WO2012122708A1

WO2012122708A1 - Method for biological treatment of sewage

Info

Publication number: WO2012122708A1
Application number: PCT/CN2011/071859
Authority: WO
Inventors: Luc Louvel; Jianming Yang; Min Yang; Junxin Liu; Benyi XIAO
Original assignee: Rhodia (China) Co., Ltd.; Research Center For Eco-Evironmental Sciences, Chinese Academy Of Sciences
Priority date: 2011-03-16
Filing date: 2011-03-16
Publication date: 2012-09-20
Also published as: CN103648988B; CN103648988A

Abstract

The invention provides a method for biological treatment of sewage, comprising: a) introducing the sewage into an aeration tank seeded with activated sludge for a sufficient aeration treatment, b) introducing the treated sludge discharged from the aeration tank into a sedimentation tank for solid-liquid separation, c) returning a part of settled sludge after solid-liquid separation to a mixing tank in which the sludge is homogeneously mixed with THPS fed into the mixing tank from a THPS storage container, and d) returning the sludge mixed with the THPS to the aeration tank, wherein the method is carried out continuously. The settlement performance of sludge can be improved and the yield of excess sludge during the biological treatment can be reduced by the method.

Description

METHOD FOR BIOLOGICAL TREATMENT OF SEWAGE

Field of the Invention

The present invention relates to a technique for treatment of sewage, in particular a method for biological treatment of sewage.

Background of the Invention

In company with industrial progress, the composition of sewage has been becoming more and more complicated. Some organic substances and toxic substances difficult to be degraded need to be treated by microbiological methods. Microorganisms can grow and propagate in the sewage by obtaining nutrients from the sewage while degrading the harmful substances, thereby purifying the sewage. Biological treatment of sewage is a purification process of sewage, which utilizes the biotic activities of the microorganisms to decompose organic pollutants dissolved or suspended in the sewage. The technology of sewage biological treatment is much attractive due to the following notable advantages: low energy consumption, high efficiency, low cost, convenience and reliability of operation as well as no secondary pollution. With such a technology, the organic pollutants in sewage can be converted into Ο0₂, water and sludge. It is estimated that 100 g sludge would be produced when 1 ton of municipal sewage is treated. Therefore, the biological treatment of sewage would produce a large amount of sludge. For example, in 2006, about 1.73 million tons of dry sludge was produced in China (Statistical Gazette, 2001) and about 1.3 million tons of dry sludge was produced in UK (www.wateruk.org, 2006). Disposal of so much sludge is one of the most complex problems for sewage plants. It is estimated that the cost for treatment and disposal of the sludge may account for 20-50%, even up to 60% of the total operating cost of a sewage plant. Thus, the processing and disposal of sludge is a key concern of a sewage plant. In view of the present situation of processing and disposal of sludge, the fundamental approach to resolve the sludge problem is to reduce the yield of excess sludge during the sewage treatment process as far as possible. Since by doing so, the amount of sludge to be disposed can be reduced from the beginning, researchers and sewage plants have paid a lot of attention to the research on the method for reducing the yield of excess sludge during the sewage treatment process.

At present, the methods for reducing the yield of sludge during a sewage treatment process are mainly based on the so-called "uncoupling growth". In these methods, the gap between the energy levels of catabolism and anabolism of the sludge microorganisms is enlarged by addition of an uncoupling agent so as to reduce the energy available for the anabolism to inhibit the anabolism, which reduces the growth rate of the sludge microorganisms and thus reduce the yield of excess sludge. Since such a sludge reduction method may be carried out in a conventional sewage biological treatment plant by incorporating a delivery device for the uncoupling agent without significant modification thereof, the input cost is very low. Accordingly, such a method has profound environmental and economic significance. The uncoupling agent currently used includes nitrophenols such as 2,4-dinitrophenol (dNP), para-nitrophenol (p-NP), meta-nitrophenol (m-NP) and the like; chlorophenols such as para-chlorophenol (p-CP), meta-chlorophenol(m-CP), pentachlorophenol (PCP) and the like; 3,3 ',4',5-tetrachlorosalicylanilide (TCS); aminophenols; and the like. Generally, these uncoupling agents are fat-soluble weak acids and may contaminate the water body more or less.

Tetrakis(hydroxymethyl)phosphonium sulphate (THPS) is a water soluble phosphonium salt biocide found in 1980s. THPS has the following advantages: a broad bactericidal spectrum, high efficiency and rapid bactericidal effect. Moreover, after use, it can be rapidly oxidized into non-bactericidal and almost non-toxic trihydroxymethyl phosphine oxide which may readily be further biodegraded into orthophosphate. THPS gained the US "President Green Chemistry Award" in 1997 due to its advantages in terms of toxicity, environment friendliness and safety over the traditional bactericides, which have been demonstrated by a great deal of data provided for registry in EPA. The major advantages of THPS include water-solubility, low toxicity, low recommended criterion for disposal and rapid degradation into completely non-toxic substance in environment without biological accumulation. Hence, it is widely used in cooling water system, water system of oil fields, fire control sprinkling system, paper manufacture and the like.

WO 2004/1 13236 discloses use of THPS as an uncoupling agent for controlling bacterial biomass in an aqueous system, and further discloses a method for controlling the bacterial biomass in an aqueous system with THPS.

Description of Invention

The object of the present invention is to provide an improved method for a biological treatment of sewage using THPS as an uncoupling agent, by which settlement performance of sludge can be improved and yield of excess sludge during the biological treatment can be reduced.

In order to achieve the above object, the present invention provides a method for a biological treatment of sewage, comprising a) introducing the sewage into an aeration tank seeded with activated sludge for a sufficient aeration treatment; b) introducing the treated sludge discharged from the aeration tank into a sedimentation tank for solid-liquid separation; c) returning a part of settled sludge after solid-liquid separation to a mixing tank in which the sludge is homogeneously mixed with THPS fed into the mixing tank from a THPS storage container; and d) returning the sludge mixed with the THPS to the aeration tank; wherein the method is carried out continuously.

As used herein, the term "sewage" includes any industrial sewage and municipal sewage.

As used herein, the term "yield of excess sludge" means the amount of excess sludge ( Kg ) produced by removal of per unit (Kg) of BOD₅ in the sewage.

As used herein, the term "return sludge ratio" means a ratio of flow rate of the sludge returned to the aeration tank to flow rate of sewage entering the aeration tank (v/v).

As used herein, the term "residence time of the returned sludge in the mixing tank" means the time required for once renewal of all returned sludge in the mixing tank.

As used herein, the term "volumetric loading of THPS" means the amount of THPS per liter of returned sludge per day, which is expressed as mg THPS/L returned sludge -day. The volumetric loading of THPS can be calculated from the following formula:

Volumetric loading of THPS = [the concentration of the solution of THPS (mg/L) * the flow rate of the solution of THPS into mixing tank (ml/h) * 24 (h)/1000 (ml/L)]/ [the flow rate of the sewage into aeration tank (L/h) * return sludge ratio * 24 h].

Except steps c) and d), the method according to the present invention can be carried out in the same manner as the conventional biological treatment process of sewage.

There is no limitation on size and shape of the aeration tank. The size of the aeration tank depends on the amount of sewage to be treated. The sludge concentration, namely the mixed liquor suspended solids (MLSS), in the aeration tank is preferably 1500-4000 mg/L, more preferably 2000-3000 mg/L. The dissolved oxygen concentration in the aeration tank is preferably maintained at 1.5-3.5 mg/L, more preferably 2-3 mg/L.

In an embodiment of the present invention, THPS can be in the form of a solution of THPS, preferably of an aqueous solution of THPS

In an embodiment of the present invention, the volumetric loading of THPS ranges from 0.0001 mg THPS/L returned sludge-day to 0.003 mg THPS/L returned sludge-day, preferably from 0.00015 mg THPS/L returned sludge -day to 0.0025 mg THPS/L returned sludge -day, more preferably from 0.0002 mg THPS/L returned sludge-day to 0.002mg THPS/L returned sludge-day, particularly preferably from 0.0003 mg THPS/L returned sludge-day to 0.0009mg THPS/L returned sludge-day, the most preferably from 0.0004 mg THPS/L returned sludge-day to 0.0008mg THPS/L returned sludge-day.

In an embodiment of the present invention, a part of settled sludge is returned after solid-liquid separation to the mixing tank in a return sludge ratio of 50-350%, preferably 100-300%, more preferably 150-225%.

In an embodiment of the present invention, THPS and the returned sludge are fed into the mixing tank in such directions that the angle there between is 90 ± 5 degrees, preferably in directions perpendicular to each other.

In an embodiment of the present invention, the solution of THPS has a concentration of 1 -50 mg THPS/L, preferably 1.5-7 mg THPS/L, more preferably 2-5 mg THPS/L.

THPS may be formulated with one or more selected from a surfactant; an antifoam; a scale inhibitor; a correction inhibitor; a biocide; a flocculent and a dispersant; for example, formulated with bicarbonates, more preferably sodium bicarbonate and/or potassium bicarbonate for better dispersion of THPS in the returned sludge. The concentration of bicarbonates in the solution of THPS may be 0.2-20 g/L, preferably 2-5 g/L.

In an embodiment of the present invention, the residence time of the returned sludge in the mixing tank is 1-25 minutes, preferably 3-20 minutes, more preferably 4-15 minutes, particularly more preferably 5-7 minutes.

The volume of the mixing tank depends on the amount of the returned sludge, and the mixing tank may be in various shapes. In an embodiment of the present invention, said mixing tank is in a cylindrical shape.

In an embodiment of the present invention, said mixing tank is equipped with a stirrer, such as a paddle stirrer, preferably with the shaft of the stirrer positioned at the vertical axis of the mixing tank.

In an embodiment of the present invention, said returned sludge is fed into the mixing tank horizontally at a distance of 1/5*H-1/3*H, preferably 1/4*H from the bottom of the mixing tank, wherein H is the height of the mixing tank.

In an embodiment of the present invention, said THPS is fed into the mixing tank vertically from the top of the mixing tank at a distance of 1/4*R-1/2*R, preferably 1/3 *R from the shaft of the stirrer, wherein R is the radius of the mixing tank.

As compared with the traditional activated sludge treatment technology, the advantageous effects of the present invention include: the yield of excess sludge during the biological treatment of sewage can be reduced by about 10-55%, preferably 20-55%, more preferably 25-55% and most preferably 30-55%, and the settlement performance of the sludge (expressed by SV30, which means the volume percent of the sludge after settling the mixed liquor from the aeration tank in a graduated cylinder for 30 minutes; the lower the SV30 is, the better the settlement performance is) can be improved by about 5-35%, preferably 10-35%, more preferably 15-35% according to the present invention.

The efficiency of sewage treatment of the method according to the present invention is substantively the same as that of the traditional activated sludge treatment method in terms of removals of COD, five-day biochemical oxygen demand (BOD₅), suspended solids (SS), total nitrogen (TN), total phosphorus (TP), NH4+-N, etc. The method of the present invention also has the following advantages: simple equipments, convenience of operation, low investment and operating cost and the like. Brief Description of Drawing

Fig. l is a flow scheme of the method according to one embodiment of the present invention.

Examples

The invention will now be described by way of non-limiting examples with reference to accompanying figure.

Example 1

In this example, a completely mixed activated sludge process (see Fig. 1) was carried out on a laboratory scale. For comparison, six experiments were carried out using six sets of identical experimental equipments under the same conditions except that the THPS aqueous solutions with different THPS concentration as shown in table 1 were used. In the experimental equipment, an aeration tank 3 having an effective volume of 500L, and a sedimentation tank 8 having an effective volume of 100L were used. The aeration tank 3 was seeded with settled sludge from a sedimentation tank of the sewage plant for the central park of Eco-Environmental Research Center of Chinese Academy of Sciences. The domestic sewage from the central park of Eco-Environmental Research Center of Chinese Academy of Sciences was used as test sewage. A sewage reservoir 1 having 2 m of volume was used as the sewage storage tank.

After entering the sewage reservoir 1 , the sewage was continuously pumped from the sewage reservoir 1 into the aeration tank 3 via a sewage feeding pump 2 at a flow rate of 1.0 L/min for aeration treatment therein. The sludge concentration in the aeration tank 3 is 2500-3400 mg/L and the dissolved oxygen concentration in the aeration tank 3 is 1.5-3.5 mg/L. The residence time of sewage in the aeration tank 3 was 9 hours and the residence time of the sludge in the aeration tank 3 was 10 days. The treated sewage from the aeration tank 3 was fed into the sedimentation tank 8 for solid-liquid separation. The supernatant was discharged through a drainage outlet at upper portion of the sedimentation tank, while a part of settled sludge was discharged through the pump 9 from the bottom of the sedimentation tank 8, and the other part of settled sludge was returned to a mixing tank 7 (in a cylindrical form) in a return sludge ratio of 200% via a pump 10 to be mixed with the THPS aqueous solution fed into the mixing tank 7 from the THPS storage tank 4 via a pump 5. The mixing tank 7 has 10L of effective volume which is 1/50 of that of the aeration tank 3 and has a dimension of dxh=l 5cm* 15cm. The mixing tank 7 was equipped with a paddle stirrer 6 whose shaft was located at the vertical axis of the mixing tank. The concentration of the THPS aqueous solutions fed into the mixing tank 7 were 0 mg THPS/L, lmg THPS/L, 2mg THPS/L, 3.5mg THPS/L, 5mg THPS/L and 8mg THPS/L, respectively. The THPS aqueous solution was fed into the mixing tank 7 at a flow rate of 18 ml/h. The volumetric loadings of THPS were 0 mg THPS/L returned sludge-day, 0.00015 mg THPS/L returned sludge-day, 0.0003 mg THPS/L returned sludge-day, 0.000525 mg THPS/L returned sludge-day, 0.00075 mg THPS/L returned sludge-day, 0.0012 mg THPS/L returned sludge-day, respectively. The returned sludge was fed into the mixing tank horizontally at a distance of 1/4*H from the bottom of the mixing tank 7, while the THPS aqueous solution was fed into the mixing tank 7 vertically from the top of the mixing tank 7 at a distance of 1/2*R from the shaft of the stirrer 6. The returned sludge and the THPS aqueous solution were fed into the mixing tank 7 in two directions perpendicular to each other in order to facilitate mixing. The residence time of the returned sludge in the mixing tank was 5.5 minutes. The returned sludge homogeneously mixed with the THPS aqueous solution was returned to the aeration tank 3. The experiments ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN and SS values in the effluent water, five experiments with addition of THPS were similar to the experiment without addition of THPS, while TP values in the experiments with addition of THPS were slightly higher than that of the experiment without addition of THPS. In the experiments with addition of THPS, the average yield of excess sludge was reduced and the settlement performance of sludge was improved, as compared with the experiment without addition of THPS (see tables 1 and 2 for the details).

Table 1 : comparison of the water quality between the feeding sewag the effluent water

* : The numerical values in the brackets were mean values. Table 2 Concentration

of THPS in the

0 1 mg/L 2 mg/L 3.5 mg/L 5 mg/L 8 mg/L THPS tank

(mg/L)

Volumetric

loading of

THPS (mg

0 0.00015 0.0003 0.000525 0.00075 0.0012 THPS/L

returned

sludge-day)

Average yield 0.95 0.84 0.66 0.45 0.63 0.85 of excess

sludge* (kg

VSS/kg BOD₅)

Reduction of - 12 31 53 34 1 1 excess sludge

** (%)

sv₃₀ 30 28 24 19.5 23 27

* : Average yield of excess sludge - sum of the yield of excess sludge throughout the experiment/days of experiment.

** : Reduction of the excess sludge = (the yield of excess sludge from the system without addition of THPS - the yield of excess sludge from the system with addition of THPS) / the yield of excess sludge from the system without addition of THPS x 100%.

Example 2

In this example, a completely mixed activated sludge process (see Fig. 1) was also carried out on a laboratory scale. For determining the effect of the return sludge ratio on the reduction of the excess sludge and SV30, five experiments were carried out using five sets of the same experimental equipments under the same conditions as in Example 1 , except that the concentration of the THPS aqueous solution used in all these five experiments was 3.5mg /L, and different return sludge ratios as shown in table 3 were employed in these experiments. The experiments also ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN, TP and SS values in the effluent water, the experimental results were similar to those of Example 1. The average yield of excess sludge, reduction of the excess sludge and SV₃₀ were shown in table 3.

Table 3 : effect of return sludge ratio on the reduction of the excess slud

Example 3

In this example, a completely mixed activated sludge process (see Fig. 1) was also carried out on a laboratory scale. For determining the effect of the residence time of the returned sludge in the mixing tank on the reduction of the excess sludge and SV₃₀, five experiments were carried out using five sets of the same experimental equipments under the same conditions as in Example 1 , except that the concentration of the THPS aqueous solution fed into the mixing tank 7 for all these five experiments was 3.5 mg THPS/L, i.e., the volumetric loading of THPS was 0.000525 mg THPS/L returned sludge-day, and different residence time of the returned sludge in the mixing tank as shown in table 4 were employed in these experiments. The experiments also ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN, TP and SS values in the effluent water, the experimental results were similar to those of Example 1. The average yield of excess sludge, reduction of the excess sludge and SV30 were shown in table 4.

Table 4: effect of residence time of the returned sludge in the mixing tank on the reduction of the excess sludge and SV30

Example 4

In this example, a completely mixed activated sludge process (see Fig. 1) was also carried out on a laboratory scale. For determining the effect of the site at which the returned sludge was fed into the mixing tank on the reduction of the excess sludge and SV30, five experiments were carried out using five sets of the same experimental equipments under the same conditions as in Example 3, except that the residence time of the returned sludge in the mixing tank for all these five equipments was 6 minutes, and the returned sludge was fed into the mixing tank at different sites as shown in table 5. The experiments also ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN, TP and SS values in the effluent water, the experimental results were similar to those of Example 1. The average yield of excess sludge, reduction of the excess sludge and SV30 were shown in table 5.

Table 5 : effect of the site at which the returned sludge was fed into the mixing tank on the reduction of the excess sludge and SV₃₀

Example 5

In this example, a completely mixed activated sludge process (see Fig. 1) was also carried out on a laboratory scale. For determining the effect of the site at which the THPS aqueous solution was fed into the mixing tank on the reduction of the excess sludge and SV30, five experiments were carried out using five sets of the same experimental equipments under the same conditions as in Example 3, except that the returned sludge was fed into the mixing tank horizontally at a distance of 1/4*H from the bottom of the mixing tank in all these five experiments, and the THPS aqueous solution was fed into the mixing tank at different sites as shown in table 6. The experiments also ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN, TP and SS values in the effluent water, the experimental results were similar to those of Example 1. The average yield of excess sludge, reduction of the excess sludge and SV₃₀ were shown in table 6. Table 6: effect of the site at which the THPS aqueous solution was fed into the mixing tank on the reduction of the excess sludge and SV30

Example 6

In this example, a completely mixed activated sludge process was also carried out on a laboratory scale. One experiment was carried out using the same experimental equipment as in Example 1 but without the mixing tank 7 and under the same conditions as in Example 1 except that the returned sludge from the sedimentation tank was recycled directly into the aeration tank and the THPS aqueous solution was fed directly from the THPS storage tank 4 into the aeration tank to be mixed with the activated sludge therein. The concentration of the THPS aqueous solution used in this experiment was 3.5mg/L. The experiment also ran steadily for two months with stable quality of effluent water (i.e., the supernatant) being achieved throughout the experiment procedure. In terms of COD, BOD₅, NH₄ ⁺-N, TN, TP and SS values in the effluent water, the experimental results were similar to those of Example 1. The comparison of experimental results between this example and Example 1 in terms of yield of excess sludge, reduction of the excess sludge and SV₃₀ were shown in table 7. Table 7

The comparison of experimental results shows that as compared with adding the THPS aqueous solution directly into the aeration tank, better effect in terms of the reduction of the excess sludge and SV30 has been achieved by mixing the THPS aqueous solution with the returned sludge in the mixing tank.

Claims

1. A method for biological treatment of sewage, comprising:

a) introducing the sewage into an aeration tank seeded with activated sludge for a sufficient aeration treatment;

b) introducing the treated sludge discharged from the aeration tank into a sedimentation tank for solid-liquid separation;

c) returning a part of settled sludge after solid-liquid separation to a mixing tank in which the sludge is homogeneously mixed with THPS fed into the mixing tank from a THPS storage container; and

d) returning the sludge mixed with the THPS to the aeration tank; wherein the method is carried out continuously.

2. The method according to claim 1 , wherein said THPS is in the form of a solution of THPS, preferably of an aqueous solution of THPS.

3. The method according to claim 1 or 2, wherein volumetric loading of THPS ranges from 0.0001 mg THPS/L returned sludge-day to 0.003 mg THPS/L returned sludge-day, preferably from 0.00015 mg THPS/L returned sludge-day to 0.0025 mg THPS/L returned sludge-day, more preferably from 0.0002 mg THPS/L returned sludge-day to 0.002mg THPS/L returned sludge-day, particularly preferably from 0.0003 mg THPS/L returned sludge-day to 0.0009mg THPS/L returned sludge-day, the most preferably from 0.0004 mg THPS/L returned sludge-day to 0.0008mg THPS/L returned sludge-day.

4. The method according to any one of claims 1 to 3, wherein said part of settled sludge is returned after solid-liquid separation to the mixing tank in a return sludge ratio of 50-350%, preferably 100-300%, more preferably 150-225%.

5. The method according to any one of claims 1 to 4, wherein said THPS and said returned sludge are fed into the mixing tank in such directions that the angle there between is 90 ± 5 degrees, preferably in directions perpendicular to each other.

6. The method according to any one of claims 2 to 5, wherein said solution of THPS has a concentration of 1-50 mg THPS/L, preferably 1.5-7 mg THPS/L, more preferably 2-5 mg THPS/L.

7. The method according to any one of claims 1 to 6, wherein said THPS is formulated with one or more selected from a surfactant; an antifoam; a scale inhibitor; a correction inhibitor; a biocide; a flocculent and a dispersant; preferably formulated with bicarbonates, more preferably sodium bicarbonate and/or potassium bicarbonate.

8. The method according to any one of claims 1 to 7, wherein the residence time of said returned sludge in the mixing tank is 1-25 minutes, preferably 3-20 minutes, more preferably 4-15 minutes, particularly more preferably 5-7 minutes.

9. The method according to any one of claims 1 to 8, wherein said mixing tank is equipped with a stirrer, such as a paddle stirrer, preferably with the shaft of the stirrer positioned at the vertical axis of the mixing tank.

10. The method according to claim 9, wherein said mixing tank is in a cylindrical shape, and said THPS is fed into the mixing tank vertically from the top of the mixing tank at a distance of 1/4*R- 1/2*R, preferably 1/3 *R from the shaft of the stirrer, wherein R is the radius of the mixing tank.

1 1. The method according to any one of claims 1- 10, wherein said returned sludge is fed into the mixing tank horizontally at a distance of 1/5*H-1/3 *H, preferably 1/4*H from the bottom of the mixing tank, wherein H is the height of the mixing tank.