Developments in anaerobic digestion of organic wastes in Israel
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Biological Wastes 26 (1988) 275-284
Developments in Anaerobic Digestion of Organic Wastes in Israel
S. Kimchiefl S. Tarrefl E. Lumbroso, b M. Green ~ & G. Sheleff
~Department of Environmental Engineering and Water Resources, Technion-- Israel Institute of Technology, Haifa 32 000, Israel
bOrganic Recycling Ltd, PO Box 5604, Herzlyia 46497, Israel
An overview of the research conducted at the Technion, Israel, on recycling organic wastes, using anaerobic digestion to produce valuable by-products and provide an ecologically sound solution to their disposal, is presented. The development of a thermophilic process, a two-step thermophilic-mesophilic process, an integrative piggery waste process and applications of the solid and liquid by-products are discussed.
The intensive agricultural practices applied in Israel have generated growing quantities of ecologically offensive wastes. This, along with the rapid rise in energy costs during the 1970s, gave the impetus to expand research on agricultural pollution abatement and fuel production using anaerobic digestion. Israel's poor natural resource assets and collective farming infrastructure made for a promising opportunity for application of this technology.
Organic wastes, when processed by anaerobic digestion, can be converted into valuable product streams. The first, methane-rich biogas, can easily replace, or work in tandem with, existing fuels. Concurrent to biogas production, high-value liquid and solid by-products are made during the separation of the digested waste. The liquid can be used for its soil conditioner and fertilizer value. The liquid's potential fertilizer value in terms of nitrogen is about 22 000 tons per year. This represents more than
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276 S. Kimchie, S. Tarre, E. Lumbroso, M. Green, G. Shelef
40% of Israel's nitrogen demands for fertilization (FAO Fertilizer Yearbook, 1985). In addition, the liquid fraction has been used successfully as an amendment for fresh-water fish ponds (Lumbroso et al., 1980).
The solid fraction, after proper aerobic maturation, has been successfully applied as a valuable plant growth medium (called EGMA--Enriched Growth Medium for Agriculture). EGMA's uniform consistency, fertilizer value and enhancement of plant resistance to disease has in most instances made it the preferred organic component in horticultural mixed potting media. Due to EGMA's high market value (about $50 per m 3 wet matter), anaerobic installations have become increasingly attractive.
DEVELOPMENT OF ANAEROBIC DIGESTION IN ISRAEL
In 1975 we began an intensive and long-term programme to develop anaerobic technologies for the treatment of agricultural and municipal wastes. Using cow manure as the first substrate, reactor systems were constructed on the bench and pilot-plant scales. Full-scale plants were built in Israel and abroad according to the research and development results. The laboratory units were used for the initial screening of substrates and operating conditions, while the pilot-plant dealt with the engineering aspects of materials handling, pumping and mixing, digester configuration, separation and the development of gas and digested-slurry applications. As a result of this ongoing research programme three main processes were developed: a thermophilic process, a two-step process and an integrative process for piggery-waste effluents. The essential features of each of these processes will be reviewed in this paper.
THE THERMOPHILIC PROCESS
An important achievement in the application of anaerobic digestion to agricultural wastes was the development of a thermophilic process (Shelef et aL, 1980). Thermophilic digestion has the advantage of being capable of producing large volumes of gas at high organic loadings and short retention times, along with a high degree of pathogen destruction.
Thermophilic (55C) and conventional mesophilic (35C) systems were thoroughly compared using multiple laboratory units. The systems (4 liters working volume each) were examined over a wide range of hydraulic retention times (6-16 days) and feed-solids concentrations (3-18% total solids), i.e. organic solids loading of up to 24 g of volatile solids/liter/d. The results of these experiments are shown in Fig. 1.
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Digesters of up to 20 m 3 working volume, operated in batch or continuous operation mode, were used. The biogas production profile of a second stage, non-heated 1 m 3 pilot-plant reactor, fed with the effluent of a thermophilic cow manure digester, is given in Fig. 2. Figure 2A displays the spontaneous temperature decline of the digester, from about 55 to 35C, during the 18 day period. During this gradual cooling, the conditions in the reactor change from thermophilic to mesophilic. This transition is coupled with the appearance of a 'wave' of biogas production. The 'wave' starts after 6-7 days of retention time in the second stage reactor, coming to a peak after another 3-4days and then gradually slowing. The main source of mesophilic bacteria, which are active at this stage, is the cow manure.
Maximum utilization of the organic residues and of the waste heat from the thermophilic digester effluent is achieved at the second stage of the process, entitled also 'Residual Digestion Stage'. The economy of the 'residual stage' is based on its simple design (unheated, uninsulated,
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unmixed), low price, and high biogas productivity (up to 80% of the biogas yield of the first thermophilic stage, see Fig. 2B). Table 1 summarizes a feasibility comparison of several design alternatives in three retention times: 8 day, 16 day, and 24 days. The 16 day and 24 day are subdivided into two operation alternatives: one is a conventional CSTR (continuously stirred tank reactor) for the whole retention time period while the second alternative includes an 8 day retention time in a thermophilic CSTR step and the rest of the period in a second-step reactor. The figures are based on the calculations done for a 500 cow dairy. Table 1 indicates that by extending the retention time in both alternatives, the increase in net gas productions in the two-step system exceeds the conventional 'CSTR only' system.
TABLE l Feasibility Comparison Between One-step Thermophilic CSTR Process and Two-step
System. Figures Based on 500 Head Dairy Cowshed
Parameter Digester Retention Time (RT) a CSTR--55C Two-step system b
8 days 16 days 24 days 16 days 24 days
Gross daily gas production (m 3) 540 745 830 720 870 Net daily gas production (m 3) 360 550 625 540 690 NEPC c 1.57 1.48 1.27 2.0 2.4
a Retention time for two-step system includes first-step thermophilic digestion with 8 day RT. h Second-step reactor is a low cost, plastic bag, plug-flow type anaerobic digester. c NEPC (net energy per unit cost) values are given in millions of kcals.
Net energy production per unit cost (NEPC), as defined by Hashimoto and Chen (1980), was used as an economic index to compare the feasibility of the various alternatives. As indicated in Table 1, the value of the economic index drops with the extension of retention time in the 'CSTR only' system, while there is an increase in NEPC values as the retention time extends from 8 to 16 and to 24 days for the two-step system. In practical terms, the advent of the residual digestion justifies the integration of digested slurry and biogas storage under the same simple plastic cover, as demonstrated by the low cost, 50m 3 plastic bag, plug-flow type reactor at the pilot-plant. In addition, the quality of the digested slurry improves as reflected by the drop in Salmonel la by 2-3 log numbers over the thermophilic CSTR reductions (Klinger & Marchaim, 1985). The cation-exchange capacity and water- holding capacity characteristics of the solids also improve (Raviv et al., 1987; Tarre et al., 1987).
280 S. Kimchie, S. Tarre, E. Lumbroso, M. Green, G. Shelef
BY-PRODUCT DEVELOPMENT AND APPLICATIONS
Early on in the research programme, applications for the by-products were identified and investigated. Digested slurry was successfully tested as a livestock fodder, a component in particle board, an agricultural fertilizer and fresh water fish culture amendment (Marchaim & Criden, 1981).
To ease digestedzslurry handling and to create more than one product stream, a separation technology was employed. Horizontal vibrating screens were chosen because of the degree of control obtained when rinsing and sieving digested cow manure.
Piles of freshly prepared fibrous solids (EGMA) spontaneously autoheat to 50-60C due to exothermic aerobic microbial breakdown (Tarre et al., 1987). This maturation process continues for up to two months in static piles, but can be reduced to one month using temperature control. Figure 3 illustrates the differences between the crude-fiber profile of typical EGMA produced at the industrial facility of Kibbutz Yagur before and after aerobic maturation. The aerobic process concentrates the refractory components (ash and lignin) at the expense of cellulose and hemicellulose. The amount of volatile solids decomposed for one- and two-stage digested cow manure during temperature controlled maturation (550C) is presented in Fig. 4. Due to the extension of digestion time, the fibrous solids derived from the two- stage system are more stable, accounting for the differences in volatile solids reduction.
The properties and uses of EGMA (Enriched Growth Medium for
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Fig. 4. Weekly volatile solids reduction during aerobic maturation of EGMA separated from one-stage (thermophilic) and two-stage (thermophilic + mesophilic) digested cow manure. 1, first week; 2, second week; 3, third week; 4, fourth week; 5 and 6, fifth and sixth
week; Cure., cumulative VS destruction.
Agriculture, also known in Israel under the Hebrew name 'cabutz') as the organic component in growing media for intensive horticulture have been documented (Chen et al., 1983; Raviv et al., 1983). Plant growth in matured EGMA was superior to the growth in fresh EGMA and peat, with cucumbers, tomatoes and peppers (Inbar et al., 1985). Matured EGMA's suppressiveness against soilborne fungi has been demonstrated (Chen et al., 1988). Additionally, fresh EGMA has been shown to be an ideal casing material for mushroom cultivation (Levanon et al., 1988).
ANAEROBIC DIGESTION OF PIGGERY-WASTE EFFLUENTS (PWE)
Pig husbandry in Israel is small (about 70 000 head), but concentrated into four main breeding centers. The severe water and air pollution problems created by the piggery effluents can be successfully treated using an integrative approach developed by our research group. The process developed, outlined in Fig. 5, is characterized by primary mechanical separation followed by separate treatment for the liquid and solids fractions.
The liquid fraction, after a short time of settling, represents over 85% of the original waste volume, but contains only a quarter of the original COD. The liquid from the settler is anaerobically treated by a high-rate digester of
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the UASB (Upflow Anaerobic Sludge Blanket) type. Using this reactor, over 93 % of the inftuent COD is removed. The UASB effluent is now evaluated as a diluted liquid fertilizer, either for direct use or after an additional 'polishing' stage. Reduction of COD in the liquid fraction by the various treatment steps is illustrated in Fig. 6.
Anaerobic digestion in Israel 283
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The separated solid fraction is anaerobically digested in a daily-fed CSTR type digester. Gas yields of 0"29 m 3 biogas (60% methane) per kg of volatile matter fed, were obtained under thermophilic (55C) conditions at a retention time of 10days and a volumetric organic loading of 10"9 kg VS/ m3/d. The digested slurry from the CSTR was aerobically treated yielding a highly stable and odorless material which was successfully used as an EGMA product.
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284 S. Kimchie, S. Tarre, E. Lumbroso, M. Green, G. Shelef
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