Liquid purification or separation – Processes – Treatment by living organism
Patent
1993-08-26
1997-01-14
Lithgow, Thomas M.
Liquid purification or separation
Processes
Treatment by living organism
210604, 210605, 210607, 210609, 210630, 210621, 210631, 210903, 210906, C02F 328, C02F 1112, C02F 1114
Patent
active
055935907
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the treatment of a starting mixture of liquid and solid organic wastes, which mixture is susceptible to at least partial microbiological degradation, with formation of biogas.
2. Description of Related Art
It has already been proposed (French Patent 2,541,669) to mix sludge obtained from the liquid phase of wastes by anaerobic fermentation (and optionally subsequent aerobic treatment) with the solid components intended for composting. However, this process has the disadvantage that separate thickening must be provided for relatively small amounts of sludge, or that undesirably large amounts of liquid are introduced into the composting.
Owing to its CO.sub.2 content, it has already been proposed (EP-A1-0302852) to use engine exhaust gas, possibly from the combustion of biogas, for pH reduction in intermediate stages of fermentation or rotting. In the first case, expensive fuel gas or fuel oil must be used; in the second case, the CO.sub.2 in any case present in the biogas is carried along as ballast and must be heated.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a process in which these disadvantages are avoided, which surprisingly is achieved at least partially by the the following steps:
mechanically separating a starting mixture of liquid and solid wastes that is susceptible to at least partial microbiological degradation, with formation of biogas, into a first sludge phase with enriched dry substance and a first liquid phase containing dilute solids;
subjecting the first liquid phase to anaerobic treatment, leading to the formation of biogas and to the development of a second sludge phase and a second liquid phase;
subjecting the first sludge phase to aerobic treatment resulting in the formation of compost, fertilizer, or feed; and
recirculating at least part of the second sludge phase into the starting mixture, thereby causing the formation of biogas in the starting mixture and facilitating decomposition of higher compounds contained in the starting mixture.
In the anaerobic treatment as well as in any subsequent aerobic treatment of the liquid phase, only a small amount of sludge is obtained; a not insignificant part of the organic wastes and the major part of the nitrogen and phosphorus remain dissolved, pollute the outflow sewer or the wastewater treatment plant and are lost to the production of fertilizers or the like.
It has therefore also been proposed to precipitate these substances from the liquid phase by chemical methods, in particular by flocculation; however, the resulting precipitate is either disposed of in a landfill or incinerated, once again both the groundwater and the atmosphere being polluted.
According to the invention, it has proven particularly expedient to absorb the CO.sub.2 contained in the biogas in the starting mixture and/or in one of the subsequent liquid phases with simultaneous reduction of their pH; on the one hand, some of the 30 to 40% by volume of the CO.sub.2 in the biogas is thus subjected to direct chemical reaction without detours and achieves an object for whose achievement special, expensive measures would otherwise be necessary; on the other hand, the biogas is enriched and thus attains a higher calorific value, i.e. becomes more valuable.
The precipitate obtained by chemical precipitation or flocculation in one of the liquid phases has to date been disposed of in a landfill or incinerated, as mentioned above; in connection with the genera of the process according to the invention, this precipitate--if necessary after thickening--can now be added to the solid components and/or--preferably--the starting mixture, with the result that not only is the pollution of the environment (groundwater, atmosphere) avoided but in addition the valuable substances (organic humus formers, nitrogen and phosphorous compounds) contained in the precipitate are retained and can be returned to the soil.
If--as is preferred--the precipitate is added to the starti
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Lithgow Thomas M.
Technoagrar Consulting AG
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