Water

The two most common treatment processes are Flocculation and Sedimentation. 

Water from particular sources such as rivers, creeks, dams and weirs contain suspended particles of clay and other matter that needs to be removed to make the water clear. The clarity or cloudiness of the water is measured by the turbidity of the water; the more turbid the water, the more particles it contains and therefore the more treatment it requires to make it clear and sparkling. 

Sedimentation involves letting large particles in water settle to the bottom of a large storage tank to form sludge. This sludge is removed and pumped to drying beds to de-water. 

Flocculation is a similar process to sedimentation except it involves adding a coagulant such as alum to help clarify the water. 

As particles suspended in water are negatively charged (repel other/similar particles with the same charge) they do not settle very easily. 

The alum helps the suspended particles join to form larger particle or “floc”. The quantity of the alum is controlled so that it clarifies (clears) the water which flows from the top of the tank and the particles sink to the bottom to for a sludge. 

Information sourced from: Australian Water Association, We All Use Water, At the water treatment plant, brochure 9. 

Most drinking water is filtered after it has been treated. 

The most common process is to allow it to filter through a bed of sand. Filters need to be backwashed regularly. 

Pretreatment 

Bore water passes through a feed pump which pressurises the water for the pre-treatment stage. The pre-treatment stage consists of multimedia filtration, anti-scalant dosing and bag and cartridge filtration. 

Initially, the feed water passes through three fibreglass Multimedia vessels containing gravel, sand and anthracite. These filters suspend solids and colloidal material and are routinely backwashed to remove any deposited matter. 

Anti-scalant dosing occurs to reduce carbonate, sulfate and calcium fluoride scaling of the membrane. Scaling reduces the efficiency of the membranes and can also decrease the working life of the membranes. 

The bag and cartridge stage of the pre-treatment process removes any suspended solids remaining after multimedia filtration. The bag filters remove particles larger than 5 micron (0.005mm) and the cartridge filters remove particles larger than 1 micron (0.001mm). 

Desalination

Desalination is the removal of salts from ocean or brackish waters using various technologies. Electrodialysis Reversal (EDR), Multiple Stage Flash (MSF) and Reverse Osmosis (RO) are some of the technologies that are capable of removing nutrients and salts that make the water unfit for human consumption. 

Reverse Osmosis is the main process used in the Western Downs. 

Principles of Reverse Osmosis 

The phenomenon of osmosis occurs when pure water flows from a dilute saline solution through a membrane into a higher concentrated saline solution. 

A semi-permeable membrane is placed between the two compartments. Semi-permeable means that the membrane is permeable, or penetrable, by some species or substances, but not to others. Assuming that this membrane is permeable to water but not to salt, a salt solution is placed in one compartment and pure water in another compartment. The membrane will allow water to permeate through it to either side, but salt cannot pass through the membrane. 

As a fundamental rule of nature, this system will try to reach equilibrium where the same concentration (saltiness) of water exists on both sides of the membrane. The only way to reach equilibrium is for water to pass from the pure water compartment to the salt-water compartment, in order to dilute the salt solution. 

Osmosis can cause a rise in the height of the salt solution. This height will increase until the pressure of the column of salt solution is so high that the force of this water column (head) stops the water flow. The equilibrium point of this water column height, in terms of water pressure against the membrane is called osmotic pressure. 

If a force is applied to this column of water, the direction of the water flow through the membrane can be reversed. This is the basis of the term, “Reverse Osmosis”. Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt. 

Within the Desalination Plant, the high-pressure pump forces the saline feed through the spiral-wound membranes to produce a product stream (permeate) and a high-salinity brine stream (concentrate, or reject). 

The processed outcome of these two (2) streams results in a 80% recovery rate, or alternatively, a waste rate of 20%. This waste is transferred to evaporation ponds. 

Although higher recovery rates are possible, production costs are greatly increased due to the increased amount of membranes required and associated pumping costs. 

These pressure vessel arrays contain the spiral-wound membranes which are the foundation for the reverse osmosis process. 

Post Treatment 

The water produced through the reverse osmosis process is close to pure water. The process does not remove the carbon dioxide that is naturally present in the feed water. Conversely, salts contributing to the alkalinity are strongly rejected. Carbon dioxide in its dissolved form causes a decrease in the pH levels due to its acidic quality. Aeration supplemented by caustic dosing of the permeate is conducted in order to increase the pH to an acceptable level. 

A certain amount of the initial feed water by-passes the RO process and is re-introduced into the permeate in order to increase the alkalinity of the water, thus reducing the need for chemicals. Achieving the correct rate of alkalinity is required to produce a slightly scaling as opposed to a corrosive product. 

Management of Concentrate Waste 

Due to the nature of Dalby’s reverse osmosis process, there is a saline by-product that represents approximately 20% of the total feed water. 

Considerable research has gone into the most appropriate way to manage this by-product. As a result of this research, a series of large evaporation ponds have been developed in close proximity to the plant. Monitoring of the ponds and associated infrastructure is conducted daily and comprises visual inspection of pipe work, monitoring of evaporation rates, salinity testing and monitoring of near by test bores.