Untreated River Water Research Articles

Neural Network Simulation of the Chemical Oxygen Demand Reduction in a Biological Activated‐Carbon Filter

AbstractThis paper is primarily aimed at encouraging further use of neural networks by the water‐ and wastewater treatment industry. The study demonstrates the principle of using a network method of simulating the performance of a biological activated‐carbon filter based on a biological water‐quality assessment and measurements of pH and dissolved oxygen during the bio‐regeneration mode with untreated river water. Protozoa, worms, rotifers, bacteria, fungi and algae were used as biological parameters. The neural network model could reasonably estimate the chemical oxygen demand reduction in an exhausted filter. The neural network model gave much better results than a second‐order polynomial regression model; however, a much larger database is required than is currently available.

Mixed enzyme (laccase/tyrosinase)-based remote electrochemical biosensor for monitoring phenolic compounds

An electrochemical biosensor for remote continuous monitoring of phenolic compounds in environmental analysis is described. The probe relies on rapid and sensitive amperometric detection at a submersible biosensor assembly, connected to a 50 ft long shielded cable. The enzymes laccase and tyrosinase were used as individual sensors and also as a bienzymatic sensor; these enzymes were immobilized chemically on the carbon fiber transducer. The analysis was based on the amperometric detection of the enzymatic products at a potential of −0.10 V vs. Ag/AgCl. Operational conditions were optimized to meet the requirements of remote operations. Tests with untreated river water spiked with phenolic compounds gave results similar to those obtained with synthetic buffer solutions. The remote laccase biosensor allowed the convenient quantification of guaiacol and chloroguaiacol at levels down to 22 and 9 nmol L−1, respectively. The co-immobilization of laccase and tyrosinase allowed the efficient detection of a larger group of phenolic compounds.

Evaluation and validation of a commercial ELISA for diazinon in surface waters.

The performance of a commercially available microtiter plate ELISA kit for the determination of diazinon was evaluated for sensitivity, selectivity, intra-assay repeatability, accuracy, and matrix effects in fortified distilled water and filtered and unfiltered environmental surface water samples. Repeatability and reproducibility studies show that the kit satisfies current EPA criteria for the assessment of analytical methods. Mean recoveries from spiked samples averaged 80.3, 95.5, and 103.5% from distilled, unfiltered surface, and filtered surface waters, respectively. The experimentally determined method detection limit (MDL) for the commercial diazinon microtiter plate format (0.0159 microg L(-)(1)) was comparable to the least detectable dose (LDD) established by the manufacturer (0.022 microg L(-)(1)). Specificity studies indicate that the diazinon polyclonal antibody can readily distinguish the target compound from other structurally similar organophosphorus analogues, with the exception of diazoxon. Cross-reactivity with the oxon was approximately 29%, while reactivity with pirimiphos-methyl, pirimiphos-ethyl, and chlorpyrifos-ethyl was negligible. A slight matrix effect was discovered to be present in both filtered and unfiltered environmental water matrixes, but its effect on the immunoassays is insignificant within experimental error. For validation of the microtiter plate ELISA format, environmental surface and storm runoff water samples were collected, split, and analyzed directly by ELISA and by liquid-liquid extraction followed by GC (California State Department of Food and Agriculture method EM 46.0). Results of the two analytical methods were then compared statistically. A close correlation was found between methods for unspiked and untreated river water samples (r = 0.969) while a much less robust correlation was obtained for runoff waters (r = 0.728). Results from runoff waters exhibit a particularly high positive bias for the ELISA method relative to the GC method. Cross-reactivity of diazoxon and probably other unidentified cross-reacting components may be responsible for the exaggerated account of the target analyte in surface and runoff waters. While excellent for screening purposes, further study is required to elucidate and quantify the factors responsible for the consistent overestimation of ELISA results before the kit can be employed routinely for regulatory compliance monitoring.

Denitrification by indigenous microbial populations of river water measured using membrane inlet mass spectrometry.

The major process that reduces nitrate levels in soils and water is denitrification, which converts nitrate and nitrite into gaseous forms of nitrogen, which are then released into the atmosphere. This study used membrane inlet mass spectrometry (MIMS) to investigate denitrification in river water bacterial isolates supplied with nitrate and succinate as an energy source as well as in the total population by provision of different carbon compounds to untreated river water samples. Substantial variation was observed in the gases detected with nitrogen, nitrous oxide and nitric oxide all being produced by one or more of the isolates. The indigenous river population as a whole was found to respond very differently to the addition of different carbon sources. Peak nitrogen levels differed by nearly 1 mmol 1(-1) and nitrous oxide by approximately 0.5 mmol 1(-1) depending on which carbon source was supplied. Nitric oxide was only detected when glycerol was supplied as the carbon source. These results demonstrate the complex interactions involved in nitrogen cycling and suggest that with careful management it may be possible to stimulate particular consortia of micro-organisms to reduce more nitrate to harmless nitrogen rather than nitrous oxide, a known greenhouse gas.

Speciation of Submicrogram per Liter Levels of Arsenic in Water: On-Site Species Separation Integrated with Sample Collection

Speciation of arsenic is crucial for assessing health implications from arsenic ingestion and for effective removal of arsenic from water. We report a method for the speciation of submicrogram per liter levels of arsenic in water. The method incorporates water sample collection with on-site arsenic species separation. The method is based on selective retention of arsenic species on specific solid-phase cartridges followed by selective elution and hydride generation atomic fluorescence analysis of the arsenic species. The use of a membrane filter, a resin-based strong cation-exchange cartridge, and a silica-based strong anion-exchange cartridge allows for the speciation of particulate arsenic and soluble arsenite, arsenate, monomethylarsonate, and dimethylarsinate species. Detection limit is on the order of 0.05 μg/L. The method is suitable for direct water sample collection and on-site separation of arsenic species by flowing a measured volume of water sample through the filter and cartridges connected in serial. A particular advantage of this approach is to maintain the integrity of original arsenic species in the sample. It overcomes the common problem of instability of arsenic species after water sampling and during sample storage and handling. Applications of the method are demonstrated to the speciation of arsenic in well water, raw (untreated) river water, bottled water, and a standard reference material (SRM 1643d). Results agree well with the certified values of the SRM.

Biofouling prevention with pulsed electric fields

Temporary immobilization of aquatic nuisance species through application of short electric pulses has been explored as a method to prevent biofouling in cooling water systems where untreated lake, river, or sea water is used. In laboratory experiments, electrical pulses with amplitudes on the order of kilovolts/centimeter and submicrosecond duration were found to be most effective in stunning hydrazoans, a common aquatic nuisance species. Varying pulse amplitude and repetition rate allows us to control the stunning time in a temporal range from minutes to hours. The temporary immobilization is assumed to be caused by reversible membrane breakdown. This assumption is supported by results of measurements of the energy required for stunning. Based on the data obtained in laboratory experiments, field experiments in a tidal mater environment have been performed. The flow velocity was such that the residence time of the aquatic nuisance species in the system was approximately half a minute. The results showed that the pulsed electric field method provides full protection against biofouling when pulses of 0.77 /spl mu/s width and 6 kV/cm amplitude are applied to the water at the inlet of such a cooling water system. Even at amplitudes of 1 kV/cm, the protection is still in the 90% range, at an energy expenditure of 1 kWh for the treatment of 60 000 gallons of water.

Remote Biosensor for In-Situ MOnitoring of Organophosphate Nerve Agents

A remote electrochemical biosensor for field monitoring of organophosphate nerve agents is described. The new sensor relies on the coupling of the effective biocatalytic action of organophosphorus hydrolase (OPH) with a submersible amperometric probe design. This combination results in a fast, sensitive, selective, and stable response at large sample-instrument distances. Such attractive performance is illustrated for direct measurements of micromolar levels of paraoxon and methyl parathion in untreated river water samples. Unlike multi-step inhibition biosensors, the remote OPH probe offers single-step direct measurements, and is thus highly suitable for the continuous monitoring task. Variables relevant to field operations are discussed, along with prospects for remote monitoring and early detection of nerve agents.

Speciation and sorptive behaviour of nickel in an organic-rich estuary (Beaulieu, UK)

Abstract The speciation of dissolved and particulate Ni has been determined along an axial transect of the Beaulieu Estuary, southern England, using adsorptive cathodic stripping voltammetry (ACSV). Total dissolved concentrations ranged from about 80 nM in the river end-member to about 15 nM in the marine end-member, and the ACSV-non-labile fraction, comprising Ni complexes whose alpha coefficients exceed 10 9.2 , contributed between about 15 and 60% of the total dissolved pool. The axial distributions of total dissolved Ni and species thereof, and dissolved organic carbon, could be defined in terms of simple end-member mixing, suggesting little biogeochemical reactivity in the estuary. Particulate Ni was dominated (≥70%) by the fraction available to acetic acid, and a distinct seaward decrease from about 35 to 2 μg g −1 could, likewise, be largely accounted for by mixing of (particle) end-members of different composition. The uptake of Ni by Beaulieu particles was monitored in a series of laboratory simulations employing 63 Ni and liquid scintillation counting. Adsorption was dependent to some degree on salinity, pH and particle type, but most importantly on the presence of dissolved organic matter. Thus, particle–water distribution coefficients ( K D s) were almost doubled when artificial river and sea waters, containing no pre-existing dissolved organic matter, were used in place of organic-rich Beaulieu end-members, and increased by seven-fold when UV-irradiated river water was used in place of untreated river water. These observations were interpreted in terms of the formation of small (sub-colloidal) and relatively strong organic complexes which are relatively resistant to adsorption, and a simple model was developed, incorporating the K D s, to predict the ratio of this fraction of dissolved Ni to the remaining fraction of dissolved Ni as a function of suspended particle concentration. The ACSV (field) measurements of labile and non-labile dissolved Ni from the Beaulieu conformed with the model prediction when K D s for UV-irradiated and untreated river waters were employed, implying that the ACSV-non-labile fraction of dissolved Ni (log 10 α>9.2) affords a good indication of the fraction which is resistant to adsorption. It is suggested that the low biogeochemical reactivity of Ni in the Beaulieu results from a combination of its low particle affinity, maintained by the presence of specific dissolved organic ligands, and small suspended sediment fluxes throughout the estuary. The distributions of Ni in other estuaries are examined and implications for the more general estuarine behaviour of Ni are discussed.

Remote electrochemical sensor for monitoring TNT in natural waters

An electrochemical sensor for remote continuous monitoring of the explosive 2,4,6-trinitrotoluene (TNT) is described. The probe relies on rapid and sensitive square-wave voltammetry at a submersible carbon-fiber working-electrode assembly, connected to a 50ft-long shielded cable. Operational conditions have been optimized to meet the specific requirements of remote operations. Tests with untreated river water, groundwater, and drinking water samples, spiked with TNT, displayed results similar to those obtained with synthetic buffer solutions. Due to the low detection potential of TNT, coexisting sample constituents do not contribute to the response. The use of ultrafast square-wave voltammetric scanning, without a regeneration step, leads to a rapid detection of dynamic concentration changes, low ppb detection limits, and high stability. Prospects for using the electrochemical probe for real-time monitoring of TNT are discussed.

Electrochemical biosensor for detecting DNA sequences from the pathogenic protozoan Cryptosporidium parvum

An electrochemical hybridization biosensor was developed for the detection of short DNA fragments specific to the deadly waterborne pathogen Cryptosporidium. The sensor relies on the immobilization of a 38-mer oligonucleotide unique to the Cryptosporidium DNA onto the carbon-paste transducer, and employs a highly sensitive chronopotentiometric transduction mode for monitoring the hybridization event. Variables of the probe-immobilization, hybridization and indicator-detection steps are optimized to offer convenient quantitation of ng ml −1 levels of the Cryptosporidium DNA target, in connection with short (3–15 min) hybridization times. The suitability for direct detection of the spiked Cryptosporidium DNA target in untreated drinking and river water samples is demonstrated. Similar performance characteristics are observed at DNA-coated microfabricated thick-film carbon strips. This and similar developments hold great promise for field screening of Cryptosporidium and other microorganisms in environmental samples.

Survival of cryptosporidium parvum, escherichia coli, faecal enterococci and clostridium perfringens in river water: influence of temperature and autochthonous microorganisms

Oocysts of Cryptosporidium parvum can survive for several months in surface water, one of the main factors determining their success in environmental transmission and thus their health hazard via water. Several factors in the environment, e.g. temperature, presence of predators and exo-enzymes will probably influence oocyst survival. The high persistence of oocysts may also limit the value of traditional faecal indicator bacteria. The aim of this study was to determine the rate at which C parvum oocysts, E coli, faecal enterococci and C perfringens spores die in surface water and the influence of temperature and the presence of autochthonous (micro)organisms on the die-off rate. Microcosms with autoclaved river water were inoculated with the organisms. Microcosms with untreated river water were inoculated with concentrated primary effluent containing the bacteria and with C parvum oocysts. Microcosms were incubated at 5°C or 15°C at 100rpm. Viability of oocysts was monitored by in vitro excystation and dye-exclusion; viability of the bacteria was determined on appropriate selective media. When pseudo first-order die-off kinetics were assumed, the die-off rate of oocysts at 5°C was 0.010 log10/d and at 15°C, 0.006–0.024 log10/d. These rates underestimate die-off since oocyst disintegration was not accounted for. Incubation in autoclaved or untreated water did influence the die-off rate of oocysts at 15°C but not at 5°C. The die-off rate of E coli and enterococci was faster in the non-sterile river water than in autoclaved water at both temperatures. At 15°C, E coli (and possibly E faecium) even multiplied in autoclaved water. In untreated river water, the die-off of E coli and enterococci was approximately 10x faster than die-off of oocysts but die-off rates of C perfringens were lower than those of oocysts. As for oocysts, die-off of the bacteria and spores was faster at 15°C than at 5°C. Oocysts are very persistent in river water: the time required for a 10x reduction in viability being 40–160d at 15°C and 100d at 5°C. Biological/biochemical activity influenced oocyst survival at 15°C and survival of both vegetative bacteria at 5 and 15°C. The rapid die-off of E coli and enterococci makes them less suitable as indicators of oocyst presence in water. As C perfringens survived longer in untreated river water than oocysts, it may prove useful as an indicator of the presence of C parvum.

Survival of Cryptosporidium parvum, Escherichia coli, faecal enterococci and Clostridium perfringens in river water: influence of temperature and autochthonous microorganisms

Occysts of Cryprosporidium parvum can survive for several months in surface water, one of the main factors determining their success in environmental transmission and thus their health hazard via water. Several factors in the environment, e.g. temperature, presence of predators and exo-enzymes will probably influence oocyst survival. The high persistence of oocysts may also limit the value of traditional faecal indicator bacteria. The aim of this study was to determine the rate at which C. parvum oocysts, E. coli, faecal enterococci and C perfringens spores die in surface water and the influence of temperature and the presence of autochthonous (micro)orgsnisms on the die-off rate. Microcosms with autoclaved river water were inoculated with the organisms. Microcosms with untreated river water were inoculated with concentrated primary effluent containing the bacteria and with C parvum oocysts. Microcosms were incubated at 5°C or 15°C at 100rpm. Viability of oocysts was monitored by in vitro excystation and dye-exclusion; viability of the bacteria was determined on appropriate selective media. When pseudo first-order die-off kinetics were assumed, the die-off rate of oocysts at 5°C was 0.010 log10/d and at 15°C, 0.006–0.024 log10/d. These rates underestimate die-off since oocyst disintegration was not accounted for. Incubation in autoclaved or untreated water did influence the die-off rate of oocysts at 15°C but not at 5°C. The die-off rate of E coli and enterococci was faster in the non-sterile river water than in autoclaved water at both temperatures. At 15°C, E coli (and possibly E faecium) even multiplied in autoclaved water. In untreated river water, the die-off of E coli and enterococci was approximately 10x faster than die-off of oocysts but die-off rates of C perfringens were lower than those of oocysts. As for oocysts, die-off of the bacteria and spores was faster at 15°C than at 5°C. Oocysts are very persistent in river water the time required for a 10x reduction in viability being 40–160d at 15°C and 100d at 5°C. Biological/biochemical activity influenced oocyst survival at 15°C and survival of both vegetative bacteria at 5 and 15°C, The rapid die-off of E coli and enterococci makes them less suitable as indicators of oocyst presence in water. As C perfringens survived longer in untreated river water than oocysts, it may prove useful as an indicator of the presence of C parvum.

Screen-printed tyrosinase-containing electrodes for the biosensing of enzyme inhibitors

Disposable amperometric inhibition biosensors have been microfabricated by screen printing a tyrosinase-containing carbon ink. The decrease in the substrate (catechol) steady-state current, caused by the addition of various pesticides and herbicides, offers convenient quantitation of micromolar levels of these pollutants. Unlike esterasebased disposable strips, the tyrosinase thick-film devices can be fabricated by incorporating the enzyme within the carbon ink. and do not require a prolonged incubation step in the presence of the inhibitor. The effect of experimental variables, such as the enzyme loading or substrate concentration, is assessed. Applicability to an untreated river water sample is illustrated. Such use of single-use devices for monitoring toxins addresses the problem of irreversible enzyme inhibition, and holds great promise for on-site field analysis.

Levels of uranium in waters from some Indian cities determined by fission track analysis

This paper describes the results of measurements on the traces of uranium found in 24 water samples collected from various sources in four important cities of Uttar Pradesh (India). The sources of water samples were municipal taps, private hand pumps, jet pumps and natural rivers, etc. Melinex-O plastic sheets of thickness ≌ 100 microns were used as detectors for the registration of fission tracks produced in the 235U (n,f) reactions. In the case of treated drinking water samples collected from taps supplied by the water works departments of various cities (basic sources for these samples being tube-wells, dams and rivers in different cities as specifically indicated in the paper), the values of uranium concentrations were found to lie between 0.87 ± 0.01 and 11.36 ± 0.34 μg/l. For the untreated water samples collected from private jet pumps and hand pumps, the U-contents were found to vary from 2.02 ± 0.03 to 9.07 ± 0.10 μg/l. The levels of uranium concentration in untreated river water samples of various cities collected directly from the river streams were found to range from 0.92 ± 0.01 to 5.37 ± 0.06 μg/l. The average uranium intake resulting from the drinking tap waters in these cities has also been calculated. They are found to be much less than the maximum permissible limit of 40 mg/day in every case reported in this paper.

Electrocatalytic modified electrode for remote monitoring of hydrazines

A remote electrochemical sensor for field monitoring of hydrazine compounds is described. The new submersible probe relies on the coupling of an effective electrocatalytic modified electrode to a 50 ft long shielded cable. The catalytic surface, based on an electorpolymerized film of 3.4-dihydroxybenzaldehyde, offers low-potential detection of hydrazine compounds. Such catalytic activity results in high sensitivity and selectivity, and hence offers convenient measurements of micromolar hydrazine concentrations in untreated groundwater, river water or lake water samples. Coexisting sample constituents do not contrubute to the response. Operational conditions have been optimized to meet the spacific requirements of remote operation. The concept seems suited for the remote monitoring of other contaminants via the judicious choice of the surface modifier.

Evolution of 131I from freshwater and its partitioning in simple aquatic microcosms

Our investigation quantifies iodine evolution from freshwater to the atmosphere. A factorial experimental design was employed in which the distribution of 131I was determined after 7 days in microcosms containing either (1) water (untreated Winnipeg River water), (2) Chlorella in a nutrient solution, (3) water and sediment (aerobic), or (4) water, sediment and N 2 gas (anaerobic). The 131I released from the microcosms to the atmosphere was retained by triethylenediamine (TEDA) impregnated charcoal traps. Evolution accounted for < 0.5% of the total 131I inventory and was in the order Chlorella > water > water and sediment > water, sediment and N 2 gas. Combining all treatments, the evolution coefficient had a geometric mean (G.M.) of 4.5 × 10 −5 day −1 with a geometric standard deviation (G.S.D.) of 3.7. In the water treatment, 97% of the 131I remained in solution, whereas 22% was algal-bound in the Chlorella treatment, and 80% and 7% were sediment-bound under aerobic and anaerobic conditions, respectively. A partition coefficient ( K d) value of 163 l · kg −1 dry weight (dry wt.) was obtained for the aerobic organic sediment, whereas a value of 3 l · kg −1 dry wt. was obtained for the anaerobic microcosms. On the basis of our K d values, the water to sediment transfer rate for iodine in Canadian Shield lakes would have a G.M. of 0.002 day −1 and G.S.D. of 7.8. We conclude that evolution of iodine generally constitutes only a minor component of losses from freshwater. Under aerobic conditions iodine is lost to the sediments, but under low oxygen conditions most of it remains in solution.

Murray River water, raised cyanobacterial cell counts, and gastrointestinal and dermatological symptoms.

To investigate whether exposure to Murray River and allied water sources during a period of raised cyanobacterial cell counts was associated with gastrointestinal and dermatological symptoms. A case-control study selecting gastrointestinal and dermatological cases and controls from subjects attending 21 general practitioners in eight Murray River towns. The association between the proportion of consultations for such symptoms and mean log cyanobacterial count was also examined. 102 gastrointestinal cases, 86 dermatological cases and 132 controls. The relative odds of gastrointestinal and dermatological symptoms, respectively, as opposed to no such symptoms, according to water-contact history during the week preceding the medical consultation. After adjusting for concurrent risk factors, subjects drinking chlorinated river water rather than rain water had a raised risk of gastrointestinal symptoms (P = 0.008), and those using untreated river water for domestic purposes rather than rain water had a raised risk of gastrointestinal (P = 0.034) and of dermatological (P = 0.048) symptoms. The proportion of consultations for gastrointestinal and dermatological symptoms correlated on a weekly basis with the mean log cyanobacterial cell count, although statistical significance was not achieved for the correlation with dermatological consultations or for separate reaches of the river. The raised risks of gastrointestinal and dermatological symptoms in those using Murray River water for drinking and other domestic purposes are consistent with causal relationships. However, the evidence for adverse health effects is, at best, only suggestive. Further research is indicated.

Determination of Volatile Organic Compounds at the Parts per Trillion Level in Complex Aqueous Matrixes Using Membrane Introduction Mass Spectrometry

Recent advances in membrane introduction mass spectrometry (MIMS) have demonstrated the utility of MIMS in the trace level analysis of volatile organic compounds in water. The present study details the performance of an ion trap mass spectrometer fitted with a capillary membrane probe in the direct analysis of volatile organic compounds in water using flow injection techniques. Detection limits for 59 volatile organic compounds listed in EPA method 524.2 were determined to be in the mid to low parts per trillion (pptr) range using MIMS. Analyses of these compounds in untreated river water, sea water, and matrices containing strong acids and bases are demonstrated. The matrix studies show that there are no adverse effects on compound identification or detection limits resulting from matrix interferences in MIMS. Further experiments demonstrate the utility of MIMS as a direct analysis technique for on-line monitoring of trihalomethane formation in water treatment facilities. Detection limits of 100 pptr were obtained for the four trihalomethanes, with repetitive sample times averaging 6 min/sample. Quantitation of the total trihalomethane content was performed in one step, demonstrating further increases in analytical speed over traditional analysis methods. 26 refs., 9 figs., 2 tabs.

Identification of Odour Problems in the River Dee: A Case Study

ABSTRACTAn intermittent strong vegetable/cabbage odour was noted on raw water abstracted from the River Dee (NW England). The source of the odour was traced to an industrial discharge from an animal feed manufacturer to a tributary of the River Dee. Analysis of water samples by CLSA GC‐MS revealed the likely causes of the odour to be dimethyl disulphide and dimethyl trisulphide. 2‐hydroxy‐4‐(methylthio)butanoic acid (HMB), a hydroxy methionine analogue growth supplement used by the feed manufacturer was a likely precursor to the odour problem. Studies conducted on the behaviour of HMB in untreated water showed it to be rapidly degraded. Degradation products included dimethyl disulphide, dimethyl trisulphide and methyl(methylthio)methyl disulphide. The odour of water samples incubated with HMB changed from that of potato to cabbage/sprout, indicating that breakdown of this compound was responsible for the vegetable odour on untreated river water.

THE AQUEOUS PHOTOLYSIS OF TRICLOPYR

The aqueous photolysis of triclopyr was examined in both pH 7-buffered water and natural river water under artificial lights and midsummer sunlight (40°N latitude). The pseudo-first-order half-lives in pH 7-buffered water and natural river water averaged 0.5 and 1.3 d, respectively, at 25°C. Diffuse mercury lamps and midsummer sunlight produced similar rates of triclopyr decay and photoproduct distribution for a given solution. The photolysis of untreated river water produced a significantly different distribution of photoproducts compared to sterile, pH 7-buffered water. The photolysis of triclopyr in river water generated oxamic acid as the major photoproduct and a number of other low-molecular-weight carboxylic acids as minor products. Photolysis in sterile, pH 7-buffered water produced 5-chloro-3,6-dihydroxy-2-pyridinyloxyacetic acid as the major photoproduct, with minor amounts of oxamic acid and other low-molecular-weight acids.