Mechanical Dehydration up to the Limit

Krauss, W., Bucher Unipektin AG, Switzerland



Bacteria from a young SAS which only removed carbon from contaminated, but well aerated water had not been under stress and could be dehydrated mechanically up to app. DS ~ 30 %. Due to the fact that 70 % of the cytoplasm is water, the tightest possible sphere packing of bacteria is limited to their water content within the cell. Stressed Gram positive bacteria could form a extracellulare peptidoglycane layer which contains 10 times more mass than the bacteria itself. Several processes for mechanical or thermal treatment try to crack the molecular (polymer) structure of the bacteria. This releases ribosomes, plasmids etc. to supply other anaerobic bacteria for digestion and metabolism.

The enhanced treatment reduces the organic content of i.e. municipal sludge and increases the minerals content. Lime, chalkstone, silicates, gypsum, clay etc could be dehydrated mechanically up to DS ~ 70 %. As more as the organics in the sludge are reduced as better it could be dewatered mechanically and approaches the mechanical limite of the equipment.

The Kemicond process for sludge conditioning i.e. consists of chemical treatment with hydrogen peroxide at a pH ~ 4.5, followed by a dewatering unit. It is shown that chemical treatment can improve the dewaterability of digested sludge. It is assumed that the slight oxidation potential of peroxide could cleave the polymeric structure of the peptidoglycans around bacteria cell walls to polysaccharides and proteins. In this case dewaterability increases and it is possible to achieve an increase in DS content of filter cake which affects a reduction of the sludge volume. Cost savings for sludge disposal can justify and amortize the additional investment and operational costs for chemical treatment.

Keywords: Dewatering, Extracellulare Polymeric Substances [EPS], Sludge Treatment


Limitation of dewatering

The limit could be caused by process technology, biology, chemistry or any combination of these effects.
Sludge as a solid residue of a modern biological treatment is an individual creation of individual feed, individual composition and the microbiology and biocenosis widely is influenced by i.e. aeration, denitrification, bio-P, autolysis, (an)aerobic digestion and finally rotting.
Beside this (bio-)physical interactions the addition of chemicals like aluminium salts as a remedy against filamentous bacteria could influence the well-being of bacteria during their lifetime. Gram positive bacteria respond to stress by creating a gel layer around them to protect their cell wall or to increase their surface to capture nutritive molecules.
This layer is known since 1885, when Mr. Gram added a colorant to different bacteria cultures. It is assumed that the EPS consists of a peptidoglycan structure with cross linked polysaccharides and proteins like a jellyfish. This structure could retain enormous amounts of water – a jellyfish i.e. consists of only 2-4 % of solids which fixes 96-98 % of water.

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