Wastewater treatment plant design for fluctuating flows, Queensland, Australia

Project summary

Supporting the Australian Government’s strategic partnerships with the Australia–Singapore Military Training Initiative (ASMTI), the existing Shoalwater Bay Training Area in Central Queensland was upgraded. As a result of sporadic personnel visitation to the site, intermittent loading and fluctuating diurnal flows were a known complexity when considering the design of an effective wastewater treatment solution. Working with Laing O’Rourke, Pensar supported the design and construction of a suitable wastewater treatment solution.

The background

The substantial expansion of the existing Shoalwater Bay Training Area in Central Queensland to support the growing ASMTI will see up to 14,000 Singapore Armed Forces personnel on-site. Personnel will conduct unilateral training in nine-week training windows, up to twice a year, presenting an intermittent load and fluctuating flow challenge for the wastewater infrastructure at the facility.

Over a two-year period, Pensar was engaged in the design and construction of wastewater infrastructure that permitted a large buffering capacity to meet the increased diurnal flows of typical camp routines when the base is in use, while enabling it to endure extended stand down periods of little to no flow, in some cases up to nine months. In addition, the design needed to facilitate potential future growth of the training facility and meet the effluent discharge target requirements of the Great Barrier Reef region.

The challenges

The project scope included the development of a septic tank process for primary treatment; a filtration system for the removal of suspended solids and biological oxygen demand; ammonia stripping for nutrient removal and chemical dosing systems; and the proposal of a combination of irrigation and surface water release for the final effluent disposal.

Delivering this scope presented a series of challenges owing to the site-specific context in which it was to be delivered. Adaptation solutions to a traditional facultative wastewater sewage plant were required to overcome constraints. A traditional lagoon waste stabilisation pond was not viable because it would die during the expected periods of inactivity at the training base. During training exercises, there will be large diurnal peaks due to the inherent nature of military routines, presenting capacity challenges versus the periods of inactivity.

Additionally, lagoons are notoriously known for their high levels of suspended solids and inefficient ammonia removal. Because of the intense wet season rainfall and extended dry periods at the site’s location, the lagoon system would be exposed to highly variable hydraulic and environmental conditions. Heavy rainfall can disturb settled solids and dilute the treatment process, while long dry periods reduce inflow and can destabilise biological activity. These fluctuations risk overloading or underfeeding the lagoon, making it difficult to maintain consistent treatment performance.

The solutions

Pensar’s tailored design solution provided the Department of Defence with a significantly reduced ongoing operational expenditure, eliminating the requirement of full-time wastewater treatment plant operators, and delivered suitable final effluent for irrigation use at the training base.

Through extensive research on similar environments and other military wastewater assets, collaboration, through optioneering workshops, and process of elimination, a facultative waste stabilisation pond (WSP) solution was approved. Pensar’s final infrastructure design included two primary aerated facultative lagoons, which utilise aerobic and anaerobic biological activities and are supplemented with mechanical aeration. This was followed by a secondary sediment pond that was also enhanced with mechanical aeration.

The primary lagoons were designed with a deeper anaerobic zone for increased solids retention to enable slow digestion and release of organics. By sequestering and retaining large volumes of primary sludge during camp use, the lagoon intends to sustain life during the expected down periods, drawing the parallel to an intermittent fasting diet.

The impact

By creating highly oxygenated surface levels using mechanical aerators and artificially controlling the lagoon pH levels, the aim was to accelerate the rate at which ammonia broke down. These conditions are also advantageous to preventing the creation of anaerobic byproducts and minimising odour.

Two different recycle loops on the lagoon systems allow the asset to continue operating throughout the quiet periods in between base use. The result of this process will also provide the client with a significantly cleaner final effluent, which will be used as irrigation on the training base land.