Requirement and distribution of trace elements in mesophilic anaerobic digestion.

Abstract

For AD process control and optimisation, significant attention must be paid to long-term process stability. TE are essential and play a crucial role in the metabolism of anaerobic microorganisms. The TE requirement has been recognised widely, but the challenge about how to optimise TE dosing, which is a multifaceted question involving metal chemistry, physical interactions of metal and solids, microbiology and technology optimisation, still remained. The availability of TE for microbial activities is very important to obtain efficient and stable biogas processes. Therefore, the factors regulating TE bioavailability need to be further understood in order to determine proper TE dosage to be supplied in anaerobic systems. Moreover, an appropriate quantification of the required TE dosing and suitable ranges during long-term operation has been given little attention. Therefore, this research aimed to quantify the required TE dosing ranges under designated operational conditions in mesophilic AD using model substrate. A special attention was given to identify the critical TE concentrations and the dynamic changes of their distribution in different fractionations over the course of TE washing process. TE availability was examined via digester performance and TE fractionation using sequential extraction. Because of the fluctuation of TE concentrations in real-world organic waste, model substrate comprised of whole milk powder, whole egg powder and rice flour on a 20:20:60 ratio (VS basis) was employed. The low nutrient contents indicated the suitability of substrate used for this research. TKN concentration in feed was unlikely to induce inhibitory effect and therefore this would not become an interfering issue when investigating the TE effect on AD. The requirement of TE for long-term AD were determined in eight 5-L CSTRs at OLR of 3.0 kg VS m-3 d -1 , HRT of 33.3 days and VS of 10% in the feed. The digesters were subject to 8 different TE dosing strategies in sufficient amount. Among them, digester with no TE addition and with full set of 11 TEs were acted as control. Co, Ni and Fe were selected as the main TE to investigate due to the long recognition of their role on stable AD, especially when TAN is not high. The results confirmed that single element dosing of Co or Ni was unable to prevent VFA accumulation when baseline TE concentrations were Co 0.01, Ni 0.03, Fe 2.88, Se 0.04 and Mo 0.06 mg kg-1 FM. The digester with both Co and Ni supplementation could not maintain long-term stable performance either, but its VFA accumulation appeared later than the control digester or digesters with single TE dosing. Fe showed an antagonistic effect when supplemented with either Co or Ni and reduced their availability, whilst at the same time it proved to be an essential TE. When supplemented with a mix of Co, Ni and Fe digesters operated well for 400 days but showed VFA accumulation after 12 HRTs. Se was also found to be essential for long-term stable operation of this substrate. Digesters supplemented with Co 1.0, Ni 1.0, Fe 10 and Se 0.1 mg kg-1 FM was confirmed to be sufficient for stable performance. Other TE provided by model substrate was sufficient and there was no evidence that any of them was required in concentration greater than their baseline level. The system operated for 18 HRTs and clearly showed that the TE requirement was uncoupled from the hydraulic characteristics of the digester. This suggested that the chemical species and bioavailability of the TEs is a critical element in their function and is, to a certain degree, independent of washout as the results showed that nutrients are not lost from the system simply as a hydraulic function. The critical concentrations of Co, Ni, Fe and Se to maintain stable digestion at moderate OLR when the rest TE existed in sufficient quantity (Co 1.0, Ni 1.0, Fe 10 and Se 0.1 mg kg-1 FM) were quantified. Four digesters were operated at OLR of 3.0 kg VS m-3 d -1 , HRTs of 33.3 day, supplied with sufficient TE dosing to establish digestion baseline. The general approach of this study was to supplement 3 from 4 TE in sufficient amount in order to identify the minimum requirement for selected TE. The critical TE concentration will induce VFA accumulation, then, step-wise increased of selected TE dosing strength when it was re-introduced to recover process stability. Results indicated that the impact of Fe deficiency appeared earlier while Ni, Co and Se seemed to affect the process at later stages under the higher OLR. Co concentration became critical at its baseline level of 0.01 mg kg-1 FM at OLR 3.5 kg VS m-3 d -1 . Ni was critical at 0.03 mg kg-1 FM, equal to the baseline concentration at OLR 3.0 kg VS m-3 d -1 . Ni at the strength of 0.6-0.8 mg kg-1 FM is recommended to maintain stability at OLR 3.5 kg VS m-3 d -1 . Fe was critical at 5.0 and 6.2 mg kg-1 FM at OLR of 3.0 and 3.5 kg VS m-3 d -1 , respectively. Se was critical at its baseline level of 0.06 mg kg-1 FM at OLR 4.0 kg VS m-3 d -1 . This critical concentration, however, was not sustainable and not sufficient if instability initiated for instance by OLR increase. A sufficient safety factor should be applied if the critical concentrations are to be used for developing the TE supplementation strategies. Recovery and continuing stable operation, however, required much higher TE strength compared with their critical concentrations at high level of VFA. After long-term washed out, TE deficiency appeared earlier in higher OLR digester. In parallel with the work as abovementioned, the fractionations of Co, Ni, Fe and Se in AD under different operational conditions were assessed, in order to identify the dynamic changes of their distribution over a range of main fractions (i.e. in liquid, organically bound, precipitated with sulphide, and in microbial biomass) when their total concentrations were gradually decreased in digesters over time. Results illustrated that microbial biomass maintained relatively stable amount of Co and Ni for metabolic activities although all the extracellular TE fractions were washed-out gradually. Sulphide fraction competed with intracellular fraction for Fe. This caused the accumulation of VFA when the total Fe concentration was still high. The fact that TE availability to certain extent decoupled from simple hydraulic washing out effect was explained by the high affinity of microbial biomass for TE in this experiment. Apart from its readily biodegradability, another important feature of the model substrate was that OLR was able to decouple from HRT by adjusting the organic matter to water ratio. This allowed the investigation of the effect of OLR on the requirement of Co, Ni and Fe supplementation for stable AD without the interference of HRT (constant at 33.3 days). This trial was started at OLR 1.0 and step-wise increased to 3.0 Kg VS m-3 d -1 . The initial TE dosing was set at the strength of Co 0.03, Ni 0.03 and Fe 0.3 mg kg-1 FM and it was increased later in response to digester performance and loading increase. Se dosing strength was kept constant at 0.1 mg kg-1 FM. This ensured that Se was sufficient up to an OLR of 3.0 kg VS m-3 d -1 . Performance of digesters at different OLR levels was therefore directly related to the dosing strengths of Co Ni and Fe. Results indicated that the critical TE levels (defined as the concentration when VFA start to appear) appeared to be much higher than those obtained from washing-out experiment as aforementioned. When OLR increases, the concentration of microbial biomass increases accordingly if HRT is fixed, and microbes may have to take more TE from their environment. But when TE concentration in their environment is low, there are two problems: 1) the concentration gradient or energy required to find and carry TE to the inside of the cells; 2) the availability/speciation of TE in their environment. Therefore, the total TE concentration used to control this set of experiment should be much higher than that of the effective TE which should belong to microbeincorporated TE. The minimum Co, Ni and Fe requirement relevant to OLR when Se was supplied in sufficient quantity are quantified as follow: OLR 1.0 kg VS m-3 d -1 , a mix of Co 0.03, Ni 0.06 and Fe 0.6 mg kg-1 FM; OLR 1.5 kg VS m-3 d -1 , a mix of Co 0.06, Ni 0.06 and Fe 0.6 mg kg-1 FM; OLR 2 kg VS m-3 d -1 , a mix of Co 0.09, Ni 0.09 and Fe 0.9 mg kg-1 FM and OLR 2.5 kg VS m-3 d -1 , a mix of Co 0.12, Ni 0.12 and Fe 1.2 mg kg-1 FM. Not only the total strength, but the optimal ratio of a mix of Co, Ni and Fe in supplementation mixture played a significant role on digester stability. The ratio of Co+Ni and Fe in supplementation mixture in range of 1:5-1:6.67 was found optimal to recover process stability and ensure stable performance. Higher ratio (1:2.5, 1:3.33, 1:3.75 and 1:4) gave rise to disturbance and function decline probably due to less amount of available Fe. The research provided new insight on optimising essential TE supplementation to mesophilic anaerobic digestion, considering the availability of TE for microbial activities, which is very important to obtain efficient and long-term stable biogas processes.

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