Wastewater or discharge from industries, including sewage, contain various organic pollutants. These industrial effluent streams need to be treated to remove the pollutants before it is discharged back into the environment.
One method of treating wastewater is anaerobic wastewater treatment.
What is Anaerobic Wastewater treatment?
Anaerobic water treatment is a biological process that breaks down organic contaminants found in wastewater using microorganisms in the absence of oxygen.
However, these organisms use oxygen found in the oxides introduced in the system or that which is present in the organic matter found in the wastewater, slurries, and sludge. The anaerobic microorganisms, or “anaerobes” convert the organic pollutants present in wastewater into biogas. Most of the biogas comprises methane and carbon dioxide. The biogas can be used as an alternative for fuel after it is treated to reduce or eliminate hydrogen sulphide.
Anaerobic treatment occurs in many stages and it’s popular because it stabilises water in the first stage without too much biomass production.
Anaerobic wastewater treatment is used to treat wastewater from various industries, including food production, paper and pulp, and sewage with high concentrations of organic material, often prior to aerobic treatment.
How does it work?
The anaerobic wastewater treatment process consists of two stages:
1. Acidification stage: In this initial stage, the anaerobes break down complex organic compounds into acid or short-chain volatile acids.
2. Methane production phase: This second stage consists of two steps:
- AcetogenesisThe anaerobes synthesize the acids to form acetate, carbon dioxide and hydrogen gas.
- MethanogenesisThe anaerobes will act upon the newly created molecules to form carbon dioxide and gas. The byproducts can be used for fuel if needed, while the wastewater is routed for further treatment or discharge.
The anaerobic treatment systems can be designed and configured as single-or-multi-stage units, depending on the application and requirements of the facility.
Basically, an anaerobic treatment cycle involves wastewater entering a bioreactor receptacle containing a thick semi-solid substance called sludge, full of anaerobic bacteria and other microorganisms which break down the organic contaminants present in the wastewater.
Advantages of anaerobic systems
There are several advantages of the anaerobic wastewater treatment system compared to the aerobic systems:
- Lower cost of sludge treatment and disposal as biomass production is six to eight times lower than aerobic processes.
- Lower cost of nutrient supplementation as fewer nutrients are required because of lower biomass production.
- Anaerobic systems require lower energy for its operation.
- A small reactor surface is required as anaerobic processes can handle higher volumetric loading rates with a small reactor volume.
- Anaerobic systems produce biomass which is an alternative source of fuel.
- Quick response to substrates after a long shutdown period.
Types of Anaerobic Treatment:
Common type of anaerobic wastewater treatment systems are:
These are large man-made ponds almost 20 feet deep. Wastewater is introduced at the bottom of the lagoon, where it settles and forms a semi-solid sludge layer and an upper liquid layer.
The liquid layer prevents oxygen from reaching the sludge below, allowing anaerobic digestion to break the organic contaminants in the wastewater. Optimal conditions—warm water temperatures and neutral pH—are maintained for the anaerobic bacteria to effectively work on the contaminants, resulting in shorter wastewater detention time.
Anaerobic lagoons are widely used for the treatment of agricultural wastewater as a result of meat reduction, for the treatment of industrial wastewater, and as the primary treatment for municipal wastewater.
Anaerobic sludge blanket reactors
Wastewater is passed through a layer of suspended sludge particles where the anaerobes digest the organic contaminants in the wastewater, multiplying and collecting as larger granules which then settles to the bottom of the reactor tank. Meanwhile, the treated effluent flows upward and out of the unit and the resultant biogas is collected by collection hoods throughout the process.
Anaerobic sludge blanket reactors are available in a few different forms and configurations:
- Upflow anaerobic sludge blanket (UASBs)
- Expanded granular sludge beds (EGSBs)
- Anaerobic baffled reactors (ABRs)
Anaerobic filter reactors
A reactor tank is fitted with a filter medium of some sort where anaerobic micro-organisms will establish themselves to form a biofilm. The biofilm takes several months to become established and ready for treatment at full capacity. Common material used for filter media include plastic films and particles, gravel, pumice, bricks and other materials, which vary from system to system.
During the treatment cycle, wastewater is passed through the filter media, where the anaerobes in the biofilm will capture organic material from the wastewater and act upon them.
Over time, the filter media will get clogged with excess biofilm and particulate buildup, requiring maintenance, backwashing and cleaning to maintain optimum operations.
How is anaerobic treatment different from aerobic treatment?
Both treatment systems use microorganisms to break down organic contaminants in wastewater. However, the major difference is that aerobic systems require oxygen while anaerobic processes do not. Each type of treatment process is suited for certain wastewater streams and also differs in the type of waste byproducts that result from each treatment.
The differences in system design between the two processes ensures that anaerobic systems have a lower operational cost and energy requirements but they tend to be slower and need more capital upfront.