Thermal hydrolysis has been successfully used for over 20 years as a pre-treatment to anaerobic digestion. It has allowed digesters to be operated at typically double their loading rates, thus reducing the size of new build facilities, or doubling capacity of existing ones, and fundamentally improved dewatering. This improved dewatering has resulted in significantly lower biosolids recycling costs and, if used, smaller energy requirements for downstream dryers and incinerators.
However, looking through the literature reveals that the initial aim of thermal hydrolysis was not to improve the performance of anaerobic digestion, but rather to improve dewatering. Attempts during the 1950s to the mid 1970s found that when raw undigested sludge was exposed to conditions of thermal hydrolysis, the resultant material dewatered routinely above 50% DS, and depending on sludge type, as high as 60% DS. Subsequent work has shown that, by reintroducing materials known to influence dewatering, downstream anaerobic digestion deteriorates the dewatering potential of the biosolids.
Although use of thermal hydrolysis with digestion improves dewatering by approximately 10% points compared with a case with no pre-treatment, the dewaterability would have been higher without the digestion process at all.
Considering this, Cambi have developed a process known as SolidStream®, whereby the thermal hydrolysis unit is installed downstream of the digestion plant, immediately prior to dewatering. In this instance the digested sludge is dewatered hot, and the centrate, now high in biodegradable COD, is recycled to the digester inlet and digested. This paper describes full-scale operating data from Amperverband in Germany, where the technology is installed. Although the results from third party analysis exceed even those of thermal hydrolysis, there is a current challenge with demonstrating that the technology is aligned with the US EPA’s interpretation of Class A under the 503 regulations.
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