Method and associated plant for the disposal of the liquid from the bottom of dumping tanks for solid urban refuse

Abstract: Method consisting of a stage involving drawing off the settled percolate for concentration of the percolate with a stream of hot air to extract water vapour and volatile organic substances, a stage involving the return of the concentrated percolate into the settling tank for subsequent oxidation, a stage involving the extraction of the residual sludge; and plant comprising a settling tank (V) with an optional preheater (R) and flue gas/liquid heat exchanger (BT), a pipe to extract the percolate, a mixing concentrator (C) supplied by an appropriate flue gas/air exchanger (AT), a drawing off pipe capable of conducting the vapours of organic substances to an appropriate combustion chamber (CC).

Description: The invention relates to a method and associated plant in accordance with the method for the disposal of the liquid from the bottom of dumping tanks for solid urban refuse, namely a process and assemblage of equipment for the treatment of the liquid residue, or 'percolate' as it is known, which becomes deposited at the bottom of containers, either natural or artificial, for solid urban refuse. In the processes known from the prior art for the treatment of percolate, the liquid, extracted from the refuse collecting tanks by means of drainage tubes, is introduced into suitable purification equipment from which is extracted a dry residue for subsequent incineration: the said liquid consisting of mixtures of the most varied substances or compounds, both organic and inorganic, arising from reactions of the most varied nature, for example physico-chemical, biological or other which have occurred due to the mixing together of or heterogenous and chance contact between the refuse in the said tanks, into which is also collected and indeterminable quantity of water arising from atmospheric precipitation or also from surface infiltration or from condensation of the water vapour contained in the atmosphere or from other sources. Also known is treatment consisting of direct combustion of the percolate in incineration plants used for the thermal destruction of solid refuse. Moreover, chemical plants for the purification of the percolate have high setting up and running costs; as for direct combustion of the percolate, it must be said that the high temperatures used, due inter alia to the necessity of not releasing harmful compounds, thus from the order of 950°C and up to 1200°C and above, maintained for a period of time of not less than 2 seconds with a flue gas velocity in excess of 10 m/s, bring about the thermal destruction of all the pollutants contained in the liquid, so causing, in particular, the deleterious release into the flue gases, in free or combined form, of the toxic metals usually contained in the liquid, such as for example mercury, zinc, tin, copper, lead with the consequent requirement to filter the flue gases themselves in order to avoid the release of the pollutants into the atmosphere; this being achieved, for example, by means of wet filters of the milk of lime type, textile filters, electrical filters; all this bringing with it considerable complications for the plant and consequent high costs; furthermore the considerable operating temperatures dictate the use of costly materials for lining the combustion chambers of the incineration furnaces and of sophisticated auxiliary and control equipment for the associated disposal process. Patent EP-A-0094547 utilizes a plant which comprises vacuum and/or multistage rectifying columns for percolate evaporation, heat exchangers for heat yield to the percolate, and a condensed liquid removal system for vapour extraction. Such prior art is open to considerable improvement with respect to the possibility of eliminating the disadvantages mentioned above. The aim of the present invention is to allow the realisation of a treatment method for the percolate, which method proceeds according to the following stages: A) Drawing off the liquid consisting of the already settled percolate and concentration of said liquid through mixing with a stream of hot air that causes the evaporation of water as well as of volatile organic substances dissolved into the percolate. B) Separation of the percolate concentrated in this manner from the saturated air containing water vapours and vapours of organic substances for the subsequent oxidation of these latter substances in a suitable combustion chamber and return of the concentrated percolate to the bottom of the dumping tank where the settling out proceeds. C) Extraction of the residual sludge deposited during settling of the percolate for subsequent dispatch to toxic waste storage facilities, the heavier pollutants having been concentrated in the sludge. Oxidation of the vapours of the organic substances can occur at ambient temperature when the presence of the associated emissions in the atmosphere can be tolerated; the said vapours may also be oxidized by combustion at temperatures indicatively between 150°C and 500°C, by means of using a liquid or gaseous fuel, such as for example biogas, advantageously produced on site by anaerobic fermentation of organic refuse. In the case of oxidation by combustion of the said vapours of organic substances, recirculation of the flue gases is advantageously provided for, dependent on the thermal regime predetermined by the combustion process, to preheat the vapours destined for combustion and, subsequently, of the percolate to be extracted from the settling tanks. Another aim of the present invention is to allow the realisation of plant in accordance with the above method, consisting of a percolate settling tank fitted with a percolate drawing off pipe to conduct it to an appropriate mixing concentrator supplied with a stream of hot air and equipped with a sloping bottom surface to collect the concentrated percolate: between the sections of the said mixing concentrator, through which are conducted both the percolate to be concentrated and stream of hot air, is inserted the end of a pipe to draw off the vapours, the other end of which pipe communicates with the combustion chamber for oxidation of the extracted vapours. In the version of the plant for oxidation of the vapours at ambient temperature, it is arranged that the vapours extracted by the said mixing concentrator are vented directly to the atmosphere. The advantages of this invention are: high efficiency of the disposal plant with the possibility of treating considerable flows of liquid percolate; economical operation, due mainly to the restriction of heat levels, with the consequent possibility of using low grade, and therefore low cost, materials for the plant components; absence of harmful substances or compounds in the flue gases and consequent possibility of their direct release into the environment without the necessity for costly and cumbersome filtration equipment; possibility of the convenient use of the biogas produced in the dumps, which biogas is not otherwise always economically usable. Other aims and advantages will be disclosed by the specification which follows and by the attached drawings, which illustrate schematically and by way of example an embodiment of the method and plant in accordance with the object of the invention. With reference to the said drawings: Figure 1 is a schematic view of a plant with an oxidation stage by combustion of the vapours of the organic substances and preheating of the said vapours and the percolate; Figure 2 is a schematic view of the plant without the flue gas/vapour heat exchanger; Figure 3 is a schematic view of the plant without the flue gas/vapour and flue gas/liquid heat exchangers; Figure 4 is a vertical section, partially cut away and enlarged, of the percolate concentrator. The following are indicated: CC, the combustion chamber; CC1, the probe to sense the temperature in the combustion chamber, CC, for controlling the flow rate of fuel to the burner CC2, advantageously of the air blast type; AT, the high temperature flue gas/air heat exchanger for preheating the feed air; AT1, the air fan to supply the exchanger AT; MT, the medium temperature flue gas/vapour heat exchanger for optionally preheating the mixture of water vapour and vapours of organic substances to be sent for oxidation by combustion; BT, the low temperature flue gas/liquid heat exchanger for optionally preheating the percolate; C, the percolate concentrator for mixing the percolate with the stream of hot air coming from exchanger AT; V, the percolate settling tank, communicating with the associated preheating chamber V1, equipped with heater R, via a weir V2; V3 a vertical dividing wall, with a gap at the bottom, between the feed chamber for the concentrated percolate from the mixing concentrator C and conducted to the settling tank; a, the fuel supply circuit to the combustion chamber CC; a1, a fuel flow control valve, installed in the associated supply circuit a; b, the feed pipes for the water vapour and the vapours of organic substances interconnecting the mixing concentrator C with the exchanger MT and this latter component with the combustion chamber CC; b1, a valve to control the flow rate of the gaseous mixture consisting of water vapour, vapours of organic substances and air, installed in the section of pipe b between the percolator mixing concentrator C and the flue gas/vapour heat exchanger MT; c, the circuit for the circulation of the flue gases extracted from the combustion chamber CC to the succession of exchangers AT, MT, BT for subsequent venting to the atmosphere; c1, a by-pass in pipe c interconnecting the inlet and outlet sections of the flue gas/vapour exchanger MT, for optionally shutting off the exchanger from the flue gas circulation circuit; c2, the associated air flow stop valve; c3, a by-pass in pipe c interconnecting the sections of c upstream and downstream from the heat exchanger BT for optionally shutting off this exchanger from the flue gas circulation circuit; c4, the associated flue gas flow stop valve; c5, a fan to draw the flue gases along the final section of circuit c and to send them to the stack; d, the pipes to supply air to the exchanger AT and from there to the mixing concentrator C; f, the pipes for the extraction of the percolate from the settling tank V to supply the said concentrator and for return from the concentrator to the settling tank of the percolate which has been concentrated with the heavier pollutant particles; f1, the percolate suction pump installed in the associated pipe f for the extraction of the percolate; g, the feed and return pipes for the percolate preheating liquid, for example water, interconnecting the percolate heater R with the associated exchanger BT; gl, the associated circulation pump for the exchange liquid for preheating the percolate; T1, the percolate preheating temperature; T2, the associated settling temperature; T3, the temperature of the exchange liquid on leaving the heater R; T4, the temperature of the said liquid on entry into heater R; T5, the air intake temperature to the associated exchanger AT; T6, the air temperature on leaving the said exchanger; T7, the temperature of the mixture of water vapour and vapours of organic substances on leaving the associated concentrator C; T8, the temperature of the said mixture on entry into the combustion chamber CC; T9, the temperature of the flue gases on leaving the said combustion chamber; T10, the temperature of the flue gases from the said combustion chamber on leaving flue gas/air exchanger AT; T11, the temperature of the flue gases on leaving the flue gas/vapour exchanger MT; T12, the temperature at which the flue gases are vented to the atmosphere on leaving the flue gas/liquid exchanger BT; the values of temperatures T1, T2 varying principally in dependance on the external ambient temperature and the degree of agitation of the percolate within the settling tank; the value of the air intake temperature T5 being highly variable depending on the location of the plant and the associated climatic conditions; C1 (figure 4), the base of the concentrator C, sloping towards the side wall C2 of the concentrator to direct the percolate C3, the concentration of which in the heavier pollutants has been increased, towards the associated discharge orifice C4, into which is inserted the extraction pipe f for the return of this percolate to the settling liquid; C5, the pipe for the drawing off the water vapour and vapours of volatile organic substances from the concentrator C, which pipe consists of an end section inside the body of the concentrator having a curved, downwards facing elbow equipped with an associated intake orifice C6, advantageously located in a horizontal plate resting in the lower-mid part in height of the body of the concentrator C and coaxial with it; C7, the optional transverse support for pipe C5 to the body C2 of the concentrator C; C8, a droplet sepatator fitted at the external end of the drawing off pipe C5 at a level with the side wall of the body C2 of the concentrator, which separator consists of a housing C9, tapered at its far end to connect with pipe b which feeds the vapours to the combustion chamber CC, there being within the separator a droplet interception surface consisting of a vertical arrangement of crosspieces C10, each crosspiece being formed by a pair of pitched surfaces converging at the top so as to form a plurality of short, superposed deviated passages to achieve the inertial separation of the droplets against the pitched surfaces: the vapour being drawn out from within the concentrator due to the effect of the negative pressure maintained within the flue gas circulation circuit by the associated fan c5; f2, an outlet pipe for the droplets collected from the bottom of the associated separator C8 and for their return inside the lower part of the body C2 of the connector C; f3, the end section of pipe f which draws off the percolate from the settling tank V, inserted tranversally into the body of the said concentrator above the vapour drawing off pipe C5: the said end section being equipped with a plurality of spray jets f4 to disperse the percolate as minute droplets having a component of motion advantageously contrary to the motion of the stream of hot air; C11, the inlet section for hot air into the concentrator C at the upper end of the concentrator body C2; C12, a distributor of air within the concentrator body, which distributor is fitted transversally between the air inlet section C11 and the arrangement of spray jets f4 and which consists of an array¹ of blades C13 to deflect the hot air coming from the flue gas/air exchanger AT, the said blades being inclined in relation to the vertical axis of the concentrator C, the ends of the blades being attached to an associated peripheral support C14, itself attached to the external side wall of the concentrator body C2: the said deflecting crosspieces conferring a downwards vortical motion to the stream of hot air in order to improve the exchange of heat with the minute droplets of percolate released into the fluid blast by the spray jets f4. In the plant diagram for the version of the method with oxidation of the vapours of organic substances at ambient temperature, it is arranged that the air entering the concentrator C is heated by means of an independent source of heat, which source may also be sufficient to supply the exchanger BT. Example 1 Temperature (T9) of flue gases on leaving combustion chamber between 450°C and 500°C. Example 2 Temperature of flue gases on leaving combustion chamber (T9) between 300°C and 450°C; no vapour preheating stage on leaving concentrator (C) and direct introduction of vapours into combustion chamber. Example 3 Temperature of flue gases on leaving combustion chamber (T9) between 150°C and 300°C; no vapour preheating stage on leaving concentrator nor percolate preheating stage. The exchange of heat within the mixing concentrator C (Figure 4) proceeds in the following manner: the percolate, drawn up from the settling tank V by the associated pump f1, is introduced into the concentrator in the form of minute droplets sprayed upwards by the sprays f4 countercurrently to the stream of hot air introduced from above through the associated inlet section C11, to which air is imparted a downwards vortical motion by the deflecting blades C13: the mixture of water and volatile organic substances obtained by the heat exchange between the percolate and hot air, driven down towards the lower part of the concentrator, is drawn off through the associated elbowed vapour extraction pipe C5 by the negative pressure maintained in the flue gas circulation circuit by means of the fan c5, while the non-evaporated liquid, consisting of percolate with a high concentration of heavy pollutants, is collected on the base C1 in order to be returned to the percolate settling tank, together with the droplets entrained by the gaseous mixture and extracted by the droplet separator C8. In practice, the materials, dimensions and details of execution may be different from, but technically equivalent to, those stated without falling outside the legal scope of this invention. Thus, the temperatures of the percolate and exchange fluids may also be appreciably different from those stated, due for example to the atmosphere or climatic conditions prevailing at the location in which the method is realised, or because of design choices made for the associated plant, without this comprising a substantial alteration of the method itself.

Claims: METHOD for the disposal of the liquid from the bottom of dumping tanks for solid urban refuse, characterised in that it proceeds according to the stages stated below: A) Drawing off the liquid consisting of the already settled percolate and concentration of said liquid through mixing with a stream of hot air that causes the evaporation of water as well as of volatile organic substances dissolved into the percolate. B) Separation of the percolate concentrated in this manner from the saturated air containing water vapours and vapours of organic substances for the subsequent oxidation of these latter substances in a suitable combustion chamber and return of the concentrated percolate to the bottom of the dumping tank where the settling out proceeds. C) Extraction of the residual sludge deposited during settling of the percolate for subsequent dispatch to toxic waste storage facilities, the heavier pollutants having been concentrated in the sludge. Method according to claim 1, characterised in that oxidation of the vapours of organic substances occurs by combustion at temperatures between 150°C and 500°C, with optional recirculation of the flue gases in order to preheat the vapours to be sent for combustion and the percolate stored in the settling tanks. Method according to claim 2, characterised in that, a combustion temperature of between 450°C and 500°C being arranged, the temperatures of the exchange fluids and the percolate are: Method according to claim 2, characterised in that, a combustion temperature of between 300°C and 450°C being arranged with no vapour preheating stage on leaving the concentrator (C), the temperatures of the exchange fluids and the percolate are: Method according to claim 2, characterised in that, a combustion temperature of between 150°C and 300°C being arranged with no vapour preheating stage on leaving the concentrator (C) nor percolate preheating stage, the temperatures of the exchange fluids and the percolate are: Plant in accordance with the method according to claim 1, characterised in that it consists of a percolate settling tank fitted with a percolate drawing off pipe to conduct it to a mixing concentrator supplied with a stream of hot air and equipped with a sloping bottom surface to collect the concentrated percolate: between the sections of the said mixing concentrator, through which are conducted both the percolate to be concentrated and the stream of hot air, there being inserted the end of a pipe to draw off the vapours, the other end of which pipe communicates with the combustion chamber. Plant according to claim 6, characterised in that between the sections of the said mixing concentrator, through which are conducted both the percolate to be concentrated and the stream of hot air, is inserted the end of a pipe to draw off the vapours, which pipe communicates with the outside environment. Plant according to claim 6, characterised in that, there being a feed pipe for the hot air stream mixing concentrator (C) at the upper end of the concentrator, with extraction of the vapour by means of a pipe at the mid point of the concentrator, the percolate settling tank (V) communicates by means of a substantially vertical separating wall (V3) with a gap at the bottom with the feed chamber for the concentrated percolate (C3) extracted from the mixing concentrator. Plant according to claim 8, characterised in that the said percolate settling tank (V) communicates with the associated preheating chamber by means of a weir (V2) in order to house in this chamber a heater (R), which is fitted in an associated exchange liquid circulation circuit (g), into which liquid is transferred heat by means of a flue gas/liquid exchanger (BT) fitted in the flue gas circulation circuit (c), advantageously in the section of this circuit immediately upstream from the stack. Plant according to claim 6, characterised in that the mixing concentrator (C) consists of a hollow body (C2), elongated along its vertical axis, having a base (C1) inclined downwards towards the outside to make the percolate (C3), the concentration of which in the heavier pollutants has been increased, flow towards the discharge orifice (C4), into which is inserted the extraction pipe (f) for the return of the said percolate to the settling liquid. Plant according to claim 10, characterised in that, within the body (C2) of the said mixing concentrator is inserted a pipe (C5) for drawing of the water vapour and vapours of volatile organic substances, which pipe consists of an end section having a curved elbow advantageously turned in the opposite direction to the hot air inlet pipe, which elbow is equipped with an appropriate intake orifice (C6), advantageously located in a horizontal plate resting in the lower-mid part in height of the concentrator. Plant according to claim 11, characterised in that in the external end section of the pipe for drawing off the vapours (C5), close to the side surface of the body of the mixing concentrator (C) is fitted a droplet separator consisting of a housing (C9), tapered at its far end to connect with pipe b which feeds the vapours to the combustion chamber (CC), there being within the separator a droplet interception surface: from the bottom of the said housing there leading a discharge pipe (f2), the other end of which is inserted into the body of the concentrator (C) close to the bottom of it. Plant according to claim 12, characterised in that the said droplet interception surface consists of a vertical arrangement of horizontal crosspieces (C10), each crosspiece consisting of a pair of pitched surfaces converging at the top having a the cross-section of an upturned 'V', so as to form a plurality of longitudinal passages with vertically deviated flow. Plant according to claim 11, characterised in that between the vapour extraction pipe (C5) and the air supply orifice (C11) is inserted tranversally into the said mixing concentrator the end section (f3) of the pipe (f) for drawing off the percolate from the settling tank (V): the said pipe being equipped with spray jets (f4) to disperse the liquid as minute droplets having a component of motion advantageously opposed to the motion of the stream of hot air. Plant according to claim 14, characterised in that between the air supply orifice (C11) and the pipe supplying the percolate to the concentrator (C) is inserted an air distributor (C12) consisting of an array (C13) of horizontal air flow deflection blades to impart a downwards vortical motion, the ends of which blades are attached to an appropriate peripheral support (C14). Plant according to claim 6, characterised in that in the section of the vapour drawing off pipe (b) between the mixing concentrator (C) and the combustion chamber is fitted a flue gas/vapour heat exchanger (MT) to preheat the vapours by means of the combustion products. Plant according to claim 6, characterised in that in the section of the flue gas circulation circuit (c) immediately downstream from the combustion chamber (CC) is fitted a flue gas/air heat exchanger (AT) to heat the air sent to the mixing concentrator (C). Plant according to claim 9, characterised in that in the flue gas circulation circuit (c) is fitted a by-pass (c3), equipped with an appropriate stop valve (c4), interconnecting the sections upstream and downstream from the flue gas/liquid exchanger (BT). Plant according to claim 16, characterised in that in the flue gas circulation circuit (c) is fitted a by-pass (c1), equipped with an appropriate stop valve (c2), interconnecting the sections upstream and downstream from the flue gas/vapour exchanger (MT).

Inventors: BONICELLI IVO DOMENICO, BORTOLANI GIUSEPPE, BORTOLANI, GIUSEPPE
Applicants: TECHNAIR SRL, TECHNAIR S.R.L. IMPIANTI TECNOLOGICI - MODENA
Application year: 1992
Main classification: C02F1
IPC codes: C02F1, C02F11