This technical guide explains what biological wastewater treatment is, how it works, and how it’s used to improve the quality of industrial waste water streams prior to discharge. The guide looks at the main types of treatment including aerobic, anaerobic and anoxic; the importance of Biological Oxygen Demand or BOD, and the different types of waste water treatment technologies currently including various types of bioreactor.
Why should you treat wastewater?
Any company producing industrial waste typically has to incorporate some sort of wastewater treatment system to ensure compliance with their local environmental and waste discharge regulations.
The ideal wastewater treatment system will take into account the potential for environmental harm, the health of staff and the general public, the process or products which the facility is making or processing, and both upfront capital and ongoing operating costs.
Thinking about treating waste products before discharging any wastewater into the local drainage system will also keep operations on the right side of the law, and avoid the hefty fines often associated with breaching environmental legislation.
Biological wastewater treatment
Biological wastewater treatment is usually the second stage in the cleaning process and comes after larger particles have been removed through the filtering or settlement stages.
Biological wastewater treatment is an effective way of breaking down and eliminating organic waste, typical of the waste products produced in the food and drink, chemical, oil and gas industries.
Biological treatment is complex and has many different aspects to it.
Read on for a brief run-through of the most common methods of biological wastewater treatment methods.
What is biological wastewater treatment?
The simple version of the answer to this is that a typical biological wastewater treatment system uses bacteria and other microbes to clean contaminated water so that it passes predetermined standards.
The bacteria used in the processes uses the pollutants found in the wastewater as food.
As it consumes the pollutants it begins to create particles which start to stick together to create larger clumps.
This process allows the organic matter to eventually settle out of the wastewater solution, producing a sludge which can be easily disposed of as a solid waste.
Types of biological treatment
There are three main types of biological wastewater treatment:
Biological Oxygen Demand
Biological Oxygen Demand, or BOD, is the name of the measurement used to quantify the amount of dissolved oxygen needed by the anaerobic bacteria in the system to break down organic matter.
A high level of BOD means that there is a high level of biodegradable material in the water.
This can be caused by several things including industrial discharge pollutants, domestic sewage, or run-off from fertilisers.
If pollutant levels are very high, BOD can also remove the oxygen required by other aquatic lifeforms.
This can cause the death of fish and other aquatic life, accelerate the growth of blooms of harmful algae and result in serious damage to the ecosystem in the area where the untreated or poorly treated wastewater is discharged.
This potential for environmental damage is what drives the requirements to treat waste before final discharge.
Selecting the best wastewater treatment system
The type of wastewater produced by your operation, coupled with the discharge requirements in your particular area will determine which type of biological treatment is needed, and in which order the treatments should be sequenced.
Biological treatment systems therefore take an existing biological process, and optimise this to make it easier to remove contamination in industrial wastewater.
The best systems can replace, or be used alongside, other physical and chemical wastewater treatment processes.
Microorganisms and wastewater
The exact composition of a biological wastewater treatment system will depend on the chemical composition of the raw wastewater and might be made up several different steps.
The system will also include procedures which can keep biomass growth in check.
For example, engineers will often keep an eye on and adjust aeration to keep dissolved oxygen levels constant and at the correct rates to keep the system running efficiently.
As well as monitoring dissolved oxygen levels, operators have to balance other aspects of the system such as nutrients, temperature, flow and pH levels.
Trying to balance a wide range of factors in a biological treatment process can become very complex, very quickly.
There are several key technologies in used in the treatment of wastewater and these are explored below.
Aerobic wastewater treatment technologies
Common types of aerobic treatment technologies.
Activated sludge is a system that has been around since the start of the 20th century.
It’s the most common biological treatment used in large utility type water treatment plants but also has a place in other industrial settings.
Wastewater first flows into an aeration tank, where oxygen is pumped into the water to feed the freely-floating bacteria and other microorganisms.
These then break the organic material down to form biological solids which form clumps known as flocs.
Flocs can then be removed from the wastewater through the sedimentation process.
The disadvantage of activated sludge processes is that they require a lot of space, and can produce large amounts of sludge.
On the plus side, they are cheap to build and maintain when compared with other options.
Fixed-bed bioreactors (FBBRs)
Fixed-bed bioreactors or FBBRs are a technology that was developed in the 1970s and 80s.
FBBRs comprise a series of tanks with multiple chambers, packed with a porous material such as ceramic, foam or plastic.
The wastewater flows through the various chambers, with contaminants being eaten by microbes along the way.
The design of the chambers and porous material allows FBBRs to hold more microbes in the same space, making this a space-saving technology when compared with other options.
It’s also energy efficient, and ideal for treating wastewater at all BOD levels.
Sludge disposal costs are low and FBBRs have a long lifespan too.
Water can flow through the system without plugging or channeling.
There’s also the option to add other biological processes such as nitrification or desalination into the chambers, and the set-up of the system can be tailored for your specific requirements.
Moving-bed bioreactors or MBBRs are a Norwegian invention which came into use in the late 1980s.
This technology is now being used in many countries for treating both domestic wastewater and industrial wastewater.
A typical MBBR set-up comprises aeration tanks filled with small biofilm carriers of different shapes and sizes.
As the biofilm carriers are MBBRs are suspended and moving, this allows high BOD wastewater to be treated in a smaller area.
There’s usually a second stage in the process after a MBBR is used.
Excess sludge settles into a slurry which can be removed by vacuum, or solids can be pressed into solids for removal and disposal.
MBBRs are typically used in the first step of the treatment process, or used in situations where the quality of the effluent is of lesser importance.
MBBRs are typically used to treat wastewater from food and drink factories, meat processing or packing plants, oil refineries or petrochemical sites.
Membrane bioreactors (MBRs)
Membrane bioreactors or MBRs were developed in the 1990s.
Membrane modules are submerged into the aeration tank, and air is used to scour the submerged membrane to keep them clear.
This is an advanced treatment method which combines conventional activated sludge and membrane filtering to remove solids rather than depending on sedimentation.
MBRs produce a higher quality of effluent when compared with conventional activated sludge plants, and take up far less space.
The design of the system will depend on the type of wastewater produced and what type of end result is required.
A typical MBR will have both aerobic and anaerobic treatment tanks, a system for aeration, a tank with a membrane, and an ultrafiltration membrane.
Although effective, the downside of a MBR system is that it is expensive to build initially, and also involves higher operating and maintenance costs.
Biological trickling filters
Biological trickling filters can be used to remove organic contamination from either wastewater or air.
The air or water is passed through some type of medium which will allow biofilm to collect on its surface.
This biofilm, composed of both anaerobic and aerobic bacteria, will break down organic contamination.
Gravel, foam, sand or ceramics might be used to create these systems.
These filters are more commonly found in water treatment plants but can be very effective in any situation where keeping smells to a minimum is essential.
Anaerobic wastewater treatment technologies
With up-flow anaerobic sludge blankets, or UASBs, the theory is that wastewater enters at the base of the system and up through a blanket of sludge before into a upper gas separator, where biogas is sucked away.
Suspended solids settle and return to the lower levels of the system, as cleaned effluent flows away from the top.
The biogas, a mix of methane and carbon dioxide, is burned off, or can be used to generate electricity for using elsewhere in the plant.
UASB creates less sludge than an aerobic system, requiring less frequent cleaning and emptying of sludge.
However, maintaining this type of plant is skilled work and experienced engineers must keep on top of conditions to ensure it works properly.
Anaerobic digesters can break down organic waste without the need for oxygen.
This process is most commonly used in sewage treatment, and frequently found digesters include covered lagoons, suspended or submerged media, stirred tank reactors and fixed film.
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