Conventional wastewater treatment involves several physical, chemical and biological processes to remove solids, organic matter and nutrients from wastewater. Primary treatment removes coarse solids and other large materials from wastewater.
At this stage, municipal wastewater treatment technologies eliminate organic and inorganic solids and floatable components by allowing them to settle and skimming them out. They also reduce biochemical oxygen demand (BOD) and total suspended solids (TSS) levels before pumping wastewater into the aerobic or anaerobic digestion process.
Key steps of primary wastewater treatment are:
This blog post discusses several established and emerging technologies municipal wastewater plants use during the primary treatment stage.
The primary treatment stage generally removes around 25-50% of incoming BOD, 50-70% of TSS and 65% of oil and grease from wastewater. Primary sedimentation also removes some organic nitrogen, organic phosphorus and heavy metals associated with solids. Examples of established primary stage treatment technologies used at municipal wastewater treatment plants (WWTPs) across the U.S. include:
Grit affects the downward treatment of wastewater and subsequently increases WWTP maintenance and operating costs. The conventional design for headworks grit removal equipment at WWTPs operates under a standard assumption about particle size. Customary designs presume the majority of grit particles exceeds 212 microns in diameter with a specific gravity of 2.65. However, systems designed around these accepted standards only remove 30-50% of incoming grit. Some WWTPs with advanced grit removal systems such as Grit King, PISTA Grit, HeadCell and Hydro-Grit can remove as much as 85-95% of all grit entering the plant.
Many municipal WWTPs remove grit and grease from rectangular tanks via grit removal traveling bridges. Dividing each tank into two sections separates grit settling processes from grease collection. An air-injection system inside the grit channel separates inorganic and other floating particles. The bridge travels back and forth, conveying settled grit into a collection hopper while moving in one direction and collecting scum and floating particles in the other.
A majority of large WWTPs use mechanically cleaned screening systems to remove large solids, rags and debris. With openings of 6 millimeters (mm) and larger, coarse screens typically filter out the largest particles. Cylindrical drum screens rotate in the flow channel to retain fine solids. Fine screens, with openings from 1.5 mm to 6 mm, remove materials large enough to cause operational or maintenance issues in downstream processes. Micro screens, with openings from 0.2 mm to 1.5 mm, further reduce suspended solids. Fixed and movable plates called step screens, with 3 mm minimum openings, are designed to handle large flow rates of sewage when placed across the width of the flow channel to separate suspended solids.
A chemically enhanced primary treatment for wastewater uses coagulants like alum and iron salts, or flocculants such as poly aluminum chloride and polyacrylamide. These chemicals combine with solids in wastewater to accelerate settling. After settling and removing all solids, the wastewater is disinfected and neutralized. This primary treatment process equips WWTPs to handle unexpected wet weather flows without increasing biological treatment capacity.
Apart from established municipal wastewater treatment technologies, some emerging primary stage technologies are testing a pilot or demonstration scale at various WWTPs.
Some innovative and emerging technologies for the primary wastewater treatment stage include:
Just as no two WWTPs are identical, the choice of primary treatment technology varies. A wastewater engineer can best evaluate the efficacy and cost-efficiency of established or emerging municipal wastewater treatment technologies to help municipal leaders make informed choices.
At Fehr Graham, we understand the challenges municipalities face to comply with effluent limits and implement effective treatment technologies within budget. We have provided wastewater treatment solutions to communities across the Midwest since 1973. From designing individualized, cost-effective WWTP solutions to securing financing for treatment technologies, we provide end-to-end assistance with municipal wastewater treatment plants to improve process quality while also increasing efficiency for the communities they serve.