Assimilable Organic Carbon Concentration Research Articles

Effect of Alfalfa Seed Washing on the Organic Carbon Concentration in Chlorinated and Ozonated Water

The bioassays assimilable organic carbon (AOC) and coliform growth response are better indexes than biological oxygen demand to determine water quality and water's ability to support the growth of bacteria. Ozonated (5 mg/liter) and chlorinated tap water were used to wash alfalfa seeds for 30 min. After washing in the ozonated tap water, the AOC concentration increased 25-fold, whereas the dissolved ozone decreased to undetectable levels. The AOC levels for the chlorinated water after washing the seeds also increased. These increases are due to ozone's strong oxidizing ability to break down refractory, large-molecular-weightcompounds, forming smaller ones, which are readily used as nutrient sources for microorganisms. This same phenomenon was observed when using ozone in the treatment of drinking water. The AOC value increased from 1,176 to 1,758 μgC-eq/liter after the reconditioned wastewater was ozonated. When the ozonated wastewater was inoculated with Salmonella serotypes, the cells survived and increased generation times were observed. The increased nutrients would now become more readily available to any pathogenic microorganisms located on alfalfa seed surface as seen with the increase in the inoculated levels of Salmonella in the ozonated wastewater. If the washing process using ozonated water is not followed by the recommended hypochlorite treatment or continually purged with ozone, pathogen growth is still possible.

Impact of UV disinfection on microbially available phosphorus, organic carbon, and microbial growth in drinking water

UV irradiation at a wavelength of 253.7 nm (UV 254) is commonly used for drinking water disinfection. UV radiation is known to convert organically combined phosphorus to orthophosphate and to degrade natural organic matter. We studied if UV disinfection increases the amount of microbially available forms of organic carbon and phosphorus in drinking waters with different characteristics, and if these changes in water chemical quality could enhance the microbial growth in drinking water. The UV 254 dose (15–50 mW s/cm 2) used in waterworks reduced the concentration of assimilable organic carbon and the sum of the molecular size fractions. The release of microbially available phosphorus needed higher doses (204 mW s/cm 2) of UV 254 radiation. Of bacteria in drinking water, 90% were inactivated with UV 254-irradiation doses below 50 mW s/cm 2. A high dose (501 mW s/cm 2) of UV 254 radiation inhibited the microbial growth in water.

Biofouling reduction in recirculating cooling systems through biofiltration of process water

Biofouling is a serious problem in industrial recirculating cooling systems. It damages equipment, through biocorrosion, and causes clogging and increased energy consumption, through decreased heat transfer. In this research a fixed-bed biofilter was developed which removed assimilable organic carbon (AOC) from process water, thus limiting the major substrate for the growth of biofouling. The biofilter was tested in a laboratory model recirculating cooling water system, including a heat exchanger and a cooling tower. A second identical model system without a biofilter served as a reference. Both installations were challenged with organic carbon (sucrose and yeast extract) to provoke biofouling. The biofilter improved the quality of the recirculating cooling water by reducing the AOC content, the ATP concentration, bacterial numbers (30–40 fold) and the turbidity (OD 660). The process of biofouling in the heat exchangers, the process water pipelines and the cooling towers, was monitored by protein increase, heat transfer resistance, and chlorine demanded for maintenance. This revealed that biofouling was lower in the system with the biofilter compared to the reference installation. It was concluded that AOC removal through biofiltration provides an attractive, environmental-friendly means to reduce biofouling in industrial cooling systems.

Evaluation of assimilable organic carbon in the Yodo River and the effect of coagulation on its control

Biodegradable organic compounds in Yodo River water, the coagulated water and those ozonated waters were characterized on the concentration of assimilable organic carbon (AOC), the apparent molecular size and the sensitivity to E260 (UV absorbance at the wavelength of 260 nm). High molecular (>5,000 Da) organic compounds which were insensitive to E260 were contained with relatively high concentration in Yodo River, whereas they were not detectable as AOC. The AOC concentration in Yodo River was reduced by coagulation because the phosphorus concentration sharply decreased. The AOC concentration rapidly increased with the reduction of E260 and slowly with the depolymerization of high molecular organic compounds during the ozonation. However, 40-50% of AOC increasing with the ozonation was reduced by the coagulation. Furthermore, batch incubations of native bacteria living in the Yodo River were conducted in order to examine the validity of the apparent AOC concentration as an indicator of the regrowth potential. The AOC concentration calculated as acetate carbon equivalent would not exactly represent the concentration of the available carbon for the assimilation. However, maximum cell numbers for the incubation period were generally proportional to apparent AOC concentrations even if phosphorus limitation. In conclusion, it is suggested that coagulation is an effective process in controlling the concentration of AOC and is a useful indicator to evaluate the regrowth potential of heterotrophic bacteria in Yodo River.

Biostability and microbiological quality in a chloraminated distribution system

In this study, two components were evaluated: biostability, measured by assimilable organic carbon (AOC) concentration; and microbiological quality, determined by heterotrophic plate counts and coliform counts. The study centered around a full‐scale chloraminated distribution system. Results from full‐scale monitoring showed that the water in the system was biologically stable and there was no significant microbial activity in the system in spite of a moderate to high AOC concentration of 162 + 24 μg/L as acetate‐carbon. Batch experiment results indicated that the treated water containing monochloramine was biologically stable, and removal of the residual chlorine resulted in rapid consumption of AOC and bacterial regrowth. This study suggested that the treated water's AOC concentration may not be the dominant factor for controlling bacterial regrowth in all distribution systems. Biostability can be achieved in a water supply containing a moderate to high AOC concentration by maintaining a high disinfectant residual throughout the system.

Biofilm formation potentials in drinking waters treated by different advanced treatment processes

When biodegradable organic matter and other nutrients, such as ammonia and phosphorus, are not sufficiently removed during water treatment, bacteria may proliferate in the water distribution system. Bacterial regrowth deteriorates water quality (taste and odor), accelerates corrosion, and potentially increases the risk of microbial diseases. Therefore, this research was conducted to evaluate the impact of four different advanced water treatment processes, including biological treatments such as a rotating biofilm membrane reactor (RBMR) and a biological activated carbon (BAC) filter and ultrafiltration (UF), on reduction of nutrient levels and biofilm formation potentials of the treated water entering model distribution systems (annular reactors). Our results revealed that biological treatments significantly improved the “biostability” of water leaving from the treatment plant. On average, The RBMR and BAC filter reduced easily assimilable organic carbon (AOC) concentration by half when compared with conventional treatment (multi-media filtration; MF) and ultrafiltration (from 35-49 to 18-23 mg C L-1). Consequently, biofilm formation potential was reduced by a factor of 5 to 10 (from 3,200-5,100 to 490-710 pg ATP cm-2). With respect to “biostability” of water, ultrafiltration was less effective in reducing AOC concentrations. In addition, the impact of chlorine disinfection on biofilm accumulation and AOC levels in the distribution system were studied.

An investigation on biological stability of product water generated by lab-scale and pilot-scale distillation systems.

Lab-scale and pilot-scale distillation systems were assessed in this study and the effectiveness of the treatment processes on Assimilable Organic Carbon (AOC) removal was also investigated. Seawater as one of the alternative water sources was used as the feed water and the resulting product water was intended for industry use. Acidification, ion trapping and anti-scaling chemical dosing were adopted as the pre-treatment processes to the feed water before distillation for pilot-plant system. The AOC assay was used to evaluate the efficiencies of various unit processes for AOC removal. Heterotrophic Plate Counts (HPC) was adopted in conjunction with AOC to determine the biological stability of the water samples. It is noted that the distillation system was able to produce a product water with an acceptable AOC concentration of less than 20 micrograms acetate-C L-1. This would suggest that the distillation system was capable of delivering a biologically stable product water. The results from lab-scale system indicated that pretreatment such as acidification, chlorination and antiscalant would increase AOC, although this does not affect the general performance of distillation system.

Investigation of assimilable organic carbon (AOC) and bacterial regrowth in drinking water distribution system

This paper investigated the variation of assimilable organic carbon (AOC) concentrations in water from several typical water treatment plants and distribution systems in a northern city of China. It is concluded from this study that: (1) The AOC in most of the product water of the studied water treatment plants and the water from the associated distribution systems could not meet the biostability criteria of 50–100 μg/L. (2) Only 4% of the measured AOC concentrations were less than 100 μg/L. However, about half of the measured AOC values were less than 200 μg/L. (3) Better source water quality resulted in lower AOC concentrations. (4) The variation of AOC concentrations in distribution systems was affected by chlorine oxidation and bacterial activity: the former resulted in an increase of AOC value while the latter led to a reduction in AOC. (5) The variation of AOC concentration followed different patterns in different distribution systems or different seasons due to their respective operational characteristics. (6) Less than 30% of AOC could be removed by a conventional treatment process, whereas 30–60% with a maximum of 50–60% could be removed by granular activated carbon (GAC). (7) The observation via scanning electron microscope (SEM) on distribution pipe tubercle samples demonstrated that the pipe inner wall was not smooth and bacteria multiplied in the crevice as well as in the interior wall of distribution pipes.

Case Study: Ozonation and distribution system biostability

Biodegradable organic matter that is not removed during water treatment can lead to the proliferation of bacteria within the distribution system. This, in turn, can deteriorate water quality, accelerate corrosion rates of pipes, and potentially increase the incidence of bacteriological diseases. The main objective of this project was to collect and analyze full‐scale system data on short‐term and long‐term responses to the implementation of ozonation with respect to its impact on bacterial regrowth potential as quantified by assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC). A major observation made during the sampling period was that ozonation caused a significant increase in the AOC concentration of the distribution system (more than 200% in the short term and more than 100% in the long term). Use of ozone also caused a significant increase in the bacterial counts within the distribution system for both the long and short terms. In contrast, ozonation affected BDOC and dissolved organic carbon (DOC) concentrations only slightly compared with its impact on AOC concentrations. However, this may be because of the source water, which had low DOC and BDOC concentrations (1.2 and 0.11 mg/L, respectively).

Bacterial growth in distribution systems: effect of assimilable organic carbon and biodegradable dissolved organic carbon.

Two distribution systems, one treating water by ozonation and another treating water by nanofiltration in parallel with lime softening, were monitored for bacterial growth. Both systems kept disinfectant residuals such as chlorine and chloramine in their respective distribution systems. Bacterial growth was assessed by heterotrophic plate counts (HPC) on R2A agar. In the distribution systems fed by ozonated water, HPCs were correlated (R2 = 0.96) using an exponential model with the assimilable organic carbon (AOC) at each sampling site. Also, it was observed that ozonation caused a significant increase in the AOC concentration of the distribution system (over 100% increase) as well as a significant increase in the bacterial counts of the distribution system (average increase over 100%). The HPCs from the distribution systems fed by nanofiltration in parallel with lime-softening water also displayed an exponential correlation (R2 = 0.73) with an exponential model based on AOC. No significant correlation was found between bacteria growth on R2A agar and BDOC concentrations. Therefore, in agreement with previous work, bacterial growth in the distribution systems was found to correlate with AOC concentrations.

Microbially available organic carbon, phosphorus, and microbial growth in ozonated drinking water

Ozonation is a disinfection technique commonly used in the treatment of drinking water. It destroys harmful microbes, but it also degrades organic matter in water, increasing the bioavailability of organic matter. Recently, it was found that not only organic carbon but also phosphorus can limit the microbial growth in drinking water, which contains high amount of organic matter. We used a bioassay to analyze whether ozone could also increase the microbially available phosphorus (MAP) in drinking water, and whether MAP in ozone-treated water was associated with the growth of heterotrophic microbes. We found that both assimilable organic carbon and MAP concentrations were increased by ozone treatment. In ozonated water, microbial growth was mainly limited by phosphorus, and even minor changes in MAP concentration dramatically increased the growth potential of heterotrophic microbes. In this study, ozonation increased the MAP by 0.08–0.73 μg P/l, resulting in an increase of 80,000–730,000 CFU/ml in water samples. In contrast to MAP, the content of assimilable organic carbon (AOC potential) did not correlate with microbial growth. The results show that in water treatment not only AOC potential but also MAP should be considered as an important factor that can limit microbial growth in drinking water.

Biofouling potential of chemicals used for scale control in RO and NF membranes

The potential of 14 different antiscalants (AS) to promote biofouling was determined with the easily assimilable organic carbon (AOC) test and the Biomass Production Potential (BPP) test, respectively. The AOC concentrations of slow sand filtrate supplemented with 50 mg/l of AS ranged from 4 to 112 μg of C/l, and maximum concentrations of adenosinetriphosphate (ATP) from 7 to 380 ng/l, respectively. Concentrations of AOC and BPP were not related to the organic carbon content of the AS's tested. Biofilm Formation Rate (BFR) values of three selected AS's, as determined with the biofilm monitor at a concentration of 2.5 mg/l, were 245, 18.1 and <1.0 pg ATP/cm 2.d, respectively and confirmed the ranking as based on the AOC and BPP tests. Dosage (2.5 mg/l) of one selected AS with a relatively high growth potential caused severe fouling in a 4″ RO membrane element supplied with tap water. ATP concentrations on the membrane, including spacers, were 4 times higher in the element supplied with tap water containing AS than in the control. These experimental results agreed with practical experiences, viz. severe biofouling was observed with an AS with high AOC and BPP values and a high BFR value, but no operational problems were observed with an AS with low values for these parameters. Practical experiences also showed that impure mineral acids can cause biofouling. Therefore, selection of chemicals used for scale control on the basis of their growth-promoting properties is needed to limit operational problems. Quality classes based on growth potential tests are proposed for AS's, but further comparison between test results and practice is needed to improve this classification.

Assessing biodegradable organic matter

High temperature, high BOM concentration, and low disinfectant residual promote regrowth of coliforms in distribution systems.This article summarizes data collected during various surveys that assessed four aspects of biodegradable organic matter (BOM) in drinking water: (1) BOM concentrations entering distribution systems, (2) the relationship between concentrations of assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC), (3) field tests of a simple method of measuring BDOC using bioreactors, and (4) the effect of BOM on coliform occurrences in distribution networks. Overall, average concentrations of AOC and BDOC in plant effluent were 100 μg/L and 0.32 mg/L, respectively. A statistically significant but weak relationship between AOC and BDOC concentrations suggests that both parameters need to be monitored during biostability studies because they provide different pieces of information. BDOC bioreactors are useful for measuring BDOC concentrations at water utilities. However, they require a long time to colonize and must adapt to water at the site where they are used. Coliform occurrences were associated with three factors: temperature, disinfectant type and concentration, and AOC concentration. When threshold values for these factors were exceeded (15oC, AOC &gt; 100 μg/L, and dead‐end disinfectant residuals &lt; 0.5 mg/L for free chlorine or 1.0 mg/L for chloramines), the probability of coliform occurrences greatly increased.

Bacterial biofilm formation on polyvinyl chloride, polyethylene and stainless steel exposed to ozonated water

The formation of biofilm on polyvinyl chloride (PVC), polyethylene (PE) and stainless steel surfaces was studied in bank infiltrated and chemically purified waters without (control water) and with ozonation. Ozonation increased the concentration of assimilable organic carbon (AOC) in water (78 vs 450 μgC/l). Ozonation of water increased the viable numbers and cell volume of heterotrophic bacteria on the surfaces. The highest bacterial count, 7.7×10 5 cfu/cm 2, was detected on PVC surface after three weeks’ exposure to ozonated water. The numbers of heterotrophic bacteria on surfaces reflected the bacterial numbers in water. There was no difference in total cell count on different surfaces and in control and ozonated waters. Direct staining of microbes on the surfaces produced higher cell count per cm 2 than enumerating microbes detached from surfaces by sonication. In general, the accumulation of biofilm on different surface materials was quite similar. Only the cell volume was slightly higher on PE than on PVC surface.

Impacts of the reduction of nutrient levels on bacterial water quality in distribution systems.

This study evaluated the impacts of reducing nutrient levels on bacterial water quality in drinking water. Two American Water System facilities (sites NJ102a and IN610) with histories of coliform problems were involved, and each water utility received two pilot distribution systems (annular reactors). One reactor simulated the conventional treatment conditions (control), while the other reactor was used to assess the effect of biological filtration and subsequent reduced biodegradable organic matter levels on suspended (water column) and biofilm bacterial concentrations in the distribution systems. Biodegradable organic matter levels were reduced approximately by half after biological treatment. For site NJ102a, the geometric mean of the assimilable organic carbon concentrations was 217 microg/liter in the plant effluent and 91 microg/liter after biological filtration. For both sites, plant effluent biodegradable dissolved organic carbon levels averaged 0.45 mg/liter, versus 0.19 to 0.22 mg/liter following biological treatment. Biological treatment improved the stability of free chlorine residuals, while it had little effect on chloramine consumption patterns. High bacterial levels from the biological filters resulted in higher bacterial concentrations entering the test reactors than entering the control reactors. On average, biofilms in the model systems were reduced by 1 log unit (from 1.4 x 10(5) to 1.4 x 10(4) CFU/cm(2)) and 0.5-log unit (from 2.7 x 10(5) to 7.8 x 10(4) CFU/cm(2)) by biological treatment at sites NJ102a and IN610, respectively. Interestingly, it required several months of biological treatment before there was an observable impact on bacterial water quality in the system, suggesting that the effect of the treatment change was influenced by other factors (i.e., pipe conditions or disinfection, etc.).

The effect of water treatment processes on the biological stability of potable water

The AOC (assimilable organic carbon) method was used in this study to evaluate the biological stability of potable water. The potable water was drawn from a waterworks located in a petrochemical industrial area in China. Infrared spectra analysis was adopted in this study to examine the relationship between treatment process and organic compounds in water. By monitoring the AOC concentration in groundwater, potable water and the effluent of each treatment process (biological pre-treatment, ozonation, GAC adsorption), the following were noted: (1) a considerable AOC concentration could still be detected in potable water after advanced treatment processes, such as ozonation and activated carbon adsorption. Relatively poor biological stability was also noted in the potable water; (2) GAC was the most effective process with a total AOC removal efficiency of more than 80%. The corresponding biotreatment removal efficiency was only 45%. However, ozonation yielded a negative overall removal efficiency of −119%; (3) while biological pre-treatment and GAC adsorption contributed towards a biologically stable water, ozonation yielded oxidation products which were biologically unstable. Ozonation, if used, should therefore be combined with the GAC or biological processes. Treatment processes (like ozonation) which increased the amount of organics in carbonyl group would likely lead to poor product water biological stability.

Determination of assimilable organic carbon in humus-rich drinking waters

The potential for microbial growth in drinking waters is considered to be associated with the content of easily assimilable organic carbon (AOC). The bacterial test for the determination of the AOC concentration presumes that microbial growth in the test is limited by the availability of organic carbon. However, in the northern latitudes inorganic nutrients, particularly phosphorus, may limit the microbial growth in drinking waters. In our studies, addition of an inorganic nutrient mixture (N, P, K, S, Ca, Mg) to soft, humus-rich drinking waters increased the AOC yield in most of the drinking waters. The assimilable organic carbon determined with the addition of nutrients describes that pool of organic carbon potentially available for microbial growth (AOC potential). The AOC yield when there is no without addition of inorganic nutrients describes that part of organic carbon (AOC native) which bacteria can utilize in the prevailing conditions and it is composed of the availability of both organic substrates and inorganic nutrients. We found that the bacterial strains, Pseudomonas fluorescens P 17 and Aquaspirillum NOX, used in the standard AOC method, can also be applied in humus-rich drinking waters.

A new sensitive bioassay for determination of microbially available phosphorus in water

The content of assimilable organic carbon has been proposed to control the growth of microbes in drinking water. However, recent results have shown that there are regions where it is predominantly phosphorus which determines the extent of microbial growth in drinking waters. Even a very low concentration of phosphorus (below 1 &mgr;g of P liter-1) can promote extensive microbial growth. We present here a new sensitive method to determine microbially available phosphorus concentrations in water down to 0.08 &mgr;g of P liter-1. The method is a bioassay in which the analysis of phosphorus in a water sample is based on maximum growth of Pseudomonas fluorescens P17 when the energy supply and inorganic nutrients, with the exception of phosphorus, do not limit bacterial growth. Maximum growth (CFU) in the water sample is related to the concentration of phosphorus with the factor 373,200 +/- 9,400 CFU/&mgr;g of PO4-P. A linear relationship was found between cell growth and phosphorus concentration between 0.05 to 10 &mgr;g of PO4-P liter-1. The content of microbially available phosphorus in Finnish drinking waters varied from 0.1 to 10.2 &mgr;g of P liter-1 (median, 0.60 &mgr;g of P liter-1).

Potential for biofilm development in drinking water distribution systems

Regrowth of micro-organisms in drinking water distribution systems is caused by the utilisation of biodegradable compounds which are either present in treated water or originate from materials in contact with drinking water. In the Netherlands most drinking water is distributed without disinfectant residual and regrowth is limited by achieving biostable drinking water. A combination of methods is used to assess the biostability of drinking water. These methods are: (1) determination of the concentration of easily assimilable organic carbon (AOC); and (2) assessment of the biofilm formation rate (BFR). Assimilated organic carbon concentrations in drinking water in the Netherlands range from a few μg C/l in slow sand filtrates and in ground water supplies to values of ∼ 50 μg C/l in supplies using ozonation in water treatment. Biofilm formation rate values were found to range from < 1 pg ATP/cm(2)/d in supplies using anaerobic ground water as the source. Increase of heterotrophic plate counts is limited at AOC values below 10 μg C/l. At BFR values below 10 pg ATP/cm(2)/d the risk of exceeding the guideline value for aeromonads (90 percentile < 200 c.f.u./100 ml) is less than 20%. Calculations based on the decrease of the AOC concentration observed in distributions systems confirm that very low concentrations of AOC can cause considerable biofilm formation on the pipe wall. The methods for assessing the biostability of drinking water combine with the assessment of the Biofilm Formation Potential of materials in contact with drinking water, thus providing a framework, the Unified Biofilm Approach, for evaluating the biostability of drinking water and materials.

MICROBIAL GROWTH IN DRINKING WATERS TREATED WITH OZONE, OZONE/HYDROGEN PEROXIDE OR CHLORINE

The effects of ozonation, hydrogen peroxide treatment and chlorination on the quality of organic matter and microbiological growth in drinking water were studied in a pilot plant. All oxidants degraded organic matter (humus) increasing the concentration of assimilable organic carbon (AOC). Even though the oxidants efficiently decreased microbial numbers, microbial regrowth occurred in all drinking waters. Microbial regrowth was strong especially after ozone/hydrogen peroxide treatment. Chlorination combined with ozone and hydrogen peroxide decreased microbial numbers most efficiently. However, microbial growth started even in chlorinated samples within 2-3 days.