The Thermal Hydrolysis Process (THP) as a pre-treatment for anaerobic digestion (AD) was first explained at the European Biosolids conference in 1997. Since then THP has become a standard process in UK and Ireland with over 35% of all sludge being treated this way. These projects were in the 300,000 to 3 million PE range. The challenge was to make economical systems for smaller digestion projects without losing system efficiency or reliability.
In designing smaller systems for the 100,000 to 300,000 p.e. range, the following factors were considered as essential to retain system efficiency: maintaining batch action in the continuous process; using steam injection not heat exchangers; maintaining heat recovery using a pre-heat tank for energy efficiency; optimum dry solids feed; ability for flash steam explosion; smooth steam profile for waste heat use; maintain high availability.
In designing smaller systems the following factors were considered important in keeping installed cost down: standardising design for assembly line production; fitting the systems in a standard ISO container footprint; rapid installation for minimum project and site costs.
The new systems are based on 2m3 reactors and the operating method is identical to larger systems but at a greatly reduced cycle time. The batch can still be 20- 30 minutes and has the benefit of being self cleaning compared to continuous systems at the same high dry solids.
The new B2 x 4 system was installed and commissioned by United Utilities at their Leigh and Burnley sites in 2016. This paper focuses on the performance of the Leigh plant and the lessons learnt over 3 years of operation.
THP and AD performance. Lab data at the end of 2016 showed lower than expected performance in the digesters with 54% VSR and low gas production- but inspection of the lab data showed an abnormally low specific COD of the sludge at 1.45 (compared to 1.75 as the norm). Since that time, the VSR has risen and AD results are as per normal for THP. Dewatering has been good with an enhanced treated status and a crumbly cake above 30% DS.
The First Amtreat process on THP centrate. Amtreat is a high rate activated sludge process for denitrification and was set up with both carbon source dosing and alkalinity adjustment. However the Carbon from the pre-dewatering plus the free VFA from the final dewatering are sufficient C source to avoid any addition, The very high alkalinity of the centrate (about 10,000mg/l) from post dewatering is sufficient to provide the chemical buffer needed for the process. So, the process has the advantage of needing no supplementary additions without the need for special annamox bacteria. It has proven to be simple and robust.
The THP equipment. Installation proved to be simple as a plug and play system – the difficulties were more on the ancillary side where the CHP and boiler system were a long time in the permitting process. The system has proven to do what is said on the can and has achieved the 96 cycles per day expected (24 tds per day). Annual inspections have shown that the system runs very cleanly with little sludge bake-on and no concerns with higher DS% feed. This compares well with other one pass systems that need daily or continuous cleaning. Operators objected to the prefabricated kiosk arrangement as it was considered too restrictive for access.
Lessons learnt applied mark 2 systems. The desire to get all the equipment into one ISO container led to some compromises on capacity and redundancy. Improvements on the next systems being installed are:
Redundant pumps on reactor feed and digester feed; MCC and compressor moved to a small separate skid; GRP enclosure not used – system supplied as free standing system that can go in a building if needed; improved capacity from 143m3/day to 170m3/day and a capacity of 28 tds per day.
This improved system has been installed at Franklin, Ohio WWTW and will be operational in 2020/21.
Anaerobic Digestion, Dewatering, Thermal Hydrolysis, CHP, steam explosion
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