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During water treatment processes, water is pumped backwards through a backwash sand filter, often using intermittent compressed air. The backwashing of the sand filter is a preventive measure to ensure reusability of the filter media.
Typically, backwashing is automated in water treatment plants using local programmable logic controllers (PLCs). After a set time interval, the backwash cycle is triggered. This happens when the turbidity of the filter effluent exceeds a certain reference value or a certain level of differential pressure (head loss) across the filter.
In the loop system, oxygen and other dirt and debris can cause pipes to break down and mechanical equipment to fail. This includes regulators, valves, boilers, controls, fittings, and other forms of mechanical equipment. Corrosion, wear and tear as well as sludge buildup can be increased by such malfunctions.
Impurities can negatively affect the lifespan and efficiency of water treatment filters and related components. Backwashing can effectively remove oxygen and filter granules. As a result, system fluids can transfer heat more efficiently, resulting in improved boiler efficiency. In addition, backwashing reduces corrosion risks significantly.
In essence, a good backwash is one in which two things are achieved. Firstly, it expands the bed by 30 to 40%. Secondly, it stratifies the carbon particles at the end of the operation. To achieve the former, you need to make sure that the bed expands. You cannot simply rely on the manufacturer’s reported graphs. These graphs give an approximate idea of the flows with which various percentages of expansion are achieved. However, a final adjustment must be made for each particular case.
If the above mentioned verification is not done, there is a risk that the bed will not expand, or that the carbon will be dragged out of the adsorber. When backwashing is done correctly, as the bed expands, the particles move up and down. This way they rub against each other, thus achieving a better and faster cleaning of this surface.
Air is used to expand and agitate the filter media, breaking up the compacted material. Clean backwash water is pumped in, removing the loose material from the filters. The particles are then returned to the wastewater treatment process.
Using blowers in filter backwashing can reduce the amount of time the process is run. This is possible because the use of blowers allows for cleaning of the filters in a more efficient manner, ultimately reducing operational and energy costs.
Air assist is preferred in backwashing as it provides a more vigorous washing action. Air and water are applied for several minutes as the bed is agitated. The wash water that has risen from the backwashing process is removed. Then, a second lower pressure backwash, without air assist, makes sure bubbles are removed from the filtering media.
Backwash water is either discharged to sanitary sewers without treatment, or treated and repurposed within the plant. The water used for backwashing contains high levels of particulate matter. The most common treatment processes are coagulation, flocculation, and sedimentation.
High molecular weight synthetic organic polymers are sometimes added to facilitate the formation of settleable floc. Failure of a backwash treatment process can result in the reintroduction of poor quality water into the main water purification stream. This leads to the production of poor quality treated drinking water, and premature ancillary equipment failure.
There are two common types of filter backwashing: continuous backwashing and semi continuous backwashing.
Backwashing occurs continuously when raw water enters the tank near the bottom. Then, the water flows up through the media bed, filtering suspended solids out. The filtrate passes over the effluent as it reaches the top of the filter. In order to clean and transfer waste solids, a portion of the filtrate is diverted through the filter media.
In this process, water is pumped below the sand filter; it's then sucked out from the top with a separate pump. Next, the dirty water from the backwash is sent to the main distribution inlet header for reprocessing. An aeration blower provides air used in a semi-continuous and pulsed bed filter. The backwashing process begins once the suspended solids in the effluent cease to filter to an acceptable level of the head.
Robuschi is the industry’s global leading specialist of engineered products. We ensure reliable, quality, and cost effective solutions for water and wastewater applications. Whatever type of filter backwashing you choose for your wastewater treatment plant, Robuschi has the solution for you.
We select the best technology according to your wastewater treatment plant. We can present you with the most efficient blower, making calculations with considerations on lifetime costs, including cost of purchase, electricity usage, and service (downtime) costs. Do you need more insights?
