 |
Two of our 620 C models. The competitors system would
have had approximately 15 times more exposed hatch area. |
Elementary, one-dimensional, conductive heat transfer equations.
Integrating and rearranging, we obtain:
This is a simple conductive heat transfer equation stating that the transfer of energy as heat (q) :
Decreases:
- As the thickness of the transfer medium(insulation) increases. (x and x1)
Increases:
- as the temperature difference between the warm side and cold side increases (T and T1)
- increases as thermal conductivity of the medium he increases (km), and
- increases as the area of transfer surface increases (A)
Obviously the converse of each statement above is also true. For example, the heat transferred will decrease if the area of transfer surface decreases.
The equations are simply a concise way of stating something we all know to be true based on our everyday experience.
We all understand that, in general, bigger windows in our homes equate to higher heating bills in the winter because windows are not very thermally efficient and even the best windows allow much more heat to escape than well constructed walls. We also understand that thicker insulation will result in less heat loss and that it requires more energy to keep the house warm when it is cold outside than when it is warm outside.
The equation above describes heat transfer by conduction. For example, a metal spoon gets hot if you put one end in a fire because the heat from the fire is conducted along the length of the metal spoon. I have not presented equations for other mechanisms of heat transfer, convective heat transfer, for instance. Heat transfer by free or forced convection is why we blow on hot soup to cool it. Moving air across the surface of the hot liquid results in convective transfer of heat from the soup to the air.
So, where am I going with all this, you ask......
If the other variables are kept equal, and we vary only the surface area of the access openings it is simple to show (lets bring this back to onsite treatment systems) that greater access hatch area results in greater heat loss from the treatment system and a resulting lower temperature inside.
The effect of temperature on biological activity can be described by the Arrhenius Equation:
Here is the Arrhenius equation in graphical form. I set all the variables to a constant so we could investigate k as a function of T, k=f(T). k is the temperature dependant rate coefficient. As k goes up, microbial activity goes up. As k goes down, microbial activity goes down. This is why we refrigerate our food; colder temperatures slow the action of microorganisms that lead to food spoilage. But, in a wastewater treatment system, we want the microorganisms active so they can break down the pollutants in the wastewater.
In the graph above you can see that as the temperature increases, the value of the rate coefficient increases. Keep in mind that to create the graph, I set all the variables on the right hand side of the Arrhenius equation to a constant. Therefore, the numerical values of temperature on the horizontal axis serve no practical purpose other than to show increasing or decreasing temperature.
Remember, wastewater treatment systems do their job because microorganisms utilize constituents in the wastewater as a food source. The vast majority of wastewater treatment system are "
biological wastewater treatment systems" that experience declines in efficiency as their internal temperature declines.
So what causes a decline in temperature? Heat loss. And, as the system loses heat, its temperature goes down; as its temperature goes down, microbial activity slows. And, as microbial slows, the ability of the system to treat the wastewater diminishes.
With this in mind it is possible to understand the types of onsite systems not particularly well suited for cold climates. By cold, I mean anywhere temperatures can be expected to drop below about 40 degrees F.
Suspended Growth Systems: These types of onsite systems utilize an air blower to force outside air into a treatment tank. If the outside air is cold, the efficiency of these systems decreases dramatically. And, since most of the air-bubbler systems have no means of operational control other than blowing more air, the only thing the operator can do to affect treatment actually makes things worse because it just drives the temperature down further. A big manufacturer of bubbler systems tried to take on Montana winters a few years ago. As I recall the data, not a single one of their systems even came close to meeting Montana's nitrogen reduction standard. Another large manufacturer was included in a long-term, govt-funded DNRC (Dept. of Natural Resources and Conservation) field study in Montana, and that system was removed early by the manufacturer because it never worked and the results were pretty embarrassing. Incidentally, that same system is now listed as a "Best Available Technology" in Maryland. As far as I can tell, the factors inhibiting that particular system's ability to provide effective treatment results have not been resolved, which may partially explain why Maryland's current governor is calling for a moratorium on all onsite systems...apparently, the systems bestowed with the "Best Available Technology" qualifier aren't quite getting the job done in Maryland. The onsite wastewater treatment industry is one of the few industries where, instead of regulatory agencies setting necessary water protection standards and industry rising to meet the challenge, industry dictates standards to regulatory agencies through a combination of lobbying, political maneuvering, over-involvement in the standard-setting processes, and, ultimately, downright inability to meet the higher standards. But enough of my rant...let's get back to our regularly scheduled program
Fixed Film Systems (FFS): Fixed film systems utilize some type of media where microorganisms can attach and grow. Fixed film systems are widely regarded as being superior to suspended growth systems because they provide a higher degree of treatment more reliably than suspended growth systems. Not all FFS are created equal however.
The heartbeat of any FFS is the media it utilizes. Let's consider a few points about media because the type of media used often dictates the overall system design.
I think most people would agree that because wastewater is going to be applied to the media, it should be resistant to decomposition. Clearly, organic media like peat moss, straw, leaves, plant fiber, is not a good idea because over time these will begin to decompose and the owner will need to dig the system up, scoop out the old rotten media and replace it with new media. Owners of these systems enjoy the luxury of getting to repeat this process over and over. Heat transfer in these system may also be excessive because as the media becomes saturated, it loses its thermal efficiency. Think about the difference in how you feel outside on a cold day in dry clothes versus wearing soaking wet clothes.
Several FFS are based upon one type of fabric or another. In a few of these the fabric is rolled up, in others it is hung in sheets, I have seen several that use little scraps of patchwork dumped into a tank. They generally do not experience decomposition seen in organic media but may experience biological fouling due to the small pore spaces in the fabric. What does this have to do with thermal efficiency?
If the media has an inherent potential to break down or clog, it will be necessary to remove the media from the tank in which it is installed and either clean it or replace it. In your minds eye, picture a tank full of decomposed and/or clogged septic system media. It's not pretty and removal is absolutely not a clean job. But, in order to facilitate removal, manufacturers require large hatches on these tanks; a person simply cannot remove a ton of septic-saturated peat moss through a 2 ft access opening. So, they use hatches that generally cover the entire surface area of the tank. These large hatches tell you something about the system:
- Heat transport out of the treatment unit will be greater in systems with large hatches compared to systems with smaller area hatches.
- Biological activity and hence,wastewater treatment, may be negatively impacted due to lower system temperature.
- Because tanks are more costly to construct with large hatches, their inclusion suggests that at some point the media will need to be removed or cleaned or both.
- If it becomes necessary to inspect the system in cold weather, opening a huge cover will expose the entire treatment system to potentially damaging frigid air. Anyone who has ever spent time in sub-zero temperatures knows what I am talking about.
I think the problematic nature of many fixed film systems stems from the fact that the treatment media was not specifically designed to treat wastewater but rather was borrowed from some other application(I am thinking aquarium filters, yard landscaping, clean water filters) and force-fit to onsite systems. If the treatment media, the most critical component of the system, is not ideally suited for the task, then the system must be designed to manage the flaws of the media. This has the result of treating symptoms rather than dealing with the root cause of the problem. It reminds me when I was a kid and tried to build a go-kart from an old riding lawnmower. It would "go" but it certainly wasn't a go-kart.
MetaRocks were designed specifically to treat wastewater and they do this exceedingly well. MetaRocks do not decompose and will easily treat wastewater that would quickly cause fabric, peat or plant fiber systems to fail hopelessly. Since MetaRocks do not need to be removed or replaced , we can use smaller hatches and our systems stay warm even in brutal Rocky Mountain winter. Furthermore, the robust design does not just make Eliminite a superior cold weather system. Independent testing shows superior secondary and tertiary treatment levels in warm climates as well. The bottom line is if you are looking for 80-90% nitrogen removal at the best price, we have your wastewater treatment system.
.jpg){kind=link}