improved alkalimetric monitoring for anaerobic digestion of high-strength wastes

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 .

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il

Improved alkalimetric monitoring for anaerobic digestion of

high-strength wastes

L. E. Ripley, W. C. Boyle, J. C. Converse

Anaerobic digestion of high-strength industrial or agricultural residues becomes more attractive as higher influent concentra

tions and shorter hydraulic retention times (HRTs) reduce capital and operating costs. Unfortunately, such a strategy also poses

an increased risk of process failure caused by inhibition of the

methanogenic bacteria. Early detection of such inhibition is par

ticularly important when limited experience with digestion of a

specific waste precludes use of operating guidelines or rules of

thumb. Although various biochemical parameters (such as

adenosine triphosphate or dehydrogenase activity) have been

proposed to monitor methanogen activity and digester stability,

these tend to be more useful for research applications than for

routine field monitoring. Successful anaerobic digestion of high strength wastes requires development of a simple, inexpensive,

yet sensitive monitoring technique to allow rapid detection of

process instability.

Poultry manure is a high-strength waste that presents both a

disposal problem and an energy-recovery opportunity. The

U. S. currently has approximately 280 million egg-laying hens

in production, each hen also produces 110 to 150 g (0.25 to 0.33

lb) of manure per day. A bench-scale poultry manure digestion

study was undertaken to meet the following two objectives:

Methane optimization. Measure methane production rates

in identical digesters with different HRTs and different influent solids concentrations. All other variables (mixing, heating, and

influent composition) were held constant throughout the study. Simple process monitoring. Evaluate methods to measure

the biological stability of anaerobic digestion so that the onset

of a process upset can be quickly, easily, and inexpensively de

tected, and remedial action taken before failure.

This paper addresses the second of these objectives, in detail.

CURRENT MONITORING METHODS

An organically overloaded anaerobic digester exhibits several

symptoms that traditionally have been used to indicate a process

upset. An increase in the C02 fraction of the digester off-gas or

a decrease in the digester pH results from destruction of bicar

bonate buffering and volatile acids build-up. Unfortunately,

neither the off-gas C02 fraction nor the digester pH changes quickly with the onset of digester stress. The two parameters

used most frequently to monitor digester stability are alkalinity

and volatile acids (VA) concentration; however, both parameters

have drawbacks.

Alkalinity. The most common way to measure digester al

kalinity is method 403 outlined in "Standard Methods."1 Fol

lowing settling or centrifugation, the sample supernatant is ti

trated with standardized H2S04 or HC1 to an endpoint of pH 4.3. Titration to pH 4.3 measures not only the bicarbonate buff

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.

Besides its use in process monitoring, the method

distinguishes stable configurations with good methane

yields from others with poor yields.

To prevent VA buffering from being included in the alkalinity measurement, Jenkins et al.2 proposed that digester supernatant

be titrated to an endpoint of pH 5.75. It was argued that, while

specification of a precise endpoint was somewhat arbitrary, a

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

centration. The significance or the usefulness of monitoring the

alkalinity between pH 5.75 and pH 4.3 was not examined.

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

H2S04, then boiled to remove C02 (and thus, bicarbonates), then back-titrated between pH 4.0 and 7.0 with standardized

NaOH.

"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

suitable only for routine control. Steam distillation (method 504B) is more accurate, but the procedure is more time-con

suming.

A third VA method in "Standard Methods"1 is Chromato

graphie separation (method 504A) using adsorption on silicic acid and elution with n-butanol in chloroform. The procedure

is difficult, however, because careful preparation of the silicic

acid, a C02- free atmosphere, and frequent standardization of

the NaOH titrant are all necessary.

A final method for determination of VA concentrations is gas chromatography. While it requires expensive equipment and a

406 Journal WPCF, Volume 58, Number 5



_Process Research

degree of analytical skill, gas chromatography can accurately

measure each acid. Knowledge of the acid distribution can be as useful as total concentration.

Monitoring requirements. The ideal monitoring technique for

any anaerobic process would be simple to perform and would

require minimal equipment, yet would be sensitive enough to

indicate an upset before imminent digester failure. Simplicity is

particularly important in digestion of agricultural residues, as

the farmer/operator generally has limited time and analytical expertise. Unfortunately, none of the previously described

methods meets all of the criteria for simplicity, expense, and

sensitivity.

PROCESS BIOCHEMISTRY

The biochemical transformations involved in the anaerobic

digestion of poultry manure are not markedly different from those of any other high-strength waste, except for the presence

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

indicated that free ammonia concentrations from 300 to 350

mg/L NH3-N inhibited methane production rates in poultry manure digestion only slightly; the ammonia is probably more

significant as a buffer than as an inhibitor.

Figure 1 is a simplified diagram of the biochemical reactions in poultry manure digestion. Note that three of the components

(ammonia, VA, and bicarbonate) are in boxes to indicate their

participation in digester buffering. Georgacakis et al5 studied the buffering in swine manure digesters, and stated that stable

digester operation depends on a balance of carbon (VA and

bicarbonate) against nitrogen (ammonia) buffering. Equally im

portant, is the need to maximize the contribution of bicarbonate

to carbonaceous buffering.

EXPERIMENTAL DESIGN AND METHODS

Because the process monitoring evaluation was conducted as

a corollary to the methane optimization investigation, the ex

perimental design was developed to measure methane yield for

a wide range of HRTs and feed concentrations. Figure 2 shows

the condition and duration for each of the 12 test configurations,

CHEMICAL TRANSFORMATIONS IN POULTRY MANURE DIGESTION

ORGANICS INORGANICS

PROTEIN, URIC ACID LIMESTONE, CARBOHYDRATES AMMONIA

^-1AMM0NIA"H-^7

IVOLATI LE ACIDSI ̂^ IBICARBONAT?]

y

Figure 1?Chemical transformations in poultry manure digestion.

10 20 30 40 50 HRT (DAYS)

Figure 2?Experimental design: steady-state sampling configurations.

with HRTs from 10 to 50 days, and feed concentrations from 4.0 to 7.4% volatile solids (VS). For comparison purposes, the

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

mixing. The remaining portion of the effluent was withdrawn from the bottom of the digester while the mixer continued to

operate at high speed. The decant operation increased the VS

retention time by approximately 50% over the HRT for each

point.

Apparatus and feed. Three 5-L digesters were built from 14

cm ID acrylic tubes fitted with conical bottoms and 2.54-cm

polyvinyl chloride sampling valves. The digester contents were

mixed twice daily by multiple propellers and the digester tem

peratures were maintained at 35 ? 1?C by thermoswitch-con

trolled heating tapes. Gas was measured by brine displacement

and adjusted to standard conditions. A more detailed description of the experimental apparatus was presented elsewhere.4

To ensure consistent feed composition throughout the 19

month study, 200 L of poultry manure were collected from the area beneath the cages of 70-week-old layers at a large com

mercial egg farm. The manure was slurried, then frozen in 2-L

batches until needed, when it was thawed and further diluted to the desired VS concentration.

Sampling and analytical methods. The digesters were fed daily and operated for several retention times to reach steady-state at

each test configuration. The principal parameter used to deter

mine when equilibrium conditions were attained was alkalinity.

Influent and effluent samples were analyzed several times each

week for total solids (TS) and VS, Kjeldahl and ammonia ni

trogen, and total organic carbon. Gas composition was char

acterized using a gas partitioner.

The parameters chosen to evaluate process monitoring were

alkalinity to pH 4.3 and 5.75, as well as VA measured by gas chromatography. Influent and effluent samples were spun for

May 1986 407



Ripley et al.

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

for subsequent VA measurement. Two drops of an organo-silicon

emulsion were added to the alkalinity sample before titration with 0.6 TV H2S04 ; the pH was measured and standardized at

pH 7.0 and 4.0. At the end of each steady-state sampling period,

an additional sample was titrated by increments with 0.6 N

NaOH to develop complete titration and buffering curves.

A gas Chromatograph with a stainless steel column (10%

SP1200/l%/H3PO4 on 80/100 chromosorb W AW, Supelco) was used to measure VA. The machine was fitted with a removable

glass wool pre-column in the flash heater to extend column life.

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

peak areas plotted on an integrating recorder, then converted to

total volatile acids (TVA) expressed as HAc.

RESULTS AND DISCUSSION

Steady-state buffer intensity. Although the sensitivity of any process monitoring tool is most important during dynamic con

ditions, for example, when a decrease in HRT or an increase in

feed concentration raises the organic loading rate, it can still be

informative to examine parameter behavior during different

steady-state periods. Because the primary monitoring tool was

alkalinity, it is useful to see how both the magnitude and the distribution of alkalinity varied throughout the 12 test config urations.

Figures 3-7 show the alkalinity and acidity titration curves

(between pH 10 and pH 3) measured at the end of five of the

steady-state periods. The most obvious difference between the

various curves was the difference in the magnitude of the alka

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.

The change in shape of the titration curves was not quite as

obvious as the change in magnitude. The change in shape was

independent of the magnitude of the alkalinity, and resulted from differences in the relative contributions of bicarbonate and

VA. Because the slope of a titration curve at any pH depends

on the concentration of buffer acting at or near that pH, a plot

-200 -100 0 100 200 300 0 100 200 300 400 1350 MEQ/L 0(MEQ/L-PH)

Figure 3?Titration and buffering curves for 7.0% VS and 50-day HRT

digester.

5.5% VS,50day HRT I I

_i_i_i_j_i_i_j_LA_i-1-1-1 -200 -100 0 100 200 300 0 100 200 300 400

MEQ/L ?(MEQ/LPH)


digester.

of buffer intensity indicates the contributions of different buff

ering species. The buffer intensity, ?, can be determined empir

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

intensity curves. Note that all of the steady-state buffer curves

demonstrated large peaks near pH 9.5 and 6.5 to indicate the

buffering effects of ammonia and bicarbonate, respectively. In

particular, note the difference in the bicarbonate peak between

Figures 4 and 5, which show the 50-day HRT and the 10-day HRT-decant digesters at 5.5% VS. The longer retention time

resulted in an increase from 230 meq/L pH to 386 meq/L pH. While there is no VA peak in the 50-day HRT digesters (Figures

3 and 4), a hump in VA concentration is evident between pH 4.0 and 5.5 in the 10-day HRT-decant digester (Figure 5). Figure 6 indicates a moderate VA buffering effect in the 15-day HRT

digester, an effect which was converted to bicarbonate buffering

after the digester had operated for 3 weeks without feed

(Figure 7). Partial and intermediate alkalinity. While titration and buffer

intensity curves conveniently show the relative magnitudes of

bicarbonate and VA, the development of such curves is too te

dious for routine operation; therefore, titration to two endpoints

becomes more attractive. Titration from the original sample pH

-200 -100 0 100 200 300 0 100 200 300 MEQ/L ?(MEQ/LPH}

Figure 5?Titration and buffering curves for 5.5% VS and 10-day HRT?

decant digester.




_Process Research

4.0% VS, 15 day HRT

-200 -100 0 100 200 300 MEQ/L

0 100 200 300 ?{ MEQ/L' PH)


digester.

to pH 5.75, or partial alkalinity (PA), results in an alkalinity that corresponds roughly to bicarbonate alkalinity, as demon

strated by Jenkins.2 Dashed contour lines are used in Figure 8 to show average PA concentrations for each of the steady-state

configurations. If the partial alkalinity were strictly conservative

(a result of influent bicarbonate or carbonate concentrations), the contour lines would be horizontal. The actual contours in

dicate that for a given feed concentration, the PA value increases

with longer HRTs, as would be expected with more conversion

of the substrate to bicarbonate. As the HRT approaches 50 days, however, substrate conversion reaches its maximum and the PA

curves become horizontal.

Titration from pH 5.75 to 4.3, or intermediate alkalinity (IA), approximates the VA alkalinity. Figure 9 shows average IA values

for the steady-state configurations, again with interpolated con

tour lines. The IA contour trend is quite different from the PA

trend, as the IA line nearly follows the organic loading rate line

at the right of the plot then shifts to a vertical line as the HRT

approaches 10 days. Figure 10 presents a regression of TVA, as

measured by the gas Chromatograph against the IA, to show how

the IA approximated VA. Each of the 50 data points represents the average of three IA measurements and two TVA measure

ments during a single week of steady-state sampling. The dashed

10

9

8

7

c L

6 -

5

4 -

?i i i r~

4.0% VS, 15 day HRT 3 WEEKS AFTER LAST FEEDING

1 -200 -100 0 100 MEQ/L

200 300 0 100 200 500 ?(MEQ/LPH)

600


digester, 3 weeks after last feeding.

O 10 20 30 40 50 HRT (DAYS)

Figure 8?Steady-state partial alkalinities.

line through the origin indicates the expected correlation if one

equivalent of TVA (60 g H Ac) reacted as one equivalent of IA

(50 g CaC03). The solid line shows the actual regression, with a correlation coefficient of 0.98. The line does not pass through

the origin, probably as a result of titration of some bicarbonate

below pH 5.75.

Dynamic alkalinity response. As each steady-state sampling

period was completed during the project, the digesters were sub

7.5

7.0

>

2 6.0

rr

_; 5.5 LU

O

o 5.0 o o

i??4-5

4.0 h

3.61

i?./' ,0/ cm/loading RATE

'2.56 U-(gVS/LDAY)

INTERMEDIATE ALKALINITY

g/L as CaC03 COMPLETELY MIXED

A DECANT

0 10 20 30 40 50 HRT (DAYS)

Figure 9?Steady-state intermediate alkalinities.

May 1986 409



Ripley et al.

5^ STEADY-STATE DATA REGRESSION EQUATION

TVA = l.84(IA)-l.98 n=50 r = 0.98

o < X 3 o

o?

< >

?/ / /?

/ / ?/

/ /

/

rtf,

0 12 3 IA(g/L as CaC03)

Figure 10?Correlation of steady-state TVA and I A.

jected to step changes, which usually increased their organic

loading rates. The most dramatic step change was the one ex

perienced when the digester operating at 5.5% VS, 15-day HRT with decant, received a 50% feed rate increase to bring it to a

10-day HRT with decant. Gas production rose for several days, then reached a plateau as the CH4 fraction dropped from its usual 55 to 60% range to 44%. Figure 11 shows the response of the IA and the PA, as well as the total alkalinity (TA), and the

14

7il2 o o o

U|0 V) < -i 8 V.

> 6

z 3 4 <

< 2

0

.8

PARTIAL= -?pH 5.75

Y

10 20 30 40 50 60 70 80 90 100 TIME (DAYS)

Figure 11?Alkalinity response to HRT step-change.

IA-to-PA ratio (IA:PA). The TA declined after the step change, but the decline was small compared to the magnitude; the TA

dropped 16% from its antecedant concentration of 14 300 mg/ L CaC03. The PA responded more clearly to the change, de

creasing about 42%. The step change was most visible, however,

in the I A, which increased steeply to twice its original concen tration. Approximately 6 weeks after the step change, the CH4

production rate began to climb again; the alkalinities showed that the digester was recovering and reaching a new steady-state

condition.

Comparison of the IA and TVA values during the stress period showed that the correlation demonstrated previously for steady state periods began to diverge. During the plateau of the stress

period, the TVA increased to 9500 mg/L HAc, while the IA leveled at 5500 mg/L CaC03 (Figure 12). The divergence of the

parameters possibly resulted when the acetic acid fraction of the

TVA dropped from its usual 90 to 99% range to 70%, thus in

creasing the average pK of the volatile acids. While this precludes

use of the IA measurement as a replacement for gas chroma

tography, the IA test still gives a semi-quantitative estimate of

TVA concentrations. It should also be noted that the step change in Figures 11 and 12 was presented as a "worst case" example;

the other step changes resulted in more moderate TVA and al

kalinity changes.

IA:PA. Because successful digester operation depends on both

maintenance of adequate bicarbonate buffering and avoidance

of excessive VA concentrations, the VA-to-alkalinity ratio (VA:

Alk) has been used to monitor anaerobic digestion of municipal sludge. VA:Alk values of 0.1 to 0.35 have been considered typical of well-operated municipal digesters.7 Instead of selecting either

TA, PA, or IA individually as a monitoring tool, it was decided to combine the latter two into a dimensionless ratio that provided an indicator analogous to VA:Alk ratio. IA:PA requires only one simple analytical procedure, rather than two separate mea

surements. Additionally, because the ratio is dimensionless, any

analytical errors that may arise from incorrect titrant standard

ization or sample volume measurement are avoided.

0

Figure

10 20 30 40 50 60 70 80 90 TIME (DAYS)

12?TVA and IA response to HRT step-change.




_Process Research

IA:PA is plotted beneath the various alkalinities in Figure 11. IA:PA increased from 0.25 to 0.8 during the stress period, then returned to an average of 0.37 during the following steady-state period. If the IA and PA are combined, a change in either tends to be exaggerated, and is thus more easily detected during a

process upset.

In addition to its usefulness as a dynamic monitoring tool,

IA:PA is a convenient indicator to compare various steady-state

configurations. The ratio provides a rule-of-thumb to distinguish

the stable configurations with good methane yields from marginal configurations with poor yields. All but two of the steady-state

configurations demonstrated methane yields from 226 to 263 mL CH4/g VS added. The 4.0% VS, 15-day HRT digester yielded only 206 mL CH4/g VS added, and the 4.0% VS, 10-day HRT

digester yielded only 175 mL CH4/g VS added. In addition to these reduced CH4 yields, the two 4.0% VS configurations were

difficult to operate because of foaming, which occurred imme

diately after feeding. The 5.5% VS, 10-day HRT-decant digester also had foaming problems, but to a lesser extent. Figure 13

demonstrates an empirical relationship between overall digester

performance and the I A: PA, which suggests that a poultry ma

nure digester should be operated at an I A: PA less than 0.3.

Application of the IA:PA monitoring tool to digestion of other

high-strength wastes requires recognition of the fact that the ratio

of VA to bicarbonate (and ammonia) varies with influent com

position as well as digester operation. The success of the method

depends on the ability to detect VA by alkalimetric titration, without significant interference by other ions (such as soluble

organic nitrogen). To identify the contributing buffer species, it is useful to examine the titration/buffering curve of the digester.

0 10 20 30 40 50 HRT (DAYS)

Figure 13?Steady-state I A/PA ratios.

CONCLUSIONS

Titration of the centrifuged sample to an endpoint of pH 5.75 (PA), and then to pH 4.3 (IA) makes it possible to distinguish the relative buffering contributions of both bicarbonate and VA in the anaerobic digestion of a high-strength waste such as poultry

manure. Additionally, IA provides a rapid, semi-quantitative

estimate of TVA concentration.

IA:PA is a simple, inexpensive, yet sensitive digestion mon

itoring tool which increases rapidly with a process upset then decreases with recovery. IA:PA is analogous to the VA-to-Al

kalinity ratio, but it does not require VA measurement. The

ratio is dimensionless; there is no need for precise titrant stan

dardization and sample volume measurement.

I A: PA is also useful to compare steady-state operating con

figurations in a bench-scale digestion study. Successful digestion of poultry manure occurred with IA:PA below 0.3.

ACKNOWLEDGMENTS

Credits. Funding for this project was provided by the Wis consin Department of Administration, Division of State Energy; the University-Industrial Research Program, the College of

Agricultural and Life Sciences, and the College of Engineering of the University of Wisconsin at Madison. Laboratory assistance

was given by Mary Jo Moubry, Mark Biel, Tom Weiland, and Linda Rogalinski, and start-up participation by Nancy Mohr Kmet. This paper was presented at the 40th Purdue Industrial Waste Conference, May 1985.

Authors. Leonard E. Ripley and William C. Boyle are research

assistant and chairman and professor, respectively, in the De

partment of Civil and Environmental Engineering, University of Wisconsin at Madison. James C. Converse is a professor in

the Department of Agricultural Engineering, University of Wis consin at Madison. Correspondence should be sent to Leonard

E. Ripley, Department of Civil and Environmental Engineering, University of Wisconsin at Madison, Madison, WI 53706.

REFERENCES

1. "Standard Methods for the Examination of Water and Wastewater."

15th Ed., Am. Public Health Assoc. (1980). 2. Jenkins, S. R., et al, "Measuring anaerobic sludge digestion and

growth by a simple alkalimetric titration." /. Water Pollut. Control

Fed., 55, 448 (1983). 3. DiLallo, R., and Albertson, O. E., "Volatile acids by direct titration."

J. Water Pollut. Control Fed., 33, 356 (1961). 4. Ripley, L. E., et al, "The Effects of Ammonia Nitrogen on the

Anaerobic Digestion of Poultry Manure." Proc, 39th Indust. Waste

Conf., Purdue Univ., May 1984, Butterworth Publ., Boston, Mass.

(1985). 5. Georgacakis, D., et al, "Buffer Stability in Manure Digesters." Agrie.

Wastes, 4, 427 (1982). 6. Stumm, W., and Morgan, J. J., "Aquatic Chemistry." Wiley Inter

science, New York, N. Y. (1981). 7. U.S. Environ. Prot. Agency, "Anaerobic Sludge Digestion Operations

Manual." Sect. 4-17(1976).

May 1986 411



improved alkalimetric monitoring for anaerobic digestion of high-strength wastes

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