The effect of pH control on ammonium release in anaerobic digestion

Abstract

Anaerobic digestion is a process whereby microorganisms break down waste material into simpler compounds, while simultaneously producing biogas that is generally high in methane and carbon dioxide content (60 –70 % and 30 –40 %, respectively) and a nutrient-rich by-product called the digestate. The energy potential of this process cannot be overstated; it is estimated that a feed of one ton of biowaste generally translates to an electricity yield of 250 kWh. However, the digestate is a typically overlooked by-product in anaerobic digestion studies due to a fixation on the energy-dense biogas product. The digestate can be in either liquid or solid form, this study focused on the liquid digestate. The liquid digestate is generally high in valuable nutrients like nitrogen, potassium, and phosphorus which are essential for plant growth. This indicates that the liquid digestate can be an effective fertiliser. Currently, farmers habitually apply digestate on farmlands because of its high macronutrient concentration. However, an underexplored avenue is the use of liquid digestate as an organic fertiliser for soilless agriculture. Although soilless agriculture provides a multitude of advantages over conventional agriculture, such as providing better nutrient distribution while also saving on both land and water, it is still dependant on harmful mineral fertilisers. Given the major growth occurring in the soilless agriculture sector and the need for more sustainable fertilisation strategies in food production processes, liquid digestate has been promoted to a more prominent stream in the circular production of human nutrition. In this regard, it is important to understand the rate and extent of fertiliser production in anaerobic digestion process.

In this study, the pH of the anaerobic digestion process was controlled at three different set points (6, 7, and 8) for three different substrates (banana peels, cow dung, and red lentils) in order to determine the ammonium release characteristics at each set point. This was achieved by using two different set-ups; One system employed a daily pH adjustment (Daily dosing system or DDS) while the other system (Continuous Dosing Set-up or CDS) employed pH corrections every minute. The results indicated that a pH of 7 is the optimal set point for both ammonium release as well as the gas production rate. This pH value provided average percentage differences of 20 % and 22 % in terms of ammonium release and gas production when compared to the runs that were performed without pH control. In terms of a comparative analysis between precise pH control, that was performed every minute and pH control that was performed once a day, there were differences present i in the gas production profiles, with the CDS providing enhanced rates compared to the DDS. The CDS provided an average percentage increase of 50 % compared to the DDS in terms of gas production. However, there was a negligible difference in the ammonium release rate.