Variability of invertebrate abundance in drinking water distribution systems in the Netherlands in relation to biostability and sediment volumes

A survey of invertebrates in drinking water from treatment works, internal taps and hydrants on mains was carried out by almost all water companies in the Netherlands from September 1993 to August 1995. Aquatic sow bugs (Asellidae, 1-12 mm) and oligochaeta worms (Oligochaeta, 1-100 mm), both known to have caused rare though embarrassing consumer complaints, were found to form 98% of the mean biomass in water flushed from mains. Their numbers in the mains water ranged up to 1500 (mean 37) Asellidae m⁻³ and up to 9900 (mean 135) Oligochaeta m⁻³. Smaller crustaceans (0.5-2 mm) dominated the numbers in water from mains. e.g. water fleas (Cladocera and Copepoda up to 14,000 m⁻³). Common invertebrates in treated water and in tap water were Rotifera (<1 mm) and nematode worms (Nematoda, <2 mm). No Asellidae, large Oligochaeta (>5 mm) or other large invertebrates were found in 1560 samples of 200 l treated water or tap water.Large variations in invertebrate abundance were found within and between distribution systems. Of the variability of mean biomass in mains per system, 55%, 60% and 63% could statistically be explained by differences in the Biofilm Formation Rate, non-particulate organic matter and the permanganate index of the treated water of the treatment works respectively. A similar correlation was found between mean invertebrate biomass and mean sediment volumes in the distribution systems (R² = 52%).     

Analyzing trophic transfer of metals in stream food webs using nitrogen isotopes

This study examines detrimental effects of acid mine drainage (AMD) on stream invertebrate communities and tests for a direct relationship between trophic position and accumulation of three metals (Fe, Cu, Zn) by stream invertebrates in situ. On two dates in each of seven stream sites, we measured food chain length, mean trophic level, taxa richness, and trophic position of stream macroinvertebrates comprising the food webs using stable nitrogen isotope ratios. Metals in tissue of representatives of 35 taxa were measured by ICP-OES. Our results are the first direct comparison of uptake of these metals in stream invertebrate taxa according to trophic position as identified by delta15N. As predicted, metal concentrations were generally greater in water and insects from sites adjacent to mining activity and invertebrate taxa richness was significantly lower. Taxa richness increased with distance away from contaminated headwaters. Despite reductions in diversity at sites nearest AMD, food chain length and mean trophic level did not differ between streams. The relationship between trophic position and metal accumulation differed considerably among metals. Specifically, Fe declined (biodilution) and Zn increased (biomagnification) with trophic level, but trophic position had no effect on Cu levels in these insects. Our results highlight fundamental differences in trophic transfer of specific metals through aquatic food webs and identify ecologically important impacts of AMD on stream invertebrates.     

Size fractionation of metals in runoff from residential and highway storm sewers

Stormwater sampling for particulate, colloidal, and dissolved metals was conducted for several storms at six outfalls in Monmouth County, NJ. Samples were initially sequentially filtered through 5 microm, 0.45 microm, and 10 kDa filters. Of the heavy metals, Cu and Zn were mostly either dissolved (<10 kDa) (20-100%) or in the particulate size fractions >5 microm (0->70%). Pb and Cr were associated exclusively with particles >5 microm in size. Fe, Al, and Si were found mostly in larger size fractions (>70%), with smaller amounts 0.45-5 microm in size. Preliminary data from a small set of samples passed through coarser filters suggested that metals may actually be largely associated with particles larger than 20 microm. Variable and sometimes large dissolved fractions of Cu and Zn can contribute to erratic metals removal by structural best management practices (e.g., wet ponds, detention basins). The size fractionation of stormwater constituents has implications for the design and performance of stormwater control structures and the aquatic toxicity risks posed by the metals. The results demonstrate the importance of obtaining particle size data when planning stormwater treatment.     

Aquatic and terrestrial invertebrate community responses to drying in chalk streams

Temporary streams are dynamic ecosystems that shift between wet and dry states and include the ‘winterbourne’ chalk streams of south England. Our understanding of temporary stream biodiversity is biased, with most research to date exploring aquatic invertebrate communities in benthic sediments during flowing phases. We surveyed the invertebrate communities of the Candover Brook chalk stream, comparing aquatic (benthic, hyporheic) and terrestrial communities in reaches with different flow permanence regimes. We used kick and Bou–Rouch sampling methods to collect aquatic invertebrates, and compared the terrestrial communities characterised by pitfall traps and ground searches and in different seasons. Although aquatic taxa richness was lower in temporary compared to perennial reaches, the total biodiversity of temporary stream channels was enhanced by contributions from both aquatic and terrestrial species, including several of conservation interest. We recommend that both aquatic and terrestrial communities should be considered in research and monitoring to characterise the biodiversity and ecological quality of temporary streams.     

Assessing land-use effects on water quality, in-stream habitat, riparian ecosystems and biodiversity in Patagonian northwest streams

Changes in land-use practices have affected the integrity and quality of water resources worldwide. In Patagonia there is a strong concern about the ecological status of surface waters because these changes are rapidly occurring in the region. To test the hypothesis that greater intensity of land-use will have negative effects on water quality, stream habitat and biodiversity we assessed benthic macroinvertebrates, riparian/littoral invertebrates, fish and birds from the riparian corridor and environmental variables of 15 rivers (Patagonia) subjected to a gradient of land-use practices (non-managed native forest, managed native forest, pine plantations, pasture, urbanization). A total of 158 macroinvertebrate taxa, 105 riparian/littoral invertebrate taxa, 5 fish species, 34 bird species, and 15 aquatic plant species, were recorded considering all sites. Urban land-use produced the most significant changes in streams including physical features, conductivity, nutrients, habitat condition, riparian quality and invertebrate metrics. Pasture and managed native forest sites appeared in an intermediate situation. The highest values of fish and bird abundance and diversity were observed at disturbed sites; this might be explained by the opportunistic behavior displayed by these communities which let them take advantage of increased trophic resources in these environments. As expected, non-managed native forest sites showed the highest integrity of ecological conditions and also great biodiversity of benthic communities. Macroinvertebrate metrics that reflected good water quality were positively related to forest land cover and negatively related to urban and pasture land cover. However, by offering stream edge areas, pasture sites still supported rich communities of riparian/littoral invertebrates, increasing overall biodiversity. Macroinvertebrates were good indicators of land-use impact and water quality conditions and resulted useful tools to early alert of disturbances in streams. Fish and birds having a greater ability of dispersion and capacity to move quickly from disturbances would reflect changes at a higher scale.     

Water mains renewal planning framework for small to medium sized water utilities: a life cycle cost analysis approach

For the last few decades, concerns have repeatedly been raised about deteriorating water mains in Canada. Small to medium sized water utilities are generally impacted more due to lack of technical and financial resources. This paper presents a user-friendly life cycle cost (LCC) analysis-based decision support tool to help these utility managers to prioritize water mains rehabilitation or replacement (R/R) strategies. The deterioration curves for water mains of different materials and sizes have been developed based on their likelihood of failure. The proposed model is implemented for the water supply network of City of Kelowna (Canada). It compares the costs of various R/R scenarios for each pipe over its life cycle and suggests the most cost-effective decision to the managers to efficiently allocate their limited resources.     

Probability of detecting and quantifying faecal contaminations of drinking water by periodically sampling for E. coli: a simulation model study

Drinking water supply companies monitor the presence of Escherichia coli in drinking water to verify the effectiveness of measures that prevent faecal contamination of drinking water. Data are lacking, however, on the sensitivity of the monitoring programmes, as designed under the EU Drinking Water Directive. In this study, the sensitivity of such a monitoring programme was evaluated by hydraulic model simulations of contamination events and calculations of the detection probability of the actual sampling programme of 2002. In the hydraulic model simulations of 16-h periods of 1l h(-1) ingress of untreated domestic sewage, the spread of the contamination through the network and the E. coli concentration dynamics were calculated. The results show that when large parts of the sewage reach reservoirs, e.g. when they originate from the treatment plant or a trunk main, mean detection probabilities are 55-65%. When the contamination does not reach any of the reservoirs, however, the detection probability varies from 0% (when no sampling site is reached) to 13% (when multiple sites are reached). Mean detection probabilities of nine simulated ingress incidents in mains are 5.5% with an SD of 6.5%. In reality, these detection probabilities are probably lower as the study assumed no inactivation or clustering of E. coli, 100% recovery efficiency of the E. coli detection methods and immediate mixing of contaminations in mains and reservoirs. The described method provides a starting point for automated evaluations and optimisations of sampling programmes.     

Experience of Cast-Iron Mains Rehabilitation in the Gas Industry

Since 1987 British Gas plc has undertaken an extensive programme of medium pressure (up to 2 bar) cast-iron mains replacement in urban locations, using a variety of rehabilitation techniques.
This paper describes the experience of North Thames Region of British Gas plc, over the period 1987–1991, when the rehabilitation of over 230 km of mains was undertaken in the size range 300–460 mm.
The techniques which were used favoured medium density polyethylene pipe wherever possible. However, about 100 km of rehabilitation was undertaken using hoselining, usually where mains were larger than 460 mm dia., or where aboveground access was restricted, or if the mains configuration included many changes of direction.
This paper deals with (a) various aspects of operating rehabilitation contracts from a public utilities viewpoint, (b) the special factors relating to the inspection and testing of hoselined mains including factors found to contribute to failure, and (c) some of the controls that have been introduced to improve reliability of rehabilitation systems.     

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.     

Physicochemical and microbiological quality of water from a pilot domestic rainwater harvesting facility in Ireland

Dublin Institute of Technology personnel were commissioned in 2005 by the Department of Environment, Heritage and Local Government in Ireland to assess the feasibility of utilizing harvested rainwater to replace treated mains water, for nonpotable uses. A pilot project was set up. The project involved the design, installation, commissioning and monitoring of rainwater harvesting facilities in a rural housing development. A monitoring programme was carried out to examine the physicochemical and microbiological quality of the harvested rainwater. Harvested rainwater was sampled monthly and tested. Analysis of the harvested rainwater quality showed a consistently high quality of raw water in general compliance with the requirements of the European Communities (Quality of Bathing Water) Regulations for 100% of samples and the European Communities (Drinking Water) Regulations, 2007 for 37% of sampling dates.     

Quality assurance and quality control practices for rehabilitation of sewer and water mains

As part of the US Environmental Protection Agency (EPA)'s Aging Water Infrastructure Research Program, several areas of research are being pursued including a review of quality assurance and quality control (QA/QC) practices and acceptance testing during the installation of rehabilitation systems (USEPA 2011). The objectives of this research effort were to collect, analyze and summarize information on the installation and QA/QC practices for the trenchless rehabilitation of sewer and water transmission mains. In addition, consideration was given to practices related to water service lines, sewer service laterals, force mains, siphons, sewer manholes, pumping stations, associated wet wells and other appurtenances. This review was accomplished primarily by conducting interviews directly with rehabilitation technology vendors, design engineers and water and wastewater utilities that have a track record of using trenchless rehabilitation technologies within their network. This paper provides an overview of how QA/QC issues have been handled in North America for trenchless rehabilitation technologies.     

Introduction of the leak-before-break (LBB) concept for cast iron water pipes on the basis of laboratory experiments

Failure of cast iron water mains in Australia is a common occurrence. Water utilities are seeking solutions to optimise the renewal and rehabilitation of ageing cast iron critical water mains (diameter ≥ 300 mm). Failure pressures of three large-diameter cast iron pipe specimens (600 mm in diameter) were tested. A large corrosion patch was machined onto each pipe section to initiate pipe failure. A large corrosion patch and significant reduction (>90%) of pipe wall thickness were needed to fail the tested pipe specimens. All three pipes under test exhibited leaking before bursting, indicating that the concept of leak-before-break (LBB) could be used for pipe failure prevention. In addition, LBB provides valuable information that could be added to the pipe asset database to make better management decisions on repair or replacement along with all other asset information. The study also found that small corrosion patches (<50 mm) with low remaining wall thickness may cause pipe leaks, but are less likely to cause major pipe bursts.     

Invertebrate mercury bioaccumulation in permanent, seasonal, and flooded rice wetlands within California's Central Valley

We examined methylmercury (MeHg) bioavailability in four of the most predominant wetland habitats in California's Central Valley agricultural region during the spring and summer: white rice, wild rice, permanent wetlands, and shallowly-flooded fallow fields. We sampled MeHg and total mercury (THg) concentrations in two aquatic macroinvertebrate taxa at the inlets, centers, and outlets of four replicated wetland habitats (8 wetlands total) during two time periods bounding the rice growing season and corresponding to flood-up and pre-harvest (96 total samples). In general, THg concentrations (mean ± standard error) in Notonectidae (Notonecta, back swimmers; 1.18 ± 0.08 µg g^− 1 dry weight [dw]) were higher than in Corixidae (Corisella, water boatmen; 0.89 ± 0.06 µg g^− 1 dw, MeHg: 0.74 ± 0.05 µg g^− 1 dw). MeHg concentrations were correlated with THg concentrations in Corixidae (R² = 0.80) and 88% of THg was in the MeHg form. Wetland habitat type had an important influence on THg concentrations in aquatic invertebrates, but this effect depended on the sampling time period and taxa. In particular, THg concentrations in Notonectidae, but not Corixidae, were higher in permanent wetlands than in white rice, wild rice, or shallowly-flooded fallow fields. THg concentrations in Notonectidae were higher at the end of the rice growing season than near the time of flood-up, whereas THg concentrations in Corixidae did not differ between time periods. The effect of wetland habitat type was more prevalent near the end of the rice growing season, when Notonectidae THg concentrations were highest in permanent wetlands. Additionally, invertebrate THg concentrations were higher at water outlets than at inlets of wetlands. Our results indicate that although invertebrate THg concentrations increased from the time of flood-up to draw-down of wetlands, temporarily flooded habitats such as white rice, wild rice, and shallowly-flooded fallow fields did not have higher THg or MeHg concentrations in invertebrates than permanent wetlands.     

Effects of backwashing on the prosobranch snail Potamopyrgus jenkinsi Smith in granular activated carbon (GAC) adsorbers

Granular activated carbon (GAC) adsorbers are often the penultimate stage of surface water treatment and provide ideal habitats for invertebrates. Proliferation of chlorine-resistant invertebrates in GAC adsorbers may lead to their efflux into distribution systems, possibly resulting in contamination of customers' tap water. GAC adsorber sampling and laboratory experiments were undertaken to determine the effects of routine backwashing on GAC adsorber populations of the chlorine-resistant snail Potamopyrgus jenkinsi at a water treatment works. GAC adsorber sampling results suggested that routine backwashing altered the spatial distribution of snails, but not their overall abundance. In small-scale glass columns 40-50% of the smallest (0.3-0.6 mm shell height) juvenile snails were removed by a GAC backwash bed expansion of 30-40%; however, bed expansions of greater than 20% were not possible in the GAC adsorbers.     

Biodegradation of the cyanobacterial toxin microcystin LR in natural water and biologically active slow sand filters

A bacterium (MJ-PV) previously demonstrated to degrade the cyanobacterial toxin microcystin LR, was investigated for bioremediation applications in natural water microcosms and biologically active slow sand filters. Enhanced degradation of microcystin LR was observed with inoculated (1 x 10(6) cell/mL) treatments of river water dosed with microcystin LR (>80% degradation within 2 days) compared to uninoculated controls. Inoculation of MJ-PV at lower concentrations (1 x 10(2)-1 x 10(5) cells/mL) also demonstrated enhanced microcystin LR degradation over control treatments. Polymerase chain reactions (PCR) specifically targeting amplification of 16S rDNA of MJ-PV and the gene responsible for initial degradation of microcystin LR (mlrA) were successfully applied to monitor the presence of the bacterium in experimental trials. No amplified products indicative of an endemic MJ-PV population were observed in uninoculated treatments indicating other bacterial strains were active in degradation of microcystin LR. Pilot scale biologically active slow sand filters demonstrated degradation of microcystin LR irrespective of MJ-PV bacterial inoculation. PCR analysis detected the MJ-PV population at all locations within the sand filters where microcystin degradation was measured. Despite not observing enhanced degradation of microcystin LR in inoculated columns compared to uninoculated column, these studies demonstrate the effectiveness of a low-technology water treatment system like biologically active slow sand filters for removal of microcystins from reticulated water supplies.     

Detection of enteric viruses, Giardia and Cryptosporidium in two different types of drinking water treatment facilities

In this study, two types of drinking water treatment facilities (two conventional drinking water treatment plants (DWTPs) and two compact units (Cus)) were compared referring to their production capacity. Water samples were collected from three main points: (a) different water treatment steps (b) washings of sand filters and (c) distribution system at different distances from the water treatment plants. Both viruses and protozoa were concentrated from each water sample by adsorption and accumulation on the same nitrocellulose membrane filters (0.45 microm pore size). Enteroviruses were detected by plaque infectivity assay in BGM cells and HAV, HEV and Norovirus were detected by RT-PCR. Giardia and Cryptosporidium were detected by conventional staining methods and PCR. The results revealed that enterovirus load at the intake ranged between 10-15 PFU/L for the two compact units and between 4.5 and 75 PFU/L for the two conventional DWTPs. The virus load in distribution system of the first type DWTPs at 1 km from the plant was the same as that of the intake. Viruses in the other type of treatment plants CUs at 1, 5 and 7 km, were much reduced. Investigation of raw water sediments of the two DWTPs showed enterovirus counts between 12 and 17.5 PFU/L. Virus count was reduced in sand of filters after washing. Giardia cysts were equally detected by microscopy and PCR in only intake samples of EL-Hawamdia CU (33.3%) and Meet Fares DWTP (50%). Cryptosporidium oocysts were equally detected by microscopy and PCR in intake samples of Abo EL-Nomros CU (100%), EL-Hawamdia CU (66.7%) and Fowa DWTP (50%). At Meet Fares DWTP three positive intake samples for Cryptosporidium were detected by PCR, compared with only two positive samples by microscopy. Giardia cysts and Cryptosporidium oocysts were detected in raw water sediment and sand of filters before washing. Only one sample from Meet Fares DWTP sand of filters after washing was positive for both Giardia and Cryptosporidium. It can be concluded that the poor microbial quality of the water may be due to improper operational skills and management of the various water treatment plants (especially at the two high capacity treatment plants).     

Evaluation of a high-volume portable bioaerosol sampler in laboratory and field environments

This study investigated the physical and biological performances of a portable centrifugal sampler for viable bioaerosols, RCS High Flow. The performance of the test sampler in the laboratory and field environments was compared with that of a reference sampler, BioSampler. The laboratory experiments with non-biological particles of KCl, oleic acid, and polystyrene latex showed that the test sampler's collection efficiency is about 22% for 0.5-microm particles, 48% for 1.0-microm particles, and approximately 100% for particles of 2.5 microm and larger. These tests indicated that the sampler's cut-off size (d50) was 1.1 microm. The test sampler's physical performance when collecting the spores and vegetative cells of Bacillus subtilis var. niger (BG) was similar to that when collecting non-biological particles of the same size. In the laboratory tests, the RCS High Flow sampler was found to enumerate approximately 40% of BG spores and cells relative to the reference sampler, BioSampler. A similar ratio was found during testing in an indoor environment. This ratio decreased to below 10% when testing was performed in an outdoor environment. We hypothesize that the test sampler's underperformance compared with the BioSampler could be caused by the damage to sensitive microorganisms during the collection process, test sampler's sensitivity to wind direction and speed as well as break-up of particle aggregates during the impingement process in BioSampler, which resulted in more colony-forming units (CFUs) being counted by the reference sampler than by the test sampler. Overall, when the RCS High Plus is used to sample culturable airborne microorganisms, the results obtained may have to be adjusted to avoid potential underestimation of microorganism concentration in the air. PRACTICAL IMPLICATIONS: The laboratory testing of the RCS High Flow bioaerosol sampler showed that the sampler collects 1 microm particles and larger with an efficiency of 50% and higher; the efficiency reaches approximately 100% for particles of 2.5 microm and larger. When considering this result, most of the airborne fungal spores would be collected with an efficiency between 50 and 100%. The field testing, however, indicated that the RCS High Flow sampler recovered from 41 to 71% of microorganisms collected relative to the reference sampler, Biosampler, and this ratio dropped to below 5% during outdoor testing. Thus, while the RCS High Flow sampler offers certain advantages over other samplers for viable bioaerosols--it is lightweight, battery operated, and collects viable microorganisms at a high flow rate directly on agar media, the results obtained may have to be adjusted to avoid potential underestimation of microorganism concentration in the air, especially if sampling is performed outdoors.     

Influence of intensive fishing on the partitioning of mercury and methylmercury in three lakes of Northern Québec

It has been demonstrated that intensive fishing, i.e., removing more than 25% of the fish biomass, can reduce mercury levels in predator fish in a lake. We test here the hypothesis that, by removing an important part of the fish biomass from a lake, a significant amount of methylmercury can be eliminated, therefore reducing the mercury available to the remaining biota, at least in the short term. A mass burden approach is used to evaluate the partitioning of total mercury and methylmercury in natural lake ecosystems. Three small natural lakes from the James Bay territory, in northern Québec, Canada, were selected for intensive fishing. Mercury (Hg) and methylmercury (MeHg) concentrations were evaluated for sediments, water column (dissolved fraction and suspended particulate matter), plankton, aquatic invertebrates, and fish. Biomasses were determined for fish, plankton, and aquatic invertebrates. Two case scenarios are presented using different mercury contributions from the sediment component (1 cm depth, and no sediment). Our results for the scenario including the sediment contribution show that lake sediments represent over 98% of the total mercury while the biotic components represent less than 0.1% of the same burden. For methylmercury, fish account for up to 5% of the burden, while sediments make up 84.6% to 93.1%. If we put aside the sediment contribution, the methylmercury in fish partitioning can represent up to 48%. As for invertebrates, they can account for up to 48% of the total MeHg burden. We do not observe any change in the partitionings or the quantities of Hg and MeHg before and after fishing in either of the two case scenarios even when we do not take into account dynamics of the ecosystems. This will be all the more the case when the dynamics of the system are included in the analyses. Therefore, biological parameters such as growth rates or fish diet must be considered.     

Bioaccumulation patterns of methyl mercury and essential fatty acids in lacustrine planktonic food webs and fish

Organisms of the planktonic food web convey essential nutrients as well as contaminants to animals at higher trophic levels. We measured concentrations of methyl mercury (MeHg) and essential fatty acids (EFAs, key nutrients for aquatic food webs) in four size categories of planktonic organisms - seston (10-64 microm), micro-(100-200 microm), meso-(200-500 microm), and macrozooplankton (>500 microm) - as well as total mercury (THg) and EFAs in rainbow trout (Oncorhynchus mykiss) in coastal lakes. We demonstrate that, in all lakes during this summer sampling, MeHg concentrations of planktonic organisms increase significantly with plankton size, independent of their taxonomic composition, and that their MeHg accumulation patterns predict significantly THg concentrations in rainbow trout (R2=0.71, p<0.05). However, concentrations of total EFAs do not follow this pattern. Total EFAs increased from seston to mesozooplankton but decreased in the largest zooplankton size fraction. Moreover, concentrations of individual EFA compounds in rainbow trout are consistently lower, with the exception of docosahexaenoic acid, than those in macrozooplankton. The continuous increase of MeHg concentrations in aquatic organisms, therefore, differs from patterns of EFA accumulation in zooplankton and fish. We interpret these contrasting accumulation patterns of MeHg and EFA compounds as the inability of aquatic organisms to regulate the assimilation of dietary MeHg, whereas the rate of EFA retention may be controlled to optimize their physiological performance. Therefore, we conclude that bioaccumulation patterns of Hg in these aquatic food webs are not controlled by lipid solubility and/or the retention of EFA compounds.     

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.