Treatment of Ammonium-Rich Digestate from Methane Fermentation Using Aerobic Granular Sludge

Abstract

Digestate produced by cofermentation of agricultural waste and manure can be difficult to dispose of because its high ammonium content impedes its use in agriculture due to generation of odor and overfertilization. This study investigated the possibility of treating such nitrogen-rich digestate with aerobic granular sludge depending on the nitrogen load in the reactor. At nitrogen loading rate of 1.0 g TN/(L·day), the nitrogen removal efficiency was high (64.9 ± 9.8%), ammonium nitrogen was completely oxidized, and nitrate was the main nitrification product. At nitrogen loading rate of 3.4 g TN/(L·day), ammonium oxidization was still good (93.6 ± 2.0%), but the percentage of partial nitrification was high (over 68%) and nitrogen removal efficiency worsened to 30.2 ± 2.6%. Despite this, the overall amount of nitrogen removed was 0.86 g TN/(L·day) and was over nearly two times higher than at the lower nitrogen loading rate. At both nitrogen loading rates, in the effluent nitrogen in a form of suspended solids predominated. To diminish the overall N loading in the effluent, treatment is therefore recommended enabling removal of solids, e.g., microfiltration, should be applied, or the digestate should be separated into solid and liquid phases, and only the liquid fraction should be subjected to biological treatment. At high N load in aerobic granules, a very versatile community of N-metabolizing microorganisms was present. More than 50% of all bacteria in aerobic granules were able to metabolize nitrogen, and the predominant genera (35%) was Thauera, which indicated that stable ammonium removal was achieved mostly as a result of heterotrophic nitrification.

Treatment of Liquid Phase of Digestate from Agricultural Biogas Plant in a System with Aerobic Granules and Ultrafiltration

“…In the granule samples, bacteria of the first stage of autotrophic nitrification were not identified. Oxidation of ammonium nitrogen was most probably carried out by organisms belonging to Thauera sp., which are not only able to carry out heterotrophic nitrification under conditions of high biomass loads with organic and nitrogen compounds [9], but are also the dominant taxa in systems where either autotrophic or heterotrophic denitrification occurs [36]. The main advantage of heterotrophic nitrification is that nitrification and denitrification can take place simultaneously, so a smaller buffering capacity is necessary.…”

“…Arcanobacterium sp. are facultative anaerobes that degrade sugars (e.g., glucose and lactose) [41] and were found to be an important component of digestate from an agricultural biogas plant [9].…”

“…or Paracoccus sp. may play an important role in ammonium removal [9]. According to Cydzik-Kwiatkowska [10], heterotrophic nitrifiers predominated in granules with increasing organic loads of the reactor.…”

“…The initial study on the treatment of digestate from agricultural biogas plant has shown that treating unseparated digestate at nitrogen loading rates of 3.4 g TN/(L·d) and 1.0 g TN/(L·d), despite high efficiency of ammonium removal, is not very effective due to the high content of suspended solids in the digestate which hinder the purification process and result in a high pollutant load in the effluent [9]. Thus, the aim of this study was to remove ammonium nitrogen from only the separated liquid phase of digestate with high nitrogen and organics loads in a granular sludge batch reactor (GSBR) and to polish biologically purified effluent in an ultrafiltration unit (UF) to remove suspended solids and significantly decrease the pollutant load in the effluent.…”

Aerobic granular sludge formation in a sequencing batch reactor treating agro-industrial digestate

“…and the long sludge age in granular sludge systems should ensure the growth of ammonium oxidizing bacteria [18]. Aerobic granular sludge has been recently applied to the treatment of diluted digestate produced by co-fermentation of corn-silage and liquid manure (3:7, v/v) [18]: the digestate was previously diluted with rainwater, and aerobic granular sludge was able to achieve an ammonium removal efficiency of 98.5 ± 1.2 and 93.6 ± 2.0% at a nitrogen loading rate of 1.0 and 3.4 g TN L −1 d −1 , respectively.…”