Surfactants are the key ingredient of detergents. Surfactants are the surface-active agents, which make water "wetter"—that is, a better cleaning agent. Surfactants concentrate at the interfaces of water with gases, solids (dirt), and immiscible liquids (oils).
The ability of surfactants to concentrate at an interface is due to their molecular structure (Figures 19.15 and 19.16). Surfactants contain a polar or ionic group (head) with a strong affinity for water and a nonpolar or non-ionic group (tail) with an aversion to water. The tail is a hydrocarbon group.
Although surfactants contribute to operational problems, other ingredients in detergents also contribute to operational problems. Most commercial solid detergents contain 10-30% surfactant. An additional critical component of a detergent
FIGURE 19.16 Isomers of LAS (linear sulfonate). LAS has a benzene ring that may be attached anywhere on the carbon chain between the two terminal carbon atoms. LAS is used in many laundry detergents.
is the "builder." An example of a builder is polyphosphate. The builder binds to hardness ions such as calcium in the water. The builder makes the detergent solution alkaline and improves the action of the surfactant.
Other ingredients in detergents may include alkalis, anticorrosive compounds, bleaches, brighteners, dyes, fabric softeners, foam stabilizers, fragrances, ion exchangers, soil-suspending compounds, and enzymes to degrade lipids (lipase) and starches (cellulases). Several undesired effects of slowly degrading detergents or slug discharges of detergents include
• Deflocculation of the biomass
• Deflocculation of colloids
• Emulsification of fats, oils, and grease
• Flotation of solids
• Foam production (decreased surface tension of wastewater)
Toxicity caused by surfactants is influenced by several factors including the molecular structure of the surfactant, water hardness, temperature, and dissolved oxygen. The most important factors are the molecular structure of the surfactant and dissolved oxygen concentration.
Surfactant toxicity is probably due to the damage that surfactants cause to cellular proteins and the cell membrane. Even when surfactants present no toxicity concern, low levels of surfactants may increase the uptake of other wastes.
With respect to molecular structure, surfactants can be broadly placed into three groups, anionic, non-ionic, and cationic.Anionic surfactants are the most widely used and the most commonly discharged surfactants to wastewater treatment processes. A commonly used group of surfactants are the sulfonates.
Non-ionic surfactants have attracted less attention than anionic surfactants. Generally, non-ionic surfactants as well as anionic surfactants tend to be more toxic at lower concentrations than cationic surfactants. Cationic surfactants are used mostly as medical and laboratory disinfectants.
Surfactants differ from many toxic wastes. They are not uniformly distributed in water. They concentrate at surfaces. An additional difference is that one surfactant may exist in a large number of isomers or molecular structures. These isomers are responsible for the variation in reported results of surfactant toxicity. Also, loss of surfactant toxicity during testing may occur as surfactants are degraded in all but short testing periods.
The molecular structure of a surfactant, especially anionic surfactants, greatly influences toxicity. LAS (1-benzenesulfonate), for example, has a benzene ring that may be attached at any point on the alkyl (carbon) chain except the terminal or end carbons. A "hard" surfactant such as ABS (alkylbenzenesulfonate) is branched and the benzene ring is attached at an end or terminal carbon unit on the alkyl chain. A hard surfactant is a persistent or slowly degradable surfactant. Significant impacts of hard surfactants are undesired foaming and destruction of bacteria, protozoan, and metazoa.
Water hardness also affects surfactant toxicity. The toxicity of an anionic surfactant may increase or decrease with increasing water hardness. The toxicity of an non-ionic surfactant usually is not affected by charges in water hardness.
The toxicity of surfactants generally increases with increasing wastewater temperature. The increase in toxicity may be due to the increase in rate at which the surfactant attacks the cell and the decrease in ability of the wastewater to hold dissolved oxygen with increasing temperature. Low concentrations of anionic surfactants and non-ionic surfactants that are not toxic during cold wastewater temperatures may be toxic during warm wastewater temperatures.
Dissolved oxygen concentration affects the toxicity of surfactants as well as other toxic wastes. With increasing dissolved oxygen concentration, surfactants exert decreasing toxicity.
Salinity consists of dissolved sodium (Na+), potassium (K+), and magnesium (Mg2+). Salinity influences the osmoregulatory ability of a cell—that is, the ability of the cell to import materials and export wastes. During stress conditions such as the presence surfactants, changes in salinity may increase or decrease the impact of surfactant toxicity.
Total suspended solids (TSS) also influence surfactant toxicity. Because some suspended solids such as kaolin clay adsorb surfactants, increasing TSS concentration may reduce surfactant toxicity. The adsorbed surfactants are removed from the waste stream when the solids settle in the secondary clarifiers.
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