Water & Wastewater Asia Nov/Dec 2018

Water & Wastewater Asia • November / December 2018 OPINION | 45 What to consider when selecting a filter-press pump High-pressure Air-Operated Double-Diaphragm (AODD) pumps from Almatec ® are able to deliver operational advantages necessary for optimised operation of chamber filter presses. By Peter Schüten, Almatec Product Manager I n many industrial processes that feature the handling of liquids, the substances that are being transferred may occur in an aqueous or “muddy” state, with the liquid portion needing to be removed. This liquid-removal stage may take place during the production process — for example, in the filtration of edible oils or yeast solutions — but it more frequently occurs at the end of a process like wastewater treatment when the waste sludge that has formed must be withdrawn. Since the disposal of such sludges is calculated based on weight and volume, their thickening and drying is particularly lucrative for the processor. During the thickening and drying process, the sludges are treated using chemicals and/or physical processes so that the waste sludge forms into flake-like solids. After adjusting the pH level to neutral or alkaline via the use of milk of lime, the subsequent dewatering process separates the water for disposal, with the remaining volume of sludge significantly reduced. The simplest technology for this process involves collection and thickening of the sludge through the use of gravity. However, it is significantly more effective to use technical drying methods via processes such as centrifuges and evaporators, or more commonly, chamber filter presses (Figure 1). The operational principle of the chamber filter press relies on the use of a number of plastic frames that are pressed together under high pressure. Inside the frames are hollow chambers — from which the press gets its name — that are surrounded by filter cloth. When the sludge is fed into the chambers using pressure, a “filter cake” forms inside the chambers and the filtrate flows through the filter cloths into drainage channels. When all the chambers are completely filled, the sludge feed stops. The press is then opened and the solid filter cakes can be removed. After closing, the press is ready for a new pressing process. To fill these presses, filter material and pressure is required. The pressure — usually between 8 and 15 pressure bars (116 to 218 psig) at its peak — should be even in order not to destroy the sludge flocs during feeding. The flocs should also have enough free space in the feed area. In addition to the constantly increasing counter-pressure that occurs until the end of the pressing, a further constraint is the eventuality that an empty-running sludge tank can lead to dry running of the pump being used to generate pressure. To build pressure, different systems of displacement pumps are frequently used, including piston diaphragm pumps, which are appliedmost frequently for large presses. In these large and costly units, one or two diaphragms are hydraulically actuated and feed the sludge into the press through a series of valves. These pumps require large overhead costs even for small plants, such as an air-pressure vessel to equalise the feed rate and a maximum pressure monitor or bypass. Eccentric screw pumps are also used, either as self-regulating pumps (with motors that are electronically controlled via a frequency converter) or as cyclical systems (where an air-pressure vessel is “charged” by the pump). This valve-free procedure is advantageous when processing large sludge quantities and when long fibers prevent the use of valves. However, there are operational constraints that need to be considered in small- and medium-sized plants due to their sensitivity to abrasion and dry running. The space required to use this system is also considerable. The list of filter-press pumps also includes hose-diaphragm piston pumps, which function in a similar way to piston diaphragm pumps but with crimped hoses rather Figure 1: A typical chamber filter press being fed by a high- pressure air-operated double-diaphragm (AODD) pump (Almatec ® AHD Series)