Hydraulic retention time influence on azo dye and sulfate removal during the sequential anaerobic-aerobic treatment of real textile wastewater.

In the present study, we evaluate the behavior of real textile wastewater treatment using a system composed of two sequential pilot-scale reactors (anaerobic followed by aerobic) during 622 days. The work focused on the competition between color and sulfate removal processes, when the hydraulic retention time (HRT) was increased in the anaerobic/aerobic reactors from 16/12 hours in phase I (PI) to 4/3 days in phase II (PII). The organic matter was successfully removed in both phases through the system, and the highest efficiency (75%) was achieved in PII. The increase in the HRT did not improve azo dye degradation under anaerobic conditions. Instead, it favored sulfate reduction, which removal efficiency increased from 26% in PI to 75% in PII. Aromatic amines were detected in the anaerobic reactor effluent and removed in the aerobic reactor.

Anaerobic treatment of crude glycerol from biodiesel production.

In this study, we evaluated the use of an up-flow anaerobic sludge blanket (UASB) reactor to treat crude glycerol obtained from cottonseed biodiesel production. The laboratory-scale UASB reactor (7.0 L) was operated at ambient temperature of 26.5°C with chemical oxygen demand (COD) concentrations between 0.5 and 8.0 g/L. The volatile fatty acid contents, pH, inorganic salt contents and biogas production were monitored during a 280-day experimental period. Molecular biology techniques were used to assess the microbial diversity in the bioreactor. The reactor achieved COD removal efficiencies of up to 92% except during one phase when the efficiency decreased to 81%. Biogas production remained stable throughout the experimental period, when the fraction converted to methane reached values as high as 68%. The profile of the denaturing gradient gel electrophoresis (DGGE) bands suggested slight changes in the microbial community during reactor operation. The overall results indicated that the crude glycerol from biodiesel production can serve as a suitable substrate for anaerobic degradation with a stable reactor performance and biogas production as long as the applied organic loads are up to 8.06 kg COD/m3·d.

The role of matrix metalloproteinases in dental erosion.

This review discusses the role of matrix metalloproteinases (MMPs) in the development of dentin erosion and the protective effects of MMP inhibitors, based on recent evidence from in vitro and in situ studies. MMPs are present in both dentin and saliva and play an important role in dentin erosion progression. Enzymatic removal of the organic matrix by MMPs increases the demineralization process, since the demineralized organic matrix has been shown to hamper ionic diffusion after an acidic challenge. Recent evidence from in vitro and in situ studies has shown a protective role of MMP inhibitors against dentin erosion and erosion plus abrasion. The inhibitors tested were green tea and its active epigallocatechin-gallate (EGCG), ferrous sulfate, and chlorhexidine. They have been tested in dentifrices, solutions, and gels. The latter led to a more pronounced protective effect against dentin erosion and erosion plus abrasion. The protection was long-lasting and could be observed after up to 10 days of severe erosive and erosive-plus-abrasive challenges in situ. Thus, the use of MMP inhibitors has emerged as an important preventive tool against dentin erosion. Clinical studies should be conducted to confirm the results obtained and to give support to the establishment of clinical protocols of use.

Activity of matrix metalloproteinases in bovine versus human dentine.

Metalloproteinases (MMPs) have been implicated with metabolism of collagen in physiological and pathological processes in human dentine. As bovine teeth have been used as a substitute for human teeth in laboratory analysis, this study evaluated the activity of MMP-2 and -9 in bovine versus human dentine. Bovine and human dentine fragments, from crowns and roots, were powderized. Protein extraction was performed by two protocols: a neutral extraction with guanidine-HCl/EDTA (pH 7.4) and an acidic extraction with citric acid (pH 2.3). Gelatinolytic activities of extracts were revealed by zymography. MMP-2 and -9 were detected in crown and root dentine from bovine and human teeth. Total activities of MMP-2 were 11.4 ± 2.2, 14.6 ± 2.0, 9.7 ± 1.2 and 12.4 ± 0.9 ng/ml for bovine root, human root, bovine crown and human crown dentine, respectively. Corresponding activities for MMP-9 were 14.9 ± 2.0, 15.3 ± 1.3, 15.4 ± 1.3 and 15.5 ± 1.3 ng/ml, respectively. Bovine dentine was found to be a reliable substrate for studies involving the activity of MMP-2 and -9.

Textile effluent treatment in a UASB reactor followed by submerged aerated biofiltration.

An upflow anaerobic sludge bed (UASB)-submerged aerated biofilter (SAB) system that treats effluents from a jeans factory was evaluated. The 210-day operational period was divided into three phases (PI, PII and PIII), each with a different hydraulic retention time (HRT in h) and organic loading rate (OLR in kg COD/m3.d). In PI, the best performance was achieved using the UASB (HRT 24, OLR 1.3) with COD and color removal efficiencies of 59 and 64%, respectively; the corresponding values were 77 and 86% for the final effluent. In PII, the efficiencies were 50 and 55% using the UASB (HRT 16, OLR 1.2), respectively, and 69 and 81% for the final system effluent, respectively. In PIII, the UASB (HRT 12 and ORL 3.2) showed the poorest performance; the efficiencies decreased to 48 and 50%, respectively. The same phenomenon occurred in the system with corresponding efficiencies decreasing to 69 and 61%. Throughout the experiment, the system removal efficiencies were between 57 and 88% for nitrogen and between 14 and 63% for sulfate. The final effluent showed relatively non-toxicity or moderate toxicity using Daphnia magna as an indicator. Therefore, the overall results showed that the use of a sequential anaerobic-aerobic system is promising for treatment of textile industrial wastewater.

Preventive effect of commercial desensitizing toothpastes on bovine enamel erosion in vitro.

This study evaluated in vitro commercial desensitizing toothpastes with respect to the prevention of erosion and explored the effect of their agents alone or in combination with fluoride. Bovine enamel blocks were randomly allocated to five groups of 20 and exposed to: Sensodyne ProNamel (1,425 ppm F as NaF, 5% KNO(3)), Sensodyne Original (no fluoride, 10% SrCl(2)), Colgate Sensitive (1,450 ppm F as sodium monofluorophosphate, 5% K citrate), Crest (fluoride-only toothpaste, 1,100 ppm F as NaF) and water (negative control). A second experiment was conducted with experimental dentifrices containing fluoride (NaF, 1,100 ppm F), 10% SrCl(2), 5% KNO(3 )or 5% K citrate alone or the latter three combined with F. The samples were submitted to four cycles, alternating demineralization (cola, 10 min) and remineralization (artificial saliva, 1 h). Before and between cyclic de- and remineralization, blocks were treated with slurries of the respective toothpastes or water (1 min). Erosive tissue loss was analyzed by profilometry. Data were analyzed by Kruskal-Wallis and Dunn's tests (p < 0.05). The mean erosion depth (+/- SE, mum) was significantly less for Colgate Sensitive (0.04 +/- 0.00), Sensodyne Original (0.06 +/- 0.01) and Crest (0.07 +/- 0.01) than for Sensodyne ProNamel (2.36 +/- 0.25) or water (2.92 +/- 0.24), which did not significantly differ from each other. Both F and the desensitizing agents alone reduced erosion, but no additive effect was found. In addition, the combination of F and KNO(3) did not reduce erosion. These in vitro results suggest that the presence of fluoride or desensitizing substances in toothpastes, alone or in combination, can reduce erosion of enamel, but this is not valid for all the formulations.

Effect of iron on matrix metalloproteinase inhibition and on the prevention of dentine erosion.

It is known that some metal salts can inhibit matrix metalloproteinase (MMP) activity, but the effect of iron has not been tested yet. On the other hand, it has recently been suggested that MMP inhibition might influence dentine erosion. Based on this, the aims of this study were: (1) to test in vitro the effect of FeSO(4) on MMP-2 and -9 activity, and (2) to evaluate in situ the effect of FeSO(4) gel on dentine erosion. MMP-2 and -9 activities were analysed zymographically in buffers containing FeSO(4) in concentrations ranging between 0.05 and 1.5 mmol/l or not. Volunteers (n = 10) wore devices containing bovine dentine blocks (n = 60) previously treated with the following gel treatments: FeSO(4) (1 mmol/l FeSO(4)), F (NaF 1.23%; positive control) and placebo (negative control). The gels were applied once and removed after 1 min. Erosion was performed extraorally with Coca-Cola 4 times per day for 5 min over 5 days. Dentine wear was evaluated by profilometry. The data were analysed by Kruskal-Wallis and Dunn's tests (p < 0.05). FeSO(4) inhibited both MMP-2 (IC(50) = 0.75 mmol/l) and MMP-9 (IC(50) = 0.50 mmol/l) activities. In the in situ experiment, the mean wear (+/- SD) found for the F gel (0.79 +/- 0.08 microm) was significantly reduced in more than 50% when compared to the placebo gel (1.77 +/- 0.33 microm), but the FeSO(4) gel completely inhibited the wear (0.05 +/- 0.02 mum). Since FeSO(4) was able to inhibit MMP in vitro, it is possible that the prevention of dentine wear by the FeSO(4) gel in situ might be due to MMP inhibition, which should be investigated in further studies.

Microaerated UASB reactor treating textile wastewater: The core microbiome and removal of azo dye Direct Black 22.

Sequential anaerobic and aerobic processes have been recommended to treat textile wastewater reliably. In this work, the focus was on finding an energetically more competitive system to remove tetra-azo dye Direct Black 22 (DB22). We operated two upflow anaerobic sludge blanket (UASB) reactors (R1 and R2) in three phases (PI, PII, and PIII). R1 was operated as a conventional UASB, while R2 was microaerated in the upper part (0.18 ±â€¯0.05 mg O2. L-1), aiming to remove DB22 simultaneously with the aromatic amine byproducts. PI consisted of feeding reactors with synthetic textile wastewater (STW), PII had higher salinity in the STW, and PIII was the same as PII, plus sulfate. The results showed that color and COD removal efficiencies were similar for both reactors (67-72% for R1 and 59-78% for R2) without a substantial influence of oxygen in R2. However, microaeration played a crucial role in R2 by removing the anaerobically formed aromatic amines; during PIII, the effluent was 16 times less toxic than that of R1. The microbial community that developed in the sludge bed of both reactors was quite similar, with the core microbiome represented by Trichococcus, Syntrophus and Methanosaeta genera. The increase in salinity in PII and PIII promoted a shift in the microbial community, excluding salty-sensitive genera from the core microbiome. The putative genera Brevundimonas and Ornatilinea were associated to aromatic amine microaerobic removal. Therefore, there is a potential application of a compact microaerated anaerobic system for textile wastewater treatment.

Production of 1,3-propanediol from pure and crude glycerol using a UASB reactor with attached biomass in silicone support.

The 1,3-propanediol (1,3-PDO) yield and productivity from glycerol were studied over a 155-day period. A UASB reactor that also contained silicone support for biomass attachment was used to evaluate the optimal operational conditions and microbiota development. The highest average 1,3-PDO yield was 0.54 and 0.48 mol.mol-gly-1 when reactor pH was 5.0-5.5 and the applied loading rate was 18 and 20 g-gly.L-1.d-1 using the pure and crude substrate, respectively. The productivity was close to 7.5 g.L-1.d-1 for both substrates; therefore, the direct use of crude glycerol can be valorized in practice. Clostridium was the predominant genus for 1,3-PDO production and C. pasteurianum was dominant in the biofilm. Using crude glycerol, C. beijerinckii dropped strongly; some Clostridium population was then replaced by Klebsiella pneumoniae and Lactobacillus spp. The good process performance and the advances in the microbiota knowledge are steps forward to obtain a more cost-effective system in practice.

The effect of an experimental 4% Tif4 varnish compared to NaF varnishes and 4% TiF4 solution on dental erosion in vitro.

This in vitro study assessed the effect of an experimental 4% TiF(4) varnish compared to commercial NaF and NaF/CaF(2) varnishes and 4% TiF(4) solution on enamel erosion. For this, 72 bovine enamel specimens were randomly allocated to the following treatments: NaF varnish(2.26% F), NaF/CaF(2) varnish (5.63% F), 4% TiF(4) varnish (2.45% F), F-free placebo varnish, 4% TiF(4 )solution (2.45% F) and control (not treated). The varnishes were applied in a thin layer and removed after 6 h. The solution was applied to the enamel surface for 1 min. Then, the specimens were alternately de- and remineralized (6 times/day) in an artificial mouth for 5 days at 37 degrees C. Demineralization was performed with the beverage Sprite (1 min, 3 ml/min) and remineralization with artificial saliva (day: 59 min, 0.5 ml/min; during the night: 0.1 ml/min). The mean daily increment of erosion and the cumulative erosion data were tested using ANOVA and ANCOVA, respectively, followed by Tukey's test (alpha = 0.05). The mean daily erosion increments and cumulative erosion (micrometers) were significantly less for the TiF(4) varnish (0.30 +/- 0.11/0.65 +/- 0.75) than for the NaF varnish (0.58 +/- 0.11/1.47 +/- 1.07) or the NaF/CaF(2) varnish (0.62 +/- 0.10/1.68 +/- 1.17), which in turn showed significantly less erosion than the placebo varnish (0.78 +/- 0.12/2.05 +/- 1.43), TiF(4) solution (0.86 +/- 0.11/ 2.05 +/- 1.49) and control (0.77 +/- 0.16/2.06 +/- 1.49). In conclusion, the TiF(4) varnish seems to be a promising treatment to reduce enamel loss under mild erosive conditions.

Structure and dynamics of the phytoplankton community within a maturation pond in a semiarid region.

In northeastern Brazil, stabilization ponds are very suitable for wastewater treatment because of the relative great land availability and environmental conditions (e.g., high temperature) favorable for microorganism optimal development. However, blooms of potentially toxic cyanobacteria may affect the use of these treatment ponds due to resulting effluent poor quality. The objective of this study was to evaluate the dynamics of phytoplankton communities and the occurrence of cyanobacteria in a maturation pond located immediately after a series of two ponds. Temperature, dissolved oxygen, pH, BOD, N, and P were measured during a period of four months when samples were collected from the surface and the bottom of 7 sampling points distributed inside the pond. The phytoplankton of collected samples was also identified and classified using a conventional optical microscopy. Analysis of variance and Tukey test were used to evaluate the results. The three phytoplankton divisions found (Cyanophyta, Chlorophyta, and Euglenophyta) did not change considerably through surface and bottom. However, they changed greatly over the sampled months; great dominance of Cyanophyta was found at April and October, while Chlorophyta dominated the lagoon in September. Low superficial organic loads (between 78 and 109 kg BOD.ha-1.d-1) and N:P ≤ 10 were the determinant factors that favored the predominance of Cyanophyta. The presence of two potentially toxic species of Cyanophyta, Oscillatoria sp. and Microcystis aeruginosa, indicates that caution is required when considering the final destination of treated effluent and suggests a need to assess the risks and benefits associated with the use of the treatment technology.

Removal of E. coli and helminth eggs in UASB: Polishing pond systems in Brazil.

Ponds following anaerobic reactors, such as Upflow Anaerobic Sludge Blanket (UASB) reactors, have been termed polishing ponds in the literature. The present paper analyses the removal of E. coli and helminth eggs in five UASB-polishing pond systems in Brazil. Since there were ponds in series, the total number of ponds was 10. The ponds had average retention times varying from 2 to 21 days, and depths ranging from 0.40 to 2.00 m. The shallow ponds in series, even with low retention times, were able to produce effluents complying with the coliform WHO guidelines for unrestricted irrigation (< or = 1000 MPN/100 ml). An equation for the coliform decay coefficient was proposed: Kb (dispersed flow) = 0.710H(-0955) (20 degrees C). The equation highlights the inverse relationship between the pond depth and the decay coefficient. All polishing pond systems were able to produce effluents with helminth eggs concentrations predominantly equal to zero, and satisfying the WHO guidelines for unrestricted and restricted irrigation (< or = 1 egg/L, arithmetic mean). The approximate range of helminth eggs removal efficiency was predicted satisfactorily.

The role of matrix metalloproteinases and cysteine-cathepsins on the progression of dentine erosion.

OBJECTIVE: To evaluate in vitro the effect of the inhibition of endogenous dentinal enzymes (matrix metalloproteinases-MMPs and cysteine cathepsins-CCs) on dentine erosion. DESIGN: Dentine blocks (4mm×4mm×2mm) from sound human teeth were randomly divided into 7 groups (n=17) according to the treatment: MMP- and CC-inhibitor chlorhexidine digluconate (CHX, 10mM); MMP-inhibitor galardin (G, 0.2mM); specific cathepsin B inhibitor (CCB, 0.2mM); non-specific CC inhibitor (CCE-64, 0.5µM); fluoride (F, 1.23% NaF); placebo (P) and untreated (UT). Inhibitors were applied as gels once for 1min. Specimens were submitted to 5 days of pH cycling including the erosive challenge (Coke, pH 2.64, 90s/day) and remineralisation (artificial saliva). Demineralised organic surface loss was determined profilometrically. Demineralised organic matrix (DOM) was removed with collagenase and the profile was re-evaluated in the absence of collagen fibrils. The differences in profilometric results and DOM thickness among the groups were analysed with ANOVA and Tukey's test (p<0.05). RESULTS: Loss of demineralised tissue (µm, mean±SD) was: CHX 8.4±1.7 b, G 8.6±1.9 b, CCB 9.6±1.4 a, CCE-64 9.9±1.3 a, F 9.9±1.7 a, P 10.9±2.2 a, UT 11.0±1.5 a. Loss of mineralised tissue was: CHX 15.4±2.2 b, G 16.0±1.8 b, CCB 17.6±2.4 a, CCE-64 17.6±2.0 a, F 17.3±2.8 a, P 19.1±2.1 a, UT 18.9±2.4 a. MMP-inhibitors significantly reduced organic matrix and mineral loss in comparison to all the other groups (p<0.05). No statistically significant differences were found in the thickness of the remaining DOM (p=0.845). CONCLUSION: Dentine endogenous MMPs seem to be the main enzymes responsible for DOM loss and erosion.

Integrated measures for preservation, restoration and improvement of the environmental conditions of the Lagoon Olho d'Agua basin.

The Lagoon Olho d'Agua in Pernambuco State, Northeast Brazil has received increasing environmental concern due to significant stress from pollution in the catchment. The existing environmental problems are the result of great pressure from a broad range of human activities, especially in the last 10 years. Serious pollution exists mainly from some industrial and urban activities, which increased intensively after the eighties. There is a strong social and economical pressure for housing and construction near the lagoon, due to the available land nearby beaches and estuarine zone, and recently by growing tourism activities. Uncontrolled land use by low-income communities and the pressure for construction by developers have led to landfilling and to deterioration of water quality in the lagoon catchment. Improvement of the environmental conditions in the catchment needs integrated measures. Guidelines and some specific actions involving several institutions have been established and refer to sanitation and urban infrastructure as the main priorities. A main target is the construction of low-cost sewage system with smaller and decentralised treatment plants.

Anaerobic treatment of low-strength brewery wastewater in expanded granular sludge bed reactor.

Anaerobic treatment of low-strength brewery wastewater, with influent total chemical oxygen demand (COD) (CODin) concentrations ranging from 550 to 825 mg/L, was investigated in a pilot-scale 225.5-L expanded granular sludge bed (EGSB) reactor. In an experiment in which the temperature was lowered stepwise from 30 to 12 degrees C, the COD removal efficiency decreased from 73 to 35%, at organic loading rates (OLR) of 11-16.5 g COD/L/d. The applied hydraulic retention time (HRT) and liquid upflow velocity (Vup) were 1.2 h and 5.8 m/h, respectively. Under these conditions, the acidified fraction of the CODin varied from 45 to 90%. In addition to the expected drop in reactor performance, problems with sludge retention were also observed. In a subsequent experiment set at 20 degrees C, COD removal efficiencies exceeding 80% were obtained at an OLR up to 12.6 g COD/L/d, with CODin between 630 and 715 mg/L. The values of HRT and Vup applied were 2.1-1.2 h, and 4.4-7.2 m/h, respectively. The acidified fraction of the CODin was above 90%, but sludge washout was not significant. These results indicate that the EGSB potentials can be further explored for the anaerobic treatment of low-strength brewery wastewater, even at lower temperatures.

Domestic sewage treatment in full-scale UASBB plant at Mangueira, Recife, Pernambuco.

The anaerobic technology application for domestic sewage treatment in Pernambuco State (Brazil) is relatively recent. Some UASB reactors of less than 250 m3 were built in the Recife Metropolitan Region (RMR) in the 1990s. Mangueira (18,000 inhabitants) was the first neighborhood where the municipality built a plant with a bigger UASB reactor of 810 m3. It was intended to evaluate the performance and verify if such technology would be feasible. The objective would be the possible application of UASB reactors to the RMR, according to the new sewage master-plan under elaboration that would benefit about 3 million inhabitants. The monitoring of the Mangueira UASB reactor over 30 months showed that satisfactory results were obtained. Three distinct operational phases occurred, in which efficiency varied from 60% up to 90% based on COD removal. The results were very dependent on the operation and maintenance, either in the plant or in the sewage collection. Significant amount of inert solids was measured inside the reactor. Despite the operational problems, the UASB was shown to be very robust and stable. Under high fluctuation of influent concentrations (150-750 mg COD/L) during the period, resulting in applied organic loading rate of 0.5 to 2.5 kg COD/m3.d, the average values of COD removal efficiency did not change significantly. An active biomass with specific methanogenic activity varying from 0.18 to 0.25 g COD/g VSS.d was measured at the end of the period.

Post-treatment of UASB effluent in an expanded granular sludge bed reactor type using flocculent sludge.

The performance of an EGSB (expanded granular sludge bed) reactor type using flocculent sludge was evaluated for the post-treatment of effluent from UASB reactor treating domestic sewage. A pilot-scale 157.5-L EGSB reactor was monitored during a 331-day period. The original concept of the EGSB reactor was based on granular sludge use and by applying of high upflow liquid velocities (Vup). However, even using flocculent sludge from a UASB full-scale (Mangueira Plant, Recife, Brazil), good mixing conditions and high retention of biomass were achieved. By applying a 4-h hydraulic retention time and Vup values up to 3.75 m/h, effluent chemical oxygen demand concentrations in the EGSB were below 87 mg/L and 55 mg/L, for total and filtered samples, respectively. Total suspended solid (TSS) concentrations in the effluent were below 32 mg/L. Good performance and stable reactor operation during the whole experimental period were observed. Therefore, the EGSB reactor type using flocculent sludge can also be used for post-treatment of very dilute anaerobic effluent from reactors treating domestic sewage.

The effect of the supplementation with a primary carbon source on the resistance to oxygen exposure of methanogenic sludge.

Anaerobic methanogenic consortia have a considerable resistance to oxygen exposure. Yet, most research has been focused on the study of the tolerance to oxygen of anaerobic immobilized biomass. Less is known on the potential of the anaerobic suspended biomass for withstanding exposure to oxygen and the effect of a primary degradable substrate on such resistance. Thus, the objective of this work was to determine the effect of the amount of a primary degradable substrate (sucrose) on the resistance of a methanogenic suspended biomass to oxygen exposure. It was found that the inhibition of disperse anaerobic sludge by oxygen exposure decreases when the concentration of the supplemented carbon source increases. This is in agreement with the fact that aerobic respiration of the added substrate by the facultative heterotrophic bacteria, always present in this type of sludge, has been found in previous studies as one of the main mechanisms protecting methanogens against O2. From a practical point of view, this suggests that aeration of anaerobic systems should be possible without inhibiting the activity of methanogenic bacteria if an adequate ratio between oxygen and COD feeding is maintained. Such a ratio will depend however on the wastewater initial COD concentration.

Color, organic matter and sulfate removal from textile effluents by anaerobic and aerobic processes.

An upflow anaerobic sludge blanket (UASB)-submerged aerated biofilter (SAB) system was evaluated to remove color and chemical oxygen demand (COD) from real textile effluent. The system was operated for 335 days in three phases (P-1, P-2, P-3) with total hydraulic retention time varying from 21 h to 14 h. The results showed that high sulfate levels (>300 mg SO4(2-)/L) impaired the dye reduction. The best color removal efficiencies of 30% and 96% for the UASB and the reactor system, respectively, were obtained in P-1; the SAB higher efficiency was associated with adsorption. The best COD removal efficiency of 71% for the reactor system was obtained in P-2. Precipitation of some material composed mostly of sulfur (98%) and some metals occurred in the UASB. However, the precipitated sulfur was again oxidized in the SAB. The system also showed an effective toxicity reduction in tests (Daphnia magna) with the treated effluent.

Sodium fluoride inhibits MMP-2 and MMP-9.

The importance of fluoride (F) in preventing dental caries by favorably interfering in the demineralization-remineralization processes is well-established, but its ability to inhibit matrix metalloproteinases (MMPs), which could also help to prevent dentin caries, has not been investigated. This study assessed the ability of F to inhibit salivary and purified human gelatinases MMPs-2 and -9. Saliva was collected from 10 healthy individuals. Pooled saliva was centrifuged, and supernatants were incubated for 1 hr at 37°C and subjected to zymography. Sodium fluoride (50-275 ppm F) was added to the incubation buffer. The reversibility of the inhibition of MMPs-2 and -9 by NaF was tested by the addition of NaF (250-5,000 ppm F) to the incubation buffer, after which an additional incubation was performed in the absence of F. F decreased the activities of pro- and active forms of salivary and purified human MMPs in a dose-response manner. Purified gelatinases were completely inhibited by 200 ppm F (IC50 = 100 and 75 ppm F for MMPs-2 and -9, respectively), and salivary MMP-9 by 275 ppm F (IC50 = 200 ppm F). Inhibition was partially reversible at 250-1,500 ppm F, but was irreversible at 5,000 ppm F. This is the first study to describe the ability of NaF to inhibit MMPs completely.