Two-Stage Digester for High Solids Waste
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Solution Overview
Problem
Conventional anaerobic digestion processes require significant water dilution for treating high-solids animal waste, leading to inefficiencies and operational challenges, such as sandbar formation and high water usage, especially in arid regions, and existing dry digestion methods suffer from low hydrolysis rates and methane production efficiency.
Innovation Solution
A two-stage digester system where hydrolysis and methanogenesis occur separately, with leachate recirculation between a leachate bay reactor and a storage tank, and subsequent cycling through a high-rate fixed film reactor for optimized biogas production, while also incorporating a struvite system for ammonium and phosphate management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional complete mix digestion process is used to treat high solids waste, then the waste can be treated, but large quantities of water (9000 cubic meters for 1000 metric tons) are required for dilution
Solution Approach 1:
The digestion process is divided into two separate stages: a hydrolysis stage where solid waste is converted to liquid leachate, and a methanogenesis stage where the liquid leachate is treated. This segmentation allows the high solids waste to be processed without requiring large quantities of dilution water, as the hydrolysis stage operates on the solid material directly and produces liquid leachate that can be treated in the second stage.
2Reliability
If large quantities of water are used for dilution in conventional digestion, then waste treatment is achieved, but sandbars form at the bottom of the digester interrupting stable operation
Solution Approach 1:
The harmful inorganic particles (sand and soil) are extracted and separated from the organic waste material during the hydrolysis stage. The leachate that drains from the solid waste bed contains the dissolved organic compounds while the heavier inorganic particles remain with the solid residue, preventing them from being distributed throughout the digester and forming sandbars.
3Quantity of substance
If dry digestion batch systems are used to reduce water usage, then water consumption decreases, but hydrolysis rates are low resulting in long retention times
Solution Approach 1:
A liquid intermediary (leachate) is introduced to facilitate the hydrolysis process. The leachate circulates through the solid waste bed, providing a liquid medium that enhances contact between microorganisms and solid organic material, thereby significantly increasing hydrolysis rates compared to dry batch systems, while still maintaining much lower overall water consumption than conventional dilution methods.
4Quantity of substance
If leachate production with recirculation is used, then water usage is reduced, but methane production efficiency is low due to pH sensitivity of methanogens
Solution Approach 1:
The system separates the hydrolysis function (which produces acidified leachate) from the methanogenesis function (which requires neutral pH). By segmenting the process into two stages with different pH requirements, the system can optimize each stage for its specific function while maintaining high methane production efficiency in the second stage.
Solution Approach 2:
The pH parameter is allowed to vary between stages: the first stage operates at lower pH to facilitate hydrolysis and acid production, while the second stage operates at higher pH (7.0-8.5) to optimize methanogen activity and methane production. This parameter change between stages resolves the conflict between hydrolysis efficiency and methane production efficiency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces water usage, prevents substrate clogging, and enhances methane production efficiency, allowing for effective treatment of high-solids waste with improved process control and reduced operational costs.
Implementation Method 1
The leachate bay reactor facilitates hydrolysis
Implementation Method 2
cycling leachate from the leachate storage tank to a fixed film environment in a high rate reactor for methogenesis
Implementation Method 3
During operation, water is absorbed into the waste material until saturation
Data Source
AI summary
A digester with separate stages for hydrolysis and methanogenesis is disclosed. The digester includes a leachate bay reactor, which may be configured as modular, multi-bay units. Waste material is added to the leachate bay reactor. A leachate storage tank is fluidically connected to the leachate bay reactor, which facilitates hydrolysis. Leachate from the leachate storage tank is recirculated through the leachate bay reactor. A high rate methanogenic reactor is fluidically connected to the leachate storage tank to cycle the leachate in a fixed film environment for biogas production from solubilized organic matter. The reactor may be operated in an anaerobic digestion mode, or a dual aerobic-anaerobic digestion mode. The reactor may also include a struvite system having a crystallizer unit and a separation unit to reduce ammonium and phosphate accumulation during operation.