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.     

Optimisation and significance of ATP analysis for measuring active biomass in granular activated carbon filters used in water treatment

A method for determining the concentration of active microbial biomass in granular activated carbon (GAC) filters used in water treatment was developed to facilitate studies on the interactions between adsorption processes and biological activity in such filters. High-energy sonication at a power input of 40 W was applied to GAC samples for the detachment of biomass which was measured as adenosine triphosphate (ATP). Modelling of biomass removal indicated that a series of six to eight sonication treatments of 2 min each yielded more than 90% of the attached active biomass. The ATP concentrations in 30 different GAC filters at nine treatment plants in The Netherlands ranged from 25 to 5000 ng ATP cm(-3) GAC, with the highest concentrations at long filter run times and pretreatment with ozone. A similar concentration range was observed in nine rapid sand (RS) filters. ATP concentrations correlated significantly (p<0.05) with total direct bacterial cell counts in each of these filter types, but the median value of the ATP content per cell in GAC filters (2.1 x 10(-8) ng ATP/cell) was much lower than in the RS filters (3.6 x 10(-7) ng ATP/cell). Average biofilm concentrations ranging from 500 to 10(5) pg ATP cm(-2) were calculated assuming spherical shapes for the GAC particles but values were about 20 times lower when the surface of pores >1 microm diameter is included in these calculations. The quantitative biomass analysis with ATP enables direct comparisons with biofilm concentrations reported for spiral wound membranes used in water treatment, for distribution system pipes and other aquatic environments.     

Measurement of biomass activity in drinking water biofilters using a respirometric method

A simple respirometric method was developed and applied for the measurement of biomass activity in bench-scale drinking water biofilters. The results obtained with the new method, i.e. biomass respiration potential (BRP), indicated a high sensitivity allowing the quantification of the activity of low amounts of biomass. The analysis of duplicate samples showed a reasonable reproducibility, i.e. average coefficient of variation of 14% (n = 19). The calculation of the ratio between biomass activity and the amount of viable biomass (phospholipid) at different filter depths indicated a substantial increase of this ratio with filter depth. This indicated an increased biomass activity per unit amount of viable biomass deeper in the biofilters, where biofilm thickness is low. The comparison of the filter profiles of biomass activity and dissolved biodegradable organic matter (BOM), expressed as theoretical oxygen demand, showed a high correlation between these profiles. Consequently, BRP results appear to be good indicators of the BOM removal capacity of the filter biomass. Therefore, BRP results can potentially be used in certain cases instead of BOM measurements for the assessment of the BOM removal capacity of drinking water biofilters, operated under different conditions. This is important because of the relative complexity of the measurements of BOM surrogates, e.g. assimilable organic carbon and biodegradable dissolved organic carbon, and BOM components.     

Transport of nanoparticles with dispersant through biofilm coated drinking water sand filters

This article characterizes, experimentally and theoretically, the transport and retention of engineered nanoparticles (NP) through sand filters at drinking water treatment plants (DWTPs) under realistic conditions. The transport of four commonly used NPs (ZnO, CeO_2, TiO_2, and Ag, with bare surfaces and coating agents) through filter beds filled with sands from either acid washed and calcined, freshly acquired filter media, and used filter media from active filter media, were investigated. The study was conducted using water obtained upstream of the sand filter at DWTP. The results have shown that capping agents have a determinant importance in the colloidal stability and transport of NPs through the different filter media. The presence of the biofilm in used filter media increased adsorption of NPs but its effects in retaining capped NPs was less significant. The data was used to build a mathematical model based on the advection-dispersion equation. The model was used to simulate the performance of a scale-up sand filter and the effects on filtration cycle of traditional sand filtration system used in DWTPs.     

Rapid and direct estimation of active biomass on granular activated carbon through adenosine tri-phosphate (ATP) determination

Granular activated carbon (GAC) filtration is used during drinking water treatment for the removal of micropollutants such as taste and odour compounds, halogenated hydrocarbons, pesticides and pharmaceuticals. In addition, the active microbial biomass established on GAC is responsible for the removal of biodegradable dissolved organic carbon compounds present in water or formed during oxidation (e.g., ozonation and chlorination) processes. In order to conduct correct kinetic evaluations of DOC removal during drinking water treatment, and to assess the state and performance of full-scale GAC filter installations, an accurate and sensitive method for active biomass determination on GAC is required. We have developed a straight-forward method based on direct measurement of the total adenosine tri-phosphate (ATP) content of a GAC sample and other support media. In this method, we have combined flow-cytometric absolute cell counting and ATP analysis to derive case-specific ATP/cell conversion values. In this study, we present the detailed standardisation of the ATP method. An uncertainty assessment has shown that heterogeneous colonisation of the GAC particles makes the largest contribution to the combined standard uncertainty of the method. The method was applied for the investigation of biofilm formation during the start-up period of a GAC pilot-scale plant treating Lake Zurich water. A rapid increase in the biomass of up to 1.1 x 10(10)cells/g GAC dry weight (DW) within the first 33 days was observed, followed by a slight decrease to an average steady-state concentration of 7.9 x 10(9)cells/g GAC DW. It was shown that the method can be used to determine the biomass attached to the GAC for both stable and developing biofilms.     

Development of biomass in a drinking water granular active carbon (GAC) filter

Indigenous bacteria are essential for the performance of drinking water biofilters, yet this biological component remains poorly characterized. In the present study we followed biofilm formation and development in a granular activated carbon (GAC) filter on pilot-scale during the first six months of operation. GAC particles were sampled from four different depths (10, 45, 80 and 115 cm) and attached biomass was measured with adenosine tri-phosphate (ATP) analysis. The attached biomass accumulated rapidly on the GAC particles throughout all levels in the filter during the first 90 days of operation and maintained a steady state afterward. Vertical gradients of biomass density and growth rates were observed during start-up and also in steady state. During steady state, biomass concentrations ranged between 0.8-1.83 x 10⁻⁶ g ATP/g GAC in the filter, and 22% of the influent dissolved organic carbon (DOC) was removed. Concomitant biomass production was about 1.8 × 10¹² cells/m²h, which represents a yield of 1.26 × 10⁶ cells/μg. The bacteria assimilated only about 3% of the removed carbon as biomass. At one point during the operational period, a natural 5-fold increase in the influent phytoplankton concentration occurred. As a result, influent assimilable organic carbon concentrations increased and suspended bacteria in the filter effluent increased 3-fold as the direct consequence of increased growth in the biofilter. This study shows that the combination of different analytical methods allows detailed quantification of the microbiological activity in drinking water biofilters.     

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.     

Demonstration of mass transfer and pH effects in a nitrifying biofilm

A bench-scale nitrifying trickling filter (surface AREA = 0.5 m²) was developed to permit evaluation of diffusion of oxygen within a biofilm, the pH dependence of ammonium oxidation and external mass transfer. In addition, a biofilm model was developed and verified for homogeneous nitrifying biofilms of varied thickness and for thin nitrifying biofilms covered by heterotrophic biofilms. The model uses literature values for the pH dependence of Monod coefficients for Nitrosomonas and Nitrobacter.

The diffusion coefficient of oxygen in the biofilm was found to be 40–80% of the value in pure water. Due to mass transfer resistance, the biomass ·sees” a lower pH than is measured in the water film passing over it. The surface uptake rate of ammonia is used as an indicator of pH gradients within the biofilm system. With the help of oxygen limitation experiments, the location of nitrifying biomass within mixed biofilms (heterotrophic, autotrophic) can be determined.

The biofilm model predicts ammonium uptake rate of a trickling filter as a function of the bicarbonate concentration in the water film.     

饮用水生物强化过滤工艺生物膜特性研究

以水厂沉淀池出水为原水,对生物强化过滤工艺的生物膜形成过程进行了研究,探讨了不同滤料介质组成的生物滤柱的生物膜特性,分析了膜形成过程中污染物的去除效果和滤柱生物量的变化情况,并对膜形成过程的影响因素进行了讨论.结果表明,在活性炭-石英砂滤料上生物膜形成效果要优于无烟煤-石英砂双层滤料和石英砂单层滤料;反冲洗水含氯对生物膜形成有负面影响,对无烟煤-石英砂滤柱的影响尤为显著;可以CODMn和NO-2的去除率作为生物膜成熟的评价指标.     

生物预处理富营养化水源水的试验研究

采用生物陶粒滤柱作为预处理工艺,考察了其对滏阳河水的处理效果以及影响因素,并确定了运行参数。试验结果表明:生物滤柱对各种藻的去除效果不同,且去除率随滤速的上升而降低;有机物的去除率受滤速影响不大,但随着温度的上升而提高。生物预处理不但能够降低混凝剂的用量,而且使溶解氧增大,改善了水质。     

沸石-无烟煤生物滤池联合处理微污染水试验研究

考察了沸石-无烟煤双层滤料生物滤池处理微污染水的运行效能.结果表明,沸石-无烟煤生物滤池可以有效提高出水水质,对CODMn、NH3-N及浊度的去除率分别达到39.5%,93.6%和91.3%,而且工作区间主要在滤层上部40 cm内;反冲洗对滤料表面附着的微生物膜影响很小,生物膜在反冲洗后1.5 h内能恢复到反冲洗前的水平.     

The effectiveness of biomass hold-up and packing surface in trickling filters

The experimental performance characteristics of a filter bed containing 2 in. Biopac are assessed in terms of the overall rate of BOD removal by biomass films, liquid phase mass transfer and wetted area. The bed was operated with large biomass hold-ups and this determined both the internal geometry of the bed and the liquid flow pattern.Experimentation involving the effect of flow-rate on filter performance suggests that not only was the wetted area dependent on flow-rate in the range investigated and substantially less than the specific surface area of the packing but it was also lower than that associated with very small biomass hold-ups, i.e. thin biomass layers covering the packing pieces without affecting the internal geometry of the bed.The loss of potential performance attributable to the wetted area being less than the specific surface area of the packing depended both on flow-rate and biomass hold-up. At high biomass hold-ups the largest loss of efficiency when compared with the theoretical efficiency of the biomass-substrate-packing system was as much as 55%. The equivalent figure at the same flow-rate, for low biomass hold-ups is 20%. The actual removal efficiencies approach the theoretical efficiencies in the region of the minimum wetting rate.The values obtained for the wetted areas and their dependency on flow-rate support the theory proposed for packed-bed biological film reactors and a methodology for the interpretation of experimental data based upon laboratory scale experimentation for characterization of the BOD-biomass system and pilot-scale experimentation for characterization of the packings.     

折流曝气生物滤池的微生物特性研究

考察了折流曝气生物滤池(BBAF)内的生物膜和微生物种群的分布特征.结果表明:BBAF各单池的比质生物量和比面生物量均随运行时间的增加而增加;沿流程方向各单池的比面生物量逐级递减,而比质生物量呈N字型变化;各单池的生物膜活性沿流程方向也呈N字型变化;由于各单池之间存在浓度梯度,可形成各自的微生物优势种属,有利于BBAF达到最佳运行效果.     

The sensitivity of fixed-bed biological perchlorate removal to changes in operating conditions and water quality characteristics

Flow rate, electron donor addition, and biomass control were evaluated in order to optimize perchlorate (ClO₄⁻) removal from drinking water using biologically active carbon (BAC) filtration. Influent dissolved oxygen (DO) was lowered from ambient conditions to approximately 2.5 mg/L for all experiments using a nitrogen sparge. When influent nitrate concentration was 0-2.0 mg/L, 1.6-2.8 mg/L as carbon of acetate or ethanol was required to achieve and sustain the complete removal of 50 μg/L perchlorate in a BAC filter. Most or all of the exogenous acetate and ethanol was removed during biofiltration. When a 72-h electron donor feed failure was simulated, a maximum perchlorate breakthrough of 18 μg/L was observed and, once electron donor was reapplied, 9 days were required to reestablish complete perchlorate removal. During a 24-h electron donor feed failure simulation, the maximum effluent perchlorate concentration detected was 6.7 μg/L. Within 24 h of reactivating the electron donor, the filter regained its capacity to consistently remove 50 μg/L perchlorate to below detection. Although biomass growth diminished the filter's ability to consistently remove perchlorate, a cleaning procedure immediately restored stable, complete perchlorate removal. This cleaning procedure was required approximately every 50 days (4800 bed volumes) when influent DO concentration was 2.5 mg/L. Empty-bed contact time (EBCT) experiments showed that 80% perchlorate removal was achieved using a 5-min EBCT, and complete perchlorate removal was observed for an EBCT of 9 min. It was also demonstrated that BAC filtration consistently removed perchlorate to below detection for influent perchlorate concentrations ranging from 10 to 300 μg/L, influent sulfate concentrations between 0 and 220 mg/L, influent pH values of 6.5-9.0, and operating temperatures of 5-22°C.     

Molecular characterization of microbial populations in groundwater sources and sand filters for drinking water production

In full-scale drinking water production from groundwater, subsurface aeration is an effective means of enhancing the often troublesome process of nitrification. Until now the exact mechanism, however, has been unknown. By studying the microbial population we can improve the understanding of this process. Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments of bacteria, archaea and ammonia-oxidizing bacteria was used to characterize the microbial populations in raw groundwater and trickling filters of an active nitrifying surface aerated system and an inactive non-surface aerated system. Only in the active filter were nitrifying microorganisms found above the detection limit of the method. In ammonia oxidation in this groundwater filter both bacteria and archaea played a role, while members belonging to the genus Nitrospira were the only nitrite-oxidizing species found. The subsurface aerated groundwater did not contain any of the nitrifying organisms active in the filter above the detection limit, but did contain Gallionella species that might play a major role in iron oxidation in the filter.     

Treatment of water closet flush water for recycle and reuse

Results from the operation of a 37.8 m³/d extended aeration and sand filtration system in the closed‐loop treatment of water closet flush water are presented.

The system has operated for four and one‐half years at 95 percent recycle. During this period over 30,000 m³ of flush water was treated and reused. Water inputs into the recycle system resulted from liquid human wastes plus wastage form potable water uses. Wasted potable water inputs were from wash basins, water fountains, and custodial services.

Operation of both the biological treatment unit and the pressure sand filter followed acceptable conventional practice. Chemicals were not required to adjust pH, aid solids settleability or enhance treatment efficiency. Equilibrium concentrations followed predicted values except for pH, alkalinity, ammonia, nitrite and nitrate. The concentration of these constituents was governed by urine content after flushing, agitation before biological treatment, temperature and biological oxidation. Variations in nitrogen (ammonia, nitrite and nitrate), pH and alkalinity that were observed could be accounted for through fundamental biological, chemical and physical relationships. The pH throughout the entire recycle system varied between 5.5 and 8.4. Recycled water pH rose from a preflush pH of approximately 7.0 to a pH of 8.4 immediately after flushing. The biological unit lowered the pH and functioned between pH values of 5.5 and 7.0. A slight rise in pH between the biological unit (through storage and filtration) and water closets was observed.

The predominate biomass in the biological unit was fungi. Biological solids were threadlike; however, they readily separated by gravity settling. Wastage of biological solids from the biological unit in the recycle‐reuse system was the same experienced for a comparable biological unit used to treat water closet wastewater that was not recycled.

Results from this study have conclusively demonstrated on a full‐scale basis the acceptability of using biological oxidation and sand filtration as a treatment train in the reuse of water closet wastewater with a recycle ratio of 20.     

A portable virus concentrator for testing water in the field

A system is described for concentrating viruses from large volumes of water. The system consists of a water pump, an electric generator, a series of clarifiers, a virus adsorbent, a virus reconcentrator, a 5- and a 1-gal pressure vessel with a small tank of nitrogen as a source of positive pressure, and ancillary equipment, all mounted on 2-wheel carts for easy portability. Standardization of the system was achieved by use of minute amounts of poliovirus. The virus was added to dechlorinated city tap water so that it could not be detected unless the virus was first concentrated. In the system, raw tap water containing virus is serially passed through clarifying filters of porosities of 1–5 μm to remove particulate matter, and then through a 1-μm cotton textile filter to electrostatically remove submicron ferric and other heavy metallic complexes. These filters do not detectably remove virus. Salts are then added to the running tap water to enhance the adsorption of virus to a fibreglass or cellulose acetate filter. Raw water could be processed at the rate of 300 gallons per hour, with total virus removal from the water and with 80 per cent elution of the virus from the adsorbent.     

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.     

臭氧——浮滤池工艺强化处理引黄水库水中有机物的试验研究

分别采用预臭氧-浮滤池工艺和臭氧气浮-浮滤池工艺对鹊山水库水进行了试验研究.预臭氧-浮滤池工艺试验结果表明,臭氧预处理对浊度去除影响不大;臭氧预氧化对UV<,254>的去除效率很高,平均去除率为76.47%,经气浮、活性炭过滤之后,总去除率可达到100%;臭氧预氧化对COD<,Mn>有一定的去除效果,总去除率较无臭氧预处理平均提高7.0%,出水平均值降至1.87 mg/L.臭氧气浮-浮滤池工艺试验表明,臭氧气浮对浊度去除影响不大,臭氧的强氧化性主要表现在对UV254的去除上,其平均去除率比浮滤池工艺提高了9.8%.     

饮用水中二甲基异莰醇的应急处理技术研究

针对郑州市S水厂原水二甲基异莰醇(2-MIB)季节性升高问题,采用在原水中投加粉末活性炭(PAC)或二氧化氯(ClO2)的应急处理技术。结果表明,在原水泵站投加PAC可以有效控制水体的臭味;与单层石英砂滤层相比,石英砂加活性炭滤层对2-MIB的去除率可增加56%;为防止活性炭吸附效率降低,应避免PAC与ClO2同时使用。