WO2010010071A2

WO2010010071A2 - Treatment system of sewage sludge and relevant energetic utilization for cogeneration.

Info

Publication number: WO2010010071A2
Application number: PCT/EP2009/059319
Authority: WO
Inventors: Gennaro Pieralisi
Original assignee: Nuova M.A.I.P. Macchine Agricole Industriali Pieralisi Societa' Per Azioni
Priority date: 2008-07-22
Filing date: 2009-07-20
Publication date: 2010-01-28
Also published as: ITMC20080135A1; WO2010010071A3

Abstract

The present invention relates to a treatment plant and process of sewage sludge, comprising the following operations: separation of liquid sludge (20) from sewage (10), grinding of urban organic solid waste (100), mixing and homogenization of ground organic solid waste (104) with liquid sludge (20), anaerobic digestion of amalgamated mixture (105) of organic waste and liquid sludge so as to produce biogas (30), centrifugal dehydration of liquid sludge (21 ) from anaerobic digestion to obtain dehydrated sludge (22) with dry product concentration of approximately 25% and feeding of biogas (30) in electricity-generating set (8) comprising a gas engine or turbine (T) and a generator (G) connected to the turbine (T) for production of electricity (40).

Description

Treatment system of sewage sludge and relevant energetic utilization for coαeneration.

The present patent application for industrial invention relates to a treatment plant and process of sewage sludge from civil housing settlements. Sewage is an effluent contaminated by organic and/or inorganic substances. As required by the environmental regulations in force, sewage must be subjected to a special treatment process in which contaminants are removed to produce a clarified/depurated effluent that can be reintroduced into the environment, for instance discharged in rivers, channels or directly into the sea. The sewage treatment process provides for natural and aided sedimentation process by clari-flocculation and treatment with active sludge (active biomass) mixed to water to interact with contaminants and favour natural depuration processes. The sludge produced at every purification stage is separated from water in clarification by physical sedimentation. The sludge from the sewage treatment plant is almost at liquid state (approx. 2% dry product concentration).

The liquid sludge is subjected to thickening by aided gravity (to increase dry product concentration) and then digestion (normally anaerobic) that originates gas (biogas basically composed of methane). The result is stabilised sludge, which is more mineralized to the detriment of the organic part transformed in biogas. However, sludge cannot be taken directly to the dumping ground, since it would involve serious problems of percolated liquid. Being basically liquid, said sludge must be subjected to dehydration by means of suitable mechanical means, normally a centrifugal separator decanter (hereinafter defined as "decanter") that delivers dehydrated sludge with approximately 25% dry product concentration. Then the dehydrated sludge is taken to the dumping ground (or incinerator). The treatment and disposal process of sewage sludge is extremely expensive and has a very high environmental impact because of dumping. The dumping cost is related at least proportionally to the quantities of sludge produced and dumped. The environmental impact is related to the quality of the sludge (the best sludge is sludge with higher dry product concentration that produces less percolated liquid).

Patent US 6,171 ,499 discloses a method for the treatment and energetic upgrading of urban and industrial sludge issued from the sewage treatment plant, wherein the hot gases produced by the turbogenerator are thermically recovered in a heat exchanger to advantage of sludge drying. Patent application DE 10107712 discloses a gas generation plant from sludge, as mix of biogas generated by the digester and gas produced from gasification of same sludge.

Patent application WO2006/116658 discloses systems and methods to efficiently dry sludge and/or generate energy from sludge by using waste heat.

Said known documents disclose plants that use anaerobic digesters to produce biogas from sludge. However, it must be considered that sludge produces a limited quantity of gas, which is not sufficient to satisfy the energy demand of the plant. In particular, DE 10107712 provides for a supplement of gas production through gasification of dried sludge, with a process that is complicated, expensive and difficult to manage, requiring a large quantity of energy. The purpose of the present invention is to eliminate the drawbacks of the known technique, by disclosing a sewage sludge treatment system that is efficient, efficacious and inexpensive.

Another purpose of the present invention is to use the treatment system of sewage sludge for energetic purposes by using suitable means for cogeneration of thermal and electrical energy, which can be both used to manage the plant independently. It is understood that saving on running costs is obtained by basically reducing the energy consumption from external sources to special situations during the operation of the plant (start-up and peak). These purposes have been achieved according to the invention, with the plant and process whose characteristics are respectively claimed in the enclosed independent claims 1 and 7.

Advantageous embodiments are disclosed in the dependent claims. Based on experimental tests carried out on sludge treatment plants of known type, the applicant has ascertained that the gas production from sludge was not sufficient and the energy produced by said gas did not cover the energy consumption of the plant. The applicant has tried to enrich sludge with other substances with high gas- generating intrinsic characteristics, without using the gasification process of dried sludge provided for in patent DE10107712.

A good solution appeared to be the utilization of waste from litter of farming animals that generate high quantities of gas during anaerobic digestion. However, no satisfactory results were obtained from said waste, since practical problems were encountered with the digester due to the high content of straw (excessive quantity of solids) with functional clogging on the bottom of the digester and consequent digestion problems.

The applicant has surprisingly discovered that the best gas production results were obtained by mixing urban organic solid waste with the sludge obtained from sewage treatment. After being mechanically pre-treated by grinding, said urban organic solid waste do not create problems in mixing or amalgamation with sludge, in spite of the presence of solids that create no problems on the bottom of the digester and during the anaerobic digestion process. Moreover, it must be considered that urban organic solid waste can be easily found and made available at the urban sewage treatment plant, since they are managed by the same municipality that is usually in charge of managing sewage purifying plants. Therefore, said waste is easy to access and the process of the invention also allows for saving on traditional waste disposal costs. The organic solid waste has been ground with size lower than 6 mm and amalgamated and homogenized with sludge in quantity equal to 10-30% in weight of sludge. Said mixture of organic waste and sludge has been loaded in the anaerobic digester.

The results have been surprising, as shown in the example below, which refers to a sewage treatment plant for a population of 50 000 people. According to the known technique, the plant has shown a daily average production of sludge in the digester of 1 15 m3/day (with 4% dry product); and a biogas production of 96 Nm3/h

The known plant was modified according to the invention, with the addition of 30 m3/day of organic solid waste (with approx. 9.2% dry product), which was treated mechanically and mixed to 115 m3/day of sludge. The addition of organic solid sludge is approximately 26% of the sludge, for a total of 145 m3/day. The biogas production was 175 Nm3/h. Such an increment is due to the higher contents of volatile solids in the organic solid waste compared to the corresponding contents in the liquid sludge issued from sewage purification. Obviously, the biogas production increases in this case of approximately 83%, this value being proportional to the addition percentage of solid sludge to liquid sludge.

By extrapolation, in 10 ÷ 30% range of waste addition, the biogas production increment is 32 ÷ 96%, respectively. The advantages of the treatment system of sewage sludge according to the invention are evident, it being possible to achieve great saving on the disposal system in dumping ground (or incinerator) of sewage sludge and of organic solids, while obtaining clean energy from said sludge and waste to cover the energy requirements of the plant. Additional characteristics of the invention will become more evident after a detailed description that refers to a merely illustrative, not limiting, embodiment, as shown in the enclosed figures, wherein: Fig. 1 is a block diagram of the treatment plant of sewage and sludge from sewage according to the invention; and Fig. 2 is a hydraulic and electrical flow diagram of the treatment plant of sewage and sludge according to the invention. With reference to the enclosed figures, the treatment plant of sewage and sludge according to the invention is disclosed, being indicated with numeral

(1 )-

The plant (1 ) comprises a sewage purifying plant (2), a biogas generation plant (200), an electricity-generating set (8) for cogeneration of electrical energy and thermal energy and optionally a dryer (7) to dry sludge, which is useful and suitable to send dehydrated sludge to a dumping ground inexpensively. Although in the following example the electricity-generating set (8) is a turbogenerator, said electricity-generating set (8) can also be a gas internal combustion engine.

The purifying plant (2) comprises a sewage treatment plant (3) of known type, adapted to separate liquid sludge from sewage to obtain purified water that can be introduced in the environment. Therefore, the sewage treatment plant (3) receives sewage (10) and produces purified water (1 1 ) that is introduced in the environment and liquid sludge (20) with approximately 2% dry product concentration. Liquid sludge (20) is sent to a dynamic thickener (4) that is a circular area with scraper bridge that performs sludge concentration by gravity to reduce the sludge volume.

Then liquid sludge (20) issued from the thickener (4) are sent to a collection and mixing pit (102) that is part of the biogas generation plant (200). The urban organic solid waste (100) is fed to a mechanical crusher (101 ). By "urban organic solid waste" we mean all organic waste produced by the population of a town and collected by the municipal cleansing service, with differentiated garbage collection system. Said urban organic solid waste is collected and managed by the municipality that is generally in charge of the sewage purifying plant. Given the fact that the municipality must bear a cost for the disposal of urban organic solid waste, a suitable disposal system of said waste, either traditional or innovative one, as the one proposed in the present invention will be advantageous. The mechanical crusher (101 ) is of hammer type with perforated grid or bars, of known type (such as the one disclosed in patent application EP1479302 in the name of the same applicant). The crusher (101 ) produces organic waste with maximum size ranging from 5 to 6 mm.

Advantageously, the crusher (101 ) can be double. In such a case, the second crusher is provided with a grid from which solid waste with maximum diameter of 2-3 mm is obtained. The crusher (101 ) produces ground organic waste (104) (with size lower than 3 mm) suitable to be amalgamated with liquid sludge (20) issued from the sewage treatment plant. The ground organic waste (104) issued from the crusher is fed to the pit (102) where it is mixed and amalgamated with liquid sludge. Mixing is made with a submerged recirculation pump (103). Advantageously, the pump (103) can be of the type provided with additional grinding means, in alignment with the rotor of the pump to additionally grind the mixture of sludge and organic waste and homogenize the sludge. Advantageously, a certain amount of organic waste (104) equal to approximately 10-30% in weight of liquid sludge (20) is fed in the pit (102). Said range is the ideal compromise to take fullest advantage of the biogas generation potential of the sludge-waste mixture, without compromising the functionality of the digester, in line with the requirements of energy balance of the entire plant.

The well-amalgamated and homogenized mixture of sludge and organic waste (105) is fed to a anaerobic digester (5) in which it is subjected to biochemical conversion process, in absence of oxygen, consisting in the demolition by micro-organisms of complex organic substances (lipids, protides, glucides) contained in vegetables and subproducts of animal origin contained in liquid sludge (20) and especially in solid organic waste (100). Said anaerobic process favours the production of biogas (30) generally composed of methane for 50-70% and CO₂ and other components for the remaining part. Then the mixture of liquid sludge and organic waste (21 ) issued from the anaerobic digester (5) is sent to a dehydration centrifugal (6) that is part of the purifying plant (2). The centrifuge or decanter (6) separates the dehydrated sludge from water. The centrifugal water (12) is reintroduced in circulation upstream the sewage treatment plant (3).

The centrifuge (6) delivers dehydrated sludge (22) with dry product concentration of about 25%. The dehydrated sludge (22) is sent to a dryer (7) in which it comes in contact with a flow of hot air (and fumes) that makes a good part of the water contained in the dehydrated sludge (22) evaporate. The flow of hot air (and fumes) of the dryer is obtained by means of burners that must be powered with fuel (80), such as methane. As a result, the dryer (7) delivers evaporated water (70) and dried sludge (23) with dry product concentration of approximately 90%.

The dried sludge (23) can be dumped, with a considerably lower cost than the dehydrated sludge (22) from the centrifuge (6).

The biogas (30) from the anaerobic digester (5) is sent to a turbogenerator (8) provided with a gas engine, that is to say a turbine (T) (or internal combustion engine) connected to a generator (G) for production of electricity (40). The electricity (40) produced by the turbogenerator (8) can be used to power the treatment plant (1 ) and/or can be sold to the electrical network operator. To that end, as shown in Fig. 2, the electricity (40) delivered by the turbogenerator (8) is fed over three lines: a first line (41 ) connected to the purifying plant (2), a second line (42) connected to the dryer (7) and a third line (43) connected to the gas generation plant (200). Moreover, it must be noted that the turbine (T) must be cooled and therefore hot water is produced by means of a water cooling system. Instead, the anaerobic digester (5) must be heated by means of a heat exchanger (S) to obtain and improve performance.

Therefore, the hot water (51 ) from the turbine (T) is sent to the heat exchanger (S) provided in the anaerobic digester (5). The cold water (52) from the exchanger (S) is reintroduced in circulation in the turbine (T) to cool the turbine. Moreover, it must be considered that hot exhaust gases (60) and hot washing gases (61 ) are delivered by the turbine (T). Said hot gases (60, 61 ) from the turbine (T) are conveyed in a conduit and sent to the dryer (7) to be combined with the flow of hot air (and fumes) produced by the dryer (7) to favour the sludge drying process, thus saving on the fuel (80) needed for the burners of the dryer.

The treatment plant (1 ) of sewage and sludge according to the present invention can be used to purify municipal sewage.

For illustrative purposes, in case of sewage produced by a town with 50 000 population, the anaerobic digester (5) according to the invention produces 175 Nm³/h of biogas that is associated with thermal power of 1 050 000 kcal/h (equal to 1221 kWh/h) and consequently the turbogenerator (8) produces 366 kWh/h of electricity (considering that the electricity produced by the turbogenerator is 30% of the thermal energy introduced in the turbine). The purifying plant (2) and the gas generation plant (200) consume 225 kWh/h, the dryer (7) consumes 25 kWh/h, and the pre-treatment system of solid waste consumes 50 kWh/h, for a total of 300 kWh/h. Therefore, the electricity produced by the turbogenerator (8) is sufficient for the entire treatment plant (1 ). Compared to the 366 kWh/h that can be produced by the turbogenerator, the energy difference (366 - 300 kWh/h) can be used, rather than for producing extra electricity, as thermal power for the dryer that must bear a higher load of sludge to be dried (sludge mixed with waste) than the known technique (sludge only).

Otherwise, without the addition of urban solid waste, the relevant plant needs 40 kWh/h from an external electricity supplier (electrical network). Moreover, it must be considered that the 366 kWh/h of electricity produced by the turbogenerator (8) are classified as clean energy (the so-called "green" energy), because they do not originate from the combustion of hydrocarbons. Consequently, said clean energy is subjected to an integrative contribution given by government authorities through the electrical network operator. Evidently, electricity and fuel (methane) from an external supplier are necessary during the start-up phases of the plant (dryer, digester and generator).

Numerous variations and modifications can be made to the present embodiment of the invention by an expert of the field, while still falling within the scope of the invention as claimed in the enclosed claims.

Claims

1 ) Treatment plant (1 ) of waste waters and sludge from sewage, comprising: a purifying plant (2) comprising: treatment means (3) of sewage (10) to separate liquid sludge (20) from purified water (1 1 ), and centrifugal dehydration means (6) to centrifuge said liquid sludge (20) from the treatment means (3), a gas generation plant (200) comprising an anaerobic digester (5) situated between the treatment plant (3) and the centrifuge (6) to receive liquid sludge (20) and produce biogas (30) by means of an anaerobic process, and an electricity-generating set (8) comprising a gas internal combustion engine or a turbine (T) powered with the biogas (30) produced by the anaerobic digester (5), and a generator (G) connected to the turbine (T) for production of electricity (40), characterised in that said gas generation plant (200) also comprises:

- at least one crusher (101 ) to grind urban organic solid waste (100),

- a mixing and homogenization pit (102) to mix and homogenize said urban organic solid waste (100) with liquid sludge (20) from the treatment means (3), to obtain an amalgamated mixture (105) of sludge and organic waste, and

- feeding means to feed said sludge and organic waste mixture (105) to the anaerobic digester (5).

2) Plant (1 ) according to claim 1 , characterised in that the crusher (101 ) comprises at least a perforated grid or sieve with holes or spaces (in case of bar chamber) to issue ground product, so as to obtain organic solid waste with size lower than 6 mm.

3) Plant (1 ) according to claim 2, characterised in that it comprises a second crusher with perforated grid or sieve with holes or spaces (in case of bar chamber) to issue ground product, so as to obtain organic solid waste with size lower than 3 mm. 4) Plant (1 ) according to any one of preceding claims, characterised in that said mixing and homogenization pit (102) comprises a submerged recirculation pump (103) to mix and homogenize said sludge and organic waste mix (105). 5) Plant (1 ) as claimed in any one of preceding claims, characterised in that said anaerobic digester (5) comprises a heat exchanger (S) connected to an hydraulic cooling system of the turbine (T) or internal combustion engine of the electricity-generating set (8) so that the hot water (51 ) from the turbine (T) is sent to the heat exchanger (S) of the anaerobic digester (5). 6) Plant (1 ) according to any one of preceding claims, characterised in that it also comprises a dryer (7) installed downstream said centrifuge (6) to dry the dehydrated sludge (22) from said centrifuge (6), said dryer (7) comprising a connection duct (60, 61 ) with said internal combustion engine or turbine (T) of the electricity-generating set to receive hot gases from the turbine (T). 7) Treatment process of sewage sludge, comprising the following operations: separation of liquid sludge (20) from sewage (10) to obtain purified water (1 1 ) to be introduced in the environment, and grinding of urban organic solid waste (100) to obtain ground organic waste (104), mixing and homogenization of ground organic solid waste (104) with liquid sludge (20) to obtain an amalgamated mix (105) of organic waste and liquid sludge, anaerobic digestion of said amalgamated mix (105) of organic waste and liquid sludge to produce biogas (30), centrifugal dehydration of liquid sludge (21 ) from anaerobic digestion to obtain dehydrated sludge (22) with dry product concentration of approximately 25%, feeding of biogas (30) in an electricity-generating set (8) comprising a gas engine or turbine (T) and a generator (G) connected to the turbine (T) for production of electricity (40). 8) Process according to claim 7, characterised in that a quantity of ground organic solid waste (104) equal to 10 - 30% in weight compared to the weight of liquid sludge (20) is mixed during the mixing operation.

9) Process according to claim 7, characterised in that organic waste is ground with size lower than 6 mm, preferably 3 mm, during the grinding operation.

10) Process according to any one of claims 7 to 9, characterised in that it comprises heat exchange to liquid sludge (20) during the anaerobic process to favour the production of biogas, wherein said heat is taken from the cooling system of the engine or turbine (T) of the electricity-generating set (8). 11 ) Process according to any one of claims 7 to 10, characterised in that it also comprises a drying phase of dehydrated sludge (22) through contact with a hot air flow after said dehydration phase, so as to obtain dried sludge (23) with dry product concentration of approximately 90%, wherein hot gases (60, 61 ) from the turbine (T) of the turbogenerator (8) are mixed with said hot air flow that comes in contact with sludge to favour sludge drying process.