By Ivan Drako
Stabilization of organic materials and biosolids, primary clarifier raw sludge and waste activated sludge that are generated at wastewater treatment plants, is achieved mainly in anaerobic digesters that use suspended-growth treatment processes. The main objective of stabilization is to reduce the volume of solids for disposal, make the digested sludge less odorous and putrescible and reduce the content of pathogenic organisms and helminths (parasitic worms).
Anaerobic digestion (AD) occurs through several sequential biochemical reactions, including hydrolysis, acidogenesis, acetogeneses and methanogenesis. The product of each reaction depends on the previous biochemical reaction products and process conditions, including substrate composition, temperature and pH. Based on the temperature range maintained during organic material stabilization and the present biological community (groups of anaerobic microorganisms involved in the process), anaerobic digestion may operate under mesophilic (30°C – 38°C) or thermophilic (49°C – 57°C) temperatures.
For referenced anaerobic digestion operation temperatures and established opinion on the process energy demand for AD reactor heating, the mesophilic AD is widely used, in most cases because of its supposedly lower demand for anaerobic digestion heating and better stabilization. This opinion is inconclusive and is qualitative, rather than quantitative, due to lack of comprehensive data or explicit analysis.
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The objective of this article is to compare the heating requirements of both mesophilic and thermophilic anaerobic digestion operating modes and provide some analysis and conclusions based on the results.
Key features of both anaerobic digestion modes are provided in Table 1. This data will be used for the analysis and discussion that follows.
Table 1: Operating and controlling parameters.
*Meets the “Class A” biosolid classification set by the United States Environmental Protection Agency.| Parameter | Mesophilic AD | Thermophilic AD |
|---|---|---|
| Temperature, °C | 35 – 38 | 50 – 55 |
| Tolerated Temperature Fluctuation, °C | 3 – 5 | 1 – 2 |
| Heating Requirement (qualitative) | lower | higher |
| Hydraulic Retention Time (HRT), day | 15 – 25 | 3 – 10 |
| Max COD Reduction, % | 65 – 85 | 85 – 95 |
| Max BOD5 Reduction, % | 60 – 80 | 80 – 90 |
| Max Organic Material Reduction, % | 45 – 55 | 55 – 70 |
| Biogas Production (Nm3/1000kg dry org. material) | 920 – 980 | 950 – 1000 |
| Methane Gas Content, % | 60 – 70 | 70 – 85 |
| Pathogen and Helminth Destruction, % | 60 – 70 | Complete* |
| Daily Solid Feeding Volume, volume | 1 | 2 |
Mesophilic and thermophilic anaerobic digestion reactors
For the comparative analysis of mesophilic and thermophilic anaerobic digestion, we will use a reactor with a fixed concrete roof and conical floor slab, 33.5 m diameter, 10.8 m operating water depth, and 12.2 m side wall height. The above grade wall height and below grade wall depth of the digester structure is approximately equal. The above grade structure is a thermally insulated structure. Its below grade structure has thermal insulation between the structure and the soil installed to the frost line depth only. There is no insulation in the deeper digester wall section, or beneath the base slab. Total operation volume is 9,840 m3.
Biochemical reaction rates increase with increased temperatures. This is in accordance with the Arrhenius relationship whereby the process rate is increased 1.5 times for each 10°C temperature rise. Therefore, thermophilic anaerobic digestion is faster than mesophilic AD for the same or greater volatile solids (VS) reduction. As the operating temperature is increased, the minimum mean cell residence time is reduced significantly. This means that, at a higher temperature, the system can be operated at lower mean cell residence and hydraulic retention times. Also, a smaller reactor volume can be used.
Based on this precondition, for the comparative analysis, a reactor having a smaller reactor volume, two-time smaller operation volume of 4,920 m3, is also reviewed as a thermophilic anaerobic digester (called further as “50% Volume”). The reactor has a similar operational characteristic (digested sludge volume by process time combination effect) as the mesophilic AD reactor of the 9,840 m3 volume. It has the following physical dimensions: 26.6 m diameter, 8.6 m operating water depth, and 10.1 m side wall height. The roof and floor construction, thermal insulation and installation conditions are the same as the digester reactor described previously.
For the comparative analysis, some applied assumptions are summarized in Table 2.
Table 2: Applied assumptions.
*The required sludge temperature is achieved by transferring heat from hot water to the sludge via a heat exchanger, followed by recirculation of the sludge to the digester to keep it constantly suspended and to prevent solids and temperature stratification in the digester reactor.| Parameter | Mesophilic AD | Thermophilic AD |
|---|---|---|
| Sludge Temperature, °C * | 37 | 55 |
| Outdoor Temperature Variation, °C | –35 to +40 | –35 to +40 |
| HRT, days | 15 | 7.5 |
| Total Solids (TS) in Primary Sludge, % | 5 | 5 |
| Volatile Solids in Primary Sludge, % of TS | 65 | 65 |
| Organic Material Reduction, % | 50.0 | 62.5 |
| Biogas Production (Nm3/1000kg dry org. material) | 950 | 975 |
| Methane Gas Content, % | 65 | 77.5 |
Analytical results
Thermodynamic analyses conducted on the digester structure to calculate heat losses for each season’s temperature, anaerobic digestion mode and reactor size are presented as sludge temperature rises that should be provided in the heat exchanger to compensate for the outward heat dissipation through the digester structure and piping over time as shown in Figure 1.
Hourly heat requirements to support digestion for each season’s temperature, anaerobic digestion mode and reactor size are presented in Figure 2. Heat energy demand required to digest one m3 of sludge within the respective digestion period (HRT) for each season’s temperature, anaerobic digestion mode and reactor size are presented in Figure 3.
The heat loss balance of a digester is affected also by the addition of raw primary sludge and total waste activated sludge (TWAS) that may take place one or two times per day. The added “fresh” sludge quantity is usually limited to some pre-assigned volume in order not to upset the digestion process.
Figure 4 shows the estimated digester sludge temperature drop when the quantity of the added sludge varies from 1% to 6 % of the total digester operation volume. As a conservative approach, it was assumed that the average temperature of raw primary sludge is +15°C and that the sludge is added as one slug to the digester and instantaneously mixed.
For our case, the volume of the added sludge would be 1.5% and 3% and the temperature drop would be around 0.30°C and 0.65°C for mesophilic and thermophilic AD, respectively. These temperature drops appear to be less than the tolerated temperature ranges referenced in Table 1. Therefore, the digestion process would not be compromised.
Table 3 provides analysis of digestion products yielded for both mesophilic and thermophilic anaerobic digestion modes. It is assumed that the weight of one m3 of sludge is approximately one tonne. Also, that the yielded quantity of methane (part of biogas yield) was converted to heat that could be obtained by burning methane in a digester boiler for the generation of hot water as the energy source for heating the digester sludge in the heat exchanger.
Table 3: Analysis of digestion products.
| Parameter | Digestion Mode | ||
| Mesophilic | Thermophilic | Thermophilic, 50% Volume | |
| Digester Volume, m3 | 9,840 | 9,840 | 4,920 |
| TS / Digester, t | 492 | 492 | 246 |
| Total VS / Digester, t | 320 | 320 | 160 |
| Organic Material Reduced, t | 160 | 200 | 100 |
| Biogas Yield, Nm3/Digester Volume | 151,905 | 194,878 | 97,439 |
| Methane Yield, Nm3/Digester Volume | 98,738 | 151,031 | 75,515 |
| Total Calorific Value, kW/Digester Load | 1,033,196 | 1,580,382 | 790,191 |
| Methane Yield, Nm3/kg VS reduced | 0.309 | 0.472 | 0.472 |
| Calorific Value, kW/ kg VS reduced | 3.23 | 4.94 | 4.94 |
Discussions and conclusions
Desktop analyses to review the heating energy demand associated with mesophilic and thermophilic anaerobic digestion modes have led to several conclusions and recommendations.
Thermodynamic analyses conducted on the digester structure to estimate heat losses associated with the two reviewed digestion modes have shown that instantaneous sludge temperature rises maintained in the heat exchanger (Figure 1) and hourly heat requirements (Figure 2) are greater for a thermophilic digester rather than for a mesophilic digester, while both have equal physical dimensions and volume.
Thermophilic anaerobic digestion carried out in a two-time smaller reactor than a mesophilic digester has comparable temperature rises and hourly heat losses as in the mesophilic digester.
Heat demand required to digest one m3 of primary sludge within the respective digestion period (HRT) is greater for mesophilic anaerobic digestion due to a longer HRT than the thermophilic anaerobic digestion HRT. The average heat requirement difference is around 25% for equal volume reactors and around 100% for a two-time smaller reactor, “50% Volume” (Figure 3).
Addition of raw primary sludge and TWAS to an operating digester and the following temperature drops are below the tolerable temperature fluctuation for each anaerobic digestion mode.
Organic material reduction and methane gas production is greater for the thermophilic anaerobic digestion mode than for the mesophilic anaerobic digestion mode at 25% and 50%, respectively. As a result, the calorific value of the organic material reduction yield is greater for the thermophilic anaerobic digestion rather than for its mesophilic counterpart at approximately 50% per 1 kg of reduced volatile solids.
Conducted comparative analysis has demonstrated that the overall heating requirement of thermophilic digestion, mostly due to the shorter HRT, is lower in the long run as compared with the heating requirement associated with mesophilic digestion. Greater reduction of organic material in the thermophilic digestion results in greater methane yield (calorific value surplus as compared with the mesophilic anaerobic digestion). This calorific surplus is a benefit in favour of the thermophilic digestion application.
Ivan Drako is with WSP. This article appears in ES&E Magazine’s February 2020 issue. References are available upon request.
