A method for preparing engineering packing material from sludge in a water supply plant

Abstract

This invention belongs to the field of sludge treatment, specifically relating to a method for preparing engineering filler from sludge in water treatment plants. The method employs a two-step process: first, "conditioning and modifying" the sludge, followed by "solidification and stabilization." The conditioning and modifying agent for the sludge comprises the following components in parts by weight: regenerated micro powder: 100 parts; attapulgite: 8-36 parts (attapulgite fineness: 180-360 mesh); coupling agent: 3-9 parts; water-repellent agent: 3-7 parts; water-absorbing resin: 2-6 parts; oxidant: 1.5-4.0 parts. This invention, through the synergistic effect of specific components, produces a conditioning and modifying agent that, after conditioning and modifying the sludge in water treatment plants, yields modified sludge. Further solidification and stabilization of the modified sludge allows for the preparation of inorganic binders, i.e., regenerated engineering filler, which possesses high mechanical properties and achieves the resource-based treatment of sludge from water treatment plants. This not only solves environmental pollution problems but also yields materials with multiple applications, representing a significant advancement in technology.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of sludge treatment, and particularly relates to a method for preparing engineering filler from waterworks sludge. BACKGROUND

[0002] Urban water supply and drainage facilities play an important role in the development of urban modernization process, and are an important prerequisite for protecting human living environment and improving people's material living standards, and are one of the important symbols for measuring the level of modern cities. Since the organic matter content of sewage plant sludge is high, energy recovery is currently mainly used for utilization at home and abroad, and the resource utilization technology is relatively mature. In comparison, the organic matter content of waterworks sludge is low, and the energy recovery value is not high, and at present, landfill treatment is mainly used, which not only occupies land and pollutes the environment, but also produces a large amount of fugitive greenhouse gases, and the research on resource utilization technology of waterworks sludge is urgently needed.

[0003] The water content of waterworks sludge is high and is in a viscous state, which greatly limits the further resource utilization of waterworks sludge, and reducing the water content and viscosity of sludge to improve the sludge properties is a key pretreatment link for carrying out sludge resource utilization. The conventional drying technology of sludge includes airing, drying and adding chemical agents, etc. Among them, although the cost of airing is low, it has limitations such as occupying a large amount of land, being greatly affected by weather and greatly affecting the environment. Although the drying method has the advantages of fast drying speed, it has the disadvantages of limited treatment capacity and high cost. In addition, adding quicklime to sludge to reduce the water content of sludge and improve the sludge properties by using the dual action of quicklime of water absorption and heat release is the most common chemical method, which has the advantages of simple process and easy to be applied on a large scale, but also has the disadvantages of high quicklime content and large dust, and the high alkalinity of modified sludge also limits the further resource utilization of modified sludge.

[0004] In view of the fact that the high water content and viscous state of waterworks sludge limit its resource utilization, it is necessary to carry out necessary conditioning and modification pretreatment on the sludge, and at the same time, in order to meet the needs of subsequent multi-path resource utilization of modified sludge, it is urgent to develop a low-alkali / alkali-free sludge efficient conditioning modifier to improve the resource utilization level of waterworks sludge. SUMMARY

[0005] The present application provides a method for preparing engineering filler from waterworks sludge, which overcomes the defects of low resource utilization and high alkalinity of the conditioning modifier in the prior art. The conditioning modifier of the present application conditions and modifies the waterworks sludge without significantly affecting the pH value of the sludge, realizes the plastic reduction and dispersion of the sludge, and improves the sludge property of the modified sludge.

[0006] The present application mainly solves the above technical problems through the following technical solutions:

[0007] The present application provides a conditioning modifier for waterworks sludge, which comprises the following components in parts by weight:

[0008] Regenerated micro powder: 100 parts;

[0009] Pillared-attapulgite clay: 8-36 parts; the fineness of the pillared-attapulgite clay is 180-360 mesh;

[0010] Coupling agent: 3-9 parts;

[0011] Hydrophobic agent: 3-7 parts;

[0012] Water-absorbing resin: 2-6 parts;

[0013] Oxidizing agent: 1.5-4.0 parts.

[0014] The present application can adsorb part of the free water in the sludge, reduce the hydrophilicity of the sludge, adjust the particle size distribution of the sludge, achieve plastic reduction effect on the original viscous sludge, and make the sludge uniformly dispersed, thereby improving the subsequent modification effect on the sludge, by the cooperation of the above six specific contents and components.

[0015] The inventor has experienced many failed experiments in the development process to obtain the above specific component formula, for example, experiments without adding regenerated micro powder or water-absorbing resin, etc. The sludge conditioning modifier prepared in the experiments cannot achieve uniform dispersion and modification effect on the waterworks sludge, and cannot achieve the effect of being used as inorganic binder.

[0016] In the present application, the grade of the regenerated micro powder is preferably grade I or grade II, which means that the technical index meets the requirements of grade I or grade II specified in “Regenerated Micro Powder for Concrete and Mortar” JG / T 573-2020.

[0017] In the present application, the parts of the pillared-attapulgite clay are preferably 10-30 parts, for example, 18 parts or 26 parts.

[0018] In the present application, the inventor has found in experiments that when the fineness of the pillared-attapulgite clay is not within the range of 180-360 mesh, the conditioning modifier will reduce the adjustment effect on the particle size distribution of the waterworks sludge, and will also significantly affect the transformation of free water in the system; and when the fineness is less than 180 mesh, not only the water absorption effect will be reduced, but also the mechanical properties of the modified sludge material will be reduced.

[0019] In the present application, the fineness of the pillared-attapulgite clay is preferably 200-340 mesh, for example, 220 mesh or 320 mesh.

[0020] In the present application, the coupling agent is preferably a silane coupling agent, such as vinyl triethoxysilane. The silane coupling agent is preferably a silane coupling agent ethanol solution. The mass concentration of the silane coupling agent in the silane coupling agent ethanol solution is preferably 10%, which refers to the mass ratio of the silane coupling agent to the silane coupling agent ethanol solution.

[0021] In the present application, the amount of the coupling agent is preferably 4-9 parts, such as 5 or 7 parts.

[0022] In the present application, the hydrophobic agent is preferably a stearate, more preferably one or more of zinc stearate, calcium stearate and sodium stearate. When the hydrophobic agent is zinc stearate and calcium stearate, the weight ratio of the zinc stearate to the calcium stearate is preferably 1:1.

[0023] In the present application, the amount of the hydrophobic agent is preferably 4-6 parts, such as 4 or 5 parts.

[0024] In the present application, the water-absorbing resin is preferably a high-molecular water-absorbing resin. The high-molecular water-absorbing resin is preferably a high-molecular water-absorbing resin with low cross-linking type polyacrylate as the main chemical component.

[0025] In the present application, the amount of the water-absorbing resin is preferably 3-5 parts, such as 3 parts or 4 parts.

[0026] In the present application, the fineness of the water-absorbing resin is preferably 120-260 mesh, such as 120 mesh, 180 mesh, 200 mesh or 260 mesh.

[0027] In the present application, the oxidizing agent is preferably a ferrate, such as potassium ferrate.

[0028] In the present application, the amount of the oxidizing agent is preferably 2-3.5 parts, such as 2.6 parts or 3.2 parts.

[0029] In the present application, the type and amount of the above-mentioned preferred coupling agent, the type, amount and mesh number of the water-absorbing resin, and the type and amount of the oxidizing agent are preferred cases made by the inventors during the research and development process, i.e. the specific embodiments below are preferred embodiments. Compared with the conditioning modifiers prepared by other raw materials in the art, the conditioning modifier can achieve higher compressive strength when used as inorganic binding material.

[0030] In a specific embodiment of the present application, the conditioning modifier comprises the following components in parts by weight: 100 parts of I-grade regenerated micro powder; 8 parts of attapulgite with a fineness of 180 mesh; 3 parts of a 10% mass concentration of a vinyl triethoxysilane ethanol solution; 3 parts of zinc stearate; 6 parts of water-absorbing resin with a fineness of 260 mesh; and 1.5 parts of potassium ferrate.

[0031] In a specific embodiment of the present application, the conditioning modifier comprises the following components by weight fraction: 100 parts of Grade II regenerated micro powder; 26 parts of attapulgite with fineness of 320 mesh; 7 parts of 10% vinyl triethoxysilane ethanol solution; 7 parts of sodium stearate; 4 parts of water-absorbing resin with fineness of 200 mesh; and 2.6 parts of potassium ferrate.

[0032] In a specific embodiment of the present application, the conditioning modifier comprises the following components by weight fraction: 100 parts of Grade I regenerated micro powder; 36 parts of attapulgite with fineness of 220 mesh; 5 parts of 10% vinyl triethoxysilane ethanol solution; 5 parts of calcium stearate; 2 parts of water-absorbing resin with fineness of 120 mesh; and 4.0 parts of potassium ferrate.

[0033] In a specific embodiment of the present application, the conditioning modifier comprises the following components by weight fraction: 100 parts of Grade II regenerated micro powder; 18 parts of attapulgite with fineness of 360 mesh; 9 parts of 10% vinyl triethoxysilane ethanol solution; 4 parts of mixture of zinc stearate and calcium stearate; 3 parts of water-absorbing resin with fineness of 180 mesh; and 3.2 parts of potassium ferrate.

[0034] The present application also provides a preparation method of the conditioning modifier, which comprises the following steps:

[0035] (1) mixing the mixture 1 and the hydrophobic agent for the first time to obtain a mixture 2;

[0036] (2) mixing the mixture 2 with the water-absorbing resin and the oxidizing agent for the second time to obtain the conditioning modifier;

[0037] The mixture 1 is the regenerated micro powder, the attapulgite and the coupling agent.

[0038] In the present application, the preparation method of the mixture 1 can be conventional in the art, which generally comprises uniformly mixing the regenerated micro powder, the attapulgite and the coupling agent. The mixing time can be conventional in the art, preferably 4-7 min. In the present application, the mixing is preferably achieved by pulverization.

[0039] In the present application, the first mixing time can be conventional in the art, preferably 3-6 min.

[0040] In the present application, the first mixing can be conventional in the art, preferably achieved by pulverization. The pulverization can be achieved by using conventional equipment in the art, preferably a ball mill.

[0041] In the present application, the second mixing time can be conventional in the art, preferably 2-4 min.

[0042] In the present application, the second mixing can be conventional in the art, preferably by milling. The milling can use conventional equipment in the art, preferably a ball mill.

[0043] In the present application, preferably, after the second mixing, it further comprises sealed storage in a dry environment.

[0044] In the present application, the waterworks sludge is generally obtained after dewatering treatment. The dewatering treatment is, for example, centrifugal dewatering or plate and frame filter dewatering.

[0045] In the present application, the waterworks sludge generally comprises organic matter, mineral matter and water.

[0046] Preferably, the content of the organic matter is 10% to 35%, for example, 11.8% or 23.9%. The content of the organic matter is measured by weight method according to the “Urban Sewage Treatment Plant Sludge Inspection Method” CJ / T 221.

[0047] Preferably, the content of the water is 55% to 80%, for example, 58.6% or 77.2%. The content of the water refers to the percentage of the mass of water to the total mass of the waterworks sludge.

[0048] Preferably, the pH value of the waterworks sludge is 7 to 8, for example, 7.33 or 7.83.

[0049] In the present application, the ratio of the mass of the conditioning modifier to the mass of the waterworks sludge is preferably greater than 0.1, for example, 0.2 or 0.45.

[0050] In the present application, when the sludge material is used as inorganic binder, preferably, Portland cement is further added. The Portland cement is, for example, PO42.5. Preferably, the percentage of the mass of the Portland cement to the mass of the waterworks sludge is 12% to 20%, for example, 15%.

[0051] The reagents and raw materials used in the present application are commercially available.

[0052] The positive progress effect of the present application is that:

[0053] (1) The conditioning modifier of the present application does not significantly affect the pH value of the sludge, realizes the plastic reduction and dispersion of the sludge, and makes the viscous sludge into granular, homogeneous and modified sludge with appropriate water content after conditioning modification, thereby improving the sludge quality resource property, through the mutual cooperation between the effects of water migration, particle size distribution adjustment, surface chemical modification and organic matter oxidation and decomposition in the waterworks sludge.

[0054] (2) The product preparation process is simple, efficient, suitable for centralized treatment of sludge in waterworks, and has great popularization and application potential. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 Figures of property changes of sludge of Example 1 before and after modification. DETAILED DESCRIPTION

[0056] The application will be further described by way of examples, but the application is not limited to the examples. The experimental methods in the following examples are not specified, and are selected according to conventional methods and conditions, or according to the instructions of the products.

[0057] Example 1

[0058] Each raw material was weighed according to the weight ratio of 100 parts of I-grade regenerated micro powder, 8 parts of attapulgite with fineness of 180 mesh, 3 parts of 10% vinyltriethoxysilane ethanol solution, 3 parts of zinc stearate, 6 parts of water-absorbing resin with fineness of 260 mesh, and 1.5 parts of potassium ferrate.

[0059] First, the regenerated micro powder, attapulgite and vinyltriethoxysilane ethanol solution were added to the ball mill in the above proportions and ground for 4 min.

[0060] Then, zinc stearate was added and ground for another 3 min, and finally, water-absorbing resin and potassium ferrate were added and ground for 4 min to obtain the product S-1 of this example.

[0061] Example 2

[0062] Each raw material was weighed according to the weight ratio of 100 parts of I-grade regenerated micro powder, 8 parts of attapulgite with fineness of 180 mesh, 3 parts of 10% vinyltriethoxysilane ethanol solution, 3 parts of zinc stearate, 6 parts of water-absorbing resin with fineness of 260 mesh, and 1.5 parts of potassium ferrate.

[0063] First, the regenerated micro powder, attapulgite and vinyltriethoxysilane ethanol solution were added to the ball mill in the above proportions and ground for 6 min.

[0064] Then, sodium stearate was added and ground for another 4 min, and finally, water-absorbing resin and potassium ferrate were added and ground for 2 min to obtain the product S-2 of this example.

[0065] Example 3

[0066] Each raw material was weighed according to the weight ratio of 100 parts of I-grade regenerated micro powder, 36 parts of attapulgite with fineness of 220 mesh, 5 parts of 10% vinyltriethoxysilane ethanol solution, 5 parts of calcium stearate, 2 parts of water-absorbing resin with fineness of 120 mesh, and 4.0 parts of potassium ferrate.

[0067] The regenerated micro-powder, attapulgite and the vinyl triethoxysilane ethanol solution are added into the ball mill in the above-mentioned proportions and ground for 7 minutes.

[0068] The calcium stearate is then added and ground for another 6 minutes, and finally the water-absorbing resin and the potassium ferrate are added and ground for 3 minutes to obtain the product S-3 of this example.

[0069] Example 4

[0070] The regenerated micro-powder, attapulgite and the vinyl triethoxysilane ethanol solution are added into the ball mill in the above-mentioned proportions and ground for 7 minutes.

[0071] The regenerated micro-powder, attapulgite and the vinyl triethoxysilane ethanol solution are added into the ball mill in the above-mentioned proportions and ground for 7 minutes.

[0072] The calcium stearate is then added and ground for another 6 minutes, and finally the water-absorbing resin and the potassium ferrate are added and ground for 3 minutes to obtain the product S-3 of this example.

[0073] Figure 1 The changes in the properties of the sludge of Example 1 before and after the conditioning modification are shown in the graph of Figure 1. Figure 1 As shown in Figure 1, before the conditioning modifier of Example 1 is added, the sludge of the waterworks is in a sticky and agglomerated state, and after the conditioning modification, the sludge of the waterworks achieves a uniform and loose effect.

[0074] Two sludges from waterworks in Shanghai are selected to test the implementation effect, and the two sludges are denoted as W1 and W2. W1 is the sludge produced by centrifugal dewatering, with a water content of 77.2%, a pH of 7.33 and an organic matter content of 23.9%; W2 is the sludge produced by plate and frame filter dewatering, with a water content of 58.6%, a pH of 7.83 and an organic matter content of 11.8%. Quicklime is selected as a comparative sample, and the effect of the conditioning modifier of Examples 1-4 on the conditioning modification of the sludge is tested.

[0075] Modified sludge used as engineering filler:

[0076] The raw sludge W1 was mixed with lime and the product of Example 1-4, respectively, at a mixing amount of 45 wt.%, and after mixing, curing and turning, five groups of modified sludge were obtained. Then, 15 wt.% (wt.% refers to the mass of P·O 42.5 cement to the total mass of the sludge of the waterworks) of P·O 42.5 cement was added to the five groups of modified sludge for solidification and reinforcement. The unconfined compressive strength test pieces were formed according to the method specified in the Highway Engineering Inorganic Binder Stabilized Material Test Procedure JTG E51-2009, and the 7d strength of the test pieces was tested. The results are shown in Table 1.

[0077] The raw sludge W2 was mixed with lime and the product of Example 1-4, respectively, at a mixing amount of 20 wt.%, and after mixing, curing and turning, five groups of modified sludge were obtained. Then, 15 wt.% (wt.% refers to the mass of P·O 42.5 cement to the total mass of the sludge of the waterworks) of P·O 42.5 cement was added to the five groups of modified sludge for solidification and reinforcement. The unconfined compressive strength test pieces were formed according to the method specified in the Highway Engineering Inorganic Binder Stabilized Material Test Procedure JTG E51-2009, and the 7d strength of the test pieces was tested. The results are shown in Table 1.

[0078] Table 1 Influence of lime and the tempering modifier of Example 1-4 on the modified sludge used as engineering filler

[0079]

[0080] As shown in Table 1, after the lime-modified sludge is solidified and reinforced with cement, the 7d unconfined compressive strength of the solidified sludge is low, and the solidification and reinforcement effect is poor. However, after the sludge modified by the product of the present application is solidified and reinforced with cement, the 7d unconfined compressive strength is significantly improved. When W1 is used as the raw sludge, the strength of the solidified sludge after the sludge modified by the product of the present application is solidified and reinforced with cement is 2.0-2.8 times that of the control group. When W2 is used as the raw sludge, the strength of the solidified sludge after the sludge modified by the product of the present application is solidified and reinforced with cement is 2.2-2.8 times that of the lime group, which is the modified sludge in which lime is added to the sludge in Table 1.

[0081] The above are only the preferred embodiments of the present application, and do not limit the implementation and protection scope of the present application. It should be realized by those skilled in the art that any equivalent replacement and obvious changes made according to the content of the present application should be included in the protection scope of the present application.

Claims

1. A method of preparing an engineering filler from waterworks sludge, characterized in that, The waterworks sludge is treated by the conditioning modifier to obtain modified sludge, and then the modified sludge is treated by solidification and stabilization, wherein the conditioning modifier comprises the following components in parts by weight: regenerated powder: 100 parts; palygorskite: 8-36 parts; the fineness of the palygorskite is 180-360 mesh; coupling agent: 3-9 parts; hydrophobic agent: 3-7 parts; water-absorbing resin: 2-6 parts; oxidizing agent: 1.5-4.0 parts, and the oxidizing agent is potassium ferrate; The waterworks sludge is treated by the conditioning modifier to obtain modified sludge, and then the modified sludge is treated by solidification and stabilization, wherein the conditioning modifier comprises the following components in parts by weight: The waterworks sludge is treated by the conditioning modifier to obtain modified sludge, and then the modified sludge is treated by solidification and stabilization, wherein the conditioning modifier comprises the following components in parts by weight: The mass ratio of the conditioning modifier to the waterworks sludge is greater than 0.1; The coupling agent is a vinyl triethoxysilane ethanol solution, and the mass concentration of vinyl triethoxysilane in the vinyl triethoxysilane ethanol solution is 10%; 2. The method of preparing engineered fill from water treatment plant sludge according to claim 1, characterized in that, The hydrophobic agent is one or more of zinc stearate, calcium stearate and sodium stearate.

3. The method of claim 1, wherein the waterworks sludge is prepared into the engineering filler. The conditioning modifier is added to the waterworks sludge during the treatment, and after mixing, curing and turning, the modified sludge is obtained, and then P.O 42.5 Portland cement is added to the modified sludge for solidification and reinforcement treatment, and a non-side-limit compressive strength test piece is formed according to the method specified in the Highway Engineering Inorganic Binder Stabilized Material Test Regulations JTGE51-2009. The preparation method of the conditioning modifier comprises the following steps: (1) the mixture 1 and the hydrophobic agent are mixed for the first time to obtain a mixture 2; the first mixing is realized by grinding; and the first mixing time is 3-6 min; (2) the mixture 2 is mixed with the water-absorbing resin and the oxidizing agent for the second time to obtain the conditioning modifier; the second mixing is realized by grinding; and the second mixing time is 2-4 min; 4. The method of claim 1, wherein the waterworks sludge is prepared into the engineering filler. The mixture 1 is obtained by mixing the regenerated powder, the palygorskite and the coupling agent; the mixing is realized by grinding; and the mixing time is 4-7 min. The palygorskite is 10-30 parts; And / or, the fineness of the palygorskite is 200-340 mesh; And / or, the coupling agent is 4-9 parts; And / or, the hydrophobic agent is 4-6 parts; And / or, the water-absorbing resin is 3-5 parts; And / or, the fineness of the water-absorbing resin is 120-260 mesh; 5. The method of claim 1, wherein the waterworks sludge is prepared by a process comprising the steps of: (a) mixing the waterworks sludge with a binder; (b) drying the mixture; and (c) crushing the dried mixture. And / or, the oxidizing agent is 2-3.5 parts. The palygorskite is 18 parts or 26 parts; And / or, the fineness of the palygorskite is 220 mesh or 320 mesh; And / or, the coupling agent is 5 parts or 7 parts; And / or, the hydrophobic agent is 4 parts or 5 parts; And / or, the water-absorbing resin is 3 parts or 4 parts; And / or, the fineness of the water-absorbent resin is 120 mesh, 180 mesh, 200 mesh or 260 mesh; And / or, the proportion of the oxidizing agent is 2.6 parts or 3.2 parts.

6. The method of claim 1, wherein the waterworks sludge is prepared by a process comprising the steps of: The grade of the regenerated micro powder is grade I or grade II; the grade I or grade II refers to that the technical index meets the grade I or grade II specified in "Regenerated Micro Powder for Concrete and Mortar" JG / T573-2020; ​ And / or, when the hydrophobic agent is zinc stearate and calcium stearate, the weight ratio of the zinc stearate and the calcium stearate is 1:1; And / or, the water-absorbent resin is a high molecular water-absorbent resin.

7. The method of claim 6, wherein the waterworks sludge is prepared by a process comprising the steps of: The high molecular water-absorbent resin is a low cross-linking type polyacrylate.

8. The method of claim 1, wherein the waterworks sludge is prepared by a process comprising the steps of: (a) mixing the waterworks sludge with a binder; (b) drying the mixture; and (c) crushing the dried mixture. The tempering modifier is composed of the following components in parts by weight: 100 parts of grade I regenerated micro powder; 8 parts of attapulgite with a fineness of 180 mesh; 3 parts of a vinyl triethoxysilane ethanol solution with a mass concentration of 10%; 3 parts of zinc stearate; 6 parts of water-absorbent resin with a fineness of 260 mesh; 1.5 parts of potassium ferrate; Or, the tempering modifier is composed of the following components in parts by weight: 100 parts of grade II regenerated micro powder; 26 parts of attapulgite with a fineness of 320 mesh; 7 parts of a vinyl triethoxysilane ethanol solution with a mass concentration of 10%; 7 parts of sodium stearate; 4 parts of water-absorbent resin with a fineness of 200 mesh; 2.6 parts of potassium ferrate; Or, the tempering modifier is composed of the following components in parts by weight: 100 parts of grade I regenerated micro powder; 36 parts of attapulgite with a fineness of 220 mesh; 5 parts of a vinyl triethoxysilane ethanol solution with a mass concentration of 10%; 5 parts of calcium stearate; 2 parts of water-absorbent resin with a fineness of 120 mesh; 4.0 parts of potassium ferrate; Or, the tempering modifier is composed of the following components in parts by weight: 100 parts of grade II regenerated micro powder; 18 parts of attapulgite with a fineness of 360 mesh; 9 parts of a vinyl triethoxysilane ethanol solution with a mass concentration of 10%; 4 parts of a mixture of zinc stearate and calcium stearate; 3 parts of water-absorbent resin with a fineness of 180 mesh; 3.2 parts of potassium ferrate.

9. The method of claim 1, wherein the water treatment plant sludge is prepared engineering filler is characterized by, The dewatering treatment is centrifugal dewatering and / or plate and frame filter dewatering; And / or, the content of the organic matter is 11.8% or 23.9%; And / or, the content of the water is 58.6% or 77.2%, And / or, the pH value of the waterworks sludge is 7-8.

10. The method of claim 1, wherein the water treatment plant sludge is prepared engineering filler is characterized by, The pH value of the waterworks sludge is 7.33 or 7.83.

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