Isotope signatures of N2O in a mixed microbial population system: constraints on N2O producing pathways in wastewater treatment.

We present measurements of site preference (SP) and bulk (15)N/(14)N ratios (δ(15)N(bulk)(N2O)) of nitrous oxide (N(2)O) by quantum cascade laser absorption spectroscopy (QCLAS) as a powerful tool to investigate N(2)O production pathways in biological wastewater treatment. QCLAS enables high-precision N(2)O isotopomer analysis in real time. This allowed us to trace short-term fluctuations in SP and δ(15)N(bulk)(N2O) and, hence, microbial transformation pathways during individual batch experiments with activated sludge from a pilot-scale facility treating municipal wastewater. On the basis of previous work with microbial pure cultures, we demonstrate that N(2)O emitted during ammonia (NH(4)(+)) oxidation with a SP of -5.8 to 5.6 ‰ derives mostly from nitrite (NO(2)(-)) reduction (e.g., nitrifier denitrification), with a minor contribution from hydroxylamine (NH(2)OH) oxidation at the beginning of the experiments. SP of N(2)O produced under anoxic conditions was always positive (1.2 to 26.1 ‰), and SP values at the high end of this spectrum (24.9 to 26.1 ‰) are indicative of N(2)O reductase activity. The measured δ(15)N(bulk)(N2O) at the initiation of the NH(4)(+) oxidation experiments ranged between -42.3 and -57.6 ‰ (corresponding to a nitrogen isotope effect Δδ(15)N = δ(15)N(substrate) - δ(15)N(bulk)(N2O) of 43.5 to 58.8 ‰), which is considerably higher than under denitrifying conditions (δ(15)N(bulk)(N2O) 2.4 to -17 ‰; Δδ(15)N = 0.1 to 19.5 ‰). During the course of all NH(4)(+) oxidation and nitrate (NO(3)(-)) reduction experiments, δ(15)N(bulk)(N2O) increased significantly, indicating net (15)N enrichment in the dissolved inorganic nitrogen substrates (NH(4)(+), NO(3)(-)) and transfer into the N(2)O pool. The decrease in δ(15)N(bulk)(N2O) during NO(2)(-) and NH(2)OH oxidation experiments is best explained by inverse fractionation during the oxidation of NO(2)(-) to NO(3)(-).

Elimination of micropollutants during post-treatment of hospital wastewater with powdered activated carbon, ozone, and UV.

A pilot-scale hospital wastewater treatment plant consisting of a primary clarifier, membrane bioreactor, and five post-treatment technologies including ozone (O3), O3/H2O2, powdered activated carbon (PAC), and low pressure UV light with and without TiO2 was operated to test the elimination efficiencies for 56 micropollutants. The extent of the elimination of the selected micropollutants (pharmaceuticals, metabolites and industrial chemicals) was successfully correlated to physical-chemical properties or molecular structure. By mass loading, 95% of all measured micropollutants in the biologically treated hospital wastewater feeding the post-treatments consisted of iodinated contrast media (ICM). The elimination of ICM by the tested post-treatment technologies was 50-65% when using 1.08 g O3/gDOC, 23 mg/L PAC, or a UV dose of 2400 J/m(2) (254 nm). For the total load of analyzed pharmaceuticals and metabolites excluding ICM the elimination by ozonation, PAC, and UV at the same conditions was 90%, 86%, and 33%, respectively. Thus, the majority of analyzed substances can be efficiently eliminated by ozonation (which also provides disinfection) or PAC (which provides micropollutants removal, not only transformation). Some micropollutants recalcitrant to those two post-treatments, such as the ICM diatrizoate, can be substantially removed only by high doses of UV (96% at 7200 J/m(2)). The tested combined treatments (O3/H2O2 and UV/TiO2) did not improve the elimination compared to the single treatments (O3 and UV).

Hospital wastewater treatment by membrane bioreactor: performance and efficiency for organic micropollutant elimination.

A pilot-scale membrane bioreactor (MBR) was installed and operated for one year at a Swiss hospital. It was fed an influent directly from the hospital's sanitary collection system. To study the efficiency of micropollutant elimination in raw hospital wastewater that comprises a complex matrix with micropollutant concentrations ranging from low ng/L to low mg/L, an automated online SPE-HPLC-MS/MS analytical method was developed. Among the 68 target analytes were the following: 56 pharmaceuticals (antibiotics, antimycotics, antivirals, iodinated X-ray contrast media, antiinflamatory, cytostatics, diuretics, beta blockers, anesthetics, analgesics, antiepileptics, antidepressants, and others), 10 metabolites, and 2 corrosion inhibitors. The MBR influent contained the majority of those target analytes. The micropollutant elimination efficiency was assessed through continuous flow-proportional sampling of the MBR influent and continuous time-proportional sampling of the MBR effluent. An overall load elimination of all pharmaceuticals and metabolites in the MBR was 22%, as over 80% of the load was due to persistent iodinated contrast media. No inhibition by antibacterial agents or disinfectants from the hospital was observed in the MBR. The hospital wastewater was found to be a dynamic system in which conjugates of pharmaceuticals deconjugate and biological transformation products are formed, which in some cases are pharmaceuticals themselves.

Improving nutrient removal while reducing energy use at three Swiss WWTPs using advanced control.

Aeration consumes about 60% of the total energy use of a wastewater treatment plant (WWTP) and therefore is a major contributor to its carbon footprint. Introducing advanced process control can help plants to reduce their carbon footprint and at the same time improve effluent quality through making available unused capacity for denitrification, if the ammonia concentration is below a certain set-point. Monitoring and control concepts are cost-saving alternatives to the extension of reactor volume. However, they also involve the risk of violation of the effluent limits due to measuring errors, unsuitable control concepts or inadequate implementation of the monitoring and control system. Dynamic simulation is a suitable tool to analyze the plant and to design tailored measuring and control systems. During this work, extensive data collection, modeling and full-scale implementation of aeration control algorithms were carried out at three conventional activated sludge plants with fixed pre-denitrification and nitrification reactor zones. Full-scale energy savings in the range of 16-20% could be achieved together with an increase of total nitrogen removal of 40%.

Behavior of metallic silver nanoparticles in a pilot wastewater treatment plant.

We investigated the behavior of metallic silver nanoparticles (Ag-NP) in a pilot wastewater treatment plant (WWTP) fed with municipal wastewater. The treatment plant consisted of a nonaerated and an aerated tank and a secondary clarifier. The average hydraulic retention time including the secondary clarifier was 1 day and the sludge age was 14 days. Ag-NP were spiked into the nonaerated tank and samples were collected from the aerated tank and from the effluent. Ag concentrations determined by inductively coupled plasma-mass spectrometry (ICP-MS) were in good agreement with predictions based on mass balance considerations. Transmission electron microscopy (TEM) analyses confirmed that nanoscale Ag particles were sorbed to wastewater biosolids, both in the sludge and in the effluent. Freely dispersed nanoscale Ag particles were only observed in the effluent during the initial pulse spike. X-ray absorption spectroscopy (XAS) measurements indicated that most Ag in the sludge and in the effluent was present as Ag(2)S. Results from batch experiments suggested that Ag-NP transformation to Ag(2)S occured in the nonaerated tank within less than 2 h. Physical and chemical transformations of Ag-NP in WWTPs control the fate, the transport and also the toxicity and the bioavailability of Ag-NP and therefore must be considered in future risk assessments.

Combined nitritation-anammox: advances in understanding process stability.

Efficient nitrogen removal from wastewater containing high concentrations of ammonium but little organic substrate has recently been demonstrated by several full-scale applications of the combined nitritation-anammox process. While the process efficiency is in most cases very good, process instabilities have been observed to result in temporary process failures. In the current study, conditions resulting in instability and strategies to regain efficient operation were evaluated. First, data from full-scale operation is presented, showing a sudden partial loss of activity followed by recovery within less than 1 month. Results from laboratory-scale experiments indicate that these dynamics observed in full scale can be caused by partial inhibition of the ammonia oxidizing bacteria (AOB), while anammox inhibition is a secondary effect due to temporarily reduced O(2) depletion. Complete anammox inhibition is observed at 0.2 mg O(2) · L(-1), resulting in NO(2)(-) accumulation. However, this inhibition of anammox is reversible within minutes after O(2) depletion. Thus, variable AOB activity was identified as the key to reactor stability. With appropriate interpretation of the online NH(4)(+) signal, accumulation of NO(2)(-) can be detected indirectly and used to signal an imbalance of O(2) supply and AOB activity (no suitable online NO(2)(-) electrode is currently available). Second, increased abundance of nitrite-oxidizing bacteria (NOB; competing with anammox for NO(2)(-)) is known as another cause of instability. Based on a comparison of parallel full-scale reactors, it is suggested that an infrequent and short-term increased O(2) supply (e.g., for maintenance of aerators) that exceeds prompt depletion of oxygen by AOB may have caused increased NOB abundance. The volumetric air supply as a proxy for O(2) supply thus needs to be linked to AOB activity. Further, NOB can be washed out of the system during regular operation if the system is operated at a sludge age in the range of 45 days and by controlling the air supply according to the NO(3)(-) concentration in the treated effluent. Early detection of growing NOB abundance while the population is still low can help guide process operation and it is suggested that molecular methods of quantifying NOB abundance should be tested.

N2O emissions: modeling the effect of process configuration and diurnal loading patterns.

The objective of this research was to develop a mechanistic model for quantifying N2O emissions from activated sludge plants and demonstrate how this may be used to evaluate the effects of process configuration and diurnal loading patterns. The model describes the mechanistic link between the factors recognized to correlate positively with N2O emissions. The primary factors are the presence of ammonia and nitrite accumulation. Low dissolved oxygen concentrations also may be implicated through differential impacts on ammonia-oxidizing bacteria (AOB) versus nitrite-oxidizing bacteria (NOB) activity. Factors promoting N2O emissions at treatment plants are discussed below. The model was applied to data from laboratory and pilot-scale systems. From a practical standpoint, plant configuration (e.g., plug-flow versus complete-mix), influent loading patterns (and peak load), and certain operating strategies (e.g., handling of return streams) are all important in determining N2O emissions.

Data reconciliation for wastewater treatment plant simulation studies-planning for high-quality data and typical sources of errors.

Model results are only as good as the data fed as input or used for calibration. Data reconciliation for wastewater treatment modeling is a demanding task, and standardized approaches are lacking. This paper suggests a procedure to obtain high-quality data sets for model-based studies. The proposed approach starts with the collection of existing historical data, followed by the planning of additional measurements for reliability checks, a data reconciliation step, and it ends with an intensive measuring campaign. With the suggested method, it should be possible to detect, isolate, and finally identify systematic measurement errors leading to verified and qualitative data sets. To allow mass balances to be calculated or other reliability checks to be applied, few additional measurements must be introduced in addition to routine measurements. The intensive measurement campaign should be started only after all mass balances applied to the historical data are closed or the faults have been detected, isolated, and identified. In addition to the procedure itself, an overview of typical sources of errors is given.

N2O emission in full-scale wastewater treatment: Proposing a refined monitoring strategy.

Nitrous oxide (N2O) emissions from wastewater treatment contribute significantly to greenhouse gas emissions. They have been shown to exhibit a strong seasonal and daily profile in previously conducted monitoring campaigns. However, only two year-long online monitoring campaigns have been published to date. Based on three monitoring campaigns on three full-scale wastewater treatment plants (WWTPs) with different activated sludge configurations, each of which lasted at least one year, we propose a refined monitoring strategy for long-term emission monitoring with multiple flux chambers on open tanks. Our monitoring campaigns confirm that the N2O emissions exhibited a strong seasonal profile and were substantial on all three plants (1-2.4% of the total nitrogen load). These results confirm that N2O is the most important greenhouse gas emission from wastewater treatment. The temporal variation was more distinct than the spatial variation within aeration tanks. Nevertheless, multiple monitoring spots along a single lane are crucial to assess representative emission factors in flow-through systems. Sequencing batch reactor systems were shown to exhibit comparable emissions within one reactor but significant variation between parallel reactors. The results indicate that considerable emission differences between lanes are to be expected in cases of inhomogeneous loading and discontinuous feeding. For example, N2O emission could be shown to depend on the amount of treated reject water: lanes without emitted <1% of the influent load, while parallel lanes emitted around 3%. In case of inhomogeneous loading, monitoring of multiple lanes is required. Our study enables robust planning of monitoring campaigns on WWTPs with open tanks. Extensive full-scale emission monitoring campaigns are important as a basis for reliable decisions about reducing the climate impact of wastewater treatment. More specifically, such data sets help us to define general emission factors for wastewater treatment plants and to construct and critically evaluate N2O emission models.

Extension of ASM3 for two-step nitrification and denitrification and its calibration and validation with batch tests and pilot scale data.

Although traditionally not taken into account by most of activated sludge models the production of nitrite as an intermediate of the nitrification-denitrification processes becomes of interest in some specific plant operational situations or in case of high sensitivity of the receiving ecosystems. The Activated Sludge Model No.3 (ASM3) was therefore extended for two-step nitrification and two-step denitrification in order to better describe nitrite dynamics especially during the treatment of communal wastewater. Nitrite was included as a new model compound and as an intermediate product of biological processes, both for heterotrophic and autotrophic bacteria. Two new model compounds replace X(A), the original autotrophic biomass: Ammonium Oxidizing Bacteria, X(AOB) and Nitrite Oxidizing Bacteria, X(NOB). Growth and decay processes of nitrifiers were split into AOB and NOB processes (3 additional processes) and heterotrophic anoxic processes were also doubled in order to account for two-step denitrification (4 additional processes). Default values from literature as well as laboratory measurements were considered for the choice of kinetic and stoichiometric parameters. The model was calibrated and validated with laboratory scale tests in batch reactors and with data from an Eawag activated sludge pilot plant configured conventionally with nitrification and pre-denitrification for the treatment of communal wastewater.

Fate of beta blockers and psycho-active drugs in conventional wastewater treatment.

The removal of beta blockers and psycho-active drugs was investigated in a representative conventional German WWTP by long-term measurement campaigns along different biological treatment processes. The activated sludge treatment with an elevated SRT of 18 d was the only process which led to a significant removal of certain beta blockers and psycho-active drugs. The removal efficiency was below 60% for all compounds except for the natural opium alkaloids codeine and morphine being removed by more than 80%. Primary biological transformation and sorption onto sludge as the main removal mechanisms were examined in lab-scale batch experiments. Sorption onto activated sludge was found to be negligible (<3%). The biological transformation could be described by pseudo-first order kinetics and the transformation constants k(biol) were used to predict the removal of beta blockers and psycho-active drugs in an activated sludge unit with a model. For most compounds the removal efficiencies measured on the full-scale WWTP were within the 95% confidence intervals predicted by the model. The results from full-scale measurements and modeling indicate that biological transformation in the nitrification tank together with parameters such as the sludge retention time and the temperature is crucial regarding the biological transformation of beta blockers and psycho-active drugs in conventional WWTPs.

The fate of selected micropollutants in a single-house MBR.

Membrane bioreactor (MBR) technology is an interesting option for single-house wastewater treatment or small communities. Because typically a very high effluent quality is achieved with respect to pathogens, suspended solids, organics and nitrogen, the permeate is well suited for reuse. Little is known about the fate of micropollutants in such small systems. The differences between centralized and decentralized biological wastewater treatment with respect to micropollutants are manifold: besides the operational parameters like hydraulic and sludge retention time, the main difference is in the load variation. While the influent load is expected to be more or less constant in large catchments, it varies strongly in small MBRs due to irregular consumption (e.g. of medication by individuals). Concentrations of micropollutants are higher by a factor 50-1000 than in centralized treatment. It is also unknown how reliable degradation of micropollutants is in case of irregular exposure. In this study, two experiments were conducted in a small MBR treating the wastewater of a three-person household. During normal operation of the treatment plant, 25 pharmaceuticals (antibiotics, antiphlogistics, lipid regulators, iodinated contrast media and hormones) that had not been used by members of the household were added in concentrations typical for municipal wastewater. The removal of most substances was in the same range as for centralized wastewater treatment. It was shown that biological transformation was the main elimination process while adsorption to the activated sludge was negligible for most substances due to the low sludge production at high sludge retention time. No appreciable lag for inducing biological degradation was observed. The high hydraulic and sludge residence time had a positive effect on the elimination of slowly degradable substances, but this was partly compensated by the lower biological activity. An experiment with antibiotics concentrations typical for decentralized treatment (between 500 and 1000 microg l(-1); sulfamethoxazole, sulfapyridine, trimethoprim, clarithromycin, roxithromycin) did not show an inhibitory effect on either nitrification or denitrification.

Assessing wastewater dilution in small rivers with high resolution conductivity probes.

Installation and maintenance of flow gauging stations to assess wastewater dilution in small creeks is expensive. The method outlined in this paper provides a flexible and cheap alternative for situations with dilution factors smaller than 10 and unambiguous flow direction. It is shown that conductivity profiles from three sampling locations enable accurate identification of the dilution factor with an uncertainty of +/-10-30%. Furthermore, much insight can be gained on the temporal behavior of both, the receiving water and the wastewater treatment system by combined analysis of conductivity and temperature data. In this case study the data also enabled identification and tracking down of illegal polluters.

Reducing the natural color of membrane bioreactor permeate with activated carbon or ozone.

The suitability of two membrane bioreactors for on-site wastewater treatment and reuse in Switzerland was investigated. The treated wastewater was used for toilet flushing and gardening, with water recycling rates of 30% (single family house) and almost 100% (toilets in a cable car station) respectively. Due to the recycling, an increase in a natural yellowish-brown color was observed, leading to double flushing of the toilets, higher cleaning requirements and increased permeate production. Color removal with ozone, powdered (PAC) and granulated (GAC) activated carbon was assessed in laboratory and field experiments. PAC was added directly into the MBR, whereas ozonation and GAC were applied to the permeate. The dosage of ozone or activated carbon depended on the recycling rate and color intensity. If color removal is necessary, PAC is the option best suited to small treatment plants, with a requirement of 30-50 g m(-3) for 30% and 100 g m(-3) for 100% water recycling.

Biological nutrient removal in a small-scale MBR treating household wastewater.

The biological nutrient-removal potential of an on-site Membrane bioreactor (MBR) located in the basement of a four-person house treating domestic wastewater was investigated. The reactor consists of two tanks in series. This treatment plant differs from other conventional MBRs by a highly fluctuating influent water flow and a lack of pretreatment. During the first period, the first reactor was operated as a primary clarifier, resulting in nitrogen and phosphorus removals of 50% and 25%, respectively. Primary sludge production and bad odors in the basement were further disadvantages. When using the first reactor as an anaerobic/anoxic reactor by recycling activated sludge and mixing the first reactor, nitrogen and phosphorus removals of over 90% and 70% were achieved, respectively. By applying a dynamic model of the plant, the return sludge ratio was identified as the most important parameter. With a return sludge ratio of about 1.2, optimal PAO growth and phosphorous removal up to 90% was reached. Since only activated sludge is produced with this operational mode, on-site sludge dewatering is possible. During vacation periods without loading, the Bio-P activity is kept constant if the aeration is reduced to 5-20 min d(-1).

Modelling nitrite in wastewater treatment systems: a discussion of different modelling concepts.

Originally presented at the 1st IWA/WEF Wastewater Treatment Modelling Seminar (WWTmod 2008), this contribution has been updated to also include the valuable feedback that was received during the Modelling Seminar. This paper addresses a number of basic issues concerning the modelling of nitrite in key processes involved in biological wastewater water treatment. To this end, we review different model concepts (together with model structures and corresponding parameter sets) proposed for processes such as two-step nitrification/denitrification, anaerobic ammonium oxidation and phosphorus uptake processes. After critically discussing these models with respect to their assumptions and parameter sets, common points of agreement as well as disagreement were elucidated. From this discussion a general picture of the state-of-the-art in the modelling of nitrite is provided. Taking this into account, a number of recommendations are provided to focus further research and development on nitrite modelling in biological wastewater treatment.

Evaluation of a full-scale wastewater treatment plant upgraded with ozonation and biological post-treatments: Abatement of micropollutants, formation of transformation products and oxidation by-products.

To protect the ecosystem and drinking water resources in Switzerland and in the countries of the downstream catchments, a new Swiss water protection act entered into force in 2016 aiming to reduce the discharge of micropollutants from wastewater treatment plants (WWTPs). As a consequence, selected WWTPs must be upgraded by an advanced treatment for micropollutant abatement with suitable and economic options such as (powdered) activated carbon treatment or ozonation. WWTP Neugut (105'000 people equivalent) was the first WWTP in Switzerland to implement a long-term full-scale ozonation. Differing specific ozone doses in the range of 0.35-0.97 g O3/g DOC were applied to determine the adequate ozone dose to fulfill the requirements of the Swiss water protection act. Based on this assessment, a specific ozone dose of 0.55 g O3/g DOC is recommended at this plant to ensure an average abatement of the twelve selected indicator substances by ≥80% over the whole treatment. A monitoring of 550 substances confirmed that this dose was very efficient to abate a broad range of micropollutants by >79% on average. After ozonation, an additional biological post-treatment is required to eliminate possible negative ecotoxicological effects generated during ozonation caused by biodegradable ozonation transformation products (OTPs) and oxidation by-products (OBPs). Three biological treatments (sand filtration, moving bed, fixed bed) and granular activated carbon (GAC, fresh and pre-loaded) filtration were evaluated as post-treatments after ozonation. In parallel, a fresh GAC filter directly connected to the effluent of the secondary clarifier was assessed. Among the three purely biological post-treatments, the sand filtration performed best in terms of removal of dissolved organic carbon (DOC), assimilable organic carbon (AOC) and total suspended solids (TSS). The fresh activated carbon filtration ensured a significant additional micropollutants abatement after ozonation due to sorption. The relative abatement of the indicator substances ranged between 20 and 89% after 27'000 bed volumes (BV) and was still substantial at 50'000 BV. In an identical GAC filter running in parallel and being fed with the effluent of the secondary clarifier, the elimination was less efficient. Seven primary OTPs (chlorothiazide and six N-oxides) formed during ozonation could be quantified thanks to available reference standards. Their concentration decreased with increasing specific ozone doses with the concomitant formation of other OTPs. The seven OTPs were found to be stable compounds and were not abated in the biological post-treatments. They were sorbed in the fresh GAC filter, but less efficiently than the corresponding parent compounds. Two OBPs, bromate (BrO3-) and N-nitrosodimethylamine (NDMA), were formed during ozonation but did not exceeded 5 µg/L for bromate and 30 ng/L for NDMA at the recommended specific ozone dose of 0.55 g O3/g DOC. NDMA was well abated in all post-treatments (minimum 41% during fixed bed filtration, maximum 83% during fresh GAC filtration), while bromate was very stable as expected.

Ozonation reduces sludge production and improves denitrification.

The effectiveness of partial ozonation of return activated sludge for enhancing denitrification and waste sludge minimization were examined. A pair of nitrifying sequencing batch reactors was operated in either aerobic or alternating anoxic/aerobic conditions, with one control and one ozonated reactor in each set. The amount of solids produced decreased with the ozone dose. Biomass in the anoxic/aerobic reactor was easier to destroy (up to 25% of the initial excess sludge) than in the aerobic (10%) one, generating approximately twice as much soluble COD by cell lysis. Denitrification rate improved up to 60% due to additional carbon released by ozonation. Nitrification rates deteriorated much more in the aerobic than in the alternating reactor, possibly as a result of direct destruction of nitrifying autotrophs as well as competition created by growth of heterotrophs receiving the additional COD. Overall, ozonation provided the expected benefits in denitrification and had less impact on nitrification in the alternating reactors.

Fate of sulfonamides, macrolides, and trimethoprim in different wastewater treatment technologies.

The elimination of sulfonamides, macrolides and trimethoprim from raw wastewater was investigated in several municipal wastewater treatment plants. Primary treatment provided no significant elimination for the investigated substances. Similar eliminations were observed in the secondary treatment of two conventional activated sludge (CAS) systems and a fixed-bed reactor (FBR). Sulfamethoxazole, including the fraction present as N4-acetyl-sulfamethoxazole, was eliminated by approximately 60% in comparison to about 80% in a membrane bioreactor (MBR) independently of the solid retention time (SRT), indicating a positive correlation of the observed elimination to the organic substrate concentration. The elimination for macrolides and trimethoprim varied significantly between the different sampling campaigns in the two CAS systems and in the FBR. In the MBR, these analytes were eliminated up to 50% at SRT of 16+/-2 and 33+/-3 d. Trimethoprim, clarithromycin and dehydro-erythromycin showed a higher elimination of up to 90% at a SRT of 60-80 d indicating a correlation with reduced substrate loading (SL). Together with the high SRT, the SL may lead to an increased biodiversity of the active biomass, resulting in a broader range of degradation pathways available. Two investigated sand filters showed different elimination behavior. One led to a significant elimination of most macrolides (17-23%) and trimethoprim (74+/-14%), while no elimination was observed in the other sand filter investigated.