What is a sewage sludge?
Sewage sludge is a semi-solid material or residual byproduct of industrial or municipal wastewater treatment. The term "septage" also refers to sludge from simple wastewater treatment but is associated with simple on-site sanitation systems such as septic tanks.
When fresh sewage enters a primary settling tank, approximately half of the suspended solid matter settles out in an hour and a half. This collection of solids is known as raw sludge or primary solids, and it is said to be "fresh" before anaerobic processes become active. When anaerobic bacteria take over, the sludge quickly becomes putrescent and must be eliminated from the sedimentation tank.
This is completed in one of two ways. Typically, mechanical scrapers continuously extract fresh sludge from the bottom of a hopper-shaped tank and transport it to separate sludge-digestion tanks.In some treatment plants, a Foul condensate tank is used: sludge settles through a slot into the lower storey or digestion chamber, where anaerobic bacteria decompose it, culminating in liquefaction and a reduction in sludge volume.
The secondary treatment process also produces sludge, which is primarily made up of bacteria and protozoa with entrained fine solids and is eliminated by settlement in secondary settlement tanks. Both sludge streams are typically combined and treated anaerobic or aerobically at either elevated or ambient temperatures. After a long period of digestion, the result is known as "digested" sludge, which can be disposed of by drying and then landfilling.
Pollutants in Sewage Sludge
· Pathogens:Bacteria in Class A sludge products can actually grow back in certain environments. Pathogens in untreated sewage sludge could easily go undetected. Pathogens do not pose a significant health risk if sewage sludge is treated properly and site-specific management practises are followed.
· Micro pollutants:Micropollutants are compounds found in the aquatic and terrestrial environments at concentrations of up to a microgram per litre and milli-gramme per kilogramme, respectively, and are considered potential threats to environmental ecosystems. They can accumulate in sewage sludge. Each of these disposal options has a plethora of potential, human health and environmental consequences. Several organic micropollutants, including endocrine disrupting compounds, pharmaceuticals, and per-fluorinated compounds, have been identified in sewage sludge samples from around the world at concentrations of hundreds of mg/Kg of dried sludge. Sterols and other hormones have been discovered as well.
· Heavy metals:One of the main concerns in treated sludge is the concentration of metals (lead, arsenic, cadmium, thallium, and so on); certain metals are regulated, while others are not. To reduce the metal content and meet the regulatory limit, leaching methods can be used.
The EPA published the Targeted National Sewage Sludge Study, which details the levels of metals, chemicals, hormones, and other materials found in a statistical sample of sewage sludge. Among the highlights are:
• The EPA estimates that lead, arsenic, chromium, and cadmium are present in detectable quantities in 100 percent of national sewage sludge, but thallium is only present in 94.1 percent of sludge.
• Silver is present at a rate of 20 mg/kg of sludge on average, with some sludge containing up to 200 milligrams of silver per kilogramme of sludge; one outlier demonstrated a silver lode of 800–900 mg per kg of sludge.
• Barium is available at a rate of 500 mg/kg, with manganese present at a rate of 1 g/kg sludge.
Sewage Treatment
The process of removing contaminants from wastewater is known as sewage sludge treatment. Sewage sludge is produced during the treatment of wastewater in sewage treatment plants and is divided into two types: raw primary sludge and secondary sludge, also known as activated sludge in the case of the activated sludge process.
Typically, sewage sludge is treated with one or more of the following steps: lime stabilisation, thickening, dewatering, drying, anaerobic digestion, or composting. Some treatment processes, such as composting and alkaline stabilisation, may affect contaminant strength and concentration: depending on the process and the contaminant in question, treatment may decrease or, in some cases, increase contaminant bioavailability and/or solubility.
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