Conveying Raw Wastewater Over Long Distances - Technical Issues and Solutions

A range of factors are driving water utilities to centralise their wastewater treatment, including stricter consent and licencing conditions for treatment plants, discharges, and re-use, enhancement of resource recovery, and adaptation to a changing climate. This forces many cities around the globe to pump wastewater considerable distances. This is further exacerbated as urban boundaries extend to accommodate growing populations.

Constraints of urban environments, geotechnical conditions, and environmental, heritage, and statutory planning considerations restricts route options, often resulting in complex hydraulic profiles that incorporate pumped sections, pressure gravity sections and part-full gravity sections.

Schemes are often designed on a 50-year population horizon with assets appropriate for 100 years of service. A key driver for these schemes is to provide the backbone wastewater service to growth areas. Schemes linked to growth projects are challenged by high upfront capital cost and a risk that development may not occur quickly enough to generate the required start-up flows to manage flushing and septicity. Equally, the start-up flow can be a fraction of the design population meaning hydraulic design and pump selection must consider a broad operating range. The commercial terms imposed on developers by asset owners relating to developer contributions, land sale and house construction timeframes are a key consideration in mitigating slow build-out and high capital cost.

Understanding how an asset owner’s maintenance regime, particularly pump replacement timeframes, aligns with population growth offers a means of identifying natural points in the asset lifecycle where pumps can be upsized as part of a standard maintenance replacement programme to suit the catchment development. Identifying how to ‘sweat’ an asset is essential in identifying the tipping point where provision for greater population requires a full scheme upgrade or pipeline duplication. Pressure main velocity range is a good place to focus for this assessment.

The potential for generation and release of nuisance odour increases with long distance pumping. Longer rising mains are more susceptible to generating odours due to longer residence times and the increased potential for septicity. Air expelled during filling of free-draining sections are another source of release of potentially odorous air. Air valve locations need to consider the functional requirement to facilitate emptying, filling and limit transient pressures, alongside the potential to generate odour complaints from customers and ensure that air valves have sufficient pressure to maintain a seal under a variety of flow scenarios.

It may take an extended period of pumping to generate self-cleansing velocities along the whole pipeline to ensure that sediment does not accumulate, and wall slimes are maintained within the system’s hydraulic limitations. Sufficient volume must be allowed for in the pump sump or parallel storage provided as catchments develop. Optimising the flushing regime to flush the longest section of sagged pipeline, as opposed to the full line, may help to reduce the flushing volume required.

This paper describes the delivery of some major long-distance conveyance schemes in Australia and the United Kingdom, and discusses the design and operational challenges faced and the solutions to overcome them.