Tubular Digesters Research Articles

Temperature Control Effect on Cheese Whey Anaerobic Digestion with Low-Cost Tubular Digesters

Cheese whey (CW) is a worldwide abundant by-product of the cheese industry, which can be used for biogas production if further processing is not performed to produce other valuable food products. This study evaluates biogas production from CW in low-cost, tubular reactors, thus comparing the effect of temperature control. CW was monodigested in two tubular reactors at the pilot scale: one of them with temperature control (30 ± 3 °C) and the other one working at environmental conditions. The results show that CW could be monodigested in pilot scale tubular reactors, thus yielding high methane. Temperature control (30 ± 3 °C) at the pilot scale led to higher methane yields under all tested operating conditions, thus reaching 565.8 ± 20.9 L kg−1VS at an Organic Loading Rate (OLR) of 0.416 ± 0.160 kgVS L−1 d−1, which was higher than the maximum yield obtained without temperature control (445.6 ± 21.9 L kg−1VS) at 0.212 ± 0.020 kgVS L−1 d−1. Methane yield differences were attributed to the increase in temperature, thus leading to a more stable process and a higher degradation capacity. The increase in temperature is only worthwhile if adequate thermal insulation is used between the digester and the soil; otherwise, the increase in biogas production will not meet the digester’s heat demand. The anaerobic monodigestion of CW in low-cost tubular reactors is a promising alternative for CW valorization, thus leading to high biogas yields, which can be used in several energy applications replacing fossil fuels.

Revolutionizing biogas generation: Polyethylene tubular digesters for household pig farms

Manure decomposition from animal waste, including farm sludge, is a significant source of methane (CH4) and carbon dioxide (CO2) emissions, aggravating global warming. Addressing this issue is vital for the environment and pivotal in achieving sustainable development goals by combating pollution from agricultural activities. One promising solution is biogas production, which offers threefold benefits including mitigation of global warming, assurance of energy security, and efficient waste management. This can be achieved by optimizing the process using substrates that yield high biogas output while ensuring low water usage and retention. This study focuses on pig farms' biogas potential of liquid and solid manure fractions performed with laboratory-scale batch digesters and enhanced polyethylene tubular digesters for evaluation. From the screening system, the biogas output from pig slurry resulted in CH4 and CO2 in 45 days, achieving 61.44 and 36.35%, respectively. After the initial screening experiment, polyethylene tubular digesters were implemented for biogas production at household pig farms and produced through fermentation in polyethylene tubular digesters under anaerobic conditions and are mainly composed of CH4 (60–64%) and CO2 (29–38%). This study suggested that the pig slurry could be a reliable biomass energy source for biogas and applicable to householders.

Benefits and risks of agricultural reuse of digestates from plastic tubular digesters in Colombia.

The aim of this study is to characterize the digestates from three plastic tubular digesters implemented in Colombia fed with: i) cattle manure; ii) cattle manure mixed with cheese whey; iii) pig manure. All the digesters worked under psychrophilic conditions. Physico-chemical characteristics, heavy metals, pathogens, and agronomic quality were investigated. All the digestates were characterized by physico-chemical characteristics and nutrients concentration suitable for their reuse as biofertilizer. However, these digestates may only partially replace a mineral fertilizer due to the high nutrients dilution. Heavy metals were under the detection limit of the analytical method (Pb, Hg, Ni, Mo, Cd, Chromium VI) or present at low concentration (Cu, Zn, As, Se) in all the digestates. Biodegradable organic matter and pathogens (coliform, helminths and Salmonella spp.) analysis proved that all the digestates should be post-treated before soil application in order to prevent environmental and health risks, and also to reduce residual phytotoxicity effects. The digestate from pig manure had a higher nutrient percentage (0.2, 0.6 and 0.05 % w/w of total N, P2O5 and K2O, respectively), but also higher residual phytotoxicity than the other digestates. Co-digestion seemed not to significantly improve the digestate fertilizing potential. Finally, further studies should address how to improve fertilizing potential of digestates from plastic tubular digesters, avoiding environmental and health risks.

CUSTO ENERGÉTICO DE CONSTRUÇÃO DE BIODIGESTORES PARA O MANEJO E TRATAMENTO DE RESÍDUOS DA SUINOCULTURA

Pela praticidade e simplicidade de construção, nos últimos anos, os biodigestores tubulares, também conhecidos como canadense, têm sido utilizados para o manejo e tratamento de resíduos da suinocultura. Contudo, a escassez de informações acerca da composição energética destas estruturas limita a realização de estudos relacionados às análises energéticas nesses biossistemas. O objetivo deste trabalho foi contabilizar a energia empregada na construção de biodigestores anaeróbios de fluxo tubular utilizados para o manejo e tratamento de resíduos da suinocultura. Para tanto, foram quantificados os componentes envolvidos na construção dos biodigestores e, em seguida, para transformação em unidades de energia desses componentes, multiplicou-se a quantidade destes pelos seus respectivos coeficientes energéticos. Os resultados indicaram que a construção dos biodigestores consumiu 490.179,59 MJ de energia, sendo que o componente energético mais expressivo no consumo energético foram as geomembranas flexíveis de PVC (29,33%), seguido pelas tubulações de PVC e lonas de impermeabilização com 28,45% e 21,06% da energia consumida, respectivamente. Considerando o volume útil dos biodigestores, o índice energético determinado foi de 225,89 MJ m-3.

Evaluation of the low technology tubular digesters in the production of biogas from slaughterhouse wastewater treatment

The aim of this research is to identify the operational conditions and performance of low cost tubular digesters producing biogas from the slaughterhouse wastewater treatment. Three full scale sequenced tubular digesters (of more than 8 m3 each), operating in a slaughterhouse in Cochabamba (Bolivia), have been monitored during one year. The Organic Load Rate (OLR) varied from 0.04 to 1.13 kgSV/m3d (0.04–1.07 kgCOD/m3d), and the Hydraulic Retention Time (HRT) from 3.2 to 87.4 days. The passive solar heating design considered drove the slurry temperature to 27 °C, 8 °C more than mean ambient temperature. The first digester has been filled with soda bottle plastic rings in order to increase the internal effective surface and evaluate, obtaining that an increment in 67% of the internal effective surface, more than doubles the biogas production and enables higher OLRs. A Biogas Production Rate (BPR) peak of 0.2 m3/m3d was observed for OLR = 0.37 kgSV/m3d, HRT = 9.7d and Specific Biogas Production (SBP) = 0.55 m3/kgSV. COD removal achieves values above 70% from HRT>19 d. A comparison with other anaerobic digestion technologies demonstrates that low cost tubular digesters with biofilm carriers, are competitive for OLR<0.5 kg COD/m3d of biogas production, and for 0.25 kgCOD/m3d, when the focus is on COD removal.

Does the use of tubular digesters to treat livestock waste lower the risk of infection from Cryptosporidium parvum and Giardia lamblia?

Worldwide, high incidences of cryptosporidiosis and giardiasis are attributed to livestock waste. Quantitative microbial risk assessment can be used to estimate the risk of livestock related infections from Cryptosporidium parvum and Giardia lamblia. The objective of this paper was to assess the occupational and public health risks associated with management of raw and anaerobically digested livestock waste in two rural communities in Costa Rica based on fomite, soil and crop contamination and livestock waste management exposure pathways. Risks related to cattle waste were greater than swine waste due to cattle shedding more (oo)cysts. Cryptosporidium parvum also posed a greater risk than Giardia lamblia in all exposure pathways due to livestock shedding high loads of Cryptosporidium parvum oocysts and oocysts' lower inactivation rates during anaerobic digestion compared with Giardia lamblia cysts. The risk of infection from exposure to contaminated soil and crops was significantly lower for a community using tubular anaerobic digesters to treat livestock waste compared to a community where the untreated waste was applied to soil. The results indicate that treatment of livestock waste in small-scale tubular anaerobic digesters has the potential to significantly decrease the risk of infection below the World Health Organization's acceptable individual annual risk of infection (10-4).

Viability and fate of Cryptosporidium parvum and Giardia lamblia in tubular anaerobic digesters

In many developing countries where pathogenic diseases of animal waste origin, such as giardiasis and cryptosporidiosis, are often prevalent, facilities are limited to treat livestock waste. However, household-scale anaerobic digesters are currently being promoted for bioenergy production from livestock manure. Since the effluent is often used as a fertilizer for food crops, it is critical to understand the effect of environmental conditions within household-scale digesters on the viability of Cryptosporidium parvum oocysts and Giardia lamblia cysts. In this study, key environmental parameters affecting (oo)cyst inactivation were measured in four tubular anaerobic digesters, which are a type of household-scale digester promoted for treatment of swine waste in rural Costa Rica. Interviews and participant observations were used to understand digester operation and maintenance procedures. Ambient temperatures (21–24°C), near-neutral pH, total ammonia nitrogen (TAN) concentrations<250mg/L and hydraulic retention times (HRTs) between 23 and 180days were observed. Laboratory (oo)cysts inactivation studies were performed in bench-scale digesters, which were maintained under conditions similar to those observed in the field. Apparent first-order inactivation rate coefficients for Giardia lamblia and Cryptosporidium parvum were 0.155±0.041 and 0.054±0.006day−1, respectively. Temperature and volatile fatty acids were the main factors contributing to Cryptosporidium parvum and Giardia lamblia inactivation. A mathematical model was developed that predicts the concentration of (oo)cysts in the liquid effluent of tubular digesters like those observed in Costa Rica. A mathematical model was developed that predicts the concentration of (oo)cysts in the liquid effluent of tubular digesters like those observed in Costa Rica. Two dimensionless groups can be used to predict the performance of the digesters for inactivating pathogens; both dimensionless groups depend upon the average HRT in the digester. This is the first study to combine mathematical modeling with qualitative analysis, field and laboratory studies to predict the concentrations of (oo)cysts in tubular digester effluents.

Study of Household Biodigesters

Tunnel or tubular type of bio-digester has been widely developed in many countries for producing a renewable clean fuel gas named biogas. This digester type only requires minimum maintenance and simple construction for ensuring its lower cost compared to others with similar capacity. Mostly, polyethylene (PE) plastic bag is used for construction materials of the tubular digesters. In this research, the tubular type has been constructed using large diameter PVC (Poly Vinyl Chloride) pipe for increasing the durability and safety. The performance of PVC digesters have been tested by measuring biogas production rate and CH concentration in the gas. COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand) drop between inlet and outlet sludge have been measured for examining the digester ef- ficiency. Then, the performance was compared with PE plastic digesters data from several literatures which still needs some improvements for enhancing its capability and performance.

Use of physical and biological process models to understand the performance of tubular anaerobic digesters

Tubular anaerobic digesters are used in developing countries to produce biogas from livestock waste. In this research, field measurements and physical and biological process modeling studies were used to investigate transport and transformation mechanisms for particulate and soluble organic matter in household-scale tubular digesters in the Monteverde region of Costa Rica. Greater than 75% removal of volatile solids and biochemical oxygen demand (BOD5) were observed. The high effluent quality was attributed to the formation of a biologically active floccular sludge layer, which allowed for separation of hydraulic and mean cell residence times (HRT and MCRT). A reduced order transport model was developed and validated using field tracer study data. Key assumptions of the reduced order model were verified via computational fluid dynamics (CFD) analysis. The mean HRT predicted by the reduced order model was 23 days and was in good agreement with the tracer experiment. A simplified floccular sludge biological process model was developed and used to estimate an average MCRT of 115 days. The results showed that household-scale tubular anaerobic digesters can provide enough biogas to meet households’ cooking energy needs, which was consistent with field results. This is the first study to combine mathematical modeling with field studies of tubular anaerobic digester performance.

How to report biogas production when monitoring small-scale digesters in field

The aim of this research was to evaluate the error originated when biogas production from field monitoring digesters, influenced by the diurnal temperature cycle, was normalized to standard conditions for pressure and temperature (273.15 K and 100 kPa) from local conditions. The biogas production data is often reported without indicating if done under local conditions, whether these conditions have been standardized and, if they have actually been standardized, the standard temperature and pressure is not indicated. In this research ambient and biogas temperature, as well as biogas production were monitored with a 30 min frequency during three consecutive days, in three different tubular digesters. Normalization was realized using the high frequency data collected as reference values, and also using daily biogas production with mean daily biogas, ambient and nearby meteorological station temperatures. The outcome of this research shows that normalization of biogas production can be obtained using daily biogas production and the daily mean ambient temperature with an overestimation by no more than 1.5%, in comparison to the normalization achieved by using high frequency data from biogas temperature and production. Using mean daily ambient temperature or mean daily biogas temperature results in the same overestimation, while using mean daily ambient temperature from a nearby airport weather station pushes the overestimation up to 2.7%. So, if ambient temperature and altitude is identified, biogas production reported in local conditions can be normalized.

The influence of users' behavior on biogas production from low cost tubular digesters: A technical and socio-cultural field analysis

The aim of this paper is to understand the influence of the user behavior on tubular digesters performance, through a technical and a social approach in the Bolivian context. Fifteen domestic digesters were evaluated, from which 6 were installed in the Altiplano and other 6 in the Andean Valleys. Data about slurry temperature, feedstock and biogas quality were collected from these 12 digesters, while daily biogas production and feeding pattern were also monitored from further three digesters in the valleys. Because of changes in user behavior along the monitoring period and particular characteristics of the digesters monitored, 5 complete patterns of biogas production and digester management were established. Furthermore, the results of a socio-cultural study with Andean families about the perception of poverty, their needs and the role played by digesters in their expectations in improving life quality, are correlated to the obtained technical data. The technical evaluation shows how the digester management seems to have a seasonal performance throughout the year according to the agricultural calendar. This means that families are more interested in using bioslurry in crops and agricultural improvements than in the use of biogas. The Bolivian government subsidy on liquefied gas seems to be one of the key issues to understand these results. Finally, data also reveals how the thermal behavior of tubular digesters adapted to cold climate that use a passive solar design, is similar to the thermal behavior of valley digesters, and therefore intends to add the same passive heating techniques for warm and tropical climates, to increase slurry temperatures and achieve a higher biogas production.

Cow, sheep and llama manure at psychrophilic anaerobic co-digestion with low cost tubular digesters in cold climate and high altitude

The aim of this research is to evaluate the co-digestion of cow and llama manure combined with sheep manure, in psychrophilic conditions and real field low cost tubular digesters adapted to cold climate. Four digesters were monitored in cold climate conditions; one fed with cow manure, a second one with llama manure, the third one with co-digestion of cow–sheep manure and the fourth one was fed with llama–sheep manure. The slurry had a mean temperature of 16.6°C, the organic load rate was 0.44kgvsm−3d−1 and the hydraulic retention time was 80days. After one hundred days biogas production was stable, as was the methane content and the pH of the effluent. The co-digestion of cow–sheep manure results in a biogas production increase of 100% compared to the mono-digestion of cow manure, while co-digestion of llama–sheep manure results in a decrease of 50% in biogas production with respect to mono-digestion of llama manure.

Improvement through low cost biofilm carrier in anaerobic tubular digestion in cold climate regions

The aim of this research is to evaluate the increase of biogas production with low cost tubular digesters in cold climates using PET rings inside the reactor. Two similar digesters have been operated and monitored in cold weather conditions and have been fed with cow manure. Digester 1 was filled with PET – rings as a biofilm carrier, Digester 2 was kept as a reference. Through the PET – rings the functional surface could be increased by a factor 4.2. The results show that 44% more biogas per KgSV has been produced with the biofilm carrier in use (0.33m3/kgSV) (reference digester −0.23m3/kgSV), at an organic load rate of 0.26kgSV/m3/d. The thermal performance shows that with an adaptation of the low cost tubular digester the slurry temperature can be raised up to 16.6°C (average) by surrounding temperature of 6.1°C (average) without using any active heating system.

Low cost tubular digesters as appropriate technology for widespread application: Results and lessons learned from Bolivia

This paper presents the results and lessons learned from four and a half years of implementing low cost tubular digesters in Bolivia. The selection of this technology is justified in comparison with other popular technologies such as fixed dome or floating drum digesters. The highlighted weakness of the tubular model (its short life expectancy), is transformed into a strength, making the low cost tubular digester an appropriate technology for widespread application. The experiences in Bolivia show that the success of biogas programs depend more on socio-economic factors than on the validated technology selected, suggesting that local circumstances are a critical, and often underestimated, factor to be taken into consideration in the praxis. Finally, some testimonies of the use of biol (bio-slurry or effluent) are reported, identifying the high potential of this anaerobic digestion product that provides a food sovereignty approach, reduced expansion of the agricultural frontier, increased agricultural productivity and hence family income, that other household energizing systems do not have. A brief report of lessons learned is also included.

Towards thermal design optimization of tubular digesters in cold climates: A heat transfer model

A cold climate, low cost, tubular digester is monitored and temperatures from different parts of the slurry, greenhouse, and adobe walls are presented, discussing the thermal performance of the digester. The slurry exhibits a vertical gradient of 6°C, with a mean value of 24.5°C, while the ambient temperature varies from 10°C to 30°C, showing the efficiency of the system as a solar heat collector with thermal inertia. A simple time-dependent thermal model is developed using inputs of solar radiation, wind velocity, ambient temperature, and digester geometry. The model outputs include temperatures of the slurry, the biogas, its holding membrane and the greenhouse air, wall and cover. Radiative, convective and conductive heat transfer phenomena are considered between all system elements. The model has 0.47°C (2%) standard error for the average slurry temperature. This model can be used to predict the influence of geometry and materials on the performance of the digester.

Design methodology for low cost tubular digesters

The aim of this paper is to present a novel, universal, methodology for the design of low cost tubular digesters. This method improves on the established methodology by avoiding assumptions that tend to reduce the final hydraulic retention time (HRT) of digesters once installed. This work recommends designing the digester using trench cross-sectional area and proposes an optimization of the trench dimensions with respect to the angle of the walls and the relationship between the length of the biogas bell and the top width of the trench. The influence of the biogas pressure is considered. A simple geometrical analysis is presented that, by parameterization, can be applied in a wide range of situations.

Codigestion of cow and guinea pig manure in low-cost tubular digesters at high altitude

The aim of this research was to improve the anaerobic digestion of cow and guinea pig manure in low-cost unheated tubular digesters implemented at high altitude, by comparing different operating conditions and codigesting both manures. Additionally, the effect of the greenhouse on process temperature was investigated. The study was carried out in a pilot plant with 3 full-scale digesters located at the Peruvian Andes (2800 m.a.s.l.). In terms of biogas production, the codigestion ( 0.08 m biogas 3 m digester − 3 d − 1 ) outperformed guinea pig manure digestion ( 0.03 m biogas 3 m digester − 3 d − 1 ), but did not improve cow manure digestion ( 0.12 m biogas 3 m digester − 3 d − 1 ), which may be due to the characteristics of guinea pig manure. Therefore, codigestion with other local organic wastes, such as cheese whey, should be explored. The temperature measured inside the digester greenhouse (15–60 °C) was always higher than ambient temperature (10–30 °C); however the liquor temperature remained fairly constant (around 20 °C), suggesting that the greenhouse effect on process temperature was doubtful. Taking into account that the greenhouse increases implementation costs, it would be worth comparing process performance with and without greenhouse in a future study.

Agricultural reuse of the digestate from low-cost tubular digesters in rural Andean communities

This research aimed at assessing the properties of guinea pig manure digestate from low-cost tubular digesters for crops fertilization in rural Andean communities. To this end, field trials were carried out to evaluate the effect of the digestate on two common Andean crops: potato ( Solanum tuberosum) and forage ( Lolium multiflorum and Trifolium pratense L.). The potato yield (20–25 t ha −1) increased by 27.5% with digestate, by 15.1% with pre-compost and by 10.3% with the mixture, compared to the control. The forage yield (20–21 t ha −1) increased by 1.4% with digestate – 50% dose, and by 8.8% with digestate – 100% dose and digestate – 150% dose, compared to the control. The results suggest that the digestate is an appropriate substitute of manure pre-compost for potato fertilization. The results with forage indicate that it can be applied in a range of doses, according to the amount produced by the digester. Currently, manure is either used for cooking or as fertilizer. With low-cost tubular digesters implementation, it could be used to feed the digester, using the digestate for crops fertilization and biogas for cooking; improving household living conditions and protecting the environment. Since soil properties in rural Andean communities differ from experimental layouts, the effect of fertilizers should be re-evaluated in-situ in future research studies.

Reduced hydraulic retention times in low-cost tubular digesters: Two issues

Low-cost tubular digesters have been implemented in several developing countries. One of the problems reported from field surveys is that biogas generation that does not meet the user’s expectations. This report provides two reasons for the discrepancy between the biogas generation rate estimated in the design phase and the actual rate measured after construction, due to a lower final hydraulic retention time (HRT). The hydraulic retention time is normally determined from the liquid volume calculated from the cylindrical shape of the bag and not from the trench dimensions. The result is a reduction in HRT of 6%–51%, depending on the dimensions of the trench recommended by various authors. Another factor that is not normally considered is the influence of the biogas pressure on the liquid level inside the digester which negatively affects the liquid volume of the digester, reducing HRT by as much as 15%.

Psychrophilic anaerobic digestion of guinea pig manure in low-cost tubular digesters at high altitude

Guinea pig is one of the most common livestock in rural communities of the Andes. The aim of this research was to study the anaerobic digestion of guinea pig manure in low-cost unheated tubular digesters at high altitude. To this end, the performance of two pilot digesters was monitored during 7 months; and two greenhouse designs were compared. In the dome roof digester the temperature and biogas production were significantly higher than in the shed roof digester. However, the biogas production rate was low (0.04 m biogas 3 m digester - 3 d −1), which is attributed to the low organic loading rate (0.6 kg VS m digester - 3 d −1) and temperature (23 °C) of the system, among other factors. In a preliminary fertilization study, the potato yield per hectare was increased by 100% using the effluent as biofertilizer. Improving manure management techniques, increasing the organic loading rate and co digesting other substrates may be considered to enhance the process.