Single-stage Digestion Research Articles

Kraft-mill biosolids treatment by conventional and biphasic fermentation

Kraft primary and mixed primary-activated sludges were gasified by thermophilic (50 °C) high-rate and mesophilic two-phase digestion. Kraft sludges were deficient in nitrogen, phosphorus, and other unidentified nutrients. Municipal sludge proved to be a much better nutrient source than inorganic salts. Performance of the mesophilic (35°C), two-phase anaerobic digestion (TPAD) system was about the same as that of thermophilic single-stage, high-rate digestion at a 10-day HRT as judged in terms of methane yield (0.17 SCM/kg VS added) and VS reduction efficiency (43%). The mesophilic TPAD process yielded a higher methane-content (65%) fuel gas; it is capable of exhibiting higher net-energy recovery efficiency than thermophilic single-stage digestion.

The anaerobic treatment of a ligno-cellulosic substrate offering little natural pH buffering capacity

The stability and operational performance of single stage digestion with and without liquor recycle and two stage digestion were assessed using a mixture of paper and wood as the digestion substrate. Attempts to maintain stable digestion in both single stage reactors were unsuccessful due to the inherently low natural buffering capacity exhibited; this resulted in a rapid souring of the reactor due to unbuffered volatile fatty acid (VFA) accumulation. The use of lime to control pH was unsatisfactory due to interference with the carbonate/bicarbonate equilibrium resulting in wide oscillations in the control parameter. The two stage system overcame the pH stability problems allowing stable operation for a period of 200 days without any requirement for pH control; this was attributed to the rapid flushing of VFA from the first stage reactor into the second stage, where efficient conversion to methane was established. Reactor performance was judged to be satisfactory with the breakdown of 53% of influent volatile solids. It was concluded that the reactor configuration of the two stage system offers the potential for the treatment of cellulosic wastes with a sub-optimal carbon to nitrogen ratio for conventional digestion.

The Release of Heavy Metals from Digested Sludge by Centrifugation

The effect of centrifugal shear on the quality of the centrates produced from both mesophilic and thermophilic sludges was examined. The sludges were obtained from two single-stage digesters and from a thermophilic/mesophilic dual digestion system. Three centrifugal forces were used; 1000, 2000 and 3000 g. Increasing the magnitude of the centrifugal force had a slight effect on the concentration of ammoniacal-nitrogen in the centrates. It also caused a decrease in the COD concentrations and an increase in the metal (Ni, Cu, Zn) concentrations. Although there were differences between the single-stage and dual digestion systems, the quality of centrates produced from thermophilic sludges was worse than those from mesophilic sludges. The data for metals showed that nickel was different from the other two metals in that it was less tightly bound. It also showed that, although the concentrations produced were less than those frequently encountered in municipal sewage, they were of a sufficient magnitude to have the potential for the inhibition of side-stream bio-oxidation.

The anaerobic treatment of a ligno-cellulosic substrate offering little natural pH buffering capacity

The stability and operational performance of single stage digestion with and without liquor recycle and two stage digestion were assessed using a mixture of paper and wood as the digestion substrate. Attempts to maintain stable digestion in both single stage reactors were unsuccessful due to the inherently low natural buffering capacity exhibited; this resulted in a rapid souring of the reactor due to unbuffered volatile fatty acid (VFA) accumulation. The use of lime to control pH was unsatisfactory due to interference with the carbonate/bicarbonate equilibrium resulting in wide oscillations in the control parameter. The two stage system overcame the pH stability problems allowing stable operation for a period of 200 days without any requirement for pH control; this was attributed to the rapid flushing of VFA from the first stage reactor into the second stage, where efficient conversion to methane was established. Reactor performance was judged to be satisfactory with the breakdown of 53% of influent volatile solids. It was concluded that the reactor configuration of the two stage system offers the potential for the treatment of cellulosic wastes with a sub-optimal carbon to nitrogen ratio for conventional digestion.

Biomethanation of tobacco waste

Tobacco is an important cash crop produced in considerable quantities in India. During cigarette manufacturing large quantities of tobacco waste are produced annually and its disposal is a serious ecological problem. In the present study a process for the biodegradation of tobacco waste, under methanogenic conditions, was developed so that environmental pollution can be reduced while producing biogas as a useful energy source. The methanogenic bacterial consortium developed for the anaerobic degradation of tobacco waste without any chemical pretreatment in a single stage digester at 15 days HRT (hydraulic retention time) has been successfully scaled-up to install a 10 m 3 biogas plant. The gas yields (1 kg −1 TS fed day −1) varied from 169 to 282 depending on the ambient temperatures. The methane content in the biogas produced was 60%. The pH and VFA content of the digested slurry were 6·8 ± 0·1 and 599 mg litre −1, respectively. Nicotine, COD and BOD reductions were 75, 60 and 80%, respectively.

Increased Stability of Anaerobic Digestion by Controlled Recirculation

The treatment characteristics of municipal sludge were investigated by the anaerobic activated sludge digestion (AASD) system. This study used the suspended growth system and mesophilic temperature in the digestors and separators; the system achieves a more stable and improved process; such a process configuration offers the possibility of a substantial reduction in the total volume necessary for efficient stabilization. This study presents data indicating that the AASS system is feasible. In general, with an applied solids concentration of TS= 2%, the nonbiodegradable portion of the substrate concentration contained in the primary and secondary sludge was found to be 40.6% and 35.1% on the basis or TVS and COD, respectively. This study also provides evidence that the reactions at a recycling ratio of R=1 and R=3 are considerably more stable than those achieved in conventional or other recycling ratio digestors with a HRT of 9 days or longer. The gas production and bioactivity is also higher than that normally produced by the conventional single-stage digestion system. The experimental results also indicate that the dilution rate exceeds the maximum specific growth rate as the HRT is decreased from 9 days to 6 days. The significant saving in reactor volume and enhanced methane generation should offset the energy required for digested sludge recycling.

USING A CELLULOLYTIC COMPLEX TO ESTIMATE IN VITRO DIGESTIBILITY OF BIRDSFOOT TREFOIL

Digesting forages with a prepared cellulase solution (PCS) has been shown to be a rapid method of obtaining dry matter digestibility (DMD) estimates in many species. This procedure has not been previously used to obtain DMD estimates of broadleaf birdsfoot trefoil (Lotus corniculatus L.). A broad based population consisting of 19 accessions, representing 11 countries of origin, three germplasm pools, and two domesticated cultivars of birdsfoot trefoil was evaluated. The PCS was produced from culturing Trichoderma reesei Simmons culture number QM9414. Fiber and PCS determinations were compared to an in vitro rumen fermentation (IVDMD) procedure. The fiber components measured were; neutral detergent fiber (NDF), acid detergent fiber (ADF), and hemicellulose (HEMI). The PCS procedures involved digestion using a PCS without any pre- or post-treatment (CS), acid-pepsin digestion followed by a single-stage digestion in PCS (APCS), acid detergent solution (ADS) followed by a two-stage digestion in PCS (ADSCS), and neutral detergent solution (NDS) followed by a two-stage digestion in PCS (NDSCS). All of the fiber and PCS procedures were significantly (P ≤ 0.001) correlated with IVDMD except HEMI. The CS procedure underestimated IVDMD by 199 g kg−1; however, it had the best regression coefficient (b = 0.99), had the second best correlation coefficient (r = 0.86), and required less time than the other PCS procedures. The APCS procedure provided the best estimate of IVDMD. It overestimated the IVDMD mean of birdsfoot trefoil by only 30 g kg−1 and was the most accurate (residual standard deviation (RSD) = 22 g kg−1) approach. The CS procedure is recommended when a ranking of samples is desired and actual values of DMD are not of primary interest.Key words: Fiber digestion, germplasm, in vitro dry matter digestibility, Lotus corniculatus L., Trichodenna reesei Simmons

In situ methane enrichment in anaerobic digestion

A major cost consideration in the use of anaerobic digestion to convert biomass and waste to utility-grade gas is the expense of separating CO(2) from the product gas. Anaerobic digestion has a number of inherent properties that can be exploited to increase the methane content of the gas directly produced by the digester, the most important of which is the high solubility of CO(2)(40-60 times that of methane) in water under digestion conditions. The methane enrichment concept examined in this study involved the recirculation of a liquid stream from the digester through a CO(2) desorption process and the return of the liquid stream back to the digester for absorption of additional CO(2) produced by the conversion of organic materials. A steady-state equilibrium model predicted that a digester gas methane content exceeding 94% could be achieved with this scheme using modest recirculation rates provided a desorption process could be designed to achieve a 60+% CO(2) removal efficiency in the degassing of the liquid recycle stream. Using fixed-film laboratory digesters operated on synthetic feedstocks, the technique of methane enrichment was tested under pressurized and unpressurized conditions. A 93 + 2% methane gas stream was produced from a volatile-acid-fed bench-scale digester simulating the methanogenic stage of two-phase digestion under conditions of (1) a pH swing achieved without caustic addition that allowed digestion at pH 7. 5 and air stripping at pH 6. 5-7. 0, (2) digester pressurization to 30 psig, and (3) a recycle rate of 0. 33 L/L reactor/day. Significant but lower levels of methane enrichment were achieved with the single-stage digester at the low experimental recycle rate. However, the narrow range among all experiments of CO(2) desorption efficiencies achieved in air stripping the recycle stream (35-60% CO(2) removal) suggests that comparable methane enrichment-may be achieved with unpressurized single-stage digestion using greater recycle rates. A materials balance analysis of data from an unpressurized, single-stage digester employing no chemical addition and using laboratory degassing efficiencies indicated that 94% methane could be produced at recycle rates of less than 1. 4 L/L reactor/day with a methane loss of less than 2%.

Anaerobic digestion of native cellulosic wastes

Anaerobic digestion is an extensively used bioconversion process to produce gaseous fuel from native lignocellulosic materials. It consists essentially of two steps; acidogenesis and methanogenesis. Most conventional anaerobic digesters are single-stage systems. Animal waste, agricultural residues, sewage sludge and industrial effluents are suitable as feedstock. Biogas productivity in single stage digestion ranges from 0.5–1.5 m3/m3/day at mesophilic (20–40°C) and 1.0–2.5 m3/m3/day at thermophilic (40–60°C) temperatures, and about 30–50% of the volatile solids are converted to biogas. In two-stage systems, acidogenesis is separated from methanogenesis, which improves cellulose hydrolysis and process efficiency. Recent advances in digester design include the introduction of the upflow anaerobic sludge blanket, and fluidized-bed and fixed-film bioreactors, which are operated at much higher loading. Process efficiency can be as high as 97%. An overview of these technologies is presented.

Heat Treatment and Anaerobic Digestion of Refuse

Alkaline heat treatment was evaluated as a pretreatment process for increasing the anaerobic digestibility of municipal refuse. The goal was an increased methane yield from refuse lignocellulose. Batch digestibility assays were employed to ascertain the separate effects of solids concentration (0.79 - 10.0 percent), sodium hydroxide concentration (75 - 300 meq/l), and temperature (25 - 250°C) on the methane yield from refuse. Consumption of added alkali was observed and found to be relatively constant per unit mass of refuse solids. Various semi-continuous reactor configurations were evaluated. These studies resulted in investigation of a two-stage digestion system, with the second digester fed alkaline-heat-treated (180 meq/l NaOH, 200°C, 2.1 percent solids, 1 hour) effluent from the first. Such a system operated on newsprint was capable of increasing the methane yield by 80 percent, compared with the yield from a single-stage digester fed raw newsprint. A major limitation appears to be digester inhibition noticeable whenever the concentration of heat-treated lignin products exceeds 1 g/l.

Multiple staged methane recovery from solid wastes

Abstract Multiple staging of anaerobic digestion has been investigated as a means of optimizing the bioconversion of solid waste to methane fuel gas. Stage separation was achieved by pH control of the acid production stage. The results obtained from a laboratory‐scale experimental program are promising. The acidogenic stage was operable at 22°C without sacrifice of methane productivity. There was essentially no difference between the gross energy conversion efficiency of the single vs. multiple stage systems. The methane production stage off‐gases were enriched for methane with a partial pressure averaging 0.8. Multiple staging provided greater stability than did the single stage digesters.