Toxicity of Leachate

Leachate consists of a mixture of organic and inorganic compounds, many of which have a hazardous impact on the environment. Assessment of their toxicity as separate substances is insufficient and risks underestimation. Therefore, the toxicity evaluation must examine leachate as a complete mixture and is performed by means of biological screening methods (biotests).

Toxicity tests are generally grouped as acute and chronic. Acute toxicity is measured at short-time exposure and it is expressed as mortality (for fish), immobility (for crustaceans), and reduced photosynthesis (algae), or reduced light emission (bacteria). Chronic toxicity evaluates long-time exposure at concentrations proven to be nonlethal in acute toxicity tests. For bacteria, algae, and small crustaceans, which have a short life-cycle, chronic tests may comprise several generations. For organisms with longer life-cycles such as fish, tests are focused on important life stages such as reproduction, embryo and larval growth, and survival [21]. The most popular biological screening methods are presented in Table 4.

Species used for toxicity examination display different levels of sensitivity. This is seen in Fig. 1, which presents data from leachate toxicity tests conducted at eight landfills in Sweden [21]. For these eight landfills, the most sensitive species were algae, followed by crustacean, fish, plant, and bacteria (Microtox). However, for other landfills, this "sensitivity chain" can be completely different [21].

Effect concentrations are given as percentage of leachate responsible (EC or LC). For Microtox tests, the percentage of inhibition is reported as 100% in the leachate [21].

Table 4 The Most Popular Biotests Applied for Leachate Toxicity Evaluation

Trophic

Organism

Effect parameter

level

Guppy

Rainbow trout Fathead minnow

Reduced survival of larvae or fingerlings

Crustacea

Daphnia magna Ceriodaphnia dubia Mysidopsis

Reduced survival of larvae

Plants

Lemna minor (Duckweed)

Inhibition of growth (chlorophyll a and weight)

Radish

Reduced germination or seedling growth

Sorghum

Algae

Selenastrum capricornutum

Inhibition of photosynthesis

Nitzschia palea

Inhibition of cell growth

Skeletonema costatum

Bacteria

Photobacterium phosphoreum

(Microtox)

Inhibition of light emission

Salmonella typhimurium (Ames test)

Revertants

Lhte<fi*t fiavstl

Lhte<fi*t fiavstl

S 100

Figure 1 Leachate biotests of eight sanitary landfills in Sweden. Effect concentrations are given as percentage of leachate responsible for 50% effect (EC or LC). For Microtox tests percentage of inhibition in 100% leachate is reported (from Ref. 21).

S 100

Figure 1 Leachate biotests of eight sanitary landfills in Sweden. Effect concentrations are given as percentage of leachate responsible for 50% effect (EC or LC). For Microtox tests percentage of inhibition in 100% leachate is reported (from Ref. 21).

As shown in Fig. 1, particular organisms display different levels of sensitivity to toxic substances in leachate. Investigations in leachate toxicity show that no organism or species would be completely resistant to the toxic effect of leachate components [21-24]. These results are not confirmed by the presence of biocenoses formed at biological treatment processes. Biocenoses active in biological treatment processes are very differentiated, mixed bacterial populations (apart from organisms from higher trophic levels, which are not as essential to treatment effectiveness). The normally demonstrate much higher resistance to toxic substances than test organisms used in ecotoxicology. On the other hand, biocenoses are less resistant to other factors such as organic compounds overloading, which can disturb homeostasis.

Sensitivity of activated sludge to leachate can be estimated by means of oxygen uptake rate and/or dehydrogenase activity measurements. Both parameters are good indicators of the whole metabolism of microorganisms because they reflect processes arising in the respiratory chain, where all metabolic pathways converge. Therefore, any disturbance in any metabolic pathway exerts its influence on processes in the respiratory chain. Sensitivity of activated sludge to toxic substances is measured as inhibition of oxygen uptake rate or dehydrogenase activity resulting from biomass contact with different concentrations of examined substances [20,25].

An example of lack of sensitivity of activated sludge to leachate is presented in Fig. 2 and 3, which show the examination results of leachate from three landfills in Poland's region of Upper Silesia [20]. An increase of oxygen uptake rate and dehydrogenase activity was observed. This was independent of the percentage and origin of leachate found in an activated sludge sample. Oxygen uptake rate grew from 3 to 48% in comparison with a control sample. The increase of oxygen uptake rate depended on amount and quality of introduced leachate and amount of biodegradable compounds in leachate, as well as on activity of activated sludge measured as oxygen uptake rate before leachate introduction.

Results of dehydrogenase activity measurements confirmed data obtained in experiments with oxygen uptake rate. None of the leachate doses introduced into the activated sludge samples affected the decrease of dehydrogenase activity. This confirmed the lack of sensitivity of acitvated sludge to components of examined leachate. Even the growth of activity was observed with the increase of leachate doses in the samples. All these results proved that concentrations of xenobiotics present in leachate did not inhibit metabolic activity of microorganisms, but simultaneously only small amounts of these components could be used in biological processes of oxidation by nonadapted biocenosis [20].

Figure 2 Activated sludge oxygen uptake rate increase influenced by different percentage of leachate from landfill in Di^^^Gornieza (Poland) (from Ref. 20).

Figure 3 Activated sludge dehydrogenase activity increase influenced by different percentage of leachate from three landfills in Poland. Series 1, leachate from landfill in ^'■'■"■■'Gornicza; series 2, leachate from landfill in Siemianowice; series 3, leachate from landfill in ^■■k^-v.,,-,-(from Ref. 15).

Figure 3 Activated sludge dehydrogenase activity increase influenced by different percentage of leachate from three landfills in Poland. Series 1, leachate from landfill in ^'■'■"■■'Gornicza; series 2, leachate from landfill in Siemianowice; series 3, leachate from landfill in ^■■k^-v.,,-,-(from Ref. 15).

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