Anaerobic wastewater treatment has a well-deserved reputation for efficient, low-odor treatment of streams with high concentrations of organic contaminants. If you’re investigating biological treatment processes, you’ve no doubt asked questions like “What is anaerobic wastewater treatment?” and “How does anaerobic wastewater treatment work?”
This article will provide an approachable introduction to anaerobic treatment, and help to clarify how it works and why it’s used.
What is anaerobic wastewater treatment?
Anaerobic wastewater treatment is a biological process where microorganisms degrade organic contaminants in the absence of oxygen. In a basic anaerobic treatment cycle, wastewater enters a bioreactor receptacle. The bioreactor contains a thick, semi-solid substance known as sludge, which is comprised of anaerobic bacteria and other microorganisms. These anaerobic microorganisms, or “anaerobes,” digest the biodegradable matter present in the wastewater, resulting in an effluent with lower biological oxygen demand (BOD), chemical oxygen demand (COD), and/or total suspended solids (TSS), as well as biogas byproducts.
Anaerobic wastewater treatment is used to treat a variety of industrial effluent streams from agricultural, food and beverage, dairy, pulp and paper, and textile industries, as well as municipal sewage sludge and wastewater. Anaerobic technologies are typically deployed for streams with high concentrations of organic material (measured as high BOD, COD, or TSS), often prior to aerobic treatment. Anaerobic treatment is also used for specialized applications, such as treatment of waste streams with inorganics or chlorinated organics, and is well-suited for treating warm industrial wastewater.
How does anaerobic wastewater treatment work?
Anaerobic wastewater treatment is a type of biological treatment where anaerobic microorganisms are used to break down and remove organic contaminants from wastewater. While anaerobic treatment systems may take a variety of forms, they generally include some form of bioreactor or repository capable of maintaining the oxygen-free environment needed to support the process of anaerobic digestion.
The anaerobic wastewater treatment process consists of two stages: an acidification phase followed by a methane production phase, with both processes occurring in dynamic equilibrium. In the initial acid-forming phase, anaerobes break down complex organic compounds into simpler, short-chain volatile organic acids. The second phase, known as the methane-production phase, consists of two steps: acetogenesis, where anaerobes synthesize organic acids to form acetate, hydrogen gas, and carbon dioxide; and methanogenesis, where the anaerobic microorganisms then act upon these newly-formed molecules to form methane gas and carbon dioxide. These byproducts can be reclaimed for use as fuel, while the wastewater can be routed for further treatment and/or discharge.
Depending upon specific application needs and facility requirements, anaerobic digester systems can be designed as single- or multi-stage units, meaning that they can be configured with a separate acidification tank and bioreactor unit. Common types of anaerobic wastewater treatment systems include the following:
Anaerobic lagoons are large man-made ponds, typically ranging between 1-2 acres in size, and up to 20 feet deep. They are used widely for treatment of agricultural wastewater resulting from meat production, as well as treatment of other industrial wastewater streams, and as a primary treatment step in municipal wastewater treatment. Wastewater is typically piped into the bottom of the lagoon, where it settles out to form an upper liquid layer, and a semi-solid sludge layer. The liquid layer prevents oxygen from reaching the sludge layer, allowing a process of anaerobic digestion to break down the organic materials in the wastewater. On average, this process can take as little as a few weeks, or up to six months to bring BOD/COD levels to the target range. Anaerobic bacteria favor certain environmental conditions, such as warm water temperatures (85-95° F) and a near-neutral pH, therefore, maintaining optimal conditions will enhance the rate of anaerobic microorganism activity, resulting in a shorter wastewater detention time. The rate of anaerobic respiration can also be limited by a number of factors, including fluctuations in BOD/COD concentration, and presence of substances such as sodium, potassium, calcium, and magnesium.
Anaerobic sludge blanket reactors
Sludge blanket reactors are a type of anaerobic treatment where wastewater is passed through a free-floating “blanket” of suspended sludge particles. As the anaerobes in the sludge digest the organic constituents in the wastewater, they multiply and collect into larger granules that settle to the bottom of the reactor tank, and can be recycled for future cycles. The treated effluent flows upward and out of the unit. Biogases resulting from the degradation process are collected by collection hoods throughout the treatment cycle.
Anaerobic sludge blanket reactors are available in a few different forms, including:
- Upflow anaerobic sludge blanket (UASBs): In UASB treatment, wastewater is pumped into the bottom of a UASB bioreactor with upward flow applied. This causes the sludge blanket to float as the wastewater flows through it.
- Expanded granular sludge beds (EGSBs): EGSBs are very similar to UASB technology, with the key distinguishing factor being that the wastewater is recirculated through the system to promote greater contact with the sludge. They are also typically taller than UASBs, and influent flows are sustained at a higher velocity. As a result, EGSBs are able to treat streams with higher loads of organics comparative to UASB systems.
- Anaerobic baffled reactors (ABRs): ABRs are constructed with semi-enclosed compartments that are separated by alternating baffles. The baffles interrupt the smooth flow of the wastewater stream, encouraging greater contact with the sludge blanket as it travels from the reactor inlet to outlet.
Anaerobic filter reactors
Anaerobic filter reactors are comprised of a reactor tank outfitted with a fixed filter medium of some kind. Anaerobic microorganisms are allowed to establish themselves on the filter media, forming what is known as a biofilm. Filter media vary from one system to the next, with common materials including plastic films and particles, as well as gravel, pumice, bricks and other materials. New filter media must be inoculated with anaerobes, and the biofilm may take several months to become established to the point that it is ready for treatment at full capacity.
During treatment cycles, the wastewater stream is passed through the filter media, which serves to capture particles from the stream, while also providing ample surface area for exposing anaerobes in the biofilm to the organic materials present in the stream. Filter reactor performance must be carefully monitored over time, as the filter media will eventually become clogged with excess biofilm and particulate buildup, requiring maintenance steps such as backwashing and cleaning to maintain optimal performance.
Can SAMCO help?
SAMCO has over 40 years’ experience custom-designing and manufacturing biological wastewater treatment systems for a range of industries and applications, so please feel free to reach out to us with your questions. Contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your specific water treatment system needs.
To learn more about the services and technologies that SAMCO offers, visit our page on wastewater treatment solutions here.