Acidification of In-Storage-Psychrophilic-Anaerobic-Digestion (ISPAD) process to reduce ammonia volatilization: Model development and validation

• Mild acidification of anaerobic systems reduces ammonia volatilization of digestate.
• Optimized organic loading can acidify highly buffered anaerobic digestion systems.
• Digestate acidification to a pH 6.0 was achieved with 13 kg glucose/m3.
• The difference between experimental data and model prediction should be explored.

In-Storage-Psychrophilic-Anaerobic-Digestion (ISPAD) is an ambient temperature treatment system for wastewaters stored for over 100 days under temperate climates, which produces a nitrogen rich digestate susceptible to ammonia (NH3) volatilization. Present acidification techniques reducing NH3 volatilization are not only expensive and with secondary environmental effects, but do not apply to ISPAD relying on batch-to-batch inoculation. The objectives of this study were to identify and validate sequential organic loading (OL) strategies producing imbalances in acidogen and methanogen growth, acidifying ISPAD content one week before emptying to a pH of 6, while also preserving the inoculation potential. This acidification process is challenging as wastewaters often offer a high buffering capacity and ISPAD operational practices foster low microbial populations. A model simulating the ISPAD pH regime was used to optimize 3 different sequential OLs to decrease the ISPAD pH to 6.0. All 3 strategies were compared in terms of biogas production, volatile fatty acid (VFA) concentration, microbial activity, glucose consumption, and pH decrease. Laboratory validation of the model outputs confirmed that a sequential OL of 13 kg glucose/m3 of ISPAD content over 4 days could indeed reduce the pH to 6.0. Such OL competes feasibly with present acidification techniques. Nevertheless, more research is required to explain the 3-day lag between the model results and the experimental data.