One of the objectives of the DESAL+ project was to perform a study on the water post treatment system in the desalination plant of Porto Santo Island and propose solutions for an identified problem.
To correct the pH, and thus the corrosion potential of the product water, a certain percentage of the permeate is passed through five limestone contactors. At the exit of the desalination plant, the saturation pH is about 9,1 with a Langelier saturation index (LSI) of -0,43. The analysis of the water supply network reveals that the pH value and the LSI was lowering along the network. The lowest LSI of -1,33 was found in one water distribution tank for a pH value of 8,2. It was obvious that the water was becoming more aggressive along the water supply network.
The study has the objective of proposing a solution to stabilize the pH and the LSI along the water distribution network.
After collecting operational data of the desalination plant, and taking analysis in nine different collecting points including water storage tanks, a first report was elaborated by ITC and FCCA.
The report suggested that the water was absorbing CO2 from the atmosphere and that was the main reason for the pH value decrease. The suggested solution was to dose 18 mg/l CO2 before the limestone contactors and thus increasing the alkalinity and water hardness. A contact period of 12 to 15 minutes in the contactors was also suggested. The original design of the existing contactors was for descending flux. The report suggested that ascending flux was more efficient.
To validate the suggested solution some tests had to be made. One of the contactors was disconnected from the post-treatment system. One new flowmeter and a small CO2 dosing system was installed. Some pipe rearrangement was done to allow the tests with ascending and descending flux.
After performing the first tests water was storage in one reserve tank and then analysed in the ARM water Lab. The operational parameters of the first tests are the ones showed in Table nº 1.
Table 1 – Operational parameters of the first tests.
The first results, as expected, showed an increase in alkalinity and hardness, as can be observed in Table nº 2.
Table 2 – Test results
Surprisingly, the second test, with higher flow and less contact time, reveals another increase in alkalinity and an even better LSI. The water hardness measured in French degrees was increased from 2,3 to 6,6. Further tests and analysis will be performed to understand this difference.
Besides the good results obtain so far, further tests are to be perform with descending flux, which corresponds to the original design of the contactors. In addition, it will be installed a static mixer to increase de CO2 mixing with the permeate water, as the actual distance between the injection point and the contactor hasn´t the optimal distance due to installation difficulties. In conjunction with the static mixer, further tests will be perform with higher velocities and less contact time to optimize the actual contactors, taking them to their treatment capacity limits.