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Wastewater Myths
On Rules Of Thumb And Diminishing Returns

The good old trusty sludge judge.
I’ve heard so many “rules of thumb” about operating wastewater plants that I’ve lost count. Never let your secondaries go above 2 feet, don’t over aerate or you’ll sheer your floc, and the list goes on and on. I’ve found that in most cases these are good rules to start at but they definitely aren’t the end all be all of operating a wastewater plant. Often times people focus so much on maintaining these unwritten rules that they lose sight of the bigger picture. The entire process is linked together and to get tunnel vision on maintaining a rule of thumb simply because you’ve been told to is absurd. Sometimes you’ll have to run the clarifiers higher, especially during rain events. And guess what? You’ll likely be fine. I had one operator try and dial in the secondary clarifier depths to nearly a tenth of a foot and did everything he could to make sure they were all nearly the same. I told him to stop obsessing over it and as long as they are within 1 or 2 feet of each other it’ll be fine. When he kept arguing with me, I asked him what he was accomplishing by attempting to keep them so close together. Operational it made no difference and we had ample evidence to indicate that. But he was stuck on this idea they all had to be uniform. Sometimes you’ll have no choice but to over aerate as well. A lot of plants do not have sophisticated blowers with VFDs and air valves with actuators on them. It’s awesome when you do have such luxuries as you can dial air down to the .1 ppm with those systems. Unfortunately, not everywhere has those so you’ll either be spending a ton of time and frustration trying to dial those valves in by hand and still struggle to get the air balanced in a meaningful way, or you’ll just have to accept the fact you’re going to have higher D.Os and maybe some pin floc in your secondaries.
Data driven operators aren’t excluded from this either. You can have every spreadsheet and formula available and still fall victim to diminishing returns. A different time, a different place. One colleague loved data and made sure the plant’s MCRT was always dialed in, constantly tracked all the metrics (F/M, sludge age, etc), while the other colleague operated by visual inspection of the MLSS in the aeration tanks. In most cases the guy who just wasted off the appearance of the aeration tank kept the plant in line just as well as the data-centric guy. Point being, these systems are fairly resilient and there is a point of diminishing returns. Constantly obsessing over every single equation and making sure they are dialed in isn’t always useful. Sometimes you’ll deviate well outside the recommended operating parameters of a plant and still maintain compliance. Sometimes one metric will be out of whack, while the others are “dialed” in, and still maintain compliance. More importantly there are absolutely going to be times when you’re operating outside of ideal conditions. Whether that is rain events or if you’re forced to modify your process control strategy due to a landfill that stops taking municipal biosolids cake due to a consent order from the state. That last part is true story and really makes wasting difficult. So what do you do in these less than ideal situations? You adapt and do what is necessary. You get creative and use every available storage tank you have on site. You cut wasting back as much as possible while trying your best to maintain compliance. You’d be surprised how far you can deviate outside the norm and still be in compliance.
One of the facilities I used to work at literally had about a week’s worth of capacity of sludge storage. Anytime the hauler couldn’t haul (which was incredibly frequent), we would have to use whatever back up space we had. And that required us to run the sludge backwards through the system to an old thickener tank that was no longer in use. That bought us a few days. If that would have continued then we would have obviously had to start getting even more creative and attempt to find alternative places to get rid of the sludge. The point is we didn’t hold fast and firm to normal operating guidelines that we had been told. We got creative and started to operate outside of normal guidelines. We ignored rules of thumb. Fast forward to my current plant. Remember the whole issue I stated above about not being able to get rid of my cake because of the landfill consent order? That was a major problem because I already had full sludge storage tanks, and now I had to scramble and get data to all of these different landfills. That’s another story for another time, but the point is that I was forced to cut back wasting. And guess what? I remained in compliance in very less than ideal situations for that first month while we wasted less, depths rose, and I couldn’t follow any “rule of thumb”. I had to actually operate.
There is often way too much focus on “dialing” in a plant. I’ve sat in meetings and discussions where we were arguing over very minor changes that didn’t improve operational efficiency one bit. Yet people would argue that the RAS return rate had to be exactly this percentage, or the depths had to be exactly even across the board (which is hydraulically kind of difficult in most places), and every tank had to be in service just because that’s what they were told. Case in point, my current plant had always run three final clarifiers. I was curious as to why as I found the plant to be normally hydraulically underloaded, so I tested out running only two final clarifiers during the dry months. Guess what happened? Nothing. All of the lab data indicated the plant was operating the same, in fact there was some settling improvements. I strongly believe the tinkerer operators out there love to make changes because it makes them feel like they are doing something, when in actuality they would be better served doing some preventative maintenance. There is a point when you just have the plant dialed in and you don’t need to make any more changes, but sometimes that is just too easy and the ego cant handle it. If you’re in compliance, I see no reason to follow some of these wastewater myths we’ve all heard. You know your plant better than anyone, so act accordingly and operate it.
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