2nd Generation THP -- Intermediate THP at a Large WWTW

EXECUTIVE SUMMARY (100 words maximum) Thermal Hydrolysis process (THP) is a well stablished technology with more than 25 years of experience. Different configurations of the biosolids line have been trialled to maximise the efficiency of the mass and energy balance. One successful configuration is the Intermediate THP (ITHP) arrangement, with THP in between two anaerobic digestion phases. A full ITHP biosolids line was built and is now in operation giving the expected preliminary results as those found at lab scale (Rus et al., 2017): an average 510Nm3/tDS biogas yield, 60-65%VSR, and an increase in dewatering of 7 percentual points so far (from 18%DS to 25%DS). INTRODUCTION Thermal hydrolysis process (THP) has proven to be an efficient pre-treatment for sludge before anaerobic digestion (AD), by thermally enhancing hydrolysis, the rate limiting step in AD (Shana et al., 2013). Early research (Shana et al., 2012, Mills et al., 2014) showed that a new configuration of THP and AD technology can further enhance the efficiency of THP in the biosolids line. The positive results found at lab scale led to the construction of the ITHP pilot plant at Basingstoke STW, UK, to evaluate its viability at a more realistic scale in Rus et al., (2017). The success at the large pilot plant took this process one step further, with several full-scale options being considered by the water industry. In 2020, one of the municipal wastewater treatment plant decided ITHP was the best option for their site and the concept has now been implemented there. This paper covers the initial phases of commissioning and preliminary results obtained YEAR CASE STUDY WAS IMPLEMENTED 2021 to 2024 CASE STUDY DETAIL Specific Issue: A municipality in Europe needed to develop a new biosolids strategy for their wastewater treatment plant. The sludge design throughput was to be 11,729 tonnes of dry solids per year (tDS/y), with an average of 23tDS/d going into the site at the time (2021). The plant has two 8,000 m3 digesters and one existing 7,500 m3 digester. The municipality was interested in maximizing biogas generation for electrical energy production and heat and improving dewaterability of the final digestate to minimise final cake haulage costs to incineration. Approach: In 2021, the municipality started their process of construction of an ITHP facility which included design, execution, completion, pre-commissioning, remedying defects, and staff training (Figure 1). They already were running the first two digesters in parallel and the 3rd one in series therefore they did not need to rearrange the biosolids flow line (figure 2). The required additional parts were a dewatering stage between 1st stage digestion and the THP to bring digestate from digesters 1 and 2 from 3% to 16%DS. The THP unit (1 stream of CambiTHP B2), and a boiler for steam generation. Methods or summary of activities: The company had historical data from 2021 where site performance was collected prior to the installation of the Cambi THP system in the spring of 2023. This data gathered information on flows and loads into the site and 1st stage AD (2 digesters) and 2nd stage AD, final dewatering dry solids, biogas generation and CHP electrical output was collected. Biogas yields and volatile solids reduction (VSR) was calculated. In June 2023, the THP plant was commissioned, and a guarantee period started later in 2024. The same data was collected for this period. Summary of outcomes and measurable impacts: Figures 3 and 4 show the overall VSR percentage and gas yields in terms of m3 biogas per Volatile solids in the feed (m3/tVS). The green line shows the point in time where the THP started to operate. Steady state was reached in March 2024. The hard blue lines are the average for the periods delimitated by the green line (introduction of the THP). The light blue shadow is the standard deviation (SD) for that same period. During the ramp up of the ITHP plant, the 2nd stage digester reacted very well, with a steady increase in performance both in terms of VSR and associated biogas yields. The transition from a conventional AD to an enhanced AD (i.e. preceded by THP) was smooth. Once steady state was reached, the guarantee period started. We can observe the average VSR during this period was 60% and gas yields stabilised around 550m3/tVS fed. This was what Cambi expected to obtain given the initial conditions of 40%VSR and 400m3/tVS fed. The average throughput during guarantee period was 22.8tDS/d equivalent to 8322 tDS/year, successfully maintaining a similar throughput to the previous 2 years (figure 5). On final dewatering, Figure 6 shows preliminary results, with most recent data showing 25%DS from 17.7%DS average prior to ITHP. Since the plant requires several retention times for the process to stabilise, we expect dewatering to reach steady state in the coming months. Important lessons learnt and critical success factors The key factors in delivering this project were to ensure the THP is embedded correctly into the Biosolids line, with a good control on pre-THP dewatering. Ensure there is good knowledge on the expected increase in the rate of biogas release in the 2nd stage AD (after the THP). Including in the design a robust mixing system which allows for a quick extraction of biogas from the digestate column. During installation and commissioning of the THP, good communication with the plant operations team was essential, with support during ramp up when needed and weekly data output observation. Reaching steady state was achieved successfully as planned thanks to a detailed, non-aggressive plan for ramp up was in place. Training and engagement of operations from the earliest stages of project proposal through to delivery is essential to ensure they feel ownership of the new technology at their plant. The extent to which the outcomes are sustainable Overall, the preliminary results obtained from commissioning of the ITHP have been very encouraging. Table 1 summarises preliminary outcomes. Savings of haulage and incineration together with the increase in biogas generation make the CapEx investment cost efficient. Nevertheless, these are initial results. Data will continue to be gathered and presented to give an update of steady state ITHP performances at the plant.