Centralized municipal wastewater (sewer) treatment versus Onsite Sewage Disposal Systems (OSDS, septic tanks)
After we flush our toilets, most of us don’t want to think too much about how the wastewater from our homes is treated. The 100 gallons of human wastewater we each produce every day is generally not important to us; mostly, we just want it to go away. Around the world, scientists and engineers have designed, installed and operate systems that afford the civilized world this modern day sanitary convenience. But if transporting our waste away from our homes is sanitary, where does it go, what happens to it and the places where it goes?
Tossing out the bath (and toilet) water
Throughout human history, residents of haphazardly designed cities gave almost no thought as to how to properly and adequately dispose of human wastewater. For example, in early urban areas, “chamber pots” were literally dumped out of residential windows and into gutters along cobble stone streets where a forgiving rainfall would hopefully soon wash the stinking waste to the closest river or water body. In many cases, drinking water was also collected from these water bodies where wastewater was draining. Many of the pandemic human disease outbreaks that ravaged urban society in the past, such as cholera and typhoid, were directly related to unsanitary wastewater treatment and drinking water contaminated with human waste. Public health microbiology is a field of science that developed in the 1800s and early 1900s, largely out of our need to understand public health threats from diseases resulting from water and food contaminated with pathogens from our own waste.
Big city or small town wastewater treatment?
Our modern-day domestic wastewater is usually treated in one of two ways. First, vast centralized collection and treatment systems were built in dense urban areas, where large volumes of wastewater (millions of gallons per day) are “batch” treated in a sequential series of physical, biological and chemical processes in large aerated tanks, known as “Activated Sludge” treatment. Treated wastewater is often further sanitized to destroy pathogens, and then either released or reused. Alternatively, in more rural settings, and in poorly managed suburban settings, smaller onsite sewage disposal systems (OSDS), more commonly known as “septic tanks” store, slowly diffuse, and in some cases partially treat human wastewater for individual homes before it slowly seeps out of drainfields into adjacent ground and surface waters.
Centralized domestic wastewater treatment systems (“sewer”) and OSDS (“septic”) each have advantages and disadvantages. While large municipal wastewater treatment facilities process, treat, and sanitize large volumes of wastewater daily in cities across the world, they are often prone to leaking collection system pipes, failures in wastewater treatment at large processing facilities and other symptoms of poor system operation and maintenance. As a result, either chronic leaking of wastewater from antiquated pipes or dumping of untreated or partially treated sewage contaminate local surface and groundwaters. Large rainfall events often disrupt domestic wastewater treatment facilities as the result of vast stormwater inflows to where wastewater facilities cannot efficiently treat both, resulting in millions of gallons of partially treated effluent being dumped into adjacent surface waters – a problem we have become too familiar with here in Florida.
In rural settings where large centralized treatment systems are not practical, septic systems (OSDS) are an economically viable alternative. However, OSDS lack the efficiency of treating pathogens or significantly removing nitrogen and phosphorus, pollutants that are human health and environmental threats. Conventional OSDS are not designed to treat pathogens or significantly reduce nutrient loading from drainfields under many conditions: 1) where water tables are uniformly or seasonally high, 2) in low organic or sandy soils, and 3) where high density uses of OSDS contribute significant loads of pathogens and nutrients to adjacent ground waters and surface waters. In rural settings, residents often also draw their drinking water from wells that are just too close to their septic tank drainfields. This results in the potential of drinking well water contaminated with septic tank effluent. More common, however, is that increasing numbers of poorly maintained, uninspected, and overwhelmed OSDS in failure-prone areas deliver untreated and partially treated human waste to local groundwater and surface waters resulting in pollution and subsequent ecosystem impacts.
Florida’s wastewater treatment
Here in Florida, over 30% of the residents treat their wastewater with septic tanks, with nearly 3 million OSDS serving residences in the state. The Florida Department of Health regulates the use of OSDS as it relates to public health. For more info on OSDS, see: http://www.floridahealth.gov/environmental-health/onsite-sewage/index.html. Alternatively, most urban residents in the state are serviced through domestic wastewater treatment facilities. Florida has nearly 2,000 wastewater treatment facilities managed by cities, counties and private utilities, that are regulated by the Florida Department of Environmental Protection. For more info see: https://floridadep.gov/water/domestic-wastewater
This post was authored by Peter Barile, Science Director for the American Water Security Project.