On-line monitoring of a two-stage anaerobic digestion process using a BOD analyzer

A computer-controlled biochemical oxygen demand (BOD) analyzer has been developed for fast estimation of biochemical oxygen demand (BODst) automatically with the purpose of on-line monitoring of a process for conversion of biomass under field conditions. The instrument was tested by on-line monitoring of the connecting stream between two stages of a two-stage anaerobic process in laboratory scale. In the first stage, hydrolysis of sugar beet leaves and its conversion into volatile fatty acids and other low molecular weight substrates took place. The effluent from the first reactor was used as a feed stream to the second stage, i.e. an anaerobic contact reactor. The feed stream was sampled intermittently, diluted and analyzed by the BOD analyzer automatically in order to estimate the organic loading rate to the reactor. The results from this study demonstrated that the BOD analyzer could be a stand-alone and promising sensor device for rapid on-line monitoring of easily biodegradable organic substances in biological treatment processes.     

Mathematical modeling of non-ideal mixing continuous flow reactors for anaerobic digestion of cattle manure

Most conventional digesters used for animal wastewater treatment include continuously stirred-tank reactors. While imperfect mixing patterns are more common than ideal ones in real reactors, anaerobic digestion models often assume complete mixing conditions. Therefore, their applicability appears to be limited. In this study, a mathematical model for anaerobic digestion of cattle manure was developed to describe the dynamic behavior of non-ideal mixing continuous flow reactors. The microbial kinetic model includes an enzymatic hydrolysis step and four microbial growth steps, together with the effects of substrate inhibition, pH and thermodynamic considerations. The biokinetic expressions were linked to a simple two-region liquid mixing model, which considered the reactor volume in two separate sections, the flow-through and the retention regions. Deviations from an ideal completely mixed regime were represented by changing the relative volume of the flow-through region (a) and the ratio of the internal exchange flow rate to the feed flow rate (b). The effects of the hydraulic retention time, the composition of feed, the initial conditions of the reactor and the degree of mixing on process performance can be evaluated by the dynamic model. The simulation results under different conditions showed that deviations from the ideal mixing regime decreased the methane yield and resulted in a reduced performance of the anaerobic reactors. The evaluation of the impact of the characteristic mixing parameters (a) and (b) on the anaerobic digestion of cattle manure showed that both liquid mixing parameters had significant effects on reactor performance.     

High-Rate two-phase process for the anaerobic degradation of cellulose, employing rumen microorganisms for an efficient acidogenesis

A novel two-stage anaerobic process for the microbial conversion of cellulose into biogas has been developed. In the first phase, a mixed population of rumen bacteria and ciliates was used in the hydrolysis and fermentation of cellulose. The volatile fatty acids (VFA) produced in this acidogenic reactor were subsequently converted into biogas in a UASB-type methanogenic reactor.A stepwise increase of the loading rate from 11.9 to 25.8 g volatile solids/L reactor volume/day (g VS/L/day) did not affect the degradation efficiency in the acidogenic reactor, whereas the methanogenic reactor appeared to be overloaded at the highest loading rate. Cellulose digestion was almost complete at all loading rates applied. The two-stage anaerobic process was also tested with a closed fluid circuit. In this instance total methane production was 0.438 L CH(4)g VS added, which is equivalent to 98% of the theoretical value. The application of rumen microorganisms in combination with a high-rate methane reactor is proposed as a means of efficient anaerobic degradation of cellulosic residues to methane. Because this newly developed two-phase system is based on processes and microorganisms from the ruminant, it will be referred to as "Rumen Derived Anaerobic Digestion" (RUDAD-) process.     

A hybrid anaerobic solid-liquid bioreactor for food waste digestion

A hybrid anaerobic solid-liquid (HASL) bioreactor is an enhanced two-phase anaerobic system, that consists of a solid waste reactor as the acidification reactor and a wastewater reactor, i.e. an upflow anaerobic sludge blanket (UASB) reactor as the methanogenic reactor. Food waste digestion in HASL bioreactors with pre-acidification and HASL operation stages was investigated in two separate runs. After 8 days of pre-acidification in Run A and 4 days in Run B, total volatile fatty acid (TVFA) and chemical oxygen demand (COD) concentrations in the leachates of both acidification reactors were similar. During HASL operation stage, TVFA and COD removal in the methanogenic phase were 77–100% and 75–95%, respectively. Some 99% of the total methane generated was from the methanogenic phase with a content of 68–70% methane. At the end of operation, about 59–60% of the added volatile solids (VS) were removed with a methane yield of 0.25 l g^–1 VS.     

Anaerobic membrane reactor with phase separation for the treatment of cheese whey

Two-phase anaerobic digestion of cheese whey was investigated in a system consisting of a stirred acidogenic reactor followed by a stirred methanogenic reactor, the latter being coupled to a membrane filtration system to enable removal of soluble effluent whilst retaining solids. The acidogenic reactor was operated at a hydraulic retention time (HRT) of one day, giving maximum acidification of 52.25% with up to 5 g/l volatile fatty acids, of which 63.7% was acetic acid and 24.7% was propionic acid. The methanogenic reactor received an organic load up to 19.78 g COD/ld, corresponding to a HRT of 4 days, at which 79% CODs and 83% BOD(5) removal efficiencies were obtained. Average removals of COD, BOD(5) and TSS in the two-phase anaerobic digestion process were 98.5%, 99% and 100%, respectively. The daily biogas production exceeded 10 times reactor volume and biogas methane content was greater than 70%.     

Studies on reaction techniques concerning reactor design for the anaerobic degradation of complex substrates with the example of the methanation of effluents in the fermentation industry

Two fixed-bed loop reactors were used to evaluate singleand separated-phase anaerobic treatments of a high strength waste-water from ethanol fermentation. The one-phase system consisted of an anaerobic fixed-bed loop reactor containing both acidogenic as well as methanogenic populations allowing a complete conversion of the carbon source into gaseous end products and biomass.The two-phase system consisted of a second fixed-bed loop reactor operated as a methanogenic unit, which was proceeded by a CSTR for acidification, both connected in series allowing sequential acidogenesis and methanogenesis of the organic components. The reactors were operated under steady state and variable process conditions. By gradually increasing the feed supply in both systems, maximum turnover of COD was determined.The separated-phase system consistently gave a better quality effluent with lower suspended solids and total COD. Maximum loading rates and COD elimination of the methanogenic phase of the two-phase system was over two times higher than that of the one-phase system. Process stability was also higher.On overloading the methane reactor of the two phase system accumulation of different fatty acids within the reactor was observed. Hydrogen concentration in the biogas can be used as a reliable indicator for system overloadings. At least, continuous online monitoring of hydrogen in the methanogenic reactor gas should provide a convenient alternative to other analyses for process control.     

Methane production using whole and screened dairy manure in conventional and fixed-film reactors

The technical feasibility of adopting the fixed-film reactor concept for biogas production from screened dairy manure was investigated. The methane production capability of laboratory-scale 4-L anaerobic reactors (conventional and fixed-film) receiving screened dairy manure and operated at 35 degrees C was compared. Dairy manure filtrate with 4.4% total solids (TS) and 3.4% volatile solids (VS) (average value) was prepared from 1:1 manure-water slurry. The feed material was added intermittently at loading rates ranging from 2.34 to 25 and 2.25 to 785 g VS/L d, respectively, for the conventional and fixed-film reactors. Maximum methane production rate (L CH(4)/L d) for the conventional reactor was 0.63 L CH(4)/L d achieved at a 6-day hydraulic retention time (HRT). For the fixed-film reactor the maximum production rate was 3.53 L CH(4)/L d when operated at a loading rate of 262 g VS/L d (3 h HRT). The fixed-film reactor was capable of sustaining a loading of 785 g VS/L d (1 h HRT). The fixed-film reactor performed much better than the conventional reactors. These results indicate that a large reduction of required reactor volume is possible through application of a fixed-film concept combined with a liquid-solid separation pretreatment of dairy manure.     

Impact of leachate recirculation and recirculation volume on stabilization of municipal solid wastes in simulated anaerobic bioreactors

The effects of leachate recirculation and the recirculation rate on the anaerobic treatment of domestic solid waste was investigated in three simulated landfill anaerobic bioreactors. A single pass reactor was operated without leachate recirculation while the other two reactors were operated with leachate recirculation. The leachate recirculation rate was 9 l/day (13% of the reactor volume) in Reactor₉, while the recirculation rate was 21 l/day (30% of the reactor volume), in Reactor₂₁. pH, chemical oxygen demand (COD), volatile fatty acids (VFA), ammonium–nitrogen (NH₄–N) total and methane gas measurements in leachate samples were regularly monitored. After 220 days of anaerobic incubation, it was observed that the pH, COD, VFA concentrations, methane gas productions and methane percentages in Reactor₉ were better than the single pass reactor and Reactor₂₁. When the leachate recirculation rate was increased to three times a decrease in pH, and an increase in VFA and COD concentrations were observed in Reactor₂₁. The COD values were measured as 47 000, 39 000 and 52 000 mg/l while the VFA concentrations were 15 000, 13 000 and 21 000 mg/l, respectively, in single pass, Reactor₉ and Reactor₂₁ after 220 days of anaerobic incubation. The values of pH were 5.89, 6.44 and 6.16, respectively, after anaerobic incubation. The mean methane percentages of single pass reactor, Reactor₉ and Reactor₂₁ were 30, 50 and 40%, respectively, after 50 days of incubation. Leachate recirculation reduced the waste stabilization time and was effective in enhancing methane gas production and improving leachate. However, leachate recirculation was not effective in removing ammonia from the leachate. The amounts of COD recovered by methane were 62.9, 162.3 and 94.6 g for single pass, Reactor₉ and Reactor₂₁, respectively, at the end of 220 days of anaerobic incubation.     

Hydrogen and methane production from desugared molasses using a two‐stage thermophilic anaerobic process

Hydrogen and methane production from desugared molasses by a two‐stage thermophilic anaerobic process was investigated in a series of two up‐flow anaerobic sludge blanket (UASB) reactors. The first reactor that was dominated with hydrogen‐producing bacteria of Thermoanaerobacterium thermosaccharolyticum and Thermoanaerobacterium aciditolerans could generate a high hydrogen production rate of 5600 mL H₂/day/L, corresponding to a yield of 132 mL H₂/g volatile solid (VS). The effluent from the hydrogen reactor was further converted to methane in the second reactor with the optimal production rate of 3380 mL CH₄/day/L, corresponding to a yield of 239 mL CH₄/g VS. Aceticlastic Methanosarcina mazei was the dominant methanogen in the methanogenesis stage. This work demonstrates that biohydrogen production can be very efficiently coupled with a subsequent step of methane production using desugared molasses. Furthermore, the mixed gas with a volumetric content of 16.5% H_2, 38.7% CO₂, and 44.8% CH₄, containing approximately 15% energy by hydrogen is viable to be bio‐hythane.     

The effects of digestion temperature and temperature shock on the biogas yields from the mesophilic anaerobic digestion of swine manure

In order to obtain basic design criteria for anaerobic digesters of swine manure, the effects of different digesting temperatures, temperature shocks and feed loads, on the biogas yields and methane content were evaluated. The digester temperatures were set at 25, 30 and 35 degrees C, with four feed loads of 5%, 10%, 20% and 40% (feed volume/digester volume). At a temperature of 30 degrees C, the methane yield was reduced by only 3% compared to 35 degrees C, while a 17.4% reduction was observed when the digestion was performed at 25 degrees C. Ultimate methane yields of 327, 389 and 403 mL CH(4)/g VS(added) were obtained at 25, 30 and 35 degrees C, respectively; with moderate feed loads from 5% to 20% (V/V). From the elemental analysis of swine manure, the theoretical biogas and methane yields at standard temperature and pressure were 1.12L biogas/g VS(destroyed) and 0.724 L CH(4)/g VS(destroyed), respectively. Also, the methane content increased with increasing digestion temperatures, but only to a small degree. Temperature shocks from 35 to 30 degrees C and again from 30 to 32 degrees C led to a decrease in the biogas production rate, but it rapidly resumed the value of the control reactor. In addition, no lasting damage was observed for the digestion performance, once it had recovered.     

Eliminating methanogenic activity in hydrogen reactor to improve biogas production in a two-stage anaerobic digestion process co-digesting municipal food waste and sewage sludge

Laboratory scale two-stage anaerobic digestion process model was operated for 280 days to investigate the feasibility to produce both hydrogen and methane from a mixture feedstock (1:1 (v/v)) of municipal food waste and sewage sludge. The maximum hydrogen and methane yields obtained in the two stages were 0.93 and 9.5 mL/mL feedstock. To eliminate methanogenic activity and obtain substantial hydrogen production in the hydrogen reactor, both feedstock and mixed liquor required treatment. The heat treatment (100 °C, 10 min) for feedstock and a periodical treatment (every 2-5 weeks, either heating, removal of biomass particles or flushing with air) for mixed liquor were effective in different extent. The methane production in the second stage was significantly improved by the hydrogen production in the first stage. The maximum methane production obtained in the period of high hydrogen production was more than 2-fold of that observed in the low hydrogen production period.     

Batch and continuous biogas production from grass silage liquor

Herein batch and continuous mesophilic anaerobic digestion of grass silage liquor was studied. The continuous process was carried out in Armfield digesters with an OLR ranging from 0.851 to 1.77 kg COD m⁻³ day⁻¹. The effect of recirculation of effluent from the digester was investigated using different OLRs of grass silage liquor feed. These results showed that as the OLR increased, the methane yield decreased for the reactor with no recycle and increased for the reactor with recycle. However, the COD removal for both digesters was nearly the same at the same OLR. Overall these studies show that grass silage liquor can produce a high quality methane steam between 70% and 80% and achieve methane yields of 0.385 m³ kg⁻¹ COD.     

Digestion of sand-laden manure slurry in an upflow anaerobic solids removal (UASR) digester

Studies on the performance of a laboratory scale upflow anaerobic solids removal (UASR) digester were carried out using sand-laden cow manure slurries having total solids (TS) concentration as 50 and 100 g/l. Hydraulic retention time (HRT) was maintained as 32.4 days, which resulted in the volatile solids (VS) loading rates of 1 and 1.64 g/l d. The UASR system was designed to remove sand from the manure slurry, while anaerobically digesting biodegradable solids inside a single reactor. To enhance the contact of microorganisms and substrate, the liquor from the top of the digester was recirculated through the bed of settled solids at its bottom. Volatile solids reduction through this process was observed to be 62% and 68% in the case of feed slurries having TS concentration as 50 and 100 g/l (referred in the text as 5% and 10% feed slurries), respectively. The methane production rates were observed to be 0.22 and 0.38 l/l d, while methane yield was 0.21 and 0.27 l CH₄/g VS loaded, for 5% and 10% feed slurries, respectively. This indicates that the increase in the VS loading had a positive impact on methane production rate and methane yield. It would be of interest to study the performance of a UASR digester at higher solids loadings and with longer solids retention times. Nonetheless, the presented study showed that sand-laden manure slurries can be successfully digested in a UASR digester producing methane energy equivalent to 4 kW h per m³ of digester volume per day.     

Enhancement on biodegradation and anaerobic digestion efficiency of activated sludge using a dual irradiation process

A dual irradiation process involving aerobic thermophilic irradiation pretreatment (ATIP) and intermittent irradiation anaerobic digestion was developed to improve the digestion of waste-activated sludge. First, the effect of ATIP on further anaerobic digestion of activated sludge in batch mode was investigated. When exposed to ATIP for 24 h, the digestion reactor gave the highest methane yield, removed the most dissolved organic carbon (DOC) and showed the most effective reduction of VS compared to other irradiation times. This process was further enhanced by using an anaerobic fluidised-bed reactor packed with carbon felt in semi-continuous mode for digesting the pretreated activated sludge under intermittent irradiation conditions. Dual irradiation for 24 h followed by 60 min of anaerobic irradiation processing per day turned out to be optimal. This resulted in 65.3% of VS reduction, 83.9% of DOC removal ratio and 538 ml/g-VS of methane yield.     

Anaerobic digestion of grass silage in batch leach bed processes for methane production

Anaerobic digestion of grass silage in batch leach bed reactors, with and without a second stage upflow anaerobic sludge blanket (UASB) reactor, was evaluated. Sixty six percent of the methane potential in grass was obtained within the 55 days solids retention time in the leach bed-UASB process without pH adjustment, whereas in the one-stage leach bed process 20% of the methane potential in grass was extracted. In two-stage operation, adjustment of the pH of influent to the leach bed reactor to 6 with HCl led to inhibition of both hydrolysis/acidogenesis and methanogenesis. In the leach bed-UASB process 39% of the carbohydrates and 58% of the acid soluble lignin were solubilised within the 49 days of operation, whereas Klason lignin was most recalcitrant. The methane potential of the digestates varied from 0.141 to 0.204 m3 CH4 kg(-1) added volatile solids.     

Biogas production from potato-juice, a by-product from potato-starch processing, in upflow anaerobic sludge blanket (UASB) and expanded granular sludge bed (EGSB) reactors

In this study, the utilization of potato-juice, the organic by-product from potato-starch processing, for biogas production was investigated in batch assay and in high rate anaerobic reactors. The maximum methane potential of the potato-juice determined by batch assay was 470 mL-CH₄/gVS-added. Anaerobic digestion of potato-juice in an EGSB reactor could obtain a methane yield of 380 mL-CH₄/gVS-added at the organic loading rate of 3.2 gCOD/(L-reactor.d). In a UASB reactor, higher organic loading rate of 5.1 gCOD/(L-reactor.d) could be tolerated, however, it resulted in a lower methane yield of 240 mL-CH₄/gVS-added. The treatment of reactor effluent was also investigated. By acidification with sulfuric acid to pH lower than 5, almost 100% of the ammonia content in the effluent could be retained during the successive up-concentration process step. The reactor effluent could be up-concentrated by evaporation to minimize its volume, and later be utilized as fertilizer.     

Performance of the anaerobic baffled reactor under steady-state and shock loading conditions

The anaerobic baffled reactor (ABR) contains a granulated, mixed anaerobic culture segregated into compartments. Operation of four reactors under a range of hydraulic retention times showed that this novel reactor design offers highly efficient performance in the conversion of carbon in the feed stream to methane and carbon dioxide. The design parameter varied was the number of compartments. COD removal at 20 h retention time was routinely over 95% in all reactors, with low washout of biomass. Very high specific reaction rates were achievable (although with a loss of efficiency) at low biomass concentrations and high loading rates. In order to optimize volumetric reaction rates, a tradeoff has to be made between high biomass concentration, granule size, and the resulting mass transfer limitations. Formate is shown to be an important intermediate in the process under conditions of high loading.     

Anaerobic treatment of winery wastewater using laboratory-scale multi- and single-fed filters at ambient temperatures

A lab-scale investigation was conducted to examine the effectiveness of a multi-fed upflow anaerobic filter process for the methane production from a rice winery effluent at ambient temperatures. The experiment was carried in two identical 3.0-l upflow filters, a single-fed reactor and a multi-fed reactor. The results showed that the multi-fed reactor, operated at the ambient temperatures of 19–27 °C and influent chemical oxygen demand (COD) of 8.34–25.76 g/l, could remove over 82% of COD even at an organic loading rate (OLR) of 37.68 g-COD/l d and a short hydraulic retention time (HRT) of 8 h. This reactor produced biogas with a methane yield of 0.30–0.35 l-CH₄/g-COD_removed. The multi-fed upflow anaerobic filter was proved to be more efficient than the single-fed reactor in terms of COD removal efficiency and stability against hydraulic loading shocks. A linear-regression model with influent COD concentration and HRT terms adequately described the multi-fed upflow anaerobic filter system for the treatment of rice winery wastewater at ambient temperatures.     

Hydrogen and methane production through two-stage mesophilic anaerobic digestion of olive pulp

The present study focused on the anaerobic biohydrogen production from olive pulp (two phase olive mill wastes, TPOMW) and the subsequent anaerobic treatment of the effluent for methane production under mesophilic conditions in a two-stage process. Biohydrogen production from water-diluted (1:4) olive pulp was investigated at hydraulic retention times (HRT) of 30 h, 14.5 h and 7.5 h while methane production from the effluent of hydrogenogenic reactor was studied at 20 d, 15 d, 10 d and 5 d HRT. In comparison with previous studies, it has been shown that the thermophilic hydrogen production process was more efficient than the mesophilic one in both hydrogen production rate and yield. The methanogenic reactor was successfully operated at 20, 15 and 10 days HRT while it failed when an HRT of 5 days was applied. Methane productivity reached the maximum value of 1.13 ± 0.08 L/L/d at 10 days HRT whereas the methane yield increased with the HRT. The Anaerobic Digestion Model no. 1 (ADM1) was applied to the obtained experimental data from the methanogenic reactor to simulate the digester response at all HRT tested. The ability of the model to predict the experimental results was evident even in the case of the process failure, thus implying that the ADM1 could be a valuable tool for process design even in the case of a complex feedstock. In general, the two-stage anaerobic digestion proved to be a stable, reliable and effective process for energy recovery and stabilization treatment of olive pulp.