What Is a Biological Wastewater Treatment System and How Does It Work? (2022)

What Is a Biological Wastewater Treatment System and How Does It Work? (1)

For an industrial company producingwaste as part of its process, some type of wastewater treatment system isusually necessary to ensure safety precautions and discharge regulations aremet. The most appropriatewastewater treatment systemwill help afacility avoidharming theenvironment, human health, and a facility’s process or products (especiallyif the wastewater is being reused). It will also help the facility curb heavy fines if wastewater is beingimproperly discharged into a POTW (publicly owned treatment works) or to theenvironment (usually under a NPDES, or National Pollutant Discharge EliminationSystem, permit).

Typically used as a secondary wastewater treatment method afterthe initial larger contaminants have been settled and/or filtered out,biological wastewater treatment systems can be efficient and economical technologiesfor breaking down and removing organic contaminants from heavily organic-ladenwastes, such as those produced in the food and beverage, chemicalmanufacturing, oil and gas, and municipal industries.

But “what is abiological wastewater treatment system and how does it work?”

Since this subject can be extremely multifaceted andcomplex, this article will break down the basics as an overall introduction tosome of the more common biologicalwastewater treatment methods used industrially today.

What is a biological wastewatertreatment system?

In a simplified, top-level answer to this question, abiological wastewater treatment system isa technology that primarily uses bacteria, some protozoa, and possibly other specialtymicrobes to clean water. When these microorganisms break down organicpollutants for food, they stick together, which creates a flocculation effect allowingthe organic matter to settle out of the solution. This produces aneasier-to-manage sludge, which is then dewatered and disposed of as solidwaste.

(Video) How Do Wastewater Treatment Plants Work?

Typically broken out into three main categories, biologicalwastewater treatment can be:

  1. aerobic,when microorganisms require oxygen tobreak down organic matter to carbon dioxide and microbial biomass
  2. anaerobic,when microorganisms do not requireoxygen to break down organic matter, often forming methane, carbon dioxide,and excess biomass
  3. anoxic, whenmicroorganisms use other molecules than oxygen for growth, such as for the removalof sulfate, nitrate, nitrite, selenate, and selenite

The organic contaminants these microorganisms decompose areoften measured in biological oxygen demand, or BOD, which refers to the amountof dissolved oxygen needed by aerobic organisms to break down organic matterinto smaller molecules. High levels of BOD indicate an elevated concentrationof biodegradable materialpresent in the wastewater and can be caused bythe introduction of pollutants such as industrial discharges, domestic fecalwastes, or fertilizer runoff.

When pollutant levels are elevated, BOD candepletethe oxygen needed by other aquatic organisms to live, leading to algal blooms,fish kills, and harmful changes to the aquatic ecosystemwhere thewastewater is discharged. Because of this, many facilities are required totreat their wastes, perhaps biologically, prior to discharge—but it’s the levelof organic and inorganic pollutants in relation to their dischargerequirements that will dictate what specific unit operations a facility’sbiological wastewater treatment system will need and how they are sequenced andoperated.

In short, biological industrial wastewater treatment systems optimize the naturally occurring process of microbial decomposition to break down industrial wastewater contaminants so that they, along with other unwanted materials, can be removed. They also often replace (and are sometimes used alongside) physical and chemical treatments, which can be among the pricier treatment alternatives.

How does a biological wastewatertreatment system work?

Depending on the chemical makeup of the wastewater inrelation to the effluentrequirements, a biological wastewater treatment system might be composed ofseveral different processes and numeroustypes of microorganisms. They will also require specific operational proceduresthat will vary depending on the environment needed to keep biomass growth ratesoptimal for the specific microbial populations. For example, it often isrequired to monitor and adjust aeration to maintain a consistent dissolved oxygenlevel to keep the system’s bacteria multiplying at the appropriate rate to meetdischarge requirements.

(Video) How do wastewater treatment plants work?

In addition to dissolved oxygen, biological systems often needto be balanced for flow, load, pH, temperature, and nutrients. Balancing a combinationof system factors is where the biological treatment process can become verycomplex. Below are examples of some common types of biological wastewatertreatment systems, including a brief description of how they function withinan industrial wastewater treatment regimen to give you an idea of the types oftechnologies and systems that might benefit your industrial facility.

Aerobicwastewater treatment technologies

Activated sludge was firstdeveloped in the early 1900s in England and has become the conventionalbiological treatment process widely used in municipal applications but can alsobe used in other industrial applications. Wastewaters from the primarytreatment phase enter an aeration tank where it is aerated in the presence of suspended(freely floating) aerobic microorganisms. The organic material is broken downand consumed, forming biological solids which flocculate into larger clumps, orflocs. The suspended flocs enter a settling tank and are removed from thewastewater by sedimentation. Recycling of settled solids to the aeration tankcontrols levels of suspended solids, while excess solids are wasted as sludge. Activatedsludge treatment systems typically have larger space requirements and generatelarge amounts of sludge, with associated disposal costs, but capital andmaintenance costs are relatively low, compared to other options.

Fixed-bed bioreactors, or FBBRs,developed as forced-air industrial treatment systems in the 1970s and 80s, consistof multiple-chambered tanks in which the chambers are packed tight with porousceramic, porous foam, and/or plastic media; the wastewater passes through theimmobilized bed of media. Of all biological treatment systems, FBBRs can holdthe most contaminant-eating microbes in the smallest volume, which makes FBBRs space-savingand energy-efficient technologies ideal for treating wastewaters from medium tomedium-high BOD feed levels down to very low effluent levels. The media isengineered to have a high enough surface area to encourage a robust biofilmformation with long solids lifespan, resulting in low sludge formation andlowest sludge disposal costs. A well-engineered fixed-bed will allow wastewaterto flow through the system without channeling or plugging. Chambers can be aerobicand still have anoxic zones to achieve aerobic carbonaceous removal and full anoxicdenitrification at the same time. More advanced biological processes can befacilitated with these systems (for example, nitrification, denitrification,deselenation, sulfide-reduction, and anammox), by having unique bacterialpopulations colonize the biofilm media in separate tank chambers, which can be uniquelyconfigured to treat your facility’s specific wastewater constituents.

Moving bedbioreactors, or MBBRs, invented in the late 1980s in Norway, already hasbeen applied in over 800 applications in more than 50 countries, withapproximately half treating domestic wastewater and half treating industrial wastewater.MBBRs typically consist of aeration tanks filled with small moving polyethylenebiofilm carriers held within the vessel by media retention sieves. Today theplastic biofilm carriers come from many vendors in many sizes and shapes, are typicallyhalf- to one-inch diameter cylinders or cubes and are designed to be suspendedwith their immobilized biofilm throughout the bioreactor by aeration ormechanical mixing.

Because of the suspended moving bio-film carriers, MBBRs allowhigh BOD wastewaters to be treated in a smaller area with no plugging. MBBRsare typically followed by a secondary clarifier, but no sludge is recycled tothe process; excess sludge settles, and a slurry removed by vacuum truck, or settledsolids are filter pressed and disposed as a solid waste.

(Video) Biological Treatment of Wastewater | Wastewater Treatment Technologies | Water Pollution.

MBBRs are often used to remove the bulk of BOD load upstreamof other biological treatment processes or used in situations where effluentquality is less important; they are not used for polishing BOD to low effluentlevels. They are used for treating wastewaters produced in food and beveragefacilities, meat processing and packing plants, petrochemical facilities, andrefineries.

Membrane bioreactors,or MBRs, came into common use in the 1990s once membrane modules weresubmerged directly in the aeration tank, and air scour was implemented to keep themembranes from fouling. MBRs are advanced biological wastewater treatmenttechnologies that combine conventional suspended-growth activated sludge withmembrane filtration, rather than sedimentation, to separate and recycle thesuspended solids. As a result, MBRs operate with much higher mixed-liquorsuspended solids (MLSS) and longer solids residence times (SRTs), producing asignificantly smaller footprint with a much higher quality effluent compared toconventional activated sludge.

MBRs primarily target BOD and total suspended solids (TSS). MBRsystem design varies depending on the nature of the wastewater and thetreatment goals, but a typical MBR might consist of aerobic (or anaerobic)treatment tanks, an aeration system, mixers, a membrane tank, a clean-in-placesystem, and either a hollow fiber or flat sheet ultrafiltration membrane. As aresult of its many parts and cleaning processes, MBRs are known for highcapital, high operating, and high maintenance costs.

Biological trickling filtersare used to remove organic contaminants from both air and wastewater. Theywork by passing air or water through a media designed to collect a biofilm onits surfaces. The biofilm may be composed of both aerobic and anaerobicbacteria which breakdown organic contaminants in water or air. Some of themedia used for these systems include gravel, sand, foam, and ceramic materials.The most popular application of this technology is municipal wastewatertreatment and air remediation to remove H2S at municipal sewer plants, but theycan be used in many situations where odor control is important.

AnaerobicWastewater Treatment Technologies

Upflow anaerobicsludge blankets, or UASBs, useanaerobicbacteria to, as mentioned in the intro of this article,breakdown organics without the use of oxygen, resulting with a combustiblemethane-bearing biogas, treated effluent, and anaerobic sludge. With UASB systems,the general idea is that wastewaters are pumped into the base of the system,where the organics in the wastewater flow through a blanket of sludge beforeentering the upper gas-liquid-solids (GLS) separator, where collection hoods capturethe biogas while allowing the suspended solids to settle and return to thelower reaction zone, while the cleaned effluent overflows out of the top of thesystem. The biogas (methane and carbon dioxide) is either flared or used togenerate steam or electricity for use in other processes at the facility.

(Video) How does the new biological wastewater treatment system works at Balticovo

The UASB process creates less sludge than aerobic biosystemsand therefore needs to be cleaned out and emptied less than other biologicaltreatment systems, but they require skilled operators to maintain optimalhydraulic and anaerobic conditions for UASBs to operate properly. Expandedgranular sludge beds, or EGSBs, are a similar process, but EGSBs use a strongerupward force to encourage more wastewater-to-sludge contact.

Anaerobic digestersalso useanaerobic bacteria to breakdown organic waste without oxygen and produce biogas, mostly for sewagetreatment, and there are a variety of anaerobic digesters available. They eachperform the same process in slightly different ways. Examples include coveredlagoons, fixed film, suspended and submerged media, and continuous stirred tankreactors.

Can SAMCO help?

SAMCO has over 40 years’ experience custom-designing and manufacturing wastewater treatment systems, so please feel free to reach out to us with your questions. For more information or to get in touch,contact us here. You can also visit our website toset up a call with an engineerorrequest a quote. We can walk you through the steps for developing the proper solution and realistic costs for your biological wastewater treatment system needs.

For more information on biological wastewater treatment, see these other articles that might interest you:

  • What Are the Best Biological Wastewater Treatment Equipment Supply and Technology Companies?
  • Common Problems with Biological Wastewater Treatment Systems and How to Avoid Them
  • How Much Do Biological Wastewater Treatment Systems Cost?
  • Biological vs. Chemical Wastewater Treatment Which Is Better for Your Industrial Facility?


1. Lecture 28:Secondary Treatment Processes: Introduction to Biological Treatment of Wastewater
(IIT Kharagpur July 2018)
2. Wastewater treatment plant - How does it work?
(Karl-Uwe Schmitz)
3. RGS Nordic - Biological treatment of waste water
(RGS Nordic A/S)
4. How does the Ecoflo biofilter work? | Wastewater treatment – Septic system
(Premier Tech Water and Environment | Canada - USA)
5. Lecture 29 : Biological Treatment of Waste
(Integrated Waste Management for a Smart City)
6. Wastewater treatment basics - How does wastewater treatment work?
(Karl-Uwe Schmitz)

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