anaerobic digestion of stillage from a pilot scale wood‐to‐ethanol process ii....
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Anaerobic digestion of stillage from a pilot scalewoodtoethanol process II. Laboratoryscale digestionstudiesI.J. Callander (deceased) a , T.A. Clark a & P.N. McFarlane aa Biotechnology Section, Wood Technology Division , Forest Research Institute , Private Bag,Rotorua, New ZealandPublished online: 17 Dec 2008.
To cite this article: I.J. Callander (deceased) , T.A. Clark & P.N. McFarlane (1986) Anaerobic digestion of stillage from a pilotscale woodtoethanol process II. Laboratoryscale digestion studies, Environmental Technology Letters, 7:1-12, 397-412, DOI:10.1080/09593338609384427
To link to this article: http://dx.doi.org/10.1080/09593338609384427
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Environmental Technology Letters, Vol. 7, pp. 397-412 Science & Technology Letters, 1986
ANAEROBIC DIGESTION OF STILLAGE FROMA PILOT SCALE WOOD-TO-ETHANOL
PROCESSII. LABORATORY-SCALE DIGESTION STUDIES
I.J. Callander (deceased), T.A. Clark*, and P.N. McFarlaneBiotechnology Section, Wood Technology Division,
Forest Research Institute, Private Bag, Rotorua, New Zealand
(Received 23 June 1986; accepted 3 July 1986)
The anaerobic digestion of stillage from a pilot-scale wood-to-ethanol processwas investigated using an 8-litre, continuously-fed reactor. A methanogenicconsortium acclimated to Pinus radiata stillage was developed over 160 days bymaintaining a low organic loading rate. The loading rate was then graduallyincreased to c. 4 kg COD m-3 day-1. Nutrient, alkalinity, and mineralrequirements were quantified. Approximately 90% COD removal was obtained atspecific COD utilisation rates up to 0.5 g COD g VSS~1 day-1. The methane andtrue cell yields, per gram of soluble COD removed, were 0.313 1 CH4(STP) and0.142 g VSS. The endogenous decay coefficient was 0.0083 day-1.
Part one (1) of this two part paper described the characteristics of awood-ethanol stillage, i.e., the wastewater produced by the New Zealand ForestResearch Institute (FRI) pilot-plant process of the conversion of Pinus radiatawood to ethanol. Anaerobic digestion was selected as a suitable stillage-treatment process for study because it displayed more favourable economics thanaerobic, wet oxidation, and evaporation treatment processes (2) and because it hasbeen successfully applied to the treatment of a variety of stillages (3-14). ofthe previous studies only Good et al (3) investigated the treatment ofwood-ethanol stillage. This effluent was generated from a large demonstration-scale plant using dilute sulphuric acid hydrolysis of eucalypt wood (3), a processsimilar to that used by the FSI pilot plant. Anaerobic digestion was a suitabletreatment for this stillage which was similar in composition to the P. radiatastillage produced by the FRI pilot plant (Table 1), even though it was derivedfrom a hardwood rather than a softwood. Because of the difference in stillageorigin, the compositions of potentially toxic, lignin-derived compounds in theP. radiata stillage could be expected to differ significantly from those ofcompounds in the eucalypt stillage.
This paper describes initial laboratory studies on the anaerobic digestion ofPinus radiata wood-ethanol stillage from the FRI dilute acid hydrolysis pilotplant. The study's aims were (1) to develop an anaerobic biomass acclimated to
P. radiata stillage; (2) to quantify its nutrient, mineral, and alkalinityrequirements, and then (3), to assess the efficiency of the digestion processunder conditions of increasing organic loading rate.
TABLE 1 - Comparison of the characteristics of the wood-ethanol stillagereported by Good et al (3) and that used in the present study
Parameter(g i-l except pH)
Good et al (3)
P. radiatastillage (1)
Chemical Oxygen Demand (COD)
Total Solids (TSS)
Volatile Solids (VSS)
Nitrogen and phosphorus have been added to the eucalypt stillage, and its levelof sulphur has been adjusted.
MATERIALS AND METHODS
Stillage from typical pilot-plant operations was obtained as describedpreviously (1). Large batches (200 litres) were divided into 4-litre lots whichwere stored frozen until required. Thawed stillage was then maintained at 4Cuntil it was transferred into the digester's feed container, where a magneticstirrer ensured that the suspended material would be evenly fed into the digesteritself. To prevent fungal growth an anaerobic atmosphere was maintained above thefeed. In preparing the stillage for feeding into the digester, the sulphateconcentration was modified by barium precipitation, and additions of sodiumhydroxide and various nutrients were made, as described below.
Sulphate Concentration: The sulphate remaining in the stillage is a function ofthe temperature at which the hydrolysis liquor is neutralised. The temperatureused in the pilot-plant process was 80C, which gave sulphate concentrations ofapproximately 1800 mg 1~ . Commercially, neutralisation can be performed..at 140C, resulting in sulphate concentrations of approximately 500 mg 1To simulate the effects of commercial practice the sulphate concentration wasreduced to approximately 500 mg 1 by precipitation with barium chloride.
A 10% solution of BaCl2.2H20 was used to precipitate a stoichiometric quantityof sulphate. After mixing for 15 minutes to ensure that the reaction was complete,the barium sulphate precipitate was removed from the suspension by vacuumfiltration through Whatman No. 1 filter paper.
Nutrient Addition: During the course of this study the stillage wassupplemented with various concentrations of nitrogen (N), phosphorus (P),potassium (K), magnesium (Mg), and iron (Fe) as AR-grade urea, NH4H2P0^,KC1, HgCl2.6H20, and FeCl3.6H2O, respectively. General methanogenicfermentation indicators along with analyses of the digester effluent for solublenutrient concentrations and analyses of digester contents for precipitateaccumulation, were used to assess the requirements for these added nutrients.
Alkalinity Addition: To maintain the digester pH above 6.80 an alkalinityaddition was required. A 20% NaOH solution was added to the stillage followingsulphate removal and nutrient addition. At low organic loading rates (
The concentration of H2S in the gas was determined using Drager tubes (CH28201) in conjunction with a Drager hand pump (Dr'agerwerk A.G., Lubeck, FRG).
Analytical methods used to determine the Chemical Oxygen Demand (COD), metals,total nitrogen and phosphorus, have been described in an earlier publication (1).
Methods of Calculation
The concentration of total soluble sulphides in the digester was calculatedusing the technique of Lawrence et al (16).
The true growth yield and the endogenous decay coefficient were calculatedusing the following equations (17,18):
" V Yt t
Figure 1: Chronological plots of digester operating conditions;
(a) digester pH and alkalinity(b) volatile acid concentrations and organic loading rate (the levels
of key inorganic species are shown in tabular form)(c) soluble nitrogen and phosphorus concentrations