Dewatering Lagoon Sludge with a Geotextile Tube System

Lagoon sludge can be difficult to manage because it contains a high proportion of water, has variable solids content and often behaves differently from mechanically thickened sludge. Pumping, hauling and disposing of this material can quickly consume a regional council’s operating budget. A geotextile tube system offers a comparatively simple way to separate water from solids on site, reducing volume before reuse, treatment or disposal.

This case study follows a realistic Australian municipal project involving an ageing wastewater lagoon in regional Victoria. The example shows how site investigation, polymer selection, drainage design and weather planning influenced the outcome. It also highlights the practical decisions that engineers, operators and contractors must make when adapting dewatering technology to Australian climate conditions and biosolids requirements.

Why the Lagoon Sludge Required a Different Approach

The lagoon had accumulated approximately 4,800 cubic metres of sludge over more than a decade. Sampling showed a solids concentration of about 3.5 per cent, with a soft, organic layer near the surface and denser mineral material around the inlet zone. Conventional excavation would have required extensive temporary storage, while direct vacuum truck removal would have involved thousands of kilometres of haulage to an approved facility.

The council also wanted to avoid major disruption to the treatment process. The lagoons serve a population of roughly 9,000 people, with seasonal peaks caused by tourism and agricultural activity. A geotextile tube allowed the project team to work within a fenced area beside the lagoon, using progressive pumping rather than draining the entire pond.

The method was selected for its relatively small footprint, low energy demand and ability to retain fine particles. It was not treated as a universal solution. The project team first confirmed that the available land could support the tubes, filtrate collection system, access tracks and emergency storage needed during a wet Victorian winter.

Establishing the Design Basis

Before installation, the sludge was surveyed using depth poles, sonar readings and trial pits along the lagoon margins. The investigation identified an average sludge depth of 0.7 metres, but depths exceeded 1.4 metres near the inlet. Laboratory testing measured moisture content, volatile solids, particle size, salinity and nutrient concentrations.

The expected dewatered cake volume was calculated from dry solids rather than wet sludge volume. This distinction was important: a tube receiving dilute material may process a large volume of slurry while retaining a much smaller mass of solids. The design allowed for three medium-sized tubes, operated sequentially so that each unit could drain and consolidate before the next one was filled.

A compacted gravel pad was constructed above the seasonal groundwater level. The pad included an impermeable liner, perimeter bunds, a leachate sump and a shallow diversion drain. In parts of Australia, intense summer storms can arrive after long dry periods, so the containment design allowed for both a 1-in-20-year rainfall event and operational freeboard.

Preparing the Sludge and Polymer

The sludge was pumped through a macerating screen before entering a polymer make-down and dosing system. Screening removed rags, sticks and other debris that could damage pumps or create weak points in the geotextile fabric. The polymer was selected through bench-scale jar testing, followed by a short field trial using actual lagoon material.

The final dose was lower than the initial laboratory estimate because the site team adjusted mixing energy and polymer ageing time. Overdosing created a slippery, gelatinous floc that restricted drainage, while underdosing caused cloudy filtrate and excessive solids carryover. Operators monitored floc size visually and checked filtrate turbidity during each production run.

The Australian market includes several suppliers of geotextile tubes, pumps and water-treatment chemicals, but lead times can be significant outside Sydney, Melbourne and Brisbane. Ordering spare dosing pumps, fittings and polymer well in advance reduced the risk of downtime. The contractor also arranged a local electrician because the site had limited three-phase power.

Filling and Consolidating the Tubes

Each tube was positioned with its filling port facing the lagoon and its drainage outlets directed towards the filtrate collection channel. The first fill was deliberately conservative. Operators raised the tube in stages, allowing the fabric to settle and the drainage paths to develop before increasing the pumping rate.

During the first week, the system processed approximately 180 cubic metres of sludge slurry per day. Filtrate flowed through the geotextile fabric into a lined channel, then to a holding tank for controlled return to the wastewater plant. Returning the water too quickly could have overloaded the lagoon’s hydraulic capacity and disturbed biological treatment.

The tubes were filled during daylight hours, with overnight inspections of bunds, hoses and access routes. This arrangement suited local council work practices and reduced the likelihood of an unnoticed leak during a summer thunderstorm. An anemometer was also installed because strong winds can move lightweight fabric before the tubes develop sufficient mass.

Managing Filtrate and Environmental Risk

Filtrate quality improved as the tubes consolidated. Initial turbidity was high, but suspended solids fell substantially after polymer optimisation and several days of drainage. The return flow was metered into the treatment process rather than discharged directly to a creek or stormwater channel.

Nutrient management was part of the approval process. Testing included total nitrogen, total phosphorus, ammonia, E. coli and selected metals. The final solids were assessed against the intended end use, with no assumption that all dewatered sludge would qualify as unrestricted agricultural biosolids.

The team also considered emerging contaminants. Geotextile dewatering does not remove every dissolved substance, and it should not be presented as a complete treatment barrier. The council reviewed current research on microplastics guidance while developing its sampling programme, particularly because plastic fragments and synthetic fibres can enter wastewater through household and industrial sources.

Results, Costs and Operational Performance

After 14 weeks, the tubes had retained approximately 165 tonnes of dry solids and reduced the material requiring off-site transport by an estimated 78 per cent by volume. The dewatered cake reached an average solids concentration of 26 per cent, with higher readings in the oldest sections of the tubes. The remaining water was managed through the plant’s normal liquid treatment train.

The project cost less than the council’s estimate for full mechanical dewatering and trucking, although savings depended on using land already owned by the utility. Capital costs included the pad, liner, pumps, polymer system, pipework and monitoring equipment. Haulage costs were reduced because trucks carried denser material and made fewer trips to a licensed facility.

Labour requirements were modest after commissioning. One operator checked polymer preparation, flow rates and containment each shift, while the contractor performed weekly fabric and pipe inspections. Professional networks such as LABS news can be useful for comparing field results, training opportunities and emerging equipment practices across the water environment sector.

Lessons for Australian Utilities

The project demonstrated that geotextile tubes work best when the sludge source, solids concentration and end-use pathway are understood before equipment arrives. A tube cannot compensate for poor feed conditioning, inadequate containment or unrealistic assumptions about the final cake. The most valuable early investment was the site investigation, which prevented under-sizing and identified the need for a stronger pad near the inlet zone.

Weather planning also affected the result. The council scheduled the main filling period outside the wettest part of the year, but retained enough freeboard and storage capacity for unexpected rainfall. In northern Australia, the same approach would require careful consideration of the wet season, cyclone exposure and rapid biological changes caused by higher temperatures.

Funding and approvals should be integrated into the early programme. Although California’s infrastructure environment differs from Australia’s, the principles described in this SRF loan guidance illustrate the importance of defining project outcomes, environmental safeguards and lifecycle costs before seeking capital support. Australian councils should align this work with state environmental regulators, local procurement rules and the relevant biosolids framework.

The strongest outcome came from treating dewatering as a complete process rather than a piece of equipment. Sludge characterisation, polymer trials, hydraulic control, weather resilience and final solids management all influenced performance. For a council facing limited land and rising disposal costs, a geotextile tube system can provide a practical intermediate step between lagoon excavation and a larger permanent solids-processing facility.

The next step is to collect representative sludge samples, measure dry solids and run a controlled polymer jar test before preparing the site layout and budget.