Do Me a Solid
In my last from the desk of I wrote about the importance of the F to M ratio. F to M is a mathematical equation that lets me know the amount of food (biodegradable material) vs the amount of Mass (Bugs) that I have coming into my plant.
The food is measured in a test that I run at the plant called a B.O.D. 5 test. In the wastewater world B.O.D. = food, or organic material. As a wastewater operator I want to remove as much B.O.D. as I can before I release it back into the environment. My goal is to turn B.O.D. into bugs. When I say bugs, I mean microbes Bacteria, Flagellates, Ciliates, Rotifers, Algae, and Fungi. It is just easier to say and think of them as “bugs” at this point in the operation.
There are two main processes in the wastewater treatment plant.
One is the liquid treatment process in which the water coming into the plant called the influent is chemically and biologically treated to remove the B.O.D., ammonia, and phosphorus. Once that is removed the clean treated water is released back into the environment. This is called effluent.
The other process is solids treatment. Solid treatment is the process of removing the bugs out of the system, drying them out and eventual disposal.
Remember as long I have B.O.D. or food entering the wastewater plant, the amount of Mass or bugs is going to keep climbing. Bad things could happen to the plant if I do not remove the some of the mass daily to keep my F to M ratio in check.
The process in which I remove the bugs is called wasting. Wasting is essentially just moving material (in this case bugs) from one tank to another. When I waste bugs out of the treatment system, I move them to a tank called a digester.
The digester is a storage tank that holds the older bugs that I no longer need in the treatment system. The type of digester that we have in Shipshewana is called an aerobic digester, meaning that we provide it with oxygen. The bugs stay in the digester and are no longer fed B.O.D. so they begin to starve.
What I want to accomplish in the digester is to remove as much water as possible, and I want the bugs to start breaking down and stabilizing. When this happens, we call it digested sludge. The bugs or “sludge” as it is also called in the digester can be held in the digester for weeks or up to month or better. Periodically I shut to air off in the digester to let the sludge settle to the bottom.
Once settled I can remove the clean water from the top of the digester and run that water back through the treatment plant. This process is called decanting.
I hold the sludge in the digester for as long as the sludge gives me clean water, or when I need room in the digester for more wasting.
Playing with Polymer
Sometimes the drying beds are just not fast enough to dry the sludge on their own. When I first started working at Shipshewana I could not remove and dry solids as fast as I needed to. After some research I found out about polymer and started using it.
The Polymer that we use at Shipshewana is a cationic liquid polymer. Cationic polymers act as a positively charged bridge to the negatively charged particles in the sludge. Essentially the solids in the sludge stick together and repel the water. This cuts the drying time in the beds from two weeks or so down to five days depending on the weather.
The following are the steps needed to go to the beds:
- Step one is to mix the thick and dense liquid polymer in a polymer mixer with water to make a slurry that can be moved and mixed with the sludge. Figure 2 shows the apparatus used to mix the water and slurry.
- Step two is to inject the polymer slurry with the sludge in a sludge mixer. Figure 3 shows the apparatus used to inject polymer slurry with the sludge.
- Step three is to fill the beds with the sludge polymer mixture. Figure 3 shows the apparatus used to fill the beds with sludge polymer mixture
Then bottom of the beds are filled with sand and pea gravel that allow the water to pass through and collect into pipes.
Once in the pipes the separated water is ran back through the plant for further treatment and eventual release back into the environment.
Once the beds are filled and the sludge is dried into a cake, we use a front-end loader to clean the beds. The dried cake is disposed of into a landfill.