Hydraulics of lamella sedimentation.A study of the lamella settlers at Ringsjö water treatment plant, Sweden.

Abstract

This study investigates the lamella settlers in Ringsjö water treatment plant, Sweden, in terms of water movement and performance. The treatment plant has two blocks with differences in the flocculation and lamella settlers, where block 1 is not able to process the maximum designed flow rate without the addition of polymers. Through data analysis the problem was confirmed, showing that the two designs have the same turbidity when block 1 has approximately 20 % less flow rate than block 2. Moreover, variables such as settling velocity and turbidity were measured and used to estimate and evaluate the removal rate of the system, while the hydraulic losses of the system appear to be negligible in both lamella designs. Three future possible designs of the settlers that could increase their capacity were investigated but as the cause of the problem remains unclear, which design would be more efficient is uncertain. However, as the problem started when the coagulant was changed and currently the capacity of block 1 is increased with the use of a polymer, the possibility that the flocculation plays an important role in the cause of the problem is discussed, and it is concluded that there is a need to further investigate the floc characteristics.

Popular Abstract

Ringsjö water treatment plant produces drinking water through two blocks of production, which differs in the flocculation and separation step, in terms of volume and lamella settler’s design. The oldest block, block 1 has approximately 20 % lower flocculation basin volume and almost 60% lower lamella sedimentation basin volume, while the shape and the feed to the lamella plates also differs. It has been observed that block 1 cannot have a sufficient performance for the maximum designed flow rate, and instead it appears to have a de facto capacity 25% lower than the maximum. This study investigates the lamella settlers in Ringsjö water treatment plant, in terms of water movement and its performance, by analyzing data from the past seven years and the drawings of the lamella settlers. Furthermore the removal rate of the settlers was estimated based on the advection-diffusion equation and variables measured at the treatment plant, such as, flocs’ settling velocity and turbidity in the inlet and the outlet of the settlers. Finally, some alternative designs that could possibly increase the capacity of the block are discussed.
The analysis of data, confirms that block 1 appears to have a poorer performance than block 2, as for the same flow rate, block 1 has approximately 20% larger turbidity. This behavior is more severe during winter, as with a decrease in temperature, turbidity increases significantly. This different performance of the two blocks could be attributed to the different volume and design of the flocculation and sedimentation settlers. When observing the water flow in the lamella settlers of the two blocks, it can be seen that the feed to the lamella plates in block 1 is more complicated than block 2. The water movement was analyzed for each design and in both cases the hydraulic losses appear to be negligible. In addition, the differences in flocculation where discussed as, floc characteristics play an important role in the sedimentation process. Flocculation basins in block 1 have approximately 20% less volume than block 2, resulting in lower residence time. It is likely that the residence time in block 1, is not enough for large and well-formed flocs to be created. Currently, a polymer is added in block 1, increasing its capacity, indicating that the problem might be more related to flocculation. The theoretical removal rate estimated by the advection diffusion equation and the turbidity removal rate were calculated based on measurements conducted while a polymer was added in the process, showing that block 1 has a good performance. However, the removal rate prior to the polymer cannot be estimated, as there were no available data regarding the flocs characteristics and settling velocity.
Moreover, possible modifications of the lamella settlers that could increase the capacity were considered depending on the main cause of the problem. If this behavior of the lamella sedimentation depends on the settlers feed to the lamellas, modification in the inlet channel could have a positive effect on the settlers performance. On the other hand, if the main cause of the problem is related to the flocculation, changing the flow pattern in the lamella settler from counter-current to co-current could be beneficial, as co-current settlers are more suitable for the removal of light sludge, or the possibility to increase the residence time in the flocculation basin should be considered.
Finally, the causes of the problem were not addressed in this study but through the investigation of the differences between the two blocks, the possible causes where discussed and a floc analysis is recommended. More knowledge on the floc characteristics is significant for the separation step and it could even provide new clues regarding the problem’s cause, as it could either prove that the flocs developed in block 1 have poor settling properties or eliminate this scenario, if the flocs produced in block 1, have the same characteristics as in block 2.

• author: Lytra, Maria
• supervisor: Magnus Larson
• organization: Division of Water Resources Engineering
• course: VVRM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Lamella settling tanks, Inclined sedimentation, Ringsjö water treatment plant, Suspended solids separation
• publication/series: TVVR19/5006
• report number: 19/5006
• ISSN: 1101-9824
• language: English
• additional info: Examinor: Kenneth M. Persson
• id: 8991326
• date added to LUP: 2019-08-05 10:37:30
• date last changed: 2019-08-05 10:37:30