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Municipal Wastewater Treatment Plants

Dewatering Treatment Process:

21 Alfa Laval Centrifuges Operating in New Houston WWTP A total of 21 Sharples® PM-75000 centrifuges are now dewatering solids in Houston's new 69th Street WWTP — at a projected yearly savings of up to $500,000 over two competitive machines tested in full-scale, side-by-side trials. The centrifuges are dewatering aerobically digested sludge to the level needed (18 percent or more solids) for cost-effective production of a salable soil conditioner (Hou-Actinite) in gas-fired dryers. Before making its choice, Houston tested three different manufacturers’ models designed for sludge dewatering. These were the 29-in. diameter Sharples® PM-75000, a 36-in. low-speed centrifuge (KHD), and a 30-in. high-speed centrifuge (Bird). A series of three consecutive side-by-side trials were run. Sharples was declared the winner — on the basis of lowest projected annual cost — in each trial. Though cost for specified performance was the ultimate basis of evaluation, a closer look at the relative operation of the three machines in these trials points up significant distinctions, says Alfa Laval. The Sharples centrifuge was the only unit that operated in all 22 of the available trial runs. And of the trial runs in which all machines participated, the Sharples PM-75000 was within the City of Houston’s specification 86 percent of the time. By contrast, says Alfa Laval, the Bird centrifuge was within spec only 55 percent of the time and the KHD centrifuge met the requirements only 27 percent of the time. The specifications called for a minimum of 16 percent cake with no more than 15 lb of polymer per dry ton of solids and a minimum recovery of 92 percent at a flow rate of 120 gpm. The PM-75000 did not experience any mechanical problems during the test program. The other two centrifuges did, including the one unit that required a new gear box at the site. Alfa Laval maintains that because of its superior high G-force design, the Sharples unit averaged 0.5 percent to 1 percent drier cake than the competitive high-speed centrifuge, and produced 2 percent to 3 percent drier solids than the low-speed unit — with equivalent polymer dosage. This advantage was maintained while also delivering high recovery rates. The tests conclusively proved that the high G-force Sharples centrifuge out-performed both the competitive high-speed unit as well as the low-speed centrifuge. Of the two high-speed centrifuges tested, the Sharples PM-75000 consumed 20 percent less power than the competitive unit when running at similar G-force (Sharples at 2500 Gs; Bird at 2452 Gs). This was no doubt a reflection of the streamlined construction of the PM-75000 and the advantage of its power regeneration system over the competitive hydraulic backdrive.