ABSORBENT FOR MUNICIPAL WASTEWATER TREATMENT

Abstract

There is described a particulate carbon adsorbent comprising 60 to 90% by wt carbon, wherein the particulate carbon adsorbent is a fibrous pyrolysis product of an organic fraction of waste screenings, and wherein the fibrous pyrolysis product predominantly comprises fibres having a diameter in the range about 10-40 μm and a length in the range about 50-500 μm. A method of manufacture is also described. The particulate carbon adsorbent is useful in of odour prevention in wastewater treatment and other wastewater processes.

Claims

1. A particulate carbon adsorbent comprising 60 to 90% by wt carbon, wherein the particulate carbon adsorbent is a fibrous pyrolysis product of an organic fraction of waste screenings, and wherein the fibrous pyrolysis product predominantly comprises fibres having a diameter in the range about 10-40 μm and a length in the range about 50-500 μm.

2. The particulate carbon adsorbent of claim 1 wherein the organic fraction of waste screenings comprises on a dry matter basis at least 70% volatile solids.

3. The particulate carbon adsorbent of claim 1 wherein the organic fraction substantially comprises or consists of two or more of the group comprising: rags, paper, cellulose fibres and sanitary products, made from natural and plastic materials.

4. The particulate carbon adsorbent of claim 1 wherein >60% by weight of the fibres of the particulate carbon adsorbent have an aspect ratio of length/diameter of >5.

5. The particulate carbon adsorbent of claim 1 having a BET total surface area of >200 m2/g, a micropore area of >200 m2/g and a portion of micropore area in total surface area of >80%.

6. The particulate carbon adsorbent of claim 1 having a particle size that does not exceed 500 μm.

7. The particulate carbon adsorbent of claim 1 wherein the particulate carbon adsorbent includes a magnetic species.

8. The particulate carbon adsorbent of claim 1 having a pH of approximately 10.

9. The particulate carbon adsorbent of claim 1 further comprising magnesium and calcium.

10. The particulate carbon adsorbent of claim 9 further comprising magnesium at a concentration in the range 4-100 g/kg, and calcium at a concentration in the range 10-100 g/kg.

11. The particulate carbon adsorbent of claim 9 wherein the magnesium is in the form of MgO, and the calcium is in the form of CaCO3.

12. The particulate carbon adsorbent of claim 1 in the form of pellets or granules or blocks.

13. A method of odour prevention in wastewater treatment comprising the step of adding a particulate carbon adsorbent as defined in claim 1 to the wastewater to be treated.

14. The method of claim 13 wherein the particulate carbon adsorbent is added to the wastewater at a dose of 0.25 wt. %.

15. The method of claim 13 able to decrease the emission of total volatile organic carbon compounds (tVOC) from the wastewater by more than 85% after 25 minutes of treatment.

16. A method for adsorption of wastewater bulk pollutants comprising the step of adding a particulate carbon adsorbent as defined in claim 1 to the wastewater to be treated.

17. The method of claim 16 wherein the particulate carbon adsorbent is added to the wastewater at a dose of 10-1000 g per m.sup.3 of wastewater.

18. A method of reducing the waste requiring offsite transportation in a wastewater treatment process at a site, comprising the step of forming a particulate carbon adsorbate as defined in claim 1 from the organic fraction of wastewater screenings provided by the wastewater treatment, and using the particulate carbon adsorbate in a downstream treatment of the wastewater.

19. A method as claimed in claim 18 wherein the downstream treatment comprising one or more of the group comprising: odour prevention, sludge treatment, adsorption of wastewater bulk pollutants.

20. A method of manufacture of a particulate carbon adsorbent as defined in claim 1, the method comprising the steps: (a). providing a organic fraction of waste screenings with a dry mater content of at least 50%, (b) pyrolysing the material of step (a) at a temperature in the range of 400° C. to 900° C.; and (c) grinding the pyrolysed material of step (b).

21. The method of claim 18 further comprising the step of: (d) sorting the ground pyrolysed material of step (c)

22. The method of claim 21, wherein step (d) comprises sieving or sifting the pyrolysed material resulting from step (c), optionally wherein sieving or sifting uses an aperture between about 125 μm and about 500 μm.

23. The method of claim 20 wherein step (b) is performed at a temperature in the range 650-750° C., optionally wherein the temperature is about 700° C.

24. An anaerobic digestion process comprising the steps: (a) adding a feedstock to a bioreactor; (b) adding a microorganism composition to the bioreactor; (c) adding a particulate carbon adsorbent of claim 1 wherein the adsorbent comprises adsorbed phosphate, ammonium or other nutrients for the microorganism; (d). incubating the microorganisms within the bioreactor to produce a digestate and biogas; and (e.) removing the produced biogas and digestate.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0110] Embodiments of the present invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which.

[0111] FIG. 1: Impact of screenings-derived carbon adsorbent on the reduction of the total volatile organic compounds (tVOC) released from wastewater compared to untreated and activated carbon as reference. Measured after adsorption times of 2 min, 25 min and 45 min.

[0112] FIG. 2: Impact of screening waste-derived carbon adsorbent (at two different dosages) on the reduction of the chemical oxygen demand (a), phosphorous (b), and ammonium (c).

[0113] FIG. 3: Impact of screenings derived carbon adsorbent at two different particle sizes on biogas production.

[0114] FIG. 4: Microscopy photos of the fibrous carbon adsorbent sieved through a 500 μm sieve (a), a 125 μm sieve (b) and a 20 μm sieve (c).

[0115] FIG. 5: Examples of the manufacturing process incl. standard process (A), production of a MgO-enriched carbon adsorbent (B), use of screening waste that is contaminated with aluminium waste (C), production of a magnetic carbon adsorbent (D).

[0116] FIG. 6: Nitrogen adsorption isotherm linear plots for carbons made from cotton pads (diamonds), toilet tissue (circles), wipes (crosses) and time elapsed. Adsorbed nitrogen volume at standard temperature and pressure (STP).

[0117] FIG. 7: UV spectra of wastewater before (untreated) and after treatment with three different carbon adsorbents. Absorbance data shown are net values achieved by subtracting the absorbance data of pure water from the total absorbance.

DETAILED DESCRIPTION EXAMPLES

[0118] Both crude wastewater and the organic fraction of wastewater screening waste were obtained from Scottish Water's Seafield Wastewater Treatment Works in Edinburgh, UK. The screening matter was pre-dried by means of a screw press. The wastewater was analysed with a pH of 7.3, electrical conductivity of 1260 mS/cm, COD content of 313 mg/L, total phosphor content of 6.05 mg/L, and ammonia nitrogen content of 37.18 mg/L. Sulphide was below the detection limit of the used technique of 0.1 mg/L S.sup.2−.

Example 1—Small-Scale Production of the Particulate Carbon Adsorbent

[0119] The screening waste was turned into the fibrous carbon adsorbent of the first aspect of the present invention by following steps:

[0120] 1. Drying. 1473 g of raw screening waste was dried at 105° C. for 12 h. It was found that the raw screening waste had a water content of about 70%.

[0121] 2. Pyrolysis. 402 g of the dried screening waste was filled into a 2 litre metal crucible. The crucible was filled to the top to leave as little air as possible and, thus, the process can be considered to be conducted in the absence of air (0.005 g air per g of feedstock. The represents a lambda value of 5.4×10.sup.−4). A lid was put on the crucible and it was then placed in a muffle furnace where it was heated to 700° C. for 4 h. Afterwards the crucible was left cooling to 50° C. before it was opened, and the produced raw carbon adsorbent was removed. The dry-mass based yield of raw carbon adsorbent was about 19%.

[0122] 3. Gentle crushing To isolate the carbon fibres the raw carbon adsorbent was crushed by putting it into a plastic bag and squeezing the filled bag until the particle size was visibly reduced.

[0123] 4. Sieving. The crushed carbon adsorbent was sieved manually through flat round sieves with designated apertures sizes. Carbon adsorbent material that was too large to pass through a 500 μm size was returned to Step #3.

[0124] The properties of the final carbon adsorbent were as described above in Tables 1, 2, 3 and 4.

TABLE-US-00005 TABLE 5 illustrates an example of the chemical composition of the carbon adsorbent made from organic wastewater screenings Concentration Concentration Element (mg/kg) Element (mg/kg) C 750000 Mn 161 N 17900 Ni 4 H 16300 Pb 7 Al 1922 Zn 265 Ca 33279 As 1 Fe 2076 Mo 2 K 3558 Cd <1 Mg 4898 Cr <1 P 8155 Hg <1 Cu 80 Cs <1

Example 2: Small-Scale Particulate Carbon Odour Removal Test

[0125] Commercial activated carbon C3345 Fluka was used as reference material (particle size: 100-400 mesh, surface area: 1400 m.sup.2/g). It was obtained from Sigma Aldrich.

[0126] The adsorption trials to investigate the efficiency of the carbon adsorbent of the present invention in odour removal were conducted in 50 mL falcon tubes using following steps

[0127] 1. 50 mg of carbon adsorbent that passed a 125 μm sieve but not a 20 μm sieve was weighted into the falcon tubes. As reference another falcon tube with filled with 50 mg of the activated carbon. As Control another falcon tube was left without any adsorbent.

[0128] 2. 20 mL of crude wastewater was added into each falcon tube resulting in a mass-based ratio of adsorbent to water of 0.25%

[0129] 3. The falcon tubes were closed with lids and the contents of the falcon tubes were mixed by a laboratory vortex mixer for 20 s.

[0130] 4. The falcon tubes were left alone for adsorption times of either 2 min, 25 min, and 45 min.

[0131] 5. Odour emission measurement was done in a controlled air flow chamber. This chamber was made of plastic with a transparent lid, had a volume of 5 L and was equipped with a pump to establish a continuous air flow through the chamber at a rate of 5 L per minute representing an air exchange rate of one per minute. The chamber was operated inside a fume cupboard to ensure safe removal of all emissions. An Ingeress Air quality Monitor Detector was placed inside the chamber. The detector is equipped with a sensor for measuring the total natural and synthetic volatile organic compounds causing most odours (tVOC). To measure the odour emission, the lid of the falcon tube was opened and the tube was immediately placed inside the chamber. Reading of the tVOC measurement value was done after 4 min when the air inside the chamber was exchanged 4 times assuming a steady release of odour causing substances has been established.

[0132] 6. Between individual measurements the measurement chamber was flushed with ambient air from the lab until the tVOC value reached a stable background level.

Results

[0133] The tVOC measurement value of the Control (no adsorbent added) declined from 3.4 mg/m.sup.3 after 2 min of the experimental start point (when the adsorbent was added to the other experiments) to 2.2 mg/m.sup.3 after 25 min and to 1.7 mg/m.sup.3 after 45 min. This decline can be explained by an aeration effect due to intensive mixing, increasing the concentration of dissolved oxygen in the wastewater. Mixing was performed also for the Control despite the fact that no adsorbent was added. As described above, oxygen is known to eliminate odours from wastewater.

[0134] Compared to the Control, the fibrous carbon adsorbent removed 34% of the tVOC after 2 min, 90% after 25 min, and 93% after 45 min. These values include the signal of the background tVOC in the ambient air. After 45 min, the tVOC concentration of 0.12 mg/m.sup.3 levelled with the background value of 0.13 mg/m.sup.3, thus no further tVOC removal could be detected. Activated carbon was found to react quicker already removing 91% of the tVOC after 2 min adsorption. After 45 min, activated carbon treatment also reached the background level of tVOC.

[0135] A dosage of 2.5 g of carbon adsorbent per L of wastewater was considered suitably to remove the tVOC emissions to below the detection limit. This result was confirmed by olfactory means where odour was significantly reduced. Compared to activated carbon the adsorbent requires a longer reaction time, however they both reach the same end point. However, primary settling tanks usually have a sufficient retention time of 30-60 min.

[0136] Thus, optionally, the method is able to decrease the emission of total volatile organic carbon compounds (tVOC) from the wastewater by more than 85% after 25 minutes of treatment.

Example 3—Large Scale Production of the Particulate Carbon Adsorbent

[0137] Examples of the manufacturing process are shown in FIG. 5.

[0138] Following the standard process (A), crude wastewater undertook raw screening: Grit removal, drying, shredding, pyrolysis, removal of ferrous metals, and milling.

[0139] Grit removal is a well-established standard process in wastewater treatment based on weight-based separation. Drying can be conducted in a batch or continuous process. Preferably a two-stage process is performed: 1.sup.st mechanical separation e.g. by a screw process to a dry matter content to around 30%, 2.sup.nd thermal drying to at least 50% but preferably above 70%. To allow a uniform carbonisation, a shredding step is recommended before the pyrolysis process. An established particle size for carbonisation is below 3 cm. Pyrolysis can be done in a batch or continuous process. Because of a higher throughput and higher energy efficiency and better process control, a continuous process is preferred. The continuous process can be carried out in a pyrolysis kiln working with either a rotary kiln or screws for material transport. During the process, the screening waste is heated to a process temperature of several hundred degrees Celsius for at least 20 min. This process is often termed slow pyrolysis. The minimum temperature required for full carbonisation and to destroy micro- and macro plastics inside the screening waste is 400° C. However, the preferred temperature range is 600-800° C. and more preferably 650-750° C. A lower or higher temperature will lead a product that is less rich in micropores. During pyrolysis, small amount air or oxygen can be added into the reaction chamber with the purpose to incinerate the pyrolysis gases that are inevitably released in any pyrolysis process. This will generate heat to fuel to process. Suitably the λ (lambda) ratio for air dosage may be ≤1 (in case of the pyrolysis gases deriving from the screening waste are considered as the fuel) or ≤0.2 (in case of the waste screening material is considered as the fuel). As an alternative to using the pyrolysis gases as fuel for supplying the process heat, the process heat can also be provided by electrical heating or microwave heating. After pyrolysis, the raw carbon adsorbent is left cooling down to below 30° C. before further processing. Spraying the adsorbent with water to a % wt of 30% water accelerates the cooling down and to reduce dusting and to mitigate the risk of a carbon dust explosion or spontaneous incineration. The carbon adsorbent is milled to break down the raw carbonised tissues into isolated carbon fibres. This can be done by conventional mills like a hammer mill or cone mill.

[0140] For a high fibre recovery, the mill should have a screen with an aperture size in the range of 125 μm to 500 μm. Afterwards, the ground carbon adsorbent can be further size screened e.g. to remove dust below 20 μm. Ferrous metals can be removed by standard magnetic separation. As the carbon adsorbent has a solid particle density of around 2 g/cm.sup.3 heavy materials like glass and grit can be separated based on density e.g. by air separation.

Example 4—Use of the Fibrous Carbon Adsorbent to Eliminate Odours in Wastewater Treatment Plants

[0141] Carbon adsorbent can be added at the influent of the wastewater treatment (WWT) plant after the screening step and before the primary sludge settling tanks (primary clarifying/clarifier step), and mixed with the wastewater by hydraulic stirring. Following dosage periodic adsorption tests, 25 g/m.sup.3 wastewater can be olfactory deemed to be sufficient in substantially removing odours in the sludge settling tanks. Suitably, local environmental conditions on the testing day in summer in Scotland, can be sunny intervals at 21° C., 72% humidity, with a 6 mph NNW wind.

[0142] The carbon adsorbent can be separated from the water stream in the primary clarifier treatment step by settling to the tank bottom along with natural sludge particles. Subsequently this primary sludge can then be treated by dewatering and incineration.

Example 5—Small-Scale Particulate Carbon Adsorption Test: Phosphate (P), Ammonium Nitrogen (NH.SUB.4.—N) and COD Removal

[0143] An experiment was carried out to determine the carbon adsorbent's efficiency for phosphate (P), ammonium nitrogen (NH.sub.4—N) and COD removal. Carbon adsorbent manufactured by the method of Example 1. Wastewater from Scottish Water's Seafield Wastewater Treatment Works in Edinburgh, UK was used. To reduce the impact of suspended matter on the experiment's analytical results, the wastewater was filtered through a 0.22 μm syringe filter as pre-treatment. Filtering reduced the COD content from 313 mg/L to 97 mg/L, the P content from 6.05 mg/L to 4.30 mg/L and NH.sub.4—N from 37.18 mg/L to 32.66 mg/L. In contrast to Example 1 all carbon adsorbent particles that passed a sieve of 125 μm were used, so no lower particle size limit was defined.

[0144] The adsorption experiment was carried out in following steps

[0145] 1. 50 mL falcon tubes were filled with the carbon adsorbent in a dosage of 0 mg, 2 mg and 20 mg for designated concentrations of 0 g/L (Control), 0.1 g/L, and 1 g/L.

[0146] 2. 20 mL of filtered wastewater was filled into the falcon tubes.

[0147] 3. The falcon tubes with lids on were placed on a laboratory shaker for 60 min at 15° C.

[0148] 4. The falcon tubes were centrifuged and then filtered through 0.22 μm syringe filters to remove the carbon particles. The same was applied to the Control.

[0149] 5. Hach Lange cuvette tests were used to determine the liquors' concentration of COD, P and NH.sub.4—N.

[0150] Results (see FIG. 2a-c):

[0151] Loss of COD, P, and NH.sub.4—N in the Control can be explained by a combined effect of adsorption by the wall of the falcon tubes, gaseous losses, removal by centrifuging and filtering.

[0152] Carbon adsorbent in a dosage of 1 g/L was able to remove 48% of COD, 24% of P and 47% of NH.sub.4—N compared to the Control.

Example 6—Production of Fibrous Carbon Adsorbent With a Higher P Adsorption Capacity

[0153] Production of fibrous carbon adsorbent to remove COD, P, and NH.sub.4—N may be performed following the same method for odour elimination in wastewater treatment (i.e. the third aspect of the invention).

[0154] However, as an embodiment of the present invention, a version of the present invention was prepared with a higher P adsorption capacity. The carbon adsorbent can be enriched with Magnesium to yield a carbon adsorbent with 0.5-5% Mg using the following addition to the method used in Example 2 (FIG. 5, case B):

[0155] 1. MgCl.sub.2 can be added at a dosage of 6 g molar mass of Mg per kg of screening waste to the screening waste as MgCl.sub.2 solution prior to drying and subsequent pyrolysis. To ensure a uniform distribution, the MgCl.sub.2 solution can be added by simultaneous fine-spraying and mixing the screening waste in drum-type mixer.

[0156] 2. Afterwards, the screening waste can be dried to at least 50% Total Solids in a forced air dryer. The exhaust air contains HCl gas, so it needs to be treated by water scrubbing before it is released.

[0157] 3. During the pyrolysis step, hydrated MgCl.sub.2 reacts to form MgO, the most effective Mg form for P adsorption.

Example 7—Large-Scale Use of the Fibrous Carbon Adsorbent To Remove COD, P, and Ammonia from Wastewater

[0158] Suitably, the carbon adsorbent can be added to the WWT process at the same points as for odour removal. These are before primary settling and before sludge dewatering. In addition, carbon adsorbent that has a magnetic feature can be applied at the backend of the WWT plant as a polishing step to remove remaining contaminates before the treated water is discharged. The magnetic feature enables a magnetic separation before the water is discharged.

[0159] Example 8—The Effect of Carbon Adsorbent on Anaerobic Digestion: Small Scale Test

[0160] To test the effect of the carbon adsorbent on anaerobic digestion (AD), digestate and sewage sludge were obtained from Scottish Water's Seafield Wastewater Treatment Works in Edinburgh, UK. The carbon adsorbent was produced as described under Example 1. Two particle size ranges were tested. 20-125 μm and below 20 μm. The AD experiment was carried out in 100 mL glass syringes kept at 38° C.

[0161] Steps:

[0162] 1. Syringes were filled with 20 mL of digestate and carbon adsorbent at a dosage of 1 wt. %. A Control was run without adsorbent.

[0163] 2. The syringes were placed on a rotating wheel inside a 38° C. incubation room for 19 days.

[0164] 3. On day 4 and day 10 each syringe was fed with 1 g of sewage sludge.

[0165] 4. During the experiment the gas volume was measured every 1-3 days by means of the plunger displacement. The methane concentration of the produced gas was measured at 5 times.

[0166] Results:

[0167] The Control produced about 81 mL of methane

[0168] The particle size range of below 20 μm showed an increase of 19% in methane yield, whereas no effect was found for the larger particles. Thus, fine powder formed by the process as described herein may be particularly effective for enhancing anaerobic digestion.

Example 9—Large Scale use of Carbon Adsorbent in Anaerobic Digestion of Sewage Sludge

[0169] Production of the carbon adsorbent to increase the biogas and methane yield in anaerobic digestion can be done in the same way as for odour elimination or removal of other pollutants but the carbon adsorbent should be sieved below 20 nm. A mass dosage of 0.2% to 1% is added to sewage sludge before the AD reactor.

Porosity Analysis

[0170] Three typical materials found in screening waste were pyrolysed at 700° C. for 4 h into fibrous carbon products. These were Novon Cotton Wool Pads (cotton pads), Andrex Classic Clean Toilet Roll Tissue (toilet tissue) and Nivea Refreshing Facial Cleansing Wipes (wipes). Cotton pads and wipes are not intended for flushing as they don't break down in the sewers. The wipes are made from the plastic material polyester. After pyrolysis, the resulting carbon products were milled to a particle size below 125 nm.

[0171] Porosity characteristics of all three materials were determined by standard nitrogen adsorption technique, and are shown in Table 6 below.

TABLE-US-00006 TABLE 6 Carbon made Carbon made Carbon from cotton from toilet made from Parameter Unit pads tissue wipes BET total surface Area m.sup.2/g 615.9 549.8 557.9 Micropore Area (<2 nm) m.sup.2/g 455.7 384.5 496.0 Mesopore Area m.sup.2/g 79.0 97.3 24.1 (2 nm-50 nm) Share of micropore area % 74.0 69.9 88.9 in total surface area Total pore volume cm.sup.3/g 0.324 0.338 0.241 Micropore volume cm.sup.3/g 0.184 0.155 0.193 Average pore diameter nm 2.104 2.462 1.728

[0172] The three carbon materials are different from one other with regards to porous characteristics. These differences are apparent in the micro-porous characteristics and, separately, also the meso-porous characteristics of the samples. For example, toilet tissue derived carbon has a very distinct meso-pore distribution whereas carbon from wipes has negligible meso-porous character. Regarding the micro-pore character, toilet tissue derived carbon exhibited much slower adsorption than carbon from cotton pads and wipes, which suggests that the toilet tissue carbon is composed of particularly small micro-pores or has a considerably more restrictive micro-pore structure.

[0173] This deviation in behaviour is reflected in the nitrogen adsorption and desorption isotherm linear plots of the three carbons shown in FIG. 6. These distinct porosity properties provide distinct adsorption characteristics relevant for the removal of different odour causing substances in wastewater.

Adsorption Properties Analysed by a Photometer

[0174] The same carbons as prepared for the porosity analysis were used to determine their adsorption behaviour in wastewater from Scottish Water's Seafield Wastewater Treatment Works in Edinburgh by means of UV absorbance. Therefore, 5 mL of wastewater was amended with 50 mg of each carbon absorbance and left on a shaker for 12 hours. Afterwards the suspension was filtered, and the liquid phase was analysed by a UV photometer. The absorbance spectra shown in FIG. 7 show typical characteristics for wastewater with a peak at 220 nm for nitrate and overlapping peaks for various dissolved and suspended organic substances between 250 nm and 400 nm.

[0175] It can be seen that all three carbon materials remove a wide range of substances from wastewater. Carbon made from cotton pad and toilet tissue appear to be more effective in terms of overall removal. However, all materials show distinctive characteristics in their absorbance spectra. This supports the expectation that carbons made from a mixture of materials are capable of removing a broader range of substances than carbons made from a single material.

[0176] Although the invention has been particularly shown and described with reference to particular examples, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the scope of the present invention.

REFERENCES

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