- Category: Wastewater
NewsroomWastewater Lagoon Design
Design – Maximizing Effective Hydraulic Retention Time (HRT)
Early lagoons were designed to provide at least 120 days detention as required by KDHE, but little consideration was given to the actual path taken by the flow. Inlet pipes often went to the center of the pond, assuming that the flow would spread evenly from there. In reality, water typically takes the shortest path available, resulting in a retention time of only a few days or hours (shown below). We’ve redesigned piping and control structures in several systems to more effectively use pond volume. Sometimes, just cutting off inlet pipe is enough to help!
The cover photo shows an HDPE baffle we installed to improve treatment, and effectively split the cell in two to meet KDHE requirements (only the first cell held water before we began our improvements, and KDHE requires a minimum of two treatment cells).
In our research, we came across a computer modelling study confirming this from the Alexandria University of Egypt, the “Study of Waste Stabilization Pond Geometry for the Wastewater Treatment Efficiency”. It shows an impressive increase in treatment as more baffles are added or the aspect ratio is increased while keeping the pond volume constant – forcing the flow to take a longer path (shown below). Keep in mind, though, that this was a 2-D computer model, so while it provides a good starting point for design, it doesn’t take the effects of wind, waves, and depth into account.
3-D Modelling with a program like the Solidworks/Cosmosworks package can provide a more accurate representation of your lagoons (shown below). One limitation of Solidworks and other flow modelling programs is that wind could not be taken into account, but these results were very similar to what we found by going out in a boat and taking dissolved oxygen (D.O.), pH, and sludge thickness measurements. The Solidworks analysis combined with our measurements allowed us to confidently install aeration and mixing where it was most needed.
The next thing to consider is prevailing wind direction and strength. In “The Spatial Significance of Water Quality Indicators in Waste Stabilization Ponds” published in Water Science and Technology, the researchers analyzed the effect of wind on flow. They used 22 inlets and 14 outlets in a real-world study, so that the flow would be evenly spread and approach the ideal plug flow condition. As you might expect, with wind in the same direction as flow or with wind directly against flow, they achieved the best retention times. With strong winds transverse (perpendicular) to flow, the flow was pushed all to one side (high longitudinal dispersion) and they experienced the worst retention times.
As always, there are several other elements to consider – thermal stratification, sideslope and bottom roughness, inlet and outlet pipe depth, trees or topography blocking wind, etc. Predicting flow path is a science, but also involves a lot of educated guesswork. Keeping all the above factors in mind during design can radically improve treatment!
Richard Ammel, PE
Richard Ammel, PE, EBH Engineer, is a licensed professional engineer who has designed and inspected the construction of sewers, pump stations, and wastewater treatment systems for several small towns in Kansas. He received his Bachelor of Science (B.S.), Civil Engineering degree from Colorado School of Mines. Contact Richard if you would like to discuss your municipality’s waste water treatment challenges at 620-793-8411.