Pneumatic transportation of municipal waste – a means of improving the ecology of the city

. The article considers the issues of the impact on the environment of the city of pneumatic transportation for the removal of municipal solid waste (MSW). The disadvantages of conventional collection and transportation of MSW (container-garbage truck) are indicated, the main of which is the release of greenhouse gases into the atmosphere of the city (methane, ammonia, sulfur compounds, etc.). These problems can be eliminated by pneumatic transportation, which collects and moves waste using an air flow in an underground pipeline. As the conducted research has shown, one of the promising directions for the development of urban waste disposal is the use of a combined pneumatic intake-injection system, for the development of a schematic diagram of which the patent of the Russian Federation for invention No. 2787693 was obtained. One of the most important technological operations in the pneumatic transportation removal of MSW is the purification of the air emitted into the atmosphere from dust and harmful gaseous impurities. For this purpose, multi-stage air filters are provided in the developed pneumatic transportation system. The conducted studies have established that in order to improve the purification of the air removed from the pipeline, it is advisable to use photocatalytic purification as a filter of the last stage.


Introduction
Currently, issues related to the collection and transportation of municipal solid waste (MSW) are one of the actual environmental problems of modern cities. The usual collection and transportation of MSW, that is, with the help of containers and special garbage trucks, is accompanied by a low level of sanitary and hygienic condition of technical sites, direct human contact with waste and an unpleasant smell, significant time spent on loading and unloading garbage trucks and increased noise during these operations, ambient air pollution and congestion on roads from moving garbage trucks [8,9,11,13].
One of the indicators that negatively affect the environment is the release of greenhouse gases into the atmosphere, such as methane and carbon dioxide [3,5,10]. The source of greenhouse gas formation can be both natural processes (rotting of organic material and burning) and anthropogenic factors (exhaust gases from motor vehicles, deforestation, which are natural absorbers of carbon dioxide). The largest source of greenhouse gas generation is landfills of MSW. The main composition of landfill gas is methane (40-75%) and carbon dioxide (30-45%). In addition, landfill gas contains a large amount of toxic organic compounds, which are the source of an unpleasant odor. Every year 4-5 m3 of landfill gas is formed from one ton of MSW. These problems can be eliminated by pneumatic transportation, which collects and moves waste using an air flow in an underground pipeline [1,4,12,14].
According to the principle of operation, the existing pneumatic transportation installations are currently divided into intake (vacuum) and injection action.
In intake installations, the transported material enters the intake pipeline due to the rarefaction of air in it created by a vacuum pump, and the material can be taken simultaneously from several points.
Relatively large values of mass concentrations of MSW in intake plants can be achieved only with a small length of transportation (up to one km). To move MSW over long distances (up to ten km), it is advisable to use injection units.
In injection installations, the material moves in the air stream under the influence of overpressure created by a compressor or special blowers. Injection plants are convenient when the material from one place must be moved to several receiving points.
Developed in Russia by the MoszhilNIIproekt Institute, a vacuum-type pneumatic transportation unit has been installed and is currently being operated in the Chertanovo residential quarter of Moscow.

Materials and Methods
The presented work is based on an empirical research method, which includes analysis and comparison of currently existing means of transportation of municipal solid waste, as well as on the establishment of their advantages and disadvantages, which allowed solving the problem associated with the improvement of the pneumatic transportation method of disposal of MSW.

Results
As studies conducted at the Moscow Automobile and Road Construction State Technical University have shown, it is advisable to use a combined waste disposal scheme in which the intake and injection systems are combined into one pneumatic transportation unit to increase the efficiency of the use of pneumatic transportation systems when moving MSW [6,7]. The combined intake and injection unit ( Fig. 1) combines the advantages of the intake and injection systems. They use intake-type equipment that ensures the intake of MSW simultaneously from several garbage collection hatches; in the injection branch of the pipeline, MSW is transferred under pressure at a sufficiently high concentration and over a considerable distance. The named installation works as follows.
Waste is collected in storage shafts 6-9 garbage chutes installed in the lower part of a row of houses, which are accessed through standard garbage collection hatches 10 located on the floors, which limits the maximum size of waste particles introduced into the system to a size of 10 cm. A number of storage shafts 6-9 are connected to each other, forming a single network of transport pipeline 1-4. An air intake valve 23 is installed at the beginning of each branch of transport pipelines 1-4, i.e. at the point furthest from the transshipment station 12. The branch of transport pipelines 1-4, as a rule, connects no more than 25...30 shafts, which reduces the volume of air intake through the leakiness of the waste inlet valves, since the non-functioning transport pipelines 1-4 of the system are sealed with valves 15 located at their junction with the vacuum pipeline 5. To transport waste, after starting the vacuuming device 17, the valve 11 for entering garbage into the pipeline opens and the inlet valve 16 opens. At the same time, the transshipment station 12 creates an air flow moving through the vacuum pipeline 5 at a speed of 24 ... 26 m/s and ensuring the movement of garbage components in the direction of the injection pipeline 19. Depending on the rate of waste accumulation in storage shafts 6-9, a system is activated that alternately transports waste from storage shafts 6-9 to the transshipment station 12. By a command from the control panel 13, opens the garbage entry valve 11 and the entry valve 16 of transshipment station 12. The last is located in the center of the system, ensuring uniform pumping of air masses from the vacuum pipeline 5 to the injection pipeline 19.
Under the influence of its own weight and the static pressure drop, the entire mass of waste from the storage shafts 6-9 passes into the branch of the transport pipeline 1-4, and then through the vacuum pipeline 5 gets to the transshipment station 12. Then the inlet valves 11 and 16 are closed.
After entering the first batch of waste into the vacuum pipeline 5, the input valves 11 and 16 open by the command from the control panel 13. The subsequent opening of the input valves 11 and 16 occurs when the parameters of the air flow in the vacuum pipeline 5 are restored, i.e. after the end of non-stationary processes in it after the previous cycle of operation. Upon completion of unloading of all storage shafts 6-9 working branches, the air intake valves 23 are closed on command from the control panel 13. After emptying the storage shafts 6-9 of all branches in the form of transport pipelines 1-4, waste accumulates at the transshipment station 12, where, after passing through the sluice gate 20, it enters the injection pipeline 19. Located in front of the sluice gate 20, the vacuum-injection device 17 simultaneously intakes air from the vacuum pipeline 5 and pumps it into the pipeline 19. The waste received into the injection pipeline 19 will move in the air stream under the action of the created overpressure to the destination-the enterprise 24 for waste recycling. Before the waste goes for recycling, it passes through the drying chamber 14, where the air is separated from the garbage. The waste is delivered through the dispenser 27 to the conveyor 25 and then moved to the receiving storage hopper 26 of the enterprise 24 for garbage recycling. The air purified from the remaining impurities by means of filters located in block 28 is released into the atmosphere.
Due to the rational location of the transshipment station and the uniform pumping of air masses from the corresponding vacuum to the injection pipeline 5 and 19, the distance of waste transportation increases.
One of the most important technological operations in the pneumatic transportation removal of MSW is the purification of the air emitted into the atmosphere from dust and harmful gaseous impurities. For this purpose, multi-stage air filters are provided in the developed design of the pneumatic transportation unit (Fig. 1). The study of the influence of pneumatic transportation installations for the removal of MSW on the environment has shown that possible factors for the appearance of environmental pollution can be coarse-dispersed dust, sulfur compounds, ammonia, toxic organic compounds of domestic origin, hydrogen sulfide pathogenic bacteria and viruses (microflora).
The formation of coarse-grained dust can be facilitated by the grinding of paper and textile components of MSW as a result of transportation through a pipeline.
Sulfur compounds can be formed from organic components of municipal solid waste, that is, from food residues and other components of MSW contaminated with them, which can result in the formation of hydrogen sulfide and methyl mercaptan. Toxic organic compounds of household origin also include oxygen-containing hydrocarbons, formaldehydes, aromatic ketones and carcinogens.
Microflora during the transportation of MSW is always represented by almost all types of bacteria and viruses, including mold fungi and pathogenic bacteria -colibacillus and staphylococci.
Currently, two main types of equipment are used in the Russian Federation to protect the environment from dust and harmful gaseous impurities [15,16,17]: 1. Dry air purification units; 2. Wet air purification units.
Among dry dust collectors, the following models are distinguished: gravitational, centrifugal, inertial, electric; among wet ones -film, drip, bubbling.
Cyclone separators have received the greatest application, the principle of operation of which is based on giving the air flow a centrifugal character of movement by means of a swirler, which ensures the separation of particles depending on their density (Fig. 2, [19]).

Fig. 2. Cyclone separator
The Russian experience of using pneumatic transportation systems for waste disposal has shown the expediency of using a multi-stage exhaust air purification system.
The first stage of purification (dry) -a cyclone separator separates waste from the transporting air, which then continues to contain a certain amount of dust and mud inclusions. Therefore, it enters the second stage of purification -a water filter (wet cleaning). At the third stage -in the activated carbon filter -the air is freed from odors.
Carbon filters of the third stage capture almost all toxic air impurities with a molecular weight of 40 atomic units. However, studies [15] have shown that coal does not adsorb light compounds, which include such air pollutants as carbon monoxide, formaldehydes. In this connection, photocatalytic air purification is of practical interest as the fourth additional step [2].
Photocatalysis is a change in the rate or excitation of chemical reactions under the action of light in the presence of photocatalyst substances, which, as a result of their absorption of light quantums, are capable of causing chemical transformations of reaction participants, entering into chemical interaction with the latter and regenerating their chemical composition ( fig. 3, [20]). The essence of the method consists in the oxidation of substances on the surface of the catalyst under the action of soft ultraviolet radiation of the "A" range (with a wavelength of more than 300 nm). At the same time, toxic impurities do not accumulate on the filter, but are destroyed to harmless components of air, carbon dioxide, water and nitrogen. The photocatalytic air purifier includes a porous carrier with a TiO 2 photocatalyst applied, which is irradiated with light and through which air is blown.
The result of photocatalysis is the absorption of ammonia, hydrogen sulfide pathogenic bacteria and viruses, toxic organic compounds of household origin (oxygen-containing hydrocarbons, formaldehydes, aromatic ketones and carcinogens) from the purified air ( fig. 4, [20]). Therefore the use of multi-stage gas-dust purification of air emissions using photocatalysis in pneumatic transportation installations is justified and appropriate.

Discussion
There is a known installation for pneumatic transportation of garbage, containing a vacuum pipeline connecting the garbage chutes with a garbage unloader, under which there is a press with pressing chambers made in the form of a series of parallel pipes, which is mounted on a swivel frame, as well as a injection pipeline consisting of two sections installed with a gap and interconnected by a bypass duct having valves. To reduce pressure losses and energy consumption during the transportation of garbage briquettes, the nozzles of the pressing chambers are located vertically above the injection pipeline and are equipped with sealed jackets connected to refrigeration units. The latter are mounted on a swivel frame. At the same time, a tilter is installed in the gap between the sections of the injection pipeline, having a channel alternately connected to the nozzles of the pressing chambers and sections of the injection pipeline, and an ejector is installed above the pressing chambers to feed the garbage briquette from the pressing chamber into the channel of the tilter [18].
The disadvantage of this installation is the availability of briquetting equipment and refrigerating units with refrigerants, which increases the metal consumption and cost of construction, as well as increases the danger of use.
The developed combined pneumatic transportation installation of intake-injection action in comparison with vacuum allows to increase the number of receiving shafts, as well as to increase the range and efficiency of pneumatic transportation systems for solid municipal waste, provides better air purification from toxic gases when entering the atmosphere. In addition, as calculations have shown, the use of pneumatic transportation for the removal of MSW will reduce the need for automatic garbage trucks by approximately 70-80%.

Conclusion
1. The conducted studies have shown that the developed combined pneumatic transportation installation of intake-injection action is an effective means for removing MSW in urban conditions from the point of view of ecology and in comparison with the existing vacuum system has the following advantages: 1.1. increasing the range of transportation of MSW, which will allow them to be moved in a closed pipeline from the collection point to the recycling point; 1.2. reducing the need for garbage trucks by 70...80%, which will have a positive impact on the ecology of the city and its traffic flows.
2. The effectiveness of the use of pneumatic transportation systems depends on the correct choice and operation of gas and dust cleaning systems for air emissions, for which it is recommended to use photocatalytic cleaning in the filter block as the last stage.