Desalination vs wastewater treatmentFor countless years, westernised societies have relied on clean water from reservoirs and bores. The average person within Australia consumes 340 litres of water per day (Rapid Plas, 2018) for bathing, washing, cleaning and drinking. Current sources are incapable of sufficiently supplying demand, mostly due to increasing problems of drought, flooding and rapid population growth. Innovative solutions to this issue have been developed over the past decade to create sustainable and safe water. Desalination and wastewater treatment technologies, both have the aim to sanitise water to eliminate bacteria and viruses, while also having the ability to reliably produce potable water to support large populations. These goals are achieved through numerous chemical and biological components before being distributed to homes across the country.
What is in the water before it is collected and treated?
The source of water can determine potential hazards when ingested or consumed. Various distinct treatment processes extract contaminants from the water so it becomes innocuous. Despite how seawater may look, it contains minerals such as salt (sodium chloride) which originates from runoff of rain after it has eroded rocks. Excessive consumption of sodium chloride in the bloodstream can be toxic to the body and forces water to rush from the cells to dilute it (Poison Control, National Capital Poison Centre, 2018). Comparatively, wastewater does not require the removal of salt as there are low levels of salinity and is generally within regulations, not exceeding 250 mg/L ( . Before treatment of wastewater, there is copious amounts of solid waste found within. The origin of these solids is caused by human waste and other materials that do not belong in sewage. Other contaminants including bacteria, microorganisms and colloids are commonly found in both treatment processes and need to be removed. These impurities can carry diseases or form aesthetic imperfections (cloudiness, discoloration), therefore affecting public opinion and satisfaction on the water they consume.
How are the different components removed from the water?
Treatment methods can purify water and eradicate substances that can and cannot be seen. Dissimilar processes are applied to wastewater treatment compared to desalination. The pair both begin their treatment identically, through a screening process to filter out solid waste from their respective sources comprising of plastics, paper and vegetable matter. The Penrith wastewater plant has 5mm fine screens which continues to a forced vortex. A forced vortex causes grit (sand, rocks and gravel) to spiral to the bottom of the grit removal chamber, allowing water to pass through all steps of the treatment without causing blockage or damage to equipment. All waste is washed, compressed and put into screening bins to be taken to landfill. In comparison, the Gold Coast desalination plant has 3mm small screens, followed by a set of finer filters to remove miniscule particles. At this point, the coagulant, ferric sulphate, is added to the impurified water to remove small suspended particles (colloids). This technique works due to the positive charge of the iron in ferric sulphate that will neutralise the negative charge of the suspended solids, creating floc in the water. Although the Gold Coast plant does not use this method, stirring could essentially speed up the process of flocculation and settlement, as a result of higher collision rates between the coagulant and dissolved compounds or suspended particles.
Sodium chloride is a component of seawater that must be removed before it can be consumed. Desalination facilities can extract sodium chloride from the water through a process called reverse osmosis. Reverse osmosis is the concept of forcing a molecule through a semi-permeable membrane (only allows specific substances through) from a high concentration to low. This cannot be performed in a natural environment, as the liquid would go from a low to high concentration to dilute the solute. To successfully achieve reverse osmosis, the water must be highly pressurised by a pump to at least 50 bar (more than 50 times atmospheric pressure) (Seqwater, 2018). However, the water is not sufficiently filtered the first time seawater passes through the membrane and requires a second permeation. This system is extremely expensive to operate owed to powering pressurised pumps. The costs can be offset by the energy recovery process which uses brine (high salt concentration in water) that was not filtered out in the first pass of reverse osmosis and forces it through a turbine to produce kinetic energy. This method also disposes of bacteria, particles, colloids, organics and other minerals for health and aesthetic reasons. However, wastewater goes through other processes to remove nutrients and remaining small particles in the water.
Wastewater relies on anaerobic and aerobic processes in its secondary treatment phase and permits bacterial decomposition to occur in an intermittently decanted aerated lagoon (IDAL). First of all, the de-gritted wastewater flows into the anaerobic (absence of oxygen) zone. Due to biological techniques not being as efficient at removing phosphorus from the water, spent pickle liquor is added to assist in the process. Spent pickle liquor is an acidic mixture left over from metal treatment (Sydney Water, 2018) and typically comes in ferrous chloride or ferrous sulphate. It will mix and react with phosphorus and form iron phosphate compounds, to later separate from the wastewater in the settling phase. The product is then either discarded or sent back to the anaerobic zone to be used as seed sludge, referring to the mass of sludge that contains microorganisms, creating a faster treatment process. After the anaerobic zone, water is pumped into the intermittently decanted aerated lagoon. Air is fed into the tank through diffusers that are made up of fine pores on a membrane which bubbles can come out of, producing an oxygen filled tank. This is where microorganisms can break down organic matter, in addition to turning ammonia into nitrates and water, reducing the biological oxygen demand (BOD). This measures the oxygen consumed by microorganisms to decompose waste and can determine the effectiveness of the water treatment. The wastewater is then left to settle, and the oxygen supply is halted. Excess organic matter is used by bacteria as an energy source and in return, converts nitrates into nitrogen gas and releases it into the atmosphere. Two types of bacteria are responsible for this conversion; nitrosomonas and nitrobacter. Nitrosomonas oxidises ammonia to form nitrite while nitrobacter converts nitrite to nitrate. All sludge will settle to the bottom of the tank and get transferred back to the anaerobic zone for reuse. Before the clear water can be sent to tertiary treatment, it must be decanted using a weir.
Nitrosomonas2NH3 + 3O2 => 2NO2 + 2H2O + 2H
Nitrobacter2NO2 + O2 => 2NO3
Tertiary treatment is the final step in wastewater cleaning. This stage involves the removal of extra phosphorus, nitrogen, bacteria and viruses, warranting safe drinking water. Water gets pumped into a flash mixer where alum is added as a coagulant to create floc in the water, containing of phosphorus and group other solids together for filtration. The flash mixer is used to increase collision rates so that reactions can occur faster. The flocculated water is gravity fed through a sand filter to capture clumps and