Anaerobic Digester with Solids Recirculation for High Loading Rates
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Solution Overview
Problem
Conventional anaerobic digesters face inefficiencies due to low active biomass concentrations, which limit organic loading rates and require large tank volumes, and struggle with high-solids industrial wastewater treatment, especially in maintaining adequate food-to-microorganism ratios and handling particulate organic carbon.
Innovation Solution
The process involves an anaerobic digester system with a solid-liquid separation unit that recycles thickened solids back into the digester, decoupling solids retention time from hydraulic retention time, allowing for higher organic loading rates and effective digestion of particulate solids, while minimizing bacteria washout and optimizing biogas production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional anaerobic digesters operate with low active biomass concentrations, then the system is simpler to operate, but the organic loading rate is limited and tank volume must be increased
Solution Approach 1:
The system changes the concentration parameter of active biomass in the digester by implementing a two-stage configuration where a first digester operates at high solids concentration (10-20% TS) while a second digester operates at lower solids concentration (3-8% TS). This parameter differentiation allows the high biomass concentration digester to achieve high organic loading rates without requiring proportionally large tank volumes, as the biomass is concentrated in a smaller volume where it is most needed for degradation.
Solution Approach 2:
The anaerobic digestion system is segmented into two distinct digesters with different operational characteristics. The first digester is dedicated to handling high-solids waste and maintaining high active biomass concentrations for rapid degradation, while the second digester handles the effluent from the first at lower solids concentrations. This segmentation allows each unit to be optimized for its specific function, enabling high organic loading rates in the first digester without requiring the entire system to be scaled up proportionally.
2Productivity
If high-solids industrial wastewater is treated in conventional digesters, then the treatment capacity is improved, but the food-to-microorganism ratio becomes inadequate and particulate organic carbon digestion is insufficient
Solution Approach 1:
The system changes the solids concentration parameter between digesters to optimize the food-to-microorganism ratio. The first digester operates at high solids concentration (10-20% TS) where particulate organic carbon can be effectively degraded, while the second digester operates at lower solids concentration (3-8% TS) providing adequate dilution and F/M ratio for soluble organic matter. This parameter differentiation ensures both particulate and soluble organic carbon are digested efficiently, maintaining high treatment capacity with reliable digestion efficiency.
Solution Approach 2:
The effluent stream from the first high-solids digester acts as an intermediary that transfers partially digested organic matter to the second digester. This intermediary stream allows the system to handle high-solids waste in the first digester while providing the second digester with a more manageable substrate concentration, ensuring adequate F/M ratio and effective digestion of remaining organic carbon without requiring either digester to operate under suboptimal conditions.
3Reliability
If solids retention time is increased to maintain adequate biomass, then the digestion efficiency is improved, but the hydraulic retention time becomes insufficient and bacteria washout occurs
Solution Approach 1:
The system segments the solids and hydraulic flow paths between two digesters. The first digester maintains high solids concentration and long solids retention time for effective biomass retention and particulate organic carbon digestion. The second digester receives the effluent with shorter hydraulic retention time, allowing bacteria to be processed through the system without washout while maintaining adequate SRT in the first digester. This segmentation decouples SRT from HRT, enabling bacteria retention without requiring excessively long hydraulic retention times that would reduce treatment capacity.
Solution Approach 2:
The system changes the solids concentration parameter between digesters to optimize the relationship between solids retention time and hydraulic retention time. The first digester operates at high solids concentration (10-20% TS) where even short HRT results in adequate SRT due to the high biomass concentration. The second digester operates at lower solids concentration (3-8% TS) with longer HRT, providing a transition zone that allows bacteria to acclimate and be retained while maintaining system throughput. This parameter differentiation enables adequate biomass retention without requiring uniformly long HRT throughout the system.
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 enables efficient treatment of industrial wastewater with high chemical oxygen demand and suspended solids, maintaining a high percentage of active biomass, allowing for higher organic loading rates and reduced solids disposal costs, while producing more biogas than traditional systems.
Implementation Method 1
In a wastewater treatment system and process, feed water is processed in an anaerobic digester
Implementation Method 2
anaerobic digestion of organic wastes, preferably to also produce a useful biogas
Implementation Method 3
A solid-liquid separation device, for example a sludge screw thickener (SST), treats digestate from the digester
Data Source
AI summary
Feed water is processed in an anaerobic digester. A solid-liquid separation device, for example a sludge screw thickener, treats a stream drawn from the digester in a recirculation loop. The solids portion is returned to the digester to increase the solids retention time and the TSS concentration in the digester. A liquid portion with less than 5% of the solids in the stream is removed and optionally treated further. The flow rate to the solid-liquid separation device is preferably greater than the influent flow rate. The solid-liquid separation device may receive digestate at a TSS concentration of 4% or more and return a solids portion having a TSS concentration of over 10%. The feed water is preferably one or more industrial waste streams having a COD concentration of 20,000 to 50,000 mg/L and a TSS concentration from 1-5%. The organic loading rate may be 10-12 kg/COD/m3/day.