NL8001997A - METHOD FOR ANAEROOB COMPOSTING OF SOLID ORGANIC WASTE MATERIAL. - Google Patents

Description

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Method for the anaerobic composting of solid organic waste material.

The invention relates to a method for anaerobically composting solid organic waste material of vegetable and / or animal origin.

According to the "Winkler Prins Technical Encyclopedia" (1) p. 119 5 (1975), a long-used method for composting such solid organic waste material is aerobic composting.

The composting process consists in that the easily attackable organic compounds are broken down by microbial action under aerobic conditions. The most common way in which the composting process takes place is the open air method, in which the waste is accumulated. These heaps are moistened during the process and converted several times using a shovel or grab crane.

This method requires a lot of labor, time and space, which entails considerable costs. Other drawbacks are that, on the one hand, valuable organic material is partly converted into worthless carbon dioxide and water during aerobic composting and, on the other hand, contamination of the groundwater takes place.

According to the Journal of Water Supply and Wastewater Treatment (22) 531 (1977), low viscous and low dry matter water-containing suspensions and solutions, of organic waste materials and inorganic salts, can be fermented anaerobically. In an open pond, under the influence of optionally anaerobic bacteria, under non-strictly anaerobic conditions, part of the polysaccharides (including cellulose) present are hydrolyzed to 25 soluble monomers, which are subsequently converted into fatty acids. strictly anaerobic conditions, formed from the fatty acids methane Such a method cannot be used for the composting of solid organic waste material.

According to "HgO" (10), 296 (1977) an improvement of this anaerobic digestion of low viscous, low dry matter containing water-rich suspensions and solutions can be achieved by performing the pre-stage in a tank under strictly anaerobic conditions, but also this method is not yet suitable for composting solid organic waste material.

In these 2 embodiments of the anaerobic digestion of low viscous, low dry matter aqueous suspensions and solutions of organic material, a, preferably granular, 800 1 9 97 -2 bacterial sludge is separated from the effluent and the gas.

In contrast to low-viscous, low dry matter-containing water-rich suspensions or solutions of organic material, from solid organic waste material, if one wishes to form methane in one step, the reaction speed is very slow. The reason for this is that such a high degree of acidity is quickly reached that the environment has a strong growth inhibition for both the acid-forming and the methane-forming microorganisms. As a result, no further acid formation, nor methane formation, but odor development (h 2 S, other sulfur compounds and NH 1) takes place (see 10 Journal of the environmental engineering division, June 1978, pp. 415-422).

The variation of the percentage of dissolved COD and of the acid formation with time in such a conversion is graphically shown in Fig. 1, the dotted line indicating the content of dissolved fatty acid COD.

It can be seen from this figure that in such a case more than 25% of the maximum soluble COD is dissolved already after 3 days before acidification from a graft mixture of straw + beet pulp + dried cow manure. The COD then consists of 14% fatty acid COD. After 30 days, this appears to have increased to about 25%. Methane gas is not formed here.

Attempts have also been made to carry out the anaerobic fermentation of solid organic waste material by the above-described methods for low viscous, low dry matter suspensions or solutions of organic material, by first grinding the dry material, then suspending it in water, and subsequently anaerobically fermented. However, it has been found that the dry matter content of such a suspension cannot exceed about 5% by weight.

In addition, U.S. Department of Commerce publication PB-258499 of August 1976, page 30 et seq., Page 36, describes a method of using the effluent to obtain a dispersion of 12 wt% solids to to make the dispersion from the solid waste.

The drawbacks of these processes are that energy must be consumed to grind the solid, while the low concentration of the suspensions necessitates the use of very large reactors.

It has been found that the composting of solid organic waste materials using an anaerobic composting process without energy consumption for grinding the solid materials and with short residence times in reactors can be carried out by the solid organic waste material in at least one reactor under strict anaerobic conditions, the water-soluble 800 1 9 97 formed thereby

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-3- Fatty acids with other soluble organic and inorganic substances by largely removing rinsing with water and passing the water-rich solution formed therein to at least one auxiliary reactor, in which the organic material is converted in a manner known per se under strictly anaerobic conditions. mixture of carbon dioxide and methane, and discard the compost formed.

In the process according to the invention, the concentration of inhibitors in the reactor is reduced by rinsing with water and the pH reduction is avoided. This prevents the growth of the microbial flora from being inhibited by too high an inhibitor concentration or too low a pH of the environment.

Although the rinsing can take place with freshly supplied water, it is nevertheless preferable to use the effluent of the reactor after conversion of the fatty acids present therein in the auxiliary reactor, with the salts present in the effluent, by recirculation as rinsing liquid in the reactor. to bring.

The reported anaerobic fermentations can be carried out both in the mesophilic temperature range (5 to 45 ° C) and in the thermophilic temperature range (25 to 70 ° C).

20 Examples of solid organic material to be processed are household waste, straw and vegetable waste.

It has been found that in the case of a semi-stagnating procedure, that is to say that the liquid from the reactor is drained once every 24 hours and is changed, a COD solution curve as shown for example in Fig. 2 is obtained. In this case, the dissolved COD consists of about 30% fatty acid COD after 2 days and about 35% fatty acid COD after 7 days.

With continuous flushing, depending on the flow rate, a COD progression in the reactor effluent with time 30 as shown in FIG. 3 is obtained.

Effluent from the auxiliary reactor was used as flushing agent for the reactor.

After 10 days, therefore, little COD appears to be dissolved from the reactor and the dotted line indicating the fatty acid-COD content shows that this soluble COD consists almost entirely of fatty acids.

Gas production by such conversion in the thermophilic temperature range is shown in FIG. The completed lyn represents the gas in the reactor. This gas mainly consists of CO 2 for the first 15 days and then changes into a mixture of carbon dioxide and 800 1 9 97 ** -4-methane, the methane of which constitutes about 50% by weight. The dotted line indicates methane formation.

After 20 days, about 50% of the fermentable solid organic material has broken down and discarded as a solution.

In the practical implementation of the method it is expedient for more than one reactor to be present per auxiliary reactor. This way of working makes it possible to supply an evenly composed influent to the auxiliary reactor, which therefore works optimally.

Flushing of a reactor can also be omitted if for any reason the built-up fatty acid concentration is still too low to form a meaningful feed to the auxiliary reactor. On the other hand, the conditions in a reactor can become so favorable that it will act as a reactor and auxiliary reactor and will yield a substantial amount of methane production. In that case, it makes no sense to flush the reactor with water or effluent from the auxiliary reactor.

It hardly needs explanation that in such a combination of reactors and auxiliary reactors, the reactors operate batchwise and the auxiliary reactors operate continuously. The auxiliary reactor works faster in the system according to the invention than in the normal use of an up-flow reactor, because the effluent from the reactor, unlike the optional anaerobically acidified material of the known low-viscous, low-dry matter process containing dispersions, does not contain floating parts of slowly digestible material.

The compost material from the reactor has a C: N 25 ratio such that it is practically odorless and contains all the salts that were present in the solid organic starting material.

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Claims (4)

Method for the anaerobic composting of solid organic waste material, of vegetable and / or animal origin, characterized in that the solid organic waste material is broken down in at least one reactor under strictly anaerobic conditions, the water-soluble fatty acids formed thereby with other soluble organic and inorganic substances, is largely removed by rinsing with water and the aqueous solution formed thereby is passed to at least one auxiliary reactor, in which the organic material is converted in a known manner under strict anaerobic conditions into a mixture of carbon dioxide and methane, and discards the compost formed. 2. Process according to claim 1, characterized in that more than one reactor is used per auxiliary reactor, which are charged intermittently and which are connected to the auxiliary reactors via the closable lines. Process according to claim 1 or 2, characterized in that effluent from at least one auxiliary reactor is used to flush at least one first reactor. 4 ·. Process according to claim 1, characterized in that the rinsing is omitted as soon as the methane formation in the reactor can take place independently. 800 1 9 97