This technical guide explains how anaerobic wastewater treatment works, and how it’s used to improve the quality of industrial waste water streams. The guide looks at where this type of process is used and how it can improve treatment efficiency. It then reviews the most common types of anaerobic digester including the use of lagoons, blanket reactors and filter reactors.
Anaerobic treatment processes
Ask anyone in the water treatment industry about anaerobic wastewater treatment and they’ll reel off its many benefits: it’s efficient, doesn’t create nasty smells, and can treat water with high levels of organic contamination.
But that doesn’t really get to the heart of the issue, and explain exactly what the process it, and how it all works.
What is anaerobic wastewater treatment?
The anaerobic treatment of water is a basic biological process.
Bacteria and other microorganisms process organic contaminants, without using oxygen to do so.
In the most basic type of treatment system, wastewater flows into a bioreactor container.
This bioreactor holds a thick substance called sludge which is made from microorganisms including anaerobic bacteria.
In simple terms this anaerobic bacteria and other microorganisms contained inside the reactor “eat” the unwanted organic material in the wastewater, so that the water flowing out of the system has a lower biological oxygen demand (BOD) than the contaminated water entering.
The system also helps to remove suspended solids and biogas by-products.
Where is anaerobic wastewater treatment used?
A wide range of different industries use anaerobic wastewater treatment processes.
These range from agriculture or the food and drink industries to textiles and large municipal waste water treatment plants.
Anaerobic wastewater treatment is often the default choice for wastewater with high concentrations of organic material, and is often used as the first step before the water flows through any aerobic treatment systems.
It’s also a useful process in specialised industries and can also be used to treat water which is warm when it flows out into the anaerobic system.
How does anaerobic digestion work?
Anaerobic microbes contained in the bioreactor help to break down organic contaminants which can then be removed from the wastewater.
There are many different forms of anaerobic treatment, but all depend on a reactor or bioreactor which can provide an environment that is without oxygen, which is a prerequisite for the anaerobic digestion process to occur.
The two phases of anaerobic digestion
There are two phases to any anaerobic digestion process.
The first is the acidification phase and the second is the methane production phase.
Common types of anaerobic digester
The requirements of individual wastewater treatment facilities will determine the precise set-up of the digester system, and whether a single or multiple unit is required.
The most commonly found types of anaerobic digesters are:
Anaerobic or waste lagoons are large artificial ponds, usually up to 20 feet deep and as large as 2 acres.
This method is most commonly used to treat agricultural wastewater, as well as waste water from the meat production and processing industries.
These artificial lagoons or pools often also form the first step in drinking water treatment plants.
Water enters the lagoon through pipes at the bottom.
The incoming water eventually settles to form liquid on the top, and a sludge layer on the bottom of the lagoon.
The top water layer prevents any oxygen from getting to the sludge, so the anaerobic digestion can start to break down the organic contamination in the water.
Although the process can be completed in just a few weeks, it can take as long as six months to get the contaminants in the water down to acceptable levels.
Anaerobic digestion happens more quickly in warmer water and where the pH is near to neutral.
If there are substances in the water such as magnesium, potassium, sodium or calcium, this can also affect the speed of the digestion.
Anaerobic sludge blanket reactors
Anaerobic sludge blanket reactors are a different sort of anaerobic treatment where the wastewater flows through suspended sludge particles known as a “blanket”.
The anaerobes in the sludge digest the organic components in the water which then collect as granules at the base of the reactor tank.
Treated effluent rises up through the tank, and the biogas produced by the process can be filtered out using collection hoods.
There are a few different types of anaerobic sludge blankets, and the most common are:
Up-flow anaerobic sludge blankets (UASBs)
Wastewater is pumped into the bottom of the reactor and an upward flow is applied.
The sludge blanket then floats as the wastewater rises through it.
Expanded granular sludge beds (EGSBs)
This technology is similar to the up-flow blankets.
The main difference is that the water is recirculated back through the system so that it gets more contact with the sludge.
EGSB systems are larger, and can be used to treat streams of water which are more contaminated.
Anaerobic baffled reactors (ABRs)
ABR units are made of compartments which are separated from each other using baffles.
The baffles stop the water from flowing smoothly, and ensure greater contact with the sludge blanket as the water moves through the reactor.
Anaerobic filter reactors
Anaerobic filter reactors use a reactor tank filled with a type of filter medium where microorganisms can establish and grow to form a biofilm covering.
The type of material used to make the filter can vary, but plastic, gravel, pumice and bricks are commonly used.
When new filter material is introduced it initially must be treated with microbes, and it might take a few months for the biofilm to develop to a stage where it is suitable for use in the water treatment process.
Monitoring of the anaerobic filter reactor over time is essential, as eventually the media will become clogged with excess biofilm.
Regular cleaning, maintenance and backwashing is needed to keep anaerobic filter reactors operating properly.
Wastewater treatment solutions for business
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