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.     

Measurement and interpretation of microbial adenosine tri-phosphate (ATP) in aquatic environments

There is a widespread need for cultivation-free methods to quantify viability of natural microbial communities in aquatic environments. Adenosine tri-phosphate (ATP) is the energy currency of all living cells, and therefore a useful indicator of viability. A luminescence-based ATP kit/protocol was optimised in order to detect ATP concentrations as low as 0.0001 nM with a standard deviation of <5%. Using this method, more than 100 water samples from a variety of aquatic environments (drinking water, groundwater, bottled water, river water, lake water and wastewater effluent) were analysed for extracellular ATP and microbial ATP in comparison with flow-cytometric (FCM) parameters. Microbial ATP concentrations ranged between 3% and 97% of total ATP concentrations, and correlated well (R² = 0.8) with the concentrations of intact microbial cells (after staining with propidium iodide). From this correlation, we calculated an average ATP-per-cell value of 1.75 × 10⁻¹⁰ nmol/cell. An even better correlation (R² = 0.88) was observed between intact biovolume (derived from FCM scatter data) and microbial ATP concentrations, and an average ATP-per-biovolume value of 2.95 × 10⁻⁹ nmol/μm³ was calculated. These results support the use of ATP analysis for both routine monitoring and research purposes, and contribute towards a better interpretation of ATP data.     

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.     

Economical and ecological comparison of granular activated carbon (GAC) adsorber refill strategies

Technical constraints can leave a considerable freedom in the design of a technology, production or service strategy. Choosing between economical or ecological decision criteria then characteristically leads to controversial solutions of ideal systems. For the adaptation of granular-activated carbon (GAC) fixed beds, various technical factors determine the adsorber volume required to achieve a desired service life. In considering carbon replacement and recycling, a variety of refill strategies are available that differ in terms of refill interval, respective adsorber volume, and time-dependent use of virgin, as well as recycled GAC. Focusing on the treatment of contaminant groundwater, we compare cost-optimal reactor configurations and refill strategies to the ecologically best alternatives. Costs and consumption of GAC are quantified within a technical-economical framework. The emissions from GAC production out of hard coal, transport and recycling are equally derived through a life cycle impact assessment. It is shown how high discount rates lead to a preference of small fixed-bed volumes, and accordingly, a high number of refills. For fixed discount rates, the investigation reveals that both the economical as well as ecological assessment of refill strategies are especially sensitive to the relative valuation of virgin and recycled GAC. Since recycling results in economic and ecological benefits, optimized systems thus may differ only slightly.     

Discriminating and assessing adsorption and biodegradation removal mechanisms during granular activated carbon filtration of microcystin toxins

Microcystins are cyanobacterial toxins that are problematic for water authorities due to their resistance to conventional water treatment. Granular activated carbon (GAC) filtration has been shown to be effective in removing microcystin from water using both adsorption and biodegradation removal mechanisms; however, little is known regarding which removal mechanism predominates and to what extent. In this study, microcystin removal due to adsorption and biodegradation in GAC filtration were discriminated and assessed by commissioning three parallel laboratory columns, including a sterile GAC column, a conventional GAC column and a sand column. The results demonstrate that biodegradation is an efficient removal mechanism once it commences and that the rate of biodegradation was dependent upon temperature and initial bacterial concentration. Adsorption of microcystins was prevalent during the initial stages of the GAC columns and was modelled using the homogeneous surface diffusion model (HSDM). The HSDM provided evidence that an active biofilm present on the surface of the conventional GAC hindered adsorption of microcystin compared with the sterile GAC with no active biofilm. Up to 70% removal of microcystin-LR was still observed after 6 months of operation of the sterile GAC column, indicating that adsorption still played a vital role in the removal of this toxin.     

Modeling the behaviors of adsorption and biodegradation in biological activated carbon filters

This investigation developed a non-steady-state numerical model to differentiate the adsorption and biodegradation quantities of a biological activated carbon (BAC) column. The mechanisms considered in this model are adsorption, biodegradation, convection and diffusion. Simulations were performed to evaluate the effects of the major parameters, the packing media size and the superficial velocity, on the adsorption and biodegradation performances for the removal of dissolved organic carbon based on dimensionless analysis. The model predictions are in agreement with the experimental data by adjusting the liquid-film mass transfer coefficient (k(bf)), which has high correlation with the Stanton number. The Freundlich isotherm constant (N(F)), together with the maximum specific substrate utilization rate (k(f)) and the diffusion coefficient (D(f)), is the most sensitive variable affecting the performance of the BAC. Decreasing the particle size results in more substrate diffusing across the biofilm, and increases the ratio of adsorption rather than biodegradation.     

The removal of perchlorate from groundwater by activated carbon tailored with cationic surfactants

In rapid small-scale column tests, cationic surfactant-tailored activated carbons (ACs) effectively removed perchlorate to below detection levels for up to 30 times longer than virgin AC. By pre-loading bituminous AC with dicocodimethylammonium chloride, tallowtrimethylammonium chloride, cetyltrimethylammonium chloride, or cetylpyridinium chloride, 75 ppb perchlorate was removed for 27,000-35,000 bed volumes before the effluent perchlorate rose above 1 ppb. These tests employed a natural groundwater that also contained 30 mg/L sulfate, 26 mg/L nitrate (as NO3-), and other ions. By the time of 25 ppb perchlorate breakthrough, 7.3-10.1% of quaternary ammonium sites had perchlorate associated with them. Although some of the surfactants leached out of the tailored carbon beds (0.6-21.2% of the amount loaded), the leached surfactant could be removed to below detectable limits with a virgin AC polishing bed that chased the tailored bed.     

Characterization of natural organic matter adsorption in granular activated carbon adsorbers

The removal of natural organic matter (NOM) from lake water was studied in two pilot-scale adsorbers containing granular activated carbon (GAC) with different physical properties. To study the adsorption behavior of individual NOM fractions as a function of time and adsorber depth, NOM was fractionated by size exclusion chromatography (SEC) into biopolymers, humics, building blocks, and low molecular weight (LMW) organics, and NOM fractions were quantified by both ultraviolet and organic carbon detectors. High molecular weight biopolymers were not retained in the two adsorbers. In contrast, humic substances, building blocks and LMW organics were initially well and irreversibly removed, and their effluent concentrations increased gradually in the outlet of the adsorbers until a pseudo-steady state concentration was reached. Poor removal of biopolymers was likely a result of their comparatively large size that prevented access to the internal pore structure of the GACs. In both GAC adsorbers, adsorbability of the remaining NOM fractions, compared on the basis of partition coefficients, increased with decreasing molecular size, suggesting that increasingly larger portions of the internal GAC surface area could be accessed as the size of NOM decreased. Overall DOC uptake at pseudo-steady state differed between the two tested GACs (18.9 and 28.6 g-C/kg GAC), and the percent difference in DOC uptake closely matched the percent difference in the volume of pores with widths in the 1-50 nm range that was measured for the two fresh GACs. Despite the differences in NOM uptake capacity, individual NOM fractions were removed in similar proportions by the two GACs.     

Characterization and treatability of aerobic bacterial thermophilically treated wastewater by a conventional activated sludge and granular activated carbon

An industrial wastewater that was pretreated by an aerobic thermophilic bacterial consortium (THE) was subjected to additional treatability studies by granular activated carbon (GAC) and a conventional activated sludge (CAS). The removal of dissolved organic carbon (DOC) in both systems was generally found to be similar. While GAC was able to attain better effluent concentrations of toluene and methyl isobutyl ketone (MIBK), the CAS was much more efficient at removing acetone. Furthermore, unlike the GAC, the performance of the CAS was not influenced by the high degree of variability in the influent wastewater. Characterization of the influent thermophilic wastewater using gas chromatography-mass spectroscopy (GC/MS) was performed to quantify the micropollutants as well as to evaluate removal efficiencies from the GAC and CAS systems.     

Regeneration of granular activated carbon with adsorbed trichloroethylene using wet peroxide oxidation

The objective of this study is to clarify the regeneration of granular activated carbon (GAC) adsorbed trichloroethylene (TCE) using wet peroxide oxidation (WPO). TCE and TOC concentrations decreased during WPO, whereas Cl(-) accumulated in water indicating that TCE was not only decomposed but was also mineralized to Cl(-) and CO(2) using WPO. Regeneration efficiencies (q/q(0)) of GAC regenerated at 150, 165 and 180 degrees C (initial pH 4) were 0.36, 0.45, 0.48, respectively. In addition, regeneration efficiencies of GAC regenerated in the solution of various initial pH (2.5, 3.0, 4.0) at 180 degrees C were 0.71, 0.60, 0.48, respectively. These results suggest that regeneration of GAC is more effective at higher reaction temperature and lower initial pH of the solution. In the repeated regeneration of GAC, the adsorption capacity of GAC for TCE gradually decreased and regeneration efficiency of the regenerated GAC at sixth step was 0.40. The adsorption capacity loss of regenerated GAC is probably due to oxidation of GAC during WPO.     

Removal of MIB and geosmin using granular activated carbon with and without MIEX pre-treatment

This study assessed the impact of MIEX pre-treatment, followed by either coagulation or microfiltration (MF), on the effectiveness of pilot granular activated carbon (GAC) filters for the removal of the taste and odour compounds, 2-methylisoborneol (MIB) and geosmin, from a surface drinking water source over a 2-year period. Complete removal of MIB and geosmin was achieved by all GAC filters for the first 10 months, suggesting that the available adsorption capacity was sufficient to compensate for differences in dissolved organic carbon (DOC) entering the GAC filters.Reduction of empty bed contact time (EBCT), in all but one GAC filter, resulted in breakthrough of spiked MIB and geosmin, with initial results inconclusive regarding the impact of MIEX pre-treatment. MIB and geosmin removal increased over the ensuing 12 months until complete removal of both MIB and geosmin was again achieved in all but one GAC filter, which had been pre-chlorinated. Autoclaving and washing the GAC filters had minimal impact on geosmin removal but reduced MIB removal by 30% in all but the pre-chlorinated filter, confirming that biodegradation impacted MIB removal. The impact of biodegradation was greater than any impact on GAC adsorption arising from DOC differences due to MIEX pre-treatment. It is not clear whether, at a lower initial EBCT, MIEX pre-treatment may have impacted on the adsorption capacity of the virgin GAC.The GAC filter maintained at the longer EBCT, which was also pre-chlorinated, completely removed MIB and geosmin for the period of the study, suggesting that the greater adsorption capacity was compensating for any decrease in biological degradation.     

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.     

High loading denitrification by biological activated carbon process

High loading denitrification was studied using granular activated carbon (Calgon Filtrasorb 400, size: 0.8–1.4 mm) column with injecting carbon source (sucrose) only once a day. Under the condition of EBCT (empty bed contact time) = 80 min, C:N RATIO = 1.88, once per day injection mode of organic supply was able to sustain an average denitrification efficiency of 84 to 89% even with influent NO₃---N concentration of 80 mgl⁻¹. With an influent NO₃---N of 20 mgl⁻¹ and C:N ratio of 1.88, however, reduction of EBCT to 20 min resulted in very poor denitrification. In the latter case, 46% of the added carbon was lost in the effluent immediately after the injection. Short EBCT critically affected the process mainly due to insufficient adsorption rate. Microbial denitrification capability and fermentation might also limit the process. Extended organic injection is a possible option to improve the process efficiency. Occurrence of the sulfate reduction was limited in early phase of the cycle.     

Predominance of ammonia-oxidizing archaea on granular activated carbon used in a full-scale advanced drinking water treatment plant

Ozonation followed by granular activated carbon (GAC) is one of the advanced drinking water treatments. During GAC treatment, ammonia can be oxidized by ammonia-oxidizing microorganisms associated with GAC. However, there is little information on the abundance and diversity of ammonia-oxidizing microorganisms on GAC. In this study, the nitrification activity of GAC and the settlement of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in GAC were monitored at a new full-scale advanced drinking water treatment plant in Japan for 1 year after plant start-up. Prechlorination was implemented at the receiving well for the first 10 months of operation to treat ammonia in raw water. During this prechlorination period, levels of both AOA and AOB associated with GAC were below the quantification limit. After prechlorination was stopped, 10⁵ copies g-dry⁻¹ of AOA amoA genes were detected within 3 weeks and the quantities ultimately reached 10⁶-10⁷ copies g-dry⁻¹, while levels of AOB amoA genes still remained below the quantification limit. This observation indicates that AOA can settle in GAC rapidly without prechlorination. The nitrification activity of GAC increased concurrently with the settlement of AOA after prechlorination was stopped. Estimation of in situ cell-specific ammonia-oxidation activity for AOA on the assumption that only AOA and AOB determined can contribute to nitrification suggests that AOA may account for most of the ammonia-oxidation. However, further validation on AOB contribution is required.     

Bioaugmented membrane bioreactor (MBR) with a GAC-packed zone for high rate textile wastewater treatment

The long-term performance of a bioaugmented membrane bioreactor (MBR) containing a GAC-packed anaerobic zone for treatment of textile wastewater containing structurally different azo dyes was observed. A unique feeding strategy, consistent with the mode of evolution of separate waste streams in textile plants, was adopted to make the best use of the GAC-zone for dye removal. Dye was introduced through the GAC-zone while the rest of the colorless media was simultaneously fed through the aerobic zone. Preliminary experiments confirmed the importance of coupling the GAC-amended anaerobic zone to the aerobic MBR and also evidenced the efficacy of the adopted feeding strategy. Following this, the robustness of the process under gradually increasing dye-loading was tested. The respective average dye concentrations (mg/L) in the sample from GAC-zone and the membrane-permeate under dye-loadings of 0.1 and 1 g/L.d were as follows: GAC-zone (3, 105), permeate (0, 5). TOC concentration in membrane-permeate for the aforementioned loadings were 3 and 54 mg/L, respectively. Stable decoloration along with significant TOC removal during a period of over 7 months under extremely high dye-loadings demonstrated the superiority of the proposed hybrid process.     

Removing low ppb level perchlorate, RDX, and HMX from groundwater with cetyltrimethylammonium chloride (CTAC) pre-loaded activated carbon

Perchlorate contaminates vast amounts of groundwater throughout the United States which could potentially be used as potable water. Activated carbon pre-loaded with cetyltrimethylammonium chloride has been shown in this research to be an effective adsorbent for removing perchlorate from three low conductivity (50-66 microS/cm) groundwaters containing perchlorate (ClO(4)(-)) concentrations of 0.85, 1.0, and 5.6 parts per billion (ppb), respectively. In rapid small-scale column tests (RSSCTs), the virgin granular activated carbon (GAC) (used as a control) treated between 20,000 and 40,000 bed volumes (BV) of water. In contrast, the activated carbon that was pre-loaded with CTAC processed 170,000-270,000 BV before perchlorate was detected above 0.25 ppb in the effluent. Though this pre-loading significantly increased the capacity for perchlorate, it also diminished the GAC's capacity to remove organics. The groundwater containing 1 ppb ClO(4)(-) also contained the nitro-organics HMX (0.6 ppb) and RDX (5.5-6.6 ppb). RDX was detected in the effluent from the CTAC-pre-loaded bed after only 8000 BV had been processed whereas 308,000 BV could be processed through the virgin bed before RDX was detected. Likewise, HMX breakthrough was observed after 116,000 BV in the CTAC-pre-loaded bed while the virgin RSSCT exhibited no breakthrough of HMX during a test that was operated for 309,000 BV. However, by combining a CTAC-pre-loaded bed followed by a virgin GAC bed in series, both perchlorate and RDX could be removed for the same length of time.     

Degradation of p-nitrophenol in aqueous solution by microwave assisted oxidation process through a granular activated carbon fixed bed

A microwave (MW) assisted oxidation process was investigated for degradation of p-nitrophenol (PNP) from aqueous solution. The process consisted of a granular activated carbon (GAC) fixed bed reactor, a MW source, solution and air supply system, and a heat exchanger. The process was operated in continuous flow mode. Air was applied for oxygen supply. GAC acted as a MW energy absorption material as well as the catalyst for PNP degradation. MW power, air flow, GAC dose, and influent flow proved to be major factors which influenced PNP degradation. The results showed that PNP was degraded effectively by this new process. Under a given condition (PNP concentration 1330mg/L, MW power 500 W, influent flow 6.4 mL/min, air flow 100 mL/min), PNP removed 90%, corresponding to 80% of TOC removal. The pathway of PNP degradation was deduced based on GC-MS identification of course products. PNP experienced sequential oxidation steps and mineralized ultimately. Nitro-group of PNP converted to nitrite and nitrate. Biodegradability of the solution was improved apparently after treatment by MW assisted oxidation process, which benefit to further treatment of the solution using biochemical method.     

Assessing granular media filtration for the removal of chemical contaminants from wastewater

Granular media filtration was evaluated for the removal of a suite of chemical contaminants that can be found in wastewater. Laboratory- and pilot-scale sand and granular activated carbon (GAC) filters were trialled for their ability to remove atrazine, estrone (E1), 17α-ethynylestradiol (EE2), N-nitrosodimethylamine (NDMA), N-nitrosomorpholine (NMOR) and N-nitrosodiethylamine (NDEA). In general, sand filtration was ineffective in removing the contaminants from a tertiary treated wastewater, with the exception of E1 and EE2, where efficient removals were observed after approximately 150 d. Batch degradation experiments confirmed that the removal of E1 was through biological activity, with a pseudo-first-order degradation rate constant of 7.4 × 10⁻³ h⁻¹. GAC filtration was initially able to effectively remove all contaminants; although removals decreased over time due to competition with other organics present in the water. The only exception was atrazine where removal remained consistently high throughout the experiment. Previously unreported differences were observed in the adsorption of the three nitrosamines, with the ease of removal following the trend, NDEA > NMOR > NDMA, consistent with their hydrophobic character. In most instances the removals from the pilot-scale filters were generally in agreement with the laboratory-scale filter, suggesting that there is potential in using laboratory-scale filters as monitoring tools to evaluate the performance of pilot- and possibly full-scale sand and GAC filters at wastewater treatment plants.     

Evaluation of autotrophic growth of ammonia-oxidizers associated with granular activated carbon used for drinking water purification by DNA-stable isotope probing

Nitrification is an important biological function of granular activated carbon (GAC) used in advanced drinking water purification processes. Newly discovered ammonia-oxidizing archaea (AOA) have challenged the traditional understanding of ammonia oxidation, which considered ammonia-oxidizing bacteria (AOB) as the sole ammonia-oxidizers. Previous studies demonstrated the predominance of AOA on GAC, but the contributions of AOA and AOB to ammonia oxidation remain unclear. In the present study, DNA-stable isotope probing (DNA-SIP) was used to investigate the autotrophic growth of AOA and AOB associated with GAC at two different ammonium concentrations (0.14 mg N/L and 1.4 mg N/L). GAC samples collected from three full-scale drinking water purification plants in Tokyo, Japan, had different abundance of AOA and AOB. These samples were fed continuously with ammonium and ¹³C-bicarbonate for 14 days. The DNA-SIP analysis demonstrated that only AOA assimilated ¹³C-bicarbonate at low ammonium concentration, whereas AOA and AOB exhibited autotrophic growth at high ammonium concentration. This indicates that a lower ammonium concentration is preferable for AOA growth. Since AOA could not grow without ammonium, their autotrophic growth was coupled with ammonia oxidation. Overall, our results point towards an important role of AOA in nitrification in GAC filters treating low concentration of ammonium.     

Characterization of tributyltin in shipyard waters and removal through laboratory and full-scale treatment

Characterization and treatment studies were conducted in an effort to evaluate treatment options capable of removing tributyltin (TBT) in shipyard waters from above 1,000,000 ng/L to effluent concentrations below 50 ng/L. Laboratory studies and operation of a full-scale treatment plant were used to examine treatment options for TBT removal and included physicochemical treatment processes of coagulation-clarification, filtration, and granular activated carbon (GAC) adsorption. Significant variability was observed in TBT-containing shipyard waters (generated from different ships) in terms of their particulate solids, conductivity, and TBT and dissolved organic carbon concentrations. Laboratory tests with aluminum sulfate and ferric sulfate showed that on average 90% of TBT in shipyard waters could be removed by coagulation-flocculation-clarification under optimum conditions. No statistically significant difference was found in TBT removal capabilities between the two metal salts when compared at equivalent metal doses and coagulation pH. Much lower removals were observed for the coagulation-flocculation-clarification portion of the full-scale plant while the complete full-scale treatment plant averaged 99.8% TBT removal over a period of 3 years. While relatively high % removals were achieved, the total treatment process did not consistently remove TBT to levels that would meet the regulatory requirements proposed (50 ng/L) at the time the study was conducted. Based on the results from limited efforts to characterize the dissolved and particulate TBT fractions in the full-scale treatment plant effluent, particulate TBT was observed to be the dominant component of the effluent TBT from the full-scale plant and may be associated with polymer carry through.