Last week I sat quietly while a design engineer lectured a client about my company’s “bad science.” He became so rabid during the two hour rant, I offered to get him a soda. Not because I was in any way empathetic. The spittle on his lip was grossing me out. Sadly, he declined the offer of drink and I withstood a two-fronted assault: verbal and visual.
Charlatans exist. I just read about a fellow in Russia who claims to have invented a device that supposedly makes drinking water out of radioactive waste. In the spirit of creating jobs in the Soviet down economy, I suppose, United Russia, the nation’s ruling party, has invested millions of rubles in the former convict’s “inventions.”
A healthy skepticism is a good defensive mechanism. In my circumstance, the engineer was outraged at my ability to work with an intelligent client to make the treatment plant he designed produce an effluent three times cleaner than his mathematical model said possible: 1.2 mg/L total-nitrogen. Without chemicals, Suffield (CT) is producing better effluent with much of the engineer’s equipment disabled. Worst of all, I have shown the audacity to provide scientific explanations. An unmitigated outrage!
At the risk of getting dead fish snail-mailed my way, I’m using this forum to share one of the very theories that got my counterpart jacked up. Comments, as always, are welcome.
In Modified Ludzack-Ettinger (MLE) process designs, engineers provide pumping equipment to internally recycle 3 to 5 times the influent flow rate. Math drives the designs. By recycling 4Q, 80% of the nitrate produced during nitrification is returned to the anoxic tank for denitrification.
Notwithstanding the beauty of the mathematics, high recycle rates don’t improve nitrogen removal at the treatment plants where I hang. The opposite occurs: too much internal recycling reduces nitrate removal. This fact has perplexed many an engineer.
Most consultants, when confronted with this reality, ascribe it to the recycling of dissolved oxygen back into the anoxic tank, and a resulting increase in anoxic DO. I agree. But. I believe it to be more than that.
And, here’s where I got Rabid Engineer frothing. I believe that the recycle rate affects the hydraulic retention time in the pre-anoxic and aeration tanks. That, when the recycle rate is too high, bacteria spend too few consecutive minutes in anoxic conditions to denitrify.
Bacteria, as those of us in the wastewater biz know, are prolific little copulators. But even these little sexpots require some time to get in the mood: to court one another, enjoy a meal together, and then go about their reproductive business. If they aren’t kept in an anoxic environment long enough, the growth rate is affected and nitrate removal efficiency declines. On the other hand, slowing down the internal recycle rate gives them more time to cozy up to one another and go about their nitrate removing business.
Biology trumps mathematics! That’s my story, and I’m sticking to it.
Thanks for reading.
Grant