Filter backwash and start-up strategies for enhanced particulate removal

A significant consideration in forward planning for water treatment works design and operation concerns the effectiveness of a filtration plant in providing a barrier to particulates in the low micrometre size range, including Cryptosporidium oocysts. The performance of rapid gravity filtration plants is believed to be dependent on backwash and start-up regimes. It was the aim of this study to optimize direct sand filtration by identifying optimum filter backwash and start-up conditions which minimized the passage of particulates into the filtrate. The filter ripening period has long been identified as a cause for concern with respect to particulate passage into the filtrate; this work has shown that up to 40% of all particles that pass into supply during a 48 h run, do so in the first hour of operation. Optimum combined air water “collapse-pulsing” backwash durations were identified that reduced the number of 2–5 μm particles entering the filtrate, especially during the ripening period. Slow start-up was also found to reduce the number of 2–5 μm particles in the filtrate during the ripening period. The reductions in particulate passage resulting from a slow start was found to be media dependent, with smaller media requiring a longer slow start duration than coarser media.     

Attrition of granular filter media during backwashing with combined air and water

Attrition resistance of granular filter media is becoming increasingly important as materials such as GAC and anthracite are being more frequently used as filter media. The purpose of this study was to evaluate the attrition experienced by various media during backwashing by performing accelerated backwash tests in a pilot column, using a combined water and air backwash at combinations that gave the condition known as collapse-pulsing. Since the dominant mode of attrition was assumed to be abrasion, the effluent was sampled at a number of intervals to determine the amount of fine material in the effluent. Coal based GAC exhibited the highest weight loss (∼ 7%) and sand the least (∼ 2%).     

Estimating nitrifying biomass in drinking water filters for surface water treatment

The objective of this work is to estimate active nitrifying biomass and its main influencing factors in low-loaded biofilters based on operational data. An analytical approach based on balancing growth, decay and biomass removed by backwashing is proposed. The method is developed and applied in pilot-scale rapid sand filters for drinking water treatment. Decay rate was measured directly in the filter for different temperatures. To assess the amount of active biomass in backwash water, a technique based on respiration measurements was used. Backwash losses increased overproportional with balanced biomass in the filter. The impact of both parameters on active biomass is quantified exemplarily for a given constant nitrification rate.     

陶粒生物滤池与无烟煤滤池强化过滤效果对比

以北京某自来水厂处理密云水库蓄水的出水（混凝沉后水）为试验用水，通过中试对比研究了陶粒生物滤池与无烟煤滤池的过滤效果。结果表明，陶粒生物滤池与无烟煤滤池对水中颗粒物的去除效果接近；对于水中的浊度，无烟煤滤层对其去除效果较好，而陶粒滤层的过滤效果则较差，需要依靠石英砂的截污作用来降低浊度；陶粒对有机物的过滤效果优于无烟煤的，陶粒生物滤池对TOC的去除率为14．9％～30．2％，对CODMn的去除率为13．1％～38．5％；与无烟煤滤池相比，陶粒生物滤池具有水头损失增长快、周期较短、初滤水中颗粒物较多等缺点。     

The interface between filtration and backwashing

A conceptual model of the initial degradation phase of filtration is presented as an interface effect between backwashing and filtration. It is shown that the initial degradation of effluent quality is due to the backwash water remnants within the media and the backwash water above the media. The two peak characteristics of initial degradation due to the backwash water remnants within the media and above the media is established by an extensive experimental investigation. A mathematical model for the quality of backwash water as a function of backwash water volume is developed. Deductions made from the mathematical expression confirm the validity of some accepted facts on backwashing and also lay the basis for the peaks in initial degradation.     

福州西区水厂双阀滤池的改造

福州西区水厂将原有的双阀滤池改造为V型滤池,重点介绍了进水系统、滤板系统、反冲洗排水系统、滤后水系统、反冲洗系统的改造。实践表明,改造后滤后水平均浊度<0.3NTU,反冲洗耗水量<2%,滤池实现了全自动化运行。     

Comparison of Algal Penetration Through Rapid-Gravity Filter Beds

T ests OF FOUR types of filter media are presented, which show that granular activated carbon performs marginally less well than anthracite/sand or anthracite/sand/garnet in the removal of algae, particulate organic carbon, iron and turbidity. The lengths of run which are achieved by the two granular activated carbon filters are also shorter than those of the other two media. A three-layer filter is better than the anthracite/sand filter for particulate organic carbon, iron and turbidity removal, and the filtrate contains lower mean concentrations of algae.     

深床直接过滤工艺深度处理城市污水

采用深床直接过滤工艺对城市污水处理厂的二级出水进行了深度处理，考察了同考脱氮除磷的可行性和运行条件。研究表明，通过铁盐絮凝剂的加入，借助微絮凝直接过小可有效去除水中的PO^3-4－P（去除率＞90％）；通过在滤池前加入甲醇作为外碳源，在滤池中进行同步脱氮，气水联合反冲洗相的冲洗方式可使过滤周期长达40h左右。     

Impact of backwashing procedures on deep bed filtration productivity in drinking water treatment

Backwash procedures for deep bed filters were evaluated and compared by means of a new integrated approach based on productivity. For this, different backwash procedures were experimentally evaluated by using a pilot plant for direct filtration. A standard backwash mode as applied in practice served as a reference and effluent turbidity was used as the criterion for filter run termination. The backwash water volumes needed, duration of the filter-to-waste period, time out of operation, total volume discharged and filter run-time were determined and used to calculate average filtration velocity and average productivity.     

生物活性炭滤池的反冲洗方式研究

反冲洗是保证生物活性炭滤池成功运行的一个重要环节，对不同反冲洗方式的效果进行了比较，根据反冲洗废水浊度变化及对滤池出水水质的影响，确立了合理的反冲洗方式，并给出相关的反冲洗强度和反冲洗历时参数，以期为生物活性炭滤池的设计和运行提供参考。     

The role of aluminum in slow sand filtration

Engineering enhancement of slow sand filtration has been an enigma in large part because the mechanisms responsible for particle removal have not been well characterized. The presumed role of biological processes in the filter ripening process nearly precluded the possibility of enhancing filter performance since interventions to enhance biological activity would have required decreasing the quality of the influent water. In previous work, we documented that an acid soluble polymer controls filter performance. The new understanding that particle removal is controlled in large part by physical chemical mechanisms has expanded the possibilities of engineering slow sand filter performance. Herein, we explore the role of naturally occurring aluminum as a ripening agent for slow sand filters and the possibility of using a low dose of alum to improve filter performance or to ripen slow sand filters.     

滤池气水反冲洗应用技术研究

对气水反冲洗工艺应用过程中存在的一些主要问题，如气水反冲参数选择、反冲洗时布气均匀性，进气进水压力计算方法及气水反冲跑砂等问题进行研究，提出切实可行的解决方法。     

反硝化滤池反冲洗效能综合影响因素及微生物种群

采用正交实验考察了水冲洗强度、水单独漂洗时间、气水联合冲洗时间、气冲洗强度、滤料粒径对反硝化滤池反冲洗效能及微生物种群的综合影响。结果表明,当以反硝化滤池运行周期、耗水量、系统恢复时间作为反冲洗效能的综合评价指标时,各因素对综合评价指标影响的显著性次序为:水冲洗强度>气水联合冲洗时间>气冲洗强度>滤料粒径>水单独漂洗时间;得出气水联合反冲洗最优工况参数如下:滤料粒径为2~4 mm、气冲洗强度为15 L/(m²·s)、水冲洗强度为10 L/(m²·s)、气水联合冲洗时间为8 min。PCR-DGGE分析结果表明,反冲洗对滤料层生物量、生物种群多样性有显著影响,对滤料层生物活性、微生物种群丰度影响不显著。     

滤池反冲洗时间控制模式的分析与优化

研究了给水厂滤池反冲洗排水浊度随时间的变化情况，探讨了反冲洗时间的控制模式。结果表明，在给水厂综合运行费用中混凝剂起控制作用，反冲洗水费所占比例〈15％；反冲洗水量占水厂自用水量的比例较大（约70％～90％），降低反冲洗水量是水厂节约自用水量的关键。反冲洗工况的优化条件是指在满足滤池再生的基础上尽量缩短反冲洗时间，通过对目前我国常用反冲洗控制模式进行技术经济分析，发现定时法是适合滤池反冲洗的控制模式。     

Biofiltration for removal of BOM and residual ammonia following control of bromate formation

Nitrification was developed within a biological filter to simultaneously remove biodegradable organic matter (BOM) and residual ammonia added to control bromate formation during the ozonation of drinking water. Testing was performed at pilot-scale using three filters containing sand and anthracite filter media. BOM formed during ozonation (e.g., assimilable organic carbon (396-572 microg/L), formaldehyde (11-20 microg/L), and oxalate (83-145 microg/L)) was up to 70% removed through biofiltration. Dechlorinated backwash water was required to develop the nitrifying bacteria needed to convert the residual ammonia (0.1-0.5 mg/L NH(3)-N) to nitrite and then to nitrate. Chlorinated backwash water resulted in biofiltration without nitrification. Deep-bed filtration (empty-bed contact time (EBCT) = 8.3 min) did not enhance the development of nitrification when compared with shallow-bed filtration (EBCT = 3.2 min). Variable filtration rates between 4.8 and 14.6 m/h (2 and 6 gpm/sf) had minimal impact on BOM removal. However, conversion of ammonia to nitrite was reduced by 60% when increasing the filtration rate from 4.8 to 14.6 m/h. The results provide drinking water utilities practicing ozonation with a cost-effective alternative to remove the residual ammonia added for bromate control.     

Particle Ripening, Removal and Breakthrough in Drinking-Water Filtration: Implications for the Capture of Cryptosporidium and Giardia Cysts

An increasing number of reported outbreaks of Cryptosporidiosis, associated with the water supply, have led to an upsurge in interest in particle analysis in drinkingwater treatment. This paper describes recent observations of rapid-gravity filter performance using particle-size distribution analysis. Experimental rapid sand filters were constructed and operated using a suspension of PVC particles, and samples for particle analysis were collected at several depths and various times during the filter cycle. Ripening and early breakthrough of Cryptosporidium oocyst size particles were observed during the study, and 1 β particles exhibited the worst capture efficiency. Flow disturbances were found to reduce particle-capture efficiency including Cryptosporidium and Giardia cyst sizes.     

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 2. Studies of deep-bed filtration

Viral transport in deep-bed sand filters was studied by a new method that enables rapid and simple quantitation of labeled viruses. The residence time distribution (RTD) of viruses in the bed was compared to the RTD of a fluorescein dye under conditions that simulate a filter run. The characteristics of the RTD curves for the free dye and the labeled bacteriophages followed very different trends during the filter run. While the retention time of free dye was practically independent of the filtration stage, the average retention time of the labeled bacteriophage depended in a non-linear way on filtration time. Average virus retention time as well as virus-removal efficiency were minimal at the ripening stage, increased during the operational stage and then decreased again towards the turbidity breakthrough stage. This complex trend reflects two opposing mechanisms that dominate the behavior of the filter. During the ripening stage the accumulation of the kaolin-alum material in the filter increases the adsorption surface area and retards virus mobility. After sufficient kaolin-alum deposit is accumulated in the filter, aging and densification of the alum deposit induces size exclusion phenomenon giving faster apparent mobility of viruses in the filter bed.     

原水预臭氧化对常规处理工艺的影响

杭州南星水厂的原水经预臭氧处理后,砂滤池除铁、锰作用得以加强并发挥了生物除氨氮作用,但反冲时滤砂难以洗净,池壁还滋生了青苔;混凝效果得到强化,矾耗降低.通过生产性试验分析了原水预臭氧化对常规工艺的不利影响,认为原水由预氯化变为预臭氧化后,生物砂滤池宜改为气水反冲洗方式,斜管沉淀池和滤池宜采用遮阳方法防止池内滋生藻类,另外从成本方面考虑,用投加臭氧来降低矾耗是不经济的.     

普通快滤池改造为气水反冲洗均质滤料滤池工程实践

针对大连大沙沟净水厂原有普通快滤池过滤周期短、产水量下降、反冲洗效果不佳等问题,进行了技术改造。将原有中阻力陶瓷滤砖配水系统改为Azurfloor整体滤板小阻力长柄滤头配水,煤-砂滤料改为石英砂均质滤料,单一水反冲洗改为气水反冲洗,并完善了滤池的自动控制系统。运行情况表明,改造后滤池出水水质提高,反冲洗效果明显改善,产水量增加了10×104m3/d,运行成本减少且滤站实现了自动化运行,经济效益和社会效益显著。     

改善V型滤池跑砂的工程装置设计研究与实践

通过模型滤柱实验,研究滤料气水反冲洗过程中滤料流失量G与气冲强度L、水冲强度M和排水槽高度H之间的定量关系,由此确定设计参数设计出一种改善滤池跑砂的装置,并安装在V型滤池排水槽的两侧。生产实践应用结果表明,该装置能够起到改善V型滤池跑砂的效果,单次滤料流失量的减少率最高可达35.1%。按照定量关系确定的理论排水槽高度进行装置安装,不仅可以有效改善跑砂效果,反冲洗过程中的排水浊度也最低,反冲洗末期达到冲洗效果的时间也最短。通过经济性评估,该装置投资较省,安装简单,不需要停水施工,对生产无影响,对V型滤池具有普遍适用性。