Implications of Design Assumptions for Pressure Sewer Network Performance: A New Zealand Study

Pressure sewer networks offer a flexible alternative to more traditional gravity-based systems for the conveyance of wastewater. Some of the issues associated with pressure sewer networks (e.g., odor, septicity) arise from inappropriate design assumptions. Daily inflow volumes are a crucial part of the design of pressure sewer systems;gravity design parameters are presently used to design pressure sewer networks in New Zealand. This study analyzed flow data from six representative pressure sewer networks (approximately 24% of operating pump units in New Zealand) to identify the daily inflow volumes per connected pump unit. The results indicated that the median inflow volume was approximately 410  L/pump unit/day. This inflow is much lower than current council design standard assumptions, which range from 650 to 1,000  L/pump unit/day. Pressure sewer network designs using higher daily loading rates may result in oversized networks that are detrimental to the network’s operation and performance, especially for meeting minimum self-cleansing velocities and wastewater retention times. The data collection period included the first COVID-19 lockdown in New Zealand. Four lockdown levels were introduced, with Level 4 and Level 3 being the most restrictive and requiring all but essential workers to stay and work from home. Levels 1 and 2 allowed people to return to their place of work. The data indicated that the Level 4 lockdown period caused a 25% increase in daily inflow volumes. In comparison, the Level 3 and 2 lockdown periods increased the daily inflow volumes by 20% and 15%, respectively. The analysis also included the networks’ wet-weather responses. Minor rain events did not significantly affect the daily inflow volumes. However, gravity networks that have been retrofitted with pressure sewer networks may be more subject to aging or damaged laterals and illegal stormwater connections, both of which are likely to result in a more significant wet-weather response. The paper also discusses the issues associated with an overreliance on standardized design methods without understanding their proper application and the pitfalls of adopting gravity sewer design assumptions for pressure sewer network designs. The findings of this paper will further allow determination of the sensitivity of network design outcomes, performance, and maintenance requirements to the design methods and assumptions for pressure sewer networks, not only in New Zealand but in any country that uses the technology.