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.     

Removal of biodegradable dissolved organic carbon in a water treatment plant

A new scheme of treatment has been developed in the water treatment plant of Ivry-sur-Seine near Paris. This treatment consists of a pretreatment of preozonization, contact coagulation, coagulation on a filter, slow sand filtration, ozonization and GAC filtration. We have tested the efficiency of this new line of treatment for the removal of BDOC and have attempted to correlate the results with the removal of organic matter (global parameters). Results show a very good efficiency of slow sand filtration and we think that this treatment step allows production of a biologically stable water.     

Assessment of nitrification in groundwater filters for drinking water production by qPCR and activity measurement

In groundwater treatment for drinking water production, the causes of nitrification problems and the effectiveness of process optimization in rapid sand filters are often not clear. To assess both issues, the performance of a full-scale groundwater filter with nitrification problems and another filter with complete nitrification and pretreatment by subsurface aeration was monitored over nine months. Quantitative real-time polymerase chain reaction (qPCR) targeting the amoA gene of bacteria and archaea and activity measurements of ammonia oxidation were used to regularly evaluate water and filter sand samples. Results demonstrated that subsurface aeration stimulated the growth of ammonia-oxidizing prokaryotes (AOP) in the aquifer. Cell balances, using qPCR counts of AOP for each filter, showed that the inoculated AOP numbers from the aquifer were marginal compared with AOP numbers detected in the filter. Excessive washout of AOP was not observed and did not cause the nitrification problems. Ammonia-oxidizing archaea grew in both filters, but only in low numbers compared to bacteria. The cell-specific nitrification rate in the sand and backwash water samples was high for the subsurface aerated filter, but systematically much lower for the filter with nitrification problems. From this, we conclude that incomplete nitrification was caused by nutrient limitation.     

Removal of heavy metals from storm and surface water by slow sand filtration: the importance of speciation

The removal of heavy metals from storm and surface waters by slow sand filtration is described. The importance of speciation as a technique for exploring and improving the mechanisms of removal is identified. Laboratory-scale slow sand filters operating at conventional flow rate and depth were shown to be able to reduce concentrations of selected heavy metals (Cu, Cr, Pb and Cd) found in road runoff, surface water and sewage effluents to drinking water standard. Nitrogen, volatile solids and modified Stover speciation were used to differentiate between the potential mechanisms of removal, i.e. active biomass, organic adsorption and simple adsorption or precipitation on the surface of the sand. The data presented show that adsorption via organic ligands was the predominant mechanism for metal removal at the surface of the filter but chemical adsorption was the more important deeper in the filter. In the lower layers the adsorbed metals were more easily exchanged than the organically bound metals. The precise chemical ligands were not identified and varied from metal to metal. The most important operational factors affecting performance were therefore the concentration of organic matter, filter depth and the flow velocity.     

Slow sand filters effectively reduce Phytophthora after a pathogen switch from Fusarium and a simulated pump failure

Slow sand filtration has been shown to effectively reduce Phytophthora zoospores in irrigation water. This experiment tested the reduction of Phytophthora colony forming units (CFUs) by slow sand filtration systems after switching the pathogen contaminating plant leachate from Fusarium to Phytophthora and the resilience of the system to a short period without water, as might be caused by a pump failure. The slow sand filtration system greatly reduced Phytophthora CFUs and transmission after switching the pathogens. In addition, Phytophthora reduction by the slow sand filter was equally effective before and after the simulated pump failure. Reduction of Fusarium was not seen by the SSFs, before or after the simulated pump failure. The results suggest that slow sand filters are effective at reducing larger organisms, such as Phytophthora zoospores, even after a pump failure or a change in pathogens.     

珊瑚砂和玻璃微珠制备超高性能混凝土的试验研究

采用珊瑚砂、空心玻璃微珠两种轻质材料等比例取代石英砂制备了超高性能混凝土(UHPC),并对其表观密度和力学性能进行了研究.试验结果表明:珊瑚砂经过预处理后可以取代石英砂制备UHPC,表观密度降低了10.5％;空心玻璃微珠取代10％的石英砂制备的UHPC表观密度降低了11.5％;将珊瑚砂与空心玻璃微珠复合作为骨料制备出的...     

Pre-Filtration of Very Highly Turbid Waters Using Pebble Matrix Filtration

As A SOLUTION to very high turbidity problems associated with slow sand filters, a new pretreatment method has been developed at University College, London. Initial experiments with a model using a bed of fine sand (effective diameter d. = 0.32 mm) demonstrated that maximum loadings on slow sand filters should not exceed 25 mg/1 at a filtration velocity of 0.2 m/h for satisfactory run times (approximately 5 weeks). However, a literature survey revealed that many tropical rivers may carry several hundred (or even a few thousand) milligrammes per litre of suspended solids during monssoon periods. A need for pretreatment methods is therefore obvious.
A novel process, called pebble matrix filtration, can protect slow sand filters by reducing the suspended-solids concentration of monsoon river waters (containing up to 5000 mg/1) to below 25 mg/1. The paper briefly describes the principles lying behind the treatment process of pebble matrix filtration, and suitable operational parameters are given at flow rates of 0.72–1.56 m/h for tested suspended-solids concentrations of 500, 1000, 2000 and 5000 mg/1 kaolin clay in London tap water, with achieved run times of up to 116 h to head losses not exceeding 1.5 m. Filter cleaning is described by a method called 'drainage and backwash'.     

Capturing phosphates with iron enhanced sand filtration

Most treatment practices for urban runoff capture pollutants such as phosphorus by either settling or filtration while dissolved phosphorus, typically as phosphates, is untreated. Dissolved phosphorus, however, represents an average 45% of total phosphorus in stormwater runoff and can be more than 95%. In this study, a new stormwater treatment technology to capture phosphate, called the Minnesota Filter, is introduced. The filter comprises iron filings mixed with sand and is tested for phosphate removal from synthetic stormwater. Results indicate that sand mixed with 5% iron filings captures an average of 88% phosphate for at least 200 m of treated depth, which is significantly greater than a sand filter without iron filings. Neither incorporation of iron filings into a sand filter nor capture of phosphates onto iron filings in column experiments had a significant effect on the hydraulic conductivity of the filter at mixtures of 5% or less iron by weight. Field applications with up to 10.7% iron were operated over 1 year without detrimental effects upon hydraulic conductivity. A model is applied and fit to column studies to predict the field performance of iron-enhanced sand filters. The model predictions are verified through the predicted performance of the filters in removing phosphates in field applications. Practical applications of the technology, both existing and proposed, are presented so stormwater managers can begin implementation.     

Biomass development in slow sand filters

Microbial biomass development in the sand and schmutzdecke layer was determined in two full-scale slow sand filters, operated with and without a light excluding cover. A standard chloroform fumigation-extraction technique was adapted to routinely measure microbial biomass concentrations in the sand beds. Sand was sampled to a depth of 10 cm and schmutzdecke was also collected at the same random positions on the uncovered filter. Interstitial microbial biomass in the uncovered sand bed increased with time and decreased with sampling depth. There was a small accumulation of sand biomass with time in the covered filter, but no relationship was apparent between biomass concentration and depth in this filter. Schmutzdecke did not develop on the covered filter and was spatially highly variable in the uncovered condition compared to the consistent patterns observed in interstitial biomass production. It is speculated that microbial biomass in the sand of uncovered filters is largely related to carbon inputs from photosynthetic activity in the schmutzdecke and involves mechanisms that spatially distribute carbon substrate from the schmutzdecke to the sand. However, total organic carbon and dissolved organic carbon removals were similar in both filters suggesting that relatively small biomass populations in covered filters are sufficient to remove residual labile carbon during advanced water treatment and little further advantage to water purification and organic carbon removal is gained by the increased production of biomass in uncovered slow sand filter beds.     

BAF/O_3预处理工艺后砂滤池的除污效果

以珠江广州段源水为处理对象,考察了曝气生物滤池（BAF）／臭氧（O3）预处理工艺后砂滤池的除污效果。结果表明,砂滤池出水CODMn、NH4^＋ -N和浊度的平均值分别为2．19、0．099mg／L和0．225NTU,NO2^- -N的最高值为0．003mg／L;相对于沉淀池出水,砂滤池对上述指标的平均去除率分别为27．60％、66．88％、69．88％和98．53％。BAF和臭氧塔提高了源水的DO浓度,其对浊度和有机物的去除作用降低了砂滤池的反冲洗频率,从而有利于提高生物膜中微生物的数量和活性;臭氧氧化可提高源水的可生化性,且水中没有残留臭氧,也为砂滤池的生物降解作用提供了有利条件。     

Removal and fate of Cryptosporidium parvum, Clostridium perfringens and small-sized centric diatoms (Stephanodiscus hantzschii) in slow sand filters

The decimal elimination capacity (DEC) of slow sand filtration (SSF) for Cryptosporidium parvum was assessed to enable quantitative microbial risk analysis of a drinking water production plant. A mature pilot plant filter of 2.56m(2) was loaded with C. parvum oocysts and two other persistent organisms as potential surrogates; spores of Clostridium perfringens (SCP) and the small-sized (4-7microm) centric diatom (SSCD) Stephanodiscus hantzschii. Highly persistent micro-organisms that are retained in slow sand filters are expected to accumulate and eventually break through the filter bed. To investigate this phenomenon, a dosing period of 100 days was applied with an extended filtrate monitoring period of 150 days using large-volume sampling. Based on the breakthrough curves the DEC of the filter bed for oocysts was high and calculated to be 4.7log. During the extended filtrate monitoring period the spatial distribution of the retained organisms in the filter bed was determined. These data showed little risk of accumulation of oocysts in mature filters most likely due to predation by zooplankton. The DEC for the two surrogates, SCP and SSCD, was 3.6 and 1.8log, respectively. On basis of differences in transport behaviour, but mainly because of the high persistence compared to the persistence of oocysts, it was concluded that both spores of sulphite-reducing clostridia (incl. SCP) and SSCD are unsuited for use as surrogates for oocyst removal by slow sand filters. Further research is necessary to elucidate the role of predation in Cryptosporidium removal and the fate of consumed oocysts.     

微污染高藻湖泊水的深度处理工程设计

由于太湖水质呈现高藻、高有机物、高氨氮的＂三高＂特征,常规处理对原水中藻类、氨氮、有机物等的去除效果较差,因此充山水厂实施了深度处理工程.经试验比较,在水厂原有气浮、过滤的常规处理工艺基础上,增设了BIOSMEDI（R）生物滤池及臭氧-生物活性炭滤池等深度处理单元.介绍了充山水厂生物预处理-气浮-臭氧/生物活性炭-砂滤-消毒组合工艺的流程、设计参数及设计特点.     

Modeling antimicrobial contaminant removal in slow sand filtration

Slow sand filters are used in rural regions where source water may be subjected to antimicrobial contaminant loads from waste discharges and diffuse pollution. A numerical model (LETA) was derived to calculate aqueous antimicrobial concentrations through time and depth of a slow sand filter and estimate accumulating contaminant mass in the schmutzdecke. Input parameters include water quality variables easily quantified by water system personnel and published adsorption, partitioning, and degradation coefficients. Simulation results for the tetracycline, quinolone, and macrolide classes of antimicrobials suggested greater than 3-log removal from 1 microg/L influent concentrations within the top 40 cm of the sand column, with schmutzdecke antimicrobial concentrations comparable to other land-applied waste biosolids. A 60-day challenge experiment injecting 1 microg/L tylosin to a pilot slow sand filter showed an average 0.1mg/kg of the antimicrobial remaining in the schmutzdecke layer normally removed during filter maintenance, and this value was the same order of magnitude as the sorbed concentration predicted by the LETA model.     

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.     

Pilot-scale investigation on the removal of organic foulants in secondary effluent by slow sand filtration prior to ultrafiltration

Natural biofiltration processes have been verified as effective pre-treatment choice improving the performance of low-pressure membranes (MF/UF) in wastewater reclamation. In the present work, pilot-scale slow sand filtration (SSF) was used to simulate bank filtration at high filtration rates (from 0.25 m/h to 0.5 m/h) to filter secondary effluent prior to UF. The results showed that SSF improved the performance of UF to a large extent. Related to previous work biopolymers are considered as major dissolved organic foulants in treated wastewater. The removal of these organic foulants in slow sand filters and factors affecting the performance of SSF were investigated. It was observed that the removal of biopolymers took place mainly at the upper sand layer and was related to biological degradation. Tests on the degradability of biopolymers verified that they are biodegradable. Sixteen months monitoring of biopolymer concentration in the secondary effluent indicated that it varied seasonally. In winter season the concentration was much higher than during the summer months. Higher temperature and lower biopolymer concentration led to more effective foulants removal and more sustainable operation of SSF. During the whole experimental period, the performance of SSF was always better at filtration rate of 0.25 m/h than at 0.5 m/h. Under the present experimental conditions, SSF exhibited stable and effective biopolymer removal at temperatures higher than 15 °C, at biopolymer concentrations lower than 0.5 mg C/L and with sufficient oxygen available.     

Slow sand filtration as a technique for the tertiary treatment of municipal sewages

The investigation was designed to demonstrate the viability, or otherwise, of slow sand filtration as a means of tertiary treatment for secondary effluents derived from conventional aerobic, biological treatment processes operating with municipal wastewaters. Secondary effluents derived from both an activated-sludge plant and from a percolating filtration plant were employed.The basic slow sand filtration unit used consisted of a 140 mm i.d. perspex cylinder, 2.65 m in height containing a 950 mm depth of fine sand. Treatment rates were either 3.5 or 7.0 m d⁻¹ and the sand used was of an effective size initially of 0.3 mm and then later of 0.6 mm.This investigation has demonstrated that a laboratory-scale slow sand filtration unit is capable of consistently removing at least 90% of the suspended solids, more than 65% of the remaining BOD and over 95% of the coliform organisms from the settled effluent from an operational percolating filter plant. The length of operational run averaged 20 days at 3.5 m d⁻¹ and 13 days at 7.0 m d⁻¹. Slightly inferior results were achieved when using the settled effluent from an operational activated sludge unit.Further investigation employing a horizontal-flow gravel pre-filter demonstrated that at flows of 2 m h⁻¹ with a contact time of 33 min up to 82% of the suspended solids in the secondary effluent could be removed prior even to slow sand filtration.     

Corrosion control enhancement from a dolomite-amended slow sand filter

The associated decrease of pH in slow sand filters, due to CO2 conversion and biological activity, may produce effluent that is slightly corrosive to downstream distribution pipe material. This pilot study examined the use of a 3-cm crushed dolomite limestone media layer placed within the filter column of a slow sand filter to enhance effluent corrosion control by the introduction of beneficial dolomite dissolution products, without impacting turbidity removal efficiencies. Turbidity removal, calcium concentration, pH, conductivity, total hardness and alkalinity changes were calculated for the filter during a 60-day pilot study, and water chemistry values were used to estimate the changes of the saturation index (SI) throughout the filter run. Total hardness change through the filter was compared to change calculated by a derived equation for hardness using calcium concentrations to determine if the media was dissolving in stoichiometric proportions, and mineral service life in the filter was estimated using an assumption of stoichiometric dissolution at a constant flow rate. Effluent SI was raised an average of 30%, alkalinity was increased by 19%, and effluent pH averaged 7.7. Filter effluent complied with current turbidity regulatory requirements for the provision of potable water, and mineral service life was estimated between 7.5 and 9.5 years.     

直接过滤池过滤性能综合评价指标

本文在试验的基础上,通过对无烟煤—石英砂双层滤料与石英砂均质滤料直接过滤性能的比较,提出了较全面较实用的评价滤池运行好坏的综合评价指标。     

Plasticity Modeling of the Effect of Sample Preparation Method on Sand Response

Experimental evidence shows that different preparation methods produce sand samples with distinctly different stress-strain response. This paper explores and draws conclusions from the measured differences on the monotonic undrained triaxial response of Toyoura sand, prepared by different methods at the same values of void ratio and initial effective stresses. This is achieved by comparing data and simulations performed with a recently developed plasticity constitutive model, which accounts for the effect of inherent fabric anisotropy on the mechanical response. The inherent fabric anisotropy is represented by a second order symmetric fabric tensor, and its effect on the response at different loading directions is expressed by an appropriate dependence of certain constitutive ingredients on a joint isotropic invariant of the loading direction and the fabric tensor. Use of this constitutive scheme to simulate the aforementioned data on Toyoura sand exploits the fact that the preparation method affects both the dilatancy and the hardening response in a systematic manner. Under the premise that these effects are due to the different inherent fabric created by the preparation method, it follows that the foregoing simulations do not require changes in constitutive equations or entirely different sets of model constants. On the contrary, only model constants related to this inherent fabric anisotropy scheme need readjustment, providing insight to how the sample preparation method affects the response and what can be done to model it.     

宏细观结合的砂土各向异性破坏准则

采用宏细观结合的方法,建立一个砂土的各向异性破坏准则。该方法将细观定义的组构张量和加载应力张量结合形成一个组合张量,用组合张量的2个不变量定义一个新的各向异性状态变量,将该状态变量引入到宏观方程中可建立各向异性破坏准则。考虑组构和应力状态的关系,建立的各向异性强度准则可以描述主应力轴旋转和固定时,细观各向异性参量对砂土破坏强度的影响,而且该准则的参数有清晰的物理意义。采用多组试验数据进行验证,结果表明,在整个三维条件下,该准则能较好地反映细观参量对砂土的各向异性强度和峰值内摩擦角的影响,初步验证该方法描述各向异性的合理性和有效性。