**4.4 Cyanide**

This substance is an important toxicant to fish and other aquatic animals and its salts are frequently found in effluents from industrial wastes. Certain forms of cyanide are acutely toxic to many aquatic life forms and concentrations <0.1 mg/l can be toxic to some sensitive aquatic species. At the cellular level, cyanide blocks the oxygen consumption of metabolizing cells, which is due to inhibition of the enzyme cytochrome oxidase catalyzing the final oxidation step in cellular respiration. Cyanide forms complexes with some heavy metals such as Zn, Pb, and Cd and is highly toxic. Several cyanide-containing substances display acute toxicity toward aquatic life [38]. However, it has been observed that cyanide-containing compounds also have effects on aquatic life at sublethal concentrations.

### **4.5 Pharmaceuticals**

Pharmaceuticals are among the emerging contaminants in wastewater and are one of the most relevant group of substances having a possible impact on aquatic ecosystems due to their chemicophysical properties [44]. Water bodies that receive wastewater discharges are found to be heavily impacted by annual loadings of these substances. Pharmaceuticals along with their metabolites are readily excreted with urine and feces. While the main concern about pharmaceuticals and their metabolites is that they are being added continuously into lakes and rivers as pollutants, they can have certain adverse effects on aquatic ecosystems and harm freshwater resources including drinking water supplies on a long-term basis. Although concentrations of pharmaceutical compounds in aquatic ecosystems are low, they can cause toxic effects on organisms [45]. Uptake of pharmaceuticals into fish can occur *via* both dermal and gill surfaces for water-borne/sediment-associated pharmaceuticals, orally through the diet, or maternally, *via* the transfer of contaminants through the lipid reserve of eggs. Pharmaceutical drugs are generally designed to have low toxicity but there is the potential for unintended side effects. The active ingredients in pharmaceuticals are known to have potential risks to the aquatic ecosystem and are suspected to have direct toxicity to certain aquatic organisms. There is a global concern about the presence of estrogenic residues in the aquatic ecosystems. The source of these estrogenic residues is industrial wastes and medicines, and as additives in animal feed [46]. The effect of these traces is remarkable on aquatic animals and consequently on humans. Fishes are considered more susceptible to the high concentration of pharmaceuticals. It has been reported that substances such as diclofenac and 17a-ethinylestradiol are responsible for inducing structural disruption in the kidney and intestine and also modify the expression genes, which are associated with the process-controlling metabolism [47, 48]. Their chronic exposure to fishes might affect their survival and reproduction. Another research stated that the presence of antibiotic compounds such as sulfamethoxazole might cause chronic toxicity effects on the photosynthetic apparatus of algae [49]. Therefore, the pharmaceuticals have an effect on the survival of algae due to their rate reduction of photosynthesis by affecting the functions of chloroplasts. A large amount of dead algae lead to secondary effects on the ecosystem such as eutrophication and disruption of the food chain. It threatens the equilibrium of the entire aquatic ecosystem [50].

### **4.6 Nitrogen**

Some water-soluble forms of inorganic nitrogen, such as ionized ammonium, ammonia, nitrite, and nitrate, are present in waste streams, which can exert oxygen demand in surface water resources. Molecular ammonia or NH4OH is considered as a most toxic form of ammonia, while the dissociated ammonium ion (NH4) is relatively nontoxic. The discharge of ammonia is mostly from industries, agriculture,

*Implications of Sewage Discharge on Freshwater Ecosystems DOI: http://dx.doi.org/10.5772/intechopen.100770*

and domestic wastewater. Organic wastes contributed from these sources are responsible for the increases in oxygen demand as a result of the increase in biological decomposition and production of ammonia due to the decomposition of organic nitrogen-containing compounds. Ammonia has toxic effects on aquatic life and high concentration can impair aquatic communities [51]. It encourages eutrophication in receiving water bodies. Ammonia and nitrate are principal forms of nitrogen and in the presence of oxygen, ammonia is converted into nitrate creating low dissolved oxygen conditions in surface waters [52, 53]. Excess ammoniacal nitrogen is damaging to aquatic life due to its ability to destroy the aquatic enzyme hydrolysis reaction apart from damaging certain tissues and organs in organisms. Its elevated concentration can cause certain symptoms in aquatic organisms such as hypoxia, coma, and reduced immunity, resulting in slow growth and even large numbers of deaths [54]. Ammonia concentrations >2 mg/l are toxic to aquatic life, especially fishes. Several works done on ammonia toxicity on freshwater vegetation have shown that concentrations >2.4 mg/l inhibit photosynthesis. Further, nitrate causes a decline in amphibian populations and in adverse cases causes poor larval growth, reduced body size, and impaired swimming ability. Direct toxic effects from ammonia are those with a direct impact on individual organisms, typically death, reduced growth rate, or reduced reproductive success.

## **4.7 Heavy metals**

Heavy metals comprise one of the most toxic pollutants in aquatic ecosystems due to the detrimental impacts they display in aquatic biota [55]. The heavy metal present in sewage has severe detrimental effects on the ecological balance of the aquatic environment including organisms [56]. Fishes are among the severely affected species and cannot escape from the detrimental impacts of metals. They accumulate a considerable amount of heavy metals in their body tissues and represent a major dietary source of this element for humans. The presence of heavy metals can inhibit the growth of fish as well as its larvae, reduce the size of fish populations, and can threaten the entire fish population if present in high concentration. A high concentration of aluminum can result in osmoregulatory failure in aquatic animals like fishes [57, 58]. It has the potential to bind with fish gills causing several kinds of diseases, suffocation and ultimately death, change in blood plasma levels, and decrease in nutrient intake at gills. More residence time of water in lakes results in the accumulation of heavy metals in biota, while a significant portion finds its way into the sediments. Mercury has carcinogenic and neuro-toxic properties with the ability to accumulate in living organisms, which gradually increases in the food web. Apart from its toxic effects on humans due to biomagnification in fish, mercury compounds have certain toxic effects on aquatic animals as well.

### **4.8 Phosphorus**

One of the major pollutants found in aquatic environments is phosphorus. The average amount of phosphorus in water resources is <1 mg/l; exceeding the amounts permitted in water causes a serious threat to the environment, animals, and aquatic life. Phosphorous is one of the essential nutrients which promotes algal blooms in rivers and lakes and finally leads to eutrophication which causes oxygen depletion in water *via* algal decay, which has harmful effects on aquatic life. A little rise in the content of this nutrient influences toxin production since it increases the growth of the algae.
