Final Digestate Research Articles

Recovery of ammonia from anaerobically digested manure using gas-permeable membranes

Nitrogen (N) can be recovered from different types of wastewaters. Among these wastewaters, anaerobically digested swine manure (digestate) has the highest N content in ammonia form (NH3). It is desirable to reduce N in digestate effluents to safely incorporate them in arable soil in N vulnerable zones (NVZ) and to mitigate NH3 emissions during N land application. Additional benefit is to minimize inhibition of the anaerobic process by removing NH3 during the anaerobic digestion process. This work aimed to apply the gas-permeable membrane technology to evaluate ammonia (NH3) recovery from high-ammonia digested swine manure. Anaerobically digested swine manure with NH4+ content of 4,293 mg N L−1 was reduced by 91 % (to 381 mg N L−1) during the 32-day experiment. Although the results showed a total N recovery efficiency of 71 %, it is possible to increase this recovery efficiency to > 90 % by adjusting the area of the membrane system to match the high free ammonia concentration (FA) in digested swine manure. Moreover, final digestate pH and alkalinity were kept around 8.1 and 8,923 mgCaCO3 L−1, which are convenient for the anaerobic process or incorporation in arable soil when the process is finished.

The ability of macroalgae to stabilise and optimise the anaerobic digestion of household food waste

This study investigated the potential of seaweed waste (SW) as a sustainable feedstock for anaerobic co-digestion with food waste (FW). The study was conducted at laboratory scale using a batch test approach run over 34 days. Methane (CH4) potential assays were conducted at the following FW to SW dry mass ratios: 100:0, 90:10, 75:25, 50:50 and 0:100. Results indicated that anaerobic co-digestion of FW and SW at a mixture ratio of 90:10 produced the highest methane yield (252 cm3 g−1 of volatile solids (VS)), rates of reaction (0.08 d−1) and resulted in a better stability of the process. Predictions based on the Buswell formula suggested that all reactors were performing below the theoretical (maximum) with a greater disparity at increasing levels of seaweed in the feed, likely due to high levels of sulphur in the SW (1.73% mass fraction). The analysis of heavy metals in SW and final digestate indicated that using SW for anaerobic co-digestion with FW enhanced the process by providing trace nutrients without impacting the heavy metal content of the digestate. The analysis of carbon (C) and nitrogen (N) indicated that by using SW for co-digestion with FW, C:N optimal mass ratios were achieved. It was concluded that the addition of SW for anaerobic co-digestion of FW can be used to accelerate the bioenergy production from FW. An additional benefit will be the abatement of the negative impacts of SW in coastal areas, making the overall process more sustainable.

Innovative two-stage mesophilic/thermophilic anaerobic degradation of sonicated sludge: performances and energy balance.

This study investigates for the first time, on laboratory scale, the possible application of an innovative enhanced stabilization process based on sequential mesophilic/thermophilic anaerobic digestion of waste-activated sludge, with low-energy sonication pretreatment. The first mesophilic digestion step was conducted at short hydraulic retention time (3-5 days), in order to favor volatile fatty acid production, followed by a longer thermophilic step of 10 days to enhance the bioconversion kinetics, assuring a complete pathogen removal. The high volatile solid removals, up to 55%, noticeably higher compared to the performances of a single-stage process carried out in same conditions, can guarantee the stability of the final digestate for land application. The ultrasonic pretreatment influenced significantly the fatty acid formation and composition during the first mesophilic step, improving consequently the thermophilic conversion of these compounds into methane. Methane yield from sonicated sludge digestion reached values up to 0.2 Nm(3)/kgVSfed. Positive energy balances highlighted the possible exploitation of this innovative two-stage digestion in place of conventional single-stage processes.

Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

The thermophilic temperature has showed operational advantages in the Anaerobic Digestion process (AD). The increasing of the rate of hydrolysis, which suppose an acceleration of the overall process, and a higher hygienization of the final digestate are one of them. Dealing with OFMSW (Organic Fraction of Municipal Solid Waste) from non-selective collection systems involves additional difficulties due to their heterogeneity. The AD process of OFMSW is conducted usually in the range of 20–35% TS (dry AD). The main objective of this study was to determine the optimum Solids Retention Time (SRT) for the dry thermophilic AD of OFMSW operating in semi-continuous regime. Thus, the following SRTs have been tested: 15, 10, 8, 6, 5, 4 and 3 days. The main results have showed that the process is feasible in SRTs lower that those found in literature. Probably the history of the reactor, joined to the lower organic content in the waste, can be a key factor and the progressive decreasing of SRT is a strategy that permits to maintain stable conditions for lower SRT. The best operating conditions for thermophilic semicontinuous systems ranged in 8–5 days, with a yield of 0.33–0.34 LCH4/gVSadded and a concentration of Volatile Fatty Acids (VFA) around 100 mg HAc/L. The process becomes unstable for SRTs lower than 4 days, showing a clear decrease in methane productivity (lower than 0.2 LCH4/gVSadded) and an accumulation of VFA (more than 500 mg HAc/L). In general, SRTs below 4 days were not suitable for single stage of dry AD of OFMSW.

Semi-continuous anaerobic co-digestion of cow manure and steam-exploded Salix with recirculation of liquid digestate

The effects of recirculating the liquid fraction of the digestate during mesophilic anaerobic co-digestion of steam-exploded Salix and cow manure were investigated in laboratory-scale continuously stirred tank reactors. An average organic loading rate of 2.6 g VS L−1 d−1 and a hydraulic retention time (HRT) of 30 days were employed. Co-digestion of Salix and manure gave better methane yields than digestion of manure alone. Also, a 16% increase in the methane yield was achieved when digestate was recirculated and used instead of water to dilute the feedstock (1:1 dilution ratio). The reactor in which the larger fraction of digestate was recirculated (1:3 dilution ratio) gave the highest methane yields. Ammonia and volatile fatty acids did not reach inhibitory levels, and some potentially inhibitory compounds released during steam explosion (i.e., furfural and 5-hydroxy methyl furfural) were only detected at trace levels throughout the entire study period. However, accumulation of solids, which was more pronounced in the recycling reactors, led to decreased methane yields in those systems after three HRTs. Refraining from the use of fresh water to dilute biomass with a high-solids content and obtaining a final digestate with increased dry matter content might offer important economic benefits in full-scale processes. To ensure long-term stability in such an approach, it would be necessary to optimize separation of the fraction of digestate to be recirculated and also perform proper monitoring to avoid accumulation of solids.

Biogas final digestive byproduct applied to croplands as fertilizer contains high levels of steroid hormones

In this study we evaluate and demonstrate the occurrence of nine natural and one synthetic steroid hormone, including estrogens, androgens and progestagens in biogas final digestate byproduct (digestion liquid) commonly used as an agricultural fertilizer. We investigated two biogas sites that utilize different anaerobic digestion technologies (mesophilic and thermophilic) from swine manure and other organic wastes. Individual hormone concentration levels were observed up to 1478 ng g−1 dry weight or 22.5 mg kg−1 N with estrone and progesterone reaching highest concentration levels. Evaluation of the potential environmental burden through the application in agriculture was also assessed on the basis of predicted environmental concentrations. This study indicates that the biogas digestion process does not completely remove steroid hormones from livestock manure and use of final digestate byproduct on croplands contributes to the environmental emission of hormones.

Digestion of cattle manure: Thermogravimetric kinetic analysis for the evaluation of organic matter conversion

Anaerobic digestion of cattle manure was studied under thermophilic and mesophilic conditions with the purpose of evaluating the effect of temperature on the quality of the final digestate. Non-isothermal thermogravimetric kinetic analysis was applied for assessing organic matter conversion of biological stabilization. The mathematical approximation proves to be a useful tool for evaluating the differences attained during biological degradation. The anaerobic digestion of the organic substrate resulted in a reduction of the activation energy value obtained from the different applied kinetic models. Results obtained from thermal kinetic analysis were in accordance with those from the monitoring of the anaerobic digestion process. The higher values of methane yield reported for the mesophilic digestion in comparison to that of the thermophilic indicated a greater capability of the former process in the utilization of substrate and thus a higher conversion of organic matter which can be quantified by the activation energy value.