Method for reducing heavy metal content of sludge-based biocoke

Abstract

The disclosure relates to a treatment method for sludge utilization in a sewage treatment plant, in particular to a method for reducing heavy metal content of sludge-based biocoke. The disclosure includes following steps (1) to (5): step (1): concentrating a residual sludge produced by a municipal sewage treatment plant to be with a moisture content of 95-98%; step (2): conditioning the concentrated sludge in a sludge bioleaching tank for 48 hours, with a pH value of the sludge being reduced to below 4.5; step (3): pumping the conditioned sludge into a high-pressure diaphragm plate and frame for a press filter dewatering to obtain a dewatered cake with a moisture content less than or equal to 50%; step (4): delivering the dewatered cake into a sludge dryer for crushing, heating and drying to obtain the dried sludge with a moisture content of 15-22%; and step (5): carbonizing the dried sludge into sludge-based biocoke at a high temperature in a pyrolytic carbonization device with a carbonization temperature of 500-650° C.

Claims

1. A method comprising following steps (1) to (5): Step (1): concentrating a residual sludge produced by a sewage treatment plant; Step (2): conditioning the concentrated sludge in a sludge bioleaching tank for 36-48 hours, with a pH value of the sludge being reduced to below 4.5; Step (3): pumping the conditioned sludge into a high-pressure diaphragm plate and frame for a press filter dewatering to obtain a dewatered cake with a moisture content less than or equal to 50% w/w %; Step (4): delivering the dewatered cake into a sludge dryer for crushing, heating and drying to obtain dried sludge with a moisture content of 15-22 w/w %; and Step (5): carbonizing the dried sludge into sludge-based biocoke at a high temperature in a pyrolytic carbonization device with a carbonization temperature of 500-650° C.; wherein the step (2) further comprises the sludge bioleaching tank using a sludge bioleaching conditioning process, which comprises sludge acclimation, sludge inoculation and sludge cultivation steps; wherein in the sludge bioleaching conditioning process for the concentrated residual sludge, a leaching microbial nutrient, comprising iron disulfide, glucose, and reduced iron, is added in an amount of 5-15 w/w % of the concentrated residual sludge.

2. The method according to claim 1, wherein in the step (1), a gravity concentration in a concentration tank is applied, and after the sludge concentration, a dreg-water separation is made with a supernatant being discharged to a front influent end of sewage treatment, and the concentrated sludge is pumped to the sludge bioleaching tank by a lift pump.

3. The method according to claim 1, wherein after the sludge is conditioned by bioleaching dewatering performance of the sludge is improved and a specific resistance of the sludge is reduced.

4. The method according to claim 1, wherein in the step (3), the conditioned concentrated sludge is pumped into the high-pressure diaphragm plate and frame with a high-pressure feed pump for the press filter dewatering with a pressure of the pump of 1.6-2.0 Mpa, and after a pressure chamber is filled up with the sludge, a press pump is switched for a secondary press dewatering with a pressure of 6.0-7.0 Mpa, and the moisture content of the dewatered sludge is directly reduced to below 50 w/w % after the secondary high-pressure press dewatering.

5. The method according to claim 1, wherein in the step (4), the sludge is dried by a dryer, and the sludge is crushed to 5-10 mm by a grinder before entering the dryer, thereby causing heat absorption and drying of the sludge; and the sludge is uniformly dispersed in the dryer cavity through a belt conveyor, and heated exhaust gases are collected and waste heat is returned to the dryer for continuous utilization after a condensation and heat exchange.

6. The method according to claim 3, wherein in the step (4), a pyrolysis carbonization temperature of the dried sludge is 500-650° C., the temperature is kept constant during carbonization, and a carbonization chamber is configured to be in an oxygen-free or anoxic state.

Description

DETAILED DESCRIPTION

(1) In the following, the present disclosure will be further described in detail by way of embodiments which present explanation of the present disclosure and the present disclosure is not limited to the following embodiments.

EMBODIMENTS

(2) It should be noted that terms such as “upper”, “lower”, “left”, “right”, “middle” and “an” referenced in this specification are only for convenience of description and clarity, but not intended to limit the implementable scope of the present disclosure; and the change or adjustment to its relative relation shall be regarded as within the implementable scope of the present disclosure without any substantial change to the technical content.

(3) The method for reducing heavy metal content of sludge-based biocoke in this embodiment includes following steps (1) to (5):

(4) step (1): concentrating a residual sludge produced by a sewage treatment plant to be with a moisture content of 95-98%;

(5) step (2): conditioning the concentrated sludge in a sludge bioleaching tank for 48 hours, with a pH value of the sludge being reduced to below 4.5;

(6) step (3): pumping the conditioned sludge into a high-pressure diaphragm plate and frame for a press filter dewatering to obtain a dewatered cake with a moisture content less than or equal to 50%;

(7) step (4): delivering the dewatered cake into a sludge dryer for crushing, heating and drying to obtain the dried sludge with a moisture content of 15-22%; and

(8) step (5): carbonizing the dried sludge into sludge-based biocoke at a high temperature in a pyrolytic carbonization device with a carbonization temperature of 500-650.

(9) In the step (1) in this embodiment, only a gravity concentration in a concentration tank is applied without adding flocculants such as PAM, and after the sludge concentration, a dreg-water separation is made with a supernatant being discharged to a front influent end of the sewage treatment, and the concentrated sludge is pumped to a sludge conditioning unit by a lift pump.

(10) The step (2) in this embodiment is realized by the sludge conditioning unit using a sludge bioleaching conditioning process, which includes sludge acclimation, sludge inoculation and sludge cultivation steps, with a conditioning time of 36 to 48 hours.

(11) In the sludge bioleaching conditioning process for the incoming concentrated sludge in this embodiment, a special leaching microbial nutrient, mainly composed of iron disulfide, glucose, reduced iron and dressing and the like, can be added in an amount of 5-15% of the absolute dry sludge.

(12) After the sludge is conditioned by bioleaching in this embodiment, on the one hand, dewatering performance of the sludge can be improved and a specific resistance of the sludge can be reduced; and on the other hand, the leaching system is in a weak acid state, which can change an influence of heavy metal accumulation in the sludge and transfer the heavy metals in the sludge into a liquid phase, thereby reducing a content of the heavy metals adsorbed in the sludge.

(13) In the step (3) of this embodiment, the conditioned concentrated sludge is pumped into the high-pressure diaphragm plate and frame with a high-pressure feed pump for the press filter dewatering with a pressure of the pump of 1.6-2.0 Mpa, and after a pressure chamber is filled up with the sludge, a press pump is switched for a secondary press dewatering with a pressure of 6.0-7.0 Mpa, and the moisture content of the dewatered sludge can be directly reduced to below 50% after the secondary high-pressure press dewatering.

(14) In the step (4) of this embodiment, the sludge is dried by an energy-saving dryer, and the sludge is crushed to 5-10 mm by a grinder before entering the dryer, which is beneficial to heat absorbing and quick drying of the sludge; and the sludge is uniformly dispersed in the dryer cavity through a belt conveyor, and heated exhaust gases are collected and waste heat is returned to the dryer for continuous utilization after a condensation and heat exchange.

(15) In the step (4) of this embodiment, a pyrolysis carbonization temperature of the dried sludge is 500-650, the temperature is kept constant during carbonization, and a carbonization chamber can be in an oxygen-free or anoxic state.

(16) The following cases are realized by using this method for reducing the heavy metal content of the sludge-based biocoke.

(17) Case 1.

(18) Details of residual sludge from a municipal sewage treatment plant are shown in a table below.

(19) TABLE-US-00001 Sludge Type Municipal Sludge Sampling Point Outlet of Belt Filter Press Moisture Content 80.37% Organic Matters 42.39%

(20) The sludge of the sewage treatment plant is concentrated by a radial continuous gravity concentration, and the residual sludge of the secondary sedimentation tank is discharged into the concentration tank, and the sludge is gravity concentrated to a moisture content of about 95% without adding the concentration agent. After the sludge concentration, a dreg-water separation is made with a supernatant being discharged to the front influent end of the sewage treatment, and the concentrated sludge is pumped to the sludge conditioning unit by the lift pump.

(21) A bioleaching conditioning process is made on the sludge, including sludge domestication, sludge inoculation and sludge culture, which is the same as specific embodiments disclosed in CN201410010263.0. After the sludge enters a leaching pool, a special leaching microbial nutrient can be added in an amount which is 5-10% of the absolute dry sludge, with an aeration cultivation for 24 hours and with a conditioning time for 36 hours.

(22) After the sludge is conditioned by bioleaching, on the one hand, dewatering performance of the sludge can be improved and the specific resistance of the sludge can be reduced; and on the other hand, the leaching system is in a weak acid state, which can change the influence of heavy metal accumulation in the sludge and transfer the heavy metals in the sludge into a liquid phase, thereby detoxifying the heavy metals in the sludge.

(23) The bioleaching conditioned concentrated sludge is pumped into a high-pressure diaphragm plate and frame with a high-pressure feed pump for a press filter dewatering with a pressure of the pump of 1.6-2.0 Mpa, and after the pressure chamber is filled up with the sludge, the press pump is switched for a secondary press dehydration with a pressure of 6.0-7.0 Mpa. The moisture content of the dewatered sludge can be directly reduced to 50% after the secondary high-pressure press dewatering.

(24) The dewatered sludge is naturally air-dried with the moisture content of about 20% after air drying, crushed and sieved to control particle sizes of the dried sludge to be 5-10 mm.

(25) The dried sludge is sent to a pyrolysis carbonization device at a temperature of 550-650, the temperature is kept constant during carbonization, the carbonization chamber is a closed space, and the carbonization process is in an oxygen-free or anoxic state.

(26) The municipal sludge is made into the sludge-based biocoke after this treatment, and related parameters before and after treatment are shown in a table below.

(27) TABLE-US-00002 Water Content of Dewatered Cake Under 47.56% Ultrahigh Pressure (%) Moisture Content of Dried Sludge (%) 18.90% Carbonization Temperature (° C.) 700 Heavy metal content before and after leaching Cd 2.97 1.83 (mg/kg)(* data before and after leaching) Cu 479 86.1 Zn 1540 956 Ni 81 35.4 Pb 231 36.8

(28) Case 2.

(29) Details of residual sludge from a printing and dyeing sewage treatment plant are shown in a table below.

(30) TABLE-US-00003 Sludge Type Printing and Dyeing Sludge Sampling Point After Concentration Tank + Centrifuge Moisture Content 83.75% Organic Matters 68.47%

(31) The sludge of the printing and dyeing sewage treatment plant is concentrated by a radial continuous gravity concentration, and the residual sludge of the secondary sedimentation tank is discharged into the concentration tank, and the sludge is gravity concentrated to a moisture content of about 96.5% without adding the concentration agent. After the sludge concentration, a dreg-water separation is made with a supernatant being discharged to the front influent end of the sewage treatment, and the concentrated sludge is pumped to the sludge conditioning unit by the lift pump.

(32) A bioleaching conditioning process is made on the sludge, including sludge domestication, sludge inoculation and sludge culture, which is the same as specific embodiments disclosed in CN201410010263.0. After the sludge enters a leaching pool, a special leaching microbial nutrient can be added in an amount which is 10-15% of the absolute dry sludge, with an aeration cultivation for 24 hours and with a conditioning time for 48 hours.

(33) The bioleaching conditioned concentrated sludge is pumped into a high-pressure diaphragm plate and frame with a high-pressure feed pump for a press filter dewatering with a pressure of the pump of 1.6-2.0 Mpa, and after the pressure chamber is filled up with the sludge, the press pump is switched for a secondary press dehydration with a pressure of 6.0-7.0 Mpa. The moisture content of the dewatered sludge can be directly reduced to 50% after the secondary high-pressure press dewatering.

(34) The dewatered cake is delivered to the sludge dryer, which is of energy-saving and environment-friendly type, and the sludge is crushed to 5-10 mm by a grinder before entering the dryer, which is beneficial to heat absorbing and quick drying of the sludge. The sludge is uniformly dispersed in the dryer cavity through a belt conveyor, and heated exhaust gases are collected and waste heat is returned to the dryer for continuous utilization after a condensation and heat exchange.

(35) The dried sludge is sent to the pyrolysis carbonization device and is rapidly heated to 500-600° C., the temperature is kept constant for 2-3 hours, and the carbonization process is in the oxygen-free or anoxic state. Then the sludge-based biocoke product is obtained after cooling to normal temperature.

(36) The printing and dyeing sludge is made into the sludge-based biocoke after this treatment, and related parameters before and after treatment are shown in a table below.

(37) TABLE-US-00004 Water Content of Dewatered Cake Under 43.80% Ultrahigh Pressure (%) Moisture Content of Dried Sludge (%) 17.8% Carbonization Temperature (° C.) 650 Heavy metal content before and after leaching Cd 24.6 2.56 (mg/kg)(* data before and after leaching) Cu 230 158 Zn 610 302 Ni 319.9 56.8 Pb 40 26.9

(38) The method for reducing the heavy metal content of the sludge-based biocoke is also suitable for wastewater and sludges involving biochemical treatment processes such as meat processing wastewater and sludges, food industry wastewater and sludges, wine industry wastewater and sludges, pharmaceutical wastewater and sludges, etc.

(39) Using a sludge bioleaching conditioning process to produce the sludge-based biocoke can effectively remove accumulations of heavy metals in the sludge; with dual economic advantages of low investment and low operating cost, it opens up a market of resource utilization of the sludge, which has important economy, environmental protection and sociality.

(40) In addition, it should be noted that the specific embodiments described in this specification may have different shapes, names or the like of parts and components; and the above contents described in this specification are only examples of the structure of the present disclosure. Equivalent or simple changes made in accordance with the configurations, features and principles described in the inventive concept are included in the scope of protection of the inventive disclosure. Various modifications, supplements or similar replacements can be made to the described specific embodiments by those skilled in the art to which the present disclosure pertains, which fall within the protection scope of the present disclosure without departing from the structure of the present disclosure or beyond the scope defined by the claims.