Wastewater Treatment Terminologies

Terminologies in Wastewater Treatment

• Wastewater treatment system consists of unit operations and processes to reduce wastewater constituents to acceptable levels.
• Headworks are unit operations at the upstream end of a wastewater treatment plant and include pumping stations, flow measurement, and preliminary treatment.
• A reactor is a vessel or containment structure, along with its appurtenances where a unit operation or process occurs.

Municipal Wastewater Treatment Levels

• Municipal wastewater treatment includes preliminary, primary, secondary and tertiary treatments.

Methods of Removal

• Physical unit operations used are mainly for primary treatment.
• Physical unit operations include screening, sedimentation, floatation and membrane filtration.
• Membrane filtration is a tertiary treatment method using filters that are less than 1um.

Biological & Chemical Unit Processes

• Biological unit processes, mainly secondary, remove contaminants through biological activity.
• Aerobic processes are biological processes that require the presence of dissolved oxygen.
• Biofilters, trickling filters and activated sludge are aerobic processes.
• Anaerobic processes occur in the absence of dissolved oxygen.
• Denitrification, septic tanks and ponds are anaerobic processes.
• Denitrification is a tertiary process.
• Chemical unit processes involve removal/conversion of contaminants via the addition of chemicals, or by chemical reactions, and are tertiary treatments.
• Precipitation, Adsorption and Disinfection are examples of chemical unit processes.
• Precipitation includes lime and struvite formation.
• Adsorption is an ion exchange.
• Disinfection includes chlorination, UV light, and ozone.

Preliminary Treatment

• The purpose of preliminary treatment is to remove coarse solids, large materials, and floatable materials from raw wastewater
• This involves coarse screening, grit removal, and comminution.
• In grit chambers, velocity is maintained or air is used to prevent settling of organic solids.
• Comminutors supplement coarse screening by reducing the size of large particles for sludge removal in later processes.

Screening

• Screening removes coarse solids such as sticks, rags, and boards from wastewater.
• Screening protects pumps and other mechanical equipment and prevents clogging of valves.
• Screens can either be fine or coarse.
• Screens are cleaned manually or mechanically.
• Flow velocity should not exceed 1.0 m/s in the channel and 0.3 m/s is considered a good design.
• Head loss depends on the degree of clogging.
• Solid waste is deposited at a sanitary landfill, or it is grinded for incineration or return to flow.

Comminutor

• Comminutors are located across the flow path and shreds coarse solids down to approximately 8 mm in size.
• A comminutor is a grinder consisting of a screen or slotted basket, a rotating or oscillating cutter, and a stationary cutter.
• Solids pass through the screen and are chopped or shredded between the two cutters.
• Channel design for a comminutor is similar to that for screens.
• Head loss rarely exceeds 10 cm.

Grit Removal

• Grit chambers separate gravel, sand, and other mineral materials down to a diameter between 0.2 and 0.1 mm.
• Grit chambers protect downstream mechanical equipment from abrasion, reduce heavy deposits in pipes, and reduce digester cleaning frequency due to grit accumulation.
• Aerated grit chambers use diffused air to keep organic solids in suspension as grit settles.
• Vortex-type grit chambers create a vortex to collect grits.

Grit Chambers

• Grit chambers hydraulically remove Type-I settling particles.
• A horizontal velocity of 0.3 m/s is maintained.
• Artificial control may be required with high variations in flow rates.
• To maintain flow, a proportioning weir is used on the effluent end of the tank, or a parabolic tank section maintains steady flow of 0.3 m/s.

Flow Measurement

• Flow measurement involves measuring the flow of various sewages in pipes and open channels.
• Operative parameters depend on flow, including chemical additives, air volume, and recirculation rates.
• Knowledge about the flow is important for operating a reactor.
• Instruments used are Parshall flumes, Palmer-Bowlus flumes, and Venturi meters.

Primary Treatment

• Primary treatment removes material from incoming wastewater that will either float or readily settle out (suspended solids) by gravity.
• Removal requires a combination of unit operations such as screening, grinding, and settling.
• Preliminary treatment is an integral part of primary treatment.
• Inorganic solids are removed in grit channels, and organic suspended solids via sedimentation.
• Primary clarifiers are used to process effluent that contains significant amounts of BOD that requires further treatment.
• 25–50% of incoming BOD5, 50–70% of total suspended solids (SS), and 65% of oil and grease are removed during primary treatment.
• Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation.
• Primary treatment is the minimum pre-application treatment level required for wastewater irrigation in many industrialized countries.
• It may be considered sufficient treatment if the wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops.