Effects of Increasing Nitrogen Content on Process Stability and Reactor Performance in Anaerobic Digestion

Abstract

The aim of this study was to analyse the effect of different nitrogen increase rates in feedstock on the process stability and conversion efficiency in anaerobic digestion (AD). The research was conducted in continuously stirred tank reactors (CSTR), initially filled with two different inocula: inocula #1 with low and #2 with high nitrogen (N) concentrations. Three N feeding regimes were investigated: the “0-increase” feeding regime with a constant N amount in feeding and the regimes “0.25-increase” and “0.5-increase” where the N concentrations in feedstock were raised by 0.25 and 0.5 g·kg−1, respectively, related to fresh matter (FM) every second week. The N concentration inside the reactors increased according to the feeding regimes. The levels of inhibition (Inhibition) in specific methane yields (SMY), related to the conversion efficiency of the substrates, were quantified. At the N concentration in digestate of 10.82 ± 0.52 g·kg−1 FM measured in the reactors with inoculum #2 and “0.5-increase” feeding regime, the level of inhibition was equal to 38.99% ± 14.99%. The results show that high nitrogen increase rates in feeding regime are negatively related to the efficiency of the AD process, even if low volatile fatty acid (VFA) concentrations indicate a stable process.

Microbiome Diversity and Community-Level Change Points within Manure-based small Biogas Plants

“…Despite the benefits of using livestock manure for biogas production and the promotion by the Renewable Energy Act since 2009, still only 26.6% of the produced liquid cattle manure, 4.6% of solid cattle manure, 15.9% of swine manure and 13.4% of chicken/poultry manure were converted by anaerobic digestion in 2017 [ 24 ]. Causes for the currently low exploitation of the manure potential for biogas production lie in a number of specific challenges: Firstly, feedstock characteristics of some types of livestock manure are critical for the process of anaerobic digestion, e.g., the high lignocellulose contents of solid cattle and horse manure or the high nitrogen content of solid chicken/poultry manure [ 25 , 26 , 27 , 28 , 29 ]. Secondly, CH 4 yields mainly depend on the livestock/animal type the manure derived from [ 30 , 31 , 32 ] and are generally lower compared to crops, especially energy crops [ 33 ].…”

Use of Continuous Stirred Tank Reactors for Anaerobic Co-Digestion of Dairy and Meat Industry By-Products for Biogas Production

“…The determination of the critical threshold concentrations of total ammonia nitrogen in anaerobic digestion reactors is quite difficult as it depends on the nature and characteristics of the substrate to be digested [20]. Under high ammonia concentrations, the buffering capacity of the digester is not efficient [21]. Nitrogen content can be calculated using total Kjeldahl nitrogen (TKN), which is the conversion of organic nitrogen to ammonium nitrogen by boiling feed samples with sulphuric acid and a catalyst.…”

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

“…Ammonia acts as an inhibitor for specific enzymatic reactions. It has been shown previously that biogas plants suffer from microbial inhibition and methane losses when using N-rich substrates (Morozova et al, 2020). While concentrations above 200 mg/L are considered to hinder AD significantly, according to Chen et al (2008), lower amounts are beneficial for stabilizing AD processes, by buffering the system (Nsair et al, 2020).…”

“…The performance of AD can also benefit from a microbial adaptation, e.g. through a gradually N-increasing feeding rate, in which the viability and activity of the microbial community is investigated (Morozova et al, 2020). An issue remains for measurements at the typically high concentrations above 50 mg/L, which hinders often on line applications as it is not easily feasible to dilute the broth in an automatic manor.…”