Decarbonisation Technology - August 2024 Issue

promoted within the blended sludge prior to AD. Reactor vessels 1, 2, and 3 are operated as serial mesophilic reactors, while vessels four to six function as a serial-batch thermophilic stage. Both stages maintain a hydraulic retention time (HRT) of 1.5 days. The thermophilic stage includes a minimum five-hour batch hold time to meet US EPA standards for pathogen destruction as required by 40 CFR Part 503. In the mesophilic stage, a sludge-to-water heat exchanger is used to maintain an average temperature of 108°F across the first three reactor vessels to optimise VFA formation in the sludge. In the ensuing stage, a recirculated heat exchanger transfers heat to the sludge that has entered reactor vessel 4. The sludge recirculates through a second sludge-to-water heat exchanger in order to uniformly raise the temperature to thermophilic conditions (142°F) without hot and cold spots. Reactor vessel 4 acts as a staging tank for reactor vessels 5 and 6, which function as batch tanks. Maintaining two batch tanks is crucial to ensure continuous feeding of the anaerobic digesters. Tank mixing is supplied by an integrated gas mixing system cycling across all six tanks, while a single gas compressor provides periodic mixing to each tank sequentially. Liquid across the first four tanks is transferred by Veolia gas lift technology – the most energy- efficient mechanism to transfer liquids for this application. The gas compressor replaces the need for pumps and motors, which are ordinarily used for this function, minimising maintenance requirements. The design sequentially allows for both top and bottom partial emptying of the EBH tanks to prevent the build-up of hydrolysed scum, mitigating foam formation. As a result, the system is highly efficient and consumes less power. The EBH system is designed to be easily integrated and adaptable to a wide range of feed sludge concentration. Specifically, the technology increases the volatile solids reduction (VSR%) of sewage sludge compared with conventional mesophilic anaerobic digestion. This advantage can be harnessed to maximise biogas generation or reduce the size of the downstream digesters, or a combination of both. For the same digester retention time, a

system that integrates EBH can increase VSR% by more than 30%. The real benefit, however, is to achieve a higher VSR% in combination with a lower digester retention time. As conditioned sludge is transferred from the EBH system to the ADs, it is cooled to approximately 104°F by passing it through a cooling heat exchanger, which can be adjusted for maintaining mesophilic temperatures in the digesters during the summer and winter months. The AD system, which includes two 480,000-gallon biosolids digesters, operates within the mesophilic temperature range of 35- 40⁰C (95-104⁰F) where micro-bacteria drive the fermentation process by digesting the organic material, producing biogas. The digester contents are constantly mixed by recirculating a portion of the generated biogas. To facilitate thick slurry/sludge digestion, the digesters will utilise Veolia’s Sequential Gas Mixing system for high active volume (>90%). Sequential gas mixing is the most effective technology for completely mixing digesters and does not require any internal mechanical equipment that would require maintenance or be prone to mechanical failure. Harnessing the value of organic waste To maximise the use of the facilities, the UAJA project will also incorporate food waste processing. Two additional digesters – co-located adjacent to the sludge treatment flowsheet – will be used to anaerobically digest imported food waste slurry. Treating this additional organic material will create supplementary revenue streams, both by generating more biogas and through tipping fees, further enhancing the project’s economic viability. The UAJA will operate two separate treatment processes, allowing for the independent measurement of biogas production from each. This setup enables the accurate claiming of RIN feedstock credits for each type of biogas when it is upgraded to RNG. Ultimately, UAJA’s food waste processing initiative represents another approach to divert organics from landfills, thus reducing GHG released to the atmosphere.

Dave Swerdlyk dave.swerdlyk@veolia.com