Improving the efficiency of wastewater sludge pretreatment by high temperature hydrolysis before digestion

Summary

The results of investigating activated sludge and primary sludge thermal pretreatment with the purpose of improving the efficiency of anaerobic digestion are presented. The highest efficiency of activated sludge organics transfer into readily soluble form is ensured after thermal hydrolysis at 160°C. The solubility of primary sludge organic matter increases evenly with the temperature increase in the range of 100–200 °С. The maximum biogas output in the ultimate digestion test is increasing after activated sludge hydrolysis at 160°С, primary sludge hydrolysis – at 180–200°С. Digestion in a continuous mode after activated sludge pretreatment at 140–160 °С results in 53% gas generation increase. Any further hydrolysis temperature increase produces a negative impact on gas generation and disintegration of activated sludge ashless matter. Thermal hydrolysis of primary sludge in thermophilic mode is not efficient. Pretreated sludge dewatering performance is improved; consequently the flocculant dosage can be reduced by 0.8–1.0 kg/t DS.

Key words

energy efficiency , sludge , fermentation , biogas , dewatering performance , thermal hydrolysis

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