IMPACT OF TOXICANTS ON WASTEWATER TREATMENT

Serious concern has arisen over the release of more than 50,000 xenobiotics into the environment. Their impact on aquatic environments, including wastewaters, is generally determined by acute and chronic toxicity tests, using mostly fish and invertebrate bioassays.  However, because of the large inventory of chemicals, short-term bioassays are now being considered for handling this task. These tests are based chiefly on inhibition of the activity of enzymes, bacteria, fungi, algae, and protozoa. These enzymatic and microbial assays, also called microbiotests, are simple, rapid, cost-effective, and can be miniaturized. The advantages of microbiotests are summarized in the Table.

Toxic inhibition by organic (e.g., chlorinated organics, phenolic compounds, surfactants, pesticides) and inorganic (e.g., heavy metals, sulfides, ammonia) chemicals is a major problem encountered during the biological treatment of industrial and domestic wastewaters.

Some of the chemicals that enter wastewater treatment plants, particularly the volatile compounds, may pose a potentially health threat to plant operators. Many of the toxic chemicals or their metabolites are, however, transferred to wastewater sludges.

The application of these sludges to agricultural soils may result in the uptake and accumulation of toxic and genotoxic chemicals by crops and grazing animals, eventually posing a threat to humans.

Table: atractive features of microbiotests

Feature	Explanatory Remark
Inexpensive or cost-efficient	Cost is test-dependent and can vary from a few dollars to several hundred dollars.
Generally not labor-intensive	As opposed to steps involved in undertaking fish bioassays, for example.
High sample throughput potential	When automation technology can be applied.
Cultures easily maintained or maintenance-free	Freeze-drying technology can be applied.
Modest laboratory and incubation space requirement	As opposed to a specialized laboratory essential for fish bioassays, for example.
Insignificant postexperimental chores	Owing to disposable plasticware, which is recycled instead of having to be washed for reuse, as in the case of large experimental vessels.
Low sample volume requirements	Often, a few milliliters suffice to initiate tests instead of liters.
Sensitive/rapid responses to toxicants	Short life cycles of (micro)organisms enable endpoint measurements after just minutes or several hours of exposure to toxic chemicals.
Precise/reproducible responses	High number of assayed organisms, increased number of replicates, and error-free robotic technology are contributors to this feature.
Surrogate testing potential	Microbiotests are adequate substitutes for macrobiotests in some cases.
Portability	For cases in which microbiotests are amenable to being applied in the field.

Chemical toxicants may also adversely affect biological treatment processes (Koopman and Bitton, 1986). Toxic inhibition is sometimes a serious problem in plants treating industrial effluents. Activated sludge is the aerobic process that has been studied primarily with regard to toxic inhibition. The major effects of toxicants on activated sludge are reduced BOD and COD removal, reduced efficiency in solids separation, and modification of sludge compaction properties.

Chemical toxicants can also diminish the quality of receiving waters. Toxic wastewater effluents may threaten aquatic organisms in receiving waters, the use of which may be restricted. Guidelines are available for the levels of several heavy metals in receiving waters, but less is known as regard the levels of organic toxicants.

Some of the human-made chemicals may disrupt the endocrine system of aquatic organisms and humans. These chemicals can mimic the natural estrogens (i.e., female hormones) and can compete for the estrogen receptor sites in cells. Endocrine disrupters (ED) of varying potencies, include natural estrogens (e.g., 17b-estradiol), synthetic steroids (e.g., 17a-ethynylestradiol entering in the composition of contraceptive pills), phytoestrogens, pesticides, and alkylphenols. The latter are the biodegradation products of the nonionic surfactants alkylphenol-polyethoxylates (NPE), which are extensively used in industrial processes. Metabolites of NPE (e.g., nonylphenol) have been detected in wastewater effluents and biosolids. Nonylphenol, being more lipophilic and more persistent than NPE, tends to be well adsorbed by biosolids, where it can reach concentrations from 0,1 to more than 1200 mg/g d.w. An analysis of seven effluents in the United Kingdom showed the presence of natural and synthetic steroids consisting of 17b-estradiol (concentration range of 4–48 ng/L of effluent), estrone (concentration range of 1–76 ng/L effluent), and 17a-ethynylestradiol (concentrations range from nondetectable to 7 ng/L) (Desbrow et al., 1998). Other investigators found estrogen concentrations in tens of ng/L in sewage effluents, or river water impacted by wastewater effluents. Very low concentrations of endocrine disrupting compounds were also found in finished drinking water, but the public health significance of these findings is unknown at the present time. Purdom and colleagues (1994) demonstrated that domestic wastewater effluents were estrogenic to fish. The effluents stimulated the production of vitellogenin (VTG) in male fish exposed to wastewater effluents. Following measurement of vitellogenin blood levels in fish in Belgium, estrogenic activity was observed in a canal predominantly impacted by domestic wastewater discharges. There is not yet a systematic monitoring program for endocrine disrupters in water and wastewater treatment plants.

References:

Bitton, G. Wastewater microbiology, third edition, wiley-liss, 2005

http://www.pollutionissues.com/Ve-Z/Wastewater-Treatment.html#b

Bitton, G., and B.J. Dutka, Eds. 1986. Toxicity Testing Using Microorganisms, Vol. 1. CRC Press, Boca Raton, FL.

Bitton, G., and B. Koopman. 1992. Bacterial and enzymatic bioassays for toxicity testing in the environment. Rev. Environ. Contam. Toxicol. 125: 1–22.

Blaise, C. 2002. Use of microscopic algae in toxicity testing, pp. 3219–3230, In: Encyclopedia of Environmental Microbiology, G. Bitton, editor-in-chief, Wiley-Interscience, N.Y.

Dutka B.J., and G. Bitton, Eds. 1986. Toxicity Testing using Microorganisms, Vol 2. CRC Press, Boca Raton, FL.

Janssen, C. 1997. Alternative assays for routine toxicity assessments: A review. pp. 813–839, In: Ecotoxicology:
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Koopman, B., and G. Bitton. 1986. Toxicant screening in wastewater systems. p 101–132, In: Toxicity Testing Using Microorganisms, Vol. 2, B.J. Dutka, and G. Bitton, Eds. CRC Press, Boca Raton, FL.

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Wells, P.G., K. Lee, and C. Blaise, Eds. 1998. Microscale Testing in Aquatic Toxicology: Advances, Techniques, and Practice. CRC Press, Boca Raton, FL.