Kinetic modelling of the hydrolysis, acidogenic and methanogenic steps in the anaerobic digestion of two-phase olive pomace (TPOP)

From laboratory- to industrial-scale plants: Future of anaerobic digestion of olive mill solid wastes

In this review, the main properties of olive mill solid waste, the primary by-product of olive oil production, and its feasibility as a feedstock for anaerobic digesters operating at laboratory-, pilot- and industrial-scales are discussed in detail. Nutrient addition and thermal pretreatments were found to have the potential to address the challenges arising from the high carbon-to-nitrogen ratio, the low pH, and the high concentration of phenolic compounds. Furthermore, anaerobic co-digestion with different organic feedstocks has been identified as one of the most promising options to solve the aforementioned problems and the seasonality nature of olive waste, while improving the efficiency of anaerobic treatment plants that operate throughout the whole year. The insights generated from this study show co-digestion with wastes from animal farming to be the most environmentally and economically sustainable method for improving anaerobic digestion processes with olive mill solid waste.

Effect of organic loading rate on anaerobic digestion of raw cheese whey: experimental evaluation and mathematical modeling

Whey is a high strength waste product of cheese manufacture. Anaerobic digestion of cheese allows pollution potential reduction and simultaneously energy production. Organic loading rate (OLR) is an important operating variable for anaerobic digestion (AD) process related to system stability, waste treatment capacity and biogas production. The actual OLR depends on the types of wastes (i.e., content of chemical oxygen demand [COD]) fed into a digester. In this paper, the effect of OLR on the AD process of the raw cheese whey in a semi-continuous up-flow system is studied experimentally and with numerical simulations using a simple dynamical model calibrated with experimental data. The digester operation was performed for 90 days, doubling the OLR every 30 days, from OLR of 2.5–10 gCOD L−1 d−1. Experimental results show that the increase in OLR favors the production of biogas. However, the proportion of methane may decrease. The highest methane yield and the most considerable substrate degradation were obtained at OLR of 5 gCOD L−1 d−1 and 10 gCOD L−1 d−1, respectively. The proposed mathematical model is used to describe the dynamic behavior of key variables as COD, volatile fatty acids (VFA) and methane production. A good fit between the variables estimated by the mathematical model and experimental data was obtained, reaching determination coefficients (R2) greater than 0.8. Therefore, this model might be beneficial in predicting the maximum methane production rate and the maximum OLR that could be used without risking the AD process stability.

Effect of food to vegetable waste ratio on acidogenesis and methanogenesis during two-stage integration

The mixing ratio of food waste (FW) to vegetable waste (VW) (2:3 FW:VW ∼ 152.51 g VS and 2:1 FW:VW ∼ 137.03 gVS) was optimized using two-stage (LBR-UASB) experimental process depending upon volatile solid (VS) load. The effect of FW to VW ratio was studied in Leach Bed Reactor (LBR) towards leachate production. Results revealed that hydrolysis rate (73.11%), sCOD (3294.3 g/KgVS) and tVFA (2664 g/KgVS) yield was higher in 2:1 FW:VW ratio. Acetate, propionate, lactate and methane yield for 2:3 FW:VW (420 g/KgVS, 87 g/KgVS, 180 g/KgVS and 226.86 ml/gVS respectively) was different from 2:1 FW:VW (340 g/KgVS, 247 g/KgVS, 340 g/KgVS and 218.54 ml/gVS respectively). 2:3 FW:VW ratio depicted higher VS (53.96%) and COD (54.1%) removal than 2:1 FW:VW ratio 46.34% and 41.8% respectively. VW addition regulated pH, restricted propionate and lactate production with enhanced methanogenesis by improving acetate production in two-stage AD process which further boosted process stability and efficiency.

Hydrolysis dynamics for batch anaerobic digestion of elephant grass

Elephant grass might be a potential source of fine chemical precursors and bioenergy.

Overcoming the bottlenecks of anaerobic digestion of olive mill solid waste by two-stage fermentation

Two-stage anaerobic digestion (AD) of two-phase olive mill solid waste (OMSW) was applied for reducing the inhibiting factors by optimizing the acidification stage. Single-stage AD and co-fermentation with chicken manure were conducted coinstantaneous for direct comparison. Degradation of the polyphenols up to 61% was observed during the methanogenic stage. Nevertheless the concentration of phenolic substances was still high; the two-stage fermentation remained stable at OLR 1.5 kgVS/m³day. The buffer capacity of the system was twice as high, compared to the one-stage fermentation, without additives. The two-stage AD was a combined process - thermophilic first stage and mesophilic second stage, which pointed out to be the most profitable for AD of OMSW for the reduced hydraulic retention time (HRT) from 230 to 150 days, and three times faster than the single-stage and the co-fermentation start-up of the fermentation. The optimal HRT and incubation temperature for the first stage were determined to four days and 55°C. The performance of the two-stage AD concerning the stability of the process was followed by the co-digestion of OMSW with chicken manure as a nitrogen-rich co-substrate, which makes them viable options for waste disposal with concomitant energy recovery.

Determining anaerobic degradation kinetics from batch tests

Data obtained from a biomethane potential (BMP) test were used in order to obtain the parameters of a kinetic model of solid wastes anaerobic degradation. The proposed model considers a hydrolysis step with a first order kinetic, a Monod kinetic for the soluble organic substrate degradation and a first order decay of microorganisms. The instantaneous release of methane was assumed. The parameters of the model are determined following a direct search optimization procedure. A 'multiple-shooting' technique was used as a first step of the optimization process. The confidence interval of the parameters was determined by using Monte Carlo simulations. Also, the distribution functions of the parameters were determined. Only the hydrolysis first order constant shows a normal distribution.

Regression based state space adaptive model of two-phase anaerobic reactor

In this paper, a linear state space model for the two-phase anaerobic reactor system was developed based on historical data. Subsequently, the model was used to predict its future behavior. The state space model developed involved correlation analysis and model development. The model would be updated at every time point when a new data set became available, giving it an "adaptive" feature. The model was then applied to monitor two-phase anaerobic co-digestion of a feed comprising 2 industrial secondary sludges and 2 industrial wastewaters. The case study showed the proposed model was able to provide good predictions of various process parameters. In addition, it also predicted impending process failure and this would have allowed the operator to take necessary measures to prevent or reduce impact of such failure during plant operation.

Enhancement of volatile fatty acid production using semi-continuous anaerobic food waste fermentation without pH control

This study proposed a cost-effective and high-yield volatile fatty acid (VFA) production strategy using anaerobic food waste (FW) fermentation without pH control, which could be recommended for practical scale VFA production and FW treatment.

A review on sludge conditioning by sludge pre-treatment with a focus on advanced oxidation

The production of excess sludge by biological wastewater treatment processes has been a serious issue for the operation of wastewater treatment plants (WWTP) on both the economic and environmental sides.

Suspended growth kinetic analysis on biogas generation from newly isolated anaerobic bacterial communities for palm oil mill effluent at mesophilic temperature

The anaerobic degradation of palm oil mill effluent (POME) was carried out under mesophilic temperature in an anaerobic suspended growth closed bioreactor (ASGCB).

Alkaline post-treatment for improved sludge anaerobic digestion

Alkaline post-treatment was tested in order to improve sludge anaerobic digestion. Between the 8th and the 12th hour of a 24-h digestion cycle, 5% of sludge was extracted from a semi-continuous digester with a sludge retention time of 20 days. The sludge was then disintegrated with 0.1 mol/L NaOH and returned to the digester after neutralization. The results showed that alkaline post-treatment increased the level of soluble organic substances in the extracted sludge, particularly of volatile fatty acids and polysaccharides. This process resulted in a 33% enhancement of biogas production in comparison with the control. When the ratio of the recycled sludge was further increased to 10% or 15%, the increment of biogas yield was reduced, due to excessive inactivation of anaerobic bacteria in the digester. Alkaline post-treatment had a minimal impact on the dewaterability of digested sludge.

Comparison of two mathematical models for correlating the organic matter removal efficiency with hydraulic retention time in a hybrid anaerobic baffled reactor treating molasses

A modelling of the anaerobic digestion process of molasses was conducted in a 70-L multistage anaerobic biofilm reactor or hybrid anaerobic baffled reactor with six compartments at an operating temperature of 26 °C. Five hydraulic retention times (6, 16, 24, 72 and 120 h) were studied at a constant influent COD concentration of 10,000 mg/L. Two different kinetic models (one was based on a dispersion model with first-order kinetics for substrate consumption and the other based on a modification of the Young equation) were evaluated and compared to predict the organic matter removal efficiency or fractional conversion. The first-order kinetic constant obtained with the dispersion model was 0.28 h(-1), the Peclet dispersion number being 45, with a mean relative error of 2%. The model based on the Young equation predicted the behaviour of the reactor more accurately showing deviations lower than 10% between the theoretical and experimental values of the fractional conversion, the mean relative error being 0.9% in this case.

Biodegradation of olive-mill pomace mixed with organic fraction of municipal solid waste

This study investigated the effects of organic fraction of municipal solid waste (OFMSW) addition on the anaerobic treatment of the olive-mill pomace. Biodegradability of olive-mill pomace mixed with OFMSW was examined in anaerobic bioreactors. Only OFMSW was loaded in the first (control) bioreactor, while run 1 and run 2 bioreactors included different ratio of OFMSW and olive-mill pomace. COD, BOD(5), NH(4)-N, pH, VFA, CH(4) quantity and percentage in anaerobic bioreactors were regularly monitored. In addition, inert COD and anaerobic toxicity assay (ATA) were measured in leachate samples. The results of the study showed that 70% of OFMSW addition to olive-mill pomace has an advantage in terms of pollution parameters and methane generation. Since olive-mill pomace is not easy biodegradable, addition of high proportion of OFMSW promotes biodegradability of olive-mill pomace. Decreasing in BOD(5)/COD ratios in the run 1 and run 2 reactors carried out as 62 and 52%, respectively.

Valorisation of wastewater from two-phase olive oil extraction in fired clay brick production

Wastewater issued from oil-washing stage (OWW) in the two-phase olive oil extraction method was used to replace fresh water in clay brick manufacture. The extrusion trials were performed with one of the ceramic bodies currently being used in a local brick factory for red facing bricks (RB) production. Fresh water or OWW was added to a final consistency of 2.4 kg/cm(2), the same value as used at industrial scale for this kind of clay mixture. Comparative results of technological properties of facing bricks are presented. Results show that the products obtained with olive oil wastewater are comparable to traditional ones in terms of extrusion performance and technological properties of end products. Even dry-bending strength of the body formed by wastewater improves by 33% compared to fresh water body. In addition, heating requirements can be reduced in the range 2.4-7.3% depending on the final product. This application can alleviate environmental impacts from the olive oil extraction industry and, at the same time, result in economic savings for the brick manufacturing industry.

Bioconversion of wastes from olive oil industries by vermicomposting process using the epigeic earthworm Eisenia andrei

The present work evaluates the possible bioconversion of wet olive cake by low-cost biostabilization (vermicomposting process). Wet olive cake fresh (WOC), precomposted (WOCP), or mixed with biosolids (WOCB), were vermicomposted for 6 months to obtain organic amendments for agricultural and remediation purposes. The results showed initial differences depending on previous treatment. WOCP was initially more stable, presented a low C:N ratio, and showed more dehydrogenase and urease activity. By contrast, there was no dehydrogenase activity initially in WOC and WOCB, due to the presence of some different types of polyphenols. Finally, the end product showed relatively higher amounts of total nitrogen and humic acid and met the standard of quality for composts and vermicomposts for use both in conventional and organic agriculture and soil-restoration programs.

H2S and volatile fatty acids elimination by biofiltration: clean-up process for biogas potential use

In the present work, the main objective was to evaluate a biofiltration system for removing hydrogen sulfide (H(2)S) and volatile fatty acids (VFAs) contained in a gaseous stream from an anaerobic digestor (AD). The elimination of these compounds allowed the potential use of biogas while maintaining the methane (CH(4)) content throughout the process. The biodegradation of H(2)S was determined in the lava rock biofilter under two different empty bed residence times (EBRT). Inlet loadings lower than 200 g/m(3)h at an EBRT of 81 s yielded a complete removal, attaining an elimination capacity (EC) of 142 g/m(3)h, whereas at an EBRT of 31 s, a critical EC of 200 g/m(3)h was reached and the EC obtained exhibited a maximum value of 232 g/m(3)h. For 1500 ppmv of H(2)S, 99% removal was maintained during 90 days and complete biodegradation of VFAs was observed. A recovery of 60% as sulfate was obtained due to the constant excess of O(2) concentration in the system. Acetic and propionic acids as a sole source of carbon were also evaluated in the bioreactor at different inlet loadings (0-120 g/m(3)h) obtaining a complete removal (99%) for both. Microcosms biodegradation experiments conducted with VFAs demonstrated that acetic acid provided the highest biodegradation rate.

Kinetics of anaerobic degradation of screened dairy manure by upflow fixed bed digesters: effect of natural zeolite addition

The effect of the hydraulic retention time (HRT) on the performance of two up-flow anaerobic fixed bed digesters (UFAFBDs) packed with waste tire rubber (D1) and waste tire rubber and zeolite (D2) as micro-organism immobilization supports was studied. It was found that a first-order kinetic model described well the experimental results obtained. The kinetic constants for COD, BOD5, total solids (TS) and volatile solids (VS) removal were determined to be higher in digester D2 than in digester D1 or control. Specifically, they were 0.28 +/- 0.01, 0.32 +/- 0.02, 0.16 +/- 0.01 and 0.24 +/- 0.01 d(- 1) respectively for D1 and 0.33 +/- 0.02, 0.40 +/- 0.02, 0.21 +/- 0.01 and 0.28 +/- 0.01 d(- 1) respectively for D2. This was significant at the 95% confidence level. In addition, the first-order model was also adequate for assessing the effect of the HRT on the removal efficiency and methane production. Maximum methane yield and the first-order constant for methane production were determined and the results obtained were comparable with those obtained by other authors but operating at higher HRTs. Maximum methane yields and the kinetic constant for methane production were 11.1% and 29.4% higher in digester D2 than in D1.

Effect of the organic loading rate on the performance of anaerobic acidogenic fermentation of two-phase olive mill solid residue

A study of the effect of the organic loading rate (OLR) on the anaerobic acidogenic fermentation of two-phase olive mill solid residue (OMSR) derived from fruits with a low ripening index was carried out in a laboratory-scale completely stirred tank reactor at mesophilic temperature (35 degrees C). Eight experimental runs were carried out at OLRs of 3.2, 5.6, 7.4, 9.6, 11.0, 12.9, 14.0 and 15.1g T-COD/ld, which were equivalent to hydraulic retention times of 50.0, 28.8, 21.8, 16.9, 14.7, 12.4, 11.5 and 10.7d, respectively. The experimental results obtained demonstrated that the optimum value of OLR for the acidogenic fermentation process was 12.9 g T-COD/ld, for which a maximum production of acetic acid was achieved. It was found that inhibition of the process occurred at OLRs higher than 12.9 g T-COD/ld. This was characterized by a significant decrease in the acetic acid concentration in the effluent and an increase in the concentration of other volatile acids that may affect the methanogenic step. The process inhibition was also characterized by the plateau in the curves of the effluent substrate concentration versus the OLR applied. It was found that a first-order kinetics satisfactorily described the influence of non-acetic acid soluble organic matter concentration (S-COD( *)) on the rate of soluble organic matter conversion to acetic acid (R(S-COD)( *)), and the influence of acetic acid concentration (AcH) on the rate of acetic acid production (R(AcH)), while a potential equation type adequately described the influence of acetic acid concentration on the volumetric hydrogen production (R(ACH) ). The kinetic constant for soluble organic matter removal was 0.145 d(-1), while the constant for acetic acid formation was found to be 0.075 d(-1).