An evaporation pond provides the means to dispose of the highly saline backwash (also referred to as concentrate) that results from a reverse osmosis water desalination plant.  The concentrate is evaporated by solar radiation and the dissolved salts are precipitated to form a bed of crystallised solid.  General practice is to construct a series of small shallow ponds so that concentrate can be released into one pond while the other ponds are evaporating. 

Overview of Dalby Desalination Plant's Evaporative Pond System

At the Dalby Desalination Plant,¹ which has a capacity of 20 L/sec (320 gpm), salts removed by the semipermeable membranes are backwashed from the plant with saline reject water at the rate of 1L for every 3L of potable water produced, or 6.7 L/sec (100 gpm). The backwash is pumped to a 21-ha (52-acre) evaporation pond system located on a 40-ha (100-acre) site 1.6 km (1 mile) from the plant.

The evaporation pond system is divided into four compartments.  The ponds were proportioned such that no water overflows to the surrounding environment and constructed such that no contamination would occur to underlying groundwater resources.

The ponds have earthfill embankments around them, and around this is a flood bund (embankment) with a crest level well above the flood of record.  This flood bund addresses the Environmental Protection Agency's (EPA's) concern over downstream implications for the Condamine River basin should the saline water ponds be inundated by floodwaters.

A geotechnical investigation disclosed highly plastic dark brown alluvial silty clay overlain by 180 mm to 250 mm (7 inches to 10 inches) of topsoil.  Sporadic sand lenses were encountered at depths beyond 2 m (6.5 feet), but these were not considered to be an issue as long as excavation for the works did not reach these depths.

PB undertook detailed design and documentation, working in close consultation with Dalby Town Council.  We were also retained to provide specialist advice during the tender process for the construction works and during construction.  The design life of the pond system is deemed to be 20 years, with inflow for 15 years and 5 years to dry out.

Determining Pond Dimensions

Two of the four compartments are 7 ha (17.3 acres) in area at maximum water level and two are 3.5 ha (8.6 acres) in area (Figure 1).  The estimated pond area required for operation during the wettest 20-year period was 19.2 ha (47.4 acres).  This size took into account the decrease in evaporation rate as salt concentration increases.  We made the depth of the pond system 2.5 m (8 feet), which included:

• The minimum operational depth of 1.5 m (5 feet), based on a simulation of pond operation that accounted for the wettest continuous 20 years on record (1970 to 1989) and included a small freeboard allowance for wave action.
• A 0.5-m (1.5-foot) freeboard required by EPA
• A 0.5-m (1.5-foot) contingency provision that Council wanted for possible future expansion.

With the geometry of the ponds established, we determined the floor level (elevation) and that for the crest of the pond walls 2.5 m (8 feet) above by trial and error using 12D software.  The pond floor level was set to achieve balanced cut and fill after removal of topsoil from the entire surface within the pond area.  This required some care because a 50-mm (2-inch) change corresponded to about 13,000 m³ (17,000 cubic yards) of material, or 15 percent of the total fill requirement.  The deepest excavation for the pond floor was 1.2 m (4 feet), well above the level at which sand would be encountered.

Defences Against Cracking

The pond walls have a crest width of 3 m (10 feet), the minimum practicable for large earthmoving plant (equipment) and 1V:4H batters to limit erosion.  Normal practice in the district is to maintain bare rather than grassed surfaces to facilitate inspections for cracking.

In larger water-retaining embankments, accepted practice calls for the installation of a "filter zone."  The filter may consist of a vertical sand "chimney" to prevent the migration of soil particles should a crack develop through the embankment from drying out or movement.  While a filter was not justified for embankments this low, it was critical that the highly plastic clay be compacted at up to 2 percent wet of optimum moisture content.  This was the primary defence against drying, cracking and piping failure within water-retaining embankments and involved careful preparation of the material in the borrow area.

Figure 2:Typical section of the external pond wall. Not to Scale, which is (1V:4H)

Figure 3: Valve detail.

We gave some consideration to lining the pond floor with HDPE sheeting because of concerns about cracking (and thus leakage) when the pond was dry.  Instead, the design provided for scarifying the surface to a depth of 300 mm (12 inches) and rolling with four passes of a tamping foot roller.  This approach:

• Improves the silty-clay soil structure
• Reduces its tendency to crack
• Makes the soil non-dispersive. 

For the same reasons, the inside face of the pond walls around the perimeter was designed as a gypsum-stabilised lining 3 m (10 feet) wide.

A gravel capping similar to road sub-base was constructed on the crests
of all embankments to seal the surface against drying out and cracking,
which would allow rainfall entry and thereby induce tunnel erosion.

Figure 2 shows the pond wall cross-section.  Readers are reminded that this is not to scale (1V:4H).

The Pipe System

A delivery pipe conveys the concentrate from the treatment plant.  A feeder pipe from the delivery pipe is installed between the flood bund and pond walls with an inlet to each pond and isolating valves for manually directing water to the desired pond.  We had proposed balancing pipes between ponds initially, but Dalby Town Council determined that the manual rotation of supply from one pond to another could be done without changes more frequently than weekly.  This was considered preferable to the perceived risks associated with intermittent balancing valve operation that required an operator to be submerged in water of uncertain quality. Isolating and control valves are housed in small concrete boxes with lockable lids.  An important detail for valve pits and anchor blocks was to provide a rubber ring joint close to each point of embedment to allow for the inevitable displacements that occur seasonally in black soil country.  These details are shown in Figure 3.  For simplicity and to avoid differential settlement, adjacent valve pits and anchor blocks were designed as monolithic units, with the rubber ring joints at inlet and outlet points.

Special care was needed to avoid leakage wherever pipe work passed under embankments.  Our defences here were trenching through foundations coupled with seepage collars (Figure 4) and sand filter plugs.

The annulus between the flood bund and pond embankments will accumulate rainfall run-off.  A drainage pipe and an outlet valve were installed under the lowest part of the flood bund foundation to drain
this run-off except when the floodplain is inundated.

Construction Aspects

Pipe Work.  When pipe work is required beneath embankments, the usual approach is to install the pipes first.  However the contractor proposed completing the embankments before commencing pipe installation because:

• Earthworks for the long embankments could be carried out as a continuous process.
• Time would be saved, particularly because concrete in pipe collars needed to reach characteristic strength before backfilling.
• The embankments were not so high that the cost of six cut operations would be prohibitive. 

The merits of this approach were accepted, with the following provisos:

• All cut batters down from the crest be at 1:1 to ensure positive pressure from earthfill against cut surfaces.
• Preparation of batter surfaces be the same as for foundation surfaces (discussed below).
• All backfill be gypsum-stabilised.

The Ponds.  The pond walls were brought up to height before commencing the inner lining of gypsum-stabilised fill.  Allowing for a moisture content in the gypsum of 25 percent and a 200-mm (8-inch) depth of clay to be blended, the application rate in the borrow area was 10 kg/m2 (2 pounds per square foot) using a front-end loader.  Various blending methods were attempted.

A tractor-drawn 2-axle disc plough failed to penetrate, merely cutting 20-mm (3/4-inch)-deep grooves.  A tractor-mounted rotovator mixed the gypsum well, breaking the soil down to 10-mm to 20-mm (3/8-inch to 3/4-inch) fragments.

Trials showed that spreading by elevating scraper and kneading with four passes of the roller provided sufficient additional breakdown of the blend that good moisture conditioning resulted from only two sweeps of the water tanker.  The lining of compartment 1 was completed in this way.  For subsequent compartments, the gypsum was applied using a fertilising machine and blended with a 50-mm (2-inch)-thick layer of clay peeled off by laser-guided scrapers.

For the installation of feeder pipework, cuts battered at 1:1 were made in external pond walls down to foundation level, and then trenched as designed below a foundation.  When I inspected the trenches and the installed pipe work before backfilling commenced, and subsequently inspected the restored sections of the pond walls, I could not detect the interfaces between backfill and cut in any section.

Treatment of the pond floor was the last major activity in each compartment.  In the unlikely event that excavation
had disclosed any sand lenses, excavation and backfilling with stabilised fill would have been undertaken.  The excavation work had been done with great care and precision, however, and no sand lenses were encountered by either the contractor's personnel or supervising staff.

Epilogue

When attending the opening of the new desalination plant several months after its completion, I took the opportunity to inspect the northern end of the pond system (Figure 5).  The birdlife forming part of the small ecosystem that had developed around the shallow water in the 3.5-ha (8.6-acre) compartment 1 was pleasing to observe.  Even more pleasing was the absence of cracking on the crest.  Nor was there any significant cracking on the batter of any of the pond walls around compartment 1, or on the floor of the 7.5-ha (17.3-acre) compartment 3, which had not yet received any saline water from the treatment plant.