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分析重力過濾水廠的污泥沉降試驗
摘要：在恒壓條件下，對市政水廠進(jìn)行了重力沉降實驗和重力過濾實驗。理論分析中明確指出，污泥的沉淀對過濾率有明顯影響。當(dāng)?shù)夭煌牧鲃幼枇Q定了不同污泥濃度的沉降速率。固體的沉淀特征和當(dāng)?shù)氐目紫堵逝c該地區(qū)固體抗壓壓力有關(guān)。從沉降速率和沉降平衡的基礎(chǔ)數(shù)據(jù)上評估了重力過濾與過濾率的關(guān)系。
關(guān)鍵詞：壓縮特性;重力過濾;滲透特征，沉積;水廠污泥
一 引言
重力過濾是固液分離的一個潛在的操作方法，只涉及簡單設(shè)計和運行。這種方法對懸浮物的過濾尤其有效，在驅(qū)動力不大的情況下很容易達(dá)到分離效果。例如，在自來水廠污泥分離的過程中，增加重力增稠劑，可大大改善分離效果。以往的研究都集中在一般的加壓過濾。因此，沒有多少人注重重力過濾的行為。
沉積過程中懸浮的粒子，經(jīng)常導(dǎo)致濾餅的過濾速率發(fā)生變化。特別是，在大多數(shù)情況下，重力過濾的沉淀顆粒懸浮效果顯著。因此，重力過濾行為的分析，必須考慮到顆粒沉淀效果。通過調(diào)查了向上和向下加壓過濾的沉淀效果和其發(fā)展的理論模型來描述通量下降的行為。
此外，過濾密切相關(guān)的問題是沉積作用中有相似粒子的溶劑和滲透溶劑通過填料床的顆粒越大。在此假設(shè)的基礎(chǔ)上，根據(jù)壓縮的壓縮餅滲透率，當(dāng)?shù)氐目紫堵师藕彤?dāng)?shù)毓腆w壓縮壓力Ps的具體流動阻力，來做重力沉降實驗。結(jié)果表明，從傳統(tǒng)的壓縮滲透率數(shù)據(jù)中得到，在固體壓力相對低的地方壓縮滲透性能很好。還應(yīng)該指出，這是相當(dāng)困難的情況下在壓縮細(xì)胞通透性較低的條件下取得的結(jié)果。通過應(yīng)用超濾方法，使用壓縮滲透與超速離心機(jī)分析評估了蛋白質(zhì)溶液超濾通量衰減行為。最近，用離心機(jī)對O/ W乳液油滴的壓縮變形滲透特性進(jìn)行了評價分析，據(jù)預(yù)計，在相對低壓條件下評估重力過濾，得到重力沉降壓滲透特性使用方便。
在這篇文章中，對市政自來水廠污泥進(jìn)行批次重力過濾和重力沉降實驗，以及關(guān)于重力沉降粒子對過濾行為的影響進(jìn)行了調(diào)查，以評估的濾餅平均過濾比阻。其次在壓縮滲透的基礎(chǔ)上，針對重力沉降用沉降速度發(fā)和沉淀平衡法代替?zhèn)鹘y(tǒng)的重力沉降粒子測量。利用壓縮滲透的特點對重力過濾的過濾率下降行為進(jìn)行評價。
二 材料與方法
材料
在Kasugai水廠，增加污泥增稠劑對懸浮顆粒進(jìn)行研究。硫酸鋁中加入無機(jī)混凝劑作為增稠劑，測量用比重瓶，質(zhì)量分?jǐn)?shù)S的懸浮固體和固體的真密度ρs，分別是4.68?×?10-3 kg/m3和3.35?×103 kg/m3。
用激光散射粒度分布儀對污泥的固體顆粒粒度分布進(jìn)行了分析，測量結(jié)果如圖一所示，直徑dp與頻率f 為正態(tài)分布關(guān)系。由于混凝劑的使用，分布明顯變廣，顆粒的平均比表面積大小是65.8微米。用超純水稀釋過的污泥進(jìn)行試驗。
圖一：
重力沉降
在直徑為4.5厘米的垂直有機(jī)玻璃圓筒內(nèi)進(jìn)行重力沉降批次實驗。在實驗開始之前開始,將污泥確?；旌暇鶆?然后緩慢倒入一個沉降氣缸。測量沉積物與上清液之間的高度，所用時間沉淀時間和懸浮顆粒濃度，一個星期后測量最終平衡高度。測量沉淀速率的方法是，通過對沉淀物的高度變化與時間數(shù)據(jù)來分析，確定沉淀速率。與此同時，在沉淀平衡方法中，用平衡狀態(tài)下統(tǒng)一泥沙高度進(jìn)行分析。
重力過濾
研究中采用的重力過濾器如圖二所示，垂直氣缸內(nèi)徑和沉淀試驗采用的相同。采用的過濾介質(zhì)為非織造，平均孔徑大小為9μm，污泥瞬間被充分?jǐn)嚢?，并倒入汽缸中，開始重力過濾。同樣數(shù)量的純水作為排放濾液添加，不斷從污泥表面進(jìn)入氣缸。因此，試驗過程中重力過濾驅(qū)動器始終保持頂部高度不變，如圖所示。
圖 2 重力過濾裝置
結(jié)論
市政自來水廠污泥重力沉降過濾實驗采用批次實驗，以研究有關(guān)對重力過濾通量衰減行為的粒子沉淀效果。結(jié)果清楚地表明,形成濾餅的平均過濾比阻在中立的條件下可精確的顯示離子的沉降水平。壓縮滲透的特點，代表了當(dāng)?shù)氐木唧w流動阻力和當(dāng)?shù)毓腆w壓縮壓力函數(shù)的孔隙率，這一切都基于從最終的泥沙淤積試驗取得的沉積速度數(shù)據(jù)和綜合平衡高度。據(jù)重力過濾顯示，通量下降的現(xiàn)象是通過對滲透特性所得的沉降數(shù)據(jù)進(jìn)行良好的計算所獲得的。
Analysis of Gravity Filtration Behaviors of Waterworks Sludge Based upon Sedimentation Tests
Abstract
Both gravity filtration experiments under constant pressure conditions and gravity sedimentation experiments were conducted using the municipal waterworks sludge. It was clarified from the theoretical analysis that the effect of sedimentation on the filtration rate was noticeable for the sludge used in this study. The local specific flow resistance at various sludge concentrations was determined by the sedimentation velocity method. The local porosity was related to the local solid compressive pressure by the sedimentation equilibrium method. The decline behaviors in the filtration rate in gravity filtration accompanied by sedimentation were well evaluated only from the sedimentation data based upon the sedimentation velocity method and the sedimentation equilibrium method.
Keywords:Compression characteristics; Gravity filtration; Permeability characteristics; Sedimentation; Waterworks sludge
INTRODUCTION
Gravity filtration is one of the potential solid-liquid separation methods because of the energy-savings involving the simple design and operation. The method is especially efficient for suspension, which is relatively likely to be filtered since the available driving force of separation is not so high. For instance, in the concentration process of waterworks sludge, the separation efficiency may be significantly improved in the case that the space in the gravity thickener is used for gravity filtration. Previous studies have concentrated on the usual pressurized filtration. Hence, not much is known about the behaviors of gravity filtration.[1]
The process of sedimentation of particles in suspension above the filter cake frequently results in a variation of the filtration rate in filtration. [2-7] Especially, in gravity filtration, the effect of sedimentation of particles in suspension is remarkable in most cases.[1] Therefore, the effect of particle sedimentation must be taken into consideration in the analysis of the gravity filtration behaviors. Iritani et al.[8] investigated the effect of sedimentation on properties of upward and downward pressurized filtration and developed the theoretical model to describe the flux decline behaviors.
In addition, there is an analogy between sedimentation of particles in a solvent and permeation of a solvent through the packed bed of particles, which is closely related to filtration. On the basis of this hypothesis, the compression-permeability characteristics of the compressed cake, which represent the local porosity ε and the local specific flow resistance as functions of the local solid compressive pressure ps, are obtained from batch gravity sedimentation test.[9, 10] It was shown that the compression-permeability characteristics in the relatively low solid compressive pressure region obtained from the sedimentation test were well correlated to those obtained from the conventional compression-permeability cell data.[11] It should also be noted that it is quite difficult to obtain the compression-permeability characteristics under relatively low pressure conditions by the compression-permeability cell test. By applying the method to ultracentrifugation, Iritani et al. evaluated the flux decline behaviors of dead-end ultrafiltration of protein solutions using the compression-permeability characteristics obtained with the use of an analytical ultracentrifuge.[12, 13] More recently, the compression-permeability characteristics of deformable oil droplets in O/W emulsions were evaluated using an analytical centrifuge.[14, 15] It is expected that the use of the compression-permeability characteristics obtained from the gravity sedimentation is convenient in order to evaluate the behaviors of gravity filtration, which is conducted under relatively low pressure conditions.
In this article, gravity filtration and batch gravity sedimentation experiments are conducted using the municipal waterworks sludge, and the effect of the sedimentation of particles on the behaviors of gravity filtration is investigated in order to evaluate the true values of the average specific filtration resistance of the filter cake. Subsequently, the compression-permeability characteristics are obtained from the batch gravity sedimentation data based upon the sedimentation velocity method and sedimentation equilibrium method in place of the conventional compression-permeability cell measurements. The decline behaviors of the filtration rate in gravity filtration are evaluated from the compression-permeability characteristics thus obtained
MATERIALS AND METHODS
Materials
The sludge drawn out of the thickener in Kasugai Waterworks (Kasugai City, Japan) was employed as a test suspension in this research. Aluminum sulphate was added to the suspension in the thickener as the inorganic coagulant. The mass fraction s of solids in suspension and the true density ρs of solids, measured using a pycnometer, are 4.68 10-3 and 3.35 103 kg/m3, respectively.
The volume-based size distribution of the solid particles in the sludge was measured by a laser scattering particle size distribution analyzer (LA-920, Horiba, Ltd., Kyoto, Japan). The measured result is depicted in Fig. 1. In this graph, the frequency f is plotted semi-logarithmically against the particle diameter dp. The size distribution tends to be appreciably broad due to the use of the coagulant. The mean specific surface area size of particles is 65.8 μm. The sludge was diluted by the specified concentrations with the ultrapure, deionized water prepared by an ultrapure water system for laboratory use (Milli-Q SP, Millipore Corp., Tokyo, Japan) prior to being used as the
text suspension.
FIG. 1.Volume-based size distribution of solid particles in multiple waterworks sludge.
Gravity Sedimentation
Batch gravity sedimentation experiments were conducted by using vertical Plexiglas cylinders with 4.5-cm internal diameter. Before the experiments started, the sludge was agitated sufficiently to ensure that the contents were well mixed, and then it was gradually poured into a graduated settling cylinder. The sediment height H of the interface plane between the clear supernatant and top of the settling bed sludge was measured with the lapse of the sedimentation time θ for both various initial solid concentrations s and various initial heights H0. Final equilibrium heights H∞ were measured after a week. In the sedimentation velocity method, the initial sedimentation velocity determined from the measurements of the variation with time of H in the incipience of sedimentation was used in the data analysis. Meanwhile, in the sedimentation equilibrium method, the height H∞ of the consolidated sediment at the equilibrium state was used in the analysis.
Gravity Filtration
A schematic drawing of the gravity filter used in this research is shown in Fig. 2. The inner diameter of the vertical cylinder is the same as that employed in the sedimentation tests. The nonwoven filter cloth (FT7501SS, Shikishima Canvas Co., Ltd., Osaka, Japan) with an average pore size of 9 μm was employed as the filter medium. The instant the sludge agitated sufficiently was gradually poured into the cylinder, gravity filtration was started. The same amount of pure water as the discharged filtrate was added gently and constantly on the surface of the sludge into the cylinder. As a result, the head H0 acting as the driving force of gravity filtration was kept constant throughout the experiment, as shown in Fig. 2.
FIG. 2.Schematic diagram of experimental gravity filtration apparatus.
CONCLUSIONS
Gravity filtration experiments and batch sedimentation experiments were performed using the municipal waterworks sludge in order to examine the effect of the particle sedimentation on the flux decline behaviors of gravity filtration. The results clearly demonstrate that the accurate values of the average specific filtration resistance of the filter cake formed in gravity filtration can be obtained in view of the particle sedimentation. The compression-permeability characteristics, which represent the local specific flow resistance and the local porosity as functions of the local solid compressive pressure, were determined from the data of the initial sedimentation velocity and the final equilibrium height of the consolidated sediment obtained from batch sedimentation tests. It was revealed that the behaviors of flux decline of gravity filtration were well evaluated on the basis of the compression-permeability characteristics obtained only from the sedimentation data.
ACKNOWLEDGMENT
This work has been supported in part by a Grant-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors acknowledge with sincere gratitude the financial support leading to the publication of this article.
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