What anaerobic sludge should be used for methane fermentation?
Issues
Products
In the technology of anaerobic treatment of industrial wastewater by methane fermentation, two types of anaerobic sludge are used: classic and granulated.
Classic anaerobic sludge is digested sludge with a fragmented structure, collected and selected in the test, from a very well working sludge digester at a municipal sewage treatment plant. It has all the features of seed sludge, characterized by high biological activity in the biodegradation of complex organic compounds contained in industrial wastewater.
The second type of anaerobic sludge used in methane fermentation reactors is granular sludge, the production technology of which was developed by the Dutch, who used it in fermenters operating in UASB technology. This sludge can be grown in any fermentation chamber provided that appropriate technological conditions are maintained. As the name suggests, the structure of this sediment is granules similar to crystals of, for example, sugar.
What are the qualitative differences between these two types of sludge. Well, granulated sludge contains a larger share of organic parts, reaching up to 70% in dry matter, while classic sludge is about 50% – 55% of the share of organic parts. The advantage in the concentration of the organic part causes its higher biodegradation activity in relation to the classic sludge, i.e. it is possible to use a lower concentration of granulated sludge in the fermenter in relation to the classic sludge.
When should granular sludge be used and when should classical sludge be used?
Granulated sludge can be used in various types of digesters, both in UASB and in high-load type IC. In principle, the application criterion refers to the characteristics of the wastewater that we are to treat with this sludge, i.e. the presence of fats, suspensions or toxic substances in them. In principle, the greatest effects are obtained when the three factors mentioned above are practically absent in the wastewater at all or in minimal concentration. At higher concentrations of both fats and suspensions, the efficiency of purification decreases. In addition, using this type of sludge, it was assumed that there would be a very low concentration of digested sludge suspensions in the digested sewage, which turned out to be partially true, because the newly formed fermentation sludge is partially bound in the granule, and the remaining as a fine suspension flows out with the digested sewage. If wastewater treated in this way is directed to an aerobic reactor, it is necessary to use a settling tank to separate anaerobic sludge from digested wastewater. To sum up, granulated sludge is ideal for even highly concentrated industrial wastewater, but basically clear, free of suspensions and fats, if such are present in the wastewater.
And when do we use classic sludge? This sludge collected as seeding sludge from the municipal wastewater treatment plant as digested sludge must be tested prior to its collection in order to examine its degree of fermentation and the ability to reduce organic compounds contained in wastewater, determined by the COD indicator. This sludge is universal and has no limitations in terms of application for different characteristics of treated wastewater. It perfectly reduces suspensions and fats and tolerates the presence of toxic substances much better. It can be used in all types of fermenters. A prerequisite for proper operation and high reduction of COD in treated wastewater is its proper adaptation at the technological start-up stage. Of course, in this case, it is necessary to use a settling tank after the anaerobic reactor to separate anaerobic sludge suspensions from digested sewage.