High-Efficiency Anaerobic Digesters: Core Technology for Organic Waste Valorization
With the growing severity of environmental pollution and energy shortages, the development of biogas projects is becoming increasingly important for achieving sustainable development in terms of the environment, energy, and the economy. The selection of appropriate processes is the key to ensuring the efficient implementation of these projects
1.Classification and Characteristics of Anaerobic Digesters
Based on different hydraulic retention times (HRT), solid retention times (SRT), and microbial retention times (MRT), anaerobic digesters in biogas projects can be categorized into three main types: conventional, sludge retention, and attached film digesters.
These can be further divided into conventional digesters, continuously stirred tank reactors (CSTR), horizontal plug-flow anaerobic digesters (HCPF), and unit plug-flow anaerobic reactors (UPR).
Conventional digesters have a simple structure and are widely used, but they lack stirring devices. The raw material naturally stratifies and settles, limiting active anaerobic digestion to the active layer, leading to relatively low efficiency. In rural China, the most commonly used water pressure biogas pits belong to this type of digester.
CSTR is suitable for treating high total solids (TS) waste. It prevents stratification, ensuring uniform distribution of material and temperature. Inhibitory substances entering the digester disperse rapidly, maintaining low concentration levels. CSTRs also prevent scum formation, crusting, blockages, gas escape issues, and short-circuiting. They require low capital investment, are easy to operate and manage, and are conducive to mathematical modeling. However, they have a low volumetric loading rate, require mechanical stirring (which increases energy consumption), and the effluent quality is relatively poor.
CSTRs are ideal for high TS feedstock and were once the most widely used digesters. They can avoid material stratification, ensure even distribution of materials and temperature, and allow inhibitory substances to disperse quickly, maintaining low levels. Additionally, they prevent issues like scum, crusting, blockages, gas escape problems, and short-circuiting. The investment required is small, operational management is simple, and they are easy to model mathematically. However, because they cannot maintain an SRT and MRT longer than HRT, the required digester volume is large, volumetric loading is low, and energy efficiency is poor. Microbial loss through effluent is significant, and the effluent quality is poor, leading to a gradual decline in their application.
HCPF is suitable for treating high-concentration, high-total-solids (TS) organic wastewater. Its simple structure requires low investment, operates without stirring, and has low energy consumption. The operation is convenient with few breakdowns, and the system is highly stable. However, solids may settle at the bottom, creating large dead zones that reduce the effective volume of the digester, leading to lower HRT and SRT. It requires solid and microbial recycling as inoculum. The surface area-to-volume ratio of the digester is large, making temperature control difficult, resulting in lower efficiency and poorer effluent quality. Crusting is also common. HCPF is suitable for digesting cattle manure but is not ideal for manure with a high sediment content, such as poultry manure.
UPR addresses some of the issues associated with HCPF, such as the small reactor volume, excessive acidification due to high concentrations, and poor sand removal. UPR improves TS concentration, pre-controls acidification, enhances sand removal, increases reactor volume, and reduces investment costs.
1.2 Sludge Retention Processes
This process shares the advantages of C
STR, such as high buffering capacity, high loading rate, simple operation, and stable performance. However, it requires additional equipment to settle and recycle solids and microorganisms.
UASB is currently one of the fastest-growing digesters in the world, ideal for treating low-TS soluble wastewater. Its simple structure integrates sludge settling and recycling into one unit, without the need for stirring devices or packing material, keeping the investment and operating costs low. UASB provides long SRT and MRT, with high volumetric loading rates, stable performance, and high treatment efficiency, making it widely used. However, the three-phase separator is complex, and a distributor is needed for feeding. When subjected to shock loads, increased TS content, or excessive toxic substances, sludge loss can occur, and the operation requires a high level of technical expertise.
EGSB is an improved version of UASB with high volumetric COD loading. It performs better than other processes when treating low-concentration wastewater at low temperatures. However, due to its high upflow velocity, EGSB requires strict operational control and is unsuitable for wastewater with high suspended solids (SS). Suspended solids passing through the granular sludge bed may quickly exit with the effluent before degradation occurs.
The IC reactor is one of the most efficient anaerobic reactors in the world, combining the advantages of UASB and fluidized bed reactors (FBR). It uses the biogas generated in the reactor to create internal circulation of the fermenting liquid. IC reactors offer very high volumetric loading rates, saving on construction costs and land area. They are energy-efficient, have a short startup time, buffer pH effectively, have strong resistance to shock loads, and produce stable effluent. However, they are not suitable for feedstock with high SS content and are mainly used for treating industrial organic wastewater.
USR is suitable for high-TS feedstock. It has a simple structure and does not require a three-phase separator, sludge recycling, or stirring devices. Its efficiency is comparable to UASB, though UASB is strictly limited to soluble feedstock.
In recent years, baffled reactors have been applied in China, particularly for low-concentration domestic wastewater. However, their performance has been unsatisfactory in both theoretical and practical terms, with significant challenges in scaling up for industrial use. Further research is required. The baffles complicate the digester structure, making construction difficult and costly.
Common types of attached film digesters include anaerobic filters (AF), fluidized bed reactors (FBR), and expanded bed reactors (EBR).
AF does not require stirring and has low operational costs. It has a high loading rate, small volume, long MRT, high sludge concentration, stable operation, and low technical requirements. AF can withstand load variations and can be restarted quickly after long shutdown periods. However, the cost of packing material is high, installation is complex, and periodic replacement is required. The accumulation of microorganisms increases resistance during operation, resulting in longer startup periods. AFs are prone to clogging and short-circuiting and are only suitable for treating low-SS wastewater.
FBR and EBR are suitable for low-TS feedstock. They have higher loading rates, more stable operation, and can handle load fluctuations better. After long shutdown periods, they can restart more quickly. However, they have high energy consumption and maintenance costs. The supporting media can be flushed out, causing damage to pumps or other equipment. They also have longer startup periods and may require degassing devices to efficiently separate media particles from suspended solids in water. Currently, these reactors are primarily in the experimental research stage.
