Wastewater from various facilities can contain a wide variety of contaminants. Common domestic sewage has at least minor levels of the pollutants discussed below. Contaminants present in wastewater from other facilities, such as commercial and industrial establishments, vary widely depending on the type of activities taking place in the facility generating the wastewater.
Chloride and sulfate occur commonly in wastewater. Both of these chemicals move readily through the soil. Soils cannot adsorb chloride and sulfate anions because these chemicals are repelled from the negative surface charge of the soils. Because chemicals such as chloride and sulfate leach into the ground water the primary mechanism for reduction is dilution. If the concentration of these chemicals in ground water is too high, then the density of on-site systems in an area may be restricted.
Sodium cations are adsorbed to the soil aggregates, which are held together by organic matter and clay. If sodium levels in wastewater are too high, the sodium may disperse the organic matter and clay in the soil. Such soil dispersion changes the soil structure and reduces the rate of water movement through the soil, which can cause failure of an on-site system.
Detergent surfactants are removed from wastewater effluent by adsorption to soil particles and by biodegradation. Aerated soil conditions enhance biodegradation and increase the treatment of surfactants. Adsorption of surfactants not only removes them from the wastewater, but it also increases the time for additional biodegradation to occur.
Toxic organic compounds, such as pesticides and nonbiodegradable organic compounds, degrade slowly. Since these compounds usually are not adsorbed by the soil, they may leach and contaminate the ground water. The best way to minimize the impact of these chemicals is to keep them out of the on-site system.
Heavy metals in high concentrations are usually toxic. Such metals can slow or stop the bacterial action in the septic tank and in the treatment and disposal trench. These metals should not be put into the on-site system.
From the North Carolina Onsite Guidance Manual