- Biological wastewater treatment plants produce toxic sludge that must undergo a phosphorus recovery process. Phosphorus can be recovered from the ashes of incinerated sewage sludge.
- The hazardous gases released during combustion of the sludge are bonded during wet flue gas cleaning. The raw water is neutralized by adding a sodium hydroxide solution.
- Appropriate sodium hydroxide solution dosing requires pH measurement in the resulting raw water, as well as conductivity measurements of the sodium hydroxide solution to determine concentration.
- As a multi-channel transmitter that can monitor several measuring points at once, the Protos analyzer ensures reliable process control in combination with the SE656 and SE555 Memosens sensors.
Raw Water Treatment After Flue Gas Cleaning
The scrubber water resulting from flue gas cleaning in sludge combustion must be cleaned and neutralized before being returned to the process.
In modern biological wastewater treatment plants, sludge that mostly consists of feces-consuming microorganisms remains. The sludge contains toxic admixtures such as heavy metals, halogenated hydrocarbons, and pathogens that argue against composting or agricultural use as fertilizer. With the reorganization of sewage sludge utilization, the phosphorus recovery became mandatory in Germany. Phosphorus can be recovered from the ashes of incinerated sewage sludge. The energy gained during combustion can be used to generate electricity for centrifuges, pumps, etc. and the waste heat can be used to dry sludge. But corrosive gases that must not end up in the environment are released during the sludge combustion process. After NOx removal, the wet flue gas cleaning process ensures that the remaining pollutants are separated. The flue gas is guided against sprayed water from below in counterflow, and gaseous pollutants are bonded to the aqueous phase. The resulting raw water has a pH of approx. 2. In a second cleaning step, sodium hydroxide solution is added to bind the gases (mostly sulfur compounds) which are particularly smell-intensive. In that process the pH value should not fall below 7. Because several scrubbers are typically deployed in proximity, using multi-channel devices for pH monitoring has many economic and practical benefits.
Protos: Reliable process monitoring and high flexibility
Since several measuring points can be implemented with only one Protos-series transmitter, the analyzers have proven to be outstanding in flue gas cleaning applications. In simple cases, sodium hydroxide solution dosing can be controlled via Protos relay contacts. In the basic version, the modular transmitter has four relay outputs that meet the minimum requirement for 2-channel measurement.
If necessary, the number can be doubled using an output extension module. Protos’ modular structure ensures users maximum flexibility in all matters. For example, previously one-channel measuring points can be operated with two channels by plugging in another module. Additional modules for extra contacts, current outputs or control functions (PID) permit simple, needs-compatible retrofitting at all times.
More effective maintenance options with Memosens sensors
To determine the concentration of the sodium hydroxide solution, conductivity is measured using the SE656 toroidal sensor. The sensor with high resistance to chemicals was designed especially for heavily polluted and strongly oxidizing media. For pH measurement of the alkalization in scrubbers, which have permanent pH values of 12, we recommend using the Memosens pH glass electrode SE555, which has a service life of approx. 6 months. The option of pre-calibrating Memosens sensors in the laboratory eliminates the need for difficult on-site calibration.
Despite these complex measures, the odor impact on personnel in the immediate process surroundings would be unacceptable. This is why the buildings are purged with fresh air and the exhausted, polluted air is cleaned in a final wet scrubbing process by alkaline washing. Before the scrubber water can be re-used in the sewage treatment plant without damaging the biological stages of the plant, it is neutralized in a final step in which further pH measurements are required.
Return on Investment
- Long service life of the Memosens pH sensors
- Implementation of several measuring points with only one Protos transmitter
- The unique Protos modular concept enables easy adjustment to changing measurement requirements at all times.
- The optional full automation of measuring points that are difficult to access reduces the service effort.
- The option of dosing the sodium hydroxide solution with Protos eliminates the need for additional controls.