Secondary treatment (mostly biological wastewater treatment) is the removal of biodegradable organic matter (in solution or suspension) from sewage or similar kinds of wastewater. The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.
Secondary treatment systems are classified as fixed-film or suspended-growth systems, and as aerobic versus anaerobic. Fixed-film or attached growth systems include trickling filters, constructed wetlands, bio-towers, and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface.EPA. Washington, DC (2004). "Primer for Municipal Waste water Treatment Systems." Document no. EPA 832-R-04-001. The fixed-film principle has further developed into moving bed biofilm reactors (MBBR) and Integrated Fixed-Film Activated Sludge (IFAS) processes. Suspended-growth systems include activated sludge, which is an aerobic treatment system, based on the maintenance and recirculation of a complex biomass composed of micro-organisms (bacteria and protozoa) able to absorb and adsorb the organic matter carried in the wastewater. Constructed wetlands are also being used. An example for an anaerobic secondary treatment system is the upflow anaerobic sludge blanket reactor.
Fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems. Most of the aerobic secondary treatment systems include a secondary clarifier to settle out and separate biological floc or filter material grown in the secondary treatment bioreactor.
Definitions Influent
Primary treatment Primary treatment settling removes about half of the solids and a third of the BOD from raw sewage by settling.Abbett, p.19-28
Secondary treatment Secondary treatment is defined as the "removal of biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment." Biological nutrient removal is regarded by some sanitary engineers as secondary treatment and by others as tertiary treatment.
After this kind of treatment, the wastewater may be called secondary-treated wastewater.
Tertiary treatment
Process types
Secondary treatment systems are classified as fixed-film or suspended-growth systems. A great number of secondary treatment processes exist, see List of wastewater treatment technologies. The main ones are explained below.
Fixed film systems
Filter beds (oxidizing beds)
In older plants and those receiving variable loadings, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have large surface areas to support the biofilms that form. The liquor is typically distributed through perforated spray arms. The distributed liquor trickles through the bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biofilms of bacteria, protozoa and fungi form on the media's surfaces and eat or otherwise reduce the organic content.EPA (2004). "Primer for Municipal Waste water Treatment Systems." Document no. EPA 832-R-04-001. The filter removes a small percentage of the suspended organic matter, while the majority of the organic matter supports microorganism reproduction and cell growth from the biological oxidation and nitrification taking place in the filter. With this aerobic oxidation and nitrification, the organic solids are converted into biofilm grazed by insect larvae, snails, and worms which help maintain an optimal thickness. Overloading of beds may increase biofilm thickness leading to anaerobic conditions and possible bioclogging of the filter media and ponding on the surface.Metcalf & Eddy, pp.533-542
Rotating biological contactors
Constructed wetlands
Suspended growth systems
Activated sludge
Activated sludge is a common suspended-growth method of secondary treatment. Activated sludge plants encompass a variety of mechanisms and processes using dissolved oxygen to promote growth of biological floc that substantially removes organic material. Biological floc is an ecosystem of living biota subsisting on nutrients from the inflowing primary clarifier effluent. These mostly carbonaceous dissolved solids undergo aeration to be broken down and either biologically oxidized to carbon dioxide or converted to additional biological floc of reproducing micro-organisms. Nitrogenous dissolved solids (amino acids, ammonia, etc.) are similarly converted to biological floc or oxidized by the floc to nitrites, nitrates, and, in some processes, to nitrogen gas through denitrification. While denitrification is encouraged in some treatment processes, denitrification often impairs the settling of the floc causing poor quality effluent in many suspended aeration plants. Overflow from the activated sludge mixing chamber is sent to a clarifier where the suspended biological floc settles out while the treated water moves into tertiary treatment or disinfection. Settled floc is returned to the mixing basin to continue growing in primary effluent. Like most ecosystems, population changes among activated sludge biota can reduce treatment efficiency. Nocardia, a floating brown foam sometimes misidentified as sewage fungus, is the best known of many different fungi and protists that can overpopulate the floc and cause process upsets. Elevated concentrations of toxic wastes including pesticides, industrial metal plating waste, or extreme pH, can kill the biota of an activated sludge reactor ecosystem.Metcalf & Eddy, pp.482-533
Sequencing batch reactors One type of system that combines secondary treatment and settlement is the cyclic activated sludge (CASSBR), or sequencing batch reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage, and then mixed and aerated. The settled sludge is run off and re-aerated before a proportion is returned to the headworks.EPA (1999). "Sequencing Batch Reactors." Wastewater Technology Fact Sheet. Document no. EPA 832-F-99-073.
The disadvantage of the CASSBR process is that it requires a precise control of timing, mixing and aeration. This precision is typically achieved with computer controls linked to sensors. Such a complex, fragile system is unsuited to places where controls may be unreliable, poorly maintained, or where the power supply may be intermittent.
Package plants
Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBR plants with reduced timing sensitivity.
Membrane bioreactors
Membrane bioreactors (MBR) are activated sludge systems using a membrane liquid-solid phase separation process. The membrane component uses low pressure microfiltration or ultrafiltration membranes and eliminates the need for a secondary clarifier or filtration. The membranes are typically immersed in the aeration tank; however, some applications utilize a separate membrane tank. One of the key benefits of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in the range of 8,000–12,000 mg/L, while CAS are operated in the range of 2,000–3,000 mg/L. The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates. Thus increased sludge retention times, usually exceeding 15 days, ensure complete nitrification even in extremely cold weather.
The cost of building and operating an MBR is often higher than conventional methods of sewage treatment. Membrane filters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass peak flows. The technology has become increasingly popular for reliably pretreated waste streams and has gained wider acceptance where infiltration and inflow have been controlled, however, and the life-cycle costs have been steadily decreasing. The small footprint of MBR systems, and the high quality effluent produced, make them particularly useful for water reuse applications.EPA. Washington, DC (2007). "Membrane Bioreactors." Wastewater Management Fact Sheet.
Aerobic granulation
Aerobic granular sludge can be formed by applying specific process conditions that favour slow growing organisms such as PAOs (polyphosphate accumulating organisms) and GAOs (glycogen accumulating organisms). Another key part of granulation is selective wasting whereby slow settling floc-like sludge is discharged as waste sludge and faster settling biomass is retained. This process has been commercialized as Nereda process.
Surface-aerated lagoons or ponds
Aerated lagoons are a low technology suspended-growth method of secondary treatment using motor-driven aerators floating on the water surface to increase atmospheric oxygen transfer to the lagoon and to mix the lagoon contents. The floating surface aerators are typically rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kW·h. Aerated lagoons provide less effective mixing than conventional activated sludge systems and do not achieve the same performance level. The basins may range in depth from 1.5 to 5.0 metres. Surface-aerated basins achieve 80 to 90 percent removal of BOD with retention times of 1 to 10 days. Available at CSA Illumina website Many small municipal sewage systems in the United States (1 million gal./day or less) use aerated lagoons.Maine Department of Environmental Protection. Augusta, ME. "Aerated Lagoons – Wastewater Treatment." Maine Lagoon Systems Task Force. Accessed 2010-07-11.
Emerging technologies Biological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biological carbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer. Integrated Fixed-Film Activated Sludge Moving Bed Biofilm Reactors (MBBRs) typically requires smaller footprint than suspended-growth systems.. Brochure.
Design considerations