They are extremely dangerous. You should exercise extreme care when inspecting or looking into your tank. The final treatment step for the effluent, as it returns to the groundwater, involves the drainfield. The drainfield may be referred to as a soil absorption system, leach field or a disposal field. The wastewater slowly trickles through these materials and into the soil beneath.
Soil bacteria breakdown the effluent aerobically, which means with the presence of oxygen.
As the effluent seeps into the soil and approaches the natural groundwater it is purified by other soil components in conjunction with the bacteria. Toxins, viruses and other pollutants are removed from the wastewater. However, chemical additives, paint, grease, fats or thinners will kill the bacteria and clog the drainfield, contaminating the groundwater. Over time, these products can render your drainfield ineffective. System A malfunctioning or failed on-site wastewater disposal system is not only a nuisance but a public health hazard.