Improved Alkalimetric Monitoring for Anaerobic Digestion of High-Strength Wastes

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<ul><li><p>Improved Alkalimetric Monitoring for Anaerobic Digestion of High-Strength WastesAuthor(s): L. E. Ripley, W. C. Boyle and J. C. ConverseSource: Journal (Water Pollution Control Federation), Vol. 58, No. 5 (May, 1986), pp. 406-411Published by: Water Environment FederationStable URL: http://www.jstor.org/stable/25042933 .Accessed: 04/12/2014 12:21</p><p>Your use of the JSTOR archive indicates your acceptance of the Terms &amp; Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp</p><p> .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.</p><p> .</p><p>Water Environment Federation is collaborating with JSTOR to digitize, preserve and extend access to Journal(Water Pollution Control Federation).</p><p>http://www.jstor.org </p><p>This content downloaded from 128.83.63.20 on Thu, 4 Dec 2014 12:21:13 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/action/showPublisher?publisherCode=wefhttp://www.jstor.org/stable/25042933?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>il </p><p>Improved alkalimetric monitoring for anaerobic digestion of </p><p>high-strength wastes </p><p>L. E. Ripley, W. C. Boyle, J. C. Converse </p><p>Anaerobic digestion of high-strength industrial or agricultural residues becomes more attractive as higher influent concentra </p><p>tions and shorter hydraulic retention times (HRTs) reduce capital and operating costs. Unfortunately, such a strategy also poses </p><p>an increased risk of process failure caused by inhibition of the </p><p>methanogenic bacteria. Early detection of such inhibition is par </p><p>ticularly important when limited experience with digestion of a </p><p>specific waste precludes use of operating guidelines or rules of </p><p>thumb. Although various biochemical parameters (such as </p><p>adenosine triphosphate or dehydrogenase activity) have been </p><p>proposed to monitor methanogen activity and digester stability, </p><p>these tend to be more useful for research applications than for </p><p>routine field monitoring. Successful anaerobic digestion of high strength wastes requires development of a simple, inexpensive, </p><p>yet sensitive monitoring technique to allow rapid detection of </p><p>process instability. </p><p>Poultry manure is a high-strength waste that presents both a </p><p>disposal problem and an energy-recovery opportunity. The </p><p>U. S. currently has approximately 280 million egg-laying hens </p><p>in production, each hen also produces 110 to 150 g (0.25 to 0.33 </p><p>lb) of manure per day. A bench-scale poultry manure digestion </p><p>study was undertaken to meet the following two objectives: </p><p>Methane optimization. Measure methane production rates </p><p>in identical digesters with different HRTs and different influent solids concentrations. All other variables (mixing, heating, and </p><p>influent composition) were held constant throughout the study. Simple process monitoring. Evaluate methods to measure </p><p>the biological stability of anaerobic digestion so that the onset </p><p>of a process upset can be quickly, easily, and inexpensively de </p><p>tected, and remedial action taken before failure. </p><p>This paper addresses the second of these objectives, in detail. </p><p>CURRENT MONITORING METHODS </p><p>An organically overloaded anaerobic digester exhibits several </p><p>symptoms that traditionally have been used to indicate a process </p><p>upset. An increase in the C02 fraction of the digester off-gas or </p><p>a decrease in the digester pH results from destruction of bicar </p><p>bonate buffering and volatile acids build-up. Unfortunately, </p><p>neither the off-gas C02 fraction nor the digester pH changes quickly with the onset of digester stress. The two parameters </p><p>used most frequently to monitor digester stability are alkalinity </p><p>and volatile acids (VA) concentration; however, both parameters </p><p>have drawbacks. </p><p>Alkalinity. The most common way to measure digester al </p><p>kalinity is method 403 outlined in "Standard Methods."1 Fol </p><p>lowing settling or centrifugation, the sample supernatant is ti </p><p>trated with standardized H2S04 or HC1 to an endpoint of pH 4.3. Titration to pH 4.3 measures not only the bicarbonate buff </p><p>ering capacity of the sample, but also the VA buffering. Because acetic acid has a pK of 4.7 and propionic acid has a pK of 4.9, well below the normal digester operating pH, their buffering is not a useful part of the alkalinity. </p><p>Besides its use in process monitoring, the method </p><p>distinguishes stable configurations with good methane </p><p>yields from others with poor yields. </p><p>To prevent VA buffering from being included in the alkalinity measurement, Jenkins et al.2 proposed that digester supernatant </p><p>be titrated to an endpoint of pH 5.75. It was argued that, while </p><p>specification of a precise endpoint was somewhat arbitrary, a </p><p>pH of 5.75 led to 80% titration of the bicarbonate but less than 20% of the VA. The alkalinity to pH 5.75 was correlated strongly to the true bicarbonate alkalinity in municipal sludge digesters, that is, the alkalinity to pH 4.3 minus 0.83 times the VA con </p><p>centration. The significance or the usefulness of monitoring the </p><p>alkalinity between pH 5.75 and pH 4.3 was not examined. </p><p>Volatile acids. The VA interference in the direct titration of bicarbonate alkalinity is the basis for the VA method proposed by DiLallo and Albertson.3 They presented a procedure in which a decanted or centrifuged sample was titrated to pH 3.3 with </p><p>H2S04, then boiled to remove C02 (and thus, bicarbonates), then back-titrated between pH 4.0 and 7.0 with standardized </p><p>NaOH. </p><p>"Standard Methods"1 provides two distillation methods for VA measurement in digester sludge. Straight distillation (method 504C) is fairly rapid; however, its empirical nature makes it </p><p>suitable only for routine control. Steam distillation (method 504B) is more accurate, but the procedure is more time-con </p><p>suming. </p><p>A third VA method in "Standard Methods"1 is Chromato </p><p>graphie separation (method 504A) using adsorption on silicic acid and elution with n-butanol in chloroform. The procedure </p><p>is difficult, however, because careful preparation of the silicic </p><p>acid, a C02- free atmosphere, and frequent standardization of </p><p>the NaOH titrant are all necessary. </p><p>A final method for determination of VA concentrations is gas chromatography. While it requires expensive equipment and a </p><p>406 Journal WPCF, Volume 58, Number 5 </p><p>This content downloaded from 128.83.63.20 on Thu, 4 Dec 2014 12:21:13 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>_Process Research </p><p>degree of analytical skill, gas chromatography can accurately </p><p>measure each acid. Knowledge of the acid distribution can be as useful as total concentration. </p><p>Monitoring requirements. The ideal monitoring technique for </p><p>any anaerobic process would be simple to perform and would </p><p>require minimal equipment, yet would be sensitive enough to </p><p>indicate an upset before imminent digester failure. Simplicity is </p><p>particularly important in digestion of agricultural residues, as </p><p>the farmer/operator generally has limited time and analytical expertise. Unfortunately, none of the previously described </p><p>methods meets all of the criteria for simplicity, expense, and </p><p>sensitivity. </p><p>PROCESS BIOCHEMISTRY </p><p>The biochemical transformations involved in the anaerobic </p><p>digestion of poultry manure are not markedly different from those of any other high-strength waste, except for the presence </p><p>of CaC03 and high ammonia concentrations. CaC03 is present in the form of oyster shells or crushed limestone added to the feed to provide grit in the birds' diet, but in the manure, CaC03 increases the available carbonate buffering. Previous research4 </p><p>indicated that free ammonia concentrations from 300 to 350 </p><p>mg/L NH3-N inhibited methane production rates in poultry manure digestion only slightly; the ammonia is probably more </p><p>significant as a buffer than as an inhibitor. </p><p>Figure 1 is a simplified diagram of the biochemical reactions in poultry manure digestion. Note that three of the components </p><p>(ammonia, VA, and bicarbonate) are in boxes to indicate their </p><p>participation in digester buffering. Georgacakis et al5 studied the buffering in swine manure digesters, and stated that stable </p><p>digester operation depends on a balance of carbon (VA and </p><p>bicarbonate) against nitrogen (ammonia) buffering. Equally im </p><p>portant, is the need to maximize the contribution of bicarbonate </p><p>to carbonaceous buffering. </p><p>EXPERIMENTAL DESIGN AND METHODS </p><p>Because the process monitoring evaluation was conducted as </p><p>a corollary to the methane optimization investigation, the ex </p><p>perimental design was developed to measure methane yield for </p><p>a wide range of HRTs and feed concentrations. Figure 2 shows </p><p>the condition and duration for each of the 12 test configurations, </p><p>CHEMICAL TRANSFORMATIONS IN POULTRY MANURE DIGESTION </p><p>ORGANICS INORGANICS </p><p>PROTEIN, URIC ACID LIMESTONE, CARBOHYDRATES AMMONIA </p><p>^-1AMM0NIA"H-^7 </p><p>IVOLATI LE ACIDSI ^ IBICARBONAT?] </p><p>y </p><p>Figure 1?Chemical transformations in poultry manure digestion. </p><p>10 20 30 40 50 HRT (DAYS) </p><p>Figure 2?Experimental design: steady-state sampling configurations. </p><p>with HRTs from 10 to 50 days, and feed concentrations from 4.0 to 7.4% volatile solids (VS). For comparison purposes, the </p><p>background lines in the figure indicate organic loading rates, which ranged in this study from 1.1 to 5.6 g/L?d VS. The two decant points in Figure 2 represent digesters in which part of the daily effluent was decanted from the supernatant layer before </p><p>mixing. The remaining portion of the effluent was withdrawn from the bottom of the digester while the mixer continued to </p><p>operate at high speed. The decant operation increased the VS </p><p>retention time by approximately 50% over the HRT for each </p><p>point. </p><p>Apparatus and feed. Three 5-L digesters were built from 14 </p><p>cm ID acrylic tubes fitted with conical bottoms and 2.54-cm </p><p>polyvinyl chloride sampling valves. The digester contents were </p><p>mixed twice daily by multiple propellers and the digester tem </p><p>peratures were maintained at 35 ? 1?C by thermoswitch-con </p><p>trolled heating tapes. Gas was measured by brine displacement </p><p>and adjusted to standard conditions. A more detailed description of the experimental apparatus was presented elsewhere.4 </p><p>To ensure consistent feed composition throughout the 19 </p><p>month study, 200 L of poultry manure were collected from the area beneath the cages of 70-week-old layers at a large com </p><p>mercial egg farm. The manure was slurried, then frozen in 2-L </p><p>batches until needed, when it was thawed and further diluted to the desired VS concentration. </p><p>Sampling and analytical methods. The digesters were fed daily and operated for several retention times to reach steady-state at </p><p>each test configuration. The principal parameter used to deter </p><p>mine when equilibrium conditions were attained was alkalinity. </p><p>Influent and effluent samples were analyzed several times each </p><p>week for total solids (TS) and VS, Kjeldahl and ammonia ni </p><p>trogen, and total organic carbon. Gas composition was char </p><p>acterized using a gas partitioner. </p><p>The parameters chosen to evaluate process monitoring were </p><p>alkalinity to pH 4.3 and 5.75, as well as VA measured by gas chromatography. Influent and effluent samples were spun for </p><p>May 1986 407 </p><p>This content downloaded from 128.83.63.20 on Thu, 4 Dec 2014 12:21:13 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>Ripley et al. </p><p>20 minutes at 15 000 rpm in a centrifuge, then carefully decanted as soon as the centrifuge stopped. A 50-mL portion of the sample was taken for alkalinity titration and a 1-mL portion was frozen </p><p>for subsequent VA measurement. Two drops of an organo-silicon </p><p>emulsion were added to the alkalinity sample before titration with 0.6 TV H2S04 ; the pH was measured and standardized at </p><p>pH 7.0 and 4.0. At the end of each steady-state sampling period, </p><p>an additional sample was titrated by increments with 0.6 N </p><p>NaOH to develop complete titration and buffering curves. </p><p>A gas Chromatograph with a stainless steel column (10% </p><p>SP1200/l%/H3PO4 on 80/100 chromosorb W AW, Supelco) was used to measure VA. The machine was fitted with a removable </p><p>glass wool pre-column in the flash heater to extend column life. </p><p>Samples were acidified with H2S04 to pH 3 then filtered through a 0.45 n micropipet tip before injection. Concentrations of acetic, propionic, iso-butyric, and n-butyric acids were calculated from </p><p>peak areas plotted on an integrating recorder, then converted to </p><p>total volatile acids (TVA) expressed as HAc. </p><p>RESULTS AND DISCUSSION </p><p>Steady-state buffer intensity. Although the sensitivity of any process monitoring tool is most important during dynamic con </p><p>ditions, for example, when a decrease in HRT or an increase in </p><p>feed concentration raises the organic loading rate, it can still be </p><p>informative to examine parameter behavior during different </p><p>steady-state periods. Because the primary monitoring tool was </p><p>alkalinity, it is useful to see how both the magnitude and the distribution of alkalinity varied throughout the 12 test config urations. </p><p>Figures 3-7 show the alkalinity and acidity titration curves </p><p>(between pH 10 and pH 3) measured at the end of five of the </p><p>steady-state periods. The most obvious difference between the </p><p>various curves was the difference in the magnitude of the alka </p><p>linity. The alkalinity to pH 4.3 decreased from 395 meq/L ( 19 750 mg/L CaC03) at the 7.0% VS configuration to 215 meq/ L (10 750 mg/L CaC03) at the 4.0% VS configuration. </p><p>The change in shape of the titration curves was not quite as </p><p>obvious as the change in magnitude. The change in shape was </p><p>independent of the magnitude of the alkalinity, and resulted from differences in the relative contributions of bicarbonate and </p><p>VA. Because the slope of a titration curve at any pH depends </p><p>on the concentration of buffer acting at or near that pH, a plot </p><p>-200 -100 0 100 200 300 0 100 200 300 400 1350 MEQ/L 0(MEQ/L-PH) </p><p>Figure 3?Titration and buffering curves for 7.0% VS and 50-day HRT </p><p>digester. </p><p>5.5% VS,50day HRT I I </p><p>_i_i_i_j_i_i_j_LA_i-1-1-1 -200 -100 0 100 200 300 0 100 200 300 400 </p><p>MEQ/L ?(MEQ/LPH) </p><p>Figure 4?Titration and buffering curves for 5.5% VS and 50-day HRT </p><p>digester. </p><p>of buffer intensity indicates the contributions of different buff </p><p>ering species. The buffer intensity, ?, can be determined empir </p><p>ically as the reciprocal slope of the titration curve.6 To the right of the titration curves in Figures 3-7 are the corresponding buffer </p><p>intensity curves. Note that all of the steady-state buffer curves </p><p>demonstrated large peaks near pH 9.5 and 6.5 to indicate the </p><p>buffering effects of ammonia and bicarbonate, respectively. In </p><p>particular, note the difference in the bicarbonate peak between </p><p>Figures 4 and 5, which show the 50-day HRT...</p></li></ul>