Toxic loading

The toxic effects model uses concentration-time profiles from the respiratory and skin protection models as input to estimate casualty probabilities. Two approaches are available a simple linear dose-effect model as incorporated in RAP and a more elaborate non-linear response model, based on the Toxic Load approach. The latter provides a better description of toxic effects for agents that show significant deviations of simple Haber s law behaviour (i.e. toxic responses only depend on the concentration-time product and not on each quantity separately). 

Dose typically refers to the amount of toxic material actually retained and is sometimes referred to as the toxic load. Thus, the dose can be expressed as the product of a concentration term and a dura-tion-of-exposure term, by either of the following relationships ... 

In other cases, speeialized enzymes and genetically engineered bacteria have been used to gain temporary relief of shock and other toxic loads to a treatment system. The application of engineered bacteria may be practical... 

Isopleth—A plot of specific locations (in the three spatial coordinates—x, y, z) downwind from the release source that is corresponding to a concentration of interest (e.g., fixed by toxic load or flammable concentration). 

Gelashvilly, D.B. Bezrukova, N.V. Bezrukov, M.E. Ecotoxicological analysis of toxic load of industrial enterprises of Nizhni Novgorod to water bodies of river part of Cheboksarski reservoir. News of Samara Scientific Center of Russian Academy of Science 2000, 2, 244-251 (in Russian). 

Dry environmental quality PetroKlenz must be reconstituted with water and made into a slurry. The organisms will be ineffective if they are applied as dry cultures over the oil. The effective use of PetroKlenz in a biotreatment program is dependent upon the environmental conditions present at the site. Adverse conditions such as cold temperatures, oxygen, nitrogen, or phosphorus-deficient water, chemical toxic load, highly acidic or alkaline conditions, or excessive dilution of biomass by tides and currents, may retard or prevent biodegradation. 

It is also common in discussions of xenobiotic metabolism to find that there is an assumption that all xenobiotics must be toxic and that any metabolism of the xenobiotic must reduce the toxic load. Of course the reality is that most xenobiotics (other than drugs) will possess no significant biomolecular activity (for reasons discussed in Chapter 5) and the degradation of a substance creates one or more new compounds that are equally likely to possess significant biomolecular activity hence degradation is no guarantee of detoxification. ... 

Determination of the toxic loading of each effluent by measuring its toxic... 

Bay (Toyama Prefecture, Japan) to identify those possessing the highest toxic loads (Kusui and Blaise, 1999). ... 

The PEEP scale is a cost-effective tool to determine the toxic loading potential of a series of point source liquid wastes discharging to a common receiving environment, owing to the use of small-scale bioassays. 

Application of the PEEP index provides unambiguous capacity to discriminate between effluents having low and high toxic loading potentials. 

Numerical PEEP index values are the log10 expression of an effluent s toxic loading (= toxic potential of effluent generated with a relevant battery of toxicity tests multiplied bv effluent flow) and normally vary between 0 and 10. The PEEP scale can thus be considered as a type of environmental Richter scale for... 

Because they are readily understandable by environmental managers, industrialists and the general public, PEEP values enable rapid and enlightened decision-making to circumscribe specific effluents which should be targeted for clean-up actions in order to reduce toxic loading to aquatic environments. 

Once toxic units are calculated for all bioassays, they are integrated in the toxic print portion of the PEEP formula, which is multiplied by effluent flow datum (Q = 3213 m3/h). The product of toxic print and flow yields the toxic loading of the effluent. The resulting PEEP index value of 5.8 is then simply the log10 of the calculated effluent sample toxic loading (plus 1). The value of 1 , inserted into the PEEP formula just ahead of the toxic print, insures that the inferior scale of the PEEP index will commence at 0 for effluents which are non toxic (i.e., those where toxicity responses are absent for all of the bioassays and which yield a ETz value = 0). 

Figure 6. Relative toxic loading to the Saint-Lawrence River of the four industrial sectors made up by the 50 priority effluents identified for study under the first Saint-Lawrence River...

Figure 1. Evolution of effluent toxicity in France from 1974 to 1993 in terms of actual (discharged toxic units) and avoided (treated toxic units) toxic loading reaching surface waters.

The last step in calculating an index was to simplify the numerical values of the coefficients from each test in order to have an easier way to calculate the model. Fig. 7 compares the calculated results with the experts judgements. We also considered the possibility of including effluent flow in the index formula such that the quantity of toxic material (/. e., toxic loading) discharged to the receiving environment can be estimated. 

Hazard potential for each effluent was calculated using a mathematical formula (the PEEP index) proposed by Costan et al. (1993). This formula integrates the ecotoxic responses of the battery of tests before and after a biodegradation step. Toxicity test endpoint responses are first transformed to toxic units. The product of effluent toxicity and effluent flow (m3/h) gives the toxic loading value. The log 10 value of an effluent s toxic loading corresponds to its PEEP index. In order to rank the effluents a toxicity classification scale is generated (Tab. 11). 

To reduce the inflow of toxic substances into basin waters efficiently, the Corporation required information on which effluents posed the greatest hazard to the river. While chemical data on many of the effluents were available, their diverse composition was difficult to interpret in terms of hazard potential. Moreover, hazard is not only linked to chemical composition of an effluent, but also on the toxic effects it can have on a variety of freshwater organisms. Adverse effects on biota are also influenced by the volume of wastewater discharged at different times of the year. Hence, the Corporation searched for a cost-effective approach, based on ecotoxicological principles, to rank the various effluents (there are several hundred sources), in terms of their toxic loading, so that subsequent efforts (such as clean up actions) could be prioritized. 

The results demonstrated a wide range of toxic effects and loadings for textile effluents (Tab. 13). Inter-effluent toxicity differences could be attributed to factors such as type of industrial process, degree of waste treatment, variability of effluent composition and dilution by process waters. Toxic loads from industries 9 and 10 indicate they contribute the greatest toxic load to the receiving waters therefore based on PEEP results the first priority would be to reduce the toxic loading from industries 9 and 10. 

In brief, the PEEP index is a useful HAS to apply in comparative studies of wastewater effluents to assess their ecotoxicity and toxic loading. Some of its advantages include the fact that it considers results from different toxicity tests and endpoints, while integrating all possible antagonistic, additive or synergistic interactions that can occur between toxicants in a complex liquid sample. Furthermore, the use of a single PEEP value becomes very useful for decisionmakers who are then able to take science-based decisions to prioritize corrective actions on industries whose effluents are the most toxic for the aquatic environment. It is also noteworthy to point out that the PEEP index can be applied anywhere with any number or type of tests and endpoints to suit the needs and expertise of laboratories internationally. 

Applying the WaterTox battery of tests in Argentina, Chile and Colombia for toxicity assessment of chemical contaminants present in different types of complex matrices by means of existing, modified or developed HAS approaches has proven to be environmentally beneficial. Water and wastewater samples, sewage sludge and biosolids from municipal treatment plants and effluent toxic loads, as well as pure compounds, were effectively scored as toxic or non-toxic with the ranking systems employed, thereby allowing them to be differentiated in terms of their adverse potential. In all cases, similar bioanalytical tools were employed to conduct these evaluations. 

A completely mixed system allows a more uniform aeration of wastewater in the aeration tank. This system can sustain shock and toxic loads. 

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