SPECIAL FOCUS: OPERATORS’ FORUM
Using air-operated double-diaphragm pumps to optimize filter press operation By Paul McGarry
F
ilter presses, which were invented a century ago, are used to remove solids from a liquid stream. Sludge that is separated as the solid-laden liquid moves through the filter press can be easily removed and disposed of. The cleaned liquid can then be moved to the next stage of the production process, or recirculated back into the filter press for further processing. Industrial water and wastewater applications are ones where filter presses have proven their worth from day one. This is an application where large volumes of solids and liquids need to be separated on a daily basis. As the solid-laden slurry is pumped into the press, two chambers are filled, which facilitates the formation of thick slurry “cakes”. While the chambers fill, pressure inside the system increases to the point that when the chambers reach capacity, the liquid is filtered out with the aid of streams of compressed air or water. This “dewatering” leaves behind solid slurry, or sludge cake, that is ready for disposal. A major advantage in this cake-creation process for operators of water/ wastewater plants is that instead of having to dispose of large amounts of wet slurry, the outcome is a flow of neutralized water and a comparatively small amount of cake, which reduces disposal costs. To operate properly, filter presses require good filtration and constant system pressure so that the flakes that form the slurry cakes are not destroyed. This means that pressures of up to 100 psi (7 bar) are generally required. The filter press process in water and wastewater treatment also requires that various types of ancillary liquids be used. In many cases, these liquids, including things like milk of lime, which is very abrasive, and a variety of alkalines, cannot be released into the environment. This necessitates the need for a reliably leak40 | December 2021
Air-operated double-diaphragm pumps work well in filter-press applications, because of their simple design and method of operation.
free pumping technology that is compati- separated into the sticky, syrupy soluble with a wide range of liquid types. tion we all recognize, with a water-like byproduct that is removed. To get the CREATING A LESS-STICKY SITUATION production process started, pumps are Everyone has at least seen a depiction used to transfer the raw sap into the filof the manufacturing process for maple ter press at the beginning of the producsyrup. A series of buckets connected by tion process. plastic tubes are hung from maple trees, which are tapped, allowing their sap to SURVIVING THE DEEP FREEZE flow into the buckets. There are some industrial producWhen enough is collected, the sap is tion processes that require the finished boiled again and again, until it reaches products to be manufactured with the the point that it can be called pan- aid of components that need to be sepacake-ready maple syrup. This is a time- rated in extremely cold operating condiand labour-intensive process, but one tions. In fact, in some cases this requires that was performed in that manner for the use of a liquid like ethanol that has hundreds of years. been cooled to a temperature that can That is until recently. It has only been be as low as -40ºC. It is at the beginning in the last two or three decades that maple of these types of processes that pumps syrup production has become fully mech- are used to push a mixture of the raw anized and modernized, with many pro- material and the super-cooled ethanol ducers now utilizing a “reverse osmosis” through the filter press. production process that reduces the time Because many of these types of proand cost to manufacture maple syrup. cesses are also used in the production Enter the filter press, or as syrup manu- of goods made for human consumption, facturers have begun calling it, the “syrup they must be produced under the strict press”. Rather than repeatedly boiling the auspices of hygienic/sanitary manufacsap to create syrup, today’s process sees turing regulations. the sap passed through the filter press in Additionally, since ethanol is claswhat is called the evaporation stage. sified as an explosive substance, the As evaporation occurs, the sap is pumps used in the production of goods Environmental Science & Engineering Magazine
