Anaerobic Co-digestion System Research Articles

Technical assessment of mono-digestion and co-digestion systems for the production of biogas from anaerobic digestion in Brazil

Abstract The use of dedicated energy crops (DEC) in anaerobic co-digestion systems becomes attractive, due to their high biogas yield per tonne of fresh matter; furthermore, when mixed with animal waste, they may reach adequate carbon to nitrogen ratios (C:N). This paper presents a performance evaluation of mono-digestion and co-digestion systems for the production of biogas using cattle manure, maize silage and grass silage as feedstock. Biogas plants of large, medium and small-scale, in the electric powers between 100 and 1000 kWe were evaluated. The results showed that the optimum percentage of maize silage and grass silage for co-digestion systems with cattle manure were in the ranges of 22–65% and 18–54%, respectively. The residual heat obtained from the co-digestion and mono-digestion systems was 87% and 75% from the total heat produced respectively, which can be used into processes that demand thermal energy. In all studied systems studied, the carbon dioxide released from the generation of electricity and the burning of methane represented 3% and 97% of the total carbon dioxide avoided, respectively. Finally, the co-digestion systems present better performance in terms of size, electricity consumption and heat consumption compared to mono-digestion systems.

Enhancement of anaerobic co-digestion in acidogenic (ACIDO-DR) and methanogenic (METHA-DR) digester system

Abstract Acidogenic (ACIDO-DR) and methanogenic (METHA-DR) digesters were synchronously operated under the semi-continuous modes of mono-digestion (control) and co-digestion (mixture). The CH4 gas production rate was enhanced by 19.8% in co-digestion at an optimum ratio of 7:3 (food waste to sewage sludge). At this ratio, the maximum BMP of 620 ± 18 mL CH4 g−1 VS was attained in co-digestion but control showed lower BMP of 440 ± 14 mL CH4 g−1 VS. Co-digestion of food waste and sewage sludge was enhanced due to the adequate adjustment of C/N ratio for feed waste, improved buffer capacity and the necessary amounts of trace elements. Additionally, this system provided better buffering potential to high OLR than the one-phased system. As a result, the two-phased anaerobic co-digestion system suggested the alternatives for many struggling wastewater treatment plants, without any major remodeling of facilities, to enhance sustainable energy production that could be beneficial to public use.

Online measurement of alkalinity in anaerobic co-digestion using linear regression method

Alkalinity is a reliable indicator of process stability in anaerobic digestion system. Total alkalinity (TA) and partial alkalinity (PA) are usually monitored offline as indicators for the status of anaerobic digestion process. In order to online monitor TA and PA, the linear regression method was used as estimator to predict alkalinity via software sensor method. Parameters, namely, pH, oxidation and reduction potential (ORP), and electrical conductivity (EC), were used as input variables. EC was the most significant parameter with TA and PA. Multiple linear regression (MLR) models and simple linear regression models with EC were constructed to predict TA and PA in anaerobic co-digestion system. On the basis of the evaluation of prediction accuracy, the applications of linear regression models were better for monitoring PA than TA. MLR models provided higher accuracy for alkalinity prediction than simple linear regression models. The two MLR models based on single-phase anaerobic digestion system were also feasible to predict TA in anaerobic co-digestion systems. However, the accuracy of these models should be improved by calibrating for broad applications of linear regression method in online alkalinity measurement. Keywords: anaerobic digestion, alkalinity, online measurement, model, linear regression DOI: 10.3965/j.ijabe.20171001.2701 Citation: Bai X, Li Z F, Wang X M, He X, Cheng S K, Bai X F, et al. Online measurement of alkalinity in anaerobic co-digestion using linear regression method. Int J Agric & Biol Eng, 2017; 10(1): 176–183.

High-solid Anaerobic Co-digestion of Sewage Sludge and Cattle Manure: The Effects of Volatile Solid Ratio and pH.

High-solid anaerobic digestion is an attractive solution to the problem of sewage sludge disposal. One method that can be used to enhance the production of volatile fatty acids (VFAs) and the generation of methane from anaerobic digestion involves combining an alkaline pretreatment step with the synergistic effects of sewage sludge and cattle manure co-digestion, which improves the activity of key enzymes and microorganisms in the anaerobic co-digestion system to promote the digestion of organic waste. In this study, we describe an efficient strategy that involves adjusting the volatile solid (VS) ratio (sewage sludge/cattle manure: 3/7) and initial pH (9.0) to improve VFA production and methane generation from the co-digestion of sludge and manure. The experimental results indicate that the maximum VFA production was 98.33 g/kg-TS (total solid) at the optimal conditions. Furthermore, methane generation in a long-term semi-continuously operated reactor (at a VS ratio of 3/7 and pH of 9.0) was greater than 120.0 L/kg-TS.

Thermophilic and mesophilic temperature phase anaerobic co-digestion (TPAcD) compared with single-stage co-digestion of sewage sludge and sugar beet pulp lixiviation

The performance of temperature phase anaerobic co-digestion (TPAcD) for sewage sludge and sugar beet pulp lixiviation (using the process of exchanging the digesting substrate between spatially separated thermophilic and mesophilic digesters) was tested and compared to both single-stage mesophilic and thermophilic anaerobic co-digestion. Two Hydraulic Retention Times (HRT) were studied in the thermophilic stage of anaerobic digestion in two temperature phases, maintaining the optimum time of the mesophilic stage at 10 days, obtained as such in single-stage anaerobic co-digestion. In this way, we obtained the advantages of both temperature regimes.Volatile solids removal efficiency from the TPAcD system depended on the sludge exchange rate, but fell within the 72.6–64.6% range. This was higher than the value of 46.8% obtained with single-stage thermophilic digestion and that of 40.5% obtained with mesophilic digestion. The specific methane yield was 424–468 ml CH4 per gram of volatile solids removed, similar to that of single-stage mesophilic anaerobic digestion. The increase in microbial activity inside the reactor was directly proportional to the organic loading rate (OLR) (or inversely proportional to the HRT) and inversely proportional to the size of the microbial population in single-stage anaerobic co-digestion systems.

Chemical and biological assessment of endocrine disrupting chemicals in a full scale dairy manure anaerobic digester with thermal pretreatment

Concentrated animal feeding operations are important sources of estrogens and their conjugates, which are introduced into the environment through manure land application. In this study, concentrations of estrogens were measured in an anaerobic co-digestion system with thermal pasteurization pretreatment. Free estrogens (estrone (E1), 17α-estradiol (E2α), 17β-estradiol (E2β), estriol (E3)) were analyzed by gas chromatography with mass spectrometry (GC/MS), and conjugated estrogens (sulfate- and glucuronide-conjugates) were analyzed by liquid chromatography with tandem mass spectrometry (LC/MS/MS). Additionally, yeast estrogen screen assay was used to determine the estrogenic potential of the manure. The total hormone concentrations (mainly E1, E2α, E2β, and sulfated estrogens) were observed at concentrations up to a total of 7100ng/L in the liquid fraction, while free estrogen levels were 630ng/kg in the solid fraction of the untreated manure. The total hormone concentration did not decrease significantly during digestion, however, the relative composition of the estrogens changed from E2α (65%) being the predominant species before digestion to mostly E1 (72%) after digestion. This conversion process has important implications because E1 is more estrogenic than E2α. Total E2 equivalents associated with E1, E2α and E2β concentrations as determined by GC/MS indicate that E1 is the most important contributor to the endocrine-disruption activity of the treated manure.

An on-site prototype two-stage anaerobic digester for co-digestion of food waste and sewage sludge for biogas production from high-rise building

Up to now, there has been less results from on-site pilot-scale anaerobic co-digestion system, especially for high-rise building application. The objective of this study was to develop an on-site prototype two-stage anaerobic digester for co-digestion of food waste and sewage sludge from high-rise building for biogas production. The optimal mixing ratio of food waste to sewage sludge, obtained from laboratory-scale, was found to be 7:1. Then, the prototype two-stage anaerobic digester was further designed and constructed. The results showed that COD and total volatile solid (TVS) reduction could be achieved up to 89 and 74%, respectively with the applied hydraulic retention time (HRT) of 24 days. The methane content of biogas was 64 percent. Up to now, the biogas from on-site production has been utilized for cooking at Chulalongkorn University as a model case study for high-rise building application.

Life Cycle and Economic Assessment of Anaerobic Co-digestion of Dairy Manure and Food Waste

Abstract A comprehensive life cycle and financial analysis was conducted to evaluate the environmental and economic performance of an anaerobic co-digestion system for dairy manure and food wastes versus a control system that anaerobically digests dairy manure but landfills food wastes. The environmental impacts of co-digestion are lower than the control system for global warming (66.95%), water eutrophication (18.88%), and smog emission (68.93%), but higher for air acidification (39.38%). Co-digestion results in a net energy ratio that is 1.67 times higher than the control system. An economic analysis revealed that co-digestion is more profitable, as the calculated 25-year net value of over $8.4 million is far superior to the -$7.5 million for the control system. Considering both environmental impacts and economic indicators, manure and food waste co-digestion is a preferable solution for waste management and bioenergy production.

Outlining microbial community dynamics during temperature drop and subsequent recovery period in anaerobic co-digestion systems

To improve the stability of anaerobic reactors, more knowledge is required about how the different communities react against operating perturbations and which specific ones respond better. The objective of this work was to monitor the changes in microbial community structure of an anaerobic digester during a temperature drop by applying different complementary molecular techniques. Temperature decrease led to an increase of Bacteroidales order, Porphyromonadaceae family and Bacteroides genus and a decrease in Syntrophomonas and Clostridium genera. Once the temperature was restored, the reactor recovered the steady state performance without requiring any modification in operational conditions or in the microbiome. During the recovery period, Sedimentibacter genus and Porphyromonadaceae family played an important role in the degradation of the accumulated volatile fatty acids. The hydrogenotrophic methanogens appeared to be the keystone archaeal population at low temperatures as well as in the recovery period. This study stands out that the understanding of microbial community dynamics during temperature drop could be utilized to develop strategies for the mitigation of temperature change consequences and speed up the recovery of stable reactor performance.

A prototype single-stage anaerobic digester for co-digestion of food waste and sewage sludge from high-rise building for on-site biogas production

At present, there has been less results from pilot-scale experiments on anaerobic co-digestion system, especially as an on-site system for high-rise building application. The objective of this study was to develop a prototype single-stage anaerobic digester for the co-digestion of food waste (FW) and sewage sludge (SWS) from high-rise building for on-site biogas production. Here, the prototype system was operated at different hydraulic retention times (HRTs) of 27, 22 and 19 days, corresponding to organic loading rates of 7.9, 10.8 and 14.0 kgCOD/m3 d. The feed mixed waste ratio (FW/SWS) of 10:1 by weight was selected from the previous laboratory-scale experiment. The results of this study indicate that higher gas production rate was obtained at shorter hydraulic retention time (HRT) of 19 days; however, higher methane content of the biogas was obtained at longer HRT of 27 days. Therefore, enhancement of methane production from the co-digestion could be achieved with sufficient operating HRT. Moreover, the proposed prototype system could reduce total volatile solid up to 70 percent at HRT of 27 days. Up to now, the biogas from on-site production has been utilized for cooking at Chulalongkorn University as a model case study for high-rise building application.

Performance and microbial community dynamics in a two-phase anaerobic co-digestion system using cassava dregs and pig manure

The two-phase anaerobic co-digestion of cassava dregs (CD) with pig manure (PM) was evaluated using four sequencing batch reactors (SBRs) and a continuously stirred tank reactor (CSTR). The effect of seven different PM to CD volatile solid ratios (10:0, 8:2, 6:4, 5:5, 4:6, 2:8 and 0:10) on the acidification phase was investigated. Results indicated the concentrations of soluble chemical oxygen demand, NH4-N and volatile fatty acids increased substantially at seven ratios. Co-acidification of PM and CD performed well. Methanogenic fermentation of the acidification products at seven ratios was steady in CSTR. The highest methane yield and VS removal of 0.352m(3)/kg VSadded and 68.5% were achieved at PM:CD (4:6). The microbial population in CSTR was analyzed using molecular methods. Findings revealed that bacteria such as Firmicutes and Bacteroidetes, archaea such as Methanobacteriales and Methanomicrobiales were advantageous populations. Co-digestion of PM and CD supported higher quantity and diversity of methanogens.

Influence of mixing proportion on the solid-state anaerobic co-digestion of distiller's grains and food waste

The solid-state anaerobic co-digestion (AC) of distiller's grains (DG) and food waste (FW) for biogas production was investigated. The effects of different DG/FW (TS) ratios (10/1, 8/1, 6/1, 4/1, 1/0, and 0/1) were examined. Co-digestion with the above DG/FW ratios was found to be superior to that of mono-digestion. The AC of DG and FW had a good synergistic effect of the lower range of propionate/acetate ratio and VFA/alkalinity ratio. No methane was produced when the propionate/acetate ratio was >0.1 ± 0.01, but this inhibition was reversible. When the propionate/acetate ratio was <0.08 ± 0.01, methane production began to recover. When the volatile fatty acids (VFA)/alkalinity ratios were <0.9 ± 0.05, the AC of the influents successfully proceeded in a stable manner. However, when the VFA/alkalinity and propionate/acetate ratios were >1.25 ± 0.21 and 0.1 ± 0.01, respectively, the AC system reached an acidification crisis and failed. Therefore, the propionate/acetate and VFA/alkalinity ratios may be used as important indices for controlling anaerobic digestion (AD).

Evaluation of Anaerobic Co-digestion System of Sewage Sludge and Garbage by Life Cycle Assessment

Evaluation of Anaerobic Co-digestion System of Sewage Sludge and Garbage by Life Cycle Assessment

Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions

With increasing concerns over the limited capacity of landfills, conservation of resources, and reduction of CO 2 emissions, high-solids (dry) anaerobic digestion of organic solid waste (OSW) is attracting a great deal of attention these days. In the present work, two dry anaerobic co-digestion systems fed with different mixtures of OSW were continuously operated under mesophilic conditions. Dewatered sludge cake was used as a main seeding source. In reactor (I), which was fed with food waste (FW) and paper waste (PW), hydraulic retention time (HRT) and solid content were controlled to find the maximum treatability. At a fixed solid content of 30% total solids (TS), stable performance was maintained up to an HRT decrease to 40 d. However, the stable performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again. In further operation, instead of decreasing HRT, solid content was increased to 40% TS, which was found to be a better option to increase the treatability. The biogas production rate (BPR), CH 4 production yield (MPY) and VS reduction achieved in this condition were 5.0 m 3/m 3/d, 0.25 m 3 CH 4/g COD added, and 80%, respectively. Reactor (II) was fed with FW and livestock waste (LW), and LW content was increased during the operation. Until a 40% LW content increase, reactor (II) exhibited a stable performance. A BPR of 1.7 m 3/m 3/d, MPY of 0.26 m 3 CH 4/g COD added, and VS reduction of 72% was achieved at 40% LW content. However, when the LW content was increased to 60%, there was a significant performance drop, which was attributed to free ammonia inhibition. The performances in these two reactors were comparable to the ones achieved in the conventional wet digestion and thermophilic dry digestion processes.