Anaerobic digester wastewater

Types of Anaerobic Digesters for Wastewater

An anaerobic digester for wastewater is a device designed to treat organic matter in the absence of oxygen. Several types of anaerobic digesters exist, each with its operation and applications.

• Tunnel digester

  A tunnel digester is a horizontal, partially below-ground cylinder with a slope to promote the flow of digested material out of the digester. Its steep-sloped design helps layers to be digested at different rates, removing only the fully-digested material from the bottom of the tunnel. Although the tunnel digester has a simple design and is cheap to build, it has some drawbacks that may hinder its use. For example, it is hard to mix the contents and keep the temperature constant throughout the digester.

• Covered lagoon digester

  This type of anaerobic digester is an open lagoon covered with a plastic or inflatable membrane. The digester offers a large and flexible treatment volume that can accommodate fluctuations in the amount of incoming feedstock. The lagoon digesters are less costly to build than tank digesters. Nevertheless, the lagoon digesters are more expensive in terms of maintenance costs due to the constant need to repair and replace the membrane. Furthermore, the lagoon digesters are at the risk of losing the digested gas if the membranes develop any holes or leaks.

• Mesophilic digester

  Mesophilic digesters work best at moderate temperatures because they rely on microorganisms that flourish at those temperatures. The process takes between 15 to 30 days and results in gas production every day. Depending on the tank's size and the amount of incoming feedstock, the mesophilic digester tank must be heated regularly in order to maintain the right temperature. Thus, the operation costs of mesophilic digesters are very high, especially when there is a high demand for energy resources in such heating systems.

• Tank digester

  Tank digester: This type of anaerobic digester tank has concrete, metal, or plastic walls that are vertical or horizontal. Furthermore, there are different ways available to tanks to fill up and empty their contents, including using the pull-plug method, the floating electrode method, or mechanically pulling the electrodes through the tank.

• Dry digester

  The dry digester processes organic matter with a solid content of over 20%, such as food waste and agricultural residuals. It uses an anaerobic bacterium to break down organic materials, producing methane and carbon dioxide. After the process, the dry digester can be used as fertilizer in farms.

• Upflow anaerobic sludge blanket (UASB) digester

  The UASB digester is suitable for wastewater with a high organic load. The influent heads from the bottom of the digester to its top through a sludge bed. Here, microorganisms in the sludge bed decompose the organic matter, producing biogas in the process. The biogas rises to the surface and a part of it is used to heat the digester while the rest is captured for energy use. UASB digesters have the advantages of small area and energy efficiency. What’s more, they can efficiently treat various forms of organic industrial and municipal wastewater.

• Floating drum digester

  The floating drum digester has a metal or plastic drum that moves up and down, depending on the amount of digested gas produced. The digester is simple to set up and operate, and the volume of gas generated can be easily determined. However, the exposed drum is vulnerable to weather conditions and corrosion.

Specifications and maintenance of anaerobic digester wastewater

Specifications

• Size: Anaerobic digesters are typically larger than aerobic digesters because they have to hold waste for a longer period. Depending on the size of the plant and the amount of waste that needs to be processed, they can be anywhere from 20 to 150 feet in length and 15 to 80 feet in width.
• Temperature: Anaerobic digesters work best at certain temperatures, which can be mesophilic (around 35-40°C) or thermophilic (around 50-60°C). For each digester, temperature control is essential, and this helps to maintain the optimal microbial activity and digestion efficiency.
• pH level: The anaerobic digester has to maintain digesters' pH levels, which are typically between 6.5 and 7.5. Doing so ensures that the bacteria in the digester can thrive and break down the waste efficiently.
• gas production: Anaerobic digesters are usually utilized to produce biogas, which consists of methane, carbon dioxide, and small amounts of other gases. The digester's design will determine how much biogas it produces, and this can range from 50 to 80 percent of the methane content.
• Retention time: Depending on factors like temperature, pH, and the specific bacteria used, the retention time for waste in an anaerobic digester usually varies from 15 to 60 days.

Maintenance

• Regular Inspection: Routine inspection of the anaerobic digester should be carried out to ensure all components are functioning properly. This includes checking for leaks in the gas collection system, inspecting the mixing mechanism, and examining the integrity of the digester vessel.
• Feeding System Maintenance: Keep the feeding system clean and free from clogs. Regularly monitor the inlet valve and adjust the feed rate as necessary. Ensure proper mixing within the digester to prevent sedimentation and maintain uniform substrate distribution.
• Gas Production Monitoring: Monitor the volume and composition of the biogas produced by the digester. Regularly check the gas pressure and temperature to ensure they are within the optimal range for efficient digestion and gas production.
• Maintaining Environmental Conditions: The anaererobic digestive system's efficiency relies on environmental factors like temperature and pH balance. Keep the digester at the right temperature by regularly monitoring it and adjusting heating or cooling systems as needed. For the microorganisms to thrive and break down waste effectively, the pH must be kept within the specified range.
• Maintenance of Gas Collection System: The gas collection system should be clean and leak-free. Regularly monitor the gas collection bag or dome to ensure it is not overfilled or damaged. Gas flow rates should be measured to prevent any accumulation of pressure within the digester that could lead to disruptions in the digestion process. Also, the digester's gas collection chamber should be emptied whenever necessary to remove any settled materials or contaminants.
• Proper Handling of Digestate: The material produced from the anaerobic digestion process is called digestate, and it can be a valuable fertilizer or soil amendment. Proper handling and storage of digestate is crucial to prevent nutrient loss and contamination. Use appropriate equipment for its transportation and ensure it is stored in suitable facilities with proper nutrient management plans.

Applications of Anaerobic Digester Wastewater

An anaerobic digester, which is part of a larger wastewater treatment system, has diverse applications that go beyond just treating waste. In many municipalities, the treated wastewater from anaerobic digesters serves as a reliable source of irrigation for agriculture, especially in regions facing water shortages. By continuously supplying water for farming, cities not only help farmers but also contribute to stabilizing food production and reducing dependency on constant rainfall.

In the energy sector, anaerobic digesters are becoming crucial. They convert organic waste into biogas, a renewable energy source. This biogas is rich in methane and can be used as a substitute for natural gas, helping to lower dependence on fossil fuels. Furthermore, the leftover material from the digestion process, known as digestate, is nutrient-rich. It is being explored as a potential replacement for chemical fertilizers in agriculture, promoting more sustainable farming practices.

Industries that produce significant amounts of organic waste, such as food processing factories and farms, find anaerobic digesters beneficial as well. By using these digesters on-site, they can effectively manage their wastewater and reduce the organic content before it reaches external treatment facilities.

Even small-scale farmers can benefit from anaerobic digestion technology. Remote farms with limited access to energy sources are exploring the use of small or mini-sized anaerobic digesters. By processing farm waste and producing biogas on-site, these farms can generate their own energy, enhancing their self-sufficiency.

How to choose anaerobic digester for wastewater

While mixing and matching various configurations can help tailor the digester to a specific situation, the following key features of any wastewater digester should be considered.

• Digester Configuration

  While covered/conical reservoirs are the most common kind of digester, other configurations exist that may suit particular situations. For instance, warm regions that never face below-zero temperatures may use an open lagoon digester, which has a greater risk of methane leakage. Alternatively, a dynamic digester with a constant outflow may suit a facility that does not want to keep digester output in storage.

• Gas Storage

  For most sites, having a gas holder will increase operational flexibility, and this can come in many forms, such as an inflatable gas storage bag, a floating dome, or a gasholder drum. These will be refilled by the digester gas whenever the load is methane is present in the digester feedstock.

• Heat Recovery

  The digester may lose heat through its outer insulation, and in cold digesters, heat recovery will be necessary to maintain optimal digestion temperature. Heat recovery can be achieved through heat exchangers or heat sinks , depending on how much heat is lost and how much need to be used elsewhere in the facility.

• Pumping System

  Ensuring that the digester's outflow can be easily and quickly pumped out using pneumatic systems or other appliances is essential. Depending on the digester's configuration, varying pump strength and capacities are required.

Q&A

Q: What are the challenges of using anaerobic digester wastewater treatment?

A: One of the major challenges of using anaerobic digestion for wastewater treatment is the vulnerability of the microbial community to external factors. The pH, temperature, and organic loading rates need to be precisely controlled to maintain the activity and stability of the microbes. Another potential problem with anaerobic digesters is the digested sludge. Disposing of or reusing it can be difficult and may cause environmental concerns if proper measures are not taken.

Q: What material is used for an anaerobic digester?

A: Materials suitable for anaerobic digesters include carbon steel, stainless steel, and glass fiber reinforced plastic. Carbon steel is widely used in the construction of anaerobic digesters because of its good strength and durability, making it suitable for large-scale wastewater treatment facilities. Stainless steel, which is corrosion-resistant and robust, is often used in small and medium-sized anaerobic digesters, such as those used in kitchens or restaurants. Moreover, the antibacterial property of stainless steel can improve the efficiency of wastewater treatment. Glass fiber reinforced plastic, with its lightweight and high strength, is frequently used in the construction of anaerobic digester covers and tanks.

Q: How big is an anaerobic digester?

A: The size of an anaerobic digester varies according to the design of the plant and the flow rate of the wastewater. The digester is usually cylindrical in shape and its dimensions can be up to 10 m in diameter and 15 m in height.

Q: How long does anaerobic digestion take?

A: The anaerobic digestion process takes about 15 to 60 days, depending on the temperature, pH, and C/N ratio.