Anaerobic Digestion : Key to optimize Biogas Production

Anaerobic digestion is?a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen.

One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels. A range of anaerobic digestion technologies are converting livestock manure, municipal wastewater solids, food waste, high strength industrial wastewater and residuals, fats, oils and grease (FOG), and various other organic waste streams into biogas, 24 hours a day, 7 days a week. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland or converted into other products. Nutrients in the liquid stream are used in agriculture as fertilizer.

While there are many specific configurations for anaerobic digesters (mesophilic, thermophilic, etc.) the objective remains the same: create an environment that promotes the controlled and stable decomposition of organic matter via naturally occurring biological pathways. This is accomplished in four simultaneous stages: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis.

Methane forming bacteria are sensitive to many process conditions, including temperature, pH, and the presence of various toxins. Optimal performance occurs within a pH range of 6.8-7.2. If pH levels drop in the digester, the methane-formers can be inhibited, halting the digestion process and biogas production altogether.

Digester stability is greatly increased when high levels of alkalinity are present. Alkalinity is defined as a solutions ability to resist changes in pH in the presence of acids/bases. In anaerobic digesters, alkalinity is consumed during the production of VFA in stage 2 referenced above. Fortunately, bicarbonate alkalinity is produced as the methane-formers convert VFA into methane, as referenced in stage 3 above. Digester operators can maintain a healthy balance between VFA and alkalinity based on carefully monitored operational control of feed rates, mixing, and heating.

While anaerobic digestion processes are well understood, digester upsets or inefficient energy recovery issues are still challenges commonly faced by operators. Anaerobic digestion efficiency can be substantially improved by implementing pretreatment technologies that lyse the cells being fed into the digester. Cell lysing improves the efficiency, energy capture, environmental sustainability, and solids reduction in the digestion process. One such pretreatment technology is thermal hydrolysis process (THP). This process uses extreme pressure and heat to achieve these results. While pretreatment of anaerobic sludge can vastly improve the digestion process, the system should also be carefully monitored to prevent digester over-feeding.?

For high efficiency , the process parameters of anaerobic digestion, such as temperature, pH, Hydraulic retention time (HRT), Organic Loading Rate (OLR), and sludge retention time (SRT) have to be taken into account for the optimum conditions that allow living , grow and multiplication of bacteria.

A shutdown, cleanup and replacement of the bacterial sludge is extremely costly, calculated in the hundreds of thousands of euros for even a small overloading event. However, process optimization can result in additional income from various areas in the plant, like the sale of excess sludge or produced heat as a utility. Hence, it is highly important to continuously monitor all processes related to biogas formation to minimize waste and increase profit in many areas.

Monitoring fermentation efficiency online to track acetic acid to propionic acid ratio, fermentation rate, COD measurement to regulate Methanogenesis.

Online near-infrared spectroscopy (NIRS) can improve upon different phases in the biogas production process. Being non-destructive, the reagent-free method enables real-time knowledge of dynamic processes. Process NIRS applications suitable for bio-fermentation comprise of:

• Monitoring the fermentation process (VOA/TAC) to increase production yield
• Optimization of the bacterial feeding process through quality control of incoming feedstock
• Inspection of fermentation residue (N, P, K)
• Moisture percentage in gas

I believe most of the biogas plant profitability is decided at the very start where in the anaerobic digestion needs proper online monitoring to keep a track of small movements which tend to rise up as a problem in big way. Laboratory analysis is way too vague and includes a lot of human errors.

Lets set anaerobic digestion right to bring out the profits!

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