Method for dewatering of biological sludge using a polymeric flocculant

Abstract

A method for dewatering of biological sludge is disclosed. The method includes addition of a flocculant to a biological sludge, which includes an aqueous phase and a suspended solid organic material, flocculating and dewatering the sludge. The flocculant includes a polymer composition, which includes a cationic crosslinked first polymer, which is selected from crosslinked polyamines, and a cationic second polymer, which is a polymer obtained by polymerization of (meth)acrylamide and cationic monomers, the second cationic polymer being polymerized in presence of the cationic first polymer.

Claims

1. A method for dewatering of a biological sludge, comprising: adding a flocculant to the biological sludge, which comprises an aqueous phase and a suspended solid organic material, and flocculating the sludge, dewatering the sludge, wherein the flocculant comprises a polymer composition, which comprises: a cationic crosslinked first polymer, which is selected from crosslinked polyamines, and a cationic second polymer, which is a copolymer obtained by polymerization of (meth)acrylamide and at least one cationic monomer, the second cationic polymer being polymerized in presence of the cationic first polymer.

2. The method according to claim 1, wherein the first polymer and the second polymer are miscible with each other.

3. The method according to claim 1, wherein the polymer chains of the first polymer and second polymer are physically and inseparably entangled in during the polymerization of the second polymer.

4. The method according to claim 1, wherein the polymer composition comprises at least 1 weight-%, preferably 1-30 weight-%, more preferably 3-20 weight-% even more preferably 5-15 weight-%, of the first polymer.

5. The method according to claim 1, wherein the first polymer is selected from group comprising crosslinked: copolymers of epichlorohydrin and dimethylamine, copolymers of epichlorohydrin, dimethylamine and ethylenediamine, polyamidoamines, and polyvinylamine.

6. The method Method according to claim 1, wherein the first polymer has a weight average molecular weight at least 10 000 g/mol, preferably in the range of 10 000-350 000 g/mol, more preferably 30 000-275 000 g/mol, even more preferably 50 000-250 000 g/mol.

7. The method Method according to claim 1, wherein the first polymer has a weight average molecular weight in the range of 120 000-350 000 g/mol, preferably 125 000-275 000 g/mol, more preferably 135 000-250 000 g/mol.

8. The method Method according to claim 1, wherein the second polymer is obtained by copolymerization of (meth)acrylamide and at least 10 mol-% of cationic monomer, preferably 10-90 mol-%, more preferably 20-70 mol-%, even more preferably 30-60 mol-%, of cationic monomer, calculated from amount of monomers used for second polymer.

9. The method according to claim 1, wherein the cationic monomer for the second polymer is selected from group consisting of 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acryloylam ino)propyl] trimethylammonium chloride (APTAC), [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC), and diallyldimethylammonium chloride (DADMAC).

10. The method according to claim 1, wherein the second polymer is a crosslinked polymer, obtained by polymerizing (meth)acrylamide and cationic monomers in the presence of at least one crosslinking agent.

11. The method according to claim 10, wherein the amount of crosslinking agent is in the range of 0.25-100 mg/kg monomers, preferably 0.5-10 mg/kg monomers, more preferably 0.75-5 mg/kg monomers,

12. The method according to claim 1, wherein the second polymer is a linear polymer.

13. The method according to claim 1, wherein the polymer composition has a standard viscosity SV of 3.5-5 mPas, preferably 3.8-4.8, measured at 0.1 weight-% solids content in an aqueous NaCl solution (1 M), at 25 C., using Brookfield DVII T viscometer with UL adapter.

14. The method according to claim 1, wherein the flocculant comprising the polymer composition is added in amount of 1-40 kg/ton dry sludge, preferably 2-30 kg/ton dry sludge, preferably 4-20 kg/ton dry sludge.

15. The method according to claim 1, wherein the biological sludge is municipal wastewater sludge or agricultural sludge.

16. The method according to claim 1, wherein the biological sludge has a biological oxygen demand (BOD) >50 mg/I and/or a dry solids content in the range of 5-80 g/I.

Description

EXPERIMENTAL

[0035] Some embodiments of the invention are described in the following non-limiting examples.

Polymer Compositions Used in the Examples

[0036] Two different polymer compositions C1 and C2 were used according to invention in the following sludge dewatering Examples:

[0037] Composition C1 comprised as first polymer crosslinked polyvinylamine. The second polymer, which was polymerised in the presence of the first polymer was a copolymer of acrylamide and 30 mol-% of [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl). The amount of first polymer was 9 weight-%, as active, based on monomers of the second polymer.

[0038] Composition C2 comprised as polymer crosslinked polyvinylamine. The second polymer, which was polymerised in the presence of the first polymer was a copolymer of acrylamide and 30 mol-% of [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl). Methylenebisacrylamide was used as crosslinker in the polymerisation. The amount of first polymer was 9 weight-%, as active, based on monomers of the second polymer.

Methods Used in the Examples

[0039] The apparatuses and methods used in sludge dewatering examples are given in Table 1.

TABLE-US-00001 TABLE 1 Apparatuses and methods used in sludge dewatering examples. Property Apparatus/Standard pH Knick Portamess 911 pH Dry solids SFS 3008 Suspended solids SFS 3008 Turbidity HACH 2100AN IS Turbidimeter//ISO 7027

[0040] Gravity dewaterability of sludge was tested with Polytest. The sludge samples were filtered with Polytest cylinder of 10 cm diameter using in bottom a wire cloth having air permeability of 5400 m.sup.3/m.sup.2h. Treads/cm was 13.0/5.9. The sludge sample amount was 200-400 g, but always identical between samples compared. Mixing of the polymer composition was done with motor stirrer in baffled mixing vessel. Mixing speed was 1000 rpm and mixing time was 10-30 seconds, but always identical between samples compared.

[0041] Sludge dry solids content after centrifugation was tested with Heraeus laboratory centrifuge. For this test, sludge sample was taken from Polytest wire after gravity dewaterability testing. Sludge sample of about 6 grams was measured to a 50 mesh plastic filter that was placed on upper part of centrifuge tube. Centrifugation time was 60 seconds and rotation rate 1000 rounds per minute (rpm). After the centrifugation, reject water was collected from bottom of the centrifuge tube and centrifuged sludge from plastic filter.

Sludge Dewatering Example 1

[0042] This example simulates dewatering of biological sludge after anaerobic digestion process in municipal wastewater treatment plant with biological phosphorus removal. Dry solids content of the biological sludge was 29 g/I before dosage of the polymer composition.

[0043] Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer doses were 5, 6 and 6.5 kg/ton dry sludge. Mixing time was 10 seconds. Amount of drained water was measured after 15 seconds. Turbidity was measured from the drained reject water. Results from these experiments are presented in Table 2.

TABLE-US-00002 TABLE 2 Results of Example 1 for drainage and reject water turbidity. Dose Drainage 15 s Reject water turbidity Polymer [kg/t] [g] [NTU] R1 5.0 41.4 290 R1 6.0 54.0 205 R1 6.5 63.0 181 C1 5.0 53.3 202 C1 6.0 10.8 105 C1 6.5 97.6 102

[0044] It can be seen from the results of Table 2 that the use of polymer composition C1 according to the invention provided better performance than the reference polymer R1. Polymer composition C1 produced faster dewatering and better reject water quality than the reference polymer R1 with all the tested doses. All of these factors are important for economical sludge dewatering.

Sludge Dewatering Example 2

[0045] This example simulates centrifugation of biological sludge after anaerobic digestion process in municipal wastewater treatment plant with biological phosphorus removal. Dry solids content of the biological sludge was 29 g/I before dosage of the polymer composition.

[0046] Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Sludge dry solids after centrifugation was tested with table centrifuge as described above. Polymer doses were 5, 6 and 6.5 kg/ton dry sludge with mixing time 10 seconds and 6, 7 and 8 kg/ton dry sludge with mixing time 20 seconds. Dry solids content of the sludge was measured after 60 second centrifugation at 1000 rpm. Results from these experiments are presented in Table 3.

TABLE-US-00003 TABLE 3 Dry solids content results after centrifugation in Example 2. Dose Mixing time Dry solids Polymer [kg/t] [s] [%] R1 5.0 10 9.8 R1 6.0 10 9.9 R1 6.5 10 10.2 C1 5.0 10 10.2 C1 6.0 10 10.6 C1 6.5 10 10.7 R1 6.0 20 9.2 R1 7.0 20 9.6 R1 8.0 20 10.6 C1 6.0 20 9.4 C1 7.0 20 10.4 C1 8.0 20 10.8

[0047] It can be seen for results of Table 3 that the use of polymer composition C1 according to the invention provided better performance than the reference polymer R1. Polymer composition C1 produced higher dry solids content after centrifugation with 10 second and 20 second mixing than the reference polymer R1 with all the tested doses. Changing the mixing time from 10 seconds to 20 seconds represents increasing share forces which is required for efficient centrifugation. All of these factors are important for economical dewatering of biological sludge.

Sludge Dewatering Example 3

[0048] This example simulates dewatering of biological sludge after anaerobic digestion process in municipal wastewater treatment plant with chemical phosphorus removal. Dry solids content of the sludge was 25 g/I before dosage of the polymer composition.

[0049] Polymer compositions were diluted to 0.1% concentration before dosing to the sludge. Dewatering rate was tested with Polytest as described above. Polymer doses were 7, 8 and 9 kg/ton dry sludge with mixing time 10 seconds and 9, 10 and 11 kg/ton dry sludge with mixing time 30 seconds. Amount of drained water was measured after 10 seconds. Turbidity was measured from the drained reject water. Results are presented in Table 4.

TABLE-US-00004 TABLE 4 Results for drainage and reject water turbidity in Example 4. Dose Mixing time Drainage 10 s Reject water turbidity Polymer [kg/t] [s] [g] [NTU] R1 7.0 10 95.6 396 R1 8.0 10 125.2 301 R1 9.0 10 118.3 391 C2 7.0 10 93.9 345 C2 8.0 10 136.7 349 C2 9.0 10 130.1 286 R1 9.0 30 64.4 490 R1 10.0 30 77.9 452 R1 11.0 30 127.1 275 C2 9.0 30 84.4 382 C2 10.0 30 128.7 293 C2 11.0 30 133.8 259

[0050] It can be seen from the results of Table 4 that the use of polymer composition C2 according to the invention provided better performance than the reference polymer R1. Polymer composition C2 produced faster dewatering and better reject water quality than the reference polymer R1. Polymer composition C2 did also have much better shear resistance, which was seen in the experiments with longer mixing time causing more shear forces to the flocs. All of these factors are important for economical sludge dewatering.

[0051] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.