Brooklyn Trunk Sewer Long Term Investment Plan

The Brooklyn Trunk Sewer (BTS) was commissioned in 1964 to decommission the Spotswood Pumping Station and convey flows by gravity to the then new Brooklyn Pumping Station. Presently, the BTS conveys approximately 30% of Metropolitan Melbourne's sewage and consists of a 2.8 m diameter concrete lined tunnel that is 3,691 m long, with a depth to invert that varies between 31m and 45m below ground level. The Spotswood Pumping Station is now an exhibit at Scienceworks Museum and is open to the public. The lower half of the BTS has corroded significantly with peak concrete loss of 100 mm, over a period of approximately 58 years, and exposed reinforcement identified in numerous places. Melbourne Water wishes to reduce corrosion rates to defer significant sewer rehabilitation and/or potential duplication, if sewer relining is not feasible. These complications are further compounded by foul odour at Spotswood, which is at the junction of the BTS and major upstream connecting sewers. Ventilation and sulphide generation models have previously been built for the BTS to support decision making aimed at extending the life of the sewer. However due to the complexity of the sewerage system (which has five forced ventilation systems that interact), there was some uncertainty regarding the movement of air within the sewer as well as gas phase hydrogen sulphide (H2S) concentrations and relative humidity (RH). A comprehensive field monitoring campaign was conducted in March 2021 to understand and assess system operation and inform decision making regarding augmenting the BTS and associated ventilation systems. Tracer gas testing of the BTS using carbon monoxide was undertaken to reduce uncertainty regarding air flow movement (flowrate, direction) and interactions with upstream sewers. Pressure, H2S and RH logging, and liquid phase testing was conducted simultaneously to allow improved calibration of the sewer models (AFT Arrow Ventilation model and WATS sulphide generation model). The monitoring campaign presented challenges relating to sewer depth and diameter and number of monitoring stations (9 total). Irrespective, invaluable insight was gained relating to the behaviour of the BTS system compared to any previous modelling. Field testing was able to quantify air flow and direction in the BTS under different operating conditions and identify null points in the system which explained why particular sections of the sewer were more corroded (due to lack of air flow, high RH and H2S). Tracer gas testing also identified high sewage-air differential velocities in the BTS which is likely contributing to higher corrosion rates. Pressure monitoring identified the likely causes of odour complaints at Spotswood. Fresh air is forced into maintenance hole BTS007 which is the junction of the BTS and upstream connecting sewers and this results in the sewers being under positive pressure at times which increases the risk of odour complaints as foul air can escape through incompletely sealed covers and connections. The sewer models were calibrated using field data and used to address odour and corrosion issues for the BTS system and build on the knowledge gained from the BTS tracer gas testing project. Furthermore, multiple projects are scheduled to be delivered over the next ten years for the BTS and upstream sewers to enhance network performance and address asset management issues. The sewer models were used to optimise the delivery of these renewal/augmentation works considering the current asset condition and performance information and known projects that are likely to be delivered impacting the BTS. The key initiatives to address odour and corrosion issues for the BTS are outlined below. The sewer air-sewage differential velocity was minimised by halving the air flow rate by approximately 50% to provide a better balance between reduced H2S stripping and lower mass transfer rates of H2S into the biological slime on the surface of the sewer versus slightly lowered dilution of H2S in the gas phase (at lower air flow rates). The modelling was calibrated well for most of this complex system, but for the BTS itself it was poorer due to the interaction with tributary sewers. The modelling indicated that gas phase H2S concentrations would increase at a lower air flow rate. The WATS and SCORe corrosion rate estimation methods, indicated that corrosion rates would increase in response to minimising sewer air-sewage differential which contradicted the original hypothesis that overall corrosion rates would decrease by lowering the air flow rate through reduced turbulence and sulphide stripping. Furthermore, it was inferred that the upper most 1,300m section of the BTS could be in better condition due to the large volumes of fresh air that are forced into the sewer at BTS007 which reduces RH in this section. Higher corrosion rates (approx. 2mm/year) in the lower half of the BTS are likely due to high RH (approaching 100%) despite gas phase H2S concentrations being low (average of 1 ppm in the most corroded section). Corrosion rates in the lower half of the BTS will continue to remain unacceptably high regardless of whether the air flow rate for the BTS is reduced or maintained at current levels since RH is the dominant factor affecting corrosion. Interventions which reduce RH in this section were identified and included in the long term investment plan. Fresh air inducts were also proposed at strategic locations to dilute H2S and lower RH. Sewer modelling calibrated with sampling and system monitoring data was used to identify suitable interventions to increase the sewer headspace vacuum at Spotswood. Options that were considered included ventilating at a higher air flow rate and adding air flow restrictions to make the fans draw harder. The air flow rate for a replacement air treatment facility at Spotswood was increased from 1.8 to 7 m3/s and air curtains were strategically positioned to restrict rather than to block air movement in the sewer. Furthermore, the BTS007 fan was converted to a passive fresh air induct to avoid pressurising the sewers at Spotswood.