Discussions and conclusions
Based on the analyses conducted of the heat input requirements, the digester process control narrative was updated and utilized for programming the automated control of the sludge temperature. The proposed control system will quickly respond to the sludge heating needs and has the option of adjusting process set points when needed.
The analytical results have also led to some conclusions that may be used for digester design and operation purposes:
- a) If sludge flow monitoring and recording is not available at a digester, the increase, in time, of the temperature rise needed to maintain the digester operation temperature may be indicative of a decrease in the recirculation pump flow. This may be due to possible sludge accumulation in process pipes, or excessive buildup developing on the heat exchanger “sludge” surface. In light of this, an increase in temperature rise demand should be a trigger point for detailed review of the system operation.
- b) The heat requirement analysis should be completed for each digester and be part of the digester operation and maintenance data. Continuous recording of temperature changes in the digester reactor and further analysis may provide valuable information for long-term monitoring of the digester operation, and for scheduling maintenance and repair activities. Any visible changes in digester temperature should be analyzed and compared to the initial conditions, in order to review the digester operation and develop a troubleshooting program.
- c) In the case of a “24-hour shut-off” scenario, it appears that digester temperature drop is not significant (for the reviewed case). Availability of such data would be required for planning and implementation of maintenance or upgrades to the digester heating system. Such upgrades can be conducted without system interruption and have minimum impact on the digestion process. However, the recirculation pump must remain in operation.
- d) It is obvious that proper thermal insulation installed within the above grade digester structure provides significant heat savings. When the digester is offline for rehabilitation or upgrades, installation of new insulation with more efficient thermal characteristics would be a good practice.
- e) The below grade structure appears to account for the major portion of total heat loss. The value of heat loss is approximately 75% to 95% for winter and summer, respectively. The thermally insulated above grade structure with an almost equal surface area contributes a smaller fraction to the total heat loss (less than 25%). This is attributable to the fact that, unlike the above grade structure of the digester, the below grade structure experiences constant exposure to a steady-state lower temperature environment throughout the year.
- f) Having assumed that the digester’s below grade structure has been furnished with thermal insulation, the estimated heat loss reduction may be expected, for the reviewed case, to be in a range of 35% to 40%.
Ivan Drako, Michelle Albert, Ron Cariglia and Sean Hutchinson are with WSP. Edwin Ayson is with the City of Toronto. This article appeared in ES&E’s September/October 2015 issue.