FR2621307A1 - PROCESS FOR THE DISPOSAL AND TREATMENT OF DECANTATION SLUDGE FROM THE TREATMENT OF WASTEWATER - Google Patents

Abstract

The subject of the invention is a process for removing and treating settling sludge originating from the purification of waste water, the settling sludge then being used as fuel.

Description

The subject of the invention is a method of evacuation and treatment of sludge.

decantation from

wastewater treatment.

  In known wastewater treatment plants, large amounts of settling sludge are formed, the proportion of dry substance is generally between 4 and 6% by weight, and is sometimes even lower. Since the settling sludge cannot be handled in this way, chemical reagents are added to them for chemical conditioning of the sludge, in particular

mineral additives.

  These are intended to coagulate, flocculate and dehydrate the most suspended solids

fine and colloidal.

  Iron salts as well as calcium oxide and / or calcium hydroxide are, for example, typical packaging products of this type. They serve as support agents and promote subsequent mechanical dehydration,

  for example using chamber filter presses.

  Mechanical dehydration results in a consistent, transportable filter cake which

  is then sent to a landfill.

  The disadvantage of this process is that it requires enormous discharge capacities. In addition, products intended for the conditioning (fixing of solids) of settling sludge are to

  the cause of additional (undesirable) costs.

  The object of the present invention is therefore to propose a process for the evacuation and treatment of decantation sludge from the purification of waste water, avoiding the need for a landfill and making it possible to preferably evacuate and treat rationally. the settling sludge. In particular, it also aims to provide an environmentally friendly disposal and treatment process. In addition, it aims to condition the settling sludge as inexpensively and as little polluting as possible, preferably by reducing the use of

natural substances.

  The invention is based on the fact that the settling sludge from the purification of waste water is perfectly suitable for use as fuel in a waste incineration plant. The invention has further shown that the it is thus possible not only to dispose of the settling sludge by avoiding discharging it into a landfill, but that it can at the same time be used to reduce the rejection of SO2 in the waste incineration plant, and this , in particular due to their content of calcium hydroxide coming from the substance of

corresponding coagulation.

  The subject of the invention is therefore a process for the evacuation and treatment of at least partially dehydrated settling sludge, resulting from the purification of waste water, after conditioning with at least one coagulation substance containing Ca, such as lime or lime hydrate, the settling sludge being sent to a combustion group of a garbage incineration plant to serve as fuel and to be burned therein while fixing the SO 2 coming from the gases

combustion.

  The proportion of calcium hydroxide added to the settling sludge or formed by contact with water has the advantage not only of fixing the SO2 coming from the combustion gases, and consequently of reducing the release of SO2, but the proportion of calcium hydroxide from the settling sludge or the settling sludge filter cake also saves the CaO or Ca (OH) 2 necessary for the desulfurization of the flue gases proper, since the smoked gases already have a content reduced to SO2 at the time of desulfurization

proper.

  The process according to the invention is preferably implemented so that the settling sludge is burned in a boiler. But, a combustion of settling sludge in a hearth

  fluidized bed is also possible.

  Preferably, before being used in the combustion group, the settling sludge must be dewatered to be brought to a solid content of at least 30% by weight, preferably at least 40% by weight. For this purpose, coagulation / flocculation products such as lime hydrate or iron chloride (FeCl3) are used, but also mechanical dehydration groups. In this regard, the invention

  preferably offers the use of a filter-

  press which makes it possible to obtain decantation sludge in the form of a consistent filter cake and

  transportable, partially crumbled.

  The invention is also based on the fact that the lime hydrate used to condition the settling sludge does not necessarily have to be added in the (expensive) form of what is called slaked lime; on the contrary, a by-product containing calcium oxide and originating from the desulfurization can be used in place of the latter.

dry or almost dry.

  For example, in the direct desulfurization of the flue gases of a thermal power plant producing energy, calcium hydroxide or calcium carbonate is used as absorption product, and the calcium oxide formed after dehydration or decarbonation, fixes the SO2 contained in the smoke gases which reacts to give calcium sulfite, or after absorption of oxygen

is oxidized to calcium sulfate.

  As a by-product, a residue is finally obtained which contains, in addition to calcium sulphate (anhydrite), a high proportion of free CaO (as a result of over-stoichiometric use of the absorption product) and fly ash. The final product has the characteristic composition: CaSO4 .............. About 21% by weight CaO ............

  . About 39% by weight Fly ash ... About 40% by weight While this residue was previously evacuated in mobile silos to be transported to a landfill, or used as aggregate in the manufacture of building materials, the invention proposes, in an advantageous variant, to add this by-product to the settling sludge, before or after conditioning, to serve as..DTD: coagulation product.

  This use has various important advantages, in particular: - It avoids a polluting discharge for

  the environment of this solid matter.

  - Free lime fixes the water by giving calcium hydroxide and replaces the calcium hydroxide which must in any case be used

for sludge conditioning.

  - Calcium sulfate (anhydrite) likewise fixes water to give hydrated calcium sulfates, in particular gypsum (dihydrate of

calcium sulfate).

  - The content of dry substances is thus considerably increased before mechanical dehydration, and the two constituents thus act as a kind of auxiliary filtration product. - Fly ash which absorb water and possibly partially hydrate, play

a similar rattle.

  - It is thus easy to obtain dry substance contents of 30% by weight and more. This reduces the volume of partially dewatered settling sludge used as fuel, and increases their calorific value. As mentioned above, the by-product can come from dry desulfurization, but can also be a product from spray absorption. This advantageous embodiment of the invention makes it possible not only to evacuate and treat the settling sludge produced in a wastewater treatment plant, but also and at the same time the residue originating from a desulfurization of a power plant. thermal energy production, these two measures complement each other in a positive way,

as described.

  Calcium hydroxide in particular can be used in a variety of ways in a process according to the invention, namely firstly as an absorption product in a thermal power plant for energy production, then as a coagulation product in the wastewater treatment plant, and finally again as an absorption product in a garbage incineration plant (thermal power plant producing energy supplied

in garbage).

  Only the mixture of fly ash and reaction products obtained at the outlet of the waste incineration plant must still be sent to a final disposal and treatment operation. An embodiment of the method according to the invention is described below in more detail in

reference to the attached drawing.

  In a thermal power plant producing energy whose fuel is coal with a sulfur content of less than 1%, direct dry desulfurization is carried out using calcium hydroxide, in a manner known per se, in a range temperatures between 800 OC

and 1,000 C.

  The anhydrite (CaSO4) thus formed and the lime (CaO) formed from the absorption product are separated with the fly ash, in a filter.

electric, then extracted.

  Instead of being transported to a landfill, as provided in the prior art, this mixture of residues is added, in a conditioning device (kneading device), to decantation sludge from a wastewater treatment plant. wastewater treatment, with a characteristic content of

  dry substances between 2 and 6% by weight.

  As shown in the drawing, iron chloride is introduced into the mixer to serve as a

additional coagulation.

  All the added products absorb the water contained in the settling sludge and coagulate the fine particles in suspension, in part

  colloidal, settling sludge.

  The settling sludge thus freed from a large amount of free water is then mechanically dehydrated in a chamber filter press. While the water is drawn off, the filter cake is transported to the refuse incineration plant. The filter cake is consistent and - depending on the composition and the quantity of coagulation products - it remains for transport in the form of a filter cake or is crumbled. In the embodiment, the proportion of solid matter of

  sludge settling is 45% by weight.

  In the boiler of the garbage incineration plant, not only garbage, but also partially dewatered decantation sludge is introduced as fuel. Due to their very high proportion of solid matter, the settling sludge has a good calorific value and at the same time - as described above -

  fix the SO2 coming from the combustion gases.

  The boiler hearth is followed by a reactor stage known from the state of the art, in which the flue gases continue to be desulphurized, by using additional calcium hydroxide; in this case, however, the amount of absorption products used can be reduced due to the partial desulfurization carried out previously. Textile filters are followed by the reactor, which make it possible to extract the purified smoke gases on the one hand, and the residue consisting of fly ash and the remaining reaction product on the other, so that there is no longer any than to reprocess or

  deposit this single waste in a landfill.

  To complete, the compositions of the by-product of direct desulphurization, used here, as well as that of the decantation sludge sent to the boiler of the household waste incineration plant are indicated: Residue from direct desulphurization: 40 % by weight of fly ash; 39% by weight of CaO and

21% by weight of CaSO4.

  Settling sludge (fuel): 10% by weight of fly ash, possibly partially hydrated, 13% by weight of calcium hydroxide, 7% by weight of hydrated calcium sulphate (in particular gypsum), 15% by weight of materials

  various solids, the rest being water.

Claims (8)

  1. Process for the evacuation and treatment of at least partially dewatered settling sludge, resulting from the purification of waste water, after conditioning with at least one coagulation substance containing Ca, such as lime or lime hydrate where the settling sludge is sent to the combustion group of a garbage incineration plant to be used as fuel and burned there, while fixing the S02 from combustion gases.   2. Method according to claim 1, characterized in that before being used in the combustion group, the settling sludge is dehydrated to a solid content of at least 30% by weight, preferably d 'at least % in weight.   3. Method according to claim 1 or 2, characterized in that the settling sludge is dehydrated in a filter press to give a filter cake.   4. Method according to one of claims 1 to 3,   characterized in that the settling sludge is burned in a boiler.   5. Method according to one of claims 1 to 3,   characterized in that the settling sludge is   burned in a fluidized bed hearth.   6. Method according to one of claims 1 to 5,   characterized in that a by-product containing calcium oxide and coming from dry or almost dry desulfurization is added to the settling sludge, as a coagulation substance, before or after conditioning.   7. Method according to claim 6, characterized   in that we add to the settling sludge a sub-   product which contains not only CaO but also fly ash as well as a material containing calcium sulphate and / or calcium sulphite.   8. Method according to one of claims 6 or   7, characterized in that the sub-   produces a residue from the direct desulfurization of flue gas from a thermal power plant of energy.