIS systems being implemented by Sydney Water are effective at reducing infiltration.