Dissolved air flotation as a pretreatment solution for industrial wastewater

Working through technical presentations hosted by LABS of CWEA, members have long tracked how operators across the Pacific Rim refine their pretreatment trains, and a recent collaboration with a food processor south of Perth offers a useful window into the practical side of dissolved air flotation. The plant, which sits within the Kwinana industrial strip, runs a high-strength effluent stream that historically overwhelmed the downstream biological system during peak season.

Dissolved air flotation, or DAF, has become a go-to option for facilities that need to strip suspended solids, oils, and emulsified fats before sending water to a biological stage. The principle is straightforward: fine bubbles attach to particulates, lifting them to the surface where a skimmer removes the sludge blanket. In a country where summer temperatures routinely climb above 40 °C and biological treatment efficiency dips accordingly, removing the bulk load upstream pays real dividends.

Australian regulators expect pretreatment plants to demonstrate consistent performance, and the Western Australian Department of Water and Environmental Regulation has tightened its licencing requirements in recent years. For operators in Perth and the surrounding Peel region, that often means investing in equipment that produces a stable effluent quality regardless of the upstream variation. DAF fits that brief because it responds quickly to load changes without long lag times.

The case study that follows tracks a six-month design and install cycle, followed by twelve months of continuous operation. It draws on data shared by the site engineering team and on independent sampling carried out during commissioning. Engineers from member agencies and consultancies will recognise the familiar pattern: a tight timeline, a stubborn influent, and a technology that delivers once the parameters are dialled in correctly.

Why the plant chose DAF over other pretreatment options

The site's previous pretreatment consisted of a rotary drum screen and an equalisation tank. Solids removal hovered around 40 percent, and fats regularly escaped into the aeration basins, causing foaming and loss of mixed liquor. Operations staff trialled a dissolved air flotation pilot for eight weeks, comparing it against a primary clarifier and a membrane bioreactor in a side-by-side arrangement.

The pilot DAF unit consistently achieved more than 85 percent total suspended solids removal and cut fats, oils, and grease by over 90 percent within the first hour of contact time. The clarifier struggled with hydraulic surges during the morning production flushes, while the membrane option offered better clarity but at a capital cost that did not fit the project budget. DAF won on a combination of footprint, energy draw, and resilience to shock loads.

Another factor in the decision was the Australian preference for equipment that local technicians can service without flying in specialists from overseas. The chosen DAF supplier maintained a parts depot in Bibra Lake, which meant replacement wear components could be on site within a day. For a facility running twenty-four hours a day, that kind of local support weighed heavily in the final scoring.

Design parameters tailored to local wastewater

Engineers sized the system using a hydraulic loading rate of 12 metres per hour and an air-to-solids ratio calibrated to the plant's specific effluent profile. The wastewater carried a chemical oxygen demand of roughly 4,500 milligrams per litre and suspended solids around 1,800 milligrams per litre, with pH fluctuating between 5.5 and 8.2 depending on cleaning-in-place cycles.

Polymer dosing was a critical design input. Jar testing on site identified a cationic polyacrylamide blend that produced a strong, fast-forming floc capable of attaching to microbubbles within the contact zone. The team chose a saturated recycle flow of 15 percent to keep the bubble cloud dense enough for consistent flotation without starving the downstream biological system of dissolved oxygen.

One local consideration that shaped the design was the salinity of the incoming stream, which crept up during periods when the plant took in brine from a nearby food-grade salt supplier. Higher dissolved salts suppress bubble coalescence and can reduce flotation efficiency. The design incorporated a small coagulant pre-mix stage and a wider contact zone to compensate, ensuring performance held steady even when conductivity spiked.

Commissioning the system at the Kwinana facility

Installation took place over a twelve-week window between January and March, the period when Perth's weather is least likely to disrupt outdoor construction. The skid-mounted DAF unit arrived in two modules and was craned onto a prepared concrete pad adjacent to the existing screen building. Pipework tie-ins were completed during a planned shutdown, which the operations team had scheduled to coincide with a maintenance window at the site.

Cold commissioning progressed smoothly, but the first hot trial exposed an underestimation in the polymer dilution water supply. Operators had to install an additional line drawing from the clarified water tank to keep the polymer mix consistent. Once that was resolved, performance tracked the pilot data within five percent, with the float blanket forming a stable 60-millimetre layer across the surface.

Independent sampling during the third week of operation confirmed biochemical oxygen demand reductions of more than 60 percent and suspended solids removal of 88 percent. The downstream activated sludge plant responded almost immediately: the food-to-mass ratio stabilised, foaming incidents dropped to near zero, and the diffused aeration system could be turned down by 15 percent during off-peak hours, saving roughly $4,000 per month in blower energy alone.

Twelve months of operating data

The first year of operation generated a wealth of data that operators have shared through site visits and technical presentations. Monthly averages showed total suspended solids in the DAF effluent hovering between 110 and 160 milligrams per litre, well within the discharge consent set by the Western Australian regulator. Fats, oils, and grease readings rarely exceeded 30 milligrams per litre after the float, compared with peaks above 600 milligrams per litre in the raw influent.

Maintenance interventions were limited to weekly skimmer blade inspections and quarterly replacement of the recycle pump seals. One notable event occurred during a heatwave in late February when ambient temperatures exceeded 42 °C for three consecutive days. The team observed a temporary drop in float solids concentration, which they attributed to reduced gas solubility in the saturator. A small chiller unit on the recycle line corrected the issue, and the plant has since added that to its standard summer operating procedure.

Sludge handling improved markedly. The float sludge dewaters to roughly 12 percent dry solids without chemical conditioning, a significant gain over the old primary sludge. That consistency has simplified disposal negotiations with the receiving waste hauler and reduced the volume of material sent off site by almost a third compared with the previous year.

Training the local operations team

The success of any new pretreatment unit depends on the people who run it, and the supplier invested heavily in upskilling the Kwinana crew. Three operators completed a two-week competency programme covering saturation chemistry, polymer handling, and basic troubleshooting. The training included hands-on sessions with the plant's own skimmer and pump skids, rather than relying solely on classroom slides.

A key focus was helping the team recognise early warning signs. Operators learned to read the float blanket thickness, interpret polymer demand through jar tests, and identify when the recycle water temperature was drifting outside the optimum band. That level of ownership has paid off in reduced callouts and faster recovery from upset events.

The site has since hosted two site visits for neighbouring facilities, including a delegation from the Hunter Valley interested in similar applications for their mining service water streams. Photos and technical notes from those visits are available through the event gallery, which members have found useful when preparing their own business cases.

Maintenance demands and long-term costs

Twelve months in, the total cost of ownership has tracked close to the budgeted figure of $0.18 per kilolitre treated, including power, polymer, and routine maintenance. The largest variable cost remains polymer, which accounts for roughly 60 percent of the operating spend. The team is currently trialling a lower-dose blend that promises a 12 percent reduction without compromising float quality.

Capital payback came in at just under three years, faster than the five-year target set in the original approval. The accelerated return reflects both the energy savings in the downstream aeration basin and the avoided clean-up costs from foaming events. For Australian operators weighing similar investments, that payback window is competitive with most chemical-physical pretreatment options on the market today.

Looking ahead, the engineering team plans to integrate the DAF control system with the site's existing SCADA platform, allowing remote monitoring from the central control room in Adelaide Street. The integration work is scheduled for the next scheduled maintenance shutdown, and the project manager expects the upgrade to be commissioned within a single plant turnaround.

The next logical step for any operator considering a similar upgrade is to commission a short-duration pilot trial on their own effluent, using a containerised DAF unit that can be installed without permanent civil works.