METHOD FOR TREATING WASTEWATER

Abstract

The invention relates to a method for treating wastewater in an activated sludge process, which comprises a biological treatment step followed by a sedimentation step for separation of sludge and treated effluent water. The method comprises directing a part of the treated effluent water as a backflow from the sedimentation step to the biological treatment step, the backflow having an original pH value; adjusting the backflow pH from the original pH value to a first pH value between the sedimentation step and the biological treatment step, and adding a coagulant and/or an oxidant to the backflow after the adjustment of the backflow pH to the first pH value and before backflow's entry to the biological treatment step.

Claims

1. A method for treating wastewater in an activated sludge process, which comprises a biological treatment step followed by a sedimentation step for separation of sludge and treated effluent water, the method comprising directing a part of the treated effluent water as a backflow from the sedimentation step to the biological treatment step, the backflow having an original pH value; adjusting the original pH value of the backflow to a first pH value between the sedimentation step and the biological treatment step, and adding a coagulant and/or an oxidant to the backflow after adjusting the original pH value of the backflow to the first pH value and before backflow's entry to the biological treatment step.

2. The method according to claim 1, wherein <75 vol-%, preferably 10-60 vol-%, more preferably 10-50 vol-% of the treated water exiting the sedimentation step is directed as a backflow to the biological treatment step.

3. The method according to claim 1 that the wherein pH of the backflow pH is adjusted after the addition of the coagulant from the first pH value to a second pH value.

4. The method according to claim 3, wherein the second pH value which is within 1 pH unit from the original pH value.

5. The method according to claim 1, wherein the coagulant comprises an inorganic metal coagulant, a polymer coagulant or any combination thereof.

6. The method according to claim 5, wherein the coagulant comprises inorganic metal coagulant, which is selected from aluminium sulphate, polyaluminum chloride, iron sulphate, ferric chloride or any combination thereof.

7. The method according to claim 5, wherein the coagulant is added in an amount resulting in inorganic metal coagulant in an amount of 1 mol metal ion per 10 g to 1000 g of COD in the backflow to be treated.

8. The method according to claim 5, wherein the coagulant comprises a natural polymer coagulant, which is selected from cationic polysaccharides, chitosan and tannin-based coagulants.

9. The method according to claim 5, wherein the coagulant comprises synthetic polymer coagulant, which is a cationic polymer coagulant selected from polyamine, polyvinylamine, polyethyleneimine, polydicyandiamide (polyDCD), polydiallyldimethylammonium chloride (polyDADMAC), poly(acryloyloxyethyl trimethylammonium chloride) (polyADAM-Cl), poly (methacryloyloxyethyltrimethylammonium chloride) (polyMADAM-Cl), poly (acrylamido-N-propyltrimethylammonium chloride) (polyAPTAC), poly (methacrylamidopropyltrimethylammonium chloride) (polyMAPTAC) and/or a copolymer of (meth)acrylamide and cationic monomers selected from diallyl dimethylammonium chloride (DADMAC), [2-(acrylamido)ethyl]trimethylammonium chloride, (ADAM-Cl), [2-(methacrylamido)ethyl]trimethylammonium chloride (MADAM-Cl), [3-(acryloyloxy)-propyl]trimethylammonium chloride (APTAC) and/or [3-(methacryloyloxy)propyl]-trimethylammonium chloride (MAPTAC).

10. The method according to claim 1, wherein the first pH value is in a range of 3-6.5, preferably 4.0-6.5, more preferably 4.5-6.0.

11. The method according to claim 1, wherein the oxidant is selected from Fenton's reagent, ozone, inorganic or organic peroxy acids and chlorine compounds.

12. The method according to claim 11, wherein the oxidant is added in amount of 1 mol of active oxidant per 5 g to 100 g of COD in the backflow to be treated.

13. The method according to claim 1, wherein the wastewater is municipal wastewater and/or industrial wastewater.

14. The method according to claim 1, wherein the wastewater originates from manufacture of pulp and/or paper.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] An embodiment of the present invention will be described in more detail in the appended FIGURE, where

[0035] FIG. 1 shows a schematic process chart for one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] FIG. 1 shows schematically a process chart for wastewater treatment employing an activated sludge process, where the method according to the present invention is applied. FIG. 1 illustrates one possible non-limiting process embodiment.

[0037] In a process illustrated in FIG. 1, wastewater to be treated A is first conveyed to a pre-treatment 1, which can be, for example, a primary sedimentation for removing total suspended solids and other particulate matter from wastewater. After the pre-treatment wastewater is conveyed to a biological treatment 2 comprising an aeration tank 3 for removing majority of dissolved organic matter. The wastewater treatment process may comprise both anaerobic and aerobic treatment tanks or basins or it may comprise only aerobic treatment. In the aeration tank 3 air and/or oxygen is injected in the wastewater. Micro-organisms digest organic matter, also called as an activated sludge process. It is to be understood that the design and layout of the biological treatment may vary depending on the wastewater and/or other conditions.

[0038] After the biological treatment 2 the wastewater is conveyed to the sedimentation step 4 for sludge separation. The sedimentation step 4 comprises a solid-liquid separation unit 5, usually referred to as “secondary clarifier”, which allows the biological flocs to settle, thus separating the activated sludge from the wastewater. A part of the separated sludge is recycled back to the biological treatment step 2.

[0039] Clear filtrate, i.e. treated water, is removed from the sedimentation step. Part of this treated water is circulated back to the biological treatment step 2 as a backflow 6. The backflow pH is adjusted from the original pH value to a first pH value at suitable first location 7 between the sedimentation step 4 and the biological treatment step 2. After the adjustment of the backflow pH a coagulant and/or an oxidant is added to the backflow at a suitable second location 8 before the backflow's 6 entry to the biological treatment step 2.

[0040] The following example is merely illustrative of the principles and practices of the present invention and are not intended to limit the scope of the invention.

Example 1

[0041] Mixed papermill wastewater was used in Example 1. Main part of wastewater originated from mechanical pulp mill and other parts from paper machine, debarking process and from a coating kitchen. Wastewater was first treated in a primary sedimentation, and the secondary step was a biological activated sludge process. Wastewater for the experiments was taken after the secondary step. Activated sludge used in the experiments was taken from the aeration tank of the activated sludge process.

[0042] The experiments were carried out by using Kemira Flocculator 2000 equipment. The experiment comprised two steps.

[0043] Step 1

[0044] 0.5 liter of the sample was subjected to fast mixing for 30 s, speed 400 rpm. pH of the sample was adjusted to pH 4.0 when both 421 ppm H.sub.2SO.sub.4 and 600 ppm Fe.sub.2(SO.sub.4).sub.3 (conc. 11.5%) were added. The sample was subjected to slow mixing for 10 min, speed 40 rpm. A 5 ppm dose of cationic polyacrylamide polymer (FennoPol K8980, Kemira) was added. In the end the sample pH was raised to the neutral (pH 7.3) with 188 ppm NaOH.

[0045] Step 2

[0046] After Step 1 the treated sample was mixed with 0.5 liter of an activated sludge sample (MLSS 6.26 g/l), giving total sample volume of 1 liter. The total sample was aerated for one hour. After aeration the total sample was allowed to settle 1 hour without mixing. Treated effluent sample for analysis was taken from the settled total sample, from the clear zone above the sludge blanket.

[0047] The 0.5 liter of reference sample was treated in identical manner as described in Step 2. Treatment in Step 1 was omitted.

[0048] Following devices and methods were used for analysis of the treated effluent sample. [0049] UV absorbance measurement at 254 nm, Helios Unicam SN024 spectrophotometer in accordance. [0050] COD measured by dichromate method was analysed using Hach Lange kit tubes and DR390 Photometer. Filtered COD sample was filtered with 0.45 μm filter. [0051] Turbidity was measured by TL2360 Hach Turbidimeter. [0052] Determination of suspended solids in sludge was made according to standard methods using GF/A filter.

[0053] The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Results of Example 1 Treated Sample Reference Sample UV absorbance, 1/cm 2.272 3.181 Turbidity, FNU 12.5 10.9 COD, mg/l 206 279 COD, filtered mg/l 187 256

[0054] It can be seen form the results of Table 1 that the treated sample shows clearly reduced COD values, which indicates effective removal of organic contaminants, when part of the water from the secondary step is treated and circulated back to the biological treatment step.

[0055] 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.