CHEAPER + EASIER TO OPERATE = ?

By Steve Grimm

A small municipal wastewater plant is undergoing a long awaited upgrade. The sewer district is going to be expanded, adding two dozen, maybe more users. The aeration basins and clarifiers are going to be larger to handle the increased flows. The size of the waste sludge storage pit, which is already undersized in the operator's opinion, will remain unchanged. HUH? A trailer court installs a small-extended aeration package plant. The plant has no sludge wasting capabilities because they are told that sludge generation is so minimal that wasting is not necessary. WHAT? I don't understand what's going on here. Well, actually, I think I do. But let me start by stating that the opinions expressed in this article are my own. They should not be construed as opinions advocated by any particular person or group other than myself. Many of our smaller activated sludge plants are designed and built to run in the extended aeration mode. This mode is often referred to as the total oxidation process. The lower f/m ratio maintained in extended aeration means that the bugs aggressively compete for the little food available, and often utilize their own stored food and cell mass. This complete oxidation of BOD and cell mass produces a highly treated effluent and supposedly generates little or no sludge. Little or no sludge generation translates into minimal or in some cases non-existent sludge wasting and/or storage-digestion facilities. This in turn makes the plant easier to operate (one man/woman operation ERRRRRRRR!!!!!!!!!), and requires less of a capital investment (translation-cheaper). These are often the very words that change the minds of a reluctant board. Wait a minute! Lets think about this. As operators, do we really believe this? Is cheaper and easier really the best approach? Aren't we forgetting a few things? Some of the basics? Lets think back to the beginning. In the beginning was the Word, and the Word was… No, not that far back. How about back as far as Basic Ops? Remember when we were first introduced to activated sludge? It was basically defined as a biological process consisting of bacteria and microorganisms, dissolved oxygen, and food (wastes). It wasn't too far into the chapter that we learned we really could not control the amount of food entering the system, so we control the mass instead. How do we control mass? By wasting of course. The activated sludge course exposed us to the various ways in which we can control the process. Constant MLSS, F/M, MCRT, and sludge age, all important and useful tools. All deal with mass balance. All require excess sludge to be wasted from the system. These "bugs" are eating, growing, and reproducing. If their numbers are not controlled, eventually there will be more bugs than the available food can sustain. Then what? Endogenous Respiration. They begin to use their stored food and also begin to consume not only their neighbors, but also themselves, leaving a mass of dead bugs and inert material filling up the clarifiers. Those who are lucky enough to grab some food, do what comes natural-they reproduce also increasing the mass settling in the clarifiers. Their increased volume of sludge settles slowly producing a very clear, very clean effluent. The only problem is it is very susceptible to hydraulic surges. The slightest increase in flow, and up comes the blanket, over the weirs and out to the receiving water, violating the permit. Then next month that nice little letter from DEC arrives. You know the one… It says that last month you violated such and such and continued non-compliance could lead to regulatory action. If some sort of mass balance process control were practiced, chances are the blankets would have stayed where they belonged. The DEC would not have had to spend 34 cents to send you a letter. In reality, complete oxidation doesn't exist. Wasting is essential for proper operation. This type of scenario plays out far too often, especially in the spring of the year. Unfortunately, for the most part, it is out of the operator's control. He/She is doing the best they can with what they are given. You cannot control your mass if you don't have adequate waste sludge storage or digestion. And even with adequate storage, if you can't process the sludge either by drying beds, belt press, or bagging unit, or remove the sludge in a timely manner via bulk liquid hauling, the result is the same. Mass buildup. It's time we begin letting those who control the purse strings know what we need to perform our jobs properly. We need to be able to waste sludge. In order to waste sludge, we need sludge storage and/or digestion. We need sludge handling facilities. We need to get involved in the design of plant upgrades. We need to educate and inform not only our local politicians, but also the public and the funding agencies. I have begun to stress these issues during my conversations with representatives of Rural Development. I won't quit if you don't.

One final thought: I had this conversation with a frustrated operator the other day:
Me-"So, the snow must be just about gone, huh?"
Operator- "Yeah, my flows are way up."
Me-"And your blankets are…"
Operator-"GONE"
End of Sermon (for now)