WAS-Only Thermal Hydrolysis Projects Reach Maturity Worldwide - Having Your Cake and Heating It - For Free

Panter, K., Zikakis, D., Barber, W.P.F., Christy, P., Chauzy, J., Fountain, P., Shana, A.

Proceedings of 2018 WETEFC

Abstract

Seven of the seventy or so thermal hydrolysis process- THP- projects operating around the world use the principal of partial -or “WAS Only” thermal hydrolysis. These are at sites where class A sludge is not required and there is a reasonable existing digester volume. It is well known that Waste Activated Sludge – WAS - is extremely difficult to digester and dewater, compared to primary sludge. By hydrolyzing this fraction of the digester feed and blending it with the bypassed primary sludge the following advantages ensue:
• Virtually the same biogas production as Full THP
• Improved dewatering similar to full THP
• Less or no cooling required as hot WAS mixed with primary approximates to digester temperature
• Smaller THP system – lower capital
• Lower steam demand – easy to get free from scavenged heat from cogeneration
• 50% intensification of digestion loading where mixed sludge is fed at 7% DS but at low viscosity
The following disbenefits ensue:
• More complicated feed regime to digesters
• Not Class A
This paper gives an overview of the projects and focuses on the results from two projects where good before and after data exists – Psyttalia (Athens Greece) and Long Reach (London UK) In both cases the full-scale results were the same or similar than the lab scale predictions. Dewatering was improved by about 8-10% points and biogas increased by about 15-40% compared to conventional digestion. Mass and energy balances at both plants were greatly improved. At Psyttalia there is sufficient biogas to operate the dryers and also some CHP. At Long Reach the effective CHP generation has risen from 0.75 MWh/tDS up to 1.1 MWh/tds.

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