Final Report: Conversion of Paper Sludge to Ethanol

EPA Grant Number: R829479C006
Subproject: this is subproject number 006 , established and managed by the Center Director under grant R829479
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program
Center Director: Schumacher, Dorin
Title: Conversion of Paper Sludge to Ethanol
Investigators: Lynd, Lee
Institution: Dartmouth College
EPA Project Officer: Lasat, Mitch
Project Period: July 1, 2002 through June 30, 2004
RFA: The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program (2001) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Targeted Research

Objective:

Paper sludge is the largest solid waste stream produced by the pulp and paper industry and also is an attractive feedstock for emergent technologies based on processing of cellulosic biomass featuring enzymatic hydrolysis. The objective of this research project is to investigate the conversion of paper sludge to ethanol under industrially relevant conditions. The project had two specific objectives: (1) develop the process of converting paper sludge to ethanol including reactor design and improvement, process optimization, and test new microorganisms for glucose and xylose conversion; and (2) conduct the process design and evaluate the technical and economic viability of converting paper sludge to ethanol in full commercial scale.

Summary/Accomplishments (Outputs/Outcomes):

Improvement on Analytical Method

The analytic method for paper sludge composition that we used before was the standard two-step acid hydrolysis process called quantitative saccharification (quant-sacch). In the first step, the dry sample was hydrolyzed at 30°C in 72 percent sulfuric acid for 2 hours, and followed by diluted acid hydrolysis in 3 percent sulfuric acid at 121°C for 1 hour. The supernatant was analyzed for ethanol and soluble sugars by high performance liquid chromatography. The insoluble part was heated at 500°C. The vanished part was called the acid insoluble volatile matter, and the part that stayed was called the acid insoluble mineral. We found that the acid insoluble volatile part was always higher than the acid insoluble lignin the sludge could have contained. What was the part, other than acid insoluble lignin, in the acid volatile matter? This question had to be answered to achieve the material balance.

We improved our analytical method. When we conducted a second quant-sacch on the acid insoluble part of the first quan-sacch, we found more cellulose detected in that solid portion. If we only did quant-sacch once, that part of the cellulose would be mistakenly counted for “acid insoluble volatile matter.” Part of the acid insoluble volatile parts was unhydrolyzed cellulose from our first two-step acid hydrolysis process. Table 1 presents the compositional analysis of the same paper sludge samples using two different analytical methods. We are able to account for 96.5 percent of the substance in material balance.

Table 1. Paper Sludge Composition

Composition

Old Method
(%)

New Method
(%)

Glucan

57.0

62.0

Xylan

11.5

11.5

Mannan

2.5

2.5

Acid Soluble Lignin

0.5

0.5

Acid Insoluble Volatile Matter

8.0

3.0

Acid Soluble Mineral

10.0

10.0

Acid Insoluble Mineral

7.0

7.0

Subtotal

96.5

96.5

Redesign the Reactor

A semi-continuous reactor system that consists of a sludge feeder and fermentor has been developed in our laboratory. As indicated in Figure 1, the reactor system consists of a 4-inch diameter feed tube with a piston to advance a solid plug of paper sludge into a rotary “chopper,” which causes the advancing paper sludge to fall into a 800 mL working volume jacket stainless steel fermentor. The system is equipped with 2-inch ball valves and steam fittings so that the feed tube can be changed by isolating the fermentor, adding a full fed tube, and in-place sterilization of the feed tube-reactor junction by direct steam sterilization. A programmed logic controller was used to control the advance of the feed tube piston, the chopper, and sampling valves that allow periodic removal of the reactor content. A run with ethanol that produced 30 g/L up to 4 months was accomplished in this reactor.

Figure 1. The Reactor With the Original Design

The original designed reactor also had some operational difficulties. The common mechanical failure of our old reactor system has been solids accumulating and eventually plugging in the chopper assembly that received sludge from the advancing horizontal paper sludge plug. The problem appeared to be the diameter reduction at the 2-inch internal isolation valve used to isolate the reactor during changing of the horizontal feed tube. The second problem was the leaking of the bottom sealing where the support for the stirrer was penetrated through.

Figure 2 shows the new design of the reactor. We retrofitted the reactor by placing a 4-inch isolation valve between the feed tube and the chopper assembly. We placed the penetrates that come through top of the reactor to a new connection unit between the chopper assembly unit and the reactor and eliminated the bottom sealing.

Figure 2. Diagram of the Reactor of the New Design

The retrofit reactor has been run in three 1-month periods without mechanical failure and was much more consistent in solid delivery. Sample out volume, which reflects how much sludge is feeding to the reactor, was plotted. Results show that the reactor of the revised design was much more consistent in solid delivery than the original reactor design.

Semi-continuous Simultaneous Saccharification and Fermentation (SSF)

Semi-continuous SSF was carried out in the retrofit reactor for up to 1 month. The reactor temperature was kept at 36°C, and 1/8 of the reactor volume was replaced every 12 hours, corresponding to 4 days residence time. The sample was taken from the bottom sampler at about 80 mL, and the rest of the sample will overflow from the top sampler to keep constant reactor volume before each feeding. The enzyme loading was cellulase 20 FPU/g glucan, and β­glucosidase loading is 60 IU/g glucan. The ethanol produced at steady state is averaging 42 g/L; solid out is less than 30 g/L, which contain less than 8 g/L cellulose. The overall conversion is above 92 percent.

We also tried to do material balance of the key substance in the sludge that we fed to the reactor. The sludge feed to the reactor is an average of the sludge feed divided by how much time we fed the reactor. The cellulose in, xylan in, and acid insoluble mineral in were calculated by the composition in the sludge times the average amount of sludge per feed. The acid insoluble mineral out and cellulose out were measured by compositional analysis of the solid out. The xylan out measured was the sum of the xylose and xylan oligomer in liquid phase plus the xylan in the solid out. As shown in Table 2, we can close the material balance of xylan and acid insoluble mineral out by 95 percent and 93 percent, respectively.

Table 2. Material Balance of Run 1

Investigation of Xylan Conversion

Because the sludge studied contains a substantial amount of xylose that cannot be utilized by Saccharomyces cerevisiae, experiments were conducted to evaluate the ability of Escherichia coli KO11 to convert both glucan and xylan during SSF. We found that similar results were obtained in LB medium and in medium consisting of M9 salts supplemented with 1 percent corn steep liquor. A material balance showed that glucan, xylan, and mannan originating from the paper sludge were all converted at high yield to ethanol. E. coli KO11 showed the potential to be adopted as the microorganism to convert both glucose and xylose in paper sludge in terms of preliminary batch results. We are testing further the performance of E. coli KO11 as the ethanologen in semi­continuous culture.

Process Optimization

We also modified an existing continuously stirred tank reactor (CSTR) SSF model, built by a former lab member (Colin South) in 1995, to a discrete feeding SSF model. The reformulated model, using the same parameters used by Colin South in his CSTR SSF model, predicted that conversion can be improved by reducing the feeding frequency (residence time/feeding interval time) while keeping the residence time unchanged (Figure 3).

Figure 3. Predicted Conversion Versus Feeding Frequency When Residence Time Is 4 Days

Experimental work on optimizing the feeding frequency was conducted enlightened by the model prediction. Preliminary results were consistent with the prediction. The experimental results indicate that cellulose conversion can be increased by decreasing the feeding frequency (residence time/feeding interval) at a fixed cellulase loading. Glucan conversion of 93.4 percent at 10 FPU/g cellulose loading was achieved when the feeding frequency was 1.33 and residence time was 4 days, whereas conversion of 92 percent (Fan, et al., 2003) required 20 FPU/g cellulose loading when the feeding frequency was 8. In another set of experiments where we tested sludge B, the conversion improved from 79 percent to 92 percent using 5 FPU/g cellulose loading and 60 IU/g cellulose b-glucosidase loading when the feeding frequency was reduced from 8 to 1.33 and the residence time was held constant at 4 days.

Process Design and Economic Analysis

Technical and economic viability of an industrial facility producing ethanol from paper sludge at Nexfor Fraser’s Gorham paper mill was investigated under two scenarios. Scenario one is based on technology demonstrated at Dartmouth at lab scale. Scenario two is based on technology that is not yet demonstrated at Dartmouth, but is expected to be available for incorporation into a commercial plant with construction initiated in 2 years, assuming an aggressive development effort. Positive cashflow is realized for both scenarios, with gross income exceeding expenses by 1.8-fold in scenario one and 2.7-fold in scenario two. Total capital investment is projected to be less than $4 million, which is lower than is possible for more typical cellulosic feedstocks.

Conclusions:

This study focused on investigating conversion of paper sludge to ethanol under industrially relevant conditions. A solids-fed simultaneous saccharification and fermentation laboratory reactor system capable of aseptic, semi-continuous metered delivery of paper sludge was developed to carry out experiments with hydrolysis mediated by commercial cellulase preparations and fermentation of glucose to ethanol mediated by S. cerevisiae. Economically recoverable concentrations of ethanol were produced, and good material balance closure was achieved. Decreasing feeding frequency (feed additions per residence time) was found to allow the cellulase loading to be decreased at least two-fold with no decrease in cellulose conversion. Technical and economic viability of an industrial facility producing ethanol from paper sludge at Nexfor Fraser’s Gorham paper mill were investigated under two scenarios. Positive cashflow is realized for both scenarios, with gross income exceeding expenses by 1.8-fold in scenario one and 2.7-fold in scenario two. Results showed the potential for attractive returns at a production scale if approximately 62 percent and 35 percent sharings of capital (e.g., from government sources) are available for scenario 1 and scenario 2, respectively. Total capital investment is projected to be less than $4 million, which is lower than possible for more typical cellulosic feedstocks.


Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other subproject views: All 6 publications 1 publications in selected types All 1 journal articles
Other center views: All 211 publications 48 publications in selected types All 44 journal articles
Type Citation Sub Project Document Sources
Journal Article Fan ZL, South C, Lyford K, Munsie J, van Walsum P, Lynd LR. Conversion of paper sludge to ethanol in a semicontinuous solids-fed reactor. Bioprocess and Biosystems Engineering 2003;26(2):93-101. R829479 (2006)
R829479C006 (2004)
R829479C006 (Final)
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  • Supplemental Keywords:

    sustainable industry, waste, agricultural engineering, bioremediation, environmental engineering, new technology, innovative technology, bioaccumulation, biodegradation, bioenergy, bioengineering, biotechnology, phytoremediation, plant biotechnology, paper sludge, ethanol,, Scientific Discipline, Waste, TREATMENT/CONTROL, Sustainable Industry/Business, Treatment Technologies, Geochemistry, Technology, New/Innovative technologies, Bioremediation, Environmental Engineering, Agricultural Engineering, bioengineering, paper sludge, biodegradation, ethanol, biotechnology, plant biotechnology, remediation, semicontinuous solid state fermentation

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    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report

  • Main Center Abstract and Reports:

    R829479    The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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