FR2787102A1 - PROCESS FOR THE TREATMENT AND MANAGEMENT OF SLUDGE FROM WASTEWATER TREATMENT PLANTS - Google Patents

Abstract

This process comprises the following stages: extraction of the sludges having a volatile matter content of the order of at least 70%; anaerobic digestion of the sludge in order to obtain digested or stabilized sludge having a volatile matter content greater than or equal to 60%; thermal or chemical sludge conditioning; dewatering, in particular on filter presses or centrifuges, of the sludge from the packaging in order to obtain a dry matter content of the order of 30 to 55%, all or part of this sludge which can then be used in agricultural recovery; drying of conditioned sludge until a dry matter content of 90% or more is obtained; storage of conditioned and dried sludge or immediate use as fuel in cement works; recovery and gasification treatment of the dried and stored sludge:

Description

The present invention relates to a method for processing and managing

  sludges from urban wastewater treatment plants. It no longer applies

  particularly at large sewage treatment plants.

  At present, the biological sludge treatment process originating from treatment plants is known, which consists in extracting the sludge in order to subject it to an anaerobic digestion making it possible to obtain stabilized sludge, the latter then being conditioned either by a chemical treatment (treatment with polymers, with iron chloride and lime) or by a heat treatment, the sludge then being

  subjected to dehydration, in particular by filter presses or centrifuges.

  After this treatment, there are the problems of the disposal and use of the dewatered sludge. In this regard, we can currently envisage several sectors: - agricultural development, by spreading, which poses a large number of problems due in particular to the reluctance of the agricultural professions, to transport costs, in particular in peri-urban areas, to the possible presence of heavy metals in the sludge and the degree of hygienization of this sludge; - an incineration of which the drawbacks are known, in particular the release of carbon dioxide (greenhouse effect), the energy consumption (although this drawback tends to disappear by implementing the autothermicity of the sludge), the release of various pollutants atmospheric and, - a pure and simple landfill, sector which must however

disappear by 2002.

2 2787102

  If these sectors known and commonly used at present, and in particular agricultural recovery by spreading can be used for the recycling of sludge from small treatment plants, it is not the same

  with regard to sludge from large treatment plants.

  The present invention has therefore set itself the objective of providing a process for the treatment and management of sludge from large capacity treatment plants, this process being designed so as to cause no nuisance, while respecting the standards in vigor and allowing multiple recovery of sludge: thermal,

energetic and chemical.

  Consequently, the present invention relates to a process for the treatment and management of sludge from waste water treatment plants, characterized in that it comprises the following steps: a) extraction of sludge having a volatile matter content of 1 70%

or more.

  b) anaerobic digestion of the sludge in order to obtain digested or stabilized sludge,

  with a volatile matter content greater than or equal to 60%.

  c) conditioning of the sludge by thermal or chemical means, d) dewatering, in particular on filter presses or centrifuges of the sludge from the conditioning, in order to obtain a dry matter content of the order of 30 to 55%

  all or part of this sludge can then be used in agricultural recovery.

  e) drying the conditioned sludge, until a dry matter content of 90% or more is obtained, this drying step being preferably carried out so as to obtain granules; f) storage of granules from drying or immediate use as fuel,

especially in cement works.

  g) recovery and treatment by gasification of the stored granules: - the coke resulting from the gasification treatment which can be used as fuel, for example in a cement plant, and, - the gases produced, mixture of methane and carbon monoxide, which can:

3 2787102

  - either be burned and produce calories recovered at the sludge drying stage, - or, according to a more complex process, undergo a transformation of carbon monoxide into methanol which will be reused as a carbon source for denitrification biologically from wastewater, while methane, alone, will be used as fuel in a co-generation of electricity and calories,

  said calories can serve as an aid in the drying step.

  The process according to the invention is illustrated diagrammatically by FIG. I on

  which we find the different phases and stages specified above.

  The steps relating to the extraction of the sludge, their anaerobic digestion, their conditioning, as well as their dehydration with a view to an agricultural valorization of the sludges thus treated, are implemented in a conventional manner and well known to those skilled in the art. and under these conditions, these steps will not be described here. In this respect, we can refer to the Water Technical Memento, Edition du Cinquantenaire, 1989, edited by DEGREMONT, chap. 24: "Treatment of urban effluents" and

  in particular on pages 1309 to 1314 (Volume 2).

  During the sludge drying step with a view to obtaining a content of approximately% of dry matter, the sludge cakes originating from the dewatering step are dried thermally, by any conventional process and they are transformed into granules. These can be: used as fuel in cement works; - valued in agriculture, as an amendment; - used in landscaping and infrastructure development, - landfilled or,

  - transformed into gas during the gasification stage.

  During this gasification step, the granules produced during the previous drying step are transformed into gas in a gasification reactor of known type, operating in a controlled atmosphere. This gasification produces coke which can be used as fuel in cement works and a gaseous mixture of methane and carbon monoxide. The latter can be used:

4 2787102

  - either to supply a co-generation boiler for the production of electricity and / or heat, the electricity thus produced being returned to the network or used as an energy source in the process stages and the heat being used during drying implemented after the step of conditioning the sludge by anaerobic digestion; - either to supply a synthesis unit producing methanol from carbon monoxide, methane being used as fuel in a step

  subsequent and the calories produced reused in drying.

  This chemical synthesis unit produces a carbonaceous substrate of methanol 1o type from the gas generated during said gasification step. This substrate, as mentioned above, is used as a carbon source for purifying

  wastewater by biological denitrification.

  As noted, the method of the invention provides a

  recovery in different forms of all the sludge produced. The only sub

  product, the coke from the degassing step, which contains the mineral material and mineral carbon, being used, as described above, as

  fuel, for example in cement factories.

  Among the advantages brought by the implementation of the process described above, one can quote in particular the following ones: - multivaluation of sludges: the process indeed ensures various ways of valorization of sludges, going from the most rustic (valorization by agricultural spreading, development landscaping) to the most sophisticated (use of the carbon contained in the sludge for the denitrification of the water line) and it provides several exit doors for sludge waste; - compliance with all current regulations with regard to sludge and waste; - compliance with official recommendations relating to research into the recovery of waste makes it possible to avoid its destruction: as we have seen above, the organic matter in the sludge is transformed into electricity and heat and also into a product

2787102

  noble organic, methanol, reusable in the treatment chain (denitrification); - ecological solution perfectly acceptable by public opinion because it does not cause any nuisance (no dust, no risk of dioxin); - reduction of nuisances, especially olfactory; - possibility of decentralizing the purification, the sludge from different water treatment sites being able to be conveyed, to a sludge treatment unit implementing the process of the invention and operating on its own site; - obtaining a self-sufficient functioning of the sludge treatment unit, reuse of waste as raw materials, making it possible to provide energy and reagents to

different stages of the process.

  - Finally, the method according to the invention is particularly advantageous in urban areas where the tonnages of sludge to be spread are

  considerable and limited agricultural land areas.

  It remains to be understood that the present invention is not limited to the embodiments described and / or mentioned above but that it encompasses all of them.

O variants.

6 2787102

Claims (1)

CLAIMS I - Process for the treatment and management of sludge from wastewater treatment plants characterized in that it comprises the following stages: a) extraction of sludge having a volatile matter content of around 70% at least; b) anaerobic digestion of the sludge in order to obtain digested or stabilized sludge having a volatile matter content greater than or equal to 60%;   c) conditioning of the sludge by thermal means. or chemical d) dewatering, in particular on filter presses or centrifuges, of the sludge from the conditioning in order to obtain a dry matter content of the order of 30 to 55%, all or part of this sludge which can then be used in agricultural recovery ; e) drying of the conditioned sludge until a dry matter content of 90% or more is obtained; f) storage of conditioned and dried sludge or immediate use as combustible in cement works; g) recovery and treatment by gasification of the dried and stored sludge: - coke, resulting from the gasification treatment which can be used as fuel, in particular in cement works and, - the gases produced, consisting of a mixture of methane and carbon monoxide can: - either be burned to produce calories recovered at the sludge drying stage; - or undergo a transformation of carbon monoxide into methanol which will be used as a carbon source for the denitrification by biological means of waste water, methane, only being used as fuel in cogeneration 7 2787102   electricity and calories, said calories can be used   during the drying stage.   2- A method according to claim 1 characterized in that during the thermal drying step of the sludge, it is transformed into granules.   3 - Method according to one of claims I or 2 characterized in that   that the gasification step of the thermally dried sludge is carried out in controlled atmosphere.   4- Method according to any one of the preceding claims   characterized in that the carbon monoxide released during the gasification stage constitutes the main reagent of a chemical synthesis producing a carbonaceous substrate of the methanol type.