Selection of growth rate model for activated sludges treating phenol

Research was undertaken for the purpose of selecting a growth rate model, i.e. a function relating specific growth rate, μ, to substrate concentration, S, for acclimated heterogeneous microbial populations metabolizing a phenolic waste and to gain an insight into the expected average and range of values of the kinetic “constants” for the selected model. Various sources of seed populations were employed and batch growth curves were generated using phenol as sole source of carbon. Comparison of the fit of the data obtained from 113 growth curves to five inhibition functions failed to distinguish a function which was statistically superior to the others. Since the Haldane equation proved to be the most readily adapted to curve-fitting procedures and insertion into mass balance equations for reactor performance, it was selected as the best representation of the experimental data. The values of the three pertinent biokinetic constants were as follows: μ_max, range = 0.08–0.36 h^?1, mean = 0.19 h^?1; K_S, range = 1.3–266 mg l^?1, mean = 75 mg l^?1; K_i, range = 66–1463 mg l^?1, mean 449 mg l^?1. Analysis of chemostat performance using growth constants determined in separate batch growth studies with seed from the chemostat showed that reasonably accurate prediction of the dilution rate at which the system attains μ1, the peak or critical specific growth rate designated by the Haldane equation, could be made. This finding indicates that batch growth rate data can be used for predictive control of activated sludge reactors treating toxic wastes.     

Denitrification by packed bed reactors in the presence of chromium(VI): Resistance to inhibition

The effect of chromium Cr⁶⁺ on bacterial denitrification was investigated. The long-term influence of chromium presence was observed in packed bed reactors using methanol, ethanol, n-propanol, n-butanol, sec-butanol, tert-butanol, iso-butanol and n-pentanol as the carbon and energy sources for denitrifiers. Short-term influence was investigated by the inhibition coefficient K_i determination within activated sludge under anoxic conditions. The measured inhibition constant K_i was equal to 84.2 mg l^?1 Cr⁶⁺, independently of the kind of organic compound utilized as the electron donor for the bacterial system. The concepts of the reactor resistance to inhibition (RRI) and the resistance to inhibition (RI) have been evaluated.     

Effects of inhibitors on nitrification in a packed-bed biological flow reactor

The individual effect of trivalent arsenic, hexavalent chromium and fluoride on nitrification is studied under continuous load in a packed bed biological flow reactor. The results show that Michaelis-Menten rate expression gives the best representation of nitrification data in the absence of inhibitors. However, in the presence of inhibitors, the system follows a non-competitive mode of inhibition with the following rate expression: The values of V_max and K_s are estimated as 1.466 mg l⁻¹ min⁻¹ and 2.349 mg l⁻¹ respectively. The inhibitor constant K_i is evaluated as 273 mg l⁻¹ for trivalent arsenic, 56 mg l⁻¹ for hexavalent chromium and 1185 mg l⁻¹ for fluoride.     

Nitrification process in activated sludge with suspended marble particles

The process of nitrification in activated sludge was investigated. As the solid support for nitrifiers' growth a suspension of marble particles has been used. The results proved the possibility of successful nitrification of 100 mg l^?1 NH₄-N simultaneously with the removal of 600 mg l^?1 COD.     

Acute toxicities of five nitromusk compounds in Daphnia,algae and photoluminescent bacteria

Acute toxicities of five nitromusk compounds in Daphnia, algae and photoluminescent bacteria were investigated. In order to obtain some basic data for ecotoxicological tests, physicochemical parameters were determined. The water solubility and log K_ow values were: nitromusk xylene 0.15 mg l⁻¹ and 4.4, nitromusk ketone 0.46 mg l⁻¹ and 3.8, nitromusk ambrette 0.79 mg l⁻¹ and 4.0, nitromusk moskene 0.046 mg l⁻¹ and 4.4 and nitromusk tibetene 0.052 mg l⁻¹ and 4.3. None of these compounds exhibited a toxic effect in any of the toxicity tests even at the highest concentrations achievable except for nitromusk ambrette which was toxic to Daphnia.     

Inhibition coefficient (Ki) determination in activated sludge

A method of measurement of the inhibition coefficient, K_i, of chemicals within the activated sludge has been proposed. It makes possible the objective determination of the influence of non-competitive inhibitors on the reaction rate by means of simple respiration rate measurements with the application of a dissolved oxygen meter. The values of K_i for chromium Cr⁶⁺, for cyanide CN^? and for two pesticides—DDVP and des-methyl DDVP was determined.     

Denitrification en continu a l'aide de microorganismes immobilises sur des supports solidesContinuous denitrification by immobilized cells

In this paper, we describe a study of biological denitrification by immobilized cells. Nitrates are reduced in sterile solutions by Pseudomonas aeruginosa immobilized in a fixed bed reactor, and in synthetic waste water by mixed cultures immobilized into a fluidized bed reactor.The fixed bed reactor is a Plexiglas column filled with corn stovers (Table 1). It is 0.05 m in diameter and 0.55 in height, its volume being approx. 11. The fresh medium is injected at the base of the column and the liquid level is regulated by an overflow weir. Reactor and carrier are sterilized with ethylene-oxide. After sterilization 1 l. of a growing batch culture of Pseudomonas aeruginosa is introduced aseptically and the reactor is then fed continuously (45 ml h^?1) with fresh medium (NNO₃ = 40 mg l^?1) until the first steady state is reached.Nitrates and nitrites are determinated by means of a colorimetric method.Reactor efficiency remains constant for over 40 days. Nitrates and nitrites concentrations are measured inside the reactor for flow varying from 2 to 16 ml min^?1 (Fig. 2). Reductions of nitrates and nitrites seem to be two first-order reactions (Fig. 3 and Table 2) and constant rate increases with flow rate (Fig. 4). Until nitrate concentration reaches 960 mg/l^?1 (NNO₃) degradation is correct (Figs 5 and 6), beyond nitrites, which have been formed, seem to be inhibitor.Using this reactor, 50 mg NNO₃ have been reduced per hour and per liter of empty reactor, but it may be possible to reduce 140 mg NNO₃ l^?1 h^?1 if fresh medium contains 200 mg NNO₃ l^?1.The fluidized bed reactor is a Plexiglas column filled with earthenware. It is 0.05 m in diameter and 3.15 m in height, its volume being approx. 6.201. Fresh medium is injected at the base of the column and the liquid level is regulated by an overflow weir. Figure 7 shows the retention time of the liquid in the reactor in relation to flow. The first steady state has been reached after 2 weeks, and it has not been possible to know half life time of the column.Four experiments were conducted (Table 3) and, for each nitrate, nitrite and methanol concentrations in the reactor were measured (Fig. 8). So, it appears that reduction of nitrates and nitrites are two first-order reactions (Table 4) and that constant rate values, which are higher than in fixed bed reactor, increase with flow.The reactor is more affected by a flow shift than by a nitrate concentration shift in fresh medium, and biomass linked onto carrier is about 76 mg of dry matter g^?1 of earthenware.So, our fluidized bed column is able to reduce 560 mg NNO₃ h^?1 l^?1 of empty reactor, then retention time of liquid is less than 3 min.     

A comprehensive study on the biological treatabilities of phenol and methanol—II the effects of temperature,pH, salinity and nutrients

The effects of temperature, pH, salinity, and nutrients on bacterial activities were investigated and evaluated using a statistical method. The substrate utilization rate coefficient (k) decreased as pH deviated from neutral and as salinity increased, and the unfavorable pH and salinity alleviated the temperature effect on k. The modified Arrhenius equation, $\text{k}{}_{\mathrm{<mtext>T2</mtext>}}\text{= k}{}_{\mathrm{<mtext>T1</mtext>}}\text{θ(}{}^{\mathrm{T2?T1}}\text{)}$, was not effective in describing the temperature effect on k: the temperature coefficient (θ) ranged between 1.0–1.4 depending on the temperature range, pH, salinity, and substance (phenol or methanol). The endogeneous respiration activity was affected by various environmental factors such as pH, temperature, and salinity; however, the cell decay coefficient (k_d) turned out to be correlated to a single parameter, k. Thus, k_d = 0.066 k^0.87 and k_d = 0.0115 k^0.634, where k and k_d are based on the unit of h^?1, were proposed for the prediction of cell decay coefficient for phenol and methanol acclimated activated sludge, respectively. In batch treatment of 770 mg l^?1 of phenol and 1000 mg l^?1 of methanol as TOC, nitrogen and phosphorus did not have any recognizable effect on k, while trace elements such as Fe²⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Zn²⁺, etc. showed a slightly perceptible effect on it. The absence of extra-cellular nitrogen and phosphorus resulted in a greater cell yield; however, the cells in this condition decayed more rapidly than normal cells. The primary factor affecting the substrate decomposition rate in natural systems was pH: phenol decomposition resulted in a considerable decrease in pH so that the buffering capacity of the water was the most important factor, and methanol decomposition did not affect pH significantly so that the initial pH of the water was the most important factor. An initial lag phase was observed in 8 out of 115 phenol batch tests and 31 out of 66 methanol batch tests.     

The effect of dissolved oxygen concentration on nitrification

The effect of dissolved oxygen concentration on the rate of nitrification has been investigated by a number of researchers using both pure and mixed cultures, and cultures found in wastewater treatment systems. The maximum growth rate of both nitrification reactions are reported to be affected by dissolved oxygen concentration over the range of 0.3 mg l⁻¹ to as much as 4.0 mg l⁻¹. In some instances, it has been reported that a dissolved oxygen concentration in excess of 4.0 mg l⁻¹ is required to achieve maximum nitrification rates, while other investigators have found that only 0.5 to 1.0 mg l⁻¹ is required.It has been proposed that several factors are responsible for the wide range of reported nitrification rates with varying dissolved oxygen concentrations. Among these factors are the effects of oxygen diffusion in flocs, variation between measured results due to steady-state and dynamic measuring techniques, and double-substrate limited kinetics. This paper reviews the nitrification literature with respect to the effects of dissolved oxygen concentration, and shows that double-substrate limiting kinetics could account for the variation in the reported results.     

Utilisation de la clinoptilolite en potabilisation des eaux—elimination de l'ion NH4+Clinoptilolite in drinking water treatment for NH4+ removal

The objective of this study is to develop a technique to remove ammonium ion from water intended for potable purposes. An ion exchange method is used with a selective ion exchanger, a natural cation zeolite, clinoptilolite. Glass columns (Fig. 1) are used for laboratory experiments. These experiments show that the NH₄⁺ exchange capacity is very small compared to its total capacity 2.17 meq g^?1; its value depends essentially on the NH₄⁺ initial concentration and less on the Ca²⁺ concentration in the influent water. Figure 3 illustrates the practical exchange capacity relative to the initial concentration of ammonium ion for a soft water (Ca²⁺ = 35–50 mg l^?1). We were particularly interested in waters weak in ammonium ion concentration (NH₄⁺ = 1–3 mg l^?1). In this case and for ～1 and 2 mg l^?1 NH₄⁺ concentration in water, the practical capacity is only 0.06 and 0.108 meq g^?1 respectively. The leakage is smaller than the ECC limit (European Community Council) for drinking waters (NH₄⁺ ? 0.5 mg l^?1) and the treated volume of water to breakthrough, defined at 0.5 mg l^?1 of NH₄⁺, is ?720 BV (BV = bed volume) in both cases.In another way Fig. 6 shows that hard waters (due to Ca²⁺ ions) are more difficult to treat than soft waters. The practical capacity is smaller than before and the NH₄⁺-leakage is greater. To lessen NH₄⁺-leakage to less than 0.5 mg l^?1 for soft waters down-flow and up-flow, regeneration is used. Figure 7 shows that up-flow regeneration is more attractive than down-flow regeneration.Cycle reproducibility (Figs 4 and 5) shows that the regeneration conditions satisfied our requirements: in this case, the salt consumption is 180 eq of salt per eq of NH₄⁺ eliminated. This prompted us to try to reuse the regenerant (with NH₄⁺ ion). An increase of NH₄⁺-leakage is noticed in the presence of an NH₄⁺-residual in the regenerant. This increase is more significant with down-flow regeneration.After these laboratory experiments, we carried out a semi-industrial pilot-plant. Our objective was first to verify the laboratory results and secondly to study clinoptilolite behaviour relative to the time it was used. Two plexiglass columns comprise the pilot-plant shown in Fig. 9; soft water is used for these experiments. The first column is regenerated with fresh salt solution. The cycles obtained, considering their initial NH₄⁺-concentration, are reproduced in Fig. 10. For 2 mg l^?1 NH₄⁺ in the influent water, the leakage is about 0.2 mg l^?1 and the treated volume to breakthrough (0.5 mg l^?1 of NH₄⁺) is about 750 BV. The second column is regenerated with a recycled solution. The quality of the cycles decreases with the number of reuse of the regenerant as shown in Fig. 11. Nevertheless, it is interesting to note that after 3 reuses, the performance decrease is only 25% and the leakage, although it increases is smaller than 0.5 mg l^?1.Pilot results allowed us to propose a treatment of 30,000 m³ day^?1; the cost per cubic meter water treated, relative to NH₄⁺-removal, is about 0.165 FF (0.033 US $) for a plant and 0.77 FF (0.014 US $) for the same plant at the seaside. Using two serial columns decreased the cost by about 40–50%.     

Restoration of heavily polluted branches of the Shatt Al-Arab river, Iraq

In view of the desire to improve the water quality of the heavily polluted branches of the Shatt al-Arab River at the City of Basrah, it was proposed to maintain effective flushing as well as contracting sewerage system. The present study was conducted in order to examine the water quality of these branches in an attempt to evaluate the effectiveness of the proposed flushing system. It has been found that their waters contained very low levels of dissolved oxygen and relatively high amounts of both COD and BOD₅. The annual average water quality parameters for Basrah Branches were: dissolved oxygen 3.4 ppm; pH 7.67; hydrogen sulphide 1.4 ppm; ammonia 97 μg-at. N l⁻¹; COD 15.9 mg l⁻¹; BOD₅ 12.7 mg l⁻¹; dissolved silicates 202 μg-at. Si l⁻¹; dissolved reactive phosphate 13.4 μg-at. P-PO₄³⁻ l⁻¹; nitrate 10.4 μg-at. N-NO₃⁻ l⁻¹; nitrite 2.1 μg-at. N-NO₂⁻ l⁻¹ and chlorophyll-α 14.3 mg m⁻³. Based on our calculations, it has been concluded that the proposed system is effective, thus within a flushing cycle all of the above mentioned parameters will become within the acceptable values of the Shatt al-Arab water quality. Moreover, this system has no appreciable effect upon the water quality characteristics of the Shatt al-Arab River due to the fact that it discharges a high volume of water annually. However, It has been recommended to dredge the deposited sludge to a minimum depth of 50 cm.     

Leachate treatment with nitrification of ammonia

Results are presented on the treatment of leachate to remove ammonia by biological nitrification. Outdoor activated sludge and trickling filter pilot plants were operated for 2 years at a major co-disposal landfill. Leachate ammonia nitrogen concentrations ranged from 150 to 550 mg l^?1 while TOC concentrations ranged from 200 to 500 mg l^?1. Very little of the TOC was degradable and BOD: NH₃-N ratios were typically 1:3.Nitrification was successfully established in both plants, and curves were established for the response of the kinetics to different temperatures. Maximum ammonia removal rate in the activated sludge plant was at least 131 g N kg VSS^?1 day^?1 achieved at an average temperature of 13°C. Maximum removal rate in the trickling filter was 309 mg N m^?2 day^?1, at 16°C.Operating problems and strategies for full-scale treatment are discussed.     

A kinetic model of steady state phosphorus fixation in a batch reactor—II. Effect of pH

Measurements of phosphorus fixation at the soil/solution ratio of 250 mg 500^?1 ml at pH = 2, 3, 5, 7 and 8 in a batch reactor with steady input of phosphate were carried out. All results reached a steady state and followed a Michaelis-Menten type equation as predicted for the catalytic process. Maximum rate of reaction showed a decrease with increase of pH due to the competitive effect between OH^? and H₂PO^?₄. The maximum value of 1/K at pH = 5 showed H₂PO^?₄ may be required in phosphorus adsorption for some specific-bonding mechanism with soil.     

Comparative disinfection of treated sewage with chlorine and ozone: Effect of nitrification

Chlorine and ozone were compared in pilot plants (capacity about 3.2 m³ h⁻¹), which were fed with the same activated sludge treated and filtered water. Together with physico-chemical analysis the water was analysed for different types of microorganisms, including vegetative bacteria (total and thermotolerant coliforms, faecal streptococci and Pseudomonas aeruginosa), bacterial spores (spores of aerobic bacteria at 37°C and sulphite reducing clostridia) and bacterial viruses (somatic coliphages and F-specific bacteriophages).The average chlorine and ozone dose were, respectively, 3.65 and 15.3 mg l⁻¹ of water, while after a contact time for both of about 25 min the average residual concentrations were 1.79 and 0.35 mg l⁻¹ of water. These residuals were measured with the DPD-method. The ammonia-N concentration varied greatly (0.06–72.0 mg l⁻¹) and was used to group the data into four classes: (1) non-nitrified water, defined as water in which nitrate-N was smaller than ammonia-N; (2) moderately nitrified water, in which nitrate-N was larger than ammonia-N and the ammonia-N was higher than 2 mg l⁻¹; (3) well nitrified water, defined as water in which ammonia-N was lower than 2 mg l⁻¹; (4) very well nitrified water, in which ammonia-N was smaller than 0.5 mg l⁻¹.This classification indicated that the concentrations of most other impurities decreased with a better nitrification. Statistical analysis of the data showed also that ozone was a better disinfectant than chlorine in the case where the disinfection is based upon their residual content. The degree of nitrification had a greater effect on chlorine disinfection than on ozone disinfection.During chlorination the total residual chlorine decreased, with better nitrification; the chlorine demand increased; the composition of the residual chlorine changed very much and the inactivation of bacterial viruses improved. The vegetative bacteria showed a varying pattern; most were inactivated in moderately nitrified water, when the dichloramine concentration was highest and false positive FAC concentration was lowest of the four classes. Reduction of bacterial spores was not observed.During ozonization other effects were indicated. Reductions of most organims increased slightly with better nitrification; only reductions of F-specific bacteriophages decreased. There was also a small decrease of bacterial spores. The treated effluent had a high ozone consumption and the inactivation of the organisms was low in relation to ozone dose and residual ozone.The bromide concentration (0.3–2.9 mg l⁻¹) effected the chemistry of chlorine and ozone and had a positive effect on chlorine and ozone disinfection of total coliforms.For most types of micoorganisms the disinfection coefficients of the Selleck model and the germicidal efficiencies could be determined.     

The source of hydrogen sulfide in anoxic sediment

Putrefactive hydrogen sulfide production in the upper 4 cm of sediment in two small freshwater and eutrophic Southeast Michigan, U.S.A., lakes ranged from 0.13 to 1.51 with an average of 0.46 mg S l^?1 day^?1. Sulfate reductive production of hydrogen sulfide at the same sites ranged from 0.7 to 3.2 with an average of 1.54 mg S l^?1 day^?1. Putrefactive hydrogen sulfide production represented 5.1–53% (average of 18.3–27.6%) of the total hydrogen sulfide produced at the two lakes over an April–October study period. ³⁵S labeled substrates were used to estimate hydrogen sulfide production rates.Proteolytic bacteria averaged 2.2 × 10⁴ cells ml^?1 sediment whereas sulfate reducers averaged 4.8 × 10² cells ml^?1 sediment. Putrefactive hydrogen sulfide production correlated highly with numbers of proteolytic bacteria (r² = 0.84) but the correlation between sulfate reduction and sulfate reducing bacteria was low (r² = 0.13).Interstitial soluble inorganic sulfate, protein and organic carbon were not closely correlated with hydrogen sulfide production rates or bacteria enumeration results. Natural substrate concentrations (S_n) used to estimate hydrogen sulfide production were supported by kinetic bioassay results.     

The toxic effects of trinitrotoluene (TNT) and its primary degradation products on two species of algae and the fathead minnow

The effects of alpha trinitrotoluene (alpha TNT) and its primary degradation product (TNTcc), commonly referred to as “pink water”, were determined on members of two trophic levels. The growth responses of the algae Selenastrum capricornutum and Microcystis aeruginosa were examined through static bioassays. Death and behavioral responses of the fathead minnow (Pimephales promelas) were determined using a proportional diluter. Alpha TNT and TNTcc were both more toxic to the fathead minnow than to either species of alga. Five and 15 mg l⁻¹ alpha TNT inhibited S. capricornutum and M. aeruginosa growth, respectively. TNTcc inhibited S. capricornutum growth at concentrations above 9 mg l⁻¹; it was lethal to M. aeruginosa at 50 mg l⁻¹, but stimulated growth at lower concentrations. The 96-h lc₅₀ values based on the death response of the fathead minnow to alpha TNT and TNTcc were 2.58 and 1.60 mg l⁻¹, respectively. The 96-h ec₅₀ values based on the behavioral responses were 0.46 and 0.64 mg l⁻¹, respectively. There was no response to concentrations of 0.05 mg l⁻¹ alpha TNT and 0.07 mg l⁻¹ TNTcc.     

The effects of bubble size on microflotation

Microflotation has been shown to be an efficient process for the removal of various colloidal sized impurities from water and waste water. Laboratory studies have shown that for this purpose very small bubbles are required. This study is to determine the effects of concentration of frother (ethanol) and collector (lauric acid) upon bubble size and upon efficiency of humic acid removal.Bubble sizes were measured photographically and humic acid removals determined spectrophotometrically. Ethanol was varied from 0.25 to 7.50 ml l^?1 and lauric acid from 0 to 100 mg l^?1 concentration. Initial humic acid concentration was 50 mg l^?1 and 0.0005 m aluminum sulfate was employed as a coagulant. All experiments were conducted at an adjusted pH of 7.5 in a 400 ml batch flotation cell. Ethanol produced a greater effect on bubble size than lauric acid; small additions of ethanol (ml l^?1) reduce bubble size drastically.The investigation showed a rapid decrease in microflotation removal efficiencies when bubble diameters exceed 55–60 μm. Efficient separations are extremely rapid, being essentially complete in 30 60 s.     

Growth kinetics of the filamentous microorganism Sphaerotilus natans in a model system of a food industry wastewater

Bulking by Sphaerotilus natans has been attributed to several factors such as low dissolved oxygen in the aeration basin, wastes with high C:N ratios and phosphorus limitation; however, the occurrence of bulking has been reported in fruit, vegetable, meat and poultry wastewaters in which the ratio C:N is variable.Growth of S. natans was analyzed in a model system of a food industry wastewater (potato processing waste) that was characterized by HPLC determining that citric acid was the most important identified component. The effect of several carbon sources on S. natans growth was also studied; different C:N ratios were tested in a continuous culture system (chemostat). This strain grew in a mineral medium with citric acid as a sole carbon source, in spite of the contradictory results found in literature. Chemostat studies showed that the medium was carbon-limited when C:N ratios <19 mgCOD (mgN-NH₃)⁻¹. Monod kinetic growth coefficients, determined for this strain in chemostat were: maximum specific growth rate, μ_max=0.301 h⁻¹; Monod constant, K_S=4.6 mgCOD l⁻¹; true biomass growth yield, Y^T_X/S=0.490 mgVSS (mgCOD)⁻¹; endogenous decay rate, k_d=0.011 h⁻¹ and maintenance coefficient, m_S=0.022 mgCOD (mgVSS)⁻¹ h⁻¹. The obtained parameters were compared with literature data and the effect of glucose and citric acid as carbon sources was discussed; these parameters are useful in modeling the growth of S. natans in potato processing wastewaters (or in other effluents under carbon-limiting conditions) especially when citrate is the main component and can be used to control filamentous bulking by metabolic or kinetic selection.     

Validation of an automated fluorescein method for determining bromide in water

Surface, atmospheric precipitation and deionized water samples were spiked with μg l^?1 concentrations of bromide, and the solutions stored in polyethylene and polytetrafluoroethylene bottles. Bromide was determined periodically for 30 days. Automated fluorescein and ion chromatography methods were used to determine bromide in these prepared samples. Analysis of the data by the paired t-test indicates that the two methods are not significantly different at a probability of 95% for samples containing from 0.015 to 0.5 mg l^?1 of bromide. The correlation coefficient for the same sets of paired data is 0.9987. Recovery data, except for the surface water samples to which 0.005 mg l^?1 of bromide was added, range from 89 to 112%. There appears to be no loss of bromide from solution in either type of container.     

Toxicity of 2,4-D acid to phytoplankton

The toxic effects of 2,4-D on Phaeodactylum tricornutum (Bohlin) and Dunaliella tertiolecta (Butcher), two species of phytoplankton well suited to bioassay studies and responsive to pollutants, were studied by monitoring changes in growth in terms of cell populations, chlorophyll fluorescence and the rate of ¹⁴CO₂ assimilation. Short term bioassays, batch and continuous cultures were studied. Pure 2,4-D acid appeared more toxic than the commercial amine form of the herbicide but this may have been due to small quantities of acetone present in the solvent. Concentrations of amine herbicide in excess of 100 mg l⁻¹ extended the duration of the lag phase and inhibited growth but smaller concentrations stimulated growth, the amine being consumed by phytoplankton in preference to nitrate. Continuous culture confirmed the ability of phytoplankton to adapt slowly to herbicide concentrations even as high as 500 mg l⁻¹. It is suggested that green algae adapt more rapidly to environmental change than do diatoms.