two-phase anaerobic digestion of high-metal-content municipal-industrial sludge

Biomass 10 (1986) 97-107

Two-Phase Anaerobic Digestion of High-Metal- Content Municipal-lndustrial Sludge

S. G h o s h

university of Utah, Department of Civil Engineering, 3012 MEB, Salt Lake City, Utah 84112, USA

(Received: 3 March, 1986)

ABSTRACT

This paper presents the development of an innovative two-phase methane fermentation process that provided a mesophilic methane yield of about 0"5 SCM/kg VS (8 SCF/Ib VS) added from digestion of a municipal- industrial sludge at a system hydraulic residence time (HRT) of about 6 days compared with a yield of 0"22-0.31 SCM/kg VS (3"5-5"0 SCF/lb VS) added obtained from single-stage conventional 'high-rate' digesters operated at HRTs of 10-20 days. This innovative process has substantive beneficial impact on the production of net energy and availability of surplus digester methane.

Key words: anaerobic digestion, methane, two-phase, municipal sludge, industrial waste.

I N T R O D U C T I O N

This research was conducted with a high-metal-content and difficult-to- treat primary sludge from the South Essex Sewerage District (SESD) water pollution control plant, Salem, Massachusetts. Wastewaters received at the plant include 40-60 vol% industrial wastes, the remain- der being residential liquid wastes. Incineration, which was the sludge disposal process at the plant, is now unacceptable because it leads to the production of hexavalent chromium and other oxidized metals, and the incinerator ash containing these materials cannot be landfilled. The two- phase anaerobic stabilization process does not generate oxidized species such as Cr 6 +; it produces renewable energy and a highly stabilized resi- due, and could be an answer to the sludge disposal problems of SESD or other sewage districts.

97 Biornass 0144-4565/86/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

98 S. Ghosh

This project was undertaken several years ago with the cooperation of SESD, the Executive Office of Energy Resources (EOER), State of Massachusetts, and a number of industrial sponsors to extend the appli- cation of the patented two-phase anaerobic process 1-3 to high-solids- content problematic sludges. Results of the bench-scale process development work are presented here. Design of a 7500 liters/day (2000 gallons/day) two-phase pilot plant has been completed and operation of the plant will be started this year with support from the above industrial sponsors, and governmental and public agencies.

METHODS

Several conventional single-stage high-rate digestion runs were con- ducted to establish the feasibility of digesting high-metal and high- industrial-waste-content sludge (information on the digestibility of this sludge was not available), and to compile baseline data needed to com- pare performances of single-stage and upflow two-phase digestion pro- cesses. The primary sludges used for conventional digestion had total solids (TS) contents between about 4 wt% and 12 wt% and volatile solids concentrations between 52 wt% and 64 wt% of TS. The sludge was acidic (pH - 6.5) and contained a number of heavy metals, as indicated in Table 1.

TABLE 1 Typical Metal Concentration in Salem Primary

Sludge

Metal Concentration (rag/liter)

Cd 1.1 Ca 3570 Ba 22 Fe 584 Cu 18"8 Ni 5"7 Pb 3 l Zn 35 Cr 6 + 0 Cr 3+ 297 Hg 0.01 Se 1 "0

Two-phase anaerobic digestion of municipal-industrial sludge 99

RESULTS

Single-stage digestion

Results of the conventional digestion runs (presented in Table 2) showed that the Salem primary sludge could be digested despite the presence of heavy metals and industrial waste, and that good methane yields could be obtained at HRTs of 13 days or higher. However, methane yield decreased and volatile acids (VA) accumulated to inhibitory levels at lower HRTs and higher loadings. The highest methane yield of 0.31 m3/ kg VS (5 SCF/lb VS) added was observed at an HRT of 18 days. The highest methane production rate of 1.9 vol/culture was observed at an HRT of 6.9 days and with a feed TS concentration of 12.6 wt%. It can be concluded from these observations that the single-stage complete-mix digester exhibited unbalanced or 'sour' digestion under conditions of high loadings and short HRTs and with concentrated feeds. An advanced fermentation mode and the application of a novel reactor design were needed to obviate the difficulties of conventional digestion and to obtain increased methane production at short HRTs and high loading rates.

Innovative upfiow two-phase mesophilic (35°C) digestion

A 3 l-liter advanced two-phase system, as depicted in Fig. 1, was utilized to conduct the two-phase fermentation runs. Culture volumes of 6 liters and 22 liters were used for the acid- and methane-phase reactors. Both digester cultures were maintained at 35°C. These cultures were developed from inocula derived from a draw-off from a municipal high- rate digester in Chicago. Digester feeding, effluent withdrawal and gas collection were continuous and automated.

The two-phase system was fed with raw primary sludge, the chemical characteristics of which are reported in Table 3. The elemental composi- tion of the sludge was used to estimate the empirical formula, and to calculate the stoichiometric gas and methane yields and theoretical gas composition as follows:

Total gas yield -- 0.97 SCM/kg VS reacted Methane yield -- 0.62 SCM/kg VS reacted Carbon dioxide yield -- 0.34 SCM/kg VS reacted Methane concentration -- 64.7 mol% Carbon dioxide concentrat ion- 35.3 mol%

100 S. Ghosh

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Two-phase anaerobic digestion of municipal-industrial sludge 101

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The above theoretical yields and gas composition were calculated based on the following assumptions:

All the carbon (organic and inorganic) in the feed was biochemically reactive and was converted to and incorporated into the product gases (methane and carbon dioxide). 20% of the VS was utilized for cell bio- synthesis.

The raw sludge was slightly acidic, and was apparently a nutritionally balanced feed. The heavy metals were not present in concentrations inhi- bitory to anaerobic digestion.

The acid-phase digester was operated with concentrated bottom sludge withdrawn at the rate of about 20 vol% of the feed flow rate. There was no withdrawal of bottom sludge from the methane digester. Steady-state performance data compiled from operation of the advanced upflow two-phase system for a period of about two HRTs are presented in Tables 4 and 5. Table 4 shows that a methane yield of 0.42 SCM/ kg VS (6.7 SCF/lb) added could be obtained at a system HRT of 5.5 days. Acidogenic fermentation was predominant in the first-stage diges- ter, as expected and as indicated by the culture pH, ORP, methane pro- duction and effluent volatile acids concentrations. The methane yield

TABLE 3 Chemical Characteristics of Raw Primary Sludge Used to Operate the

Advanced Two-Phase Digestion System

Analysis Results

Total solids (wt%) 6.11 Volatile solids (dry wt% of TS) 60.98 Ash (dry wt% of TS) 39.02 Volatile suspended solids (mg/liter) 28 230 Ultimate analysis (dry wt%)

Carbon 38.4 Hydrogen 5-9 Nitrogen 2.70 Sulfur 0.88 Phosphorus 2"94 Oxygen (by difference) 16"7

Heating value (kJ/kg) (dry) 2 473 Ammonia nitrogen (mg/liter) 260 Kjeldahl nitrogen (mg/liter) 1 400 Total COD (mg/liter) 45 140 Filtrate COD (mg/liter) 6 960 Filtrate TOC (mg/liter) 2 360 Volatile acids (mg/liter)

Acetic 1 500 Propionic 810 Isobutyric 120 n-Butyric 307 Isovaleric 200 n-Valeric 40

Total as acetic 2 600 Alkalinity (mg/liter as CaCo3)

Total 11 800 Bicarbonate 9 630

pH 6'7 ORP (mV) + 307 at 33°C Metal content (rag/liter)

Cd 2"4 Ca 3 990 Cu 16"0 Ni 2"8 Pb 23 Zn 37 Cr 3+ 566 Cr 6+ 0'001 Cr (total) 566

Solid phase 561 Liquid phase 5

Ba 8-8 Hg 0'01 Se 0.5 Fe 416

Two-phase anaerobic digestion of municipal- industrial sludge 103

TABLE 4 Performance of the Advanced Two-Stage Upflow Mesophilic (35°C) Digestion System Operated at an HRT of 5"5 days (1.1 days for Acid Phase and 4.4 for Methane

Phase) with a 6.1 wt% TS-Content Feed

Operating Acid phase Methane phase System conditions~performance

Run duration (no. of HRTs) Loading rate (kg/m3-day) VSS (mg/liter)

46 33'6

5050 (overflow)

57880 (underflow)

13 10 8-7 6"9

19640 19640

Gas production Methane yield (SCM/kg 0.003 0.416 0.419

VS added) Methane production rate 0.09 3.51 2.78

(vol/culture vol-day) Gas composition (mol%)

C H 4 34"2 68"1 6-78 CO 2 61'4 31"3 31'5 N 2 4'4 0'6 0"7 H 2 0'0 0"0 0'0

Effluent characteristics pH 6.1 7.2 7.2 ORP (Ec)(mV) - 289 - 354 - 354

Alkalinity (mg/liter as CaCO3) Total 7 850 14 600 14 600 Bicarbonate 3 820 14 550 14 550

NH3-N (mg/liter) 520 720 720 Volatile acids (mg/liter)

Acetic 2 490 40 40 Propionic 1 900 21 21 Isobutyric 210 0 0 n-Butyric 560 0 0 Isovaleric 300 0 0 n-Valeric 180 0 0

Total as acetic 4 835 60 60 Ethanol (mg/liter) 20 0 0 COD (mg/liter)

Total Overflow 16 970 17 980 17 980 Underflow 67 100 - - - -

Fil trate Overflow 9 310 2 312 2 312 Underflow 8 820 - - - -

Soluble TOC (rag/liter) Overflow 3 140 610 610

104 S. Ghosh

TABLE 5 Performance of the Advanced Two-Stage Upflow Mesophilic (35°C) Digestion System Operated at an HRT of 5.9 days (1-3 days for Acid Phase and 4.6 days for Methane

Phase) with a 5.8 wt% TS-Content Feed

Operating Acid phase Methane phase System conditions~performance

Run duration ( no. of HRTs) 35 10 8 Loading rate (kg/m3-day) 28.84 7.85 6.25 Gas production

Methane yield (SCM/kg VS 0.06 0.42 0"48 added)

Methane content (mol%) 59-2 70.1 68"4 Methane production rate 1.77 3.27 2.96

(vol/vol-day) Effluent characteristics

pH 6"6 7.2 7.2 Volatile acids (mg/liter)

Acetic 643 77 77 Propionic 2251 48 48 Isobutyric 123 0 0 n-Butyric 141 0 0 Isovaleric 266 0 0 n-Valeric 79 0 0 Caproic 0 0 0

Total as acetic 2827 118 118 Ethanol (mg/liter) 0 0 0

was 67% of the theoretical value, the highest reported yield for sewage sludge or other particulate feeds 4-6 under comparable conditions of high loading, low HRT and concentrated sludge feed used in this work. The methane production rate of 3"5 vol/culture vol-day was also higher than those reported for sludge or other particulate feeds. About 69.5 wt% of the VS feed was converted to gas under the operating conditions indi- cated in Table 4; thus, the overall VS reduction was more than 69"5%.

Data reported in Table 5 showed that a still higher methane yield of 0.48 SCM/kg VS (7.7 SCF/lb VS) added could be obtained by operat- ing the system at a higher HRT and with a less concentrated feed sludge. This methane yield was 77% of the theoretical value. About 74.5% of the VS feed was converted to gas under the operating conditions indicated in Table 5; the overall VS reduction was thus greater than 74.5%. Since it has been reported that less than 80% of the sludge VS is biodegradable, the VS reductions observed in this work could be the maximum attain- able.

Two-phase anaerobic digestion of municipal-industrial sludge 105

TABLE 6 Comparison of Hypothetical Conventional and Two-Stage Upflow Mesophilic (35°C) Digestion Systems to Stabilize and Gasify 90.9 t/day (Dry Solids Basis) of Sludge at an

HRT of 5"5 days

Operating conditions~performance Conventional Two-stage u pfl o w

Feed VS (wt%) 2.2 Loading rate (kg/mLday) 4.01

Methane yield (SCM/kg VS added) 0.125 Methane production rate (vol/culture vol-day) 0.5 VS reduction to gas (%) 24

Gross methane production (m3/day) 6 797 Estimated operating energy requirement

( 10 6 kJ/day) Feed sludge heating 256 Mixing 6.33 Pumping 2.11 Heating, ventilation, lighting, other 8.44

Total 273 Net energy production ( 106 k J/day) - 20 Digester volume ( 103 m 3) 13"6

3'7 6'57 0"481 2-8

75 26621

161 0 3"2 5'3

170 823

8"3

ENGINEERING SIGNIFICANCE

The two-stage upflow digestion process has the notable advantage of producing a substantially larger quantity of methane with reduced fermenter volume (and reduced plant capital cost) relative to that of a conventional digestion process. As an example, for a 90"9 t/day sludge solids (dry) load, a conventional single-stage CSTR digestion plant operated with a 3.6 wt%TS feed slurry would require 13600 m 3 (480 000 ft 3) of fermenter volume (Table 6) for operation at a 5.5-day HRT. Based on data in Table 2, a TS concentration of 3"6 wt% was selected to obtain balanced digestion at the selected short operating HRT of 5"5 days. This process is a net energy consumer because the amount of energy required for plant operation is larger than the energy value of the produced digester methane. By comparison, the two-stage upflow digestion process operated with a 6 wt% TS feed slurry is pro- jected to require a total fermenter volume of 8270 m 3 (292 000 ft3), and it could exhibit a net energy production of 823× 106 kJ/day (780 million Btu/day). Thus, whereas there is no net energy production from conven- tional digestion, about 83% of the methane from the two-stage process is available as surplus.

106 s. Ghosh

Another important advantage of the two-stage upflow digestion pro- cess is the substantially higher volatile solids reduction that is achieved relative to that of conventional digestion (Table 6). Thus, the cost of digested sludge disposal for the two-stage system could be one-third of that for the conventional process.

CONCLUSIONS

Work with the high-metal content municipal-industrial primary sludge showed that this material could be digested to exhibit high methane yields despite the presence of a number of heavy metals. Methane yields up to 0.31 SCM/kg VS (5 SCF/lb VS added) (50% of the theoretical yield) could be obtained at an HRT of 18 days during conventional single-stage high-rate digestion. A maximum methane production rate of 1.9 vol/vol-day was obtained with conventional digestion, but with an unusually high feed TS concentration of 12.6 wt%. Unbalanced sour digestion occurred in the conventional digester at HRTs lower than 10 days and with concentrated sludge feed.

Work with an innovative two-phase system showed that an HRT between about 0.9 and 1.5 days would be optimum for upflow acid- phase digestion. An HRT between about 4 and 5 days would be opti- mum for upflow methane-phase digestion. No mechanical or compressed gas mixing was necessary for the upflow digesters. Unlike single-stage CSTR digestion, upflow two-phase systems could be operated with concentrated sludge without souring of the methane fer- mentation process. Liquefaction of organic solids and production of acetate and higher volatile acids were the predominant biochemical reac- tions in the acid-phase digester; about 50 wt% of the particulate organics were liquefied in the acid digester. There was little conversion of higher VAs to acetate in the acid-phase digester. Conversion of higher VAs to acetate and to methane were the major reactions in the methane digester. Acid-phase reactions occurred at lower pHs and higher ORPs than those of the methane-phase digester. There was no need to discharge concen- trated sludge from the bottom of the methane digester. Methane yields of 0.42 SCM/kg VS (6-7 SCF/lb VS) added and 0.48 SCM/kg VS (7.7 SCF/lb VS) added (77% of the theoretical yield) were obtained from upflow two-phase digestion at HRTs of 5.5 days and 5.9 days, respectively. The methane digester exhibited a methane production rate of 3.5 vol/vol-day with 6-7 wt% TS feeds and at HRTs of 5-6 days; higher rates are expected with more concentrated feeds and lower HRTs.

About 75% of the raw sludge VS was gasified; this could be the maxi- mum attainable feed conversion efficiency, considering that less than

Two-phase anaerobic digestion of municipal- industrial sludge 107

80% of sludge VS is normally biodegradable. Considerable energetic and economic advantages accrue from application of upflow two-phase digestion. More than 80% of the methane energy could be available as surplus energy in the case of this novel process compared with a negative energy balance for single-stage conventional digestion. Digester volume for the advanced two-phase process could be 60% of that for conven- tional digestion; similarly, the cost of residue disposal could be reduced by a factor of 3 by employing upflow two-phase digestion.

A C K N O W L E D G M E N T

The work was conducted, in part, in the author's laboratory at the Insti- tute of Gas Technology (IGT). This research was supported by Bay State Gas Company, Boston Gas Company, Brooklyn Union Gas Company, Cogenic Systems Inc., Essex County Gas Company, Valley Resources, Inc. and the Executive Office of Energy Resources, Commonwealth of Massachusetts. The efforts of Messrs M. P. Henry and S. Sajjad in helping with data collection and analysis are gratefully acknowledged.

R E F E R E N C E S

1. Ghosh, S. & Klass, D. L. (1977). Two-PhaseAnaerobic Digestion, US Patent 4,022,655.

2. Ghosh, S., Conrad, J. R. and Klass, D. L. (1975). Anaerobic acidogenesis of sewage sludge. J. Water Pollution Control Fed., 47( 1 ), 30.

3. Ghosh, S., Henry, M. P., Tarman, P. B., De Proost, V. H., Pypin, P. and Ombregt, J. P. (1983). Stabilization of high-COD industrial wastes by two- phase anaerobic digestion to maximize net energy production. Proc. of the Ind. Waste Symp., 56th Annual Conf., Water Pollution Control Fed., Atlanta, GA.

4. Fannin, K. F., Conrad, J. R., Srivastava, V., Jerger, D. E. and Chynowith, D. E (1983). Anaerobic processes. J. Water Pollution Control Fed., Literature Review Issue, 623-31, 55 (6), 623-31.

5. Fannin, K. F., Conrad, J. R., Srivastava, V., Jerger, D. E. and Chynowith, D. P. (1982). Anaerobic processes. J. Water Pollution Control Fed., Literature Review Issue, 54 (6).

6. Fannin, K. E, Conrad, J. R., Jerger, D. E., Srivastava, V., Ghosh, S. and Chynowith, D. P. (1981). Anaerobic processes. J. Water Pollution Control Fed., Literature Review Issue, 53(6).