The key results of the full-scale studies of sludge digestion technology with biomass recycle

Summary

For a number of years the specialists of R&D Centre of «Mosvodokanal» JSC have been developing and testing in a lab environment the technologies that can enhance the process of wastewater sludge digestion within the existing capacity of the treatment facilities. As a result a method was chosen which provided for thickening a part of sludge with subsequent thickened sludge returning back into the digester. The important advantage of this method is the possibility of increasing sludge retention time as dry matter without increasing the hydraulic retention time with simultaneous biomass retaining. Therefore, the required decomposition rate is provided in smaller size reactors. In the course of studying the process solutions on wastewater sludge digestion optimization the method of biomass recycle showed itself as the most economically viable. During the full-scale studies of the recycle option the decomposition of the ash-free basis increased in average to 56.6% (42.2% reference value) which was confirmed by the significant improvement of sludge dewatering properties. The average biogas yield increased by 3%. In the course of the experimental studies no adverse effects of the sludge recycling technology on the digester performance were observed. The results obtained during the full-scale studies evidence the long-term benefits of introducing the technology of wastewater sludge digestion with biomass recycle at the Moscow wastewater treatment facilities.

Key words

anaerobic fermentation , methane tank , decomposition of organic matter , biomass , sludge thickening , recycle

The efficiency of enhancing wastewater sludge digestion

Summary

The results of experimental studies of enhancing wastewater sludge digestion as applied to the process flow scheme of the Kur’ianovskie Wastewater Treatment Facilities are presented. The specialists of the R&D Centre of «Mosvodokanal» JSC tested in the laboratory and calculated the economic efficiency of three options of digestion enhancement: sludge processing in the process of thermal hydrolysis before digestion; increasing the hydraulic retention time; increasing the solids retention time. The studies were aiming at comparing the efficiency of sludge digestion when using the given technology solutions. Increasing the hydraulic retention time (HRT) is caused by sludge thickening enhancement before digesters. The experience of the Prague wastewater treatment facilities shows that thickening enhancement alone provides for efficiency improving by a few percent. This was confirmed in the process of laboratory experiments in the R&D Centre. Sludge pretreatment before digestion with the purpose of accelerating the hydrolysis of stable organic matter, e. g. thermal hydrolysis, ensures significant enhancement of the entire process by activating the slowest stage of hydrolysis; as a result, the decomposition of ash-free matter and biogas output were increased by 10–30%. However, this type of technology involves high capital expenditures. Increasing the solids retention time (SRT) is achieved by retaining solids in the digester while maintaining the hydraulic retention time (HRT). This can be ensured by retaining or recycling solids to the digester. The technology of sludge recycling serves as an example that provides for the high organic matter decomposition by means of increasing the solids retention time without any significant capital investments. Thus, following the results of the studies the given technology has been accepted as a most cost-effective.

Key words

thermal hydrolysis , wastewater sludges , sludge recirculation , digestion enhancement , decomposition of organic matter