Sometimes you take for granted that everyone working in the industry knows why our systems are so important to the environment and human health. During the last few months, we have had some questions about what is in wastewater and why it is a problem. This is where we start our basic sewage treatment workshops. We thought it would be helpful to go back to the beginning and discuss pollutants and potential problems. We will follow up these discussions over coming months looking at some specific ways to address the concerns.

Professionally designed, sited and installed conventional onsite sewage treatment systems effectively reduce or eliminate most human health and environmental hazards. This is accomplished through physical, chemical and biological processes in the septic tank, in the biomat and in the unsaturated soil zone beneath operating soil treatment trenches. 

Septic tank effluent typically moves into and through soil toward groundwater and final discharge through three zones. The infiltration zone (biomat), unsaturated soil zone (at least three feet are necessary) and a perched saturated zone or slowly permeable horizon. Treatment processes are ongoing in each of these zones and contribute to reducing or eliminating pollutant loads when final discharge to groundwater and surface water occurs. 

Here is a summary of pollutants of concern and why they can be a problem:

Total suspended solids (TSS)
If discharged directly to surface waters, TSS can negatively impact aquatic organisms in a couple of ways. Sediment can smother bottom dwelling organisms such as mussels and clams and fish eggs. Solids remaining suspended in water can block sunlight from penetrating, harming aquatic plants and reducing oxygen available in water for other organisms. If drinking water has suspended solids it can interfere with disinfection and treatment.

Biological Oxygen Demand (BOD)
BOD depletes dissolved oxygen in surface waters when organisms breaking down the organic waste consume oxygen from the water. This is harmful to not only aquatic plants but fish and other organisms.

Pathogens
Parasites, bacteria and viruses can cause communicable diseases through direct or indirect body contact or consumption of contaminated water or shellfish. They are a threat to humans and animals. Direct movement to surface waters (lakes, streams) can occur when sewage surfaces in the yard, provides potential for direct human or animal contact or is washed directly to a surface water during times of heavy precipitation. With a direct discharge to groundwater, pathogens can travel long distances, particularly in aquifers within creviced bedrock.

Nitrogen and phosphorus nutrients
Nitrogen in surface waters can contribute to eutrophication and dissolved oxygen in surface waters. In freshwater systems, nitrogen is usually not the limiting nutrient but can still contribute. In saltwater and coastal estuaries, nitrogen is the major nutrient of concern for large algal growth, including some toxic types that can cause great harm or even death if consumed by animals or humans. In areas where drinking water is obtained from groundwater, excessive nitrogen levels can cause methemoglobinemia (blue baby syndrome). Livestock (dairy cows) can also suffer health impacts and death from high nitrates.

Phosphorus is the limiting nutrient in freshwater systems for algal and weed growth. Similar large toxic algal blooms can result if levels are high enough, and of course there are the aesthetic problems and corresponding lakeshore property value loss where algae blooms — even non-toxic ones — occur.

Household cleaners
Toxic organic compounds are present in household chemicals and cleaning agents. These cleaners can interfere with the biological operation of our treatment systems, reducing their ability to treat wastewater. Transferred directly to groundwater, they can contaminate drinking water with potentially cancer-causing chemicals. If delivered directly to surface water they can harm aquatic organisms (fish, shellfish), and humans if they consume the fish and shellfish. 

Heavy metals
Heavy metals can be introduced to the wastewater stream through household use. This is not a widespread problem, but human and animal health problems can result in areas with elevated levels of arsenic or other compounds in groundwater. Similarly, elevated levels of chloride and sulfates may create negative impacts on the ability of our systems to treat the effluent. This is also true when excessive amounts of sodium are delivered to the system. It can be harmful to soil structure and the ability of the soil to accept sewage effluent. 

TAKING PRECAUTIONS

All these potential pollutants are found in a household wastewater stream. The good news is if we do things right in siting, design, installation and maintenance, soils will do a good job of removing or reducing levels so they are not a concern. Soil is not a perfect treatment system by any means. But it is very good, and we can improve conditions by educating our clients about their systems to provide the balance we desire between accepting the effluent generated from the household and providing adequate treatment. Our discussions of how to do things “right” contributes to protecting human health and the environment.