Come across references that ferric, aluminium and PACl, when dosed in a primary tank, cause problems with subsequent dewatering.
But they are dosed as dewatering aids.
Any suggestions / literature reference why they are regarded as interfering with dewatering if dosed upstream but promoting dewatering if dosed at the dewatering stage?
Jeremy one part of the problem can be that when using these coagulants in Primary settling you get a very "Hydrophilic" nature of the sludge flocs and it tend to bind more water. This mean that when you have a traditional primary sludge this can be thickened by Gravity to even 6 - 8% TS were this chemical Sludge's only can achieve 2 - 3 % TS. If on the other hand you try to dewater this sludge Mechanically on a dewatering table or in Drum Thickener though, you get very good dewater ability by just adding Polymer and can easily achieve 8 - 10% TS.
Dear Jeremy,
It is a common misunderstanding with consultants, designers and operators of (waste)water treatment plants that coagulants such as these metal salts are also thickening and dewatering aids. In most cases it is quite the opposite as indicated by Kim Helleshoj Sörensen. Hence aside from the usual jar tests for coagulation and flocculation, separate thickening and dewatering tests shall be performed with a selection of polymers.
According to my experience, chemical or electro-coagulation is not needed to enhance primary settling upstream of (advanced) activated sludge biological treatment. Instead we dose dense regenerated activated sludge with the capacity to adsorb colloids and capture suspended solids from the (equalized and conditioned) raw influent to remove up to 80% of the TSS by primary settling.
Regards,
Bruno
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