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The project aims at reducing the polluting load and stabilizing the waste sludge of a tannery near Lyon, France by two stage biomethanation, hereby producing biogas, equivalent to 31 TOE/yr, as a fringe benefit.
The innovating character of the project lies in the hazardous character of the waste to be treated biologically, because of its high chrome, as Cr+3, content. Similarily, the two-stage system choosen should increase the reliability of the biomethanation process by comparison with the one stage process.
1. A preliminary study with positive results in batch pilot reactors was conducted by the IRCHA (Institut de Recherches Chimiques Appliquees) at Vert-le-Petit, France, under grants ANRED 80.50.063 of 16 September 1980 and
COMES 8121.069.3295.
2. Start up of the biomethanation of tannery wastes has proven a slow and difficult process and required and external inoculum of animal manure (1/3 of one of the digesters volume).
3. A number of clogging problems occured during the start up period, due namely to the low diameter (30 mm) of the pipeline system around the methane digesters.
This was partly solved by enhancing the shredding from 10 mm down to 6 mm average particle size.
4. A single loading and mixing pump has proven weak for the diverse work to be done, a problem not really solved.
5. Hydrogen sulfide content proved high, up to 3 %. Some limited trials have been done for the biogas burning in the factory boiler.
6. The strictly biomethanation process proved successful. 75 % ofthe pollution load can in principle be removed by this system with concomittant reduction of 60 % of the final sludge volume. 20 % of the produced energy had to be utilized in the biogas plant itself. 80 % are available for use in the works. Optimization of the waste treatment should lead to respectively 10 % and 90 %.
7. On the basis of the optimization of the present results, the payback of a tannery waste treatment biogas plant decrease from 23 to 5 years respectively with the increase from 5 to 40 tons per week to be treated. The latter estimation does not take into account any unpolluting fringe benefits.
8. The tannery of Ronzon has decided not to take over the biogas plant for sole low economy reasons, due to too low present energy prices.
The tannery of Ronzon, at St-Symphorien sur Coise, near Lyon, has a daily polluting load of 8.5 m3 mixed sludge at 7 % total solids content with a high chrome (up to 3 %, as Cr+3) and lime content, originating for 4/5 as sludge resulting from the alumine treatment of waste water and 1/5 of crushed fleshing and cutting wastes.
The biogas plant consists of two vertical cylindrical steel digesters of 65 m3 volume each in series. Mixing will occur by liquid recirculation with 2 pumps to avoid crust formation. Upstream the sludge will be shredded below 10 mm, mixed and pre-heated at 35 C. Downstream, the biogas will be stored in a 100 m3 flexible gasholder, compressed to 3 bars and used in the main furnace of the factory, equipped with a dual-fuel burner, for heating purposes in the tannery.
The digester will operate in the completely-mixed, continuous mode without recycle. The mean residence time will be 20 d under mesophilic conditions. The expected biogas production is 1 m3 per m3 of digester volume and per day, equivalent to 31 TOE/yr. The produced biogas is expected to be treated first in a water tower to remove carbon dioxide, CO2, and, secondly, for hydrogen sulfide, H2S, removal in a ferrous-compound-containing tower. The digested sludge returns to the existing sludge treatment line to be centrifuged for dewatering before dumping. Due to too high a chrome content, the residual sludge cannot be disposed of on agricultural lands.
The expected simple payback is 12 years.

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9 Av Jules Carteret Cedex 07 B.p. 7001
69342 Lyon