Monitoring discharges into the natural environment is becoming increasingly regulated, including requirements to estimate the time, volume, and pollutant load.
Can you tell me what practices you have implemented on site to track these indicators?
=> What equipment, methods do you use to measure the flow?
=> When flow calculation comes from a mathematical relationship linking gauge height to flow, what type of combined sewer overflow systems does this apply to and what are the associated equations? Are there cases where this kind of relationship is not relevant?
=> how do you proceed with estimating or measuring the pollutant load?
Thank you in advance for your answers
Just like there are many kinds of CSOs, there are myriad ways of calculating the kind of data you are looking for.
I have tested several, so I'll tell you what makes the most sense to me.
1 – Water level and flow rate for a pipe where discharge section is clean (smooth edge with no warping, etc.)
You need to install an ultrasonic transmitter or guided-wave radar (depending on the type of manhole) which will provide the readings automatically via remote management. You can couple that with a water sensor to boost battery life in installations that do not have power. This seems to me the most reliable solution, as there is little danger of wet wipes or other debris giving you false readings.
2 - create a free overflow spillway: very reliable in terms of measuring precision, but watch out for paper or wipes than can cause blockage and distort readings.
The weir crest needs to be cleaned regularly, and there is the associated cost of installation or modification depending on the configuration.
3 – installing U- or V-shaped facing. This is reliable for measurement purposes, but difficult to attach (make sure the masonry is done properly); same problem as with a free flow spillway, watch out for wet wipes.
If discharging into a river, you need to think about the flap valve and flood sensor—this avoids incorrect readings when the river overflows the network.
Also beware of rats (they like nibbling on cables and waterproof boxes)
Thanks, that is really interesting. So if I follow you, the two systems to favor are:
- overflow orifice, i.e., a round opening in the wall of a lift station or manhole. With a level measurement, we can apply a conversion equation of height flow. On a technical level, it requires virtually no maintenance (inspections/periodic cleaning), the shape of the opening means that floating debris is dispersed by the current and does not alter the flow measurement. However, the height flow relationship is less accurate (why?).
-Free overflow spillway. The height flow conversion method is more accurate than measuring the flow through an opening. However the slightest obstruction in the overflow nappe and readings will be distorted. (I guess that means that if a 5-cm wipe gets lodged, it blocks the flow and shortens the length of the nappe, thereby changing the discharge estimate?) To ensure a reliable reading, therefore, the weir crest has to be visually inspected and cleaned often. In terms of cost, it seems pretty straightforward and relatively inexpensive?
-for a U or V-shaped notch, it must be more complicated because the wipes could get stuck at the base of the V?
Obviously we have to make do with the structures we have, but in the case of a new installation, it’s best to stick to simple openings or weir-based overflow nappes.
Hello Bilbo, glad to be of help.
Monitoring water level in a pipe is less accurate than with a free flow spillway because the discharge section has a significant impact on the relationship between height and flow. For example, depending on whether the gradient is steep or if the pipe lies flat, the calculation changes and does not take slope into consideration.
This problem does not occur with the spillway because of the free flow behind it, so the height/flow ratio is accurate (except in the case of wipes...)
In terms of installation costs, this can vary according to the configuration of the site and the nature of effluent.